X-Message-Number: 361
From att!compuserve.com!73337.2723 Tue Jun 25 02:55:37 EDT 1991
Date: 25 Jun 91 01:43:20 EDT
From: Brian Wowk <>
To: <>
Subject: Time dilation
Message-Id: <"910625054320 73337.2723 DHJ11-1"@CompuServe.COM>

To: >INTERNET: 
 
     Concerning Eric Klein and the Boston group's question about  
whether speed or acceleration causes time dilation: I am an expert on  
this subject.  In high school I won a trip to England because of a  
paper I wrote on this very question. 
 
     Consider the Twin Paradox of special relativity.  One twin  
brother accelerates away from Earth until he reaches 90% the speed of  
light, then coasts for a few years.  He then, at a great distance from  
Earth, applies reverse acceleration to return.  From the standpoint of  
special relativity, both twins see the same thing.  Each travels at  
high speed relative to the other, and each would expect the other to  
be younger upon reuniting due to time dilation. 
 
     General relativity is required to break the paradox.  The  
Equivalence Principle states that acceleration and gravity are  
physically indistinguishable.  Bearing this in mind, consider the  
travelling twin applying reverse acceleration several light years from  
Earth.  In doing this he is placing the Earth several light years "up"  
in his acceleration (gravity) field.  During this reverse acceleration  
phase, clocks on Earth will run enormously faster than his because of  
the well-known "red shift" of general relativity.  If you work through  
the red shift numbers you find, lo and behold, that the travelling  
twin will end up younger by the exact amount predicted by special  
relativity if you apply time dilation ONLY to the travelling twin. 
 
     So the answer is that fundamentally acceleration, not speed,  
causes time dilation (at least if you plan on pulling things back  
together again and comparing them at some point).  It's also worth  
mentioning that acceleration, unlike velocity, does have an absolute  
measure in the universe.  This is Mach's principle. 
 
     Mach's principle tells us that if we flit about the universe at  
high speeds, the universe will age faster than us, rather than the  
other way around.  If the lifetime of the universe is indeed limited,  
then travelling at relativistic speeds is not a good thing for  
immortalists to do. 
                                   --- Brian Wowk 

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