Could there be an absolute 'state of reference?'

[PAllen]

Lets talk about "simultaneity" a little, can we?

...

It is funny (to me) that Al explicitly presupposes that the train in actually moving (wrt the embankment) in order to explain why the passenger on the train does not "perceive them simultaneously. Obviously, if the passenger on the train also (like Al did) assumes he is actually moving, then he will factor his own motion into his subjective perceptions and "correct" for the perceived lack of simultaneity just like he might for a delayed perception of thunder.

But Al refuses to afford the poor passenger with the same superior knowledge that Al has when explaining the situation. Al knows the passenger is moving, but relegates the passenger to the role of a foil who incorrectly insists he is NOT moving. Why is that? There is definitely a reason why Al does that, but what is it?

It is just for the purpose of setting the problem up. You have to specify some initial conditions. An alternative set up would be two strikes such that they are perceived as simultaneous by the train observer. Then, the embankment observer would see them as not simultaneous.

Again, it isn't Einstein who considered two such observers equivalent - it is Newton and Galileo. Einstein just extended the principle to electromagnetic as well as mechanical phenomena.

 

[PeterDonis]

As far as I know, it is generally agreed that delays in light transmission have nothing whatsoever to do with the phenomenon of "time dilation." Time delays are just complications which need to be factored out.

In the usual sense of the term "time dilation", yes, it's what you get after you have corrected for light travel time.

A person directly under a lightning bolt with perceive the accompanying flash of light and sound of thunder "simultaneously." A person 5 miles away will see the light first, then hear the sound of thunder later. They are not "simultaneous" to him (in the order in which he subjectively perceives those two phenomena).

This is not the sense of "simultaneity" that matters for this discussion. The lightning and thunder happen at a single spatial location. "Simultaneity" in the sense that matters for this discussion (since it's the sense that is frame-dependent) refers to whether or not events at different spatial locations happen "at the same time".

Why should it be different with the lightning bolts on the track in einstein's example.

Because the lightning strikes happen at different spatial locations, whereas the lightning and thunder in your example happen at the same spatial location.

It is funny (to me) that Al explicitly presupposes that the train in actually moving (wrt the embankment) in order to explain why the passenger on the train does not "perceive them simultaneously.

No, you're mixing up the two senses of "simultaneous" again. The observer's perception also happens at a single spatial location, so whether or not he perceives the two lightning flashes "simultaneously" in this sense (i.e., light signals from the two lightning strikes arrive at his spatial location at the same event--an "event" is a point in spacetime, a single location in space at a single instant of time) is invariant, not frame-dependent. Einstein's argument simply establishes that, if the observer on the embankment receives light signals from the two lightning strikes at the same event, the observer on the train *cannot* receive them at the same event; he must receive the two light signals at two different events. In order to establish that, Einstein does not assume that the train is "actually moving"; as your own parenthetical comment shows, he only assumes that the train is moving relative to the embankment. No "actual" motion is required; just relative motion.

Obviously, if the passenger on the train also (like Al did) assumes he is actually moving, then he will factor his own motion into his subjective perceptions and "correct" for the perceived lack of simultaneity just like he might for a delayed perception of thunder.

It's quite true that the observer on the train, just like the observer on the embankment, will correct for light travel time in order to determine at what time, by his clock, each lightning strike happened. But when the train observer makes that correction, he *still* finds that the two lightning strikes happened at different times!

First, work through how the embankment observer does this correction. The points where the lightning strikes hit, on the embankment, are equidistant from him, so the light travel time to him will be the same for both. He receives light signals from both strikes at the same instant; therefore he concludes that the two strikes happened at the same time (the time he receives the light signals, minus the light travel time).

Now work it through for the train observer. In Einstein's original formulation, the lightning strikes each hit the embankment just as the ends of the train are passing the points on the embankment where the lightning strikes hit. Let's suppose that each strike leaves a mark on both the embankment and the train, so that both observers, at their leisure, can go back and verify where the strikes hit. The train observer, like the embankment observer, will then say that the two strikes happened at the same distance from him (since he is in the center of the train, equidistant from the two ends where the strikes hit). But that means the light travel time to him for both strikes is the same; yet he receives the signals at different times by his clock. Therefore the lightning strikes must have happened at different times by his clock.

 

[A.T.]

The statement that something is "meaningless" is not evidence, it is not proof

The burden of proof is on those who claim it has physical significance.

 

[PeterDonis]

The statement that something is "meaningless" is not evidence, it is not proof, it is not an argument. It is a mere assertion, standing alone.

If it's done in isolation, yes. SR does not assert that "actually moving" is meaningless in isolation. It does so in the context of a theory that explains all observations within its domain of validity (i.e., not involving gravity) *without* having to use the concept. So it's "meaningless" in the sense of being unnecessary and superfluous.

 

[A.T.]

(whether you can detect which one it is, or not).

How is that different from saying: "I believe in magic fairies, whether you can detect one, or not."?

 

[Layman]

Without friction, zero fuel would be required. The fuel is to overcome friction, not maintain inertial motion. Again, if you think energy must be expended to maintain uniform motion and that one is unambiguously moving when neither feels acceleration, you argument is not with Einstein, it is with Galileo and Newton. They established the principle of relativity, and the concept of inertia and inertial motion - that inertial motion is maintained without energy expenditure.

Well, PA, you raise a collateral, but relevant, point here. In reality, virtually all of the experiments (actual and thought) designed to "test" SR have NOT been made in inertial environments. In that sense you could almost say that SR has no application to any objects anywhere.

In the train example, Al is basically treating "uniform motion" however achieved or maintained, as "inertial motion" (which it isn't, literally speaking).

As I recall, Newton's (and also SR's) inertia included at least 3 things:

1. uniform speed
2. in a straight line
3. under the influence of no external forces.

In practice, it seems that one or two of the 3 are sufficient to designate an "inertial frame," with the primary one being that it is not accelerating (uniform speed).

 

[A.T.]

Your answer is, look to see how it got to the state it's in now.

Which is a completely useless concept for doing physics, where you often don't know how it got to the state it's in now. Physics is supposed to make predictions based on a known state, not require you to know the entire history of an object.

 

[PeterDonis]

In reality, virtually all of the experiments (actual and thought) designed to "test" SR have NOT been made in inertial environments. In that sense you could almost say that SR has no application to any objects anywhere.

Why would you say that? SR can handle non-inertial frames perfectly well. It can't handle gravity, but gravity is not the same as non-inertial frames.

(which it isn't, literally speaking).

Only because the train and the embankment are assumed to be on the surface of the Earth. But that detail can easily be handled by putting everything out in deep space, far from all other objects.

Or, you could just restate the third requirement; see below.

In practice, it seems that one or two of the 3 are sufficient to designate an "inertial frame," with the primary one being that it is not accelerating (uniform speed).

No, you need all three, but the third needs to be restated as "undergoing no net motion due to external forces". The train and embankment on the surface of the Earth are affected by external forces, but those forces are in balance so they produce no net motion (meaning, they don't cause the train and the embankment to move relative to each other).

 

[A.T.]

In reality, virtually all of the experiments (actual and thought) designed to "test" SR have NOT been made in inertial environments.

The same applies to Newton Laws of motion, as inertial frames are just an idealization, like everything in physics.

 

[Layman]

The train observer, like the embankment observer, will then say that the two strikes happened at the same distance from him (since he is in the center of the train, equidistant from the two ends where the strikes hit). But that means the light travel time to him for both strikes is the same; yet he receives the signals at different times by his clock. Therefore the lightning strikes must have happened at different times by his clock.

I don't follow this claim. This is precisely what I was getting at. You say "The train observer, like the embankment observer, will then say that the two strikes happened at the same distance from him..." then say: "that means the light travel time to him for both strikes is the same." No, it doesn't, and that's what Al was pointing out (while leaving the chump passenger in the dark with respect to the "true" situation).

What you are leaving out is that the passenger is moving toward one light flash, and away from the other, while they are in the process of being transmitted to his senses. Because he is moving "that means the light travel time to him for both strikes is [NOT] the same."

 

[Layman]

How is that different from saying: "I believe in magic fairies, whether you can detect one, or not."?

It's quite different, A. T. It is a matter of logic, of definition, actually. If I know you are a human and on that basis I conclude that you have a heart, whether I can detect it or not, then that is not the same as "believing in fairy tales."

 

[A.T.]

If I know you are a human and on that basis I conclude that you have a heart

Only because hearts have been detected in other humans. Your absolute notions have never been detected, so they are just like magic fairies.

 

[Layman]

Or, you could just restate the third requirement; see below.

No, you need all three, but the third needs to be restated as "undergoing no net motion due to external forces". The train and embankment on the surface of the Earth are affected by external forces, but those forces are in balance so they produce no net motion (meaning, they don't cause the train and the embankment to move relative to each other).

OK, Peter, I'll just accept your "restatement" as being what Newton intended (although I'm not familiar with this as a general qualification of Newton definition of inertia). But I believe your last sentence is over-generalizing. It's true that both the train and the Earth are subject to a given number of "shared" external forces. But not ALL of them are shared. Unlike the train, the stationary observer does not require the burning of mass quantities of coal to maintain his "inertial" state.

 

[micromass]

Thread locked pending moderation

 

[jtbell]

The OP's last post in this thread indicated that he was satisfied with the responses that he had received at that point, so this thread will remain closed.

 

[Dale]

The "second postulate" only works for two different observers if each assume that he is at rest and the other party is not.

Wow, this thread went really fast, and this may have already been covered before the lock, but I thought would address it directly since it is the same repeated mistake that seems to underlie all of the mistakes in this and your other thread.

This is factually incorrect. The second postulate says that the speed of light in vacuum is c in any inertial frame. There is no mention of an observer, nor any mention of a requirement that any specific object, observer, or phenomena be considered at rest. There are two and only two requirements, the light must be in vacuum, and the frame must be inertial. Given those two requirements then the postulate ensures that the speed of the light is c.

If they both use the embankment frame, the speed of light is c. If they both use the train frame, the speed of light is c. If they each use their own frame, the speed of light is c.

Before you attempt to criticize a theory it is important to understand it enough to correctly identify it's claims. You are not at that point yet. You misunderstand the theory, so your criticisms are not even properly criticisms of SR, they are criticisms of your own straw man theory. We agree that your straw man theory is wrong, but it is not SR.