Could there be an absolute 'state of reference?'

[PeterDonis]

do you doubt that ptolemic astronomy could not also be "tweaked" (as Copernican theory was) to accommodate newly observed (measured) phenomena?

If that could have been done, I'm pretty sure someone would have done it several centuries ago, when the theory was being challenged by Kepler's.

I don't.

Then please show your work.

(And I still don't see the point as it relates to the rest of this discussion.)

 

[Layman]

We will end up going there if the answer I was thinking of to my "see the problem?" question gets revealed.

Sorry, I'm have trouble keeping up. I am not deliberately "ducking" questions, as I have been accused of.

Yes, I "see the problem." SRT did NOT define "inertial" as GR did--it kept Newton's definition of it. When are "forces" acting, and what exactly is a "force" are questions that are not easily answered and which would be answered quite differently in GR as opposed to SR.

But as far as I'm concerned you question does not invalidate mine (about whether it is more reasonable, given what we know, to assume that the rocket is moving away from the earth, not vice versa.

Once the acceleration is complete, and the rocket is moving at some uniform speed, our knowledge would still tell us that it was moving away from earth, rather than the Earth moving away from it. We know it was accelerated, and is hence traveling at a great speed, relative to earth.

 

[PeterDonis]

SRT did NOT define "inertial" as GR did--it kept Newton's definition of it.

I'm not sure I understand. Are you just saying that there are no global inertial frames in GR, only local ones?

When are "forces" acting, and what exactly is a "force" are questions that are not easily answered and which would be answered quite differently in GR as opposed to SR.

You're getting warm here, but you've still missed the real problem. See below.

But as far as I'm concerned you question does not invalidate mine (about whether it is more reasonable, given what we know, to assume that the rocket is moving away from the earth, not vice versa.

Once the acceleration is complete, and the rocket is moving at some uniform speed, our knowledge would still tell us that it was moving away from earth, rather than the Earth moving away from it. We know it was accelerated, and is hence traveling at a great speed, relative to earth.

In other words, you are defining acceleration--in the relativistic sense of proper acceleration, acceleration that is actually felt--as picking out what is "actually moving" and what is not.

Now consider: you are at rest on the surface of the Earth. You drop a marble. It falls towards the ground. Which is moving, you or the marble? Once again, the practical point of view is that you are at rest and the marble is moving. And since this point of view also says the Earth is not moving, it doesn't introduce the issue that there was in the rocket ship, where saying that the marble was moving meant that you were at rest, even though you were moving relative to the Earth.

But wait: you, at rest on the surface of the Earth, are *feeling acceleration*! You feel weight; an object that feels no acceleration (i.e., is moving inertially) is weightless. So by your stated criterion, you, "at rest" on the surface of the Earth, are "actually moving", because you feel acceleration.

Now do you see the problem?

 

[PAllen]

Sorry, I'm have trouble keeping up. I am not deliberately "ducking" questions, as I have been accused of.

Yes, I "see the problem." SRT did NOT define "inertial" as GR did--it kept Newton's definition of it. When are "forces" acting, and what exactly is a "force" are questions that are not easily answered and which would be answered quite differently in GR as opposed to SR.

But as far as I'm concerned you question does not invalidate mine (about whether it is more reasonable, given what we know, to assume that the rocket is moving away from the earth, not vice versa.

Once the acceleration is complete, and the rocket is moving at some uniform speed, our knowledge would still tell us that it was moving away from earth, rather than the Earth moving away from it. We know it was accelerated, and is hence traveling at a great speed, relative to earth.

See post #97, the substantive part of which you completely ignored (as you have the substantive parts of most of my posts that ask specific questions).

 

[Layman]

This comparison illustrates, I think, what people are objecting to when someone keeps insisting that "absolute simultaneity" or some such concept is worth discussing. Why waste time discussing a concept that can be thrown away without affecting any physical predictions at all.

This is one (of many) statements I wanted to address in this thread, but haven't had time to. In effect, the GPS system implemented a system incorporating "absolute simultaneity" because it was much more practical and convenient. Some physicists have claimed that the GPS would be "impossible" to implement using only relativistic conceptions of time. I don't know if that's true, or not.

Absolute simultaneity does not require an ether, or any particular underlying physical model. It is simply a matter of establishing, however aribitrarily, a "master clock" to which all other clocks are consistently compared. This establishes a uniform "standard" for the sake of comparing various objects in various states of motion.

I've forgotten the exact details, but, as I recall, the GPS engineers used an ECI (earth-centered inertial) frame as the standard, master clock which then used to co-ordinate the times on all other (satellite) clocks.

The point is that a frame used to establish absolute simultaneity is neither "useless" nor "passé."

 

[Layman]

In other words, you are defining acceleration--in the relativistic sense of proper acceleration, acceleration that is actually felt--as picking out what is "actually moving" and what is not.

No, I'm not doing that, as far as I can tell. All kinds of ambiguities and inconsistencies can arise when you start commingling completely different theories, with diametrically-opposed definitions (of, say, "inertial" or "force" or "acceleration." I wasn't trying to get into all of that (and needless to say, neither was Al) when he was explicating on the theoretical basis of SRT. I was addressing those statements, assumptions, and claims, not ones he made later in connection with GR. As I recall, he later said that such SR concepts as the constant speed of light were not valid in the context of GR.

 

[PeterDonis]

TIn effect, the GPS system implemented a system incorporating "absolute simultaneity" because it was much more practical and convenient.

This is a different usage of the term "absolute". The GPS system incorporated a sense of simultaneity that was not the same as the "natural" sense of simultaneity of the GPS satellites. (It is also not the natural sense of simultaneity of a person standing on the surface of the rotating Earth and not moving with respect to that surface.) But it's still an arbitrary choice, and is not required by the physics. It does make things simpler, but that's just a practical matter, not a question of fundamentals.

Some physicists have claimed that the GPS would be "impossible" to implement using only relativistic conceptions of time.

By "impossible" they mean "taking a lot more time and money", not "physically impossible", and certainly not "inconsistent with physical laws and relativity".

Absolute simultaneity does not require an ether, or any particular underlying physical model. It is simply a matter of establishing, however aribitrarily, a "master clock" to which all other clocks are consistently compared. This establishes a uniform "standard" for the sake of comparing various objects in various states of motion.

And this is all perfectly consistent with relativity; this sense of "absolute" is not the sense of "absolute" that we have been discussing in this thread.

 

[PeterDonis]

No, I'm not doing that, as far as I can tell.

Then you aren't thinking very clearly. The only difference between the rocket and the Earth, in your scenario, is that the rocket fires its engines. That means its crew feels weight. That's what *you* used to distinguish its state of motion as "actually moving". By that same criterion, you, sitting on the surface of the Earth and not moving with respect to it, are "actually moving", because you feel weight just like the crew of the rocket does.

If you feel tempted to say that it's the firing of engines and the expending of fuel that counts, rather than feeling weight, you should think very carefully before trying it. It won't make your position any more tenable.

As I recall, he later said that such SR concepts as the constant speed of light were not valid in the context of GR.

That's not quite what he said; he said SR concepts such as the constant speed of light had to be properly generalized when gravity was present. That doesn't mean they're not valid; it just means they have to be properly generalized.

 

[Layman]

And this is all perfectly consistent with relativity; this sense of "absolute" is not the sense of "absolute" that we have been discussing in this thread.

So, this is all a matter of semantics and differing definitions? Wouldn't surprise me. I see the would "absolute" thrown around in these types of discussions in all kinds of contexts and apparently intended to refer to many different things. Pretty soon, I have no idea of what anyone particular person has in mind when they say "absolute."

I suppose my simple definition of absolute in this context would just be "not frame-dependent."

 

[PeterDonis]

I suppose my simple definition of absolute in this context would just be "not frame-dependent."

The usual term for that in relativity is "invariant". (I've used that term that way a number of times in this thread.) One main reason for that may well be to avoid all the issues with the word "absolute".

However, if that's your simple definition, you haven't been using the term "absolute" very clearly. For example, you've talked about "absolute simultaneity", but if "absolute" means "not frame-dependent", then this is an oxymoron taken at face value, because "simultaneity" in the usual sense is always frame-dependent. (Later on you clarified that you meant something else by "absolute simultaneity", but by then the damage had been done. And even then the term still doesn't fit, because simultaneity in the sense of choosing a clock synchronization convention is also frame-dependent, since choosing such a convention amounts to choosing a frame.) And you still seem to be clinging to some concept of "absolute motion", which is also an oxymoron as it stands, since "motion" is frame-dependent too.

 

[Layman]

Then you aren't thinking very clearly. The only difference between the rocket and the Earth, in your scenario, is that the rocket fires its engines. That means its crew feels weight. That's what *you* used to distinguish its state of motion as "actually moving". By that same criterion, you, sitting on the surface of the Earth and not moving with respect to it, are "actually moving", because you feel weight just like the crew of the rocket does.

I made no reference to acceleration, or what a person feels, at least not initially. But yes, that's definitely part of it. The baseball I hit is accelerating. Even within the context of SRT this acceleration is "absolute," as I understand it. But really I've just been addressing, as SRT does, the comparison between two objects moving relative to each other at uniform speeds. I'm just saying that at least one of these has to be actually moving (whether you can detect which one it is, or not). The train is an example. Assuming its moving at a uniform speed of 80 mph, the question of how it came to attain it's current state of motion (and what energy is required to maintain that uniform speed) are relevant factors and considerations which it would be imprudent to ignore for the sake of absolute obedience to unproven postulates.

 

[Layman]

"simultaneity" in the usual sense is always frame-dependent.

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

Here's one thing that I think is worth noting. 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.

That said, transmission delays can cause a time difference in subjective perception without any consideration of the observer's motion. 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). Yet nobody claims that the light and sound waves were not created simultaneously, even if they are not perceived by every individual at every location simultaneously. Why should it be different with the lightning bolts on the track in Einstien's example.

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?

 

[PeterDonis]

Even within the context of SRT this acceleration is "absolute," as I understand it.

Proper acceleration (i.e., felt acceleration) is invariant in SR, yes. You should really use that word instead of "absolute" to refer to such things.

really I've just been addressing, as SRT does, the comparison between two objects moving relative to each other at uniform speeds. I'm just saying that at least one of these has to be actually moving (whether you can detect which one it is, or not).

But how would you detect which one is "actually moving"? SR's answer is, the question has no meaning, because "actually moving" has no meaning; the only kind of motion SR recognizes is relative motion, since that's sufficient to account for all observations.

Your answer is, look to see how it got to the state it's in now. As you've been applying that criterion, it basically amounts to "look to see what Layman's intuition says about which object is actually moving". The only physical principle I can see that you've invoked is felt acceleration: look to see which object felt acceleration in the past. But that won't work even if we restrict to only considering objects that are currently moving inertially (so none of them feel weight). Here are a couple of counterexamples:

(1) Rocket A fires its engines and launches itself from the Earth. After a while it shuts off its engines and coasts. While it's doing that, Rocket B, which has been coasting towards the Earth, fires its engines and ends up on the same pad that Rocket A launched from. Both rockets felt acceleration in the past, so they both are "actually moving" by your criterion. But Rocket B is in the same state of motion that Rocket A was in before it launched, which according to you, is supposed to be "actually at rest". So is Rocket B "actually at rest" or "actually moving"?

(2) Rocket C is hovering high above the ground, firing its engines to maintain altitude. Then it stops its engines and starts falling. Is Rocket C "actually moving" after its engines stop? How does that square with the fact that firing its engines did not change its speed with respect to the Earth at all? (Note that in the other examples, mine and yours, rockets firing their engines *did* change their speed with respect to the Earth, which is what makes plausible the assertion that they are "actually moving" after they fire their engines.)

the question of how it came to attain it's current state of motion (and what energy is required to maintain that uniform speed) are relevant factors and considerations which it would be imprudent to ignore for the sake of absolute obedience to unproven postulates.

I have no idea what you're talking about here. Nobody has suggested that any of the things you mention about the train should be ignored. We're just saying that none of them amount to the train "actually moving", because that term has no meaning.

As for "unproven postulates", isn't that a tautology?

 

[PAllen]

I made no reference to acceleration, or what a person feels, at least not initially. But yes, that's definitely part of it. The baseball I hit is accelerating. Even within the context of SRT this acceleration is "absolute," as I understand it. But really I've just been addressing, as SRT does, the comparison between two objects moving relative to each other at uniform speeds. I'm just saying that at least one of these has to be actually moving (whether you can detect which one it is, or not). The train is an example. Assuming its moving at a uniform speed of 80 mph, the question of how it came to attain it's current state of motion (and what energy is required to maintain that uniform speed) are relevant factors and considerations which it would be imprudent to ignore for the sake of absolute obedience to unproven postulates.

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.

 

[Layman]

We're just saying that none of them amount to the train "actually moving", because that term has no meaning.

As for "unproven postulates", isn't that a tautology?

1. Yeah, its redundant.

2. The easiest thing in the world to do is to glibly assert is that a statement or concept is "meaningless." This is the tactic the logical positivists routinely resorted to when they held sway. In the end, they had produced so many contradictions that they were laughed off the philosophical stage.

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.

 

[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 Einstien'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.