Michelson–Morley experiment: Did it disprove the existence of ether?

[harrylin]

[I wrote: "It's an indirect demonstration of Lorentz contraction - based on our assumption that light propagation is fully unaffected by its motion wrt the source.]

Hmmm. What does that mean?

Either:
(1) MMX demonstrates that speed of light relative to the MMX apparatus was the same in every direction.
or
(2) MMX demonstrates that solids undergo Lorentz contraction as they move relative to the frame of reference in which the speed of light is the same in every direction.

That's by far not all:

3) Alternatively if for example transverse light propagation in the ether is reduced by the Lorentz factor by its motion wrt the source, then MMX demonstrates that solids undergo isotropic contraction - I think of that example because it's a proposed theory (by H. Varcollier, 1942 and P. Dive, 1950) that is not compatible with the relativity principle, but still close enough.

That's why we prefer instead of null results, positive evidence of the kind that Michelson was looking for - the kind we already got for the speed of light in moving media (but only well tested along the same direction), and for time dilation.

Cheers,
Harald

 

[harrylin]

So is the only difference that in SR we decide not to care that "we don't know where the ether rest frame is."

Even stronger: SR does not care if an ether exists. On purpose an ether is not part of its postulates and it's not required for the (re)derivation of the Lorentz transformations. So, the physical model is replaced by a set of rules. And if one still wants to make intuitive or physical sense of it, then one may use for example the stationary ether model or, alternatively, a physical interpretation of Minkowski Spacetime.

However, IMHO that is cheating a little, as the light postulate summarizes the consequences of the stationary ether model.

 

[thwle]

That's by far not all:

3) Alternatively if for example transverse light propagation in the ether is reduced by the Lorentz factor by its motion wrt the source, then MMX demonstrates that solids undergo isotropic contraction - I think of that example because it's a proposed theory (by H. Varcollier, 1942 and P. Dive, 1950) that is not compatible with the relativity principle, but still close enough.

That's why we prefer instead of null results, positive evidence of the kind that Michelson was looking for - the kind we already got for the speed of light in moving media (but only well tested along the same direction), and for time dilation.

Cheers,
Harald

I have to suppose you understand what your said. Have you understood the things I have said?

Admitting my uncertainty as to the meaning of the bold (above), I suppose many other interpretations are possible if enough new postulates be accepted to cobble a correspondence. If one honors Occam's principle, I maintain that only the two possibilities are available. I think we agree the first of the two can be ruled out based on evidence external to MMX. I daresay, they are logically an exhaustion of possibility.

Please tell me what you think of my reasoning in my previous post (here quoted):

"I also think the equivalence [of radar ranging and measuring stick measuring] stands to reason: If the forces between atoms in a solid are communicated at the speed of light, then they would be equivalent. Only if the forces were instantaneously communicated would they be different as calculated by Michelson and Morley."

Think of a Hydrogen molecule. What communication of forces between the two hydrogen atoms determines the distance between them? What is the speed of that communication and relative to what frame of reference is that speed? Isn't this interaction between atoms rather like echo ranging?

 

[ghwellsjr]

... [two] theories:

In one of these, Lorentz Ether Theory (LET), it is postulated that there exists only a single absolute frame of reference in which light actually travels in all directions at the speed c and only when you are at rest in that frame will your measurements reflect what is really happening. All other inertially moving observers get the same results because nature is playing tricks on them by adjusting their clocks and rulers in such a way that they get the same results as they would get if they were stationary in the ether. In fact, since we don't know where the ether rest frame is, chances are, all observers are not at rest in the ether frame and so it's a safe bet that we all have our clocks and rulers modified.

Special Relativity, SR, on the other hand, postulates that you can consider anyone of the inertial observers to be at rest in the ether frame of reference where his measurements reflect what is really going on and all the other inertially moving observers are getting their clocks and rulers modified by nature in such a way that they get the same result as they would if they were at rest in the ether but the theory says that their clocks and rulers are actually getting modified, according to the one observer which we are considering to be at rest in the ether.

So is the only difference that in SR we decide not to care that "we don't know where the ether rest frame is."

Actually, I like harrylin's suggested alternative to my second paragraph quoted above where he basically substituted the word "pretend":

Special Relativity postulates that you can pretend anyone of the inertial observers to be at rest in the ether frame of reference so that all the other inertially moving observers are getting their clocks and rulers modified by nature in such a way that they get the same result as they would if they were at rest in the ether; the theory thus says that - according to the one observer which we are considering to be at rest in the ether - their clocks and rulers are actually affected by their motion.
And just as in Newtonian mechanics, any observer can pretend to be in rest so that it's the others that move - but if we pretend all to be at rest at the same time then we mess up!

So it's not so much that "we decide not to care", it's that we can pretend that any inertial reference frame will behave exactly like the illusive ether frame. It's so much easier to do science this way than to be concerned about where that pesky ether resides.

 

[ghwellsjr]

...
Please tell me what you think of my reasoning in my previous post (here quoted):

"I also think the equivalence [of radar ranging and measuring stick measuring] stands to reason: If the forces between atoms in a solid are communicated at the speed of light, then they would be equivalent. Only if the forces were instantaneously communicated would they be different as calculated by Michelson and Morley."

Think of a Hydrogen molecule. What communication of forces between the two hydrogen atoms determines the distance between them? What is the speed of that communication and relative to what frame of reference is that speed? Isn't this interaction between atoms rather like echo ranging?

I came across this passage from Hermann Bondi's book, Relativity and Common Sense:

"THE VELOCITY OF LIGHT
On this basis, then, we can look at time standards as primary and at distance standards as quite secondary and of little importance. This does seem to be a sound procedure, particularly when we think about what our inch tapes and measuring rods are actually made of. We know that they are composed of atoms whose structure is kept in shape by electric forces. We know that these atoms have certain periods of vibration and we know that, in the materials we call very rigid, it is as a consequence of the particular periods of vibration of the atoms that different atoms keep a definite distance apart in the structure of the rod. Thus we can argue that the length of a rod is really determined by the period of the oscillation of the atoms of which it is composed, this being translated, in the usual way, through the velocity of light into distance. If we argue, as we well may, that the distances between the atoms in what we call rigid materials are the distances corresponding to the oscillations of the atoms, then we could say that those distances, too, are effectively determined by radar methods. On that basis, then, distance becomes a purely secondary quantity, time is the primary thing, and the velocity of light is in natural units necessarily equal to unity. But if we are so perverse as to choose to measure distances in feet rather than in light millimicroseconds, then we have to introduce a conventional factor of conversion which effectively defines the foot, and this we call the velocity of light."

He was not discussing length contraction at this time but his argument seems to be applicable to what you are suggesting.

However, I don't think too many scientists these days concern themselves with the mechanism of length contraction, I believe they are content to regard it as a natural consequence of Special Relativity.

 

[ghwellsjr]

...
Either:
(1) MMX demonstrates that speed of light relative to the MMX apparatus was the same in every direction.
or
(2) MMX demonstrates that solids undergo Lorentz contraction as they move relative to the frame of reference in which the speed of light is the same in every direction.

As I see it, Lorentz took interpretation (2) because he assumed an absolute ether rest frame in which MMX was not at rest while Einstein took interpretation (1) if he assumed a reference frame in which MMX was at rest or (2) if he assumed a reference frame in which MMX was not at rest (one in which the sun is at rest, for example, or any other candidate for the presummed ether).

As harrylin pointed out here:

Einstein put it as follows:

"for a co-ordinate system moving with the Earth the mirror system of Michelson and Morley is not shortened, but it is shortened for a co-ordinate system which is at rest relatively to the sun."
- http://www.bartleby.com/173/16.html

 

[harrylin]

I have to suppose you understand what your said. Have you understood the things I have said?

Yes and yes; and from your reply it appears that you still did not understand what I said. See next.

Admitting my uncertainty as to the meaning of the bold (above), I suppose many other interpretations are possible if enough new postulates be accepted to cobble a correspondence. If one honors Occam's principle, I maintain that only the two possibilities are available. I think we agree the first of the two can be ruled out based on evidence external to MMX. I daresay, they are logically an exhaustion of possibility.

My reply was to your argument which did not include Occam's razor; obviously the two first two options are not exhaustive but sure they are the simplest. Similarly Michelson based his reasoning on a number of unmentioned assumptions, using Occam's principle. And he was wrong. The evidence forced people to correct his reasoning by a new postulate, be it Lorentz contraction or the PoR. New evidence can force us to again modify our assumptions. If I'm not mistaken, Voigt even derived the third possibility from the wave equation and I have never seen a rebuttal. As I know of such options in the literature, I cannot agree that your two simplest options are "logically an exhaustion of possibility".

Please tell me what you think of my reasoning in my previous post (here quoted):

"I also think the equivalence [of radar ranging and measuring stick measuring] stands to reason: If the forces between atoms in a solid are communicated at the speed of light, then they would be equivalent. Only if the forces were instantaneously communicated would they be different as calculated by Michelson and Morley."

Think of a Hydrogen molecule. What communication of forces between the two hydrogen atoms determines the distance between them? What is the speed of that communication and relative to what frame of reference is that speed? Isn't this interaction between atoms rather like echo ranging?

Already twice I replied with yes to that! ;-)

Also Einstein admitted on the page I cited earlier that "The special theory of relativity has crystallised out from the Maxwell-Lorentz theory of electromagnetic phenomena".

 

[Buckleymanor]

"I also think the equivalence [of radar ranging and measuring stick measuring] stands to reason: If the forces between atoms in a solid are communicated at the speed of light, then they would be equivalent. Only if the forces were instantaneously communicated would they be different as calculated by Michelson and Morley."

Think of a Hydrogen molecule. What communication of forces between the two hydrogen atoms determines the distance between them? What is the speed of that communication and relative to what frame of reference is that speed? Isn't this interaction between atoms rather like echo ranging?

They would allso be different if the forces interacted slower than the speed of light.
It's hard to imagine that all materials irrespective of there mass would expand or contract at the same speed , the speed of light.
Gravitational effects upon the mass in those different materials would prevent this.

 

[roineust]

Thanks ghwellsjr,
One more pre exercise question...sorry, for asking questions indeed I don't need to prepare time in advance.

What is the relation between the current understanding of time dilation and clock synchronization? What I mean is this - If I make many times an experiment that shows me that 10 'ticks' of clock 1 (say a very precise clock etc...) are never the same as 10 'ticks' of an identical clock 2, because they are at different inertial frames - Then why can't I take these results and formalize them in such a way that I say: If there is inertial difference, 10 ticks of no.1 will never be the same as 10 ticks of no.2, and it has nothing to do with synchronization (although the results were achieved using synchronization).


Thanks,
Roi.

 

[thwle]

They would allso be different if the forces interacted slower than the speed of light.
It's hard to imagine that all materials irrespective of there mass would expand or contract at the same speed , the speed of light.
Gravitational effects upon the mass in those different materials would prevent this.

The amount of contraction would depend in the speed of communication of the forces that govern the dimensions. Instantaneous for no contraction (as assumed in MMX). I said nothing about speed of contraction.

 

[thwle]

Yes and yes; and from your reply it appears that you still did not understand what I said.

Yes, I admit that I don't. Not that I don't agree; I just can't make out what you mean.

... New evidence can force us to again modify our assumptions.

With my next post I will present new evidence showing that it is possible (1) to recognize the inertial frame of reference in which light propagates at the same speed in all directions and (2) to discover inertial frames in which it does not.

 

[harrylin]

As I see it, Lorentz took interpretation (2) because he assumed an absolute ether rest frame in which MMX was not at rest while Einstein took interpretation (1) if he assumed a reference frame in which MMX was at rest or (2) if he assumed a reference frame in which MMX was not at rest (one in which the sun is at rest, for example, or any other candidate for the presummed ether).

As harrylin pointed out here:

Thanks ghwellsjr,
One more pre exercise question...sorry, for asking questions indeed I don't need to prepare time in advance.

What is the relation between the current understanding of time dilation and clock synchronization? What I mean is this - If I make many times an experiment that shows me that 10 'ticks' of clock 1 (say a very precise clock etc...) are never the same as 10 'ticks' of an identical clock 2, because they are at different inertial frames - Then why can't I take these results and formalize them in such a way that I say: If there is inertial difference, 10 ticks of no.1 will never be the same as 10 ticks of no.2, and it has nothing to do with synchronization (although the results were achieved using synchronization).

Thanks,
Roi.

Hi Roi, it's not clear to me what you want to do, and how it relates to the topic... I agree that there is a fundamental difference, as clock synchronization is a human choice while time dilation relates to a physical phenomenon that also manifests itself for cases in which clock synchronization plays no role.
However, for inertial motion the two are interrelated. That is because if you want to measure the time between two ticks of a clock that is moving past your clocks, you need two synchronized clocks to do that. Alternatively, with only one clock, you can film the moving clock from a distance, but the correction for the propagation time is related to the clock synchronization which is simply based on a convenient speed of light assumption. So, although it implies a different speed of light in different directions wrt your frame, you can always adapt your measurement such that 10 ticks of no.1 will be (or appear to be) the same as 10 ticks of no.2. Does that help?

Harald

 

[Buckleymanor]

The amount of contraction would depend in the speed of communication of the forces that govern the dimensions. Instantaneous for no contraction (as assumed in MMX). I said nothing about speed of contraction.

Well I don't understand what you are talking about, you mentioned.

Think of a Hydrogen molecule. What communication of forces between the two hydrogen atoms determines the distance between them?

Contraction is not specificaly mentioned then what is it you are you trying to communicate.

 

[harrylin]

Hello all and Buckleymanor, for understanding the Fitzgerald contraction it can be enlightening to check out the paper which probably gave him that idea. You can find it here:

http://en.wikisource.org/wiki/Motion_of_Electrification_through_a_Dielectric

Note in particular the section on "Complete Solution ..." and eq.29.

 

[thwle]

harrylin wrote: "It's an indirect demonstration of Lorentz contraction - based on our assumption that light propagation is fully unaffected by its motion wrt the source.]

thwle replied: "Hmmm. What does that mean?" [Because thwle doesn't understand the difference between motion of light and propagation of light.]

thwle also wrote:
"Either:
(1) MMX demonstrates that speed of light relative to the MMX apparatus was the same in every direction.
or
(2) MMX demonstrates that solids undergo Lorentz contraction as they move relative to the frame of reference in which the speed of light is the same in every direction. "
[Because MMX involves no clocks thwle thinks time dilation arguments are irrelevant and that contraction is all that remains to explain the null result.]

harrylin's claimed he understood thwle, but thwle continued to doubt that harrylin did -- or that Buckleymaster did -- though ghwellsjr clearly understood thwle at least in part after at first resisting and finally expressed surprise that he had been unable to see thwle's point sooner.

thwle got to feeling a little frustrated and rashly promised to demonstrate that a unique frame or reference in which light proagates at the same speed in all directions not only exists but is discernable from other frames of reference.

Now, thwle has had the audacity to perform on his promise: See the new thread "Absolute Rest" under special and general relativity.

 

[thwle]

harrylin wrote: "It's an indirect demonstration of Lorentz contraction - based on our assumption that light propagation is fully unaffected by its motion wrt the source.]

thwle replied: "Hmmm. What does that mean?" [Because thwle doesn't understand the difference between motion of light and propagation of light.]

thwle also wrote:
"Either:
(1) MMX demonstrates that speed of light relative to the MMX apparatus was the same in every direction.
or
(2) MMX demonstrates that solids undergo Lorentz contraction as they move relative to the frame of reference in which the speed of light is the same in every direction. "
[Because MMX involves no clocks, thwle thinks time dilation arguments are irrelevant and that contraction is all that remains to explain the null result.]

harrylin claimed he understood thwle, but thwle continued to doubt that harrylin did -- or that Buckleymaster did -- though ghwellsjr clearly understood thwle at least in part (after at first resisting) and finally expressed surprise that he had been unable to see thwle's point sooner.

thwle got to feeling a little frustrated and rashly promised to demonstrate that a unique frame or reference in which light proagates at the same speed in all directions not only exists but is discernable from other frames of reference.

Now, thwle has had the audacity to perform on his promise: See the new thread "Absolute Rest" under special and general relativity.

 

[Dale]

But thwle is wrong in his analysis.

 

[PhilDSP]

Hello thwle, I'd like to add some comments.

Either:
(1) MMX demonstrates that speed of light relative to the MMX apparatus was the same in every direction.
or
(2) MMX demonstrates that solids undergo Lorentz contraction as they move relative to the frame of reference in which the speed of light is the same in every direction.

As I think Harald implied, it might be on pretty shaky logical ground to make very limiting inferences from a null result. My take on your point (1) for instance is that really, what was demonstrated was that the apparent phase of the light wave was the same in every direction. There are additional steps required to actually prove that the apparent velocity was the same. (I'm thinking in terms of Wave Mechanics here)

Please tell me what you think of my reasoning in my previous post (here quoted):

"I also think the equivalence [of radar ranging and measuring stick measuring] stands to reason: If the forces between atoms in a solid are communicated at the speed of light, then they would be equivalent. Only if the forces were instantaneously communicated would they be different as calculated by Michelson and Morley."

Think of a Hydrogen molecule. What communication of forces between the two hydrogen atoms determines the distance between them? What is the speed of that communication and relative to what frame of reference is that speed? Isn't this interaction between atoms rather like echo ranging?

We should remember that any medium apart from a pure vacuum is dispersive. That means the effective speed of EM propagation is slowed compared with c. So any inter-atom and inter-molecule force changes are not in step with light propagation and are different for each piece of the MMX apparatus.

 

[thwle]

But thwle is wrong in his analysis.

Can you be specific?

 

[thwle]

Hello thwle, I'd like to add some comments.



As I think Harald implied, it might be on pretty shaky logical ground to make very limiting inferences from a null result. My take on your point (1) for instance is that really, what was demonstrated was that the apparent phase of the light wave was the same in every direction. There are additional steps required to actually prove that the apparent velocity was the same. (I'm thinking in terms of Wave Mechanics here)

The phase going in was the same (at the half-silvered mirror beam-splitter). The phase coming out remained the same before during and after rotation. Necessary conclusion the path lengths in wavelengths could not have changed except by the same number of wavelengths. The path lengths had been set up "equal" (within a few tenths of a millimeter), so it remains uncertain whether the number of wavelengths along the paths was unchanged or did change. I tend to believe the path lengths in wavelengths was constant.

We should remember that any medium apart from a pure vacuum is dispersive. That means the effective speed of EM propagation is slowed compared with c. So any inter-atom and inter-molecule force changes are not in step with light propagation and are different for each piece of the MMX apparatus.

Do you think "dispersive medium" is an appropriate description of interatomic spaces within molecules?

 

[Dale]

Can you be specific?

Yes, I already posted specifics in the other thread. But the bottom line is that you forgot the relativity of simultaneity. Don't feel too bad about that, it is the hardest concept for students to learn. Also, the speeds of the various clocks were not the same so their time dilation factors would not be equal.

 

[PhilDSP]

Do you think "dispersive medium" is an appropriate description of interatomic spaces within molecules?

For any solid components of the MMX apparatus, most definitely yes. Even air on the surface of the Earth is dispersive but to a very limited extent. For most experiments not requiring extreme precision it can be regarded as being near enough to vacuum in optical characteristics.

 

[harrylin]

Yes, I admit that I don't. Not that I don't agree; I just can't make out what you mean.

As a matter of fact, I didn't formulate that well - sorry for that!

What I meant was that according to some theories the speed of light in the ether depends on its direction of propagation wrt to a moving source. Standard wave theory simply postulates that the speed of a wave is fully independent of the speed of the emitter, so that it is always exactly the same in all directions.

With my next post I will present new evidence showing that it is possible (1) to recognize the inertial frame of reference in which light propagates at the same speed in all directions and (2) to discover inertial frames in which it does not.

That would be a world sensation! Where is that post?

Edit: Oh I see, "Absolute rest" thread. I'll reply there!

 

[harrylin]

Hello thwle, I'd like to add some comments.

As I think Harald implied, it might be on pretty shaky logical ground to make very limiting inferences from a null result. [...]

Yes Indeed.

We should remember that any medium apart from a pure vacuum is dispersive. That means the effective speed of EM propagation is slowed compared with c. So any inter-atom and inter-molecule force changes are not in step with light propagation and are different for each piece of the MMX apparatus.

I'm afraid that you misunderstand modern scattering and refraction theory, which is still very close to Lorentz's theory. According to that, light propagation is always at speed c on a microscopic scale and interacts with matter in such a way that delayed secondary waves result in an effectively reduced macroscopic speed. I'm too lazy now to formulate it perfectly, but I guess it's good enough as a rough sketch - check an optics book for more details. Lorentz solved this problem around 1895 or so.

 

[roineust]

Thanks Harald,
Your answer is complex and I don't understand it fully.

The reason I ask it, is that if the same amount of ticks are not the same, and still light speed is considered invariant, then how could the physical laws invariance postulate, still be correct? I can show you a simple experiment diagram in this case, but I guess it is not needed, since you already know what I am asking and where I got it wrong...

Roi.

 

[PhilDSP]

I'm afraid that you misunderstand modern scattering and refraction theory, which is still very close to Lorentz's theory. According to that, light propagation is always at speed c on a microscopic scale and interacts with matter in such a way that delayed secondary waves result in an effectively reduced macroscopic speed. I'm too lazy now to formulate it perfectly, but I guess it's good enough as a rough sketch - check an optics book for more details. Lorentz solved this problem around 1895 or so.

That's not a very definitive answer or comment, if you don't mind me pointing that out as a friendly critique. I was talking about the effective speed of propagation (both macroscopically and microscopically). Born and Wolf's "Principles of Optics" devotes many pages in heavy detail on this. The diminishing of effective speed of propagation occurs even at the level of a single atom, molecule, electron or proton. Yes, from the microscopic form of the Maxwell equations you can see that mathematically some aspect of energy (partial information) does travel at c.

In considering that you may be entitled to interpret that partial energy exchange as reverberating around the particle or as a form of resonance. It is important to keep in mind that the exchange is not completed until propagated at the lower speed. And the resulting wave pattern propagates at the lower speed. If you think about the situation seriously you will likely come to the conclusion that if the larger area of medium around an atom or molecule is dispersive, then the near area in the atom or molecule's vicinity must be super-dispersive. Consider, from plasma equations, that the density of electrons or protons in the medium profoundly affects the index of refraction.

Cutting to the end of the story, though the very initial effects of force changes start to be felt by a particle at the leading edge of c, the full effect of force changes are not complete until the period of lesser propagation speed has lapsed.

 

[harrylin]

That's not a very definitive answer or comment, if you don't mind me pointing that out as a friendly critique. I was talking about the effective speed of propagation (both macroscopically and microscopically). Born and Wolf's "Principles of Optics" devotes many pages in heavy detail on this. The diminishing of effective speed of propagation occurs even at the level of a single atom, molecule, electron or proton. Yes, from the microscopic form of the Maxwell equations you can see that mathematically some aspect of energy (partial information) does travel at c.

In considering that you may be entitled to interpret that partial energy exchange as reverberating around the particle or as a form of resonance. It is important to keep in mind that the exchange is not completed until propagated at the lower speed. And the resulting wave pattern propagates at the lower speed. If you think about the situation seriously you will likely come to the conclusion that if the larger area of medium around an atom or molecule is dispersive, then the near area in the atom or molecule's vicinity must be super-dispersive. Consider, from plasma equations, that the density of electrons or protons in the medium profoundly affects the index of refraction.

Cutting to the end of the story, though the very initial effects of force changes start to be felt by a particle at the leading edge of c, the full effect of force changes are not complete until the period of lesser propagation speed has lapsed.

Sorry but you completely lost me now... what has that all to do with MMX?

For MMX the point is that the Fresnel-Fizeau equation as refined by Lorentz is valid (it has been verified along at least one direction). If it is valid, then all a dielectric does is to delay both light rays in the MMX apparatus equally.

 

[harrylin]

Thanks Harald,
Your answer is complex and I don't understand it fully.

The reason I ask it, is that if the same amount of ticks are not the same, and still light speed is considered invariant, then how could the physical laws invariance postulate, still be correct? I can show you a simple experiment diagram in this case, but I guess it is not needed, since you already know what I am asking and where I got it wrong...

Roi.

Now I must guess as to what the problem is... and I'm afraid that I don't know where you got it wrong! But often the cause of the problem is the sound bite that "light speed is invariant". Special relativity defines "speed" and so on purely operationally. Take the example of light propagating from a source S, from left to right like in one arm of a moving MMX apparatus, and let's choose it along the direction of motion. Let's imagine added clocks and means to detect light pulses.

S ----------------------------->¦ M
D <-----------------------------¦ as seen by co-moving observer

According to theory, the light propagates at +c to the right while the apparatus moves at +v to the right, and after reflection the light propagates at +c to the left. According to relativity the arm is slightly contracted so that the two-way time is only increased by the Lorentz factor and not (as Michelson calculated) by the square of the Lorentz factor.

Now, a co-moving observer will define time such that the moment a light pulse hits the mirror M, the time of a clock at that point is exactly in-between the times indicated at the times of emission and reception at point S/D. His/her measured speed of light is as a consequence isotropic by definition. If now, as relativity has it, also the observer's clocks run slower by the Lorentz factor, then his/her measured speed of light is also isotropically c.

Consequently, the laws of optics for stationary bodies will work just fine with the assumption that the apparatus is in rest.

That is just the same as with inertial motion in Newton's mechanics, which also uses the relativity principle.

Does that help?
Harald

 

[PhilDSP]

Sorry but you completely lost me now... what has that all to do with MMX?

For MMX the point is that the Fresnel-Fizeau equation as refined by Lorentz is valid (it has been verified along at least one direction). If it is valid, then all a dielectric does is to delay both light rays in the MMX apparatus equally.

I was responding to thwle's interesting observation that length contraction may be propagated rather than occur instantaneously. If that's the case then different materials would react at different speeds. And a ripple would occur in the materials. That's potentially testable, especially on the atomic level.

I agree, that the MMX is adequately described by the Fresnel-Fizeau type equations. The variations of relative velocity are far too small and too gradual to generate an observable length contraction ripple.

 

[roineust]

Harald,
Most of your last answer was even more hard for me to understand than the previous one, due to lack of knowledge on my side. Anyhow I noticed that there is a recurring theme in SR explanations, that has to do with light going one way and then (or as well) the other way back.

Please take a look at this diagram: https://www.physicsforums.com/attachment.php?attachmentid=30765&d=1292890665

In this diagram, there is only light going one way as much as I understand.

My question is, if we know for sure that the 'crystal' (e.g. a precise enough clock) is dilated, e.g. one (or ten) vibrations (or ticks) are not the same in the moving apparatus and in the stationary apparatus, as well we know for sure that the electricity (e.g. light) in both apparatus travels at the same speed, then necessarily the exact same configuration that made light sources turn on together in the stationary apparatus, will not make them turn on together in the moving apparatus, therefore physical laws invariance would be incorrect.

I think this arrangement is simple enough for me to understand. Where did I get it wrong with this diagram?

Thanks,
Roi.

 

[harrylin]

Harald,
Most of your last answer was even more hard for me to understand than the previous one, due to lack of knowledge on my side.

Dear Roi,
I assumed that you had read the introduction of Michelson's paper to which I gave the link earlier, and in which he makes the basic calculation to which I referred. Without that, much more elaboration would have been necessary.

Anyhow I noticed that there is a recurring theme in SR explanations, that has to do with light going one way and then (or as well) the other way back.

Please take a look at this diagram: https://www.physicsforums.com/attachment.php?attachmentid=30765&d=1292890665

In this diagram, there is only light going one way as much as I understand.

My question is, if we know for sure that the 'crystal' (e.g. a precise enough clock) is dilated, e.g. one (or ten) vibrations (or ticks) are not the same in the moving apparatus and in the stationary apparatus, as well we know for sure that the electricity (e.g. light) in both apparatus travels at the same speed, then necessarily the exact same configuration that made light sources turn on together in the stationary apparatus, will not make them turn on together in the moving apparatus, therefore physical laws invariance would be incorrect.

I think this arrangement is simple enough for me to understand. Where did I get it wrong with this diagram?

Thanks,
Roi.

OK, I see there a push button connected to, in parallel:
1. a crystal and wires going to a light bulb,
2. a long wire ball and wires going to another light bulb

The wire ball causes the same time delay as the crystal.

To this would apply the relativistic Fresnel-Fizeau equation we mentioned just before. The electricity will be slightly "dragged" by the moving wire. But let's keep it simple and pretend that electricity propagates at c through the wire, unhindered by the wire (the wire causes a small delay plus drag when moving but we'll ignore all that here).

Now you put the whole system in motion. First of all, you claim that the time (as measured in your stationary system) for electricity to go through the wire is still the same. Instead you must calculate roughly like Michelson did!

Say your system moves to the right while the electricity propagates to the left at speed c. The time through the wire will thus be reduced.

But we can leave out all that, for the wires are the same in both. We are left with comparing the wire ball and the crystal.

a. the crystal: yes it slows down, so the time delay will increase
b. the wire ball: difficult to calculate an unidentified wire ball. Let's make it orderly and turn it into a big zigzag - for example something like the MMX arm of my last post!
Now we are back to the basic Michelson calculation which you don't understand, which plus the Lorentz contraction term leads to the exact same time delay as that of the crystal...

OK then, here a quick sketch of that part of Michelson's calculation: the signal propagates from point 1 to the mirror and on to point 2, here the first leg is in counter speed to the system and on the return leg the signal is running in the same direction as the apparatus. The total trajectory in space is therefore less than length L on the first leg and more than length L on the return leg because the mirror and point 2 move to the right:

t1
|<---------------1 v-->
|---------------------->2
t2

For the calculation it's easier to take the relative speed of light and arm, as seen by you:
t1 = L/(c+v)
t2 = L/(c-v)
t1+t2 = T = [L(c-v)+L(c+v)] / [(c+v)(c-v)]
T = 2L * c /(c² - v²) = 2L/c * 1/(1 -v²/c²)

(For more explanation, see again his paper:
http://en.wikisource.org/wiki/On_the_Relative_Motion_of_the_Earth_and_the_Luminiferous_Ether )

However, according to Lorentz and Einstein one should also account for Lorentz contraction.

Summary:
In rest, T = 2L/c
In motion, according to Michelson, T = 2L/c * 1/(1 -v²/c²)
In motion, according to Lorentz, T = 2L/c * 1/SQRT(1 -v²/c²)

And the last is just the time dilation factor of your crystal.
Did that help?

 

[roineust]

Harald,

The first thing I don't understand is, that if it is possible for the sake of theoretical simplification, to account the electricity in wires for light then:

1. How could it matter, if the apparatus in the diagram I sent, works with the direction of velocity, or against it?
2. How does a mirror come back into the calculations, but there are no mirrors in the diagram I sent….

I understand that there are very good reason for all these matters, but you have to go very slow and simple with me, even long before it becomes math.


Thanks,
Roi.

 

[harrylin]

Harald,

The first thing I don't understand is, that if it is possible for the sake of theoretical simplification, to account the electricity in wires for light then:

1. How could it matter, if the apparatus in the diagram I sent, works with the direction of velocity, or against it?
2. How does a mirror come back into the calculations, but there are no mirrors in the diagram I sent….

I understand that there are very good reason for all these matters, but you have to go very slow and simple with me, even long before it becomes math.

Thanks,
Roi.

Hi Roi,

You cannot possibly have studied Michelson's paper. Please do! And:

1. Look at the wire ball: I simplified it to light going both ways (instead of a mirror I could have illustrated it with a glass fibre with a strong bent). That gives still the same time dilation factor for motion of the apparatus in the other direction. Or at 90 degrees, or whatever: see AGAIN the paper!
2. See 1.

Harald

 

[roineust]

OK,
I cannot understand that paper (MMX).

In any case, I attached here a new diagram with the same idea, but this time with light, instead of electric wire, because out of my very small ability to understand what you mean, I got to the conclusion, that you might have thought, that there is more than one direction of light movement in the apparatus, because the previous wire was piled in loops. So now there are only one directional light beams.

Roi.

 

[harrylin]

OK,
I cannot understand that paper (MMX).

In any case, I attached here a new diagram with the same idea, but this time with light, instead of electric wire, because out of my very small ability to understand what you mean, I got to the conclusion, that you might have thought, that there is more than one direction of light movement in the apparatus, because the previous wire was piled in loops. So now there are only one directional light beams.

Roi.

Dear Roi,

You will understand it better if you once calculated it - calculation is not only useful for making predictions! But I have been thinking how to say it without equations.

First, I'm not sure if you understood the main feature of MMX according to relativity: that is that the return time of light in an MMX apparatus in motion will increase by the time dilation factor. And that is independent of the orientation of the arms.

Secondly, about your wire ball example:
The wire ball has the signal going in all directions. It can thus be seen as many little MMX arms in series.

Thirdly, you made a new drawing, and it has again the same feature!
Here's my sketch of how I look at it:

<--------C-------B---->¦M
<----------------<------

As you can see, you make the time delay of the crystal C equal to the time delay from the signal paths B->M + M->B. That boils down to the same two-way MMX-like set-up.

Cheers,
Harald

 

[roineust]

I have attached a new diagram:
When saying that C is invariant what does it mean:

1.That because of time dilation and length contraction nullify each other it is the same speed C in an absolute sense?

2.It is not the same speed C in the absolute sense, but within the inertial frame it is never possible to measure it differently?

In case that the answer is no.2, where in the path of light, does light change its speed?

 

[harrylin]

I have attached a new diagram:
When saying that C is invariant what does it mean:

1.That because of time dilation and length contraction nullify each other it is the same speed C in an absolute sense?

2.It is not the same speed C in the absolute sense, but within the inertial frame it is never possible to measure it differently?

In case that the answer is no.2, where in the path of light, does light change its speed?

1. It is the same speed c in a "relative" sense! Special relativity assumes that such experiments as MMX and yours can not measure the speed of light relative to us in an absolute sense.

2. "Within the inertial frame" is jargon for "as measured with a standard, independent system of measurement". According to special relativity, you will with such a system always measure c for the speed of light in vacuum (that is a little different in general relativity).

And there is no "change" of speed of light anywhere! It is just differently measured with different systems of measurement, as I calculated for you several times now.

If you re-read our discussion from the start, you will perhaps understand it better this time.

 

[Dale]

I attached here a new diagram with the same idea, but this time with light, instead of electric wire, because out of my very small ability to understand what you mean, I got to the conclusion, that you might have thought, that there is more than one direction of light movement in the apparatus, because the previous wire was piled in loops. So now there are only one directional light beams.

roineust, I agree with harrylin here. You should go through the math to actually work up what happens. Start with the button push and determine the time and position for the following events:
1) button pushed
2) button signal reaches source a
3) button signal reaches source b
4) light reaches crystal
5) light leaves crystal
6) light reaches detector a
7) light reaches detector b

Then use the Lorentz transform to transform the times and positions of those 7 events to the moving frame. Finally, check to see that time dilation applies correctly for the crystal, that the speed of light applies for each light path, and that the relativity of simultaneity works out correctly.

 

[roineust]

DaleSpam thank you,
I might go over the math as you and harrylin suggest.

But since previous attempts to go over the math, did not bridge for me the gap between words and math, I am trying to remember things also by using words.

And my question in words now, is this:

Say you need to formalize in math, what is considered a physically impossible situation, regarding the crystal arrangement, that the detectors in the moving apparatus, don't turn on together, is that considered:

1. Impossible to formalize as a mathematical transformation.

2. Plainly going back to the Galilean transformation.

3. It is possible, and it is not a Galilean transformation, but this kind of transforation is called... (If this kind of option exists, and
it does not appear in the crystal arrangment, does in appear, in any physical phenomenon at all? not as a mathematical
approximation, or as a mathematical approximation as the Galilean transformation is considered to be?)

4. Neither, I don't understand (even in words) what a transformation is, or some other answer?


Thanks,
Roi.

 

[John232]

I think it would be more accurate to say that it showed that there was no ether wind (or if there was it was on an order far smaller than any theories at the time). The presence of an (a)ether stipulated a set of consequences like ether wind, ether drag, and I also seem to recall that movement through a medium affects the index of refraction. MM was just one of the many experiments that chipped away at the properties of various ether theories until you arrive at the point where an ether theory must be exceedingly complex to comply with experimental results.

I don't think it would need to be that much more complex than Special and General Relativity. The idea of spacetime replaced the aether theories. In a sense, spacetime is a type of aether that complies with experimental results. But instead of saying the aether is contracted when approuching the speed of light we say that space itself does this, so then there is no need to refferring to an aether if we know that it is actually space and time that is affected by this type of experiment.

One could say that spacetime is the medium that allows light to travel, but then spacetime is warped so that light always travels at the same speed. There would be no way to prove that it wasn't classically because relativistic effects would hide any results that show that the photon is like any regular beach ball traveling across water, for example.

Spacetime has this strange connection with photons that forces it to be bent and warped just so that the photon can say it is traveling the speed of light relative to everything else at the same time.

 

[harrylin]

[..]
thwle got to feeling a little frustrated and rashly promised to demonstrate that a unique frame or reference in which light proagates at the same speed in all directions not only exists but is discernable from other frames of reference.

Now, thwle has had the audacity to perform on his promise: See the new thread "Absolute Rest" under special and general relativity.

And now harrylin wonders what happened to thwle!

For not only did he suddenly stop posting (could be the flu or a little trip), but also there is no such thread and, to top it off, it is impossible to contact him!
and I can't even find mention of that possibility in the FAQ.

 

[Dale]

And now harrylin wonders what happened to thwle!

For not only did he suddenly stop posting (could be the flu or a little trip), but also there is no such thread and, to top it off, it is impossible to contact him!
and I can't even find mention of that possibility in the FAQ.

If you look back at one of his old posts you can see that his name is crossed out. That means that the account has been canceled, either by him or by an admin.

 

[Dale]

But since previous attempts to go over the math, did not bridge for me the gap between words and math, I am trying to remember things also by using words.

You have said this many times. Frankly, it is sounding more and more like an excuse, but that may be partly my own bias where people objecting to doing the math always seems like one of my kids trying to avoid doing their schoolwork.

Say you need to formalize in math, what is considered a physically impossible situation, regarding the crystal arrangement, that the detectors in the moving apparatus, don't turn on together, is that considered:

Just do it and see what you get. Simply follow the steps I laid out above. You will see that it all works out correctly.

 

[harrylin]

DaleSpam thank you,
I might go over the math as you and harrylin suggest.

But since previous attempts to go over the math, did not bridge for me the gap between words and math, I am trying to remember things also by using words.

Did you re-read our conversation as I suggested?

And my question in words now, is this:

Say you need to formalize in math, what is considered a physically impossible situation, regarding the crystal arrangement, that the detectors in the moving apparatus, don't turn on together, is that considered:

1. Impossible to formalize as a mathematical transformation.

2. Plainly going back to the Galilean transformation.

3. It is possible, and it is not a Galilean transformation, but this kind of transforation is called... (If this kind of option exists, and
it does not appear in the crystal arrangment, does in appear, in any physical phenomenon at all? not as a mathematical
approximation, or as a mathematical approximation as the Galilean transformation is considered to be?)

4. Neither, I don't understand (even in words) what a transformation is, or some other answer?

Thanks,
Roi.

"Say you need to formalize in math, what is considered a physically impossible situation,"

I consider that a faulty proposition. Only physically possible situations should be considered, for example the last drawing you made, it describes what happens in practice. The puzzle is then of the kind that we described, and we gave you the mathematical solution of relativity. As a matter of fact, we gave you:

1. the correct formulation of the problem and
2. the answer and
3. how to do the calculation

From your post it's not clear to me if you know the Galilean transformation or if you only know the sound bite "Galilean transformation". If it is the last, don't despair!

- Can you solve Pythogoras and so on? Then you know enough mathematics.
- Do you know how to calculate the time you need to go from A to B on a boat that sails at a certain speed on a river which flows at another speed? If so, then you know enough physics. Then you can also do the basic special relativity calculations! Harald

 

[harrylin]

If you look back at one of his old posts you can see that his name is crossed out. That means that the account has been canceled, either by him or by an admin.

Thank you! I thought that the line through his name was a fancy way he wrote it, I had not noticed before that it was not crossed out.

 

[John232]

And now harrylin wonders what happened to thwle!

For not only did he suddenly stop posting (could be the flu or a little trip), but also there is no such thread and, to top it off, it is impossible to contact him!
and I can't even find mention of that possibility in the FAQ.

If he was talking like that i can assure you he just got banned. In effect, that was what the M&M experiment did, and we know that the Earth is not at rest. I don't even think they picked up any of the acceleration that would cause curvature in the beam of light in the M&M experiment or both beams in each direction where curved by the same amount, I am not sure...

 

[harrylin]

[..]
One could say that spacetime is the medium that allows light to travel, but then spacetime is warped so that light always travels at the same speed.
[..]
Spacetime has this strange connection with photons that forces it to be bent and warped just so that the photon can say it is traveling the speed of light relative to everything else at the same time.

You can look it that way and use Minkowski's spacetime concept. Or you can stick with Lorentz's stationary ether concept with light waves (although photonic ones), which just as much complies with special relativity and experimental results.

Then the strangeness is gone and there is no warping (in special relativity), instead the relativistic behaviour is straightforward and easy to understand. Too boring perhaps?

 

[John232]

You can look it that way and use Minkowski's spacetime concept. Or you can stick with Lorentz's stationary ether concept with light waves (although photonic ones), which just as much complies with special relativity and experimental results.

Then the strangeness is gone and there is no warping (in special relativity), instead the relativistic behaviour is straightforward and easy to understand. Too boring perhaps?

I am not sure I am following you. How does that lead to no warping? I was just saying that they thought light had to have had a meduim to travel in because of its wavelike properties. It would have to have something to wave through as what they thought and is the reason why they came up with the aether theory. If aether was to equal spacetime then there would have to be warping of spacetime to comply with results. Any aether theory that attempted to comply with results would have to warp in much the same way as spactime would in relativity. Spacetime also affects waves traveling across the universe, they get stretched out along with the space they are traveling in. So to me spacetime and aether is the same thing, but the aether has to be warped by relativistic standards.

 

[harrylin]

I am not sure I am following you. How does that lead to no warping? I was just saying that they thought light had to have had a meduim to travel in because of its wavelike properties. It would have to have something to wave through as what they thought and is the reason why they came up with the aether theory. If aether was to equal spacetime then there would have to be warping of spacetime to comply with results. Any aether theory that attempted to comply with results would have to warp in much the same way as spactime would in relativity. Spacetime also affects waves traveling across the universe, they get stretched out along with the space they are traveling in. So to me spacetime and aether is the same thing, but the aether has to be warped by relativistic standards.

You talk about a "spacetime" ether - that's Minkowski's ether, and you say that it is warped. Fine. [Edit: I would say that in it different observers see events from a different angle, swapping time for space or vice versa].
Anyway, it's a literal interpretation of the equations, and in that sense it's simple. But as you said, it's also strange and "warped". In that sense it's not really simple but difficult.

I said that one may just as well stick with Lorentz's ether, which is similar to Newton's "space". That one is not warped nor does it permit to swap time for space; instead objects Lorentz contract and processes slow down. And there is nothing mystical about it, it's straightforward and quite easy to understand. However, the equations describe appearance only, so that an aspect of reality remains hidden - some people dislike that idea.

The same relativistic equations apply to both physical models.
So, take your pick, but don't mix them up!

 

[Dale]

So to me spacetime and aether is the same thing

This is a pretty useless statement. The modern concept of curved spacetime has nothing to do with the early 20th century concept of the luminiferous aether. Spacetime has only geometric properties, it has no material properties. Specifically, it does not have a velocity.