Implications of the statement "Acceleration is not relative" As a result of the discussion which ensues from this post I hope to understand the implications of this statement: "Acceleration is not relative." First, I think it will be helpful to establish some context. Context point 1: I am not an "objector" looking for a paradox, or hoping to catch relativists in a contradiction. I am asking my question in good faith, ready to learn from those who know more than I. (I reserve the right to evaluate responses, as any intellectually honest person must do, but I promise to give a fair and open-minded hearing to all.) Context point 2: Since my last post, in which I made a dumb mistake in the interpretation of the spacetime diagram (or rather, failed to check my mental image of a pole-in-barn episode against the spacetime diagram), I've worked through the problem in sufficient detail to understand my mistake, and hopefully avoid similar mistakes in future. So, you are not (I hope) wasting your time as you respond to my question. [As a side note, I have not succeeded in forming a mental picture of the "one reality represented by the spacetime diagram", as I had hoped to do. I've come to the conclusion, for now at least, that there is no way to form such a mental image from the spacetime diagram. The Lorentz transformation provides a means of predicting how an episode (collection of events) will be observed in any chosen inertial reference frame. If an extra-frame view of the episode is to be had, it is not going to come from the spacetime diagram by itself.] Context point 3: Having come to an understanding of the pole-in-barn paradox, I was naturally drawn to consider the last remaining paradox that is unresolved for me. This is the twin paradox. One might say that there are two aspects to the paradox. The first is that the twins would be of different ages when they meet at the end of the episode. That is not really a paradox; it is explained by the notion of proper time, and well illustrated by a spacetime diagram in which the earth is considered to be at rest and the rocket twin is moving. The second aspect of the twin paradox comes about when one considers the rocket twin to be at rest and the earth to be moving. In that case, it is the earth twin who will be younger, a contradiction in that both twins cannot be younger. That paradox I have not resolved, though I have read multiple explanations of it. Yesterday I decided to try again. I began with Taylor and Wheeler. The twin paradox is dealt with in section 4.6 of Spacetime Physics. The proper time for each twin is calculated, showing the age difference on return of the traveler. Then, the (always rude and unreasonable) objector says, "If there is any justice, if relativity makes any sense at all, it should be equally possible to regard you [the earthbound twin] as the stay-at-home." There follows a detailed explanation that ends with, "notice that the traveler is unique in changing frames, only the traveler suffers the terrible jolt of reversing direction of motion." But this explanation does not address the objection, for the objection is that the rocket twin should be considered at rest. A resting twin cannot reverse motion. The jolt that the resting rocket twin feels must come from some other cause than reversal of motion. I went to Born, in Einstein's Theory of Relativity. In VI-5, he deals with the objection of the resting rocket twin by asserting that only the rocket twin accelerates. Once again, it seems to me that this objection sidesteps the issue, for by definition the resting rocket twin does not accelerate. So this morning I searched on this forum, wishing to avoid being the 9,488th person to ask about the twin paradox. I found this thread. Quite quickly I saw what must be at the root of the explanation of the paradox, but which I do not recall ever seeing stated explicitly: "Acceleration is not relative." (This statement was not challenged in the first twelve pages; and anyway I think it merits its own thread.) I say "at the root of the explanation" because it is the immediate implication of the statement which avoids the paradox: The rocket twin cannot be considered at rest. (More precisely, the rocket twin cannot be considered at rest while he is accelerating. This is implicit in ghwells statement in post #161.) This statement, to one who began the study of relativity with Einstein's Relativity, is nothing short of shocking. In a religious context (which this is not, of course, but the analogy is too striking to omit mention), this would be tantamount to heresy. So I went to Einstein's book to see if I had missed something. No, I don't think so. He begins by expressing his desire to bring acceleration into the realm of the principle of relativity. In doing so, his development of the equivalence of inertial and gravitational mass is premised on the example of a man in an accelerating chest who considers himself to be at rest. I then reread Einstein's own explanation of the twin paradox. Again, the rocket twin is unambiguously at rest when the rocket frame is the reference frame. It is quite clear to me that Einstein considered acceleration to be relative, and that he considered it perfectly justifiable to consider an observer who accelerates with respect to an inertial frame to be at rest. I will take a moment here to object to the reason given for the claim that acceleration is not relative. The reason given (post #10 and following) is that acceleration may be independently measured, or felt, without reference to some other frame. But what one measures or feels is force, not acceleration. This distinction, together with the necessary equivalence of inertial mass and gravitational mass in all frames, is the basis for Einstein's claim that an observer in an accelerating frame may consider himself to be at rest in a gravitational field. What are the broader implications of the statement that acceleration is not relative? Does this mean, as it certainly would appear to mean, that modern relativity is in this very important respect not Einsteinian relativity? Are there other implications as to the meaning of the principle of relativity?