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
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.
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
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
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
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
Question: If a piece of iron would get into a particle accelerator would it be
ripped apart by the magnets into individual atoms?
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
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
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.
How does the sun move around the Earth?
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.
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
Planets distances from each other
Question: Why are planets so far from each other?
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.
Question: Can space exist without time, time without space, black holes without
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?