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From: brooks@lll-crg.ARPA (Eugene D. Brooks III)
Newsgroups: net.physics
Subject: Re: A Question on Time Dilation
Message-ID: <618@lll-crg.ARPA>
Date: Mon, 3-Jun-85 22:52:15 EDT
Article-I.D.: lll-crg.618
Posted: Mon Jun  3 22:52:15 1985
Date-Received: Thu, 6-Jun-85 10:03:34 EDT
References: <15000004@haddock.UUCP>
Organization: Lawrence Livermore Labs, CRG group
Lines: 57

> Einstein's theory about time dilation is based on the assumption 
> that the speed of electromagnetic radiation is constant 
> throughout the Universe, whether or not the object emitting the 
> radiation is in motion.  Here is an example which may clarify why 
> Einstein arrived at this assumption: suppose an object that is 
> travelling through space at a rate equal to 75 percent that of 
> light and that it was emitting a light ray in the same direction 
> it was travelling.  For that light ray's speed to remain at C, 
> the rate of time passage for the object must decrease 
> proportionally.  

This is easily understood using the contraption known as the light pulse
clock.  The device consists of a light bulb, that flashes in very short pulses,
a light detector that causes the bulb to flash when it picks up light and
a mirror placed any arbitrary distance from the bulb oriented to reflect
light from the bulb to the detector that is sitting very close to the bulb.
Each time the bulb flashes the light crosses to the mirror and back, hitting
the detector to cause the bulb to flash again.  You make the bulb flash once
and the think doesn't quit until the battery runs out.

The clock can be sitting still, moving parallel to the line connecting the
bulb to the mirror, or moving perpendicular to the line connecting the bulb
to the mirror.  Or anywhere inbetween.

If the clock is sitting still it pulses with a period given by the speed of
light and the seperation between the mirror and bulb.  This is the basic
period of the clock.

If the clock is moving perpendicular to the line connecting the bulb and the 
mirror with speed v/c you can easily work out the triangular trajectory
that the light travels in the rest frame and the rate at which the clock
fires as viewed in the rest frame.  Its slower because the light travels
farther in the rest frame.

If the clock is moving parallel to the line connecting the bulb and the mirror
with speed v/c you can again work out the firing rate of the clock in the rest
frame.  This time you will find that if fires at a different rate than above.
This is accounted for in terms of a combination of the time dilation as in the
case above with a forshortening of the distance separating the mirror and the
bulb.

If you have two such clocks and you augument them with counters to keep
track of the time, synchronize them and them send one away at speed v/c and
bring it back at speed v/c traveling to some distant place you will find
that the "moving" clock has not "aged" as much and that the age difference
does not depend on the orientation of the "moving" clock.  Its all easily
worked out with simple trig and algebra.


Of course there is one of those flakey paradoxes here and you notice that I
put "moving" in quotes in the paragraph above.  All motion is relative and
just how do the clocks keep straight on which one should "age" more slowly?
The "moving" clock experiences something that the "non moving" clock does not
and it involves Einsteins second theory.


So what does that have to do with people in space ships you say!