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From: tino@hou2f.UUCP (A.TINO)
Newsgroups: net.physics
Subject: A Question on Time Dilation
Message-ID: <480@hou2f.UUCP>
Date: Tue, 4-Jun-85 12:40:16 EDT
Article-I.D.: hou2f.480
Posted: Tue Jun  4 12:40:16 1985
Date-Received: Thu, 6-Jun-85 03:31:16 EDT
Organization: AT&T Bell Labs, Holmdel NJ
Lines: 75


>I am very puzzled over something in Einstein's theory of time 
>dilation, that is, that time passes more slowly for objects in 
>motion then for objects at rest.  Here is my understanding of th 
>theory and how Einstein arrived at it.  There has to be something 
>wrong with my thinking somewhere, though this understanding is 
>based on the book *The Universe and Dr.  Einstein*, by Lincoln 
>Barnett, a book which Einstein himself heartily praised.  

>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.  

>Yet, I find myself still asking this question:  what about a light
>ray being emitted in the opposite direction?  Can't it just as easily
>be said that time is speeding up for the object under these circumstances?
>I can't believe Einstein wouldn't have thought of this!

>			Jim Campbell
>			...!ihnp4!ima!haddock!jimc

Perhaps the book was misleading...
There's more to relativity than time dilation.  To see how 
observers moving with respect to one another can measure the same
speed for a given light beam requires an understanding of
length contraction and the relativity of simultaneity as well
as time dilation.

The special theory of relativity (STR), which deals with observers
that are in uniform relative motion, begins with two postulates:
1) the speed of light is the same for all observers, and
2) the laws of physics are the same for all observers.
(The second postulate says that unless you look out the window
you can't tell that you're moving.  Furthermore, each observer has the perfect
right to consider themselves to be at rest.)
From these postulates Einstein deduced how space and time
as measured by one observer's clocks and metersticks
relate to space and time as measured by any other observer's
clocks and metersticks.

Let's rephrase your example.  Consider two observers, A and B.
A moves east as measured by B at a speed of .75 "c".
As she moves east, A shines her flashlight eastward.
From A's point of view, she is at rest and merely shining her
flashlight.  Whether she shines it east or west or north or south
it is no surprise that she simply measures the speed of the beam to
be "c".  

From B's point of view all he sees is someone flying by shining a
flashlight.  If we believe postulate #1 we are not surprised to find
that, using his own clocks and metersticks, B measures the speed of 
light to be "c".

So far so good.  Of course the real "problem" is how can they both be
right?  Is there a way of using A's measurements to figure out what
B will measure with his clocks?  The answer is yes, that's what relativity
is all about.  When A measured the speed of the lightbeam she was finding
the ratio of the distance the light traveled (as measured by her!) to the time
the the light traveled (also as measured by her!).  In order to deduce what
B would measure as the speed of the lightbeam one has to figure out the 
distance the light traveled (according to B!) and the time it takes the light
to travel that distance (again according to B!).  If you correctly apply
the relativistic transformations -- which incorporate contraction, dilation,
and simultaneity -- you'll find that both A and B agree that they both,
using their own measuring devices, measure the same speed for light -- "c".
They better agree! -- Otherwise relativity would be inconsistent.

			Al Tino , Bell Labs at Holmdel