Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10 5/3/83; site uw-june
Path: utzoo!watmath!clyde!burl!ulysses!mhuxl!houxm!hogpc!houti!ariel!vax135!cornell!uw-beaver!uw-june!palmer
From: palmer@uw-june.UUCP
Newsgroups: net.physics
Subject: Re: Relativity questions
Message-ID: <1339@uw-june>
Date: Sun, 29-Apr-84 16:57:41 EDT
Article-I.D.: uw-june.1339
Posted: Sun Apr 29 16:57:41 1984
Date-Received: Mon, 30-Apr-84 06:06:22 EDT
References: <12647@sri-arpa.UUCP>
Organization: U. Washington, Computer Sci
Lines: 80

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 Palmer