Could there be an absolute 'state of reference?'

[PeterDonis]

[they would only not hold in a non-inertial frame].

In SR, that's true; but as I said in a previous post, GR fixes that too. The GR versions of F = MA, conservation of momentum, etc. hold in all frames, inertial and non-inertial.

 

[PeterDonis]

You would seem to be in disagreement with Peter on this point, PA.

You missed the part where I said the "Ptolemaic system" is *not* the same as "a relativistic frame of reference in which the Earth is at rest." As I said, the Ptolemaic system includes lots of extra baggage that the relativistic description in a frame in which the Earth is at rest does not.

As I understood him, he says if two theories produce the same predictions, then they are both equally valid

And this doesn't apply to the Ptolemaic system anyway, since it did *not* make the same predictions as the Copernican system, and it certainly did not make the same predictions as our current theories do.

As for the relativistic frame in which the Earth is at rest, that's not a "different theory"; it's just SR applied using a particular frame. So all this stuff about different theories producing the same predictions is irrelevant if you're just talking about using different reference frames.

 

[Layman]

The glider, being inertial, would see all laws in their simplest form - all exactly as for earth. Then, the only reason for considering the glider moving (when in the glider) would be that the Earth is bigger. I have three times asked you to answer a set of questions about the fallacy of 'bigger wins', up to galactic scales where this is no center of mass point, in principle. You have completely dodged all these questions, while repeating fallacies like F=MA, and action reaction, would not hold in a so called moving frame [they would only not hold in a non-inertial frame].

PA, I'm not saying that "bigger wins," you are. That said, I won't say size is irrelevant, either.

Suppose you fill rocket with rocket fuel, light the torch, and it starts separating from the earth. How much "force" would it take for an object the size of the Earth to "suddenly" start moving away from the rocket (rather than vice versa)?

I have no idea, but I'm sure it's a hell of a lot more than is potentially available from the rocket fuel. The only practical view is that the rocket is NOT motionless and is in fact going "away from" earth.

 

[PAllen]

You would seem to be in disagreement with Peter on this point, PA. You say the ptolemic system is "not part of science." As I understood him, he says if two theories produce the same predictions, then they are both equally valid, and hence presumably equally a "part of science."

That's not how I read what he said. First, he never commented on Ptolemaic theories at least in any recent posts (I didn't search to the beginning). Second, he agreed that physicists, given two models with identical predictions, but one has additional undetectable elements and excess complexity, choose the former. The combination of a feature being both undetectable and superfluous renders it philosophy, not science (and arguably bad philosophy as well).

 

[PeterDonis]

That's not how I read what he said.

Me neither . See post #92. Your point is well taken as well.

 

[PeterDonis]

Suppose you fill rocket with rocket fuel, light the torch, and it starts separating from the earth...The only practical view is that the rocket is NOT motionless and is in fact going "away from" earth.

And then you, inside the rocket and feeling the acceleration produced by its engine, release a small object, say a marble, and watch it fall towards the stern of the rocket. Which is moving, the marble or you? The only practical view is that the marble is NOT motionless and is in fact falling away from you.

See the problem?

 

[PAllen]

PA, I'm not saying that "bigger wins," you are. That said, I won't say size is irrelevant, either.

Suppose you fill rocket with rocket fuel, light the torch, and it starts separating from the earth. How much "force" would it take for an object the size of the Earth to "suddenly" start moving away from the rocket (rather than vice versa)?

I have no idea, but I'm sure it's a hell of a lot more than is potentially available from the rocket fuel. The only practical view is that the rocket is NOT motionless and is in fact going "away from" earth.

Again, you are mixing a complex, non-inertial frame with inertial frames. In a frame moving uniformly relative to earth, the rocket and Earth are moving together, then rocket fires and decelerates to be at rest in this moving frame. Perhaps this is the frame of an asteroid, with the rocket ending up at rest relative to the asteroid.

Again, it seems you fundamentally don't understand Newtonian physics. You can't get to relativity without understanding Newtonian physics.

 

[PeterDonis]

Again, you are mixing a complex, non-inertial frame with inertial frames.

That's a problem if you do it unthinkingly, yes, but it's actually not quite the problem I was thinking of when I posed the question in my last post. I won't give the answer away yet since I want to see if Layman can see it for himself.

 

[Layman]

And this doesn't apply to the Ptolemaic system anyway, since it did *not* make the same predictions as the Copernican system, and it certainly did not make the same predictions as our current theories do.

I'm not sure what different predictions you have in mind with respect to planetary motion, Peter. I could refer you to a lot of sources, but here's very abbreviated one from PBS, with an excerpt (it's the first one I came across).

" That Ptolemy could place Earth at the center of the universe and still predict the planets’ positions adequately was a testament to his ability as a mathematician. That he could do so while maintaining the Greek belief that the heavens were perfect—and thus that each planet moved along a circular orbit at a constant speed—is nothing short of remarkable."

http://www.pbs.org/wnet/hawking/universes/html/ptole.html

Of course I'm not talking about the method of arriving at predictive results, or about the assumptions and rationale of the theory. Just the result, such as: There will be a total eclipse of the sun as viewed from Paraguay at 7:00 P.M. on June 27, 2053.

 

[PAllen]

In SR, that's true; but as I said in a previous post, GR fixes that too. The GR versions of F = MA, conservation of momentum, etc. hold in all frames, inertial and non-inertial.

True, but I didn't want to go there. Also, of course, there are still locally inertial frames in which there is the major simplification of the connection vanishing (locally) and metric becoming Minkowski (locally). This is equivalent to inertial forces vanishing.

 

[PeterDonis]

" That Ptolemy could place Earth at the center of the universe and still predict the planets’ positions adequately was a testament to his ability as a mathematician.

"Adequately" is not the same as "with the same accuracy as Copernican theory", nor is it the same as "with the same accuracy as our current theories". Different accuracy means different predictions; measurements can distinguish less accurate from more accurate predictions. Which is exactly what they did as advances in astronomy made the Ptolemaic theory obsolete. (Which did *not*, btw, happen when Copernicus published his theory; it wasn't until Kepler that a Sun-centered system was developed that was actually more accurate in its predictions than the Ptolemaic system. It was Tycho Brahe's more accurate observations that forced Kepler to abandon Copernicus' circular orbits and consider ellipses, and that was what made the difference in predictive accuracy.)

 

[Layman]

"Adequately" is not the same as "with the same accuracy as Copernican theory", nor is it the same as "with the same accuracy as our current theories". Different accuracy means different predictions; measurements can distinguish less accurate from more accurate predictions. Which is exactly what they did as advances in astronomy made the Ptolemaic theory obsolete. (Which did *not*, btw, happen when Copernicus published his theory; it wasn't until Kepler that a Sun-centered system was developed that was actually more accurate in its predictions than the Ptolemaic system. It was Tycho Brahe's more accurate observations that forced Kepler to abandon Copernicus' circular orbits and consider ellipses, and that was what made the difference in predictive accuracy.)

Heh, Peter, I knew when I made that post that you would jump on the word "adequately." As I said then, I could find other sources, but didn't bother. But here's another, now.

"In 1543, the geocentric system met its first serious challenge with the publication of Copernicus' De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres), which posited that the Earth and the other planets instead revolved around the Sun. The geocentric system was still held for many years afterwards, as at the time the Copernican system did not offer better predictions than the geocentric system, and it posed problems for both natural philosophy and scripture. The Copernican system was no more accurate than Ptolemy's system, because it still used circular orbits."

http://en.wikipedia.org/wiki/Geocentric_model

P.S.: I see that your post already noted theoretical improvements made by kepler, etc. But do you doubt that ptolemic astronomy could not also be "tweaked" (as Copernican theory was) to accommodate newly observed (measured) phenomena? I don't.

 

[PeterDonis]

But here's another, now.

Which says the same thing I said in the post you quoted. What's your point?

 

[PeterDonis]

True, but I didn't want to go there.

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

 

[Layman]

Which says the same thing I said in the post you quoted. What's your point?

See the P.S. I added to my last post.

 

[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 einstein'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.