At rest in Einsteinian relativity

[Chestermiller]

I think some of you are being a little hard on the OP; you have the advantage of having studied relativity to the point where it seems "normal".

Most of what has been discussed is not restricted exclusively to relativity. It applies equally well to pre-relativistic Newtonian physics. So the complexities of relativity, for the most part, are not involved here.

Chet

 

[mangaroosh]

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

I presume it couldn't but have deductive consequences.

If we consider the scenario of two observers at rest relative to each other, both with light clocks; the path length of the photon in the light clock would be the same for both observers; if one of them "actually moved", then the path length of the photon would presumably have to change, if it is to travel between the midpoints of two mirrors in a light clock; either increasing or decreasing, depending on the starting motion and the subsequent movement.

This would mean that only one clock would actually be time dilated, wouldn't it i.e. both observers would not see their counterparts clock ticking slower?

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

While I think this is somewhat of an idealised notion, it doesn't really address a critical issue in the learning process, which does indeed involve peoples existing beliefs.

And a discussion on the nature of beliefs and their role within physical theories would definitely be a discussion for the philosophy forum, I think.

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?

If there were arithmetic claims being made which were based on an interpretation of numbers, which ran counter to my current understanding of numbers, then I probably would have to.

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.

I'll leave the moderating to the moderators; this is a question which pertains to expanding my understanding of relativity, so I'm not sure it is essential to move it; but again, that isn't my decision to make.

 

[mangaroosh]

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.

I get the idea that we can define a reference frame by arbitrarily choosing an object to label as being "at rest", and I get the idea that we can't determine the absolute nature of the motion of any observer - which appear to be extensions of the same idea.

For that reason I would say that they cannot determine who wins the bet, but my experience of the world leads me to believe that they could deduce that, at least one of them has to have won.


On a side note, and by no means an attempt to logically validate the point, I asked a number of people, "randomly", as we were walking, or on an escalator, whether they thought we were "actually moving" and they thought I was aksing a trick question; when I told them it was straight forward, they didn't hesitate to answer; so it does make sense to some people, on some level at least.

 

[harrylin]

[..] If we consider the scenario of two observers at rest relative to each other, both with light clocks; the path length of the photon in the light clock would be the same for both observers; if one of them "actually moved", then the path length of the photon would presumably have to change, if it is to travel between the midpoints of two mirrors in a light clock; either increasing or decreasing, depending on the starting motion and the subsequent movement.

This would mean that only one clock would actually be time dilated, wouldn't it i.e. both observers would not see their counterparts clock ticking slower? [..]

That's an entirely different argument that is, I guess, based on a "realistic" assumption about the nature of light propagation; and if you didn't - as you do here again - continue to confound acceleration with velocity, you could perhaps "build a case". If you like, we could perhaps find peer reviewed articles for you that discuss such arguments.

[..] whether they thought we were "actually moving" and they thought I was aksing a trick question [..]

Next time, ask them relative to what they think that they are "actually moving" and likely they will answer "relative to the building"/"relative to the street".

 

[mangaroosh]

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.

Cheers, I wasn't aware of that.

It's impossible to reach an inertial speed of rotation so? Is there a section of PF which would be better for learning about why that is the case?

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

Is there another name for the fallacy fallacy that takes an indefinite pronoun (the fallaciers fallacy or something like that?) I've come across it before, but thought it had a slightly different name.

It's interesting indeed, because the question is which one of us is committing 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.

Ultimately I think it is a communication problem, because I believe that a word has a certain, possible, meaning or implication, which you, and others, don't seem to agree with.

Ultimately logic stops short because we are looking to the physical world for validation of that meaning, one way or the other; I, and some others, would say that the physical world validates the meaning I think can be implied, while yourself and others suggest that it doesn't.

That is where the contextual examples come in.

Ultiately, the point I am making is that there are two causes which can give rise to the measured relative motion between two observers/objects.

The reasoning being given, as to why this isn't the case, appears to be tautological, which just leads back to the possible alternative explanations.

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 don't think the point has been shown to be incorrect at all; the issue ultimately comes down to how we interpret our experience of the physical world. There are some who have indicated in this thread that they might have a similar understanding to myself, as far as this question goes. I know from talking to other people, outside of this forum, that they also have a similar understanding.

Part of the issue is that I understand the idea that we cannot determine the absolute nature of motion, and that we can define reference frames which reflect this; I just think we can make a further deduction from relative motion, which is that, while we cannot determine the absolute nature of motion, we can determine that motion must be absolute.

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.

The human condition is such that we are all going to be guilty of personal biases and prejudices, but that doesn't necessarily invalidate what it is we are biased in favour of. It is possible to believe something which is correct, just as it is to believe something which is incorrect. There can also be the same level of attachment to beliefs about things which are true; such attachment can affect how we engage with people when they appear to challenge those beliefs.

But, as I said, my empirical experience of the physical world leads me to believe that we can make a further deduction about relative motion, other than that we cannot determine the absolute nature of it. I know that I'm not alone in that thinking, so it ultimately comes down to interpretation of experience. I believe that my reasoning is perfectly logical, but you don't seem to believe it is, because you don't share the belief about the connotation of the word "move".

If there is no means to resolving that issue, then I can probably proceed in my attept to expanding my understanding of relativity with that question shelved.

 

[mangaroosh]

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.

Indeed, this is why I said I'd say yes for the sake of argument; I presumed that you thought I was advocating the idea of absolute positions, which I wasn't, because I'm not really familiar with the idea. I was interested to see where the argument was going though, becaus it would have served to develop my understanding of same.

I am, however, familiar enough with the basic concept of a square to know that if you disect it's angles, on such a plane as you mentioned, then, where the lines cross will mark the centre.

I thought the idea of an infinite square was a bit bizarre alright, but I presumed there might be some mathematical formulation, or explanation that I wasn't aware of, that would attempt to validate putting finite boundaries on an infinite plane.

 

[mangaroosh]

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.

I think a commonplace use of the word is very relevant, because our language intrinsically reflects our experience, or interpretation of our experience, of the physical world. I also dont' think it is that imprecise, it's just that what it refers to does not lend itself easily to definition.

I don't see why relative motion between different parts of the body has no bearing on the disucssion; if it pertains to relative and absolute motion, then it presumably does.

 

[mangaroosh]

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

Indeed, the motion is ascribed to one of the objects in an active sense, while the other is passive; this results in relative motion.

The alternative, where the other object is active and the first object is passive, would also give rise to relative motion.

A third alternative is where both are actively "in motion", which would also result in relative motion.

Relative motion wouldn't occur if neither was actively "in motion".

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.

I think the issue is more fundamental than that; it comes down to the interpretation of empirical experience.

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.

Given the boundary conditions, which I am presuming means something along the lines of "the train driver starts the train and pushes the accelerator", I still think the deduction is possible.

Again, however, it's a qustion that might have to be shelved if there is no resolution to it.

 

[mangaroosh]

Mangaroosh,

I have an experiment for you to perform. Get in your car and go out on a long, straight, smooth road. Set your cruise control to a constant speed, say 100 km/hr. Now try to imagine that, rather than you and your car moving along the highway relative to the surrounding landscape, your and your car are stationary, and it is the highway and surrounding landscape that are moving backwards relative to you. I've done this experiment many times on the New Jersey Turnpike. It's a weird feeling, but I know you can do it. Now try to think of an experiment you can do within the cabin of your car to prove that you are the one that is moving at constant speed in a straight line, and not the surrounding landscape. You can't do it. You and your car are at rest relative to one another. The trees and houses and highway are at rest relative to one another. But you are moving at a constant speed in a straight line relative to the landscape, and the landscape is moving at constant speed in a straight line relative to you. That's basically all that these guys are trying to tell you.

Chet

I wasn't going to reply to any more posts tonight, because it's getting late here, but this is one that I think represents the issue at hand.

Firstly Chet, I've been meaning to get back to your PMs, but I only seem to find a limited amount of time on here, and I tend to spend it replying to threads; I'll hopefully get a chance fairly soon.

I've tried something similar here, except I wasn't in a car; I tried it while walking; I've also tried it on an escalator but I think it would probably be better to try it in a car, over a longer distance; because the brain would have more time to make the switch, which is probably necessary.


"the rub"
The thing is, I don't have trouble with the fact that no experiment can be conducted to determine if you are the one that is moving at constant speed in a straight line; or if you and your car that are stationary, while the highway and surrounding landscape are moving backwards relative to you.

The point I am trying to make is that it is surely, at the very least, one or the other. That is, in the simplest form, - which is all that is needed to make the deduction - it is either you and your car which is moving forward in a straight line, or it is the highway and surrounding landscape that are moving backwards.

A reference frame can be defined to reflect both scenarios, but surely it must be one or the other, at the very least?

 

[Dale]

I just think we can make a further deduction from relative motion, which is that, while we cannot determine the absolute nature of motion, we can determine that motion must be absolute.

So then provide a logically-sound proof that demonstrates that "motion must be absolute". You assert that such a deduction can be made, so show the chain of deductive logic that you assert exists. A deduction cannot involve a fallacy.

I believe that my reasoning is perfectly logical, but you don't seem to believe it is, because you don't share the belief about the connotation of the word "move".

Your reasoning is self-consistent, which is probably why you feel it is logical. If you assume that motion must be absolute then you can easily prove that motion must be absolute.

However, since the assumption that motion must be absolute is controversial it cannot be used in a proof without committing a fallacy. You have yet to demonstrate that you can conclude that motion must be absolute without assuming that motion must be absolute at some point.

I can probably proceed in my attept to expanding my understanding of relativity with that question shelved.

I hope so.

 

[Dale]

The alternative, where the other object is active and the first object is passive, would also give rise to relative motion.

I don't think that bringing in the terms "active" and "passive" helps at all.

 

[Michael C]

Indeed, this is why I said I'd say yes for the sake of argument; I presumed that you thought I was advocating the idea of absolute positions, which I wasn't, because I'm not really familiar with the idea. I was interested to see where the argument was going though, becaus it would have served to develop my understanding of same.

So are you happy with the idea that there is no such thing as absolute position?

If we have an infinite plane, there is no particular point that can be said to be at the centre. We can define a coordinate system centred on any chosen point, and call this point "centre" for all calculations done using that particular coordinate system. Does this seem reasonable to you?

 

[mangaroosh]

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

Can it not be extended to inertial motion also, because an object can accelerate and then plateau at an inertial speed?

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".

My understanding of the term absolute is not necessarily "agreed upon by all reference frames", it is more fundamental, and usually implies true, in an of itself, without being relative to anything else. I think that is a fairly standard definition of the term.

 

[mangaroosh]

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?

I think we can deduce that one creature must have been created before another, regardless of our hypothesis of creation; the alternative is that they were all created at the same time and didn't evolve.


It's clear that not all people have that postulate, but I think there is a huge majority of the worlds population that, if questioned, probably would. I know that is neither here nor there, but might be food for thought.

When objects A and B are moving relative to each other, I think it does tell us more than just the fact that they are moving relative to each other. I've explained it many times myself, but it ultimately seems to come down to an individuals interpretation of their empirical experience.

 

[mangaroosh]

Not your understanding, your belief. That's what this ultimately comes down to. The POR is saying that "actually moving" has no meaning in that context. You choose to believe that either the POR is wrong or that no one but you understands it.

My "belief" and my understanding are pretty much one in the same; my understanding is what leads to the formation of my belief.

I'm not sure that it can be deduced, from the experimental test of the PoR, that "actually moving" has no meaning; all that seems to be deducible is that it cannot be determined which object is "actually moving" i.e. the absolute nature of motion cannot be determined. For all we know, we could be at absolute rest, we just cannot determine if we are or not. For that reason I don't think it can, legitimately, be claimed that the PoR says that it has no meaning; or more pointedly that such can be deduced from the experimental test of the PoR.

 

[mangaroosh]

Mangaroosh, given some physical object X do you believe that the phrase "X is moving" give any physical information other than that which is already implied simply by the fact that X is an object?

Yes, I think it suggests that the object X is in motion, in an absolute sense.

If there were a light clock present with X, then we could deduce that the path length of the photon, in the light clock, changes with X's movement.

 

[russ_watters]

That's enough. In these last two posts, you've restated your incorrect reading of the PoR, which should have been corrected in the first page and your belief in absolute motion, which you choose to just believe regardless of evidence/logic. Thus, there is nowhere productive that this thread can go/should have aready if it ever would. All the answers you need/are possible have already been provided. If you ever decide you want to learn the concept, go back and reread the thread and you will - should take you all of 5 minutes once you've made the choice. Otherwise, you're just wasting our time here.

Thread locked.