trc (09/16/82)
Now that simultaneity has been settled (I hope) how about the good old twins paradox? My question here is, if both frames of reference are equally good, then why should one twin age differently than the other? In fact, it would seem to me to be reasonable to argue that, there is in fact a prefered (stationary) frame of reference, but that the only way to detect the difference is to check the relative time dilation effects between two frames. I have heard it argued that the difference is that one undergoes acceleration and the other doesnt, and that this somehow explains the effect. However, the difference in ages doesnt occur during the acceleration, for the most part, but rather during the long period of free fall. Suppose one ship starts out, and stops at some distance away. Then it, and another ship from earth take off and meet at the center. The later ship stops, and accelerates back to earth. (It has gone a bit past center, so that it can accelerate to match the other ship's velocity at exactly the center.) Now both ships have undergone the exact same accelerations, but the first to leave will be carrying a much younger twin. Another interesting effect, not necessarily related, is that a spaceship that travels away from one and near SOL velocities (NSOLV) and then turns and comes back, will appear to have taken longer to go away than to get back - that is, the information it radios back will be compressed relative to the information it sent on the way out. Thus, events will appear slowed on the way out, and actually faster on the way back. (Time dilation would still apply, but it would be modified by this effect.) Couldnt it be argued that this is in fact a real effect, and that time dilation is dependent upon direction of velocitiy? Tom Craver houti!trc
JGA@MIT-MC@sri-unix (09/16/82)
From: John G. Aspinall <JGA at MIT-MC>
Date: 15 Sep 82 19:28:39-PDT (Wed)
From: harpo!ihps3!houxi!deimos!ariel!houti!trc at Ucb-C70
Article-I.D.: houti.154
Via: Usenet; 16 Sep 82 4:08-PDT
Now that simultaneity has been settled (I hope) how about the good old
twins paradox?
My question here is, if both frames of reference are equally good,
then why should one twin age differently than the other? In fact,
it would seem to me to be reasonable to argue that, there is in fact
a prefered (stationary) frame of reference, but that the only way to
detect the difference is to check the relative time dilation effects
between two frames.
I have heard it argued that the difference is that one undergoes
acceleration and the other doesnt, and that this somehow explains
the effect. However, the difference in ages doesnt occur during
the acceleration, for the most part, but rather during the long
period of free fall.
Sigh. You have just invoked simultaneity again. That is, you have compared
their ages ("at the same time") when they're in different inertial frames.
The "stay-at-home" twin stays in one inertial frame, the
"rocket-traveller" twin switches from one inertial frame to another.
I hate to end this here, but a good explanation would take more time
and effort than I have available. I highly recommend French's explanation
in "Special Relativity" (Norton, MIT Introductory Physics Series, 1968)
on pages 154-159. He makes the asymmetry between the twins clear, and also
shows that special relativity is adequate for handling the problem.
John Aspinall.smb (09/16/82)
A comment to knowledgeable people on this newsgroup: most of us who ask apparently dumb questions aren't trying to deny or contradict relativity; we're trying to understand it. I can read a book on the subject and follow its examples -- but if I see a new "paradox" posed, I'm quite likely to be confused. Two sentences and a good reference may be all that's called for; I (at least) thank y'all for such answers. --Steve Bellovin
gwyn@BRL@sri-unix (09/17/82)
From: Doug Gwyn <gwyn@BRL> In all discussions like this, it is important to specify carefully just what is measured and how. A diagram often helps, if you don't take any distances on it as "absolute". Acceleration is not relevant. The twins paradox can be demonstrated by using three reference frames, one "stationary" (earth-bound) and two moving in opposite directions with the same speed with respect to the "stationary" frame. Then you can (1) transfer your attention from the earth-bound frame to an "outward-moving" one, (2) after a suitable interval transfer your attention to an "inward-moving" frame, and finally (3) transfer your attention to the starting point when it arrives. This thought-experiment is equivalent to the space-flying twin's itinerary, but all accelerations have been removed so that any time-dilation cannot be ascribed to an object being affected by pseudo-gravity forces or whatever. The key to the paradox when cast in this form is the availability of suitable "universal" time references in each frame; e.g. use a maser clock or something similar. Given such clocks, the time dilation claimed for the twin paradox does in fact occur in accordance with special relativity. Yet none of the clocks ever feels an acceleration. Special relativity does not answer the question, "Why can't the earth-bound observer, by symmetry, claim HE is younger than the space traveler?", except by pointing out that the situation is NOT symmetrical since by no stretch of the imagination can the space traveler (out and back) consider that he is constantly in an "inertial frame". In fact, a minimum of TWO inertial frames must be used, with a changeover in point of reference at the turn-around point of the journey. Accompanying this change of viewpoint, one must also insist that the REST OF THE UNIVERSE has suddenly started moving differently. Quite apart from questions of Mach's principle in General Relativity, the introduction of an "outside force" acting on the space traveler to turn him around (or, equivalently, to give the rest of the universe a shove) violates symmetry in a fundamental way. The detailed treatment of the problem necessarily involves agents and effects outside the province of Special Relativity. However, the conventional analysis from the viewpoint of the earth-bound frame fits within the theory's constraints and is therefore accurate.
gwyn@BRL@sri-unix (09/17/82)
From: Doug Gwyn <gwyn@BRL> Hope you liked my long explanation of the twins paradox. There is a fundamental problem with trying to "understand" relativity to the extent that things like the twins paradox seem obvious -- This paradox, and many other points, cannot be completely understood within ANY known conceptual scheme. In fact this is why Einstein kept on going, on to General Relativity, then Unified Field Theory. He saw very deeply into the situation and realized that many such questions could only be satisfactorily answered when global considerations were properly dealt with. Such things as asymptotic degrees of freedom of fields and boundary value determination are crucial to a complete comprehensive relativistic physics. This means you are right to feel that there is something hard to grasp about the twins paradox!