ஸ்டீபன் ஹவ்கிங்கிடம் இருந்து முக்கியமானவைகள்...........

Black Holes
Overview
Black holes are the densest, most massive singular objects in the universe. Formed in one of three main processes, they exert so much gravitational force that nothing - not even light - can escape their pull. Since nothing can ever come out, it is called a hole. Since not even light nor other electromagnetic radiation can escape, it is called a black hole.
Black holes come in several different sizes and types, which are discussed in the following sections.
Black Hole Formation
Current theory holds that black holes form in three main ways. The first is that if a star has more than nine solar masses when it goes supernova, then it will collapse into a black hole. The reason that a neutron star stops collapsing is the strong nuclear force, the fundamental force that keeps the center of an atom from collapsing. However, once a star is this big, the gravitational force is so strong that it overwhelms the strong nuclear and collapses the atom completely. Now there is nothing to hold back collapse of the star, and it collapses into a point (or, in theory, a ring) of infinite density.
A second way for black holes to form is that, in some rare instances, two neutron stars will be locked in a binary relationship. Because of energy lost through gravitational radiation, they will slowly spiral in towards each other, and merge. When they merge, they will almost always form a black hole.
Finally, a third way was proposed by quantum cosmologist Stephen Hawking. He theorized that trillions of black holes were produced in the Big Bang, with some still existing today. This theory is not as widely accepted as the other two.
Types and Sizes of Black Holes
A black hole is classified by the only three properties that it possesses: Mass, Spin, and Magnetic Field.
Currently, there are only two recognized mass classes of black hole: Stellar and Supermassive. The stellar black holes are star-sized and range in the 10-100 solar mass range. The supermassive black holes are at the cores of - what appear to be - every large galaxy, including our Milky Way. These range in the millions to even billions of solar masses.
Intermediate black holes are hotly debated. There has been no universally-accepted proof as to their existence, and many doubt there to be a reasonable mechanism by which they would form.
The simplest black hole has no spin and no magnetic field. This is called a Schwarzschild black hole. A black hole that has a field but no spin is called a Reissner-Nordstrøm black hole. One that has both a magnetic field and spin is called a Kerr black hole.
The differences are discussed in the next section - Black Hole Anatomy.
Black Hole Anatomy
Schwarzschild Black Holes
To begin with the simplest type, a Schwarzschild black hole has two main components - a singularity and an event horizon. The singularity is what is left of the collapsed star, and is theoretically a point of 0 dimension with infinite density but finite mass. The event horizon is a region of space that is the "boundary" of the black hole. Within it, the escape velocity is faster than light, so it is past this point that nothing can escape.
Reissner-Nordstrøm Black Holes
A step up is the Reissner-Nordstrøm black hole. It has the singularity and two event horizons. The outer event horizon is a boundary where time and space flip. This means that the singularity is no longer a point in space, but one in time. The inner event horizon flips space-time back to normal.
Kerr Black Holes
A Kerr black hole adds another feature to the anatomy - an ergosphere. The ergosphere resides in an ellipsoidal region outside the outer event horizon. The ergosphere represents the last stable orbit, and the outer boundary is called the static limit. Outside of it, a hypothetical spaceship could maneuver freely. Inside, space-time is warped in such a way that a spaceship would be drawn along by its rotation.
An interesting point that comes up in the case of a spinning black hole is that of the naked singularity. The faster the black hole rotates, the larger the inner event horizon becomes, while the outer event horizon remains the same size. They become the same size when the rotational energy equals the mass energy of the black hole. If the rotational energy were to become more than the mass energy, the event horizons would vanish and what would be left is a "naked singularity" - a black hole whose only part is the singularity.
Yet another distinguishing feature of the Kerr black hole is that, since it rotates, the 0-D point that is the singularity in the Schwarzschild and Reissner-Nordstrøm black hole is spun into a ring of 0 thickness. Interesting theoretical physics can take place around this ring singularity. One consequence is that nothing can actually fall into it unless it approaches along a trajectory along the ring's side. Any other angle and the ring actually produces an antigravity field that repels matter.
NOTE: The only physical part of a black hole is the singularity. The other parts mentioned are mathematical boundaries. There is no physical barrier called an event horizon, but it marks the boundaries between types of space under the influences of the singularity.
Extra Features
Two other features can characterize a black hole - the accretion disk and jets.
An accretion disk is matter that is drawn to the black hole. In rotating black holes and/or ones with a magnetic field, the matter forms a disk due to the mechanical forces present. In a Schwarzschild black hole, the matter would be drawn in equally from all directions, and thus would form an omni-directional accretion cloud rather than disk.
The matter in accretion disks is gradually pulled into the black hole. As it gets closer, its speed increases, and it also gains energy. Accretion disks can be heated due to internal friction to temperatures as high as 3 billion K, and emit energetic radiation such as gamma rays. This radiation can be used to "weigh" the black hole. By using the doppler effect, astronomers can determine how fast the material is revolving around the black hole, and thus can infer its mass.
Jets form in Kerr black holes that have an accretion disk. The matter is funneled into a disk-shaped torus by the hole's spin and magnetic fields, but in the very narrow regions over the black hole's poles, matter can be energized to extremely high temperatures and speeds, escaping the black hole in the form of high-speed jets.
Finding Black Holes
No black hole has actually been imaged in a telescope. Actually, this is in itself impossible because, simply by definition, one cannot see "nothing." A black hole can only be spotted by observing how the material around it acts. Through this method, astronomers have observed many black holes; they usually are found in the center of galaxies, and some believe that every galaxy harbors a black hole in its center.
The rendering at the right depicts what a binary system with a black hole might look like, with it pulling matter off its companion star to form the accretion disk. NASA created this image.
Hypothetical Journey Through a Black Hole
What would happen if you were to fall into a black hole? As the you approach the black hole, your watch would begin to run slower than the watch of your colleagues on the spaceship. Also, your comrades notice that you begin to take on a reddish color. This is due to the warping of space in the vicinity of the hole. Then, just before you "enter" the hole (pass through the outer event horizon), your friends would see you apparently "frozen" there, just outside the event horizon and to them, your watch would have stopped (if they could observe it). They would never see you enter the hole, because at that distance from the singularity, an object must travel at the speed of light to maintain its distance. Thus your dim, red image would stay frozen in their eyes for as long as the hole exists.
However, from your vantage point, as you enter the black hole, nothing has changed. As you look "out" of the hole, the universe still looks relatively normal. However, you are drawn towards the singularity, and cannot escape its grasp. At this point, modern physics does not know what would happen. The most likely outcome is that you are compacted into a miniscule size upon the singularity.
However, you would not actually survive the fall into the hole. The immense warping of space around the hole would cause a spaghettification effect - you would be pulled apart because your feet (assuming they went feet first) would be far greater than the force on your head, and they you would be pulled as one pulls dough into a rope. This would be rather unpleasant, as well as fatal.
White Holes
The idea of a white hole is the opposite of a black hole, and is entertained more in science fiction than in actual science journals. Some believe it is the "other side" of a black hole. It is theorized to spew matter and energy out. A flaw in this theory, as many scientists have noted, is that the matter ejected from the white hole would accumulate in the vicinity of the hole, and then collapse upon itself, forming a black hole.

நான் தேடிப் பிடித்து படித்தவைகள்..........

In the beginning, the Earth was flat. At least it appeared so to its first observers, hunters and gatherers, and members of early civilisations. Not totally unreasonable, one would think, because the curvature of our planet's surface is not immediately apparent. Yet we know, and it must have been not totally inconceivable even to the archaic tribesmen, that our senses occasionally deceive us. The Earth being flat brings about the problem that it must end somewhere, unless we imagine it to extend infinitely. Infinity is a rather unfathomable conception and, hence, right down to the Middle Ages people were afraid of the possibility of falling off the Earth's boundaries.
Early cosmogonies.
What lies beyond these boundaries was largely unknown and open to speculation. The starry heavens were a source of endless wonder and inspiration. Peoples from all parts of the world created their own myths, inspired by the skies and the celestial bodies. Their cosmogonies can be seen as an attempt to explain their own place in the universe. Six thousand years ago, the Sumerians believed that the Earth is at the centre of the cosmos. This belief was later carried into the Babylonian and Greek civilisations.
According to the history books, it was the Greeks who first put forward the idea that our planet is a sphere. Around 340 BC, the Greek philosopher Aristotle made a few good points in favour of this theory in On the Heavens. First, he argued that one always sees the sails of a ship coming over the horizon first and only later its hull, which suggests that the surface of the ocean is curved. Second, he realised that the eclipses of the Moon were caused by the Earth casting its shadow on the moon. Obviously, the shadow would not always appear round, if the Earth was a flat disk, unless the Sun was directly under the centre of the disk. Third, from their travels to foreign countries, the Greeks knew that the North Star appears higher on the northern firmament and lower in the south. Aristotle explained this correctly with the parallactic shift that occurs when moving between two observation points on a spherical object. Among the Greeks, the heliocentric system was proposed by the Pythagoreans and by Aristarchus of Samos (ca. 270 BC). However, Aristotle dismissed the case for heliocentrism.
Ptolemy's geocentric model of the cosmos.
The influence of Aristotle was significant. Around 150 AD, Claudius Ptolemaeus (Ptolemy) elaborated Aristotle's ideas into a complete cosmological model. He thought that the Earth was stationary at the centre of the universe and that the Sun, the stars, and all planets revolve around it in circular orbits, hence, the model is sometimes referred to as the geocentric system. Ptolemy was aware that the postulation of perfect circular orbits contradicted observation, because the planets' motion, size and brightness varied with time. To account for the observed deviations, he introduced the idea of epicycles, smaller circular orbits around imaginary centres on which planets were supposed to move while describing a revolution around Earth. This enabled astronomers to make reasonably accurate predictions about the movement of the celestial bodies, and consequently the Ptolemaic model was a great success. The system was later adopted by the Christian Church and became the dominant cosmology until the 16th century.
Ptolemy's model of the universe was that of an onion with the Earth at its centre and stars arranged in layers around it. The outer layer was thought to be like a crystal to which the fix stars were attached. The hypothesis of epicycles accounted for the observable deviations.

(You can read this article briefly at “Stephen hawking” the brief history of time.)
Copernicus.
In 1514 the Polish astronomer Nicolaus Copernicus (1473-1543) put forward an alternative model, referred to as the heliocentric system, in which the Sun is at the centre of the universe, and all planets, including Earth, revolve around it. The further apart a planet is from the Sun, the longer it takes to complete a revolution. Copernicus said that the ostensible movement of the Sun is caused by the Earth rotating around its north-to-south axis. The heliocentric system got rid of Ptolemy's obscure epicycles, whose main weakness was that they did neither account for the observed backward motion of Mars, Jupiter, and Saturn, nor for the fact that Mercury and Venus never moved more than a certain distance from the Sun. Unfortunately, the Copernican system was not inherently simpler than the geocentric system; and it did not immediately render more accurate calculations of the planet's motion.
Galileo.
The end of the Ptolemaic theory came with the invention of the telescope. With the help of this device, Galileo Galilei (1564-1642) discovered the four largest Jupiter moons. The existence of these moons demonstrated beyond doubt that not all celestial bodies revolve around the Earth, contrary to Ptolemy’s theory. Galileo confirmed the Copernican model and thus initiated a scientific revolution of great importance, much to the discontent of the Roman Catholic Church. Unsurprisingly, Galileo struggled with church authorities during much of his lifetime. In 1594 the German astronomer Johannes Kepler (1571-1630) refined the heliocentric model in his book Mysterium Cosmographicum by showing that planets move on elliptical, rather than circular orbits. Kepler also prepared the idea of gravity by explaining that the Sun exerts a force on planets that diminishes inversely with distance and causes them to move faster on their orbits, the closer they come to the Sun. This theory finally allowed predictions that matched observations.
Kepler and Newton: The paradox of the collapsing universe.
Kepler’s model became the accepted 17th century cosmology, until Isaac Newton further refined Kepler's notion of the forces between celestial bodies. Newton postulated the law of universal gravitation that applied to all bodies, whether in space or on Earth, and he supplied the mathematical foundation for it. According to Newton, bodies attract each other proportionally with their size and inverse proportionally with the square of the distance between them. He went on to demonstrate that according to this law, planets move on elliptical orbits, as previously assumed by Kepler. Unfortunately, one consequence of this theory is that the stars of the universe attract each other and thus must eventually collapse onto each other. Newton was not able to give a plausible explanation for why this did not happen.
To counter this paradox, it was inferred that the universe is infinite in space, and thus contains an infinite number of evenly distributed stars, which would on the whole create a gravitational equilibrium. This assumption, however, would still imply instability. If the balance is disturbed in one region of space, the nearest stars collapse and the gravitational pull of the resulting more massive body draws in the next cluster of stars. Clusters would collapse like a house of cards and eventually draw in the entire universe. Today we know that this is not the case, because the universe is not static as Newton thought. The cosmos is in a state of expansion and therefore, gravitational collapse is prevented.
Is the universe infinite in space and time?
The question of whether the universe has boundaries in time and space has captivated the imagination of mankind since early times. Some would say the universe had existed forever, while others would say that the universe was created and thus had a beginning in time and space. The second thesis immediately raises the question what exists beyond its temporal and spatial bounds. Could it be nothingness? But then, what is nothingness? The absence of matter, or the absence of space and time itself? The German philosopher Immanuel Kant (1724-1804) dealt intensively with this question. In his book Critique of Pure Reason he came to the conclusion that the question cannot be answered reliably within the limits of human knowledge, since thesis and antithesis are equally valid. Kant thought instead of time and space as fundamental aspects of human perception.
Big Bang - the birth of our universe. (you can get more details the brief history of time.)
Fast forward: Despite Kant's doubts thereto, it appears that modern cosmology has answered the above question. The universe we can observe is finite. It has a beginning in space and time, before which the concept of space and time has no meaning, because spacetime itself is a property of the universe. According to the Big Bang theory, the universe began about twelve to fifteen billion years ago in a violent explosion. For an incomprehensibly small fraction of a second, the universe was an infinitely dense and infinitely hot fireball. A peculiar form of energy that we don't know yet, suddenly pushed out the fabric of spacetime in a process called "inflation", which lasted for only one millionth of a second. Thereafter, the universe continued to expand but not nearly as quickly. The process of phase transition formed out the most basic forces in nature: first gravity, then the strong nuclear force, followed by the weak nuclear and electromagnetic forces. After the first second, the universe was made up of fundamental energy and particles like quarks, electrons, photons, neutrinos and other less familiar particles.
About 3 seconds after the Big Bang, nucleosynthesis set in with protons and neutrons beginning to form the nuclei of simple elements, predominantly hydrogen and helium, yet for the first 100,000 years after the initial hot explosion there was no matter of the form we know today. Instead, radiation (light, X rays, and radio waves) dominated the early universe. Following the radiation era, atoms were formed by nuclei linking up with free electrons and thus matter slowly became dominant over energy. It took 200 million years until irregularities in the primordial gas began to form galaxies and early stars out of pockets of gas condensing by virtue of gravity. The Sun of our solar system was formed out of such a pocket of gas in a spiral arm of the Milky Way galaxy roughly five billion years ago. A vast disk of gas and debris swirling around the early Sun gave birth to the planets, including Earth, which is between 4.6 and 4.5 billion years old. This is -in short- the history of our universe according to the Big Bang theory, which constitutes today's most widely accepted cosmological viewpoint.
What speaks in favor of the Big Bang theory?
A number of different observations corroborate the Big Bang theory. Edwin Hubble (1889-1953) discovered that galaxies are receding from us in all directions. He observed shifts in the spectra of light from different galaxies, which are proportional to their distance from us. The farther away the galaxy, the more its spectrum is shifted towards the low (red) end of the spectrum, which is in some way comparable to the Doppler effect. This redshift indicates recession of objects in space, or better: the ballooning of space itself. Today, there is convincing evidence for Hubble's observations. Projecting galaxy trajectories backward in time means that they converge to a high-density state, i.e. the initial fireball.

If two intelligent life forms in two different galaxies look at each other’s galaxy, they perceive the same thing. The light of the other galaxy appears redshifted in comparison to nearer objects. This is caused by ballooning space that stretches the wavelength of emitted light. The magnitude of this effect is proportional to the distance of the observed galaxy.
According to the Copernican cosmological principle, the universe appears the same in every direction from every point in space, or in more scientific terms: The universe is homogeneous and isotropic. There is overwhelming evidence for this assertion. The best evidence is provided by the almost perfect uniformity of the cosmic background radiation. This observed radiation is isotropic to a very high degree and is thought to be a remnant of the initial Big Bang explosion. The background radiation originates from an era of a few hundred thousand years after the Big Bang, when the first atoms where formed. Another piece of evidence speaking in favour of Big Bang is the abundance of light elements, like hydrogen, deuterium (heavy hydrogen), helium, and lithium. Big Bang nucleosynthesis predicts that about a quarter of the mass of the universe should be helium-4, which is in good agreement with what is observed.
Will the universe expand forever?
On basis of our understanding of the past and present universe, we can speculate about its future. The prime question is whether gravitational attraction between galaxies will one day slow the expansion and ultimately force the universe into contraction, or whether it will continue to expand and cool forever. The current rate of expansion (Hubble Constant) and the average density of the universe determine whether the gravitational force is strong enough to halt expansion. The density required to halt expansion (=critical density) is 1.1 * 10^-26 kg per cubic meter, or six hydrogen atoms per cubic meter; the relation "actual density" / "critical density" is called Omega. With Omega less than 1, the universe is called "open", i.e. forever expanding. If Omega is greater than 1 the universe is called "closed", which means that it will contract and eventually collapse in a Big Crunch. In the unlikely event that Omega = 1, the expansion of the universe will asymptotically slow down until it becomes virtually imperceptible, but it won't collapse.
Big Bang - Big Crunch?
Some scientists think it not impossible that the universe is oscillating between eras of expansion and contraction, where every Big Bang is followed by a Big Crunch. Stephen Hawking (born 1942) pointed out the possibility that such an oscillating universe must not necessarily start and end in singularities, i.e. questionable points in spacetime where physical theories, such as General Relativity, break down while energy and density levels approximate infinity. Although everything points towards Big Bang, the future reversal and contraction of the universe is rather uncertain. Big Crunch is at most a hypothesis, because only about 1/100th of the matter needed for Omega=1 can be observed.
In spite of this, galaxies and star clusters behave as if they would contain more matter than we can see. It is almost as if these objects were engulfed by invisible matter. This "dark matter" that cannot be accounted for is one of the open questions in cosmology. Dark matter makes is thought to make up 23% of the universe.
Big Rip!
Today, most cosmologists believe there is not enough matter in the universe to halt and revert expansion. Robert Caldwell of Dartmouth University has recently suggested a third alternative for the fate of the universe. His Big Rip scenario is based on astronomical observations made in the late 1990s according to which a mysterious force, labelled dark energy, is responsible for the expansion of the universe. Dark energy makes up 73% of the universe. If the rate of acceleration increases, there will be a point in time at which the repulsive force becomes so strong that it overwhelms gravity and the other fundamental forces. According to Caldwell, this will happen in 20 billion years. "The expansion becomes so fast that it literally rips apart all bound objects," Caldwell explains. "It rips apart clusters of galaxies. It rips apart stars. It rips apart planets and solar systems. And it eventually rips apart all matter." Even atoms would be torn apart in the last 10-19 seconds before the end of time. –Whether or not this scenario will become true is to be decided by future research. Until then, the field is open to speculation.
[Read On]

Physics has answered many questions about space, time, and matter. Thanks to technological advances, we have been able to look deeper and deeper into the large-scale structure of the universe and the small-scale structure of matter. From the invention of the telescope to the time of particle accelerators, insight and understanding have grown. Yet, there are still many unsolved mysteries. The contemporary models of matter, space, and time are incomplete and our picture of the world still has holes. Some of today's most challenging questions in physics are:
What is dark matter?
There seems to be a halo of mysterious invisible material engulfing galaxies, which is commonly referred to as dark matter. Scientists infer the existence of dark (=invisible) matter from the observation of its gravitational pull, which causes the stars in the outer regions of a galaxy to orbit faster than they would if there was only visible matter present. Another indication is that we see galaxies in our own local cluster moving towards each other.
The Andromeda galaxy -about 2.2 million light years away from the Milky Way- is speeding toward us at 200,000 miles per hour. This motion can only be explained by gravitational attraction, even though the mass we observe is not nearly great enough to exert that kind of pull. It follows there must be a large amount of unseen mass causing the gravitational pull -roughly equivalent to ten times the size of the Milky Way- lying between the two galaxies.
Astronomers have no idea what the dark matter is that supposedly makes up 23% of all matter in our universe. Black holes and massive neutrinos are two possible explanations. Dark matter must have played an important role in galaxy formation during the evolution of the cosmos. But, even taking into account all known and suspected black holes, there seems to be much more matter out there than we can presently see or extrapolate.
What is dark energy?
Dark energy is perhaps even more mysterious than dark matter. The discovery of dark energy goes back to 1998 when a 10-year study of supernovae took an astonishing turn. A group of scientists had recorded several dozen supernovae, including some so distant that their light had started to travel towards Earth when the universe was only a fraction of its present age. The group's goal was to measure small changes in the expansion rate of the universe, which in turn would yield clues to the origin, structure, and fate of the cosmos. Contrary to their expectation, the scientists found that the expansion of the universe is not slowing, but accelerating.
The acceleration is supposedly due to the anti-gravitational properties of the so-called dark energy. While the exact nature of this energy is presently unknown, scientists agree that dark energy is the dominant constituent of our universe, which means that it is larger than the sum of visible and dark matter. Einstein already postulated an anti-gravitational force at the beginning of the 20th century. He acknowledged that the observed matter would lead to gravitational collapse, and hence, introduced a cosmological constant to bring Relativity into line with observation. After it was discovered by Hubble that the universe is expanding, Einstein called his cosmological constant the greatest blunder of his life.
Yet, at the beginning of the 21st century it seems that anti-gravity is coming back with vengeance. A possible explanation is that the energy content of a vacuum is non-zero with a negative pressure. This negative pressure of the vacuum would grow in strength as the universe expands and it would cause the expansion to accelerate. If the acceleration does not stop, this will lead to the Big Rip scenario suggested by Caldwell, in which the universe will be literally torn apart by the anti-gravitational force in several billion years.
Home did the universe come into being?
Stephen Hawking says in the foreword of The Cosmos Explained (Cambridge, July 28, 1997): "At the Big Bang, the universe and time itself came into existence, so that this is the first cause. If we could understand the Big Bang, we would know why the universe is the way it is. It used to be thought that it was impossible to apply the laws of science to the beginning of the universe, and indeed that it was sacrilegious to try. But recent developments in unifying the two pillars of twentieth-century science, Einstein's General Theory of Relativity and the Quantum Theory, have encouraged us to believe that it may be possible to find laws that hold even at the creation of the universe. In that case, everything in the universe would be determined by the laws of science. So if we understood those laws, we would in a sense be masters of the universe."
It is uncertain whether mankind is able to develop such a theory in the near future, and it may be even more questionable whether this knowledge would indeed help us to become masters of the universe, as Stephen Hawking connotes. Obviously it is difficult to speculate on a theory that has not been developed yet. The theory might as well have no practical value at all. The great 20th century physical theories showed us that complexity and abstraction are growing, while intelligibility and practical applicability are decreasing. From a unified physical theory we can expect a more complete picture of matter, space, and time and a better understanding of the beginning of the universe. It may satisfy our curiosity in view of some big philosophical questions. Any practical value beyond this is rather uncertain.
Unified theories: How does gravity fit into the big picture?
The theory of gravity as formulated by Einstein is incompatible with the rules of quantum mechanics. Physicists encounter serious difficulties when trying to construct a quantum version of gravity. In the later years of his life, Einstein tried but failed to devise a theory that unifies gravity with quantum theory. In the 1960s, the weak nuclear force was united with electromagnetism to form the electroweak theory, which was subsequently verified in particle accelerator experiments. The next step is to create a model that unites the other fundamental forces.
Theorists are working on such a model, which they call grand unified theory (GUT). It amalgamates electromagnetism with the weak and strong nuclear interaction, but omits gravity. From GUT we expect the answer to why particles have the masses we observe. Although we observe the masses of electrons, protons, and neutrons generated through what is called "electroweak breaking," we don't know how this breaking mechanism works. GUT should be able to interpret the electroweak breaking process and thus provide an explanation for the mass of a particle.
Beyond GUT, there is a theory that accounts for all four fundamental forces in nature, including gravity. The greatest endeavour of physics is to draw hitherto unrelated and incompatible theories together into a single unified theory. The advantage of such a system is obvious: It would account for all currently known phenomena without leaving theoretical holes and it may point towards future areas of study. It is hypothesised that such a theory could create a new fundamental understanding of nature. String theory, supersymmetry, and M-theory are some candidates currently considered.
Are quarks and leptons actually fundamental, or are they made up of even more fundamental particles?
Presently it is not known whether quarks and leptons are elementary or compound particles. It seems that physicists have become more careful with announcing the fundamentality of particles after having learned that atoms, atom cores, and finally protons and neutrons are divisible. What is more, quarks and leptons are so small that they may be thought of as geometrical points in space with no spatial extension at all. This is perhaps not as miraculous as it first sounds, because after having learned from Rutherford's model that the volume of an atom is mostly made of "empty" space, it would not be too surprising to find out that matter is in fact nothing but empty space.
While the commonly accepted standard model of matter provides a very good description of the phenomena observed in experiments, the model is still incomplete. It can explain the behaviour of particles fairly well, but it cannot explain why some particles exist as they do. For example, it has been impossible to predict the mass of the top quark accurately from theoretical inference until it was determined experimentally. As mentioned before, the standard model of matter does not provide any mathematical model that allows us to calculate the observed mass.
Another question concerns the fact that there are three families of quarks and leptons. Of the three families (or generations) of particles, only the first is stable, namely that of up/down quarks, e-neutrinos, and electrons. There seems to be no need for the other two generations in the natural world, yet they exist. Theoretical physics has no explanation for the existence of the two unstable generations. Likewise, the question why there is hardly any antimatter in the observable universe remains unaccounted for. Since there is an almost perfect symmetry between matter and antimatter, one would expect some regions of the universe to be composed of matter and others of antimatter, yet almost all mass we can observe is composed of conventional matter.
Is our universe unique, or are there many universes?
Andrei Linde at Stanford has brought forward the cosmological model of a multiverse, which he calls the "self-reproducing inflationary universe." The theory is based on Alan Guth's inflation model, and it includes multiple universes woven together in some kind of spacetime foam. Each universe exists in a closed volume of space and time. Linde's model, based on advanced principles of quantum physics, defies easy visualisation. Quite simplified, it suggests quantum fluctuations in the universe's inflationary expansion period to have a wavelike character. Linde theorises that these waves can "freeze" atop one another, thus magnifying their effect.
The stacked-up quantum waves can in turn create such intense disruptions in scalar fields -the underlying fields that determine the behaviour of elementary particles- that they exceed a critical mass and start procreating new inflationary domains. The multiverse, Linde contends, is like a growing fractal, sprouting inflationary domains, with each domain spreading and cooling into a new universe.
If Linde is correct, our universe is just one of the sprouts. The theory neatly straddles two ancient ideas about the universe: that it had a definite beginning, and that it had existed forever. In Linde's view, each particular part of the multiverse, including our part, began from a singularity somewhere in the past, but that singularity was just one of an endless series that was spawned before it and will continue after it.
Will a complete physical model of the world help us to understand ultimate reality? Can we understand ultimate reality at all through science?
Some physicists believe that a complete physical model can explain everything we observe. They hold that once the fundamental laws are known and powerful computers allow us to compute models of the world by applying these laws, we can eventually deduce explanations for all phenomena. In other words, physics can lead us to understanding ultimate reality. Is this really possible?
One may doubt it. Even if we give physicists credit for their remarkable discoveries, we have to realise that their research takes place in an isolated field of knowledge. Physics does not concern itself with issues outside its own domain. For example, the subjects of biology, life, and chemistry, as well as the phenomena of mind and consciousness cannot be explained in physical terms. In addition, the following fundamental questions arise:
1. Physics deals only with what can be measured. A complete physical model must therefore necessarily produce a materialistic view of reality. Although materialists usually deny the possibility that phenomena exist which cannot be measured or somehow quantified, they may actually exist.
2. There are limits to what can be measured, as demonstrated by the Uncertainty Principle.
3. The materialist view is generally allied with reductionism. Materialists often claim that high-level phenomena, such as biological or psychological phenomena, can be reduced to physical phenomena. However, this is far from being obvious. For example, there is no generally accepted reductionist theory of consciousness. Reductionism fails in most practical cases. For example, it is practically impossible to describe the process of DNA replication in terms of subatomic properties.
4. Advanced physical models are abstract to the degree of being unintelligible to most people. Modern physics is based on higher mathematics and can hardly be put into common language, much less can it be imagined. The multidimensional worlds of Relativity and string theory, for example, are elusive to plastic imagination. The value of any science depends on how useful its models are for the thoughts and actions of humanity as a whole, hence, its usefulness leans partly on intelligibility.
[Read On]

What is light?
Light is a phenomenon that has particle and wave characteristics. Its carrier particles are called photons, which are not really particles, but massless discrete units of energy.
What is the speed of light?
The speed of light is 299,792,458 m/s in a vacuum. The symbol used in Relativity for the speed of light is "c", which probably stands for the Latin word "celeritas", meaning swift.
Is the speed of light really constant?
The speed of light is constant by definition in the sense that it is independent of the reference frame of the observer. Light travels slightly slower in a transparent medium, such as water, glass, and even air.
Can anything travel faster than light?
No. In Relativity, c puts an absolute limit to speed at which any object can travel, hence, nothing, no particle, no rocket, no space vehicle can go at faster-than-light (=superluminal) speeds. However, there are some cases where things appear to move at superluminal speeds, such as in the following examples: 1. Consider two spaceships moving each at 0.6c in opposite directions. For a stationary observer, the distance between both ships grows at faster-than-light speed. The same is true for distant galaxies that drift apart in opposite directions of the sky. 2. Another example: Consider pointing a very strong laser on the moon so that it projects a dot on the moon's service and then moving the laser rapidly towards Earth, so that it points on the floor in front of you. If you accomplish this in less than one second, the laser dot obviously travelled at superluminal speed, seeing that the average distance between the Earth and the Moon is 384,403 km.
What is matter?
The schoolbook definition would be: Matter is what takes up space and has mass. Matter as we know it is composed of molecules, which themselves are built from individual atoms. Atoms are composed of a core and one or more electrons that spin around the core in an electron cloud. The core is composed of protons and neutrons, the former have a positive electrical charge, the latter are electrically neutral. Protons and neutrons are composed of quarks, of which there are six types: up/down, charm/strange, and top/bottom. Quarks only exist in composite particles, whereas leptons can be seen as independent particles. There are six types of leptons: the electron, the muon, the tau and the three types of neutrinos. The particles that make up an atom could be seen as a stable form of locked up energy. Particles are extremely small, therefore 99.999999999999% (or maybe all) of an atom's volume is just empty space. Almost all visible matter in the universe is made of up/down quarks, electrons and (e-)-neutrinos, because the other particles are very unstable and quickly decay into the former.
How fast does an electron spin?
An electron in a hydrogen atom moves at about 2.2 million m/s. With the circumference of the n=1 state for hydrogen being about 0,33x10-9 m in size, it follows that an n=1 electron for a hydrogen atom revolves around the nucleus 6,569,372 billion times in just one second.
Are quarks and leptons all there is?
Not really. Fist of all, quarks always appear in composite particles, namely hadrons (baryons and mesons), then there is antimatter, and finally there are the four fundamental forces.
What is antimatter?
The existence of antimatter was first predicted in 1928 by Paul Dirac and has been experimentally verified by the artificial creation of the positron (e+) in a laboratory in 1933. The positron, the electron's antiparticle, carries a positive electrical charge. Not unlike the reflection in a mirror, there is exactly one antimatter particle for each known particle and they behave just like their corresponding matter particles, except they have opposite charges and/or spins. When a matter particle and antimatter particle meet, they annihilate each other into a flash of energy. The universe we can observe contains almost no antimatter. Therefore, antimatter particles are likely to meet their fate and collide with matter particles. Recent research suggests that the symmetry between matter and antimatter is less than perfect. Scientists have observed a phenomenon called charge/parity violation, which implies that antimatter presents not quite the reflection image of matter.
What are the four fundamental forces?
The four fundamental forces are gravity, the electromagnetic force, and the weak and strong nuclear forces. Any other force you can think of (magnetism, nuclear decay, friction, adhesion, etc.) is caused by one of these four fundamental forces or by a combination of them. Electromagnetism and the weak nuclear force have been shown to be two aspects of a single electroweak force.
What is gravity?
Gravity is the force that causes objects on Earth to fall down and stars and planets to attract each other. Isaac Newton quantified the gravitational force: F = mass1 * mass2 / distance². Gravity is a very weak force when compared with the other fundamental forces. The electrical repulsion between two electrons, for example, is some 10^40 times stronger than their gravitational attraction. Nevertheless, gravity is the dominant force on the large scales of interest in astronomy. Einstein describes gravitation not as a force, but as a consequence of the curvature of spacetime. This means that gravity can be explained in terms of geometry, rather than as interacting forces. The General Relativity model of gravitation is largely compatible with Newton, except that it accounts for certain phenomena such as the bending of light rays correctly, and is therefore more accurate than Newton's formula. According to General Relativity, matter tells space how to curve, while the curvature of space tells matter how to move. The carrier particle of the gravitational force is the graviton.
What is electromagnetism?
Electromagnetism is the force that causes like-charged particles to repel and oppositely-charged particles to attract each other. The carrier particle of the electromagnetic force is the photon. Photons of different energies span the electromagnetic spectrum of x rays, visible light, radio waves, and so forth. Residual electromagnetic force allows atoms to bond and form molecules.
What is the strong nuclear force?
The strong force acts between quarks to form hadrons. The nucleus of an atom is hold together on account of residual strong force, i.e. by quarks of neighbouring neutrons and protons interacting with each other. Quarks have an electromagnetic charge and another property that is called colour charge, they come in three different colour charges. The carrier particles of the strong nuclear force are called gluons. In contrast to photons, gluons have a colour charge, while composite particles like hadrons have no colour charge.
What is the weak nuclear force?
Weak interactions are responsible for the decay of massive quarks and leptons into lighter quarks and leptons. It is the primary reason why matter is mainly composed of the stable lighter particles, namely up/down quarks and electrons. Radioactivity is due to the weak nuclear force. The carrier particles of the weak force are the W+, W-, and the Z bosons.
How are carrier particles different from other particles?
The photon, gluon, and the graviton carrier particles are thought to be massless and having no electrical charge. Only the W and Z particles, mediators of the weak nuclear force, are massive, and the W+ and W- particles carry charge. Force carrier particles can only be absorbed or produced by a matter particle which is affected by that particular force. These particles allow us to explain interactions between matter.
How old is the universe?
Today's most widely accepted cosmology, the Big Bang theory, states that the universe is limited in space and time. The current estimate for the age of the universe is 13.7 billion years. This figure was computed from the cosmic microwave background (CMB) radiation data that the Wilkinson Microwave Anisotropy Probe (WMAP) captured in 2002.
What came before the Big Bang?
The Big Bang model is singular at the time of the Big Bang. This means that one cannot even define time, since spacetime is singular. In some models like the oscillating universe, suggested by Stephen Hawking, the expanding universe is just one of many phases of expansion and contraction. Other models postulate that our own universe is just one bubble in a spacetime foam containing a multitude of universes. The "multiverse" model of Linde proposes that multiple universes recursively spawn each other, like in a growing fractal. However, until now there is no observational data confirming either theory. It is indeed questionable, whether we will ever be able to gain empirical evidence speaking in favor these theories, because nothing outside our own universe can be observed directly. Hence, the question can currently not be answered by science.
How big is the universe?
The universe is constantly expanding in all directions, therefore its size cannot be stated. Scientists think it contains approximately 100 billion galaxies with each galaxy containing between 100 and 200 billion star systems. Our own galaxy, the Milky Way, is average when compared with other galaxies. It is a disk-shaped spiral galaxy of about 100,000 light-years in diameter.
What is the universe expanding into?
This question is based on the popular misconception that the universe is some curved object embedded into a higher dimensional space, and that the universe is expanding into this space. There is nothing whatsoever that we have measured or can measure that will show us anything about this larger space. Everything that we measure is within the universe, and so we see neither edge nor boundary nor centre of expansion. Thus the universe is not expanding into anything that we can see or measure.
Why is the sky dark at night?
If the universe were infinitely old, and infinite in extent, and stars could shine forever, then every direction you looked would eventually end on the surface of a star, and the whole sky would be as bright as the surface of the Sun. This is known as Olbers's paradox, named after Heinrich Wilhelm Olbers [1757-1840] who wrote about it in 1823-1826. Absorption by interstellar dust does not circumvent this paradox, since dust reradiates whatever radiation it absorbs within a few minutes, which is much less than the age of the universe. However, the universe is not infinitely old, and the expansion of the universe reduces the accumulated energy radiated by distant stars. Either one of these effects acting alone would solve Olbers's paradox, but they both act at once.
If the universe is only 13.7 billion years old, how can we see objects that are 30 billion light-years away?
This question is essentially answered by Special Relativity. When talking about the distance of a moving object, we mean the spatial separation now, with the positions of us and the object specified at the current time. In an expanding universe, this distance is now larger than the speed of light times the light travel time due to the increase of separations between objects, as the universe expands. It does not mean that any object in the universe travels faster than light.

What is light?
Light is a phenomenon that has particle and wave characteristics. Its carrier particles are called photons, which are not really particles, but massless discrete units of energy.
What is the speed of light?
The speed of light is 299,792,458 m/s in a vacuum. The symbol used in Relativity for the speed of light is "c", which probably stands for the Latin word "celeritas", meaning swift.
Is the speed of light really constant?
The speed of light is constant by definition in the sense that it is independent of the reference frame of the observer. Light travels slightly slower in a transparent medium, such as water, glass, and even air.
Can anything travel faster than light?
No. In Relativity, c puts an absolute limit to speed at which any object can travel, hence, nothing, no particle, no rocket, no space vehicle can go at faster-than-light (=superluminal) speeds. However, there are some cases where things appear to move at superluminal speeds, such as in the following examples: 1. Consider two spaceships moving each at 0.6c in opposite directions. For a stationary observer, the distance between both ships grows at faster-than-light speed. The same is true for distant galaxies that drift apart in opposite directions of the sky. 2. Another example: Consider pointing a very strong laser on the moon so that it projects a dot on the moon's service and then moving the laser rapidly towards Earth, so that it points on the floor in front of you. If you accomplish this in less than one second, the laser dot obviously travelled at superluminal speed, seeing that the average distance between the Earth and the Moon is 384,403 km.
What is matter?
The schoolbook definition would be: Matter is what takes up space and has mass. Matter as we know it is composed of molecules, which themselves are built from individual atoms. Atoms are composed of a core and one or more electrons that spin around the core in an electron cloud. The core is composed of protons and neutrons, the former have a positive electrical charge, the latter are electrically neutral. Protons and neutrons are composed of quarks, of which there are six types: up/down, charm/strange, and top/bottom. Quarks only exist in composite particles, whereas leptons can be seen as independent particles. There are six types of leptons: the electron, the muon, the tau and the three types of neutrinos. The particles that make up an atom could be seen as a stable form of locked up energy. Particles are extremely small, therefore 99.999999999999% (or maybe all) of an atom's volume is just empty space. Almost all visible matter in the universe is made of up/down quarks, electrons and (e-)-neutrinos, because the other particles are very unstable and quickly decay into the former.
How fast does an electron spin?
An electron in a hydrogen atom moves at about 2.2 million m/s. With the circumference of the n=1 state for hydrogen being about 0,33x10-9 m in size, it follows that an n=1 electron for a hydrogen atom revolves around the nucleus 6,569,372 billion times in just one second.
Are quarks and leptons all there is?
Not really. Fist of all, quarks always appear in composite particles, namely hadrons (baryons and mesons), then there is antimatter, and finally there are the four fundamental forces.
What is antimatter?
The existence of antimatter was first predicted in 1928 by Paul Dirac and has been experimentally verified by the artificial creation of the positron (e+) in a laboratory in 1933. The positron, the electron's antiparticle, carries a positive electrical charge. Not unlike the reflection in a mirror, there is exactly one antimatter particle for each known particle and they behave just like their corresponding matter particles, except they have opposite charges and/or spins. When a matter particle and antimatter particle meet, they annihilate each other into a flash of energy. The universe we can observe contains almost no antimatter. Therefore, antimatter particles are likely to meet their fate and collide with matter particles. Recent research suggests that the symmetry between matter and antimatter is less than perfect. Scientists have observed a phenomenon called charge/parity violation, which implies that antimatter presents not quite the reflection image of matter.
What are the four fundamental forces?
The four fundamental forces are gravity, the electromagnetic force, and the weak and strong nuclear forces. Any other force you can think of (magnetism, nuclear decay, friction, adhesion, etc.) is caused by one of these four fundamental forces or by a combination of them. Electromagnetism and the weak nuclear force have been shown to be two aspects of a single electroweak force.
What is gravity?
Gravity is the force that causes objects on Earth to fall down and stars and planets to attract each other. Isaac Newton quantified the gravitational force: F = mass1 * mass2 / distance². Gravity is a very weak force when compared with the other fundamental forces. The electrical repulsion between two electrons, for example, is some 10^40 times stronger than their gravitational attraction. Nevertheless, gravity is the dominant force on the large scales of interest in astronomy. Einstein describes gravitation not as a force, but as a consequence of the curvature of spacetime. This means that gravity can be explained in terms of geometry, rather than as interacting forces. The General Relativity model of gravitation is largely compatible with Newton, except that it accounts for certain phenomena such as the bending of light rays correctly, and is therefore more accurate than Newton's formula. According to General Relativity, matter tells space how to curve, while the curvature of space tells matter how to move. The carrier particle of the gravitational force is the graviton.
What is electromagnetism?
Electromagnetism is the force that causes like-charged particles to repel and oppositely-charged particles to attract each other. The carrier particle of the electromagnetic force is the photon. Photons of different energies span the electromagnetic spectrum of x rays, visible light, radio waves, and so forth. Residual electromagnetic force allows atoms to bond and form molecules.
What is the strong nuclear force?
The strong force acts between quarks to form hadrons. The nucleus of an atom is hold together on account of residual strong force, i.e. by quarks of neighbouring neutrons and protons interacting with each other. Quarks have an electromagnetic charge and another property that is called colour charge, they come in three different colour charges. The carrier particles of the strong nuclear force are called gluons. In contrast to photons, gluons have a colour charge, while composite particles like hadrons have no colour charge.
What is the weak nuclear force?
Weak interactions are responsible for the decay of massive quarks and leptons into lighter quarks and leptons. It is the primary reason why matter is mainly composed of the stable lighter particles, namely up/down quarks and electrons. Radioactivity is due to the weak nuclear force. The carrier particles of the weak force are the W+, W-, and the Z bosons.
How are carrier particles different from other particles?
The photon, gluon, and the graviton carrier particles are thought to be massless and having no electrical charge. Only the W and Z particles, mediators of the weak nuclear force, are massive, and the W+ and W- particles carry charge. Force carrier particles can only be absorbed or produced by a matter particle which is affected by that particular force. These particles allow us to explain interactions between matter.
How old is the universe?
Today's most widely accepted cosmology, the Big Bang theory, states that the universe is limited in space and time. The current estimate for the age of the universe is 13.7 billion years. This figure was computed from the cosmic microwave background (CMB) radiation data that the Wilkinson Microwave Anisotropy Probe (WMAP) captured in 2002.
What came before the Big Bang?
The Big Bang model is singular at the time of the Big Bang. This means that one cannot even define time, since spacetime is singular. In some models like the oscillating universe, suggested by Stephen Hawking, the expanding universe is just one of many phases of expansion and contraction. Other models postulate that our own universe is just one bubble in a spacetime foam containing a multitude of universes. The "multiverse" model of Linde proposes that multiple universes recursively spawn each other, like in a growing fractal. However, until now there is no observational data confirming either theory. It is indeed questionable, whether we will ever be able to gain empirical evidence speaking in favor these theories, because nothing outside our own universe can be observed directly. Hence, the question can currently not be answered by science.
How big is the universe?
The universe is constantly expanding in all directions, therefore its size cannot be stated. Scientists think it contains approximately 100 billion galaxies with each galaxy containing between 100 and 200 billion star systems. Our own galaxy, the Milky Way, is average when compared with other galaxies. It is a disk-shaped spiral galaxy of about 100,000 light-years in diameter.
What is the universe expanding into?
This question is based on the popular misconception that the universe is some curved object embedded into a higher dimensional space, and that the universe is expanding into this space. There is nothing whatsoever that we have measured or can measure that will show us anything about this larger space. Everything that we measure is within the universe, and so we see neither edge nor boundary nor centre of expansion. Thus the universe is not expanding into anything that we can see or measure.
Why is the sky dark at night?
If the universe were infinitely old, and infinite in extent, and stars could shine forever, then every direction you looked would eventually end on the surface of a star, and the whole sky would be as bright as the surface of the Sun. This is known as Olbers's paradox, named after Heinrich Wilhelm Olbers [1757-1840] who wrote about it in 1823-1826. Absorption by interstellar dust does not circumvent this paradox, since dust reradiates whatever radiation it absorbs within a few minutes, which is much less than the age of the universe. However, the universe is not infinitely old, and the expansion of the universe reduces the accumulated energy radiated by distant stars. Either one of these effects acting alone would solve Olbers's paradox, but they both act at once.
If the universe is only 13.7 billion years old, how can we see objects that are 30 billion light-years away?
This question is essentially answered by Special Relativity. When talking about the distance of a moving object, we mean the spatial separation now, with the positions of us and the object specified at the current time. In an expanding universe, this distance is now larger than the speed of light times the light travel time due to the increase of separations between objects, as the universe expands. It does not mean that any object in the universe travels faster than light.

Quantum theory evolved as a new branch of theoretical physics during the first few decades of the 20th century in an endeavour to understand the fundamental properties of matter. It began with the study of the interactions of matter and radiation. Certain radiation effects could neither be explained by classical mechanics, nor by the theory of electromagnetism. In particular, physicists were puzzled by the nature of light. Peculiar lines in the spectrum of sunlight had been discovered earlier by Joseph von Fraunhofer (1787-1826). These spectral lines were then systematically catalogued for various substances, yet nobody could explain why the spectral lines are there and why they would differ for each substance. It took about one hundred years, until a plausible explanation was supplied by quantum theory.
Quantum theory is about the nature of matter.
In contrast to Einstein's Relativity, which is about the largest things in the universe, quantum theory deals with the tiniest things we know, the particles that atoms are made of, which we call "subatomic" particles. In contrast to Relativity, quantum theory was not the work of one individual, but the collaborative effort of some of the most brilliant physicists of the 20th century, among them Niels Bohr, Erwin Schrödinger, Wolfgang Pauli, and Max Born. Two names clearly stand out: Max Planck (1858-1947) and Werner Heisenberg (1901-1976). Planck is recognised as the originator of the quantum theory, while Heisenberg formulated one of the most eminent laws of quantum theory, the Uncertainty Principle, which is occasionally also referred to as the principle of indeterminacy.
Planck's constant: Energy is not continuous.
Around 1900, Max Planck from the University of Kiel concerned himself with observations of the radiation of heated materials. He attempted to draw conclusions from the radiation to the radiating atom. On basis of empirical data, he developed a new formula which later showed remarkable agreement with accurate measurements of the spectrum of heat radiation. The result of this formula was so that energy is always emitted or absorbed in discrete units, which he called quanta. Planck developed his quantum theory further and derived a universal constant, which came to be known as Planck's constant. The resulting law states that the energy of each quantum is equal to the frequency of the radiation multiplied by the universal constant: E=f*h, where h is 6.63 * 10E-34 Js. The discovery of quanta revolutionised physics, because it contradicted conventional ideas about the nature of radiation and energy.
The atom model of Bohr.
To understand the gist of the quantum view of matter, we have to go back to the 19th century's predominant model of matter. Scientists at the time believed -like the Greek atomists- that matter is composed of indivisible, solid atoms, until Rutherford proved otherwise.
The British physicist Ernest Rutherford (1871-1937) demonstrated experimentally that the atom is not solid as previously assumed, but that it has an internal structure consisting of a small, dense nucleus about which electrons circle in orbits.

Niels Bohr (1885-1962) refined Rutherford's model by introducing different orbits in which electrons spin around the nucleus. This model is still used in chemistry. Elements are distinguished by their "atomic number", which specifies the number of protons in the nucleus of the atom. Electrons are held in their orbits through the electrical attraction between the positive nucleus and the negative electron. Bohr argued that each electron has a certain fixed amount of energy, which corresponds to its fixed orbit. Therefore, when an electron absorbs energy, it jumps to the next higher orbit rather than moving continuously between orbits. The characteristic of electrons having fixed energy quantities (quanta) is also known as the quantum theory of the atom.
The above model bears a striking similarity with the Newtonian model of our solar system. Electrons revolve around the nucleus, just as planets revolve around the Sun. It is therefore not surprising that physicists tried to apply classical mechanics to the atomic structure. The forces between nucleus and electrons were equated with the gravitational forces between celestial bodies. This idea worked quite well for the hydrogen atom, the simplest of all elements, but it failed to explain the behaviour of more complex atoms.
If matter is not infinitely divisible, why should energy be?
The idea that energy could be emitted or absorbed only in discrete energy quanta seemed odd, since it could not be fitted into the traditional framework of physics. The quantum behaviour of electrons in atoms contradicted not only classical mechanics, but also Maxwell's electromagnetic theory, which required it to radiate away energy while orbiting in a quantum energy state. Even Max Planck, who was a conservative man, initially doubted his own discovery. The traditional view was that energy flows in a continuum like a smooth, unbroken stream of water. That there should be gaps between the discrete entities of energy seemed wholly unreasonable. In fact, Planck's idea only gained credence when Einstein used it in 1905 to explain the photoelectric effect. - After all, if matter is not infinitely divisible, why should energy be?
In the course of time, physicists descended deeper into the realm of the atom. Bohr's atom model was remarkably successful in describing the spectrum of the hydrogen atom by using Planck's formula to relate different energy levels of electrons to different frequencies of light radiation. Unfortunately, it did not work well for more complex atoms, and so a more sophisticated theory had to be developed. The problem seemed to be rooted in the assumption that an electron rotates around the nucleus like a massive object revolves around a centre of gravity. De Broglie, Schrödinger, and Heisenberg showed that classical mechanics had to be abandoned in order to describe the subatomic world adequately. In an inference not less dramatic than Planck's discovery of quanta, they stated that particles don't really have a trajectory or an orbit, much less do they behave like a ball that is shot through a corridor or is whirled around on the end of a cord.
The wave-particle duality.
Just as light is thought to have a dual nature, sometimes showing the characteristic of a wave, and sometimes that of a particle (photon), quantum theory attributes a similar dual wave-particle nature to subatomic particles. Electrons that orbit around the nucleus interact with each other by showing interference patterns, not unlike those of wave interference. If the velocity of the electron is thought of as its wavelength, the crests of neighbouring electron waves amplify or cancel each other, thereby creating a pattern that corresponds to Bohr's allowed orbits.
Bohr's model of the atom was superseded by the probability cloud model that describes physical reality better. The orbital clouds are mathematical descriptions of where the electrons in an atom are most likely to be found, which means the model shows the spatial distribution of electrons. The (simplified) picture to the left shows electron probability clouds in a water molecule.
Even cloud models are only approximations. The computation of the actual distribution of electrons in an atom is extremely laborious and the result is too complicated to be illustrated in a single layer 3D model.

About misbehaved electrons, or: the probability cloud model.
The nature of electrons seems odd. Seemingly they exist in different places at different points in time, but it is impossible to say where the electron will be at a given time. At time t1 it is at point A, then at time t2 it is at point B, yet without moving from A to B. It seems to appear in different places without describing a trajectory. Therefore, even if t1 and A can be pinpointed, it is impossible to derive t2 and B from this measurement. In other words: There seems to be no causal relation between any two positions. The concept of causality cannot be applied to what is observed. In case of the electrons of an atom, the closest we can get to describing the electron's position is by giving a number for the probability of it being at a particular place. Moreover, particles have other "disturbing" properties: They have a tendency to decay into other particles or into energy, and sometimes -under special circumstances- they merge to form new particles. They do so after indeterminate time spans. Although we can make statistical assertions about a particle's lifetime, it is impossible to predict the fate of an individual particle.
What does quantum physics say about the universe?
Can we derive any new knowledge about the universe from quantum physics? After all, the entire universe is composed of an unimaginable large number of matter and energy. It seems to be of great importance to understand quantum theory properly in view of the large-scale structure of the cosmos. For example, an interesting question in this context is why the observable matter in the universe is packed together in galaxies and is not evenly distributed throughout space. Could it have to do with the quantum characteristics of energy? Are quantum effects responsible for matter forming discrete entities, instead of spreading out evenly during the birth of the universe? The answer to this question is still being debated.
If cosmological conclusions seem laboured, we might be able to derive philosophical insights from quantum physics. At least Fritjof Capra thinks this is possible when he describes the parallels between modern physics and ancient Eastern philosophy in his book The Tao of Physics. He holds that in a way, the essence of modern physics is comparable to the teachings of the ancient Eastern philosophies, such as the Chinese Tao Te Ching, the Indian Upanishads, or the Buddhist Sutras. Eastern philosophies agree in the point that ultimate reality is indescribable and unapproachable, not only in terms of common language, but also in the language of mathematics. That is, science and mathematics must fail at some stage in describing ultimate reality. We see this exemplified in the Uncertainty Principle, which is elucidated in the following section.
Molecules and atoms cannot be split into independent units. All parts interact at all levels.
The oriental scriptures agree in the point that all observable and describable realities are manifestations of the same underlying "divine" principle. Although many phenomena of the observable world are seemingly unrelated, they all go back to the same source. Things are intertwined and interdependent to an unfathomable degree, just as the particles in an atom are. Although the electrons in an atom can be thought of as individual particles, they are not really individual particles, because of the complicated wave relations that exist between them. Hence, the electron cloud model describes the atomic structure more adequately. The sum of electrons in an atom cannot be separated from its nucleus, which has a compound structure itself and can neither be regarded a separate entity. Thus, in the multiplicity of things there is unity. Matter is many things and one thing at the same time.
The Eastern scriptures say that no statement about the world is ultimately valid ("The Tao that can be told is not the eternal Tao." Tao Te Ching, Verse 1), since not even the most elaborate language is capable of rendering a perfect model of the universe. Science is often compared to a tree that branches out into many directions. The disposition of physics is that it follows the tree upward to its branches and leaves, while meta-physics follows it down to the root. Whether the branches of knowledge stretch out indefinitely is still a matter of debate. However, it appears that most scientific discoveries do not only answer questions, but also raise new ones.
The German philosopher, FriedrichHegel formulated an idea at the beginning of the 19th century that describes this process. He proposed the dialectic triad of thesis, antithesis, and synthesis, in which an idea (thesis) always contains incompleteness and thus yields a conflicting idea (antithesis). A third point of view (synthesis) arises, which overcomes the conflict by reconciling the truth contained in both, thesis and antithesis, at a higher level of understanding. The synthesis then becomes a new thesis, generates another antithesis, and the process starts over. In the next section, we shall see how 20th century physics embodies Hegel's dialectical principle. We will also take a close look at the philosophical implications of Heisenberg's Uncertainty Principle.
[Read On]

At a time when Einstein had gained international recognition, quantum theory culminated in the late 1920’s statement of the Uncertainty Principle, which says that the more precisely the position of a particle is determined, the less precisely the momentum is known in this instant, and vice versa. The above phrasing of the principle is a succinct version of the mathematically precise uncertainty relation that Heisenberg published in 1927. Since the momentum of a particle is the product of its mass and velocity, the principle is sometimes stated differently, however, its meaning remains the same: The act of measuring one magnitude of a particle, be it its mass, its velocity, or its position, causes the other magnitudes to blur. This is not due to imprecise measurements. Technology is advanced enough to hypothetically yield correct measurements. The blurring of these magnitudes is a fundamental property of nature.
The uncertainty relation describes the "blur" between the measurable quantities of a particle in mathematical terms. Like much of the math in quantum theory, it is not for the faint of heart, which is to say it is completely unintelligible to most people. Therefore we restrict ourselves to a brief account on the underlying ideas and how they developed into the "Copenhagen Interpretation", which Niels Bohr and Werner Heisenberg jointly elaborated as a complete and consistent view of quantum mechanics (the Copenhagen Interpretation refers to Bohr's place of birth).
Heisenberg: "What Schrödinger writes about the visualisability of his theory [...] is crap."
Around 1925 there were two competing mathematical theories that both attempted to explain electron orbits. Matrix mechanics developed by Heisenberg interprets the electron as a particle with quantum behaviour. It is based on sophisticated matrix computations, which introduce discontinuities and quantum jumps. In contrast, wave mechanics developed by Erwin Schrödinger interprets the electron as an energy wave. Because wave mechanics entails more familiar concepts and equations, it quickly gained popularity among scientists.
Schrödinger and Heisenberg were no too fond of each other's competing works. Schrödinger says about matrix mechanics: "I knew of [Heisenberg's] theory, of course, but I felt discouraged, not to say repelled, by the methods of transcendental algebra, which appeared difficult to me, and by the lack of visualisability." Heisenberg's comment on wave mechanics was: "The more I think about the physical portion of Schrödinger's theory, the more repulsive I find it. [...] What Schrödinger writes about the visualisability of his theory 'is probably not quite right,' in other words it's crap."
The Copenhagen Interpretation.
Despite the differences, Schrödinger published a proof in 1926, which showed that the results of matrix and wave mechanics are equivalent; they were in fact the same theory. According to the Copenhagen Interpretation, the wave and particle pictures of the atom, or the visual and causal representations, are "complementary" to each other. That is, they are mutually exclusive, yet jointly essential for a complete description of quantum events. Obviously in an experiment in the everyday world an object cannot be both a wave and a particle at the same time; it must be either one or the other, depending on the situation. In later refinements of this interpretation, the wave function of the unobserved object is a mixture of both, the wave and particle pictures, until the experimenter chooses what to observe in a given experiment.
The German physicist Werner Heisenberg (1901-1976) received the Nobel Prize in physics in 1932 for his work in nuclear physics and quantum theory. The paper on the uncertainty relation is his most important contribution to physics.

Heisenberg impressed his teachers with his ambition and brilliance. He never produced other grades than straight A's, except on one occasion: During his doctorate, professor Wien of the university of Munich gave him an F in experimental physics, because he handled the laboratory equipment clumsily. Reportedly this left Heisenberg so disconcerted that he did not speak to anyone for days.
Fate had it that a few years later, Heisenberg demonstrated the very limitations of experimental physics, which unquestionably constituted a setback for its advocates, including Professor Wien.
The observer becomes part of the observed system.
The notion of the observer becoming a part of the observed system is fundamentally new in physics. In quantum physics, the observer is no longer external and neutral, but through the act of measurement he becomes himself a part of observed reality. This marks the end of the neutrality of the experimenter. It also has huge implications on the epistemology of science: certain facts are no longer objectifiable in quantum theory. If in an exact science, such as physics, the outcome of an experiment depends on the view of the observer, then what does this imply for other fields of human knowledge? It would seem that in any faculty of science, there are different interpretations of the same phenomena. More often than occasionally, these interpretations are in conflict with each other. Does this mean that ultimate truth is unknowable?
The results of quantum theory, and particularly of Heisenberg's work, left scientists puzzled. Many felt that quantum theory had somehow "missed the point". At least Albert Einstein did so. He was an outspoken critic of quantum mechanics and is often quoted on his comment regarding the Uncertainty Principle: "The Old One (God) doesn't play dice." He also said: "I like to believe that the moon is still there even if we don't look at it." In particular, Einstein was convinced that electrons do have definite orbits, even if we cannot observe them. In a conversation with Heisenberg he said:
A conversation between Einstein and Heisenberg.
Heisenberg: "One cannot observe the electron orbits inside the atom. [...] but since it is reasonable to consider only those quantities in a theory that can be measured, it seemed natural to me to introduce them only as entities, as representatives of electron orbits, so to speak."
Einstein: "But you don't seriously believe that only observable quantities should be considered in a physical theory?"
"I thought this was the very idea that your Relativity Theory is based on?" Heisenberg asked in surprise.
"Perhaps I used this kind of reasoning," replied Einstein, "but it is nonsense nevertheless. [...] In reality the opposite is true: only the theory decides what can be observed."
We can easily see the rift between Einstein's intuitive and Heisenberg's empirical approach. Although Einstein's argumentation appears tricky, it is clear that he believes in a reality independent of what we can observe, which is in essence the view of realism. Kant's "thing in itself" comes to mind. - In contrast, Heisenberg believes that reality is what can be observed. If there are different observations, there must be different realities, which depend on the observer. Insofar Heisenberg can be regarded as an advocate of philosophical idealism, which states that the objects of perception are identical with the ideas we have about them. The idealist view denies that any particular thing has an independent real essence outside of consciousness.
Is the moon still there when nobody is looking at it?
The two philosophies seem incompatible at first. Heisenberg is in good company with famous contenders of idealistic positions, such Plato, Schopenhauer, and Husserl, but so is Albert Einstein. If we take Heisenberg's view for granted, strict causality is broken, or better: the past and future events of particles are indeterminate. One cannot calculate the precise future motion of a particle, but only a range of possibilities. Physics loses its grip. The dream of physicists, to be able to predict any future event in the universe based on its present state, meets its certain death.
If we regard reality as that which can be observed by all, we have to find that there is no objective movement of an electron around the nucleus. This viewpoint would imply that reality is created by the observer; in other words: if we take Heisenberg literally, the moon is not there when nobody is looking at it. However, we must consider the possibility that there is a subatomic reality independent of observation and that the electron may have an actual trajectory which cannot be measured. The moon may be there after all. This conflict is the philosophical essence of the Uncertainty Principle.
Relativity and quantum theory are inconsonant up to the present day, despite great efforts in creating a unified theory capable of accommodating both views. After having published his papers on Relativity, Einstein dedicated the rest of his life to working on such a unified field theory, yet without success. The physicists who followed his lead developed a new model called string theory during the 1970s and 1980s. String theory was successful to some extent in providing a mathematical model that integrates the strong and the weak nuclear forces, electromagnetism, and gravitation. In spite of this, it cannot yet be called a breakthrough, because (1) the theory has not been corroborated thoroughly by observational evidence; and (2) there is not one, but five competing string theories. The latter point has recently been addressed by M-theory, a theory that unites existing string theories in 11 dimensions.
The Zen of Quantum Theory.
We shall leave the problem of theoretical unification to the physicists and instead briefly consider a philosophical unification of Relativity and quantum theory. Is this possible? Contemplating the subatomic realm seems like a Zen exercise. The nuclear reality embodies duality and multiplicity, such as is evident in the complicated structure of atoms and particles. It transgresses the narrow world of opposites. We have to realise that in spite of the different parts and components, the subatomic world in actuality is an undivided whole, where the boundary between the observer and the observed is blurred. Object and subject have become inseparable, spatial and temporal detachment is an illusion. When the American physicist J.R. Oppenheimer (1902-1967) describes the structure of probability clouds, he almost sounds like a Zen Master: "If we ask, whether the position of the electron remains the same, we have to say no. If we ask, whether the position of an electron changes with the course of time, we have to say no. If we ask, whether the electron is in a state of rest, we have to say no. If we ask, whether the electron is in motion, we have to say no."
[Read On]

Big Bang
In the Beginning
The Big Bang model of the universe's birth is the most widely accepted model that has ever been conceived for the scientific origin of everything. No other model can predict as much with as high accuracy as the Big Bang model can.
A common question that people ask is "What happened before the Big Bang?" The phrase "in the beginning" is used here to refer to the birth of our universe with the Big Bang. In the creation of the universe, everything was compressed into an infinitesimally small point in which all physical laws that we know of do not apply. No information from any "previous" stuff could have remained intact. Therefore, for all intents and purposes, the Big Bang is considered thebeginning of everything, for we can never know if there was anything before it.
History of the Big Bang Model
The Big Bang model had its beginnings with Edwin Hubble's discovery in 1929 that, on large scales, everything in the universe is moving away from everything else. The only explanation for this was that the universe was expanding in every direction, and it was taking galaxies along with it. This is known as "Hubble's Law."
The next step towards the Big Bang model was to take this process in reverse - that is, to go back in time. If the universe is "blowing up" like a balloon as time progresses, then what would happen if you were to run the timeline backwards? What was the universe like in the past?
If the universe is currently growing, then the universe was smaller in the past. There must have been some point in time when the universe was half its current size. Then there must have been a time when it was half that size. If you continue to run time backwards, there must have been a time when the universe was an infinitesimally small point*.
This is the basic idea behind the Big Bang. All matter and energy existed in an infinitely small point of infinite density a long time ago, and has since been expanding as our universe. One important note here is that the Big Bang was not an explosion in the universe, but rather it is an explosion of the universe. Therefore, there is no "center" of the universe from where the Big Bang started.
*In calculus, this is an adaptation of "Zeno's Paradox." Through this process, Zeno presented the paradox that a runner could never actually complete a race: They go half way, then three-quarters, then seven-eighths, and so on, but never do they get to finish the race. In calculus this is paradox is solved theoretically with limits, but with the Big Bang, it can be used to show that the universe was not ever "nothing," but must have existed in an extremely tiny space at some time in the past.
Main Evidence
The Big Bang is the leading theory that almost all astrophysicists believe explains the origin of the universe. This is because all observations so far made support the Big Bang theory; there are four main lines of evidence that are most-often used.
The first was discussed above: The expansion of the universe. The universe is expanding now, so in the past it must have been smaller. If it were smaller in the past, then there probably was a time when it was infinitesimally small. One could ask why don't we think that it might be expanding now but it could have been shrinking before and we just don't know about it. The answer is that there is simply no mechanism that we know about that could accomplish this transition on a universal scale.
The second line of evidence is the Cosmic Microwave Background Radiation (CMB) that was discovered in 1965 by Arno Penzias and Robert Wilson from Bell Labs. They were working with a microwave receiver used to communicate with the Telstar satellite, but were getting noise from every direction they pointed the receiver. It was coming from all over the sky at what seemed to be exactly the same frequency. This was the first evidence for the CMB, and they later shared a Nobel Prize for this discovery.
The CMB is an "echo" left over from when the universe was approximately 300,000 years old, as predicted by the Big Bang model. As something becomes compressed, as matter was when the universe was young, it becomes hot. The actual "heat" comes from particles' movements - the faster they move, the more energetic they are, and so the more heat we see. The universe was so hot before it was 300,000 years old that atoms could not form. Because of this, photons - particles of light - could not move around, for they kept reacting with electrons - the negatively charged parts of atoms.
Therefore, during this period, the universe was effectively opaque. Once the universe had reached 300,000 years old, atoms could form, and electrons were now bound to a nucleus. Once this happened, photons could move about freely. This "first light" is the CMB, and its existence is a very strong indication that the Big Bang occurred.
The third major pillar of the Big Bang theory lies in the abundance of the different elements of the universe. The theory predicts that certain amounts of hydrogen (~72%), helium (~28%), and other elements should be made. Observations have shown almost exactly the amounts that are predicted.
The fourth piece is that the Big Bang theory is the only one that comprehensively lays down a framework for the eventual evolution of the universe as we observe it today.

எனக்குள் எழுந்த சந்தேகங்கள் உங்களுக்குளும் இருந்தால் விடை இதோ...... இங்கே........

QUEST:_Does light bend?


ANS:_ From the point of view of general relativity andEinstein's Equivalence Principle, light bends as a result of the
curvature of space-time caused by the presence of a mass. However, from the "point of view" of the photons that are moving through this curved space-time they are trying to move in a straight line but feeling forces.
The latter is an interesting point of view as photons are massless. This give credence to Einstein's point of view. The limit of this bending is reached when the central mass is great enough to cause the curvature of space-time to be so great that the light bends back in a loop of limited
height. When this limit is reached one has a black hole and the limited
height is called an event horizon. For more info. check out Stephen
Hawkings book "A brief History of Time" or Edward Teller's book "Dark
Secrets of Physics". Both have excellent discussions on these matters.
Does light refract? From a classical point of view light refracts as it
moves from one medium to another that can slow the light down. The r
refraction is limited bn the denser medium. as one approaches the critical
angle the angle of refraction approaches 90 degrees. After this total
internal reflection is achieved. This is easily demonstrated with Snell's
law from any physics text. From a quantum mechanical point of view things
get a bit dicy. Rather than try to address that in the limited space here
let me suggest reading Richard Feynman's book "QED". His concrete e
discussions are excellent. Also Teller's book has a nice discussion of
Newton's particulate theory or refraction and momentum. Have fun studying!
=========================================================

Text: What energy range are cosmic rays in are cosmic rays in when they
hit the earth? Does the rate of cosmic rays hitting the earth change over
time , if so how?

Text: As far as I know, cosmic rays come in just about any energy
range, up to extremely high energies (higher than anything that can be
produced in an accelerator on earth). That is before they hit the atmosphere
though. By the time they reach the ground I think most of the high energy
ones have scattered to produce secondary "showers". The rate of cosmic ray
influx does change with time - specifically with the solar cycle. When the
sun is active, cosmic rays are not, and vice versa.


Text: Actually, when the sun is active, it sends out streams of high
energy protons. These cause the Aurora borealis in the northern hemisphere
and present a serious radiation hazard to astronauts.




Subject: Why is sound faster in warm air?
Text: Why does sound travel faster in warm air than in cool air?

Text: The warmer the air is the greater the average mean speed of the
molecules of air. Since sound is transferred by collisions of molecules, the
quicker they move the sooner the collisions transfer the sound energy down the
path.

Text: The traveling of sound depends on the forces between the atoms or
molecules of the medium, and in a gas those force only act during the short
period when they collide. But in liquids and solids the fundamental question
is how fast the atoms jiggle around in their local positions (sound waves are
actually coordinated long-wavelength "jiggles") which gets faster the higher
the strength of the local forces keeping the atoms roughly in place. So,
sound travels fastest in the most strongly bonded materials.


Subject: What do naked singularities look like?
Text: I just read an American Scientist article by Shapiro & Teukolsky
about naked singularities forming from matter shaped as a prolate spheroid.
How can a singularity not be cloaked in a black hole? What would a naked
singularity look like in space?


Text: Well, a singularity is a very unpleasant thing in physics -
basically it means you have a place where things "blow up" - that is, diverge
to infinity, and it usually means there is something wrong with the theory.
People up till now have not been too upset about singularities in Einstein's
theories because they thought they were always cloaked in black holes. Now it
turns out they are not. Basically, a black hole is defined by the paths that
light takes. Light cannot escape from inside a black hole. Since part of
this singularity is outside a black hole, presumably light can escape from it.
But, since something in the gravitational fields is diverging at this point,
it would definitely be a very unpleasant place to even be near to. Probably
would be fascinating to drop stuff into though - you could actually see matter
being crushed and torn apart by arbitrarily large forces.



Subject: How does a voice travel over a wire?
Text: How does a voice travel over a wire? How can a picture or picture
with sound do the same? How does ISDN allow multiple independent
transmissions to occur at high speed over normal copper wires?

Text: Basically, a phone is analog (from your house to the local
switching station). Analog means that the air patterns (pulses of air) from
your mouth are transformed to analog signal. The analog signal is a electric
'wave' that represent the sound in electric form. Actually, this electric
form is a varying voltage that can duplicate your voice by moving a diaphragm
connected to magnet in the receiver on the other end. It can be thought of as
when you talk you move a magnet in a coil which produces a signal. This
signal cause a similar action on a coil in the receiver on the other end. Now
let us cover ISDN next: I said your current phone is analog to the local
switching station (in many cases anyway). From the switching station, to
other switching stations, is digital. This means that analog (wave pattern)
is translated to digital and transmitted. Well, the digital signal is much
more efficient. WHY? Well, the digital signal is translated and packaged in
a "Packet". This packet has a header that has an address. This address is
then pushed out on a high speed network (like an expressway). The expressway
is full of many signals. All actually going lets say single file, but since
each signal is en capsuled in its own packet or envelope, special devices can
grab the packets and route them to the correct destination (like exit ramps).
So, in essence, you call can share the same road as many other calls. When
you talk, your words are packaged and sent out on this busy network. ISDN
means that the whole process is digital. Your "voice" never gets transmitted
in analog. So, to send a video or voice signal, all that is needed is to
convert them to digital. Then package them and send them along. On the other
end they are unpackaged, converted back (in some cases like CD or other
digital technology this is not needed), and displayed or played. The key to


the process is bandwidth. In other words, how much data can flow at one time.
You need very high speed data paths to allow the rate of video to travel
across this network. Fiber optics have a much greater potential of carrying
large amounts of data than do copper cables. Soon (some are experimenting
with it today) it will be possible to transmit voice, data (computer stuff),
and video all over a single connection at your house. That is when ISDN comes
to the house.



________________________________________
= mc^2


What does e=mc^2 mean?
---------------------------------------
By itself, this equation tells us that, when a quantity "m" of
matter is converted to energy, the amount produced is equal to m*c^2, where c
is the speed of light. One way this equation can be obtained: when Einstein
developed the special theory of relativity, he found it predicted that mass
increases with speed. When this is applied to a calculation for the kinetic
energy KE of a moving object traveling with speed v, one obtains the result KE
= (m - m0)*c^2 where m is the (increased) mass of the object when its speed is
v, and m0 is the mass of the object when it is at rest. Thus, an increase in
kinetic energy is accompanied by an increase in mass. Furthermore, this
suggests that, even when the object is not moving, there is a "rest-mass"
energy m0*c^2 associated with it.Then the total energy (kinetic + rest-mass
energy) is given by E=mc^2 . It was Einstein's great insight to assert that
matter and energy in general are, in essence, equivalent and interchangeable.
There was other evidence for such an assertion at that time. An experiment
done in 1890 confirmed that radiation exerts a pressure when it hits and is
absorbed by an object. Interpreted in the context of relativity, one is led
to the conclusion that when an object absorbs electromagnetic radiation of
energy E, its mass is increased by E/c^2. Thus, it seems reasonable to assign
an equivalent mass to radiation. Since then, the correctness of the E=mc^2
equation, as well as the mass- energy equivalence that underlies it, has been
widely confirmed by experiment.

====================================================================

Question: I hope this seems timely . Can anyone explain, in
relatively simple terms, the reasons for Hawking's
belief in jumbled radiation emanating from black holes?
------------------------------------------------
Answer: Virtual particle-antiparticle pairs are being created all the time.
usually they disappear almost instantaneously, but if a virtual pair
is created just inside the event horizon of a black hole, then
Hawking showed that one of them could escape and become real instead
of virtual. the black hole thereby loses mass. but this process
(Hawking radiation) is only significant for very small black holes.

====================================================================

Question: If matter cannot be created or destroyed,
where did the first(present)matter come from?
------------------------------------------------
Though, it is still a big question, if the "Big Bang" was
reasonably symmetric the universe should have started out
as pure energy, or as equal parts antimatter and matter
(matter can be destroyed and turned into pure energy by
combining it with antimatter, and energy in some other
form can be converted into equal parts of matter and antimatter).
The big question is, why do we normally only see matter around
us, and not equal parts of both matter and antimatter? (Of
course it would be pretty hazardous living around stuff that
could annihilate you!). There are a bunch of theories, but as
far as I know, nobody really knows yet.


====================================================================

Question: I see the words 'chaos' and 'fractals' very often here.
and somehow I get a vague impression, that they are some kind of buzz words, that
have become popular simpley because they have a certain 'sex appeal' a
look good on a graphics screen. Some (several) years ago, when I last
read anything about these things, was in 'The mathematical Intelligenser'
from Springer, and what I read was a not very kind article about
Mandelbrot and the fuzz about fractals. So what is the truth about it?
---------------------------------------
Chaos theory: a system behaves chaotically if it behaves reproducibly
given the same initial conditions, given slightly different initial
conditions, it behaves very differently. In other words, it is very
sensitive to initial conditions. This is completely distinct from
randomness or noise.
fractals: popularized by Mandelbrot who treated them like a business
instead of science for which he was criticized. We have all seen the
pretty pictures. The essential ingredient is self-similarity which
means the pattern looks the same no matter what size scale you use.
fuzzy logic: (I am a truck) if I am going 20 MPH and the driver is applying
the brakes, then I should apply light braking pressure with a probability
of 20%, medium pressure with a probability of 50%, and high pressure
with a probability of 30%.

====================================================================
The important thing about chaos (and to a lesser extent
fractals) is that they give us a new mathematically based picture
of certain ways in which the real world behaves, which is probably
obvious to most people without the mathematics. The old ideas
(since Newton himself!) were that the universe ran like clockwork -
if you knew well enough what everything in the universe was
doing at a particular time, you could predict the future
of everything arbitrarily far. That is still sort of true (barring
randomness from quantum mechanics) - but chaos theory tells us
the immense precision that would be required to do that sort
of precision - basically it is the reason we will probably never
be able to predict the weather more than a week in advance, unless
we control it in some way... Chaos theory (and fractlas) tell
us more though - that even with this hopelessness of prediction
of specifics (where every single particle will be indefinitely into
the future) we still can get pictures of patterns that tell us
qualitatively what will happen, and that is better than nothing!

====================================================================


Question: What is anti-matter? Is it the opposite of matter
in the sense that it has a 2> force insted of a gravitational
force, and does time go slower in close
proximity to it?
---------------------------------------
Yours is a complicated question that I can best
answer with an example.
A positron is anti-matter for an electron. Try looking up positron in
an encyclopedia, and take it from there.
We believe that anti-matter and matter behave the same way with gravity,
On the surface of the earth both matter and anti-matter fall toward the
center of the earth. This belief is currently being tested experimentally.
I believe that one of the experiments is being done at Fermilab.

====================================================================
One way of viewing antimatter is as matter going backwards
in time. However, that really only works on a microscopic
scale (a microscopic piece of matter which could be thought of
as annihilating with a piece of antimatter could also be thought
of as switching directions in time, with the same effects).
Macroscopically antimatter is expected to behave exactly the
same as matter (gravity works the same, and the mass of a piece
of antimatter is always exactly the same as the mass of its
corresponding piece of matter) except that electrical things will
all be backwards, since antimatter has opposite electrical charges
from matter.

====================================================================



Question: What happens when antimatter and matter meet?
Can antimatter-matter reactions be use as an energy source
------------------------------------------------
Matter and antimatter annihilate one another when they meet,
which means that they produce a whole bunch of energy, usually
as a pair of photons. However, antimatter is rather expensive
to make, and it is very hard to keep around in any significant
quantity, so it would not make a very practical energy storage
option (and it could never be an actual SOURCE of energy because
energy had to go into making it in the first place - unless we
found somewhere in the universe where antimatter was actually
plentiful).

====================================================================

Question: If, as Einstein postulated, gravity is not really a force,
but is in fact adescription of how objects behave in curved space-time,
then why do physic
(sorry, I mean physicists) work to unify it with the other three forces?
andrew m childs
---------------------------------------
Well, perhaps what physicists should be doing is modifying the equations
for the other three forces to fit them into Einstein's model of spacetime?
That is also a hard problem, though I think progress has been made. The
problem is that the way Einstein's equations were set up, it makes
space-time itself look like something that ought to be quantum mechanically
described, just like electric fields etc are really described by quantum
mechanics. This leap of logic could be wrong - it could be that Einstein
was right all the way down to very small length-scales and very high
energies - but since we have no way of experimentally testing that
right now, the other approach looks more logically consistent, a criterion
Einstein himself would have approved of. But it could be wrong - and we
really do not have much prospect of experimental tests for several
centuries at least.

====================================================================


Question: If one could drill a hole through the center of the earth,
from pole to pole. Jump down into the hole. Would the person
eventually come to a rest in the center because of the forces
of gravity?
------------------------------------------------
Yes, one would eventually come to rest at the center BUT because
of the forces of friction (with the atmosphere in the hole). If there
were no friction, you would just oscillate from pole to pole. The
force of gravity at the center is actually ZERO because there is
equal mass in all directions (equal gravitational pull in all
directions).

====================================================================



Question: If a piece of iron would get into a particle accelerator would it be
ripped apart by the magnets into individual atoms?
jonathan
------------------------------------------------
Answer: No - actually the magnets really are not all that strong in
particle accelerators - they put humans inside magnets
of about the same strength to do MRI measurements, for
instance!
At the very start of a particle accelerator there is
an oven or gas source of some sort that produces the
original particles, and they are initially accelerated
using strong electric fields, not magnetic ones. The
magnets only kick in once the particles are already moving
at several million volts worth of energy, which is
about as far as you can get with straight electrical
fields. Actually, it is not really magnets that
do the accelerating, but certain microwave cavities
called klystrons and similar strange names.
=========================================================

Question: What makes radioactivity from a nuclear bomb so deadly? What consists
inside of nuclear bomb to make it so powerful?
------------------------------------------------
Answer: Nuclear bombs are powerful for the same reason nuclear reactors
are able to produce lots of energy out of a small amount of fuel -
the nuclear reactions involved just do give out a lot of energy,
much more per unit weight than chemical explosives. The reason
is the very small mass differences between the two sides of a
nuclear reaction get multiplied by the speed of light squared
(remember E = mc^2) which is a huge number, and so there is a lot
of energy involved.
There is lots more to it of course - in order to get the reaction
going you have to produce neutrons. If you control the neutron
production you can get a nice stable reaction going, as in a nuclear
power station. If you do not control it, you get a "chain reaction":
which explodes. And that is just for fission reactions - there is
also a fusion process that has only been successfully implemented
in bombs, because it is such a hard problem to make a reactor
that controls fusion reactions. But the energy comes from the
same kind of process (in fission it is heavy nuclei splitting apart,
in fusion it is light nuclei joining together).
Why is radioactivity from a bomb so deadly? Well, it is because of
that uncontrolled "chain reaction" that is spewing neutrons in
all directions, and those extra neutrons cause all sorts of trouble.
At least, that is the immediate effect. The nuclear reactions also
result in the release of all kinds of harmful radioactive byproducts
that last for many years. So in general it is pretty nasty stuff.

________________________________________
Question: What is the conflict between quantum mechanics and relativity theory
that I have heard about?
------------------------------------------------
Answer: According to relativity, the influence of an event can be felt
only if you can see the event, and the influence should be felt
only after it has been seen. The essential point being that
light travels faster than anything else. In quantum mechanics,
it can be shown that influence of an event may be felt before
a light signal could have reached you from the same event.
This is the main problem. In quantum mechanics, you have to
specify initial state everywhere and everything in the world
effects the evolution of a system. In relativity, only the
events within the lightcone effect the evolution.
jasjeet s bagla

________________________________________
Question: How did they figure out how to make the atom bomb work?
------------------------------------------------
Answer: They spent a lot of money :-)
Actually, I have always wondered why it was so difficult to
make the bomb work - the main principle is really simple. The
idea is that a nucleus that is splitting gives off some neutrons.
Some nuclei do this once in a while just naturally, but if
there are lots of neutrons around they have a much higher
tendency to split, and then produce more neutrons, which can
cause more nuclei to split, in a chain reaction leading to
an explosion... As soon as you have collected a critical mass
of the material (Uranium or Plutonium, for example) you should
get an explosion. I guess the hard problems are:
(1) NOT getting an explosion until you actually want it
(2) NOT getting a wimpy little explosion that blows your
material apart before the chain reaction goes very far.
Also, it was a tough problem because it is not something you
really want to do lots of trial and error on... There are
a bunch of interesting books on the beginning of the nuclear
age that you could probably find in the library. The U.S. effort
was called the "Manhattan project".

Question: How are scientists able to see and know the activity of outer
galaxies? How are scientists able to know how far away stars are?
How are scientists able to know what stars are composed of?
------------------------------------------------
Answer: Astronomers use telescopes to see things far away - does that
answer the first question? There are all kinds of telescopes
(visible, radio, infrared, x-ray, space-based, ground-based,
mountain-based..., single-lens, single-mirror, multiple-mirror,
multi-site interferometers etc.) but they all work kind of
the same way: you point it at somewhere in the sky and look
at what kind of electromagnetic signals are coming from there.
Distance can be determined very directly for nearby stars by
a technique called parallax - as the earth goes around the
sun the star looks like its moving, and the extent of the motion
goes down inversely with distance. For further stars there
are a whole sequence of things based on "color" (see next answer)
and type of star that seem to follow very standard rules.
What stars are made of is determined through "spectroscopy"
which is a detailed analysis of the different wavelengths of
light coming from the stars. Different elements emit
or absorb light at very special wavelengths, and thus leave
their "signature" in the light coming from the star.
=========================================================
Can Earth Change Speeds?

Question: Is it possible for the earth to change speeds while spinning on
its axis?
------------------------------------------------
Answer: Yes, actually the earth does very gradually change quite
a few of its orbital and rotational parameters - because
of various conservation laws of physics, however, it
cannot do this all on its own - it needs help. The help
it gets is from the moon and from the other planets,
and also from tidal interactions with the sun. The
earth has long been exchanging angular momentum with
the moon - you might be interested in looking up articles
on the moon-earth relationship - the length of the day
on earth used to be much shorter and the moon much
closer (I think that is right anyway).
=========================================================


Question: How would you calculate the age of the universe scientifically
in regards to the rotation of the moon on its axis, and the
distance from the moon to the earth?
------------------------------------------------
Answer 1: I do not think you can get a meaningful answer from
the data provided here.
=========================================================
Answer 2: Assuming the earth is younger than the universe, figuring out
something about how long the earth and moon have been together
tells you the universe must be at least that old. I believe that
the earth-moon system is more complicated than it looks because of
the earth's rotation, combined with the moon's orbit about the earth.
The axis of the earth's rotation and the moon's orbit are different,
and the gravitational forces between them cause a gradual change in
both. There is some evidence that the earth spun faster on its axis
a billion years ago or so (perhaps every 22 hours instead of 24?)
and the moon was closer in then. Newton's equations for the earth-moon
system can then confirm that this kind of trading of angular momentum
has actually taken place. This kind of problem with rotating
interacting bodies may be treatable using the Lagrange method - look
in an advanced mechanics book (Goldstein perhaps) for a discussion
of these approaches. However, the problem may be more complicated
than I think (involving energy dissipation through the tides
for example) but I do not know a good reference on the subject.
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Question:
Dear Newton,
How does the sun move around the Earth?
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Answer: The Earth moves around the sun, not the other way around.
The sun looks like it is moving around us because the Earth
is actually rotating. This was one of the big revolutions
in science about 400 years ago, when this question got settled.
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Question: How does a magnet work?
------------------------------------------------
Answer: All the magnetic fields we can create are the result of moving
charges. Electromagnets make fields through large currents
in wires we make. Permanent magnets produce fields through the
orientation of the electron orbits and spins of the atoms in
the magnet.
=========================================================

Planets distances from each other

Question: Why are planets so far from each other?
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Answer: Well, the planets are pretty far apart compared to their sizes -
the size of a planet is generally a few thousand miles (maybe 10s
of thousands for the big ones) while the distances between them
are tens or hundreds of millions of miles. Why that huge factor
of a thousand or so? It probably has a lot to do with the average
density of matter in the original dust-cloud that (we think) formed
the solar system and the sun itself. There really is not much matter
out there in space (it is "empty"!) and even the big nebulas astronomers
like to photograph are still very much less dense than the planets.
If there are nebulas that are much denser than the one our solar system
started, they might have planets closer together...
On the other hand, there could be something else going on. The
gravitational interactions between the planets even as far apart
as they are still pretty complex, and the solar system is really
not all that stable - maybe if we had extra planets closer together
long ago, they would have been ejected from the solar system by now.
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Question: Can space exist without time, time without space, black holes without
either?
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Answer: As an aside, theoretical physicists love to play with fictitious
arrangements of space and time - for example one dimension of
space and one of time makes nice little diagrams and is one of the
favorites. Undoubtedly people have investigated spaces with no time,
and "spaces" with just time and no space, but I cannot see any practical
use for them. Having both space and time makes things interesting
(motion makes no sense without both, for example). Black holes
probably can exist without time, but where would one be if there
was no space?