Relativity vs competing theories

harrylin

I did not refer to "a speech" but to a number of articles, including a speech; and you seem to contradict Einstein's precision that the ether of the general theory of relativity is transmuted conceptually into the ether of Lorentz if we substitute constants for its functions of space. But we seem to agree that the label "ether" doesn't identify competing theories, so I won't elaborate more here as that would again deviate from the topic.

harrylin

That's also correct; as I elaborated, it depends on what you mean with that. To elaborate more: with GPS time and local "Einstein synchronization" we should be able to detect the difference in standard clock synchronization (which is commonly called "relativity of simultaneity") between the ECI frame and a lab frame.

harrylin

I'm fed up with the strawman called "LET"; I now conclude that it is less than useless - thus I will from now on ban it completely.

mangaroosh

Apologies, I was probably a bit vague in the question. I thought I had read on here (may have been elsewhere) that LET would treat the slowing of the ticking clock as being a result of the mechanics of the clock being affected by the motion i.e. that the moving clock ticks slower because the photon has a longer distance to travel.


Could LET be formulated without the superfluous rest frame? For example, if we were to say that if an absolute rest frame were to exist, then the earth would be moving relative to it with a velocity v and if an absolute rest frame were to exist, a train moving relative to the earth would be moving with a velocity u relative to that absolute rest frame.

Instead of assuming that we are stopped in the absolute rest frame, and considering everything else in motion with respect to us, could we assume that if an absolute rest frame were to exist, then we would be in motion with respect to it, as would everything else?

Again, we don't need the rest frame, we can just hypothesise that if one were to exist, that we would be in motion relative to it.

ghwellsjr

Prior to Einstein, Lorentz and others spent a lot of time trying to figure out the mechanics of length contraction and time dilation but nowadays, when we speak of LET, we have striped it of everything except the idea of a preferred rest frame.
Well the idea of LET is that nature needs it in some unknown way to make things happen so there wouldn't be much point in formulating LET without that rest frame. It's not superfluous to LET.
Sure, even in SR you don't have to pick a Frame of Reference in which you are at rest or the earth is at rest or the solar system is at rest or the galaxy is at rest. You can pick one in which you are traveling at any speed in any direction (as long as it is less than the speed of light).

mangaroosh

Sorry, I'm not sure I get what you mean by "stripped it of everything".

I'm just wondering, does it actually require an absolute rest frame? Again, we could hypothesise that if one were to exist, then we would be moving relative to it, but we don't really need to assert that it actually exists, simply that we [the earth] are in motion and not at absolute rest.

Does that carry with it an implicit assumption that some reference frame is at rest?

ghwellsjr

Stripped of any characteristics of an ether to explain why lengths contract and clocks run slower. I thought I explained that. And I didn't say "everything", I said "everything except the idea of a preferred rest frame." Think of it exactly like any rest frame in Special Relativity except that there is only one in which the speed of light is actually c.
LET postulates that there exists a single rest frame in which light travels at c. There is no proof or evidence for this but neither is there any proof or evidence that denies that it can't happen.
Yes.

It's no different than Einstein's second postulate that light travels at c in any reference frame. That also has no proof or evidence but neither is there any proof or evidence that denies that it can't happen.

It sure seems to me like you're understanding this. I don't see why you keep asking the same questions and getting the same answers. Maybe you could explain why you're struggling with this.

harrylin

That is just what Einstein did in 1905, and the result was SR. For that purpose he rephrased Maxwell's assumption about light propagation in vacuum by the same assumption relative to an unidentified but presumably arbitrary Newtonian system of reference (which he identified with "empty space") and made it a postulate. I find that physically a bit messy (discarding a physical model while keeping the phenomenon that is based on it), but it did the trick.
- http://www.fourmilab.ch/etexts/einstein/specrel/www/
- http://www.bartleby.com/173/7.html
That is correct of course. See also the discussion here which highlights the philosophical differences between interpretations:
- http://www.bartleby.com/173/7.html
Obviously.

harrylin

As a correct understanding of that paper may be of general interest, I now started a discussion of it as a new topic.

Harald

mangaroosh

I thought the first rule of PF was "don't talk about fight club"

mangaroosh

I think I am getting an understanding of it, but I'm not yet certain I understand the nuances of both; so my apologies if the questions are repetitive, but I'm not sure I that I have fully understood the implications; also, the statement that LET and SR would treat the light clock [on the train] thought experiment exactly the same, kind of threw me; because I thought both offered different explanations for what happens.


I'm not sure if you expressed it exactly so, but the understanding I've garnered, from discussing it with you, would definitely be in accord with that; as mentioned though, the statement above kind of threw me. With regard to the "stripping of everything"; I probably should have been more explicit in my reference, because it was the "everything [else]" that I wasn't 100% on.


Please forgive my continual recourse to the light clock thought experiment, if it seems either naiive or monotonous; it's just that I [personally] find it quite helpful as a visual aid to addressing my understanding, or lack thereof. Essentially, the differences [as I understood them] are:

• the reciprocity of the contractions i.e. there is reciprocity under SR but not LET
  Essentially, under SR each observers clock ticks normally, from their perspective, and it's the moving clock that ticks slower; but under LET, both clocks tick slower than the rest frame clock, but the trains clock ticks slowest (assuming a higher velocity relative to the rest frame)

• -the attributed cause of the contractions i.e. under SR time slows down, under LET it's the mechanics of the clock.
  My understanding is that, under SR, the relative motion of the reference frame causes spacetime to be affected, such that lengths contract and time slows down; but under LET a slower ticking clock is ascribed to the mechanics of the clock arising from the motion relative to the rest frame - i.e. the photon has a longer distance to travel between mirrors in a moving clock.

My understanding of LET is that, if the observer on the train were in a windowless carriage - like Galileo's observer on the ship - they would not be able to determine if they were at absolute rest, so they could not tell if their clock was ticking at the "normal" rate, or if it was actually ticking slower, because they would have nothing to compare it to. If, however, they were able to see the earthbound clock, they would presumably be able to calculate that it was ticking faster (assuming the earth is traveling with a lower velocity relative to the rest frame)

I had a more detailed explanation of what I mean typed out, but I wanted to try and make it more concise. I can post that if the above isn't very clear.


I'm still a bit unclear as to the status of the second postulate; initially I thought it was absolutely set in stone, but I'm a lot less certain of that now. I fully trust what you say about it, but when others reference experiments which purportedly verify the second postulate, I wonder if there is some nuance that I haven't picked up on. To me it seems as though there is [at least limited] scope in the formulation of this postulate.

Again, just to return to the light clock thought experiment, to try and clarify my [lack of] understanding: the tick-tock of a light clock is determined by the distance the photon has to travel in the light clock; under SR this is represented by a line perpendicular to the midpoint of both mirrors. This would be the same as if the clock were at absolute rest.

I've been told that this representation of the path of the photon is not based on an assumption, but is something that is borne out by experiment; or from the verification of the second postulate; that it is because the speed of light is constant, regardless of motion relative to the source, that it has to be so, otherwise the observer on the train would measure a slower speed of light.

To me it appears as though this is not the case under LET, where, as you mention, we can just consider the train's clock from the absolute rest frame - which would be the same as the observer on the platform's view, in the Einsteinian version. In this case the tick-tock of the trains clock is not simply the perpendicular line between the midpoints of the two mirrors, rather the line as represented by the hypotenuse of a right angled triangle; and this would be as true for the observer on the train as for the observer in the absolute rest frame - even if the observer on the train could not determine that it was so.


Hopefully the above makes sense, and if I haven't entirely exhausted your patience, if you could point out anywhere I might be going wrong, I would be very greatful.

harrylin

Well yes, but the explanations are at different levels, just as Newton's explanations differ from what nowadays is called classical mechanics. As Lorentz later put it: “Einstein simply postulates what we have deduced [...] from the fundamental equations of the electromagnetic field”. Note that that remark refers to the second postulate: the PoR was not deduced from the field equations but appeared to be at odds with it.

I can see no reason for such an assumption in any interpretation of SR.

There are several formulations of it; its purpose was to provide a necessary condition based on Maxwell's electrodynamics for the derivation of the new transformation equations. A postulate based on an alternative electrodynamics theory such as by Stokes or Ritz leads to other transformations.
That is slightly inaccurate; as I think was highlighted in this thread or in another recent thread, experimental results of that time discarded all light models except that of Maxwell-Lorentz. However, in view of several other experiments such as MMX also that one was in problems, and a fix was needed to make it PoR-compatible.
Your line of thinking seems to be a preparation for MMX, as well as, potentially the later KTX:
http://en.wikipedia.org/wiki/Kennedy...ike_experiment

With such known and future experimental results was dealt with here:
http://en.wikisource.org/wiki/Electromagnetic_phenomena

Harald

mangaroosh

Hey Harry, thanks for replying. Firstly, can I assume, from the lack of comment, that what I wrote about the reciprocity of contractions and the attributed cause of contractions is accurate?
I would potentially question what is deduced from the fundamental equations, particularly because Lorentz's ether theory appears to be based on a slightly different deduction; am I right in that?


It wasn't an assumption for SR, it was the application of the principle of relativity to LET; at least that was the intention.

Would it be equally possible to formulate it as, the speed of light is constant with respect to the point of emission, but independent of the motion of the source? Would that conflict with any experimental evidence?

Apologies, I keep having to refer back to the light clock on the train thought experiment; does that mean that both the treatment of the path of the photon in SR and LET are possible?

When I say the treatment of the photon under LET, I mean my understanding as I've outlined above i.e. that the path of the photon is not necessarily the perpendicular line between the midpoints of the two mirrors, but rather the path represented by the hypotenuse of a right angled triangle.

To a certain extent it is a prep. for MMX; again, I'm wondering if the representation of the path of a photon [in a light clock] under LET were a representation of a photon in the MMX, would the null result still arise; I presume it would, but I'm not sure.

ghwellsjr

There is no difference in what we know is actually happening under SR or under LET. It is only a difference in what you are assigning to things that you can't know are happening. In SR once you assign a Frame of Reference, which defines times and distances, then the reciprocity disappears. If you select a FoR in which you are not at rest, or if you start moving after you select a FoR in which you started out at rest, then time for you is dilated and your lengths are contracted, although you can't tell that. It's only when you then transform all the distances and times (events) into a new definition (a different FoR) that what was assigned a shorter distance or a longer time in one FoR takes on a longer distance or a shorter time in the other FoR.

The easiest way I know to communicate this idea is to think just about relative motion. If A and B are in relative motion, in one FoR, A is assigned a speed of zero and B is assigned a non-zero speed. In another FoR, B is assigned a speed of zero and A is assigned non-zero speed. Do you think that using a different FoR caused A to suddenly start moving? I don't think so, I think with regard to motion, different frames make perfect sense. The same thing applies to length contraction and time dilation. The ones that have motion according to the FoR that you are using are the ones for which lengths and times take on a different meaning.

Do you think that just because in one FoR B is stationary and in another one B is moving that spacetime is affected? Nothing is happening in space or time or spacetime just because you choose to use a different FoR to assign distances and times differently such that A is stationary under one of those definitions and moving under a different definition.

In the early days, Lorentz and others were trying to determine how the mechanics of a clock would cause it to slow down as it traveled through the ether but now we don't worry about that. What we do instead is verify that the laws of physics transform intact under the Lorentz transformation. If they don't, we know they are not valid and we change them until they do. If they do, then we don't have to be concerned about the specific mechanics, it will happen automatically.

harrylin

Not really, it only means that when skimming over that part I didn't see anything shocking.
I wasn't sure about what you meant and decided to leave it. [Addendum: I agree with George's clarifications.]
However, with my remark in post #38 I meant that I decided from now on to abstain from discussions about a hypothetical competing theory to "relativity theory" as I find it too artificial - surely you notice that "ban" in my latest posts.
Historically there are different interpretations of what started out as the "Lorentz-Einstein theory of relativity" and the idea that Lorentz and Einstein proposed competing theories emerged later. In the early years they promoted the new theory together and their interpretations were not distinguished in presentations and papers. That should hardly be surprising, as physicists tend to focus on the predictions of a theory and not on "metaphysics".
Lorentz already answered that: deduced was what Einstein made a postulate. The fundamental equations suggested to Maxwell and followers that light is a wave with propagation constant c in vacuum.
Not clear what distinction you make: the speed with respect to one point is equal to the speed with respect to all points - that makes no difference.
I'm not sure that I understood your set-up, if it wasn't a bit like MMX; and as I already indicated, basic (x,t) problems like MMX and the later KTX were perfectly dealt with in Lorentz's 1904 paper.

mangaroosh

Thanks gh; I think I understand the scenario above.

If we consider both A and B as "objective" observers, which I think we can do because it is a thought "experiment". From A's perspective, he can consider himself at rest and so the instruments in B's reference frame undergo contraction; while B can also consider himself at rest and it is A's instruments that contract, form B's perspective. The instruments will contract by the same amount.That is what I meant by reciprocity.

As I understand it, under LET the contractions aren't reciprocal, in this way. If for example we were to have 3 observers: A is in the absolute rest frame; B is at rest on earth; C is on the train; where the train is moving with a higher relative velocity to the rest frame, than the earth. Then A will see the instruments in the other reference frames contract; B will see C's instruments contract but A's will "expand" [just to contrast with "contract"]; while C will see the other instruments [in both reference frames] expand.


I suppose I'm just trying to see what the logical conclusion is. If there is absolute simultaneity under LET, but there are clocks which tick at different times, then it seems like it has to be the mechanics which causes the clocks to slow down and not "time" slowing down. Under SR, it appears as though the slowing of the clocks is attributed to the slowing of time.

Does that make sense, or is it way off?

mangaroosh

It wasn't immediately clear, probably because I was reading them through the lens of "relativity vs competing theories"; though, I was wondering how you managed to quote a part of my post and leave out the important qualifier referencing LET I presumed it was intentional but I wasn't clear as to what the intention was.

I know physicists tend not to focus on the "metaphysics" but scientific theories have "metaphysical" implications, and insofar as physicists are concerned with accurately describing reality they are concerned with metaphysics; of course, where they are not concerned with accurately describing reality, they are not concerned with metaphysics. Given that the differences between [what has been described in this thread as] LET and Einsteinian relativity are effectively metaphysical, it is probably unavoidable to discuss them without discussing the metaphysics. Who knows a certain level of metaphysical consideration could actually facilitate the unification of QM and SR.


[QUOTE=harrylin;3754767]Lorentz already answered that: deduced was what Einstein made a postulate. The fundamental equations suggested to Maxwell and followers that light is a wave with propagation constant c in vacuum.[/quote
The speed of light being a constant (c) in vacuum, and the speed of light being c regardless of the motion relative to the source, appear to be somewhat different.

This is an area where I am not clear. The speed of light being constant with respect to one point, doesn't necessarily mean that it is constant with respect to all points - which I know is not what you were suggesting.

Has the latter been borne out by experimental evidence, or has it just not been contradicted by experimental evidence?

Part of my confusion arises because it seems as though LET has a different "second postulate" than Einsteinian relativity, which also fits the evidence. This leads me to believe there is limited scope for deducing a postulate with respect to the speed of light - would that be correct?

If we consider the two light clock examples, form SR and LET (as I understand it).

Under SR the path of the photon traces a line perpendicular to the two mirrors; while under LET [as I understand it] the photon traces a line as represented by the hypotenuse of a right angled triangle.

Under SR, the photon has a strictly vertical velocity component of c, and so the observer would measure the speed of light to be c, relative to the carriage.

Under LET, the photon has a velocity of c in the horizontal (or diagonal) direction - due to the motion of the train and the clock - but not a velocity of c in the vertical direction. If the observer can only measure the speed relative to his reference frame he would not measure the speed of light to be c, but some lower value.

If both theories equally explain the evidence, then it suggests that the speed of light is not necessarily constant in all directions, regardless of the motion relative to the source.


It has been suggested [by others, elsewhere] that the MMX demonstrates that the speed of light is constant in all directions, regardless of the motion relative to the source; however, if the null result of the MMX is equally explained under LET, then this seems to suggest that this isn't the case.

Again, hopefully the above makes sense.