Why would I think I'm not moving?

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Discussion Overview

The discussion revolves around the concept of motion in the context of Special Relativity Theory (SRT), particularly focusing on why an observer in an inertial frame would assume they are not moving. Participants explore the implications of this assumption, the relativity of simultaneity, and the nature of absolute motion.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants question why an observer, such as a passenger on a train, would assume they are not moving despite feeling acceleration and knowing their motion relative to the Earth.
  • Others argue that SRT allows for the assumption of no absolute motion, suggesting that both observers in relative motion can consider themselves motionless without contradiction.
  • A participant points out that if both observers assume they are motionless, at least one must be incorrect, raising questions about the nature of motion and reference frames.
  • Another participant discusses the implications of the Michelson-Morley experiment, suggesting that SRT's predictions align with experimental results while questioning the validity of alternative theories that posit absolute simultaneity.
  • Some participants assert that physics works equally in any inertial frame, implying that no single frame should be considered special or "wrong."
  • There is a discussion about the necessity of observers assuming they are not moving for SRT to hold, with some arguing that this assumption is not a requirement but a choice based on the frame of reference.
  • Concerns are raised about the implications of simultaneity and whether both observers can agree on events occurring simultaneously if they are in different frames.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of assuming one is motionless in SRT, with some asserting it is essential while others argue it is not. The discussion remains unresolved, with multiple competing perspectives on the nature of motion and reference frames.

Contextual Notes

Limitations include the dependence on definitions of motion and reference frames, as well as unresolved questions about the implications of simultaneity and the validity of alternative theories to SRT.

  • #31
PeroK said:
Suppose you were swimming upstream, just fast enough to hold your own against the flow. You are putting in all that effort against the current, but to an observer on the bank, you are stationary!

Are you moving swiftly through the water, or not moving at all?

Meanwhile, a loose buoy is moving rapidly downstream without putting in any effort.

You are moving through the water, would be my answer. Whether you're moving with respect to the shore is not the issue when you're asking about the relative motion between your body and the water.
 
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  • #32
georgir said:
.

Relativists are not saying you can't know if you're moving. They say it doesn't make sense to ask if you're moving unless you ask specifically relative to something. Motion is relative. Pick a reference frame. Any one, they are all equally "right". Some might be more convenient in a larger and more general class of cases... but none is truly and absolutely "the correct one".

Georgir, I think you are just stating generalities and completely overlooking the point I brought up, to wit:

In order for the SRT to work, each of two observers MUST (not may) BOTH simultaneously claim that only the "other guy" is moving.
 
  • #33
Contrary to common claims, there is always an "absolute" frame in SRT. It's always the one you're in.
 
  • #34
Layman said:
You are moving through the water, would be my answer. Whether you're moving with respect to the shore is not the issue when you're asking about the relative motion between your body and the water.

One last try:

This is exactly the point. You're moving (relative to the water); the man on the shore is stationary (relative to the shore). So, why prefer the water to the shore as you're absolute reference frame? If you choose the water as your reference, then you are both moving. But, the man on the shore can't feel this motion: he isn't swimming or making any effort. His natural observation is that he is not moving and the water is flowing past.

If you choose the shore, then it's vice versa: the man in the river is swimming as fast as he can just to stay motionless.

Why prefer the water to the shore of the shore to the water?

The point worth noting about SRT, it that within the universe there is no ether. If there were, then you would have absolute motion wrt the ether, which would be the natural frame of reference. The absence of a universal ether is a key experimental fact that led Einstein to suppose that there is no absolute motion.

This does not mean that there is no preferred refernce frame in given circumstances. Eg. sound always travels relative to the air; buoys float relative to the water etc. Trains move relative to the Earth.

But, universally, there is no absolute motion. An outside observer would see the train accelerate and move relative to the Earth. But, that observer would also see the train spin and orbit with the Earth. So, there would be no absolute: this train is moving at 80 mph due East and all observers in the universe will agree on this.
 
  • #35
PeroK said:
One last try:

This is exactly the point. You're moving (relative to the water); the man on the shore is stationary (relative to the shore). So, why prefer the water to the shore as you're absolute reference frame?


If you choose the water as your reference, then you are both moving. But, the man on the shore can't feel this motion: he isn't swimming or making any effort. His natural observation is that he is not moving and the water is flowing past.

If you choose the shore, then it's vice versa: the man in the river is swimming as fast as he can just to stay motionless.

Why prefer the water to the shore of the shore to the water?

The point worth noting about SRT, it that within the universe there is no ether. If there were, then you would have absolute motion wrt the ether, which would be the natural frame of reference. The absence of a universal ether is a key experimental fact that led Einstein to suppose that there is no absolute motion.

This does not mean that there is no preferred refernce frame in given circumstances. Eg. sound always travels relative to the air; buoys float relative to the water etc.


There's nothing you're saying that was not well-understood by, and well explained by, Newton. But Einstein differs from Newton, somehow. How?

Btw, I don't think you can really say that the "absence of ether" is an "experimental fact." It is a postulate, an expediency. Al just said it wasn't necessary, not that it didn't exist. In fact, in later writings, he said there MUST be an ether--he just didn't think it was as Lorentz, Maxwell, et al, thought it to be.
 
  • #36
Layman said:
In order for the SRT to work, each of two observers MUST (not may) BOTH simultaneously claim that only the "other guy" is moving.
Nope. SRT just ALLOWS them to do so. It also ALLOWS both to consider the other guy to be at rest.
 
  • #37
A.T. said:
Nope. SRT just ALLOWS them to do so. It also ALLOWS both to consider the other guy to be at rest.

Well, that's not what physicists like Baez tell me.

And, really, you don't need an authority to tell you. Just think about it. If two observers agree that one of them is moving and one of them is not (and it the same "one"), then the speed of light is not constant in all frames, etc.

If both parties agree on which one is moving, you have absolute simultaneity, not relative simultaneity.
 
  • #39
Layman said:
There's nothing you're saying that was not well-understood by, and well explained by, Newton. But Einstein differs from Newton, somehow. How?

Btw, I don't think you can really say that the "absence of ether" is an "experimental fact." It is a postulate, an expediency. Al just said it wasn't necessary, not that it didn't exist. In fact, in later writings, he said there MUST be an ether--he just didn't think it was as Lorentz, Maxwell, et al, thought it to be.

It's when you add the second postulate that the speed of light is the same for all observers that you get SRT. If it were not, then you could measure your absolute velocity by measuring the speed of light in all directions: that would tell you exactly what your universal motion is: your motion relative to the rest of the universe.

The problem is, of course, that you find the speed experimentally to be the same in all directions, so, in a way, this is one reason to conclude that you are not moving wrt the rest of the universe.

By the way, the absence of a universal ether was the subject of the famous Michelson-Morley experiment. It was critical to SRT.
 
  • #40
Layman said:
If two observers agree that one on them is moving ...
There is nothing to "agree" on here, because there so absolute meaning to "moving".
 
  • #41
PeroK said:
It's when you add the second postulate that the speed of light is the same for all observers that you get SRT.

Well, it's more than just that. Lorentz and Poincaire said the same (as far as "measurement") goes.

With respect to Michelson-Morley, the failure to "detect" an ether wind was NOT taken to prove there was no ether. The assumption that there WAS an ether, combined with the inability to detect it, is in fact what led to the LT transformations, which Einstein appropriated wholesale.
 
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  • #42
  • #43
If you are inside the train car, all the windows are covered, and the ride is very smooth, there is no experiment you can do to prove either that you are moving or that you are not moving. If you can think of one (aside from removing the covers from the windows), tell us what it is. Even if you do remove the covers from the windows, you still can't prove with physical experiments that you are the one that is moving and it is the Earth that is stationary. Your notion that you are the one that is moving is just a bias.

Chet
 
  • #44
Layman said:
Well, it's more than just that. Lorentz and Poincaire said the same (as far as "measurement") goes.

With respect to Michelson-Morley, the failure to "detect" an ether wind was NOT taken to prove there was no ether. It is in fact what led to the LT transformations, which Einstein appropriated wholesale.

So, you are saying:

a) There is an ether.

b) The speed of light depends on your motion: some observers moving against the ether will measure a higher speed of light.

c) Relativity is wrong.

d) All modern cosmology and particle physics is wrong.

e) Sometime soon, someone will come up with Newton's 4th law of motion, that will set the record straight and we'll all realize relativity was just a 100 years bad dream!

Nature and nature's laws lay hid by night,
God said: "let Newton be" and all was light!

It did not last, the devil howling "yo",
"Let Einstein be", restored the status quo!
 
  • #45
Layman said:
Well, that's not what physicists like Baez tell me.

That is not a valid reference. What did Baez say and where? It is pretty clear that you are misinterpreting it, but without a valid reference, it is hard to say how.
 
  • #46
Chestermiller said:
If you are inside the train car, all the windows are covered, and the ride is very smooth, there is no experiment you can do to prove either that you are moving or that you are not moving. If you can think of one (aside from removing the covers from the windows), tell us what it is. Even if you do remove the covers from the windows, you still can't prove with physical experiments that you are the one that is moving and it is the Earth that is stationary. Your notion that you are the one that is moving is just a bias.

Chet



I could have syphilis right now, and not know it. My "knowing" it has nothing to do with whether it exists, as a physical fact.
 
  • #47
Layman said:
In that case, the "absolute" frame is ...
If the absolute frame is different in each case, then it is not absolute.
 
  • #48

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