At rest in Einsteinian relativity

[Dale]

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?

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]

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.

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.

 

[Dale]

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.

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.

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.

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]

I think the OP is using a logical extension of some form of proof by induction (like mathematical induction)... something like this:

Assume there is a detectable absolute frame.
If two objects are in relative motion, then one must measure at least one of the two objects to be in relative motion with respect to the detectable absolute frame.
This relative distinct motion to the detectable frame would be "actual" motion.
The inference of "actual" motion holds when the absolute frame is detectable.

Now alter the assumption of the absolute frame and make it undetectable.
If two objects are in relative motion, then one must conclude at least one of the two objects to be in relative motion with respect to the undetectable absolute frame.
This relative indistinct motion to the undetectable frame would be "actual" motion.
The inference of "actual" motion holds when the absolute frame is undetectable.

Now alter the assumption of the absolute frame and make it nonexistent.
Notice that the nonexistent absolute frame and the undetectable absolute frame are indistinguishable from each other.

Therefore the inference of "actual" motion still holds when there is no absolute frame.

Maybe the OP can verify that this is the form of his thought line?

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]

[..] 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's simply wrong, as people in this forum have shown you.

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?

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.

 

[Dale]

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.

That is strange that you would say that when the reasoning you described seems to indicate the opposite:

This was explained to me as, an observer cannot determine if they are "in motion" or "at rest", in an absolute sense.

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.

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.

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

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.

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".

Certainly none of this seems to indicate that "actual motion" makes sense. In fact, quite the opposite.

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".

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

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,

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.

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".

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

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

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.

This is despite the fact that they label themselves as "at rest".

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

So is it possible that their labeling of themselves, as being "at rest", is potentially incorrect

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

[..] 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.

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...

 

[Dale]

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...

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]

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.

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]

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.

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.

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

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, let's 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?

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.

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.

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

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?

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.

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

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 definately incorrect.

Hopefully this has been clarified a bit.

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.

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

 

[Dale]

The deduction I would make is solely on the basis of relative velocities;

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]

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.

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]

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

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".

 

[mangaroosh]

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".

it's not just a case of saying relative motion exists, so absolute motion must exist though; it's a case of deducing it from the fact that relative motion exists. The point of positions would be separate, and isn't necessarily relevant.

All I'm doing is trying to make sense of something that appears to go against "common sense". Take the everyday example of walking down the road. Most people would say that they are the thing that is moving; it's not that the Earth is rotating beneath their feet and that their walking movement is causing them to remain stationary, like a person walking on a log spinning in water, or a person walking on a treadmill. They would say that it is their movement that causes the scenery to change, not the movement of the earth. I don't doubt that you can, on some level at least, understand what I am referring to.It seems to be the case that we can't actually determine which perspective is correct, but I think we can deduce that one or the other must be true i.e. either I am moving or the Earth is moving. If neither was moving, then both would be at rest relative to each other and there would be no relative motion.So, from only considering relative motion we can deduce that one of the two relatively moving objects actually has to be moving; it will always and ever be measured relatively, but surely it's not that difficult to see the point being made?EDIT: it might be helpful to pose the question: if two observers/objects start off at rest relative to each other, what must happen for there to be relative motion between them i.e. what must happen for them to start moving relative to each other?

EDIT2: if the answer is, they must start moving relative to each other; how can they start moving relative to each other without one of them moving in an absolute sense?It might be helpful to think of it in terms of you standing opposite another person; at rest relative to each other. Then one of you moves. You will measure the movement relative to the other person, but are you capable of movement; can you move so as to cause that relative motion? Is the other person capable of moving so as to cause that relative motion?

 

[harrylin]

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.

Well, the fact of the matter is that at least some of them held that their conclusions were experimentally justified and it corresponds to a currently discussed interpretation of QM (one alternative model has that reality doesn't exist!). Apparently you think that their logic must be illogical because you cannot follow their logic; and that's your good right, but I don't think that it's appropriate to make that kind of opinionated claims (lacking the "I think" or "IMHO") on physicsforums.

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.

Many people will not be satisfied with only verifying equations, they want to understand physics at a deeper level by trying to grasp the physical processes that are hidden behind the equations.

 

[harrylin]

[..] It might be helpful to think of it in terms of you standing opposite another person; at rest relative to each other. Then one of you moves. You will measure the movement relative to the other person, but are you capable of movement; can you move so as to cause that relative motion? Is the other person capable of moving so as to cause that relative motion?

I'm afraid that your discussion here - or at least the way you phrase it - is simply too philosophical for this forum... Please limit your arguments to those that have actually been discussed (or are being discussed) in the physics literature.
For example, you could rephrase that statement by saying that obviously, if you accelerate or if the other accelerates are physically different cases in relativity theory. But as you have, I think, jumped classical physics, you are perhaps stretching too far too quickly.

 

[mangaroosh]

I'm afraid that your discussion here - or at least the way you phrase it - is simply too philosophical for this forum... Please limit your arguments to those that have actually been discussed (or are being discussed) in the physics literature. For example, you could rephrase that statement by saying that obviously, if you accelerate or if the other accelerates are physically different cases.

Would it be better to start a topic in the philosophy forum, do you think?

 

[harrylin]

Would it be better to start a topic in the philosophy forum, do you think?

By being more specific and to-the-point concerning laws of physics you could discuss some of these matters here. But yes, I think that the more "fuzzy" type of discussion belongs in the philosophy forum.

 

[mangaroosh]

By being more specific and to-the-point concerning laws of physics you could discuss some of these matters here. But yes, I think that the more "fuzzy" type of discussion belongs in the philosophy forum.

[EDIT] as is undoubtedly obvious [/EDIT] I'm not that well versed in the use of some of the specific terminology, so when I present my understanding of things it tends to come across sounding "fuzzy"; I'll start one in the philosophy forum, and see how it goes.

Cheers Harry.

 

[Dale]

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

Exactly, you persistently assume the thing you want to deduce, that is a fallacy.

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.

Note all of the references to absolute motion. In SR none of these terms are defined (see "flubnubitz" above). You have made them, simply assuming that they are well-defined, without proving it. To be well-defined this paragraph should be written:

"In order for relative motion to occur, at least one has to start moving relative to any given inertial frame. We can determine which one moves relative to any inertial frame - as per the principle of relativity; and we can deduce, surely, that one of them has to have moved relative to any inertial frame."

If you say the word "move" or "rest" without saying the frame you have not said anything with meaning. If you consistently say what frame you are talking about then you will always get statements that are meaningful and logically self-consistent.

 

[Dale]

mangaroosh, here is a physical example that may help. If I say "the door is open" what I am actually talking about is the relationship between the door and its frame. If you have a door without a frame it doesn't make sense to say that it is either open or closed. Similarly, if I say "X is moving" what I am actually talking about is the relationship between X and a reference frame. If I have X without a frame it doesn't make sense to say that it is either moving or at rest.

The difference between door frames and reference frames is that each door has at most one door frame, while X can have an infinite number of reference frames. But in each case the frame is something which essential for defining the state, and statements about the state are meaningless without the frame.

 

[Dale]

Well, the fact of the matter is that at least some of them held that their conclusions were experimentally justified and it corresponds to a currently discussed interpretation of QM (one alternative model has that reality doesn't exist!). Apparently you think that their logic must be illogical because you cannot follow their logic

The equations of mechanics, EM, and QM are all relativistic (either Galilean or Special). The only way for their conclusions about absolute space to be experimentally justified would be to find experimental violations of the principle of relativity (i.e. experiments that violate the coresponding relativistic equations). None of them did that. If you believe that I am incorrect about that then please find a reference where one of them found a violation of the principle of relativity, because I am unaware of it.

I am sorry if my statements bother you, but I do not think that it is a matter of opinion which needs to be qualified by a IMO.

 

[harrylin]

The equations of mechanics, EM, and QM are all relativistic (either Galilean or Special). The only way for their conclusions about absolute space to be experimentally justified would be to find experimental violations of the principle of relativity (i.e. experiments that violate the cooresponding relativistic equations). None of them did that. If you believe that I am incorrect about that then please find a reference where one of them found a violation of the principle of relativity, because I am unaware of it.

I am sorry if my statements bother you, but I do not think that it is a matter which needs to be qualified by a IMO.

None of your statements bother me.

However, to give just one clear example, you just wrote that: "The equations of [Galilean] mechanics[..] are [..] relativistic [..]. The only way for their conclusions about absolute space to be experimentally justified would be to find experimental violations of the principle of relativity".

As Newton's mechanics uses Galilean relativity, your claim implies that he should have found an experimental violation of his theory of mechanics to justify his absolute space postulate - and I trust that you agree that that doesn't make any sense. So, I'm sorry if that bothers you; and as this isn't the right place for such a discussion, you could bring it up as a separate topic in the appropriate group - I will abstain from further off-topic comments here.

 

[Dale]

As Newton's mechanics uses Galilean relativity, your claim implies that he should have found an experimental violation of his theory of mechanics to justify his absolute space postulate

Yes, exactly.

 

[D H]

As Newton's mechanics uses Galilean relativity, your claim implies that he should have found an experimental violation of his theory of mechanics to justify his absolute space postulate

Yes, exactly.

Newton argued that we mere mortals cannot sense absolute time or absolute space. For the most part all we can sense are relative time, relative distance, relative motion. However, Newton also argued that we can catch a glimpse of this absolute time and space in the concave surface of a rotating bucket of water and in the equatorial bulge of a rotating planet.

 

[mangaroosh]

mangaroosh, here is a physical example that may help. If I say "the door is open" what I am actually talking about is the relationship between the door and its frame. If you have a door without a frame it doesn't make sense to say that it is either open or closed. Similarly, if I say "X is moving" what I am actually talking about is the relationship between X and a reference frame. If I have X without a frame it doesn't make sense to say that it is either moving or at rest.

The difference between door frames and reference frames is that each door has at most one door frame, while X can have an infinite number of reference frames. But in each case the frame is something which essential for defining the state, and statements about the state are meaningless without the frame.

Sticking with the door analogy, which is quite helpful:

Let's say that we have a door and a frame, and the door is at rest relative to the frame in the open position. How can the door change to the closed position without either the frame or the door moving; surely we can deduce that either the door, or the frame, or both, actually move so that the door goes from being open to closed?

If we imagine that, in the open position, the door is at a right angle to the frame; if we then define a reference grid such that the frame runs along the X-axis and the door along the Y-axis. If, subsequently, both the door and the frame run along the X-axis when in the closed position, would we not conclude that it was the door that moved towards the frame; if both ran along the Y-axis would we not conclude that the frame moved towards the door; if they met at different co-ordinates would we not conclude that both of them moved?

 

[Mentz114]

Sticking with the door analogy, which is quite helpful:

Let's say that we have a door and a frame, and the door is at rest relative to the frame in the open position. How can the door change to the closed position without either the frame or the door moving; surely we can deduce that either the door, or the frame, or both, actually move so that the door goes from being open to closed?

Without reference to a third frame of reference, you cannot say which of them moved.

If we imagine that, in the open position, the door is at a right angle to the frame; if we then define a reference grid such that the frame runs along the X-axis and the door along the Y-axis. If, subsequently, both the door and the frame run along the X-axis when in the closed position, would we not conclude that it was the door that moved towards the frame; if both ran along the Y-axis would we not conclude that the frame moved towards the door; if they met at different co-ordinates would we not conclude that both of them moved?

Now you've introduced another frame of reference ( your grid) and you can tell which one moved relative to that frame. Your movements are always expressed realtive to some frame. It's not possible to define movement except as relative movement.

 

[harrylin]

Newton argued that we mere mortals cannot sense absolute time or absolute space. For the most part all we can sense are relative time, relative distance, relative motion. However, Newton also argued that we can catch a glimpse of this absolute time and space in the concave surface of a rotating bucket of water and in the equatorial bulge of a rotating planet.

I'll copy-paste this discussion in a new thread in the classical physics forum. Thus that fork of the discussion can proceed here: https://www.physicsforums.com/showthread.php?p=3871579#post3871579

 

[Dale]

Let's say that we have a door and a frame, and the door is at rest relative to the frame in the open position. How can the door change to the closed position without either the frame or the door moving; surely we can deduce that either the door, or the frame, or both, actually move so that the door goes from being open to closed?

You completely missed the point of the analogy. The point is that the designation of a door being open or closed doesn't even make sense without comparison to the door frame.

Tell me, in your opinion, if there is no door frame, can the door be considered to be open or closed?

 

[Aurin]

I'm sure I'll violate all sorts of precision in my choice of words, so please forgive me in advance.

As I understand it, being at rest isn't a separable entity of kinematics. It's just a "special case" of motion where velocity is equal to 0 and that value is no more or less significant than a non-zero value of velocity. For any given object we can attribute a frame providing it any value for its velocity we choose.

As I also understand it, an object relative to a frame that suddenly changes direction, moving opposite to its original path, had at one instantaneous point attained a velocity equal to zero and thus may have been at rest in that frame for a moment. By extension, other frames never witnessed the object changing direction, but only a change in speed witnessed as speeding up or slowing down.

So if asked "what does it mean to be at rest in SR?" like this thread's original purpose, my response would surely be that an object's velocity is equal to 0 relative to the frame that provides that velocity. Is there a disjoint in that answer?

 

[russ_watters]

I see how the logic works now:

1. Start with the assumption that absolute motion and rest exist.
2. If two objects are at rest relative to each other, they may be in absolute motion or absolute rest.
3. If one starts moving relative to the other, now we can be certain that at least one is in absolute motion since you can't simultaneously have both in absolute rest.
4. Conclude therefore that absolute motion and rest exist.

The line of logic is absolutely fine in the middle, but the conclusion is #3, not #4. #4 is just an echo of the starting assumption and the test does not delve into the issue of if the starting assumption is correct.

It also frustrates me that people cling so dearly to the assumption of absolute motion/rest, particularly when every waking moment of our lives we deal in relative motion and often even multiple simultaneous relative velocities in several frames at once (ie, when driving or playing sports). None of our experiences mandate the existence of an absolute reference frame, so I have trouble even understanding where the default assumption of an absolute reference frame would come from. Perhaps it comes from the thought of a fish-tank universe with God looking through the glass. Perhaps he's there, but whether he is or not, there is no basis for assuming he is nor any value in making the assumption. If one acknowledges that he and his frame are completely undetectable, then speculation about his existence is of no value in a scientific setting. It is purely a religious belief.

But...at the risk of confusing things, I will point out that there are objective, absolute realities that have to be acknowledged here. Consider the case of a rocket launching off the earth. The rocket burns fuel, so it must be said that it is moving (accelerating) away from the Earth and not the other way around. But the absoluteness of the fuel burning doesn't require an absoluteness of motion/rest.

 

[harrylin]

[..]None of our experiences mandate the existence of an absolute reference frame, so I have trouble even understanding where the default assumption of an absolute reference frame would come from. Perhaps it comes from the thought of a fish-tank universe with God looking through the glass. Perhaps he's there, but whether he is or not, there is no basis for assuming he is nor any value in making the assumption. If one acknowledges that he and his frame are completely undetectable, then speculation about his existence is of no value in a scientific setting. It is purely a religious belief. [..]

That comment seems to be a direct attack on Newton. So, if you like, I'll add your comment to the new thread that I just started on that topic.

 

[yoron]

My take on it :)

Can you define a 'motion'?
Do you need another frame of reference to do it?

If you are on 'something' being in a uniform motion, lifting/accelerating from it, does it matter what speed it has? For what acceleration/lift you need from that object?

Think about that one for a while before answering.

If uniform motion, as measured relative something else, doesn't crave any more 'energy/momentum' relative, or if you like, no matter what 'speed/velocity' you define it to have, or make you expend more energy accelerating from it, what is a 'uniform motion'?

Einstein differed between two things, 'uniform motion' and accelerations. He called one 'relative motion', the other a 'gravity'.
=

To simplify, define a speed/velocity of something (relative some 'inertial' definition). Accelerate from it. find the energy expended. Then make the object you accelerate/lift from beget double the speed/velocity, relative that same inertial frame of reference, to then become in a 'uniform motion' again. Then lift/accelerate from it again..

Will the energy expended by you accelerating/lifting differ between those two scenarios? If it won't, what do you think a uniform motion means?