At rest in Einsteinian relativity

[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.

 

[harrylin]

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.

OK, probably you already clarified that elsewhere; if so, sorry and thanks for doing it again.

So, with "actually moving" you mean something similar as "true motion" in Newton's model?

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?

As I stated, the answer is different in SR than in 1916 GR; but I think that Einstein was forced to change his mind about it by 1920. So, your term "in Einsteinian relativity" is too poorly defined for a straightforward answer.

[..] Just on the link you provided; I'm familiar with the Twin Paradox; [..] It isn't a paradox according to GR because of the equivalence principle i.e. either gravitation or acceleration resolves the paradox.

In contrast, according to the link that I provided, Einstein's solution of induced real gravitational fields doesn't seem to make much sense to most people.

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

Yes, especially which relativity principle do you refer to? Einstein's general relativity principle which has nearly been forgotten, or the special one as he formulated it?

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.

Certainly not! That expression can be used to mean different things, but not "motion without reference to anything" - one popular modern meaning is instead "motion with reference to all inertial reference systems".

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? [..]

All those expressions relate to relative motions, such as a car relative to the road and the elevator relative to the building. So, I'm afraid that you are indeed bringing this discussion back to the ancient times of Greek philosophers.

 

[mangaroosh]

'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.

It does have to do with learning relativity though; I've been trying to outline my understanding, which I believe I have done in a fairly logical manner. There is some contention over the notion of "actual" or "absolute" motion, but it is something that, to me, seems almost self-evident; two other posters in the thread also seemed to suggest something similar. I do genuinely believe that a large proportion of people would be of the same opinion also.

It seems that my understanding does not correspond to relativity, so I'm trying to see how the issue is addressed through relativity; I don't fully undertsand how the theory can be formulated such that it doesn't matter, because I think there might be certain deductive consequences that probably do matter.

 

[harrylin]

[..]
3) The relative velocity between A and B subsequently changes to a value greater than 0.
4) Therefore either A or B moved.

For the third (or fourth) time, you must mean: Therefore either A or B accelerated. Do you disagree? This is what has been stressed in most of my last posts as well as those of several others. As long as you insist on confounding inertial motion with acceleration, no sensible discussion is possible. Motion is not the same as change of motion.

 

[Mentz114]

It does have to do with learning relativity though; I've been trying to outline my understanding, which I believe I have done in a fairly logical manner.

I can't believe you can think that. All you have done is repeat that you think there is some notion of 'actually moved' without being able to define it.

There is some contention over the notion of "actual" or "absolute" motion, but it is something that, to me, seems almost self-evident;

Where is the contention ? You are misguided if you think absolute motion has supporters on this forum.

It seems that my understanding does not correspond to relativity, so I'm trying to see how the issue is addressed through relativity;

It has no relevance in relativity whatever so you're wasting your time.

I don't fully undertsand how the theory can be formulated such that it doesn't matter,

Learn special relativity and you'll see.

because I think there might be certain deductive consequences that probably do matter.

There aren't any, as even an elementary understanding of SR would tell you.

I think you have to stop waving your hands and get down to defining what you're trying to say in a proper scientific manner.

 

[mangaroosh]

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.

I've never really had to define it before, it was something that I genuinely believed, and believe, is self-evidently true. I was hoping that by giving contextual examples it would become clear what was meant; again, examples I genuinely believed were fairly easy to comprehend.

The easiest way to think about it might be to think about it personally; imagine that you and a friend are standing about 2 feet apart, facing each other. Make a bet with your friend that the first person who moves loses the bet, and owes the other $50. Let's say your friend takes a step to the side, would you be within your rights to claim the $50?

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.

It might be worth aksing a few friends who don't have a background in science; if you are walking with them, ask them if both of you are actually moving; tell them that it isn't a trick question, it's a genuine question. Just see what answer they give.

I have thought about why people are having so much trouble understanding what I believe are very simple ideas; there are a number of reasons, some of which attributable to me, some of which attributable to those that can't understand, and some of which are attributable to the nature of the subject itself.

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.

The idea isn't necessarily "actual movement" without a reference frame; it is inevitbale that movement will take place in a reference frame, it's an unavoidable fact of the universe. The idea is that "actual movement" can be deduced from the motion within that reference frame.

The idea that movement is always thought of with respect to an assumed frame is not, I believe, entirely true; people, I would say, primarily think about the body doing the moving. They believe that they are the agent of the motion, in most cases.

I'm not sure if it clarifies the issue to speak about "the act of moving" i.e. the body which "does the action", as opposed to the passive object.

When walking down the street, there would be relative motion between you and the street, but you would be the active agent in the scenario, while the street would be the passive agent.

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.

While you could measure the distance between two points on a single object, the it's not the same issue at all.

Indeed, distance is a factor in the example; two objects a given distance from each other, then the distance begins to increase; at least one of them has to be an active agent in causing that distance to increase.

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

as the truth or facts of a situation:
we must pay attention to what young people are actually doing

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

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.

I understand that point about ER, but just address the question you asked

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?

It might be more intuitive to say that it is the car that is actually moving.

If we assume that you and the clock are both on the earth, without assuming the nature of motion of the earth, either way; if you have a light clock and the observer in the car has a light clock, then the path length of the photon will have a pre-established path length, depending on the motion of the earth, wouldn't it?

That is, if you and the car were at rest on the earth, then the photon in the light clock would have a path length dependent on the motion of the earth, wouldn't it?


If we assume that the car is actually moving, such that you and the car are moving inertially, relative to each other, then the path length of the photon will be different to the path length of the photon in your clock. Meaning that it will either tick faster or slower, depending on the motion of the earth, no?

 

[mangaroosh]

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?

I was thinking the qualifier "infinitely" was important, but if the plane is in the shape of a square then presumably you could have infinitely long lines disecting the angles of the square, which would naturally meet in the middle.

EDIT: of course, it might make more sense to ask how an infinite plane can have the shape of a square.

 

[Mentz114]

The idea that movement is always thought of with respect to an assumed frame is not, I believe, entirely true;

But it is the basis of special relativity. If you don't accept special relativity you have no business in this forum.

people, I would say, primarily think about the body doing the moving. They believe that they are the agent of the motion, in most cases.

Again, this is not relevant to special relativity.

I'm not sure if it clarifies the issue to speak about "the act of moving" i.e. the body which "does the action", as opposed to the passive object.

When walking down the street, there would be relative motion between you and the street, but you would be the active agent in the scenario, while the street would be the passive agent.

Not relevant to SR. These ideas belong elsewhere.

 

[mangaroosh]

OK, probably you already clarified that elsewhere; if so, sorry and thanks for doing it again.

No hassle at all; I don't think I mentioned it to yourself, I think it came up with Dale, but I wouldn't have stressed the point because I didn't think it wasn't aware that it might be an issue, and thinking about it, I don't think it should be.

So, with "actually moving" you mean something similar as "true motion" in Newton's model?

Unfortunately I can't say, I'm not overly familiar with Newton's model; my familiarity extends only as far as the idea that there is an absolute referecne frame against which absolute, or perhaps, true motion can be measured.

That would appear to be a contradiction in terms, to me though, to suggest that absolute motion is relative.

I'm not sure if it helps to clarify the difference between an active and a passive agent, in relative motion; for example, if you are walking down the street, there would be relative motion between you and the road, but you would be the active agent i.e. "doing the moving".

As I stated, the answer is different in SR than in 1916 GR; but I think that Einstein was forced to change his mind about it by 1920. So, your term "in Einsteinian relativity" is too poorly defined for a straightforward answer.

Apoloigies.

Just in the context of the question "Is the Earth rotating or not?", do the two theories provide different answers; what is the contemporary answer; and what is your own thinking on it?

In contrast, according to the link that I provided, Einstein's solution of induced real gravitational fields doesn't seem to make much sense to most people.

I don't doubt that.

Yes, especially which relativity principle do you refer to? Einstein's general relativity principle which has nearly been forgotten, or the special one as he formulated it?

I was referring more to the test of relativity, as it is often presented, and the often stated consequences.

Certainly not! That expression can be used to mean different things, but not "motion without reference to anything" - one popular modern meaning is instead "motion with reference to all inertial reference systems".

I was actually thinking about that, and was thinking that if something does actually move then presumably all reference frames will label it as moving, unless something else is moving inertially to it; this would bring us back to the question of which one is moving, but again, I think we could deduce that it must be one or the other.

The adjective, "absolute", however, which qualifies the noun, does mean without reference to anything; it would presumably be a kind of fundamental motion; again, a "yes or no", or "either, or" question.

I was wondering about the idea of inertial reference systems actually, and was wondering how you could determine if a system is inertial; presumably there will always be an accelerating reference system; and presumably it would always be possible to define a reference frame in which it isn't accelerating, where the accerlationg is attributed to every other object in the universe.

All those expressions relate to relative motions, such as a car relative to the road and the elevator relative to the building. So, I'm afraid that you are indeed bringing this discussion back to the ancient times of Greek philosophers.

None of them make reference to relative motion; they simply make reference to motion i.e. it is the car that is moving.

 

[mangaroosh]

For the third (or fourth) time, you must mean: Therefore either A or B accelerated. Do you disagree? This is what has been stressed in most of my last posts as well as those of several others. As long as you insist on confounding inertial motion with acceleration, no sensible discussion is possible. Motion is not the same as change of motion.

I would agree that either A or B accelerated.

But I would still think we can deduce that either A or B has to actually, or absolutely, be moving, even from inertial, relative motion.

 

[HallsofIvy]

Just in the context of the question "Is the Earth rotating or not?", do the two theories provide different answers; what is the contemporary answer; and what is your own thinking on it?

Rotation involves acceleration, it is not a constant velocity.

I don't understand why, after 189 posts, you are still simply asserting "
But I would still think we can deduce that either A or B has to actually, or absolutely, be moving, even from inertial, relative motion." Even though you have been repeatedly told that is NOT true and repeatedly told why it is not true.

 

[mangaroosh]

I can't believe you can think that. All you have done is repeat that you think there is some notion of 'actually moved' without being able to define it.

I have repeatedly tried to give contextual examples that a lot of people would have no trouble understanding. It might be possible to drop the contentious term "actual movement" and try and deduce the answer in another way.

Make a bet with your friend; stand opposite each other, facing one another, a short distance apart; make the provision of the bet that the first person to move owes the other $50. Do you think that it is possible for anyone to win the bet?

Where is the contention ? You are misguided if you think absolute motion has supporters on this forum.

The contention is manifest in this thread. Also, salvestrom's post earlier in the thread (#47 I think it was) and bahamagreen's comments seem to suggest that they at least understand the contention being made.

It has no relevance in relativity whatever so you're wasting your time.

I would presume that something which potentially has deductive consequences, has, at least, some relevance to relativity.

Learn special relativity and you'll see.

Contrary to popular belief, I am learning about relativity; there is more than one way to skin a cat.

I understand that the theory is formulated with the idea that it doesn't matter, but I struggle to see how it can be formulated such that something which might have deductive consequences doesn't matter.

There aren't any, as even an elementary understanding of SR would tell you.

I think you have to stop waving your hands and get down to defining what you're trying to say in a proper scientific manner.

I'm not the one waving hands here. The examples I've given are very straight forward, and would undoubtedly make senes to an awful lot of people. I would genuinely suggest asking someone from a non-scientific background, and perhaps even people from a scientific background, whether or not actually moving makes sense.

Try going for a walk, and ask them if they are actually moving; tell them it isn't a trick question, tell them it is a straight forward question and you're looking for a straightforward answer; just to see what they say.

Genuinely, I'm not trying to be elusive, I've never actually had to try and explain this before, so that may be why I am struggling to clarify it sufficiently, but I think it is a fairly intuitive - not that that makes it right - idea.

Again, the clarification of an active and passive agent might help some bit; as you are walking down the road there will be relative motion between you and the road; you, however, would be the active agent, while the road, the buildings etc. would be passive agents. If you pass someone walking the other way, they too would be active agents in the relative motion between you and them.

Hopefully that helps to clarify it some bit.

 

[mangaroosh]

But it is the basis of special relativity. If you don't accept special relativity you have no business in this forum.

Woudl you say that I understand relativity?

I'm guessing the answer is no.

In that case, I'm not sure how I can possibly accept something I don't understand; if I were to accept it, then the thing I would be accepting wouldn't be relativity, it would be a misunderstanding of it.

I'm here to learn about relativity, to explore it logically and rationally; once I understand it and if I find it stands up to reason then I can accept it; presumably you don't accept things you find don't stand up to reason.

Again, this is not relevant to special relativity.

But it is relevant to learning relativity, becaues it represents an existing belief that has to be unlearned.

Not relevant to SR. These ideas belong elsewhere.

Are issues with potential, deductive properties not relevant to SR?

 

[Mentz114]

I have repeatedly tried to give contextual examples that a lot of people would have no trouble understanding. It might be possible to drop the contentious term "actual movement" and try and deduce the answer in another way.

And repeatedly failed to demonstrate why this has any relevance to a theory based on relative velocity.

Make a bet with your friend; stand opposite each other, facing one another, a short distance apart; make the provision of the bet that the first person to move owes the other $50. Do you think that it is possible for anyone to win the bet?

That is just silly. You are moving the goalposts. We are talking about scientific definitions involving velocities.

The examples I've given are very straight forward, and would undoubtedly make senes to an awful lot of people. I would genuinely suggest asking someone from a non-scientific background, and perhaps even people from a scientific background, whether or not actually moving makes sense.
Try going for a walk, and ask them if they are actually moving; tell them it isn't a trick question, tell them it is a straight forward question and you're looking for a straightforward answer; just to see what they say.

Physics is not a democracy.

Genuinely, I'm not trying to be elusive, I've never actually had to try and explain this before, so that may be why I am struggling to clarify it sufficiently, but I think it is a fairly intuitive - not that that makes it right - idea.

You won't be able to explain it better. There is no scientific way of defining a useful concept that requires absolute motion.

Again, the clarification of an active and passive agent might help some bit; as you are walking down the road there will be relative motion between you and the road; you, however, would be the active agent, while the road, the buildings etc. would be passive agents. If you pass someone walking the other way, they too would be active agents in the relative motion between you and them.

The 'active/passive' notion can only be scientifically expressed as 'non-inertial/inertial' because there are no other states of motion.

 

[mangaroosh]

Rotation involves acceleration, it is not a constant velocity.

Is it not possible to rotate at a constant velocity, no?

I don't understand why, after 189 posts, you are still simply asserting "
But I would still think we can deduce that either A or B has to actually, or absolutely, be moving, even from inertial, relative motion." Even though you have been repeatedly told that is NOT true and repeatedly told why it is not true.

And I've sought to address the whys, and the discussion has grown organically. I could just as easily say that I've explained why it is true, but I recognise the fact that it is not as straight forward as I thought it might have been.

The reasons being put forward for why my understanding is wrong, is to say that motion can only ever be relative to something; I have outlined that in each case of relative motion there are, at least, two possible scenarios which can account for the relative motion. Each one involves one or another object actually moving.

The response seems to be that relative motion occurs because objects move relative to each other; that just seems to be a tautology which allows us to posit the two different scenarios again.

Bet
Just to address the bet scenario to yourself; if you stand face to face with a friend, and make a bet with the provision that whoever moves first owes the other $50; do you think it is possible for anyone to win the bet?

 

[mangaroosh]

And repeatedly failed to demonstrate why this has any relevance to a theory based on relative velocity.

Apart from having potential deductive consequences, which I presume would be relative to any theory, it is relevant to the process of learning relativity, because it represents an existing belief that would have to be unlearned, or sufficiently rationalised, to allow for the assimilation of information pertaining to relativity theory.

Conflict between existing beliefs and new ideas are one of the main stumbling blocks in the learning process.

That is just silly. You are moving the goalposts. We are talking about scientific definitions involving velocities.

How is it moving the goalposts; it's simply a continuation of the existing discussion but taking a different approach, because the current approach seems to be going round in circles.

Again, it is a fairly simple scenario which shouldn't really cause that much difficulty to answer, but might allow us to make certain deductions.

Physics is not a democracy.

I didn't think it was, although it was suggested earlier that definitions are formed on the basis of consesus.

Physics is an approach to explaining empirical experiences, and such a walk would represent an empirical experience.

You won't be able to explain it better. There is no scientific way of defining a useful concept that requires absolute motion.

That may explain why it is so difficult to explain something which is, for many people, self-evidently true; or at least, which appears to be.

The 'active/passive' notion can only be scientifically expressed as 'non-inertial/inertial' because there are no other states of motion.

Active and passive are farily well understood terms; they could easily be used as adverbs to describe a type of non-inertail/inertial motion.

 

[mangaroosh]

Wanna bet?

As is abundantly clear to all here, the discussion appears to be going round in circles. For that reason I just wanted to put something out for general consideration, as opposed to the individual discussions that have been going on. It might help to clarify the issue, it might not, but here's hoping. I've referred to it in a few posts to individuals already, but in the hope to maybe resolve the issue, I just want to post it as a general question.

The bet
If you are standing face to face with your friend, and you make a bet, with the proviso, that the first person to move owes the other $50; do you think it is possible for anyone to win the bet?

 

[Mentz114]

Apart from having potential deductive consequences,

You like this phrase "deductive consequences" but it is empty if you cannot demonstrate one.

... it is relevant to the process of learning relativity, because it represents an existing belief that would have to be unlearned, or sufficiently rationalised, to allow for the assimilation of information pertaining to relativity theory.

Physical theories don't require belief. They stand or fall on their internal consistency and agreement with experiment.

Conflict between existing beliefs and new ideas are one of the main stumbling blocks in the learning process.

In your case more of a towering wall. Would you go on a maths forum and declare you are learning arithmetic but don't understand what numbers are ?

I think you should ask for this thread to be moved to the philosophy forum. There are lots of clever people there who might grasp what mere physicists cannot.

 

[phinds]

If you are standing face to face with your friend, and you make a bet, with the proviso, that the first person to move owes the other $50; do you think it is possible for anyone to win the bet?

If they don't understand physics, yes, because they don't understand what they are ACTUALLY betting on, as you clearly do not.

If they understand physics, no.

 

[Dale]

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?

Yes. Rotation is non-inertial motion, and it can be measured without respect to any external body simply by using an accelerometer. Furthermore, if two objects are rotating relative to each other, it is possible to experimentally identify which of the two is rotating. Rotation is empirically different from inertial motion, so examples showing that rotation is absolute will not show that inertial motion is absolute.

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?

That is called proof by contradiction, or proof by counter-example. However, pointing out a flaw in a proof is not itself a proof by contradiction. Pointing out a flaw in a proof simply invalidates the proof, but does not give any information about the truth of the conclusion. I.e. you may use faulty logic in an attempt to prove a true proposition.

Interestingly, there is a fallacy called the fallacy fallacy, which is one of my favorite fallacies. Basically, the fallacy fallacy is when someone makes the claim that the conclusion of a fallacious argument is false because the argument is fallacious.
http://www.fallacyfiles.org/fallfall.html

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).

I understand your point. The problem isn't a communication problem, it is a logic problem. You have clearly "illucidated" your erroneous reasoning. Further contextual examples serve no purpose.

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 can appreciate the difficulty of learning SR, it took me 7 years of occasional study. However, in my experience working with other people to learn it is clear that a person who continues to argue an incorrect point (particularly one which has been clearly shown to be incorrect) is either unwilling or unable to learn until they change their attitude. For me, that was the case, and I have seen it other times. My motivation for changing my attitude was to read and understand the overwhelming experimental evidence:
http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

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.

I am looking forward to your subjecting SR to rational enquiry. So far, I have seen only evidence of personal biases and prejudices supported by fallacy. And despite the fact that the fallacious reasoning has been clearly identified you seem to cling to it anyway. It makes it quite difficult to believe that you are really interested in rational enquiry instead of personal validation.

 

[Michael C]

I was thinking the qualifier "infinitely" was important, but if the plane is in the shape of a square then presumably you could have infinitely long lines disecting the angles of the square, which would naturally meet in the middle.

If the plane is a finite square, there's no problem finding the middle. But what does an infinite square look like? Where are the corners? I thought I'd hit on a good analogy here but I'm afraid it's only raising more questions.

My recommendation: you really need to learn more about basic concepts in maths and classical mechanics before adventuring into relativity.

 

[Michael C]

The bet
If you are standing face to face with your friend, and you make a bet, with the proviso, that the first person to move owes the other $50; do you think it is possible for anyone to win the bet?

This is getting really silly. A commonplace, imprecise use of the word "move" does not in any way help us here. In this case it will probably be assumed that the verb "move" applied to a person means something like "produce a discernible relative motion between different parts of the body", but that really has no bearing on the present discussion.

 

[Dale]

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".

Exactly. With the qualification "relativie to the other" it is clear which one the motion is ascribed to.

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.

Correct, it is tautologically true and self-consistent. That is the reason you cannot use the existence of relative velocity to prove that absolute velocity exists. You have to make the fallacious assumption of absolute velocity, otherwise you are simply left with a tautological truth about relative velocity.

As to "why" the relative motion occurs, that is a different question. Either it is a given boundary condition in the scenario or it occurs because of some dynamics which are a result of the given boundary conditions in the scenario.

 

[Dale]

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.

This is non-inertial motion, so this scenario is not relevant to the dispute.

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.

No. Something which is absolute is agreed upon by all reference frames. I.e. it is frame invariant. There are many absolute or frame invariant quantities which are measurable. E.g. proper time, proper acceleration, spacetime interval, invariant mass, etc.

EDIT: note, this usage of the word absolute may not be standard. Others believe that "absolute" is not the opposite of "relative" but rather refers to quantities in a preferred frame. In which case proper acceleration (including rotation) would not be "absolute", but would instead be frame invariant. I am looking for a good reference on the term "absolute".

 

[Dale]

The bet
If you are standing face to face with your friend, and you make a bet, with the proviso, that the first person to move owes the other $50; do you think it is possible for anyone to win the bet?

Again, this is non-inertial motion. It is not relevant to the disagreement.

 

[harrylin]

I would agree that either A or B accelerated.

But I would still think we can deduce that either A or B has to actually, or absolutely, be moving, even from inertial, relative motion.

Obviously, IF absolute motion exists, THEN at least one of the two has to be in absolute motion. There is nobody who has any issue with that logic. However, by far not all people have that postulate; and the mere fact that A and B are moving relative to each other, doesn't tell you anything else.

It's just as with creation: if we observe different species, we could deduce that one of them must have been created before the other - IF we base our reasoning on a certain hypothesis of creation.

But surely all these things have been explained to you many times by now. What is left to discuss?