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Item 1. updated 17-JUN-1993 by SIC
An Introduction to the Physics Newsgroups on USENET
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The USENET hierarchy contains three newsgroups dedicated to the
discussion of physics and physics-related topics. These are sci.physics,
sci.physics.research, and alt.sci.physics.new-theories.
Sci.Physics is an unmoderated newsgroup dedicated to the discussion
of physics, news from the physics community, and physics-related social
issues. Sci.Physics.Research is a moderated newgroup designed to offer an
environment with less traffic and more opportunity for discussion of
serious topics in physics among experts and beginners alike. The current
moderators of sci.physics.research are John Baez (jbaez@math.mit.edu),
William Johnson(mwj@beta.lanl.gov), Cameron Randale (Dale) Bass
(crb7q@kelvin.seas.Virginia.edu), and Lee Sawyer (sawyer@utahep.uta.edu).
Alt.sci.physics.new-theories is an open forum for discussion of any
topics related to conventional or unconventional physics. In this
context, "unconventional physics" includes any ideas on physical science,
whether or not they are widely accepted by the mainstream physics community.
People from a wide variety of non-physics backgrounds, as well
as students and experts in all areas of physics participate in the ongoing
discussions on sci.physics and sci.physics.research. Professors, industrial
scientists, graduate students, etc., are all on hand to bring physics
expertise to bear on almost any question. But the only requirement for
participation is interest in physics, so feel free to post -- but before
you do, please do the following:
(1) Read this posting, a.k.a., the FAQ. It contains good answers,
contributed by the readership, to some of the most frequently asked
questions.
(2) Understand "netiquette." If you are not sure what this means,
subscribe to news.announce.newusers and read the excellent discussion of
proper net behavior that is posted there periodically.
(3) Be aware that there is another newsgroup dedicated to the discussion of
"alternative" physics. It is alt.sci.physics.new-theories, and is the
appropriate forum for discussion of physics ideas which are not widely
accepted by the physics community. Sci.Physics is not the group for such
discussions. A quick look at items posted to both groups will make the
distinction apparent.
(4) Read the responses already posted in the thread to which you want to
contribute. If a good answer is already posted, or the point you wanted
to make has already been made, let it be. Old questions have probably been
thoroughly discussed by the time you get there - save bandwidth by posting
only new information. Post to as narrow a geographic region as is
appropriate. If your comments are directed at only one person, try E-mail.
(5) Get the facts right! Opinions may differ, but facts should not. It is
very tempting for new participants to jump in with quick answers to physics
questions posed to the group. But it is very easy to end up feeling silly
when people barrage you with corrections. So before you give us all a
physics lesson you'll regret - look it up.
(6) Don't post textbook problems in the hope that someone will do your
homework for you. Do you own homework; it's good for you. On the other
hand, questions, even about elementary physics, are always welcome. So
if you want to discuss the physics which is relevent to your homework,
feel free to do so. Be warned that you may still have plenty of
work to do, trying to figure out which of the many answers you get are
correct.
(7) Be prepared for heated discussion. People have strong opinions about
the issues, and discussions can get a little "loud" at times. Don't take it
personally if someone seems to always jump all over everything you say.
Everyone was jumping all over everybody long before you got there! You
can keep the discussion at a low boil by trying to stick to the facts.
Clearly separate facts from opinion - don't let people think you are
confusing your opinions with scientific truth. And keep the focus of
discussion on the ideas, not the people who post them.
(8) Tolerate everyone. People of many different points of view, and widely
varying educational backgrounds from around the world participate in this
newsgroup. Respect for others will be returned in kind. Personal
criticism is usually not welcome.
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Item 2.
Gravitational Radiation updated: 4-May-1992 by SIC
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Gravitational Radiation is to gravity what light is to
electromagnetism. It is produced when massive bodies accelerate. You can
accelerate any body so as to produce such radiation, but due to the feeble
strength of gravity, it is entirely undetectable except when produced by
intense astrophysical sources such as supernovae, collisions of black
holes, etc. These are quite far from us, typically, but they are so
intense that they dwarf all possible laboratory sources of such radiation.
Gravitational waves have a polarization pattern that causes objects
to expand in one direction, while contracting in the perpendicular
direction. That is, they have spin two. This is because gravity waves are
fluctuations in the tensorial metric of space-time.
All oscillating radiation fields can be quantized, and in the case
of gravity, the intermediate boson is called the "graviton" in analogy
with the photon. But quantum gravity is hard, for several reasons:
(1) The quantum field theory of gravity is hard, because gauge
interactions of spin-two fields are not renormalizable. See Cheng and Li,
Gauge Theory of Elementary Particle Physics (search for "power counting").
(2) There are conceptual problems - what does it mean to quantize
geometry, or space-time?
It is possible to quantize weak fluctuations in the gravitational
field. This gives rise to the spin-2 graviton. But full quantum gravity
has so far escaped formulation. It is not likely to look much like the
other quantum field theories. In addition, there are models of gravity
which include additional bosons with different spins. Some are the
consequence of non-Einsteinian models, such as Brans-Dicke which has a
spin-0 component. Others are included by hand, to give "fifth force"
components to gravity. For example, if you want to add a weak repulsive
short range component, you will need a massive spin-1 boson. (Even-spin
bosons always attract. Odd-spin bosons can attract or repel.) If
antigravity is real, then this has implications for the boson spectrum as
well.
The spin-two polarization provides the method of detection. Most
experiments to date use a "Weber bar." This is a cylindrical, very
massive, bar suspended by fine wire, free to oscillate in response to a
passing graviton. A high-sensitivity, low noise, capacitive transducer
can turn the oscillations of the bar into an electric signal for analysis.
So far such searches have failed. But they are expected to be
insufficiently sensitive for typical radiation intensity from known types
of sources.
A more sensitive technique uses very long baseline laser
interferometry. This is the principle of LIGO (Laser Interferometric
Gravity wave Observatory). This is a two-armed detector, with
perpendicular laser beams each travelling several km before meeting to
produce an interference pattern which fluctuates if a gravity wave distorts
the geometry of the detector. To eliminate noise from seismic effects as
well as human noise sources, two detectors separated by hundreds to
thousands of miles are necessary. A coincidence measurement then provides
evidence of gravitational radiation. In order to determine the source of
the signal, a third detector, far from either of the first two, would be
necessary. Timing differences in the arrival of the signal to the three
detectors would allow triangulation of the angular position in the sky of
the signal.
The first stage of LIGO, a two detector setup in the U.S., has been
approved by Congress in 1992. LIGO researchers have started designing a
prototype detector, and are hoping to enroll another nation, probably in
Europe, to fund and be host to the third detector.
The speed of gravitational radiation (C_gw) depends upon the
specific model of Gravitation that you use. There are quite a few
competing models (all consistent with all experiments to date) including of
course Einstein's but also Brans-Dicke and several families of others.
All metric models can support gravity waves. But not all predict radiation
travelling at C_gw = C_em. (C_em is the speed of electromagnetic waves.)
There is a class of theories with "prior geometry", in which, as I
understand it, there is an additional metric which does not depend only on
the local matter density. In such theories, C_gw != C_em in general.
However, there is good evidence that C_gw is in fact at least
almost C_em. We observe high energy cosmic rays in the 10^20-10^21 eV
region. Such particles are travelling at up to (1-10^-18)*C_em. If C_gw <
C_em, then particles with C_gw < v < C_em will radiate Cerenkov
gravitational radiation into the vacuum, and decelerate from the back
reaction. So evidence of these very fast cosmic rays good evidence that
C_gw >= (1-10^-18)*C_em, very close indeed to C_em. Bottom line: in a
purely Einsteinian universe, C_gw = C_em. However, a class of models not
yet ruled out experimentally does make other predictions.
A definitive test would be produced by LIGO in coincidence with
optical measurements of some catastrophic event which generates enough
gravitational radiation to be detected. Then the "time of flight" of both
gravitons and photons from the source to the Earth could be measured, and
strict direct limits could be set on C_gw.
For more information, see Gravitational Radiation (NATO ASI -
Les Houches 1982), specifically the introductory essay by Kip Thorne.
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Item 3.
ENERGY CONSERVATION IN COSMOLOGY AND RED SHIFT updated: 10-May-1992 by SIC
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IS ENERGY CONSERVED IN OUR UNIVERSE? NO
Why? Every conserved quantity is the result of some symmetry of
nature. This is known as Noether's theorem. For example, momentum
conservation is the result of translation invariance, because position is
the variable conjugate to momentum. Energy would be conserved due to
time-translation invariance. However, in an expanding or contracting
universe, there is no time-translation invariance. Hence energy is not
conserved. If you want to learn more about this, read Goldstein's
Classical Mechanics, and look up Noether's theorem.
DOES RED-SHIFT LEAD TO ENERGY NON-CONSERVATION: SOMETIMES
There are three basic cosmological sources of red-shifted light:
(1) Very massive objects emitting light
(2) Very fast objects emitting light
(3) Expansion of the universe leading to CBR (Cosmic Background
Radiation) red-shift
About each:
(1) Light has to climb out the gravitational well of a very massive object.
It gets red-shifted as a result. As several people have commented, this
does not lead to energy non-conservation, because the photon had negative
gravitational potential energy when it was deep in the well. No problems
here. If you want to learn more about this read Misner, Thorne, and
Wheeler's Gravitation, if you dare.
(2) Fast objects moving away from you emit Doppler shifted light. No
problems here either. Energy is only one part a four-vector, so it
changes from frame to frame. However, when looked at in a Lorentz
invariant way, you can convince yourself that everything is OK here too.
If you want to learn more about this, read Taylor and Wheeler's
Spacetime Physics.
(3) CBR has red-shifted over billions of years. Each photon gets redder
and redder. And the energy is lost. This is the only case in which
red-shift leads to energy non-conservation. Several people have speculated
that radiation pressure "on the universe" causes it to expand more quickly,
and attempt to identify the missing energy with the speed at which the
universe is expanding due to radiation pressure. This argument is
completely specious. If you add more radiation to the universe you add
more energy, and the universe is now more closed than ever, and the
expansion rate slows.
If you really MUST construct a theory in which something like
energy is conserved (which is dubious in a universe without
time-translation invariance), it is possible to arbitrarily define things
so that energy has an extra term which compensates for the loss. However,
although the resultant quantity may be a constant, it is of questionable
value, and certainly is not an integral associated with time-invariance, so
it is not what everyone calls energy.
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Item 4.
EFFECTS DUE TO THE FINITE SPEED OF LIGHT updated 28-May-1992 by SIC
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There are two well known phenomena which are due to the finite
speed of electromagnetic radiation, but are essentially classical in
nature, requiring no other facts of special relativity for their
understanding.
(1) Apparent Superluminal Velocity of Galaxies
A distant object can appear to travel faster than the speed of
light relative to us, provided that it has some component of motion towards
us as well as perpendicular to our line of sight. Say that on Jan. 1 you
make a position measurement of galaxy X. One month later, you measure it
again. Assuming you know it's distance from us by some independent
measurement, you derive its linear speed, and conclude that it is moving
faster than the speed of light.
What have you forgotten? Let's say that on Jan. 1, the object is D
km from us, and that between Jan. 1 and Feb. 1, the object has moved d km
closer to us. You have assumed that the light you measured on Jan. 1 and
Feb. 1 were emitted exactly one month apart. Not so. The first light beam
had further to travel, and was actually emitted (1 + d/c) months before the
second measurement, if we measure c in km/month. The object has traveled
the given angular distance in more time than you thought. Similarly, if
the object is moving away from us, the apparent angular velocity will be
too slow, if you do not correct for this effect, which becomes significant
when the object is moving along a line close to our line of sight.
Note that most extragalactic objects are moving away from us due to
the Hubble expansion. So for most objects, you don't get superluminal
apparent velocities. But the effect is still there, and you need to take
it into account if you want to measure velocities by this technique.
References:
Considerations about the Apparent 'Superluminal Expansions' in
Astrophysics, E. Recami, A. Castellino, G.D. Maccarrone, M. Rodono,
Nuovo Cimento 93B, 119 (1986).
Apparent Superluminal Sources, Comparative Cosmology and the Cosmic
Distance Scale, Mon. Not. R. Astr. Soc. 242, 423-427 (1990).
(2) Terrell Rotation
Consider a cube moving across your field of view with speed near
the speed of light. The trailing face of the cube is edge on to your line
of sight as it passes you. However, the light from the back edge of that
face (the edge of the square farthest from you) takes longer to get to your
eye than the light from the front edge. At any given instant you are
seeing light from the front edge at time t and the back edge at time
t-(L/c), where L is the length of an edge. This means you see the back
edge where it was some time earlier. This has the effect of *rotating* the
*image* of the cube on your retina.
This does not mean that the cube itself rotates. The *image* is
rotated. And this depends only on the finite speed of light, not any other
postulate or special relativity. You can calculate the rotation angle by
noting that the side face of the cube is Lorentz contracted to L' =
L/gamma. This will correspond to a rotation angle of arccos(1/gamma).
It turns out, if you do the math for a sphere, that the amount of
apparent rotation exactly cancels the Lorentz contraction. The object
itself is flattened, but then you see *behind* it as it flies by just
enough to restore it to its original size. So the image of a sphere is
unaffected by the Lorentz flattening that it experiences.
Another implication of this is that if the object is moving at
nearly the speed of light, although it is contracted into an
infinitesimally thin pancake, you see it rotated by almost a full 90
degrees, so you see the complete backside of the object, and it doesn't
disappear from view. In the case of the sphere, you see the transverse
cross-section (which suffers no contraction), so that it still appears to
be exactly a sphere.
That it took so long historically to realize this is undoubtedly
due to the fact that although we were regularly accelerating particle beams
in 1959 to relativistic speeds, we still do not have the technology to
accelerate any macroscopic objects to speeds necessary to reveal the
effect.
References: J. Terrell, Phys Rev. _116_, 1041 (1959). For a textbook
discussion, see Marion's _Classical Dynamics_, Section 10.5.