Why would I think I'm not moving?

[Layman]

As I understand it, SRT requires me, if I'm inertial, to insist that I'm not moving. But why would I do that?

In Einstein's example designed to illustrate the relativity of simultaneity he says something to the effect that the man on the train will assume he's not moving. Why should he? He bought his ticket, he felt himself accelerate, and he KNOWS that he is moving with respect to the earth. Why in the world would he assume that he is motionless and that the Earth is moving with respect to him?

Does anyone do this in real life? Does anyone actually ask (as the old joke goes) the conductor if Chicago stops here?

 

[Layman]

A related question:

A related question: If two observers are moving with respect to each other, and each one assumes that he is motionless, doesn't at least one of them have to be wrong?

 

[pervect]

A related question: If two observers are moving with respect to each other, and each one assumes that he is motionless, doesn't at least one of them have to be wrong?

No, special relativity is about first realizing that it is perfectly consistent to assume there is no absolute motion, and then (in the modern interpretation) realizing that this assumption matches experiment, explicitly for example the Michelson Morely experiment, while other attempts to explain away the null result of the Michelson Morely experiment have issues.

Einstein starts out with the notion that the mapping between frames of reference must satisfy the axioms of an infinite group, which basically boils down to the notion that any such mapping must have an inverse. (The other group axioms are closure, associativity, and the existence of an identity transformation).

Einstein considers the possible set of such mappings, and by focusing on the speed of light being indistinguishable between moving and nonmoving fames concludes that using the Lorentz transform to transform between moving and nonmoving observers is necessary to achieve consistency. The structure of this transform involves abandoning some classical ideas, however, such as the existence of absolute time.

 

[Layman]

No, special relativity is about first realizing that it is perfectly consistent to assume there is no absolute motion, and then (in the modern interpretation) realizing that this assumption matches experiment, explicitly for example the Michelson Morely experiment, while other attempts to explain away the null result of the Michelson Morely experiment have issues.

So are both right?

I thought it had been well-established that, prediction-wise, SRT is indistinguishable from theories which posit absolute simultaneity (e.g. Sexl and Mansur's studies), so I'm not sure what your assertion that it "matches experiment" is saying. Other theories, which posit absolute simultaniey (and hence absolute motion) also "match experiment," don't they?

Put another way, isn't it also "perfectly consistent" to assume that there IS absolute motion?

I have looked at a number of discussions about the twin paradox. Time and again I see prominent, mainstream physicists readily concede that it is logically absurd to claim that "each clock is slower than the other." Isn't is just as absurd to claim that each of two observers is "motionless" when there is relative motion between them?

 

[georgir]

Your questions are not even about SRT. You can ask them about plain old Galilean relativity.

Yes, your passenger might know he is moving with respect to the Earth... but he also might not. What if the windows were shaded and the waggon was noise-proof and his acceleration was slow?
And even if he knew how he is moving with respect to the Earth, why should he care about the Earth? The important part of relativity is that inside of the waggon physics is unchanged and independent of how it moves with respect to the Earth, or how the Earth moves with respect to it, as he might view the situation.

And why do you center your measurements on the Earth at all, and not on the Sun, or the Galaxy center, or on some hypothetical aether wind that might turn out to be moving at 2 c in regards to the Earth? Sorry, I meant the Earth was moving in regards to it...

No inertial reference frame is "wrong", because physics works equally no matter which one you use. They are equivalent in formulation of the laws and in predictions, so nominating any with a special status is just adding redundant aspects to your model.

 

[A.T.]

As I understand it, SRT requires me, if I'm inertial, to insist that I'm not moving.

On the contrary, It tells you that you can use any inertial frame to do calculations based on the same laws of physics. That's Galilean relativity, which was introduced centuries before SRT:
http://en.wikipedia.org/wiki/Galilean_invariance

Other theories, which posit absolute simultaniey (and hence absolute motion) also "match experiment," don't they?

You can assume a lot of undetectable things and still match all experiments. Hence we use Occam's razor:
http://en.wikipedia.org/wiki/Occam's_razor

 

[Layman]

Your questions are not even about SRT. You can ask them about plain old Galilean relativity.

Well, I was asking a question about Einstein's assertions in his explanation of SR. If that isn't a topic that is "about SRT," then I wouldn't know what is about SRT.

And why do you center your measurements on the Earth at all, and not on the Sun...

Albert, as I understood him, was simply comparing a stationary observer on the surface of the Earth with one in a moving train. He wasn't talking about the sun, so neither was I.

 

[Layman]

On the contrary, It tells you that you can use any inertial frame to do calculations based on the same laws of physics. That's Galilean relativity, which was introduced centuries before SRT:
http://en.wikipedia.org/wiki/Galilean_invariance

I agree with that, sure, but that's not the point or the question. My understanding is that for SRT to "work out," (isotropic light speed, etc.), each observer MUST assume he is not moving. Am I wrong?

If both the guy on the embankment, and the guy on the train, agreed that it was the train that was moving, then they would also agree that the clock on the train, not the one on the earth, was truly the one moving slower. Kinda like with the GPS.

They would further agree that the two lightning bolts struck simultaneously, and there would be no "relativity of simultaneity" about it.

 

[A.T.]

My understanding is that for SRT to "work out," (isotropic light speed, etc.), each observer MUST assume he is not moving.

No, he CAN assume he is not moving. But he can pick some other inertial frame to do calculations.

They would further agree that the two lightning bolts struck simultaneously, and there would be no "relativity of simultaneity" about it.

If both agree to use the same frame, they will both agree on frame dependent things like simultaneity. That doesn't make these things frame independent.

 

[Layman]

No inertial reference frame is "wrong", because physics works equally no matter which one you use.

I suspect that you really meant that "mathematics (not physics) works equally no matter which one you use."

But math and physics are completely different "sciences," aren't they? In the physical world, trains do not run at a uniform speed of, say, 80 mph, without somebody constantly shoveling coal into a furnace--back in Al's day, anyway. It seems to me that a guy on a train would be spitting in the face of physics if he actually claimed that, as between the two, the earth, and not the train, was moving.

A guy buys a train ticket from LA to NY. He does it precisely because he expects to move from LA to NY, not because he thinks some kind of magic associated with the train will bring NY to him, while he remains motionless.

 

[jbriggs444]

I agree with that, sure, but that's not the point or the question. My understanding is that for SRT to "work out," (isotropic light speed, etc.), each observer MUST assume he is not moving. Am I wrong?

Yes. You are wrong. Each observer CAN assume that he is not moving. Not must.

Introductory presentations of S.R. tend to talk about inertial frames of reference and observers who are at rest in those frames interchangeably. In effect, observers in these presentations DO assume that they are not moving.

S.R. can be derived by exploring the requirement that the laws of physics and the speed of light do not depend on this free choice.

 

[Layman]

No, he CAN assume he is not moving. But he can pick some other inertial frame to do calculations.

My question goes beyond what one, and only one, observer assumes. The question is about what each of TWO observers must both assume, in order for SRT to hold.

 

[georgir]

Well, I was asking a question about Einstein's assertions in his explanation of SR. If that isn't a topic that is "about SRT," then I wouldn't know what is about SRT.

You are taking my post the wrong way. It does not try to critique you for being off-topic. I was just pointing out that your questions apply [and can be answered] in an even broader sense, and not just the specific one you were focused on.
[Do you have the same issues against Galilean relativity?]

Albert, as I understood him, was simply comparing a stationary observer on the surface of the Earth with one in a moving train. He wasn't talking about the sun, so neither was I.

Again, I don't get why you react so touchy...
My point here was to remind you that there are multiple candidates for "absolute" frame, and to ask you: how would you pick it?

 

[Layman]

Oops, quoted A.T. in that last post when I intended to respond to jbriggs, who said:

Yes. You are wrong. Each observer CAN assume that he is not moving. Not must.

Not sure if A.T. was making the same claim as jbriggs did.

 

[Layman]

You are taking my post the wrong way. It does not try to critique you for being off-topic. I was just pointing out that your questions apply [and can be answered] in an even broader sense, and not just the specific one you were focused on.

Well, georgir, I'm new here, so I don't know who's who and who does what. But my post was moved from the relativity forum, and I guess I assumed it was you who did it, given your post.

 

[Layman]

My point here was to remind you that there are multiple candidates for "absolute" frame, and to ask you: how would you pick it?

Well, that's a different question. My question was merely addressing a guy on a moving train, on earth, claiming that he was motionless. In that case, the "absolute" frame is obviously the surface of the earth.

The surface of the Earth is not moving with respect to the train. The train is moving with respect to the surface.

 

[A.T.]

The surface of the Earth is not moving with respect to the train. The train is moving with respect to the surface.

Both objects move relative to the other.

 

[Layman]

[Do you have the same issues against Galilean relativity?]

The same issue simply does not arise with Galileo. He readily admits that, so long as he can see the shore, the sailor will know that he is moving with respect to the land, not vice versa. Einstein wants to deny this. It is common for relativists to claim that you "can't know" if you're moving. To me, that claim is contrary to all experience and common sense.

 

[Layman]

Both objects move relative to the other.

That always true when one object is moving and one isn't. On the other hand, it's never the case when each of two objects is truly (rather than just supposedly) motionless. So that statement is missing the point.

 

[georgir]

But both are the same thing... you seem to like one more, and decide to give it special meaning, based upon some criteria that I guess involves the relative sizes of the two bodies, but it really does not matter.

One can claim that picking the mutual center of gravity in this situation is "more right". Or one might go for the cosmic microwave background instead... and that might just as well coincide with picking the train, and not the Earth like you do.

What does your choice affect? What happens if you picked "wrong"? What does "wrong" even mean... No matter what you pick, you'll get the same physics. That's relativity for you.

 

[A.T.]

the sailor will know that he is moving with respect to the land, not vice versa. Einstein wants to deny this.

No he doesn't. Of course you have a velocity relative to the land. But is just a relative velocity, not absolute one.

 

[A.T.]

truly (rather than just supposedly) motionless.

A meaningless distinction.

 

[PeroK]

The same issue simply does not arise with Galileo. He readily admits that, so long as he can see the shore, the sailor will know that he is moving with respect to the land, not vice versa. Einstein wants to deny this. It is common for relativists to claim that you "can't know" if you're moving. To me, that claim is contrary to all experience and common sense.

So, you are fully aware that the Earth is rotating on its axis, orbiting the Sun, that the Sun is orbiting the centre of the galaxy and the the galaxy is moving very quickly towards the Andromeda galaxy? You are fully aware of all these motions?

Also, consider this:

If you throw a stone, you accept that the stone moves away from you; but, when you jump, you accept that you move away from the Earth.

 

[Layman]

What does your choice affect? What happens if you picked "wrong"? What does "wrong" even mean... No matter what you pick, you'll get the same physics. That's relativity for you.

I disagree, georgir. As I said in a previous post, I think it is merely the math, and not the physics, which is the same.

A steamship on the ocean is moving with respect to a floating buoy. Does "physics" tell you that one is just as "motionless" as the other? I don't think so. Physics tells you it gunna take a lot of horsepower to keep a luxury liner moving at 20 knots, and zero input of energy to keep the buoy floating, doesn't it?

 

[Layman]

Also, consider this:

If you throw a stone, you accept that the stone moves away from you; but, when you jump, you accept that you move away from the Earth.

Exactly, PeroK. Are you suggesting that there is some inconsistency or contradiction here?

 

[jbriggs444]

Does "physics" tell you that one is just as "motionless" as the other? I don't think so. Physics tells you it gunna take a lot of horsepower to keep a luxury line moving at 20 knots, and zero input of energy to keep the buoy floating, doesn't it?

The prediction for the amount of power required to maintain a relative speed of 20 knots between a luxury liner and an ocean does not depend on whether you assume that the liner, the ocean or both are moving. That's the point of relativity. You get the same prediction regardless.

 

[PeroK]

I disagree, georgir. As I said in a previous post, I think it is merely the math, and not the physics, which is the same.

A steamship on the ocean is moving with respect to a floating buoy. Does "physics" tell you that one is just as "motionless" as the other? I don't think so. Physics tells you it gunna take a lot of horsepower to keep a luxury line moving at 20 knots, and zero input of energy to keep the buoy floating, doesn't it?

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.

 

[A.T.]

Physics tells you it gunna take a lot of horsepower to keep a luxury line moving at 20 knots, and zero input of energy to keep the buoy floating, doesn't it?

That’s because the ship moves relative to the water, while the buoy is at rest relative to the water. It's still just relative motion, and nothing absolute about it.

 

[georgir]

The same issue simply does not arise with Galileo. He readily admits that, so long as he can see the shore, the sailor will know that he is moving with respect to the land, not vice versa. Einstein wants to deny this. It is common for relativists to claim that you "can't know" if you're moving. To me, that claim is contrary to all experience and common sense.

What if the sailors sees two shores, and they are moving relative to each-other? Which of the two is "more right"? Relativity is saying it does not matter.

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".

 

[Layman]

That's the point of relativity. You get the same prediction regardless.

That may be "a" point of relativity--and of course it applies equally to gallilean relativity. But it's not the only "point," and it's not the point I'm addressing.

 

[Layman]

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.

 

[Layman]

.

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.

 

[Layman]

Contrary to common claims, there is always an "absolute" frame in SRT. It's always the one you're in.

 

[PeroK]

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.

 

[Layman]

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.