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 Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=361