# Why time DILATION?

by Artyvr
Tags: dilation, time
 Sci Advisor PF Gold P: 5,083 The case of a equatorial and polar observer is really a case of twin differential aging not just time dilation because the equatorial observer keeps returning to a fixed point in the polar observer's rest frame. In exchanging signals, the polar observer is effectively getting slightly delayed information about this 'twin' situation. As a result, the accumulated average time difference between polar and equatorial clock is invariant, not observer dependent. If two observers simply move past each other at some relative speed, each forever considers the others clock to be slow, and other observer's have various different conclusions about which clock is faster. Note, the non-inertial character of the equatorial clock must be considered (you can't consider them the origin of a coordinate system that can use the Lorentz transform or the Minkowski metric). However, since you are interested in invariant features (exchange of signals with information), you can do the analysis most simply in any inertial frame (e.g. the polar inertial frame). This frame is sufficient for computing any actual observation or measurement made by the equatorial observer. Note also, that a frame in which the earth was moving at .999c would be almost as simple to use, and would compute identical results for the behavior of exchanged signals and world line proper time information they carry. However, it would differ radically on 'time dilation' applicable to the polar and equatorial clocks.
P: 181
 Quote by Nugatory By "fixed to a point on the equator", you mean that there is a point on the equator that is at rest in the frame, right? No such frame can be inertial because it is accelerating, and I see no way to discuss it without considering the acceleration.

@Nugatory - Since in SR there is no preferred frame of reference, what is the 'point on the equator' accelerating w.r.t. to? Einstein does not refer to any acceleration at all in his example. There is a way to discuss it without considering any acceleration - a preferred IRF from which both observers' velocities are measured. The polar point has no velocity in this IRF, the equatorial point does. I believe this is implicit in Einstein's example.

 Quote by PAllen The case of a equatorial and polar observer is really a case of twin differential aging not just time dilation because the equatorial observer keeps returning to a fixed point in the polar observer's rest frame.
@PAllen - Absolutely correct, but w.r.t. what frame is the equatorial observer returning to the same point? We are talking about a single preferred IRF w.r.t. which we are looking at both observers, I think you will agree.

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A lot of discussion in this thread seems to be about differentiating the terminologies of "time dilation" and "differential aging". My understanding is that "time dilation" is just a combination of the classical Doppler effect combined with "differential aging". Would you agree?
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P: 3,941
 Quote by arindamsinha @Nugatory - Since in SR there is no preferred frame of reference, what is the 'point on the equator' accelerating w.r.t. to?
Unlike velocity, acceleration does not have to be defined with respect to something else.

In SR (and GR with a more careful definition of "acceleration") an accelerated frame and a non-inertial frame are the same thing, so if I can observe non-inertial behavior I know that I'm being accelerated - even if there's nothing else around for the acceleration to be relative to. The most natural way to for me at the equator to measure my radial acceleration towards the center of the earth due to the earth's rotation is to study the behavior of an object moving in the radial direction; it will move eastward in violation of the law of inertia and I'll know that I'm not in an inertial frame. A sensitive enough accelerometer would also do the trick; and the accelerometer reading is the position of a needle on a scale, and that has to be an invariant fact not relative to anything else.
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P: 4,787
Quote by arindamsinha
 Quote by ghwellsjr 1) Since the traveling twin's clock is essentially stopped during the trip, it will end up 10 seconds behind the Earth twin's clock when they do the synchronization verification test you described. But having answered this, it doesn't help me understand what you are saying.
Thanks for taking this up and answering the question.

I was trying to establish that velocity-based differential aging between two bodies (and agreement on the same by both), is not dependent on the moving twin coming back to origin to be 'co-located' with the stationary twin to 'compare clocks'. It can be done at any point during the traveling twin's journey using light-signals.
This is not true. Here is a diagram that depicts a similar situation to the one you proposed except that it uses the same parameters as the one in post #33 but without the moving twin returning:

You are giving preference to this one frame, the Inertial Reference Frame (IRF) in which the "stationary" twin remains inertial. In this IRF, the differential aging after the moving twin comes to mutual rest with the "stationary" twin is 4.5 quarter hours (a little over an hour) with the moving twin younger. But look at this diagram in which the moving twin is inertial during his moving portion of the trip:

Here there is a different criterion for what is simultaneous and now the stationary twin is the one that is younger by slightly over 2 quarter hours (a little over a half hour).

Of course, every IRF will show the same results for whatever light signals are exchanged between the twins, but that is not sufficient to establish unambiguous simultaneity and that's what you have to do to determine differential aging. Differential aging is answering the question, between this coordinate time and that coordinate time, what is the difference in how two observers age? Even if the two observers agree with each other because they implicitly are using their mutual rest IRF, that doesn't mean the question has been answered the same for all other IRF's. The only way for all IRF's to get the same answer is if the two observers are colocated at the first coordinate time and again colocated at the last coordinate time, (not necessarily the same location, not necessarily even at rest in the same IRF nor do the observers even have to be in mutual rest at either time). All this is simply to remove any ambiguity about simultaneity issues at the start and the end of the process.
Quote by arindamsinha
 Quote by ghwellsjr He does not say that "B could similarly be considered slower by the moving clock A" because A is not at rest in an Inertial Reference Frame (IRF).
You misunderstood me. I said A in 'its own rest frame' (which of course is an IRF as well, and A is at rest in that IRF, in the situation considered). As I said, he didn't deny it, but just that he didn't stress it, and then went on to an example where there is a clearly established stationary and moving frame - which I found very interesting.
The clock moving in a circle is constantly accelerating. If it weren't, it would go in a straight line. It has to accelerate in order to move in a circle. It is not inertial. It cannot be at rest in an IRF. Einstein only considered a single reference frame when discussing the differential tick rate of the two clocks and he did not address the issue of the differential aging.
 Quote by arindamsinha Subjective. Can you point out exactly what?
You don't have a correct idea of what Time Dilation is nor of what Differential Aging is and I suspect you don't understand Relativistic Doppler
 Quote by arindamsinha This is a SR situation we are discussing. So, without bringing in GR or acceleration, what prevents us from seeing the polar clock as 'rotating' w.r.t. an IRF fixed to a point on the equator?
I've never brought GR into the discussion but somehow you seem to associate acceleration with GR. I'm not sure you understand what acceleration is. The clock on the equator is accelerating in order to maintain a circular path. You could, if you wanted to (I don't) consider a non-inertial frame in which the equator clock is at rest and in which the pole clock is moving but I don't think you're ready for that.
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