"at rest" in Einsteinian relativity

mangaroosh

wiki - tests of SR
This doesn't make any reference to internal or external experiments, rather, co-moving experimental arrangements.The specific example you're referring to is, I think, Galileo's observer on the ship, isn't it?

When we talk about determining an observers state of motion, we refer to the idea of determining whether they are "in motion" or "at rest". This allows us to restate the above test of SR as "any uniformly moving observer in an inertial frame cannot determine if he is "in motion" or "at rest" by a co-moving experimental arrangement".

Am I right in saying that the equivalence principle extends this to accelerating reference frames, because the observer cannot determine if they are at rest in a zero gravity field, or free-falling in a gravitational field?

mangaroosh

The only link I can find is
wiki - tests of SR

My understanding of it though extends beyond just this, and comes from discussions of the idea, elsewhere.

As mentioned to Russ, when talking about determining absolute motion we can refer, as Galileo did, to determining "motion" or "rest". Such that the test can be restated as "any uniformly moving observer in an inertial frame cannot determine if he is "in motion" or "at rest" by a co-moving experimental arrangement".

I have been lead to believe that the equivalence principle extends this to accelerating reference frames/observers.

I don't think it makes any reference to statements about velocity.


Does this mean that he can only ever define a co-ordinate system in which he is "at rest"?

It was mentioned that he should be possible to define a reference frame where is traveling at, or close to the speed of light, for example; how is this possible if he measures velocity relative to himself, where the velocity will always be zero?

Relating back to the example of Albert & Henry; Albert should be able to define a reference frame in which he, not Henry, is traveling at 0.6c. How can he do this if he measures the velocity relative to himself?

Arbitrarily choosing to measure velocity relative to Henry doesn't seem like a reasonable answer, because even when he chooses to measure velocity relative to himself, he still has velocity relative to Henry, meaning that both are simultaneously true (in Albert's own frame of reference).

If Albert ascribes the velocity to himself, then he must assume that the photon in his light clock is imparted with the horizontal velocity that he ascribes to his train (or platform). This means that his instruments must contract for him to measure the speed of light to be c.

The opposite would be true if motion is ascribed to Henry.

Mentz114

Sadly, you're back. In spite of everything you've been told you still write the meaningless phrase "traveling at, or close to the speed of light". For the umpteenth time - velocities are relative. Do you understand the meaning of 'relative' ?

Suppose we have an observer, somewhere in the cosmos who experiences no proper acceleration ( i.e. is in uniform motion). All around there are other objects. Our observer has a relative velocity with respect to all of these objects. Relative velocity is a pair relationship. It is necessary to give a pair whenever you say "moving at this or that velocity". Another example of a pair relationship is distance. Distance is defined between objects and is meaningless unless two points are specified. The distance between an object and itself is always zero, as is the relative velocity between an object and itself, because this distance is not changing. Therefore you are always at rest wrt yourself.



Perhaps you believe in absolute velocity - which is a crackpot idea in my opinion.

mangaroosh

:facepalm:

are you familiar with the idea of implied meaning; saying that it is possible to define a reference frame in which you are traveling close to the speed of light implies that it is relatvie to something. I was going to try and dig out a reference from a discussion on here that made that exact point, and I'm pretty sure there was no mention of relative to anything, because the meaning was implied; but frankly, it isn't worth the effort.

We don't have to talk about traveling at the speed of light though, we can talk about Albert and Henry moving relative to each other, where the relative velocity is, say, 0.6c


Supposedly Albert should be able to define a reference frame where he is traveling at 0.6c; how is this possible if he measures the velocity relative to himself, where he will always measure the velocity to be 0?

Also, Albert can determine his motion relative to himself or anythign else, by means of a co-moving experiemtnal arrangement e.g. using a radar gun. This, however, is not the test of the PoR as referenced above, which states that he cannot determine his "absolute motion" by means of a co-moving experimental arrangement.

To say he cannot determine his absolute motion means he cannot determine if he is "in motion" or "at rest"; what does this "at rest" refer to, becauase he can determine his rest relative to himself and other objects.


Also, are you suggesting that two relatively moving observers can determine which one is actually moving i.e. that they can determine their absolute motion by means of an experiment?


As for absolute velocity, I'm not sure, but I would think that that might be a contradiction in terms, because velocity, by its very nature is relative, not absolute. I think we can deduce, however, that there must be absolute motion, because if there wasn't then there would be relative motion.

Michael C

He can define any coordinate system he chooses. If I define a reference frame that is moving at speed s with respect to me, then I am moving at speed s in the opposite direction with respect to that reference frame.

russ_watters

Great, so do you acknowledge that that doesn't match your previous wording? The wiki quote is about "absolute motion". Your formulation of the PoR did not include the word "absolute". It makes a big difference. The PoR doesn't say you can't measure a velocity RELATIVE to another object/frame.

Learning science is not a word game and our patience is wearing thin. You need to choose - rapidly - if you really want to learn what people are trying to teach you or not.

russ_watters

Even if the issue of absolute vs relative wasn't the key point of discussion in this thread, clarity is a necessary component of discussion: Be clear.

DaleSpam

So that link specifically mentioned the idea of a co-moving experiment, meaning some physical measurement of velocity where the experimental apparatus to measure the observer's velocity is also moving at the same speed and in the same direction as the observer. In such a case, the experimental result will always be the same (0) regardless of if the observer and apparatus are considered "moving" or "at rest". Is that clear now?

Huh? The terms "moving" and "co-moving" are clearly statements about velocity.

No.

By measuring his velocity relative to something other than himself.

Why not? Arbitrarily choosing to measure velocity relative to Henry is no more nor less reasonable than arbitrarily choosing to measure velocity relative to Albert.

Yes. Lengths are relative, just like velocity.

darkhorror

A frame of reference where the object's v = 0.

You are trying to make this too complicated.

russ_watters

Let me be as simple and clear as possible and see if we can end this:

"At rest" is an arbitrary choice anyone can make by choosing a reference frame other than himself and choosing to define either himself or that other reference frame as the "at rest" frame, then measuring the speed between the two frames. A statement about being at rest must include a definition of the two frames. For example, there are two common methods the police use to catch speeders (hopefully the existence of three potential frames won't confuse the issue...):

1. The police officer stands at the side of the road with a radar gun. He is standing still so he can tell he is "at rest" relative to the Earth and he thus declares himself to be "at rest", with the cars on the road moving relative to him. Thus, when he sees the measurement of 60mph on his radar gun, he says: "That car was moving at 60mph"....relative to me, who I have arbitrarily declared to be "at rest" in this system.

2. The police officer is driving in his car, following you. With his eyes, he maneuvers himself behind you at a constant distance so you are "at rest" relative to each other. He then reads his speedometer and declares that both of you are "moving" at 60mp relative to the earth.

yuiop

Sorry to be picky, but the police officer cannot be both "at rest" relative the Earth and at the same time "at rest" relative to the cars on the road, if the cars on the road are moving relative to the Earth and himself.

russ_watters

Oy, awful error. Fixed now, thanks.

bahamagreen

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

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

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

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

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

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

Austin0

Hi bahamagreen brilliant. I think you have have described the logic in a coherent form.
If not exactly the OP's thought process, it is still a clever exercise in logic whether valid or not.

A similar chain might be:
If Reality describes that which does actually exist.
And
Fantasy describes that which does not actually exist.
And
The state of actual existence, or not, is indeterminable
Then
Fantasy is indistinguishable from Reality.

DaleSpam

Or the only fantasy which is consistent with experimental evidence is one which is indistinguishable from reality.

bahamagreen

Thanks, AustinO... The OP's language on some of these points is a little arcane and I was getting frustrated that the heavy lifters here were being thrown off his train of thought; hope I captured the foundation of his position.

I like your interpretive restatement and DaleSpam's corollary extension to it very well.

If the OP will confirm or clarify that it is this epistemological indistinguishably between the existence of an undetectable absolute frame and the non-existance of an absolute frame which underlies his basis for "rest", "actual movement", and "deeper meaning", then we will have something to work with. Looking forward to seeing his and others' thoughts on this.

Mentz114

This is sophistry. There is no detectable absolute frame, no absolute motion and any argument that deduces "Therefore the inference of "actual" motion still holds when there is no absolute frame." is wrong.