@S1-A.ARPA,@MIT-MC.ARPA:Ghenis.pasa@Xerox.ARPA (06/03/85)
From: Ghenis.pasa@Xerox.ARPA >I've heard the twin paradox, and until recently I thought I had it >straight. I thought of something recently, though. When one twin >takes off, leaving the other here, why does the one in space age >more slowly? Why can't you use a reference frame travelling with >him and say that the earth is travelling at a great velocity? Am >I missing something? (Obviously I am.) The key to the twin paradox is that the travelling twin goes on a ROUND TRIP, so his frame of reference is an ACCELERATED FRAME (you cannot return to Earth without changing direction, and you cannot change direction without acceleration) whereas the stationary twin has an INERTIAL FRAME. This is what makes their frames of reference non-equivalent, thereby they will experience time differently.
@S1-A.ARPA,@MIT-MC.ARPA:TERRY%LAJ.SAINET.MFENET@LLL-MFE.ARPA (06/05/85)
From: TERRY%LAJ.SAINET.MFENET@LLL-MFE.ARPA Since the subject of special relativity's twin paradox has come up, let me add one small note. Many people believe that the "paradox" part of the twin paradox is that the two twins are no longer the same age. This is not the case. The paradox referred to as the "twin paradox" is precisely the question posed by the original posting on this subject: "Since all motion is relative, why should the twins have different ages? We can regard either twin as being stationary and the other twin as moving, therefore we can show that each twin should be younger than the other." NOW we have a paradox. The answer, as has been pointed out, is that the two twins are NOT in symmetrical situations. The traveling twin has undergone a fair amount of acceleration with respect to the Earth that the Earthbound twin has obviously not. It is precisely this acceleration that causes the traveling twin to age more slowly, and thus be younger upon return, than the Earth- bound twin. Paradox resolved. For those of you who are also on the physics mailing list, we are in a similar situation here as all the postings regarding objects such as the Moon moving much faster than the speed of light if you are sitting on a spinning turntable while watching it. As was concluded by a number of folks, accelerated (non-inertial) reference frames don't count in special relativity.
@S1-A.ARPA,@MIT-MC.ARPA:cef@cmu-cs-spice.arpa (06/05/85)
From: Charles.Fineman@CMU-CS-SPICE > The key to the twin paradox is that the travelling twin goes on a ROUND > TRIP, so his frame of reference is an ACCELERATED FRAME (you cannot > return to Earth without changing direction, and you cannot change > direction without acceleration) whereas the stationary twin has an > INERTIAL FRAME. This is what makes their frames of reference > non-equivalent, thereby they will experience time differently. What if we assume that the universe is closed? Then it would be possible to return to earth without changing your accelration. What happens then?
doug@terak.UUCP (Doug Pardee) (06/06/85)
Here's what you've been waiting for, comments from someone who doesn't
have any real knowledge of the subject...
What I had heard was that the key is "acceleration". Acceleration is
not relative. Although you can't tell visually whether you are
acclerating away from a point, or it's accelerating away from you, or
some combinations of accelerations is at work, you can certainly tell
if you're accelerating, and how much, by the G-forces that you feel.
Is it not the acceleration that causes the change in time reference?
--
Doug Pardee -- Terak Corp. -- !{ihnp4,seismo,decvax}!noao!terak!doug
^^^^^--- soon to be CalCompethan@utastro.UUCP (Ethan Vishniac) (06/07/85)
> From: Charles.Fineman@CMU-CS-SPICE > > > The key to the twin paradox is that the travelling twin goes on a ROUND > > TRIP, so his frame of reference is an ACCELERATED FRAME (you cannot > > return to Earth without changing direction, and you cannot change > > direction without acceleration) whereas the stationary twin has an > > INERTIAL FRAME. This is what makes their frames of reference > > non-equivalent, thereby they will experience time differently. > > What if we assume that the universe is closed? Then it would be possible to > return to earth without changing your accelration. What happens then? Propagation in curved space requires the application of General Relativity. In this case the answer is that it isn't possible to circumnavigate the universe travelling at less than the speed of light between the the Big Bang and the Big Crunch (chomp). However travelling *at* the speed of light it is just possible. In which case no time at all passes for the traveler and the entire history of the universe goes by for the stay at home. Why? Well basically the equivalence of all uniformly moving frames does not apply in a curved space. All that you are guaranteed is that *locally* the laws of physics will be the same for the two. However, the shape and evolution of the universe are perceived differently by the two observers. -- "Don't argue with a fool. Ethan Vishniac Borrow his money." {charm,ut-sally,ut-ngp,noao}!utastro!ethan Department of Astronomy University of Texas
seltzer@lebeef.DEC (06/11/85)
"accelerated (non-inertial) reference frames don't count in special relativity" But the Earth, revolving around the Sun, is an accelerated reference frame. Isn't it? So what does count? Has a human being ever been anywhere that would be considered an inertial reference frame?
doug@terak.UUCP (Doug Pardee) (06/11/85)
> The traveling twin has undergone a fair amount > of acceleration with respect to the Earth... Is acceleration measured "with respect to" something? I'd thought that it was absolute... -- Doug Pardee -- Terak Corp. -- !{ihnp4,seismo,decvax}!noao!terak!doug ^^^^^--- soon to be CalComp