At rest in Einsteinian relativity

[jtbell]

I would agree with saying that the Earth is actually rotating, if that is what you mean. The main reason is simple: we would have to arbitrarily assign enormous velocities (as well as very low ones) to distant star light, as well as other weird effects; a simpler set of laws of nature makes much more sense to me.

We don't even have to look that far. How would we explain the Foucault pendulum or the Coriolis effect if the Earth weren't rotating?

 

[harrylin]

We don't even have to look that far. How would we explain the Foucault pendulum or the Coriolis effect if the Earth weren't rotating?

Ah yes, I was looking unnecessarily far away.

 

[bahamagreen]

This is an interesting discussion.

It seems that if two objects are in relative motion, that is a "qualified" kind of motion; but in the end one may state that there is a motion.
The fundamental question is about the rigor of the logical extension, that this "qualified" motion, or some portion of it must logically rise to "unqualified" motion, motion inferred by relative motion but not itself relative to anything, "actual" motion, "pure" motion, absolute motion...

I understand the OP's inference, just like most people firmly understand that 2+2=4... and 160K+160K=320K, no doubt about that... yet when adding velocities simple addition is not correct.

It may actually be the case that the logic even if sound is simply not suitable for representation of measured reality; in the same way that addition in math does not get the right answer when applied to addition of velocities.

I just wonder if there is a way to indicate this disjunction in a satisfying way?

 

[mangaroosh]

That is exactly what you have received, multiple times. What could be more logical than pointing out the exact logical fallacy you are committing?

I was more trying to address the perception, which seems to abound, that I am trolling, because I know how such a perception can impact on how people engage in discussion.

I appreciate the point you are making, and that is partly the kind of discussion I think is helpful, but there seems to be either, an unwillingness, or an inability, to consider points that most people, or most lay people perhaps, would consider to be common sense, and self evidently true. While I might not be explaining myself perfectly, I don't think I would have this much trouble trying to get a point across with most other people.To address the point you make, however; I think it only appears to be begging the question if the conclusion, that moving on it's own does not make sense, is assumed. I have been trying to give everyday examples that, I think, make the distinction quite clearly, and that most people would understand; which, I feel demonstrates that it does indeed make sense to ask "which one is actually moving".

If we take the example that Harry raised, about the rotating earth. There we have the relative motion between the Earth and other objects in the universe, the sun for example. Historically, there have been two dominant scenarios which could have accounted for the relative motion; the first was that the sun moved around the earth; the other that the Earth is actually rotating.

This is a perfect example of where the relative motion is attributable to the fact that either the sun is moving, or the Earth is moving, and which should demonstrate the fact that "actually moving" makes sense.

Now, we might not be able to actually determine which is the case, but we surely can deduce that, at least, one of them has to be the case.

It is of course possible that both of them are actually moving, but it isn't a possibility that neither of them are actually moving. The movement will always manifest as relative motion, and while we may not be able to determine the absolute nature of the motion, as per the test of the PoR, I think we can deduce that the nature of the motion has to be absolute.

The existence of relative motion is a premise that we can both agree upon, so it is not fallacious to assume. Your goal is to start with that premise (and perhaps some other mutually agreeable premises) and prove that absolute motion exists.

To do so will require some series of statements, each of which are logically implied by the premises and the previous statements. The concluding statement will be something like "therefore x moved" or "therefore absolute motion exists". The premises, the logically implied statements, and the conclusion are collectively called a proof.

If you make such a statement before the final statement, then you are committing the fallacy known as begging the question. That is the problem with all of your attempted proofs up to now.

I'm not saying that assuming relative motion is fallacious, I'm saying that stating that motion has to be defined relative to something is, because that is assuming the conclusion that only relative motion makes sense.

I've tried to give real world examples which demonstrates that motion will always manifest as relative motion, but that there are two possibilities which can give rise to that relative motion; either one object moves, or the other object does.

It's like the train example, where you are on a train and the train beside you starts to pull away; you can't tell if it is your train that is moving or if it is the other train; then you realize that it was the other train that was actually moving. I think most people would understand that point.

Of course, we might not be able to determine which train was actually moving, but I think we can deduce that one of them actually was moving.

Again, both could be moving, as is the case if the Earth is actually moving, but that would just compound the point about actual motion, or absolute motion.

Please limit your examples to relevant ones. Specifically, inertial motion. It is not a topic of disagreement that non inertial motion (proper acceleration) is "absolute" in some sense.

Inertial motion isn't excluded, it's just easier to highlight when we consider two objects at rest relative to each which then start moving, relative to each other. If we have two inertial objects/observers, moving relative to each other, we can still conclude that one of them at least has to actually be moving.

Again, taking two trains that pass each other; we can have, at least, two scenarios; one where one train is stopped, on the surface of the earth, and one train is moving on the surface of the earth. There will be relative motion between the two trains because one of them is actually moving; or where both trains are moving on the surface of the earth; again, there will be relative motion because both trains are actually moving.

We can say that the movement of [EDIT] the wheels of [/EDIT] one of the trains actually makes them remain stationary i.e. the ground is moving beneath the train, like a conveyor belt and the motion of the wheels simply causes it to stay put; but in that case we can deduce that it is the Earth that is actually moving. We might not be able to determine which is the case, but we can surely deduce that it must be one or the other.

Why? Please try your proof again, now that you have been made aware of the nature of the fallacy perhaps you can avoid it.

Stating that motion has to be defined relative to something is, in itself fallacious, because it assumes the conclusion that only relative motion makes sense.I've given a number of different examples which most people would have no trouble understanding, which should hopefully demonstrate that "actually moving", or "absolute motion" perhaps, makes perfect sense.

Again, is the Earth actually rotating, or are the other objects actually revolving around the earth? Both cases would result in relative motion, but both are contrasting reasons as to why the relative motion manifests.

That we cannot determine which is correct is reflected in the idea that we can define a reference frame in which either is the case; but surely, we can deduce that, at least, one or the other must be the case; it is of course possible that everything in the universe is actually moving, and nothing is in a state of absolute rest, but that would just compound the idea of absolute motion.That would be my understanding, anyway.

 

[russ_watters]

Now, we might not be able to actually determine which is the case, but we surely can deduce that, at least, one of them has to be the case.

It is of course possible that both of them are actually moving, but it isn't a possibility that neither of them are actually moving. The movement will always manifest as relative motion, and while we may not be able to determine the absolute nature of the motion, as per the test of the PoR, I think we can deduce that the nature of the motion has to be absolute.

Again, the logic is that if two objects are in relative motion with respect to each other, then at most one can be in absolute rest and the other must therefore be in absolute motion. Well sorry, but it doesn't matter how many times it is repeated in different contexts: the logic still doesn't follow. Why? Simply because it starts with the premise that absolute motion/rest exists. You cannot prove the existence of absolute motion/rest with a thought experiment that starts with the assumption that it exists. It is circular logic that reduces to "absolute motion exists because absolute motion exists".

Stating that motion has to be defined relative to something is, in itself fallacious, because it assumes the conclusion that only relative motion makes sense.

Nonsense. Absolute motion is a special case/subset of relative motion. Motion (speed) is the change in relative distance over the change in a time interval. If one of the references is absolute, then it is absolute motion. Otherwise, it is relative motion.

And more to the point, a definition can be whatever the consensus desires it to be. That's all definitions are!

 

[mangaroosh]

Not sure, nor did I consider it, which is why I did not include it.

That's fair enough, I'm just trying to offer simple, everyday examples to demonstrate my understanding; that is one such one, which I think might help to do so.

That's perfectly wrong, as we hoped you would know from classical mechanics. It boils down to the difference between:

- being in relative motion,
and
- initiating a relative motion/a change in relative motion.

It appears that you confound the two; perhaps with "true motion" you meant the second one?

Right

Descriptions according to which is the Earth is truly rotating are much simpler than those according to which the Earth is in rest.
Moreover, Einstein's 1916 GR according to which it is equally possible to say that the Earth is in rest, has IMHO problems with consistency. In any case, the common attitude has become again to take the point of view relative to inertial reference systems, so that one regards the Earth as in motion.

I would agree with saying that the Earth is actually rotating, if that is what you mean. The main reason is simple: we would have to arbitrarily assign enormous velocities (as well as very low ones) to distant star light, as well as other weird effects; a simpler set of laws of nature makes much more sense to me.

This a great example, and one which represents an area of confusion with relativity, and which, also, can hopefully help me clarify the point I am trying to make.

My understanding was that we can't say which reference frame is correct in Einsteinian relativity; that the reference frame where the Earth is not rotating, is equally valid to the one where it is rotating; but you seem to be suggesting that there is a preferred reference frame here. It would be one I personally would agree with, but I was lead to believe that that would be at odds with Einsteinian relativity.


You mention, above, that you agree that the Earth is actually rotating, and that is precisely what I mean. It is the difference between the Earth actually rotating and the other objects in the universe revolving around the earth, which leads to the enormous velocities for distant starlight. It is the difference in the example where you're not sure if it is your own train or the train beside you that is moving out of the station; it is either the train that is moving, or the entire Earth that changes it's motion to manifest in the relative motion. Both examples give rise to the observed relative velocity, but I would have thought that we can deduce that either one or the other must be true.

 

[mangaroosh]

Again, the logic is that if two objects are in relative motion with respect to each other, then at most one can be in absolute rest and the other must therefore be in absolute motion. Well sorry, but it doesn't matter how many times it is repeated in different contexts: the logic still doesn't follow. Why? Simply because it starts with the premise that absolute motion/rest exists. You cannot prove the existence of absolute motion/rest with a thought experiment that starts with the assumption that it exists. It is circular logic that reduces to "absolute motion exists because absolute motion exists".

OK, I see what you're making, but that re-formulation doesn't necessarily represent the point.

We don't start by saying that "at most, one can be in absolute rest"; we start without any reference to absolute rest, or any assumption of absolute rest; we take the example of two objects at rest relative to each other, the absolute nature of whose motion we cannot determine, as per the PoR.

Relative motion occurs between them; again, no assumption of absolute rest or absolute motion, as neither of them has to be at absolute rest.

To say that, "in order for relative motion to occur, one of them has to move", is something that would make sense to, I dare say, most people.

For example, if you have a parked car and want it moved, there are two alternatives; move the car, or moving everything else around the car. Again, this is something I think most people would understand.

So, for our two observers at rest relative to each other; for relative motion to manifest, one of them has to move.

Regardless of which one it is, it will still manifest as relative motion; but there are two possible explanations for why the relative motion manifests; my understanding is that we can deduce that either one, or the other, has to be correct; even if we cannot determine which one it actually is.


Alternatives
Here, we can consider the alternative explanations for why relative motion occurs between the observers. Take the example of the train pulling out of the station, where, for a moment, you can't tell if it is your train that is moving, or if it is the other train.

Incidentally, that is an example that doesn't qualify the term moving, but makes sense to most people.

The two alternatives there are; 1)your train is moving; 2)the other train is moving; both would cause relative motion to manifest, but both are different explanations of it.

If we assume, for the sake of the point, that it is the other train that leaves the station (without assuming that it "actually moves", it simply leaves the station). With the second option above, the explanation for why the train leaves the station is that it is the other train that is actually moving.

The first alternative above requires that it is the earth, the buildings and your train that are actually moving, while the other train rotates its wheels to remain stationary, much like a person running on a treadmill jogs "on the spot".


Both examples would account for the relative motion, but each one requires something to actually move.

That we can define a reference frame for both cases simply reiterates the idea that we cannot determine the absolute nature of motion of either i.e. we cannot determine which one is correct; but surely we can deduce that either one, or the other, is correct.

Rotating earth
Harrry's example might be more palatable, or easier to understand. Is the Earth actually rotating, or is everything in the universe revolving around the earth?

Both would account for the relative motion, but they are two different scenarios.

Nonsense. Absolute motion is a special case/subset of relative motion. Motion (speed) is the change in relative distance over the change in a time interval. If one of the references is absolute, then it is absolute motion. Otherwise, it is relative motion.

And more to the point, a definition can be whatever the consensus desires it to be. That's all definitions are!

and the consensus on the definition of the term "absolute" is

not qualified or diminished in any way; total


Philosophy
a value or principle which is regarded as universally valid or which may be viewed without relation to other things:

http://oxforddictionaries.com/definition/absolute?q=absolute

So to define absolute motion as being relative to an absolute reference frame would be a contradiction in terms.


I would have thought the question of absolute motion would have been a "yes" or "no", or either or, answer to the question "did X move?" or "did Y move?".


Move

[no object, usually with adverbial of direction] go in a specified direction or manner; change position:
- she moved to the door
- I heard him moving about upstairs

http://oxforddictionaries.com/definition/move?q=move


The above definition of "move" might help me to clarify my understanding. If we take the example of "she moved to the door".

"She moved to the door" would represent one reference frame.

"the door moved to her" would represent the other.


In both cases the movement is attributed to either the door, or "her". We cannot determine which one is correct, so when we ask the questions:
Did she move to the door? we answer, we can't tell.
Did the door move to her? we answer, we can't tell.


But from those, to my understanding anyway, we can deduce that either she moved to the door, or the door moved to her.

Either way the act of movement is ascribed to one or the other, in an absolute sense; even though it manifests relatively.


That would be my understanding anyway, and it appears to represent a stumbling block to learning relativity. Bahamagreen might perhaps be able to put it more clearly that I can, but I think the examples I've given are fairly intelligible to most people.

 

[Michael C]

Move

http://oxforddictionaries.com/definition/move?q=move


The above definition of "move" might help me to clarify my understanding. If we take the example of "she moved to the door".

"She moved to the door" would represent one reference frame.

"the door moved to her" would represent the other.


In both cases the movement is attributed to either the door, or "her". We cannot determine which one is correct, so when we ask the questions:
Did she move to the door? we answer, we can't tell.
Did the door move to her? we answer, we can't tell.


But from those, to my understanding anyway, we can deduce that either she moved to the door, or the door moved to her.

Please define what you mean by "moved" in the last phrase. You gave a link to a dictionary definition of "move" which gives a lot of literary meanings of the word which have no place in this discussion. The only definition there that could be the start of a rigorous physical definition is this one: "change position". As I already said, "change position" only makes sense relative to a specific frame of reference.

Talking about people "moving" can confuse the issue, since a human is a complex collection of parts that can move relative to one another. Let's just imagine an inert lump of rock in space. How do you define "moving" for this rock? According to you, the rock must be either "actually moving" or "not moving". What is the difference between the two possibilities?

 

[Mentz114]

... and it appears to represent a stumbling block to learning relativity.

It is not. Special relativity is about Minkowski spacetime, the Lorentz transformation, electrodynamics and many things but has no use for your idea of 'really moved'. It is absolutely not relevant to SR.

 

[Austin0]

Move

http://oxforddictionaries.com/definition/move?q=move "She moved to the door" would represent one reference frame.

"the door moved to her" would represent the other.


In both cases the movement is attributed to either the door, or "her". We cannot determine which one is correct, so when we ask the questions:
Did she move to the door? we answer, we can't tell.
Did the door move to her? we answer, we can't tell.


But from those, to my understanding anyway, we can deduce that either she moved to the door, or the door moved to her.
Either way the act of movement is ascribed to one or the other, in an absolute sense; even though it manifests relatively.


That would be my understanding anyway, and it appears to represent a stumbling block to learning relativity. Bahamagreen might perhaps be able to put it more clearly that I can, but I think the examples I've given are fairly intelligible to most people.

In the door case and many others you have mentioned, the answer is clear cut because acceleration is involved. She is moving in a series of complex accelerations. Everybody agrees. ;-)
Do I understand your thoughts correctly in assuming when you say absolute you simply mean actual , with no implication of a quantifiable value to the motion?
If this is the case you must have realized by now that absolute is a poor choice of word for you because it has such a specific definition in science.

 

[mangaroosh]

Please define what you mean by "moved" in the last phrase. You gave a link to a dictionary definition of "move" which gives a lot of literary meanings of the word which have no place in this discussion. The only definition there that could be the start of a rigorous physical definition is this one: "change position". As I already said, "change position" only makes sense relative to a specific frame of reference.

Talking about people "moving" can confuse the issue, since a human is a complex collection of parts that can move relative to one another. Let's just imagine an inert lump of rock in space. How do you define "moving" for this rock? According to you, the rock must be either "actually moving" or "not moving". What is the difference between the two possibilities?

The best I can do is give examples that try to illucidate what I mean, because for a large portion of the population, I would be fairly sure that, my use of the terms would make perfect sense. The question of whether it is you that is actually moving when you walk down the road, or whether it is the road, the buildings, and everything else that is actually moving, as though on a conveyor belt, is one that most people wouldn't really have trouble with. The conplexity of a human being shouldn't really hinder that; if anything it helps with the question.

With regard to the inert lump of rock, let's say the Earth for example; most people wouldn't have any trouble understandng the question; is the Earth actually rotating, or is everything in the universe actually rotating around the earth? This was an historical question which was supposed to have been resolved; the answer was supposed to have been, it is the Earth that is actually rotating.

Now, we may not be able to determine if that is actually the case, or not, but we can surely deduce that either the Earth is actually rotating i.e. actually moving, or it isn't rotating and everything in the universe is moving around the earth.

Here we have two different explanations which both give rise to the observed relative motion.


This represents my current understanding, and I don't doubt it would make sense to a large section of the population; as mentioned, it was a historical question that was supposed to have been resolved i.e. was the Earth at the centre of the universe with everything in motion around it? Ptolmey's model included epicycles to explain how the planets actually moved around the earth, while the Earth remained stationary. This was discarded for the idea that the Earth actually rotates, and actually orbits around the sun i.e. it actually moves.

 

[mangaroosh]

It is not. Special relativity is about Minkowski spacetime, the Lorentz transformation, electrodynamics and many things but has no use for your idea of 'really moved'. It is absolutely not relevant to SR.

I'm not suggesting that it is included in it, but it does form part of my understanding of the world; something I genuinely believe to be self-evidently true; to be told that Einsteinian relativity disagrees with this and that it doesn't make sense inevitably causes resistance in the learning process and so it represents a stumbling block for me; and anyone who might believe similarly.

 

[Michael C]

With regard to the inert lump of rock, let's say the Earth for example; most people wouldn't have any trouble understandng the question; is the Earth actually rotating, or is everything in the universe actually rotating around the earth? This was an historical question which was supposed to have been resolved; the answer was supposed to have been, it is the Earth that is actually rotating.

I'm not talking about rotating and I'm not talking about the Earth. I just want you to define the difference between "actually moving" and "not moving" for a lump of rock floating in space. According to you, there must be a difference between these two states. What is this difference?

 

[mangaroosh]

In the door case and many others you have mentioned, the answer is clear cut because acceleration is involved. She is moving in a series of complex accelerations. Everybody agrees. ;-)
Do I understand your thoughts correctly in assuming when you say absolute you simply mean actual , with no implication of a quantifiable value to the motion?
If this is the case you must have realized by now that absolute is a poor choice of word for you because it has such a specific definition in science.

I didn't start off with the term "absolute motion", I started off saying "actually moved", which people said didn't make any sense; as the discussion progressed others used the term "absolute motion" and it became embedded. I have repeatedly tried to clarify my understanding of what I meant, and the term "absolute motion" was what others seemed to keep referring to. There was no implication of a quantifiable value to the motion, as far as I was concerned, but it probably would have deductive significance.

I wouldn't necessarily say that it is a poor choice of words, because absolute would probably be an accurate term for it; I think the suggestion that absolute motion must be relative is probably a contradiction in terms, though.

 

[mangaroosh]

I'm not talking about rotating and I'm not talking about the Earth. I just want you to define the difference between "actually moving" and "not moving" for a lump of rock floating in space. According to you, there must be a difference between these two states. What is this difference?

I'm not entirely sure how to define it, all I can do is give common examples that people generally tend to understand, to demonstrate that it does, actually, make sense.

We don't necessarily need to talk about the Earth rotating, we can take any rock and discuss the same idea; the "third rock from the sun", however, lends itself easily to discussion, given the historical context of questions about the nature of its motion. And I'm sure that rotating is just as valid as any other such example; again, it is a historical question which was fairly widely understood. Is the Earth moving around the sun, or is the sun moving around the earth?


If you are standing on the side of the road, and a car passes you; was it you that was moving, or was it the car?


Most people would have no trouble with these questions. Where they might have issue is whether their answers are correct or incorrect.


Most people would say that it is the car that is moving; but we can't actually determine if this is true. What we can deduce though, I believe, is that either you (and everything on the earth) is moving, and the cars rotating wheels are keeping it stationary - like someone jogging on the spot on a treadmill - or the car is moving.

If we throw in the fact that the Earth is moving through the universe, then it simply compounds the question of actual movement.

Again, most people wouldn't have trouble understanding this, so I am somewhat aghast at why it is causing so much trouble.

 

[mangaroosh]

This is an interesting discussion.

It seems that if two objects are in relative motion, that is a "qualified" kind of motion; but in the end one may state that there is a motion.
The fundamental question is about the rigor of the logical extension, that this "qualified" motion, or some portion of it must logically rise to "unqualified" motion, motion inferred by relative motion but not itself relative to anything, "actual" motion, "pure" motion, absolute motion...

I understand the OP's inference, just like most people firmly understand that 2+2=4... and 160K+160K=320K, no doubt about that... yet when adding velocities simple addition is not correct.

It may actually be the case that the logic even if sound is simply not suitable for representation of measured reality; in the same way that addition in math does not get the right answer when applied to addition of velocities.

I just wonder if there is a way to indicate this disjunction in a satisfying way?

Cheers bahama,

I don't fully understand the process of addition of velocities, but you've managed to state the issue much more concisely that I could have.

 

[Michael C]

I'm not entirely sure how to define it, all I can do is give common examples that people generally tend to understand, to demonstrate that it does, actually, make sense.

You keep giving examples but you don't say what, in the examples, has actually moved, nor what this "actually moved" means.

If you are standing on the side of the road, and a car passes you; was it you that was moving, or was it the car?

Most people would have no trouble with these questions. Where they might have issue is whether their answers are correct or incorrect.

Most people would say that it is the car that is moving; but we can't actually determine if this is true.

Yes, most people would say that the car was moving. So would I, in a normal, everyday situation. In a discussion on physics I might add that I am using the frame of the reference where the Earth is stationary. That's the most obvious frame to use here, and in the vast majority of mechanical problems we discuss here on Physics Forums we don't even bother to specify the frame of reference, because it is obvious for all concerned. That shouldn't blind us to the fact that we are assuming a frame of reference, though.

Again, most people wouldn't have trouble understanding this, so I am somewhat aghast at why it is causing so much trouble.

Yes, in everyday situations everybody automatically assumes a frame of reference. If we're talking about events happening on the surface of the Earth - cars running on a road or somebody walking in the woods - everybody will certainly agree on whether the car or the person is "moving" or "not moving" because they will in fact be thinking of movement relative to the Earth. If we're playing a game on the deck of a boat, we will naturally take the boat to define our frame of reference. If we're sitting in a car, we'll use the car itself to define the frame for things happening inside it (everybody will understand when we tell the kids to stop moving around on the back seat), but we'll use the road to define the frame of reference when we're thinking of our progress towards our destination (we're hardly moving at all in this traffic jam).

All this is commonplace stuff: we don't bother to say "moving with respect to frame of reference X" because it's clear what our frame of reference is. But in fact our definition of "moving" only makes sense together with a frame of reference. I'm not the only one here having big problems understanding you because you claim that there is some state called "just moving" that doesn't need a frame of reference.

What we can deduce though, I believe, is that either you (and everything on the earth) is moving, and the cars rotating wheels are keeping it stationary - like someone jogging on the spot on a treadmill - or the car is moving.

Let's say it's "really" me that is moving. How is this situation different from the one where the car is "actually moving" and I am stationary? How does this difference manifest itself?

 

[Michael C]

mangaroosh, I'm going to try an analogy. First important question: do you believe in absolute position?

To put the question more precisely, imagine a flat, two-dimensional Euclidean plane that extends infinitely in all directions. Can you give the absolute position of a point on this plane? For instance, if somebody tells you to place a mark at the centre of the plane, can you decide where this "centre" is?

 

[Dale]

there seems to be either, an unwillingness, or an inability, to consider points that most people, or most lay people perhaps, would consider to be common sense, and self evidently true.

This repeated appeal to the opinion of "most people" is also fallacious, specifically the bandwagon fallacy (http://www.fallacyfiles.org/bandwagn.html). Furthermore, I am not sure that it is correct. It is clear that you believe it, but there is no evidence to suggest that the majority of other people believe it.

If we take the example that Harry raised, about the rotating earth. There we have the relative motion between the Earth and other objects in the universe, the sun for example. Historically, there have been two dominant scenarios which could have accounted for the relative motion; the first was that the sun moved around the earth; the other that the Earth is actually rotating.

This is a perfect example of where the relative motion is attributable to the fact that either the sun is moving, or the Earth is moving, and which should demonstrate the fact that "actually moving" makes sense.

This is non-inertial motion, I agree the Earth is "actually rotating". It is irrelevant with respect to our point of disagreement regarding inertial motion.

I'm not saying that assuming relative motion is fallacious, I'm saying that stating that motion has to be defined relative to something is, because that is assuming the conclusion that only relative motion makes sense.

This does not make your proofs non-fallacious. We do agree that relative motion exists, so it can be used as a premise. We do not agree that absolute motion exists. We do not agree that only relative motion makes sense. So neither of those may be used as a premise in a proof. I have not offered any proofs presuming latter, but you repeatedly offer proofs presuming the former.

You need to be aware that the relativity of velocity is a basic part of Einstein's SR. I.e. as soon as you assume that absolute motion makes physical sense you are no longer doing SR. The likely reason that some people think you are trolling is that on the one hand (e.g. your OP) you claim that you are interested in SR, and then on the other hand you repeatedly (e.g. almost all of your other posts) violate its basic assumptions. It seems to be self-contradictory, or at least false pretenses.

Do you wish to study SR or not? If yes, then realize that the relativity of velocity is part of SR, and it is logically self-consistent (i.e. you never contradict yourself if you assume only relative motion) as well as consistent with experimental evidence (i.e. you don't contradict any available empirical evidence if you assume only relative motion). If you do not wish to study SR then you are in the wrong place.

Inertial motion isn't excluded, it's just easier to highlight when we consider two objects at rest relative to each which then start moving, relative to each other. If we have two inertial objects/observers, moving relative to each other, we can still conclude that one of them at least has to actually be moving.

So prove it, non-fallaciously.

 

[Dale]

My understanding was that we can't say which reference frame is correct in Einsteinian relativity; that the reference frame where the Earth is not rotating, is equally valid to the one where it is rotating; but you seem to be suggesting that there is a preferred reference frame here.

In SR there is a preferred set of reference frames, called inertial frames. They are related to one another via the Lorentz transform.

 

[harrylin]

[..]

You missed, again, responding to what I held (and now strongly hold) to be the main cause of the misunderstandings. I asked you:

- being in relative motion,
and
- initiating a relative motion/a change in relative motion.

It appears that you confound the two; perhaps with "true motion" you meant the second one?

[about the rotating Earth]
This a great example, and one which represents an area of confusion with relativity, and which, also, can hopefully help me clarify the point I am trying to make.

My understanding was that we can't say which reference frame is correct in Einsteinian relativity; that the reference frame where the Earth is not rotating, is equally valid to the one where it is rotating; but you seem to be suggesting that there is a preferred reference frame here.

As all to often: yes and no, mixing up things again that are related to the above non-answer by you.

1. Do you really want to discuss Einstein's 1916 version of General Relativity? If so, I think that you should read, understand and be able to comment on the concluding words of the discussion here: http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_gr.html

2. Assuming that instead you only want to discuss SR, in which the infinite set of inertial reference frames is preferred:
- obviously I am not suggesting that there is a preferred reference frame here, but that the same laws of physics can be used equally well by means of any inertial reference system for an equally precise description of the Earth's rotation and the corresponding effects

[..]It would be one I personally would agree with, but I was lead to believe that that would be at odds with Einsteinian relativity. [..]

Happily you'll by now have defined what you mean with that in the line here above.

It is the difference in the example where you're not sure if it is your own train or the train beside you that is moving out of the station; it is either the train that is moving, or the entire Earth that changes it's motion to manifest in the relative motion. Both examples give rise to the observed relative velocity, but I would have thought that we can deduce that either one or the other must be true.

Again:
- there is no issue concerning that in either classical mechanics or special relativity
and
- a steadily rotating Earth is very different from a departing train according to those theories.

And again: you do not know the answer of classical mechanics, or do you? Please start it as a question in the forum there, and keep this on hold; for until you understand this pertinent issue that will need a lot of explanation (incl. math!) about some basic concepts of classical mechanics (which are essentially unchanged in SR), without your understanding of the related concepts you will just be wasting everyone's time here. It's not useful to push understanding of motion back to the time of Zeno (sorry if that sounds rude, but by now it really feels like it).

 

[Dale]

To say that, "in order for relative motion to occur, one of them has to move", is something that would make sense to, I dare say, most people.

The non-fallacious way to say this is "in order for relative motion to occur, one of them has to move relative to the other".

To say it the way you consistently do is to presume the concept of absolute motion before proving that it exists. You cannot then turn around and say "therefore absolute motion exists" since you already assumed it.

 

[russ_watters]

I'll have more later, but recall that I said that you can say a rocket which fired its engines can be said to have "actually moved" away from earth. But if at some point in the coast phase of its journey it passes another coasting rocket, neither can claim to be the one "actually moving".

Do you understand this?

 

[Dale]

Mangaroosh, let me outline your proof for you in a way that may help you see the fallacy.

1) Consider two inertially moving objects, A and B, with relative velocity v.
2) In any inertial reference frame, if one is at rest the other is moving with velocity v.
3) Therefore, at least one is moving relative to any inertial frame.
4) Assume further that there exists some inertial frame, F, such that objects at rest in that frame are absolutely at rest and objects moving in that frame are absolutely moving.
5) Since F is an inertial frame, then by (2) at least one is moving relative to F.
6) Therefore, at least one object is absolutely moving.

The logic simply doesn't work without (4). But (4) is a topic of disagreement, which is why assuming it is fallacious.

In SR velocity is relative, and SR is self consistent. So any physical scenario consistent with SR can be stated purely in terms of relative velocities without any logical contradictions. Therefore, you cannot prove that absolute velocity is logically implied.

 

[mangaroosh]

You keep giving examples but you don't say what, in the examples, has actually moved, nor what this "actually moved" means.

The point, which is in keeping with the PoR, is that we can't say which one actually moves, but we can deduce that at least one has to have actually moved.

The examples should give a contextual understanding, and, as I say, one that most people would have no problem understanding. Does bahamagreen's post #143 clarify things any bit; he seems to have an understanding of what I am trying to get at, as does salvestrom, as indicated by his post #47.

I know that most lay people would understand the examples, and what is meant by actually moving, so I'm not sure what the issue is.

Yes, most people would say that the car was moving. So would I, in a normal, everyday situation. In a discussion on physics I might add that I am using the frame of the reference where the Earth is stationary. That's the most obvious frame to use here, and in the vast majority of mechanical problems we discuss here on Physics Forums we don't even bother to specify the frame of reference, because it is obvious for all concerned. That shouldn't blind us to the fact that we are assuming a frame of reference, though.

Most people wouldn't necessarily assume a reference frame where the Earth is stationary, they would say the car is moving even when the Earth is rotating and orbiting around the sun; they would acknowledge that the car is moving with the earth, but that the car can subsequently move along the surface of the earth; here, people would say that both the Earth and the car are actually moving, as opposed to not moving at all.

Yes, in everyday situations everybody automatically assumes a frame of reference. If we're talking about events happening on the surface of the Earth - cars running on a road or somebody walking in the woods - everybody will certainly agree on whether the car or the person is "moving" or "not moving" because they will in fact be thinking of movement relative to the Earth. If we're playing a game on the deck of a boat, we will naturally take the boat to define our frame of reference. If we're sitting in a car, we'll use the car itself to define the frame for things happening inside it (everybody will understand when we tell the kids to stop moving around on the back seat), but we'll use the road to define the frame of reference when we're thinking of our progress towards our destination (we're hardly moving at all in this traffic jam).

All this is commonplace stuff: we don't bother to say "moving with respect to frame of reference X" because it's clear what our frame of reference is. But in fact our definition of "moving" only makes sense together with a frame of reference. I'm not the only one here having big problems understanding you because you claim that there is some state called "just moving" that doesn't need a frame of reference.

While people don't necessarily think in terms of "reference frames" I wholeheartedly agree that a reference frame is indeed assumed, or implied; but so too is the idea of actual movement.

With the examples you give of relative motion, there are always two possibilities that can account for the observed relative motion; each one attributing the movement to one object or the other.

Example
It might be easier to break from the "everyday" examples and try to give a different one.

Imagine that you are standing on a metal disc, in the middle of a room; the disc can rotate freely 360°; equally, the room and everything else can rotate freely 360°.

When you rotate on the disc it has the exact same effect as when the room starts rotating.

You start to experience relative motion i.e. the spinning, between you and the room; now, either you are spinning, or the room is spinning. It is either one or the other.

We might be able to define a reference frame for both, but that simply reinforces the point that we cannot tell which one is actually spinning; but we can deduce that either it is you that is actually spinning, or it is the room that is actually spinning; it is one or the other - in the simplest form, at least.


Two possibilities which result in the same relative motion.

Let's say it's "really" me that is moving. How is this situation different from the one where the car is "actually moving" and I am stationary? How does this difference manifest itself?

If each observer were holding a light clock, as per Einstein's thought experiment, wouldn't the path length of the photon, in the clock, be affected?

 

[mangaroosh]

mangaroosh, I'm going to try an analogy. First important question: do you believe in absolute position?

To put the question more precisely, imagine a flat, two-dimensional Euclidean plane that extends infinitely in all directions. Can you give the absolute position of a point on this plane? For instance, if somebody tells you to place a mark at the centre of the plane, can you decide where this "centre" is?

If the plane is infinite, then I'm not sure, tbh. For arguments sake I would say yes, but I'm not sure how the qualifier "infinitely" affects it.

 

[mangaroosh]

This repeated appeal to the opinion of "most people" is also fallacious, specifically the bandwagon fallacy (http://www.fallacyfiles.org/bandwagn.html). Furthermore, I am not sure that it is correct. It is clear that you believe it, but there is no evidence to suggest that the majority of other people believe it.

I'm not using it as a reason as to why it makes sense, I'm simply saying that it does make sense to most people; it might perhaps be an incentive to ask some people who have no formal background in relativity, or science, and see what they say; to see if there is in fact evidence to support it.

I would imagine that most people would think it is some form of trick question, so it should probably be specified that it isn't.

This is non-inertial motion, I agree the Earth is "actually rotating". It is irrelevant with respect to our point of disagreement regarding inertial motion.

I'm not sure where the disagreement lies, if you agree that the Earth is "actually rotating". Given your responses thus far in the thread, I have to ask the question what you mean by "actually rotating". If you hadn't argued so vehemently against it I would have assumed we were in agreement, but would you say that the Earth is rotating in an absolute sense?

This does not make your proofs non-fallacious. We do agree that relative motion exists, so it can be used as a premise. We do not agree that absolute motion exists. We do not agree that only relative motion makes sense. So neither of those may be used as a premise in a proof. I have not offered any proofs presuming latter, but you repeatedly offer proofs presuming the former.

Your disagreements, thus far, have effectively taken the form of "only motion relative to something makes sense", which is why I was saying you were assuming the conclusion - I'm not sure what the formal name is, but when you attempt to disprove something by proving something else, which is contradictory to be true?

I haven't really intended to provide a rigorous logical proof; although I have tried to present a logical representation of my understanding, with recourse to contextual examples.

I appreciate your highlighting the logical fallacy that you believe is, and may perhaps be, being committed, because it helps to highlight where the issue lies. I was hoping the contextual examples would help to illucidate the point, as it seems to have done for bahamagreen, and salvesrom (although I would suspect they didn't require "my" examples to illucidate it).

You need to be aware that the relativity of velocity is a basic part of Einstein's SR. I.e. as soon as you assume that absolute motion makes physical sense you are no longer doing SR. The likely reason that some people think you are trolling is that on the one hand (e.g. your OP) you claim that you are interested in SR, and then on the other hand you repeatedly (e.g. almost all of your other posts) violate its basic assumptions. It seems to be self-contradictory, or at least false pretenses.

Do you wish to study SR or not? If yes, then realize that the relativity of velocity is part of SR, and it is logically self-consistent (i.e. you never contradict yourself if you assume only relative motion) as well as consistent with experimental evidence (i.e. you don't contradict any available empirical evidence if you assume only relative motion). If you do not wish to study SR then you are in the wrong place.

I am indeed interested in learning SR, but I think an issue might lie in the preconceptions that people might have about the learning process. A horribly contrived expression I've come across in my own teacher training is the term "expectation violation", which can be a major obstacle to effective teaching and indeed learning; where the teacher has expectations about the learning/teaching process, which do not come to fruition, it can cause frustration and anger, which can affect the overall process.

I'm not sure how my profile as a learner compares to others who visit the site, but I don't doubt that, given enough research we could all be broadly categorised. I'm approaching this as a mature learner, with a pre-existing worldview which I believe to be fairly reasonable; there are undoubtedly subconscious beliefs thrown in there, and subconscious attachment to other beliefs, that younger learners have, perhaps, not yet developed. I do try to develop self-awareness and non-attachment to those beliefs, but it isn't a simple case of switching them off, as I'm sure you can appreciate.


I am interested in learning and developing an understanding of ER that extends, primarily, to its foundational assumptions; I want to explore them and question them to see do they stand up to reason, and if they do, then I will be in position, psychologically, to accept it; unfortunately not before - as I'm sure you will agree, it is not possible to accept something that you do not understand, or that you haven't subjected to rational enquiry.

So prove it, non-fallaciously.

I'm not sure I can, but we might be able to tease out the issue to see if it is possible.

Do I take it from the above, that you agree that in the case of the non-inertial observers that one of them is "actually moving", in an absolute sense?

 

[mangaroosh]

In SR there is a preferred set of reference frames, called inertial frames. They are related to one another via the Lorentz transform.

Harry seemed to be saying that the reference frame which labels the Earth as rotating, as opposed to one which labels it as not rotating, was the preferred reference frame.

EDIT: just read Harry's next point which addresses this point; I haven't digested it yet, but just wanted to acknoweldge it.

 

[mangaroosh]

You missed, again, responding to what I held (and now strongly hold) to be the main cause of the misunderstandings. I asked you:

- being in relative motion,
and
- initiating a relative motion/a change in relative motion.

It appears that you confound the two; perhaps with "true motion" you meant the second one?

I don't see why this should be seen as an issue, because I think we can deduce that, at least, one observer has to be actually moving (or in "absolute motion") from a scenario where observers are moving relative to each other, even inertially. It doesn't necessarily require a change in relative motion, I just think it is easier to highlight.

As all to often: yes and no, mixing up things again that are related to the above non-answer by you.

1. Do you really want to discuss Einstein's 1916 version of General Relativity? If so, I think that you should read, understand and be able to comment on the concluding words of the discussion here: http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_gr.html

2. Assuming that instead you only want to discuss SR, in which the infinite set of inertial reference frames is preferred:
- obviously I am not suggesting that there is a preferred reference frame here, but that the same laws of physics can be used equally well by means of any inertial reference system for an equally precise description of the Earth's rotation and the corresponding effects

Apologies, it didn't quite sound like that in your last post.

To be honest, I'm not sure which one is the relevant theory; I'm just offering the examples which illustrate my understanding. I thought it was a more straight forward question than that though; is the Earth rotating or not?

If the answer is that it depends on the reference frame, then my understanding of this is not that "actually rotating" or "actually moving" or "absolute motion" doesn't make sense; it's that we can't tell if the Earth is actually rotating or not, but we can deduce that either the Earth is actually rotating, or everything is rotating around the earth, in an actual sense.


Just on the link you provided; I'm familiar with the Twin Paradox; how it was explained to me was that, in a nutshell, under SR it isn't a paradox because there is asymmetry between the reference frames i.e. one twin undergoes acceleration.

It isn't a paradox according to GR because of the equivalence principle i.e. either gravitation or acceleration resolves the paradox.

Happily you'll by now have defined what you mean with that in the line here above.

It might be helpful to state it in terms of the test of the principle of relativity, as mentioned earlier.

The test says that the absolute nature of motion cannot be determined by a co-moving experiment; I think we can deduce that the nature of the motion has to be absolute.

Where absolute is used in its usual sense of "without reference to anything", so "absolute motion" would be motion without reference to anything.

This can perhaps be clarified with contextual examples:
- can you move i.e. are you capable of movement?
- have you ever been in a moving car?
- have you ever stood still on an escalator and still moved?

Again:
- there is no issue concerning that in either classical mechanics or special relativity
and
- a steadily rotating Earth is very different from a departing train according to those theories.

And again: you do not know the answer of classical mechanics, or do you? Please start it as a question in the forum there, and keep this on hold; for until you understand this pertinent issue that will need a lot of explanation (incl. math!) about some basic concepts of classical mechanics (which are essentially unchanged in SR), without your understanding of the related concepts you will just be wasting everyone's time here. It's not useful to push understanding of motion back to the time of Zeno (sorry if that sounds rude, but by now it really feels like it).

Is it genuinely that complicated a question?

Are you capable of movement?

Honestly, I didn't realize it was.

 

[Mentz114]

The point, which is in keeping with the PoR, is that we can't say which one actually moves, but we can deduce that at least one has to have actually moved.

'The point' has nothing to do with the PoR. Special relativity does not require the history of the frames under consideration. SR is constituted so it does not matter which inertial frame 'actually moved'.

The Lorentz transformation does not have any terms for the 'who actually moved' thing.

I do wish you could see that what you're insisting over and over has nothing to do with relativity.

 

[Michael C]

The point, which is in keeping with the PoR, is that we can't say which one actually moves, but we can deduce that at least one has to have actually moved.

You keep on repeating the same thing. If we can deduce that one has "actually moved", then "actually moved" must have a clear definition. You haven't been able to give one.

The examples should give a contextual understanding, and, as I say, one that most people would have no problem understanding. Does bahamagreen's post #143 clarify things any bit; he seems to have an understanding of what I am trying to get at, as does salvestrom, as indicated by his post #47.

I know that most lay people would understand the examples, and what is meant by actually moving, so I'm not sure what the issue is.

You keep repeating this, but offer no evidence. I sincerely do not think that "most people" would understand what you are getting at, but that is in any case beside the point. Here you are having a discussion with some particular people, most of whom have spent considerable time reflecting on the concepts of relativity and motion. If you can't make your ideas on motion clear to us, you need to have a think about why.

Most people wouldn't necessarily assume a reference frame where the Earth is stationary, they would say the car is moving even when the Earth is rotating and orbiting around the sun; they would acknowledge that the car is moving with the earth, but that the car can subsequently move along the surface of the earth; here, people would say that both the Earth and the car are actually moving, as opposed to not moving at all.

While people don't necessarily think in terms of "reference frames" I wholeheartedly agree that a reference frame is indeed assumed, or implied; but so too is the idea of actual movement.

No, the movement is always thought of with respect to the assumed frame. The idea of "actual movement" without a reference frame simply makes no sense. You could talk about "actual movement" if it was agreed that there was an absolute rest frame, in which case the term would mean "movement relative to the absolute rest frame", but without any reference frame the term has no meaning that I can discern, nor have you been able to define any such meaning.

It's just as nonsensical as saying that the term "distance" has a meaning when applied to one object: you can measure the distance between two objects, or the distance of one object from a specified reference point, but the "actual distance" of a single object is not a meaningful concept.

With the examples you give of relative motion, there are always two possibilities that can account for the observed relative motion; each one attributing the movement to one object or the other.

Example
It might be easier to break from the "everyday" examples and try to give a different one.

Imagine that you are standing on a metal disc, in the middle of a room; the disc can rotate freely 360°; equally, the room and everything else can rotate freely 360°.

When you rotate on the disc it has the exact same effect as when the room starts rotating.

You start to experience relative motion i.e. the spinning, between you and the room; now, either you are spinning, or the room is spinning. It is either one or the other.

We might be able to define a reference frame for both, but that simply reinforces the point that we cannot tell which one is actually spinning; but we can deduce that either it is you that is actually spinning, or it is the room that is actually spinning; it is one or the other - in the simplest form, at least.

Two possibilities which result in the same relative motion.

You're still repeating the same argument. You still don't give your definition of what "actually" means in this context.

If each observer were holding a light clock, as per Einstein's thought experiment, wouldn't the path length of the photon, in the clock, be affected?

Each observer see the other's clock as running slower than their own. The situation is symmetric and does not permit us to define in an absolute sense that one of the clocks as "moving" and the other as "at rest": for each observer, their own clock is at rest and the other clock is moving.

 

[mangaroosh]

The non-fallacious way to say this is "in order for relative motion to occur, one of them has to move relative to the other".

To say it the way you consistently do is to presume the concept of absolute motion before proving that it exists. You cannot then turn around and say "therefore absolute motion exists" since you already assumed it.

Even the language in the above formulation suggests an underlying assumption that the act of moving can be ascribed to one or the other; "one of them has to move", with the qualification "relative to the other".

Would it not be a more accurate representation to say that "in order for relative motion to occur, the objects have to move relative to each other"; while self-consistent, it is merely tautological.


How I am trying to convey my understanding doesn't necessarily assume absolute motion; it offers two explanations for why relative motion can manifest; either one object moves, or the other one does.

To say that the objects move relative to each other because they move relative to each other, doesn't explain why the relative motion occurs. It's logically self-consistent, but it is merely tautological.

 

[mangaroosh]

I'll have more later, but recall that I said that you can say a rocket which fired its engines can be said to have "actually moved" away from earth. But if at some point in the coast phase of its journey it passes another coasting rocket, neither can claim to be the one "actually moving".

Do you understand this?

This I don't understand.

My understanding is that neither can verify that they are the ones "actually moving", but they could deduce that, at least, one of them has to be.

 

[Michael C]

If the plane is infinite, then I'm not sure, tbh. For arguments sake I would say yes, but I'm not sure how the qualifier "infinitely" affects it.

I can see we have a long way to go. The qualifier "infinitely" is important. If I have a finite plane, say in the shape of a square, it's very easy to define the centre of it. How would you go about defining the centre of an infinite plane?

 

[mangaroosh]

Mangaroosh, let me outline your proof for you in a way that may help you see the fallacy.

1) Consider two inertially moving objects, A and B, with relative velocity v.
2) In any inertial reference frame, if one is at rest the other is moving with velocity v.
3) Therefore, at least one is moving relative to any inertial frame.
4) Assume further that there exists some inertial frame, F, such that objects at rest in that frame are absolutely at rest and objects moving in that frame are absolutely moving.
5) Since F is an inertial frame, then by (2) at least one is moving relative to F.
6) Therefore, at least one object is absolutely moving.

The logic simply doesn't work without (4). But (4) is a topic of disagreement, which is why assuming it is fallacious.

That's not necessarily how I've outlined my understanding though.

1) Consider two inertially moving objects, A and B, with a relative velocity of 0.
2) If neither A nor B moves, their relative velocity will remain 0.
3) The relative velocity between A and B subsequently changes to a value greater than 0.
4) Therefore either A or B moved.

The only question is whether or not "A moves" or "B moves" make sense on their own.

The contextual examples given, should demonstrate that they do make sense, because we have relative motion between A and B, and two possible, fundamental explanations; A moved, which resulted in the relative motion; or B moved, which resulted in the relative motion.

If we say that there is relative motion between A and B, because A nd B moved relative to each other, we are just restating the fact that there is relative motion, without explaining how, or why there is.

In SR velocity is relative, and SR is self consistent. So any physical scenario consistent with SR can be stated purely in terms of relative velocities without any logical contradictions. Therefore, you cannot prove that absolute velocity is logically implied.

I'm not sure about absolute velocity, because absolute velocity would be a measured value, and measurement is, by its very nature, relative, not absolute. If something is absolute, or not relative, then it presumably isn't measureable, so I would imagine that it wouldn't have any measurable consequences, but would have deductive consequences.

Actual movement, or "absolute motion" as it has been termed, I think, can be deduced from relative velocity; the self-consistency of SR doesn't preclude that.