"at rest" in Einsteinian relativity

bahamagreen

I'm not making the argument, I'm just trying to identify and reveal the basis for the OP's argument for subsequent examination. Pending his reply, we don't know yet what aspect of his position may or may not be subject to error.

Just to be clear, the observation that there is no detectable absolute frame is satisfied by two existential conditions; the absolute frame exists but is undetectable, and the absolute frame does not exist and is thereby undetectable. Undetectability does not establish nor preclude its existence.

It is the indistinguishably of these two possibilities that needs to be examined to see if the inference of absolute motion may be obtained across both cases - if logically inferred in the first case, does that extend to the second?

Was not this very form of thinking the basis for much of modern physics? The atomic theory, quark model, string theory, and even Einstein's predictions of phenomena before experiments could confirm them for example seem to be very much in line with DaleSpam's corollary, "...the only fantasy which is consistent with experimental evidence..." - this phrase is almost a perfect definition of the theory and practice of science itself, isn't it?

Mentz114

Granted, but coupled with the fact that absolute velocity cannot be detected and is not required, by Occam's razor it is simpler to forget about it and avoid nugatory discussions like this.

If you take this argument further you might as well say that life itself is just a dream.

I don't agree with your last rhetorical question but I don't see any benefit in arguing about it.

DaleSpam

No. If something cannot be detected in any way then neither its presence nor its absence can be infered from failure to detect it. The fact that its presence is indistinguishable from its absence does not logically imply its presence.

Mentz114

Yep. And "about that which we know nothing we should remain silent", proposition 7 of "Tractatus Logico-Philosophicus" by Wittgenstein. Probably the only bit I understood.

bahamagreen

"If something cannot be detected in any way then neither its presence nor its absence can be inferred from failure to detect it. The fact that its presence is indistinguishable from its absence does not logically imply its presence."

That is not quite the mechanism of inference in question, so it is incidental to the argument. It is not the presence of the absolute frame that is being inferred, it is the state of rest and absolute motion being inferred from indistinguishably of a hypothetical undetectable absolute frame and a non-existent absolute frame.

The machinery that supports the inference is based on the second of these two principles:

The indiscernibility of identicals is a logical truth stating that:
For any x and y, if x is identical to y, then x and y have all the same properties.

The identity of indiscernibles is a tautology (in the strong version) stating that:
For any x and y, if x and y have all the same properties, (and no unshared properties), then x is identical to y.

Maybe the weak spot is confounding non-existence and undetectability as the "same" property, but then one must have a way of distinguishing "actual" differences between properties that present the identical result of observation and measurement. If we only have observational measures we may never know... so perhaps "about that which we know nothing we should remain silent" would lead to supporting the identical observations as stemming from the same properties.

Again, I just trying to clarify what may be the OP's argument for the sake of a coherent discussion and critique of his perspective.

DaleSpam

I do understand the distinction you are making between proofs about the existence of absolute motion vs proofs about the existence of the absolute frame, however I don't think that the absolute frame argument is "incidental to the argument" about absolute motion. The existence of an absolute frame implies the existence of absolute motion, and vice versa, by definition. So if there is no absolute frame then there is no absolute motion.

I don't think that the motion argument can stand merely by attempting to avoid the frame argument.

I actually agree with this principle, the identity of indiscernibles, and consider it to be the big problem with LET from a scientific perspective. If you limit yourself to experimentally-observable properties then the "true" frame is indiscernible from a "local" frame, and would therefore be identical. To get around that, LET postulates an experimentally-unobservable property of being at rest wrt the aether. This gives it a different "property", but the different property is non-scientific/non-observable. This invariably leads to confusing discussions like this one.

bahamagreen

It's kind of funny that we are discussing hypothetical perspectives of the OP pending his return to this thread... I hope he comes back and is able to elaborate his specific perspective with respect to some of the very incisive points you gents have presented, especially its comparison to LET.

mangaroosh

Thanks bahamagreen. I think I allowed my frustration to get the better of me and it affected my formulation of the OP, and a number of my other posts on here recently, so I don't think that has been helpful.

Context
The question stems from a philosophical discussion, on another forum, about the nature of time and the validity of presentism; the case against presentism was made solely on the basis of Einsteinian relativity, and particularly RoS; that may be part of the issue, because I am taking points that were made in that discussion to be representative of ER, and trying to develop my understanding on that basis. I'll try to distill the point, from that discussion, which lead to the question of the OP.

Basic point
The test of the PoR quoted above is that an observer cannot determine the absolute nature of their motion, by a co-moving experimental set-up. This was explained to me as, an observer cannot determine if they are "in motion" or "at rest", in an absolute sense.

The example that was used to explain this, was the everyday example of an observer on a train. It seems intuitive to say that an observer, on board a moving train, is "in motion"; but it was explained that this cannot actually be determined; the relative motion between the two could equally be attributed to the motion of the earth. The equivalence principle was used to explain how this can be extended to accelerating reference frames.

So, here we have the case where an observer cannot determine if they are "in motion" or "at rest"; but an observer can easily determine their motion relative to another object or even themselves, using a co-moving experimental set up.

While I understand the idea of being "at rest relative to something", the issue lies in the potential that, while an observer labels themselves as being "at rest" they might actually be "in motion". Again, using the example of the observer on the train; they will label themselves as being "at rest", because they are always at rest relative to themselves, but if the train is actually "in motion" then they too are "in motion" by virtue of being on board the train; this despite labeling themselves and the train, relative to which they are at rest, as "at rest". This, I think, has implications for the deductions we can draw from from the information we have about reference frames.


At this juncture, the point of "actual motion" is usually raised, to state that it doesn't make sense.

Deduction
The understanding I have arrived at, by virtue of the reasoning I have applied, is that "actual motion" does make sense, and that it also has implications for the conclusions we draw.

I hadn't really formulated the logic as you have outlined above, but it seems to be along similar lines to the understanding that my reasoning has lead me to; I'm not sure if it is precisely the same.

Without reference to an absolute reference frame, we can consider only relative motion; again, taking two observers at rest relative to each other, in order for relative motion to occur, at least one of them has to actually move, otherwise they would remain at rest relative to each other.

Again, they can determine their motion relative to the other observer, but cannot determine if it was they that actually moved i.e. they cannot determine if they are "in motion" or "at rest". This is despite the fact that they label themselves as "at rest". So is it possible that their labeling of themselves, as being "at rest", is potentially incorrect, or is it possibly obscuring other relevant information, which might affect the conclusions that are drawn?

harrylin

That's simply wrong, as people in this forum have shown you.
That's part of a centuries old discussion - even debate - of for example Newton vs. Leibniz, and later Mach and Einstein. If you are not familiar with "Newton's bucket" then you should probably start there (with perhaps questions in the classical physics forum), then read about Mach and Einstein, possibly even about Bell's theorem, and then come back with questions here and/or in the QM group.

DaleSpam

That is strange that you would say that when the reasoning you described seems to indicate the opposite:
Certainly none of this seems to indicate that "actual motion" makes sense. In fact, quite the opposite.

Here you say there is potential that it makes sense, but give no reasons supporting that assertion that there is even potential.

And here you make a mistake. In SR they never label themselves as being "at rest" only "at rest relative to the train" or "at rest relative to themselves" depending on if they are using the train's frame or their frame.

Here you simply assume, without giving any reason, that "actually 'in motion'" makes sense.

Why? If the very concept of actual movement is nonsense then there is no reason that one of them has to actually move. This is like saying that "in order for relative motion to occur at least one of them has to flubnubitz otherwise they would remain at rest relative to each other". Saying an undefined word between a bunch of defined words doesn't make the undefined word defined, it just makes the whole sentence undefined.

And a repeat of the mistake. If they are doing SR they never label themselves as "at rest", only "at rest relative to X".

In SR labeling themselves as being "at rest" is not just potentially incorrect, it is definitly incorrect.

The concept of relative motion does just fine at making all experimental predictions without the concept of absolute motion. And the concept of relative motion is logically self consistent. There seems to be no logical rationale to claim that the concept of absolute motion makes sense, and there are plenty of practical rationales to claim that it does not.

harrylin

I suppose that you mean with "making sense", based on reason, reasonable. Thus I wonder if you never read any of for example Newton, Lorentz, Langevin, Dirac(possibly), Bell - or if you simply deny that any of them had any logical rationale? Note that Bell's rationale has been a hot issue for the last decades. Also, I don't understand how you seem to suggest that a practical rationale can tell us anything about logical rationale...

DaleSpam

They all simply made assumptions that "absolute space" existed. As far as I can tell, mangaroosh is in good company, but their assumption is not logically required nor is it experimentally justified. Scientific assumptions should either be experimentally justified or logically implied by things which are experimentally justified.

I probably should have said "logically necessary" or "logically implied" rather than "making sense" which is overly strongly stated. I am just frustrated by mangaroosh's chronic obsession over LET topics which are unnecessary to an understanding of SR.

mangaroosh

Sorry, I let my frustration get the better of me in the RoS thread, and in this one. I would still maintain, however, that in determining the necessary consequences of the Lorentz transform that LET has to be considered, for the obvious reason that it also uses the LT.

There is no intended reference to LET here, however, and no intended reference to "absolute space". The deduction I would make is solely on the basis of relative velocities; the issue might lie in using common sensical ideas and trying to apply them to relativity.

I'll try and address the response above (or below, whichever way you look at it)

mangaroosh

It isn't from those that the deduction of "actual motion" is made; and that "actual motion" cannot be determined doesn't mean it doesn't make sense, or cannot be deduced, it simply means that it cannot be deduced which object is actually moving; we can deduce that at least one of them has to be though.


The problem might lie in the fact that I am taking things which seem to be common sense and applying them to relativity. The idea of movement is something I take to be common sense. While I know it can only ever be measured relative to something, the question of which object actually moves appears to be a separate, but intrinsically linked, phenomenon or question.

As per the example of the observer on the train, lets say you and I both are on a train, at rest relative to each other and the train, and the train is at rest relative to the ground. Then the train leaves the station such that it is "in motion" relative to the earth. I would deduce that either the train or the earth has to have moved, it could of course be both, but we can take the example where only one of them moves. We might not be able to determine which one actually moves, but we can surely deduce that one of them does, actually, move; as opposed to the motion being illusory.

We can do this without assuming an absolute reference frame, simply by considering the relative velocities; if the train and the earth start off at rest relative to each other, and then start moving relative to each other, then one of them has to have actually moved, otherwise the motion is illusory, or imagined.

It might be easier to explain my understanding using a slightly different example; if you and I are at rest relative to each other, and relative to the earth; then one of us starts moving relative to both; what caused the relative motion between us? Did I move, or did you move? Of course, we both move relative to each other, but what caused there to be relative motion between us?


OK, the trouble probably pertains more to the idea of the absolute nature of the motion.

When we say that an observer cannot determine the absolute nature of their motion, I understand that to mean that, they cannot determine if they are "in motion" or "at rest". This is of course different to the idea of relative motion, because the latter can easily be determined.

An example that was given to me was of an observer running on the earth; the observer cannot determine if they are running forward or if the earth is spinning beneath them, akin to someone running on a log spinning in water. While I wouldn't dispute this, I would say that either the earth or the jogger has to "actually" be in motion, in order for there to be relative motion between them.


If we go back to the example of you and I at rest relative to each other, and then moving relative to each other. I think it seems a reasonable question to ask, which one of us moved?

You might maintain that you remained where you were and that I moved; while I might argue the opposite; we cannot determine which one of us is correct, but we can surely deduce that one of us must be wrong, no?


If two observers are at rest relative to each other, what must happen in order for there to be relative motion between them?


Hopefully this has been clarified a bit.

I suppose it depends on what is meant by "absolute motion".

DaleSpam

But you cannot make the deduction you want solely on the basis of relative motion. Every time you have tried, at some point you have stopped saying "moving relative to" and started saying just "moving". When you do that you cease making your deduction solely on the basis of relative velocities.

Go back and look at your own words and see if you think that is an inaccurate characterization.

mangaroosh

The use of the term "moving" represents the idea of absolute motion, which is the deduction.

Just taking the example of the two lone observers who start off at rest relative to each other; they then start moving relative to each other - here we only consider relative motion, or velocities.

The deduction we can make on that basis, as far as I can see, is that in order for relative motion to occur, at least one has to start moving. We cannot determine which one moves - as per the principle of relativity; but we can deduce, surely, that one of them has to have moved.

EDIT: because there would be no relative motion, if one didn't start moving.

Michael C

Despite all our efforts, you keep bringing back the same argument: "relative motion exists, so absolute motion must exist".

You might just as well say "relative position exists, so absolute position exists".