RNeal.dm8%pco@HI-MULTICS.ARPA (04/24/84)
I have some questions about relativity, time, and speed and I thought
the attendees of this meetimthe attendees of this meetim ng may be able
to answer them.
First question (and we need some assumptions)-
1)the speed of light is constant in a given medium without
regard to the motion of the source.
2)the sun is moving through space in some definite direction
with a definite speed.
Therefore, the frequency of sunlight hitting the earth when it
is "following" the sun should be different than that when it is
"leading" the sun due to "red shift".
>>>Is this correct? Is it detectable?
Next (and not as well constructed either)-
Time passes at different rates due to the speed at which an
object travels (ie. the famous Twins Paradox). What reference
point is "speed" calculated from? If the same effect was seen
from either reference point, then it would not matter; but it
is not.
In the experiment with the atomic clocks on the jet and the ground
they zipped the jet around the earth, then the clocks were not the
same (the clock on the jet was behind, I think). What if the jet
flew counter to the earth's rotation? Wouldn't it in effect be
going "slower" than the ground clock? Would the time differences
reverse?
Last (and shortest, but not least)-
Since everything in the universe is moving (and some at a very
good clip according to the red shifts observed) does that mean
time passes at different rates everywhere?
Thanks for any help in setting my confused mind straight.
>RUSTY<gwyn@Brl-Vld.ARPA (04/25/84)
From: Doug Gwyn (VLD/VMB) <gwyn@Brl-Vld.ARPA> Only RELATIVE speeds matter. If the Earth were moving away from the Sun, then yes, there would be a red-shift of sunlight detected on the Earth. The proper way to put the constancy of the speed of light is "The speed of light in a vacuum is the same for all observers no matter what their state of motion". The funny mathematics of special relativity ensures that this condition is met; in particular, one cannot just add two velocities the simple way to get the net effect of combined velocity. The twin paradox is harder to explain in simple terms since more is involved than just relative velocities. These matters are fully explained in any good introduction to special relativity, such as those mentioned on this mailing list a few months back. I highly recommend studying such material rather than trying to reason out these matters intuitively.
palmer@uw-june.UUCP (04/29/84)
fnord
Rusty Neal asks some questions about relativity. Since these are the
standard questions asked by people with incomplete understanding of
special relativity (i.e., all of us, until we know enough to have
incomplete understanding of more advanced subjects) I thought I would
reply directly to the net. His questions are preceded by '***'
***First question (and we need some assumptions)-
*** 1)the speed of light is constant in a given medium without
*** regard to the motion of the source.
*** 2)the sun is moving through space in some definite direction
*** with a definite speed.
***
*** Therefore, the frequency of sunlight hitting the earth when it
*** is "following" the sun should be different than that when it is
*** "leading" the sun due to "red shift".
***
***>>>Is this correct? Is it detectable?
This is not correct. The two assumptions which should be made are:
1)The speed of light in vacuum (no medium) is a constant
independent of the motions of the source and the observer.
2)The Sun is NOT moving through space with a definite speed and
direction. Its speed depends on what its measured relative to.
There is no way to detect absolute motion, and absolute motion
is usually regarded as an invalid concept. (There is, however,
such a thing as absolute acceleration.)
There is doppler-shift ("red-shift" and "blue-shift") from the sun as
seen from Earth, but only when the Sun is moving relative to Earth.
This is usually seen at the East and West limbs of the Sun, where, due
to rotation, the Sun's surface is moving toward and away from Earth.
***Next (and not as well constructed either)-
*** Time passes at different rates due to the speed at which an
*** object travels (ie. the famous Twins Paradox). What reference
*** point is "speed" calculated from? If the same effect was seen
*** from either reference point, then it would not matter; but it
*** is not.
Time does pass at different rates due to the speed at which an
object travels. If a object A sees object B moving at a speed v, then
(after compensating for the Doppler shift) B's clocks, AS SEEN BY A,
are ticking at a rate (1-v^2/c^2)^0.5 times the rate of A's clocks
(that is, moving clocks run slow.) However, B sees A as moving at the
same speed v, and B also sees that A's clocks are running slow by the
same factor. The twin paradox arises when one of the twins stops
moving relative to the other. (there IS a difference between A
stopping and B stopping, the one that stops is the one which
accelerates, and acceleration is an absolute quantity.) If B stops
when A and B are (for instance) 10 light years away, then B will see
that A's clock stopped running slow 10 years before (when the signal
which B sees when he stops was broadcast by A's clock.) However, A
still sees B's clock running slow until 10 year's later, when the
signal, which B's clock broadcast when B stopped, arrives.
Many subtleties have been glossed over here, but that is this basis
of the paradox.
***Last (and shortest, but not least)-
*** Since everything in the universe is moving (and some at a very
*** good clip according to the red shifts observed) does that mean
*** time passes at different rates everywhere?
As seen from Earth a clock on a distant quasar seems to run slow. As
seen from the quasar, a clock on distant Earth seems to run slow.
I hope this has clarified your confusion.
--------------
"Every day its the same thing--variety. I want something different"
David Palmerdgary@ecsvax.UUCP (05/01/84)
RE the twin "paradox" ...
The classic pu\zzlement runs thisaway: "If A goes to Alpha Centauri at .995c
and comes back younger than B, the twin who stays home, OK. But howcome
(if 'all motion is relative') we can't assume B is 'at rest' and A (along
with the majority of the Cosmos) goes gallavanting away and back at .995c?
Then B would be younger than A!!"
Many textbooks discuss this problem and quite a few get it wrong. Many claim
that since accelerations are involved, General Relativity is necessary. But
this is not strictly so. By increasing the length of the trip but keeping the
accelerations constant, we can vary the relative contribution of General
Relativity effects as much as we like.
A simple way of explaining the problem is to note that special relativity
deals with inertial reference frames, and if you feel an acceleration, you're
not in a single inertial frame but moving between them. So B is (more or less)
staying in a single frame on Earth while A jumps into one frame headed out and
another coming back. (By being 'in' a frame I mean at rest in it.)
This makes it clear that A and B are NOT symmetrical and cannot easily be
exchanged.
I find this a little too simple, though. For detailed analysis, see if you
can dig up the APS book of reprints named "Special Relativity - Selected
Reprints" or something like that. There's a partial rerun of the famous
Dingle exchange in Nature (Dingle was a physicist who believed the Twin
Paradox proved Einstein wrong). You might also dig up Science News from
about 1978 which had a long exchange of letters on the subject, almost all
of which were nonsense but worth reading for amusement. The editors finally
called a halt to it by yelling, "Shut up on this relativity crap!!" or
something.
Finally, Taylor and Wheeler's Spacetime Physics is absolutely tremendous, and
may be the best physics text ever written. I recommend it for an analysis of
many well-known relativity "paradoxes."
Best,
D Gary Grady Duke University Computation Center Durham, NC 27706
(919) 286-4296 (919) 684-4146
{decvax, etc.}!mcnc!ecsvax!dgary