Relativity, speed of light and stuff

[atyy]

Just take a car or something and start moving inside the box, then measure the velocity of sound waves in your new rest frame which is different than the rest frame of the box. You will find that they are moving faster in one direction than the other, because they continue to move at the same speed in all directions in the rest frame of the box.

If I do this for light waves, then I can find many rest frames for its "stuff".

 

[JesseM]

If I do this for light waves, then I can find many rest frames for its "stuff".

If you do it for light waves you'll never find that the velocity is different in different directions in any inertial frame, which means you can't find a particular frame that qualifies as its rest frame. Any physical substance must have a single rest frame, if there's no single frame than it isn't a substance at all.

 

[atyy]

If you do it for light waves you'll never find that the velocity is different in different directions in any inertial frame, which means you can't find a particular frame that qualifies as its rest frame. Any physical substance must have a single rest frame, if there's no single frame than it isn't a substance at all.

OK, that's a little different from saying that spacetime is not a medium because it has no rest frame. It's saying that spacetime is not a medium because it has no unique rest frame. So if I take a box of air, and now declare the surrounding space to exist (Newtonian Euclidean space without gravity). Then the air in the box is not a medium, because it has no unique rest frame: for any arbitrary constant velocity of the box, there is a different inertial rest frame for the box.

 

[JesseM]

OK, that's a little different from saying that spacetime is not a medium because it has no rest frame. It's saying that spacetime is not a medium because it has no unique rest frame.

The concept of a non-unique rest frame doesn't make sense to me. I suppose for an extended physical substance, if it's non-rigid then different points in the substance could have different rest frames, and a single point of the substance could have different rest frames at different times, but to imagine a single point of matter having multiple rest frames at a single moment seems meaningless.

So if I take a box of air, and now declare the surrounding space to exist (Newtonian Euclidean space without gravity). Then the air in the box is not a medium, because it has no unique rest frame: for any arbitrary constant velocity of the box, there is a different inertial rest frame for the box.

Huh? Every point in the air would still have a unique rest frame at every moment in time, and that's all I was talking about when I said a physical medium is different from something like spacetime or the electromagnetic field. Do you imagine spacetime/the electromagnetic field can have an identifiable "rest frame" even in this limited sense?

 

[granpa]

but if you became length contracted, time dilated, and experienced loss of simultaneity as you approached mach one then you would not be able to detect any change in the speed of sound.

would the medium of sound not be an extended physical substance?

 

[JesseM]

but if you became length contracted, time dilated, and experienced loss of simultaneity as you approached mach one then you would not be able to detect any change in the speed of sound.

would the medium of sound not be an extended physical substance?

I asked 'Do you imagine spacetime/the electromagnetic field can have an identifiable "rest frame" even in this limited sense'? Are you suggesting a scenario where it would be impossible to isolate particles of air, and where all the laws of physics were symmetric with regard to a transformation that looked like the Lorentz transformation but with the speed of sound substituted for c? (If any laws were not symmetric this way, then all observers could identify a preferred frame using such a law, and then by defining distances and times in their own frames using specially-constructed rulers and clocks designed not to shrink or dilate as seen by observers at rest in the preferred frame, they'd find that sound waves had different speeds in different directions in their own rest frame thusly-defined). If so, then the rest frame of the air would have absolutely no physical consequences, and as I said back in post #109:

aether theory postulates that there are facts about the physical world which are impossible to determine experimentally, in this case the rest frame of the aether. I suppose you could take any successful theory and add to it the idea of invisible ghostly dragons which are impossible to detect with any physical instrument, and then say the evidence is "consistent with a theory involving invisible ghostly dragons", but the dragons would obviously be superfluous to the theory, and the same is true of the aether.

Here's a good post on why the notion of a completely unobservable aether not only makes it useless to science, but also is very implausible physically:

http://groups.google.com/group/sci.physics.relativity/msg/a6f110865893d962

 

[Dale]

OK, that's a little different from saying that spacetime is not a medium because it has no rest frame. It's saying that spacetime is not a medium because it has no unique rest frame.

I disagree with both you and JesseM here. Spacetime has no rest frame. Spacetime has no velocity, therefore there is no frame where its velocity is 0. I don't think that it is correct at all to say that it has a non-unique rest frame.

So if I take a box of air, and now declare the surrounding space to exist (Newtonian Euclidean space without gravity). Then the air in the box is not a medium, because it has no unique rest frame: for any arbitrary constant velocity of the box, there is a different inertial rest frame for the box.

The velocity of air is measurable (as described above) at every point, regardless of wether or not it is enclosed in a box. The rest frame is simply the frame in which the velocity is 0. So I still do not understand how you think that when air is enclosed in a box it suddenly ceases to be a medium. You are talking gibberish.

 

[JesseM]

I disagree with both you and JesseM here. Spacetime has no rest frame.

Why are you "disagreeing" with me? I clearly stated my position was that spacetime has no rest frame at all, not even in the "limited sense" that a point in a physical medium will have a definite rest frame at a single moment in time. Again:

Every point in the air would still have a unique rest frame at every moment in time, and that's all I was talking about when I said a physical medium is different from something like spacetime or the electromagnetic field. Do you imagine spacetime/the electromagnetic field can have an identifiable "rest frame" even in this limited sense?

 

[Dale]

Why are you "disagreeing" with me?

OK, I was disagreeing with atty's misinterpretation of your position. Specifically, his mis-interpretation that spacetime has a rest frame, it just has more than one rest frame.

 

[granpa]

you didnt answer my question.

would the medium of sound not be an extended physical substance?

 

[JesseM]

you didnt answer my question.

would the medium of sound not be an extended physical substance?

And you didn't answer mine, I was asking for clarification. Are you indeed proposing a substance for which it is impossible in principle to determine the rest frame of any part of it by any possible empirical method? If not, then of course it'd be a physical substance. But if so, it really depends on your definition of the word "physical", personally I would tend to think that anything which is completely outside the bounds of any possible empirical testing, and which can never have any noticeable empirical effects, is more "metaphysical" than physical. Would you say that my "invisible ghostly dragons" are physical?

 

[rbj]

I disagree with both you and JesseM here. Spacetime has no rest frame. Spacetime has no velocity, therefore there is no frame where its velocity is 0. I don't think that it is correct at all to say that it has a non-unique rest frame.

dunno what JesseM said that you're unhappy with.

i think it would be better to say that space-time has no unique rest frame and that any inertial frame of reference (including those in free-fall, if we toss GR into this) has an equal claim to being at rest. you might say that best means that none of them are a rest frame, i might say they're all rest frames.

doesn't matter, as long as we agree that these different inertial frames of reference get the same laws of physics. whether none of them are rest frames or all of them, seems to me to be a semantic issue.

 

[atyy]

Hey guys, I'll try to reply to your answers in detail a bit later. Let me first put down my former understanding, before my present confusion.

1) First we do some experiments, then we discover Newtons' laws in one reference frame.
2) Then we do experiments in a different reference frames, and discover that Galilean relativity is true - the existence of a preferred class of frames in which the laws of mechanics all look like Newton's laws.
3) Then we do Faraday sort experiments, and we discover Maxwell's equations.
4) We notice that Maxwell's equations are not covariant with Galilean relativity. So we predict that if we do the Michelson-Morley experiment, we will see different results depending on which Galilean frame we are in. At this point, we have not introduced the concept that light has a medium - the Michelson Morley experiment is not designed to see the medium (which is undefined at this point), it is designed to check that Galilean relativity holds for Maxwell's equations.
5) We obtain the null result of Michelson-Morley, and we realize that Galilean relativity does not hold for Maxwell's equations. We figure out that when doing electromagentic experiments, we need to use Lorentzian relativity.
6) We have a problem - in building the Michelson-Morley apparatus, we used mechanics (for which we had assumed Galilean relativity). So the Michelson-Morley experiment is not a pure electromagnetism experiment, and actually implies that mechanics is also not Galilean, in contradiction to our old understanding.
7) We revise our laws of mechanics to be special relativistic.

At this point, there is no medium at all. Why did we even need a medium in the days of Maxwell? Light waves have always simply propagated in space and time.

8) We do more experiments on light and discover that spacetime appears to have boundaries across which light changes its speed. Now we introduce the concept of a medium - in fact we introduce the concept of media. Spacetime has sharp boundaries (macroscopically, ie. at low energies) and can be divided into different regions. We call these different regions different media. With this definition, we call a medium in which light has the invariant speed a "vacuum".
9) If we do experiments in the vacuum only, there is no boundary, and thus there is only one medium called "vacuum", or more colloquially "spacetime". You can call it "aether" or "George Bush" or "water" - but that doesn't change the fact that there is only one medium - just like E=mc² is the same equation as Q=gt². It's simply a matter of convention that we usually don't call the vacuum "George Bush" or "water", and usually reserve those names for other media.

 

[russ_watters]

Well...
Number 2 was thought to be true, but when measurements got more sensitive, we found it wasn't (perhaps that's what you meant...).
Number 4 is incorrect wrt what it is saying about the medium. Light was thought to have a medium and the MMX was designed to measure the motion of it (thus operating on the assumption that it existed).
Number 5 - just so we're clear, it isn't just that it doesn't hold for light, Galilean relativity doesn't hold for anything.
7(a) Light was thought to have a medium for the same reason granpa thinks it has a medium - people associate waves with a medium. Prior to Einstein's relativity, it was simply assumed that if light was a wave, it had to be a wave like other waves, traveling on a medium.
8 - not sure what you mean by this. I don't know why you would think it. AFAIK, it isn't true. Note, though, you can't combine some measurements in your frame with some measurements in another frame. Perhaps that's what that is based on? The expansion of the universe screwing with our measurements? That does not provide a problem for SR. Also not sure what you mean by the media - different boundaries and regions? Huh? And a vacuum is not a medium. Quite the opposite!
9. The reason "the vacuum" or space isn't called a "medium" is it doesn't behave like a medium. Words are more than just arbitrary labels - they have definitions that mean something and must be used consistently. To call space a "medium" is to change the definition of "medium". Sure, I guess you can call it whatever you want, but you'll then have to accept that you're no longer speaking the same language as everyone else. And you can't just change the meaning of words and declare yourself right. If that were allowed, I'd just start moving the decimal points on my paychecks and declare myself rich!

 

[Hurkyl]

Disclaimer: my knowledge of the history of physics is somewhat thin.

The notion of light as waves traveling through a "luminiferous aether" had already been invented in the 17th century¹. Faraday didn't discover light was connected to EM until the mid 19th century, and Maxwell's equations didn't come out until the late 19th century. (Along with Hertz's confirmation that electromagnetically generated radio waves behaved like light)

At the time, Maxwell's theory was essentially brand new, and it contradicted both the preexisting wave theory of light through a luminiferous aether, as well as classical mechanics². The most promising lead towards fixing these flaws was to introduce a medium into Maxwell's theory. (Rewriting mechanics was certainly not a promising approach at the time!)


Every description of the Michelson–Morley experiment I've read suggests that it was attempting to detect the effects of an aether wind, contradicting your point (4). One specific point worth adding is that Maxwell's theory was mathematically incompatable with Galilean relativity; we didn't need an experiment to tell us there was a problem.


1. Even the competing particle theory of light had to invoke some sort of aether to explain diffraction
2. Because of Gailiean relativity

 

[JesseM]

One specific point worth adding is that Maxwell's theory was mathematically incompatable with Galilean relativity; we didn't need an experiment to tell us there was a problem.

Well, Maxwell's theory was incompatible with Galilean relativity if you assumed the equations were supposed to hold in every frame, but pretty much all physicists--including Maxwell--assumed they would only hold in the rest frame of the aether, and that in other frames they'd have to be modified by a Galilei transform.

 

[atyy]

The notion of light as waves traveling through a "luminiferous aether" had already been invented in the 17th century¹. Faraday didn't discover light was connected to EM until the mid 19th century, and Maxwell's equations didn't come out until the late 19th century. (Along with Hertz's confirmation that electromagnetically generated radio waves behaved like light

One specific point worth adding is that Maxwell's theory was mathematically incompatable with Galilean relativity; we didn't need an experiment to tell us there was a problem.

Yes, I am retelling history in modern terms. The idea behind these definitions is that a Principle of Relativity need not be true, and the existence and extent of such a principle is a matter for experiments. If a Principle of Relativity is true, then further experiments are needed to distinguish between various possibilities such as Galilean, Lorentzian and General Relativistic relativity.

So what I would say is that if we know Galilean relativity to be experimentally true for Newton's laws, and then we discover Maxwell's equations, that only means that Galilean relativity must be restricted to Newton's laws. It does not imply that Galilean relativity would fail for Newton's laws. It does imply the failure of Galilean relativity for Newton's laws and Maxwell's equations taken together, and implies the existence of one preferred frame of reference, which we can call "absolute space" and identify with what used to be called the "luminiferous aether". I prefer the term "absolute space", because with the benefit of hindsight, the sequence of experiments is really telling us whether a "Principle of Relativity" is experimentally true, and if it is true, what sort of relativity it is. So it is decided by experiments, not mathematics, that Galilean relativity is not true even if we restrict ourselves to the laws of mechanics.

 

[atyy]

8 - not sure what you mean by this. I don't know why you would think it. AFAIK, it isn't true. Note, though, you can't combine some measurements in your frame with some measurements in another frame. Perhaps that's what that is based on? The expansion of the universe screwing with our measurements? That does not provide a problem for SR. Also not sure what you mean by the media - different boundaries and regions? Huh? And a vacuum is not a medium. Quite the opposite!

This is all within one reference frame. By boundaries in spacetime, I just mean that there are materials like glass, in which light travels at a different speed than in vacuum. The boundary between vacuum and glass is a boundary between two regions of spacetime.

9. The reason "the vacuum" or space

I normally use "vacuum" and "spacetime" interchangeably. However there are times when it is useful to distinguish between them. For example, "a piece of glass in vacuum" and "a piece of glass in spacetime" normally mean the same thing to me. However, I sometimes distinguish between "vacuum" and "spacetime". For example, I sometimes wish to say that "spacetime exists at each location in the piece of glass", whereas I would not say "vacuum exists at each point in the piece of glass". In the above post I chose to distinguish between "vacuum" and "spacetime", which is why I said "vacuum" could be colloquially called "spacetime".

To call space a "medium" is to change the definition of "medium". Sure, I guess you can call it whatever you want, but you'll then have to accept that you're no longer speaking the same language as everyone else. And you can't just change the meaning of words and declare yourself right.

I called vacuum and glass two different media (I did not call "spacetime" a "medium"). This allows me to make statements like "the speed of light changes when crossing from one medium to another". It is true that it is also sensible to define "medium" as "a region of spacetime in which the speed of light is not the invariant speed". But then I would have to say "the speed of light changes when crossing media boundaries and when crossing from vacuum into a medium". But this is unduly cumbersome. In fact, there are many times when we talk as if spacetime were a medium - we do say the "refractive index of vacuum", the "permittivity of free space" and the "permeability of free space". So whether we need to define "medium" to include or exclude "vacuum" is a matter of notational convenience and the subject we are discussing. It is pretty much like "inertial frame" - if we are talking about Newtonian relativity, we use "inertial frame" - if we are talking about Newtonian mechanics, Special relativity and General relativity, then we may say that there is no "inertial frame" in Newtonian mechanics or Special relativity - those theories have "Galilean frames", "Lorentzian frames" - only General relativity has "inertial frames".

 

[atyy]

In this post, let me go back to my normal lax usage and let "spacetime" be the same thing as "vacuum". In colloquial physics speak, I normally think that light does not require a medium to propagate in vacuum. A common question about this point of view is, "Why isn't the electromagnetic field itself a medium". To which my reply is that the medium must exist in all regions into which the wave can travel, and the electromagnetic wave can travel into regions where the electromagnetic field has been zero at all previous times. This is usually enough to stop people from wanting to identify the electromagnetic field as the medium.

Now, if a person insists that waves must travel in a medium, I confess that it is difficult for me to see where his difficulty is. However, this does not help me clarify the person's understanding. So I try to go halfway and see if the person's difficulty would go away if I identified the vacuum as a medium, and just explaining that it's fine that he thinks that way, but since we already have a perfectly good term called "vacuum", there is no need to introduce a new term for the medium of light. So the key point is: we do not need to postulate an additional medium for light, it simply propagates in spacetime in the sense that we do not need to add a new term to our equations describing the propagation of light - those equations already give us the ability to predict all current experimental results. If you wish to call spacetime a medium, and agree that no new term has to be added to our equations based on current experimental evidence, then that's not a problem. All the physics remains the same, and I can't see what harm is done.

Now in relation to one of my earlier posts, I think the psychological difficulty for the public can be mitigated if we don't even use the word "medium" in describing what the Michelson-Morley experiment tells us, and to interpret those experiments - from our modern vantage point - as really telling us about the Principle of Relativity.

 

[Hurkyl]

Yes, I am retelling history in modern terms. The idea behind these definitions is that a Principle of Relativity need not be true, and the existence and extent of such a principle is a matter for experiments.

And there had been 250 years of experiments supporting Galilean relativity.

So what I would say is that if we know Galilean relativity to be experimentally true for Newton's laws, and then we discover Maxwell's equations, that only means that Galilean relativity must be restricted to Newton's laws.

And you would be wrong to say so: centuries of empirical verification don't get invalidated every time someone postulates a new idea. The correct inference at the time was that there was that Maxwell's theory was missing something (such as an aether). (Of course, new inferences are usually tested, just on the off chance they are wrong)

It does imply the failure of Galilean relativity for Newton's laws and Maxwell's equations taken together, and implies the existence of one preferred frame of reference, which we can call "absolute space" and identify with what used to be called the "luminiferous aether".

Wrong. The luminiferous aether was a (hypothetical) medium, expected to operate under Galilean-invariant dynamics. It is very different from the idea of absolute space.



Reading between the lines, it almost seems as if you are trying to reformulate science into some utopian methodology that desprately tries to avoid making mistakes. But that isn't how things work -- instead, science blazes forth making the best inferences it can with the information available, while continually performing experiments in order to detect our failures and bolster confidence in our successes.

 

[atyy]

Well, Maxwell's theory was incompatible with Galilean relativity if you assumed the equations were supposed to hold in every frame, but pretty much all physicists--including Maxwell--assumed they would only hold in the rest frame of the aether, and that in other frames they'd have to be modified by a Galilei transform.

And you would be wrong to say so: centuries of empirical verification don't get invalidated every time someone postulates a new idea. The correct inference at the time was that there was that Maxwell's theory was missing something (such as an aether). (Of course, new inferences are usually tested, just on the off chance they are wrong)

I agree with JesseM's statement, and I don't think I disagree with Hurkl substantively. I agree that "absolute space" may be just as misleading as "luminiferous aether", since it was originally a concept formulated for Newton's laws. How about equating the "luminiferous aether" simply with a "preferred reference frame"? I believe this is equivalent to "a rest frame of its own" which JesseM suggested many posts back, and which I thought was one reasonable definition of a "medium".

There are two everyday uses of the word "medium". One is along the lines of "a rest frame of its own". The second meaning is, for a medium like air, that it is made of atoms - "real stuff".

A non-null Michelson-Morley result would have been consistent with no new physics. The frame in which Maxwell's equations hold would define a preferred reference frame, and we would get the laws in other frames by a Galilean transformation.

So it would have implied the existence of a "medium" called the "luminiferous aether" in the first sense, but not the second. And I suspect that when people ask questions about a medium, part of the confusion is that they are asking about both senses of the word. It is worth pointing out that Maxwell and others did try to invent an atomic theory for the electromagnetic field. The Michelson-Morley experiment does not speak to a "medium" in that sense.

 

[Hurkyl]

How about equating the "luminiferous aether" simply with a "preferred reference frame"?

No. That would be wrong, because...

There are two everyday uses of the word "medium". One is along the lines of "a rest frame of its own". The second meaning is, for a medium like air, that it is made of atoms - "real stuff".

it was meant in the second sense, as in the preexisting wave theory of light (which, incidentally, had just scored some major successes in the time leading up to our story). Being at rest w.r.t. the aether only makes sense locally -- just like being at rest w.r.t. the water in a river, or being at rest w.r.t. with the wind.

This wikipedia article is consistent with my understanding of history. Note that they are very clearly talking about a medium, not a notion of what we would call absolute space or preferred reference frame.

 

[atyy]

Where do you come up with your ideas on history? Can you cite some references?

I have never been talking about real history.

Reading between the lines, it almost seems as if you are trying to reformulate science into some utopian methodology that desprately tries to avoid making mistakes. But that isn't how things work -- instead, science blazes forth making the best inferences it can with the information available, while continually performing experiments in order to detect our failures and bolster confidence in our successes.

The second quote of yours is closer to what I've been trying to talk about (but no, I am not trying to axiomatize the scientific method). I have never been talking about real history. I have only been talking about the quasi-history of physics textbooks in which Newtonian mechanics, Maxwell's equations and then quantum mechanics and special relativity are introduced in a "standard" order. I have been talking about what previous experiments, including the Michelson-Morley experiment, mean *for us*.

As an example of quasi-history, a typical example is that Einstein derived the Lorentz transformations from:
1) Principle of Relativity
2) Constancy of the speed of light.

 

[JesseM]

As an example of quasi-history, a typical example is that Einstein derived the Lorentz transformations from:
1) Principle of Relativity
2) Constancy of the speed of light.

I realize your point isn't about what is or isn't true historically, but I just want to point out that it's not right to call this "quasi-history", since in his original 1905 paper he did just that! Of course he wasn't the first to derive the Lorentz transformations, but the paper shows how they could be derived from these two assumptions.

 

[atyy]

This wikipedia article is consistent with my understanding of history. Note that they are very clearly talking about a medium, not a notion of what we would call absolute space or preferred reference frame.

Hurkyl, the reason I am not talking about historical definitions of a "medium" is I want to know why people *now* ask questions about light not needing a "medium". What do they mean? What is the source of their confusion, or inability to accept the statement that "light does not need a medium to propagate". I believe that some of the confusion has to do with physics, some has to do with semantics. What is the best way to make current physics intuitive to them?

 

[atyy]

I realize your point isn't about what is or isn't true historically, but I just want to point out that it's not right to call this "quasi-history", since in his original 1905 paper he did just that! Of course he wasn't the first to derive the Lorentz transformations, but the paper how they could be derived from these two assumptions.

Well, I was thinking of Rindler writing that the second axiom is nowhere stated in the paper even though it is logically necessary (Relativity: Special, General, and Cosmological, OUP 2006). I agree with both you and Rindler, it comes down to finessing what "statement of an axiom" means - which is why I avoid talking about history! And also why I tried to avoid the extremely fine discussion about axioms that you, rbj and DaleSpam were having - but I seem to have somehow gotten into it! Anyway, thanks for your new clarifications about what you mean by "a rest frame of its own", I'm still thinking about it and will get back to that later.

 

[russ_watters]

Hurkyl, the reason I am not talking about historical definitions of a "medium" is I want to know why people *now* ask questions about light not needing a "medium". What do they mean? What is the source of their confusion, or inability to accept the statement that "light does not need a medium to propagate". I believe that some of the confusion has to do with physics, some has to do with semantics. What is the best way to make current physics intuitive to them?

It is simply what I said before: people are unable to deal with the idea that light can be wavelike without thinking that it travels on a medium like sound. It is no more complicated than that.

 

[JesseM]

Well, I was thinking of Rindler writing that the second axiom is nowhere stated in the paper even though it is logically necessary (Relativity: Special, General, and Cosmological, OUP 2006).

Well, in section 2 of the paper he lists two numbered assumptions, and the second assumption doesn't explicitly say light moves at c in all frames, just that it moves at c in what he calls the "stationary" frame, which he earlier defined as one in which Newton's laws of motion hold good:

The following reflexions are based on the principle of relativity and on the principle of the constancy of the velocity of light. These two principles we define as follows:--

1. The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of co-ordinates in uniform translatory motion.

2. Any ray of light moves in the "stationary'' system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body.

Still, given that the stationary frame was defined to by any frame where Newton's laws of motions hold, I don't really see how it could be possible that light could move at c in this frame without postulate 1 implying that it must move at c in every other inertial frame.

 

[atyy]

OK, I think I now understand JesseM's definition of a "medium" as something having "a rest frame of its own". My old understanding was only partially technically correct, and completely wrong as to what motivated the definition. I'm summarizing my current understanding here. I also took into account comments from DaleSpam and rbj. Thank you all for commenting!

I'm going to use JesseM's idea that a medium has "a rest frame of its own", but with slightly different terminology:
an isotropic homogeneous medium defines a preferred reference frame in which the speed of a wave in any direction is the same.

If I have interpreted JesseM correctly, I think the uncontroversial part is:

1. I assume a Newtonian space in which Galilean relativity holds, and assign cartesian coordinates. Instead of a box of air, I am going to use a box of solid material, and consider low energy sound waves in the solid. If three solid boxes are put at different positions along the y-axis and pushed in the x direction with different velocities, each box defines a preferred reference frame, and we have three preferred reference frames at any time.

2. If we have only one box, and we declare the box to be the whole universe, then we have a single preferred reference frame.

OK, now I want to play with some definitions, and this will be a matter of taste, so I don't expect agreement:

3. In #2, since there is only one preferred reference frame, we can instead of saying that space is filled with a medium, just define the preferred reference frame to be a property of space itself. In this case, we don't need a "medium" for sound. Further justification for saying that it is the geometry of space is that even in this case, we don't have just one preferred reference frame, but a preferred class of reference frames related to each other by space translation and rotation.

4. If we apply the operational definition of a medium in #1 to light (now in Minkowski space), we get that we have a class of preferred reference frames, which are of course the inertial frames related by Lorentz transformations. In this case, "preferred class of reference frames" is standard nomenclature, but "equivalence class of rest frames" or "medium" is nonstandard. However, if one likes the idea that a photon has zero rest mass, then it doesn't seem so bad to say that the "preferred class of reference frames" are a "preferred class of rest frames".

5. Formally, there is a medium in situation 4, but by comparison with situation 3, we can get rid of the medium and just say that these are properties of space itself.

6. When Nobel Laureates like Wilczek or Laughlin say that quantum field theory is a descendent of the aether, they are referring to the aether or medium as being made of atoms, *not* as defining a preferred reference frame. This is because "stuff" like electrons and protons have all become waves, just like light. Furthermore, electrons, protons and photons are not fundamental, they are excitations of electron, proton and photon fields that permeate all space, and which have resting states (not frames) that are non-zero at each point in space. So the main difference between a "medium" and a "medium" is "a preferred class of reference frames" versus "stuff, like electrons and protons".

7. When a person says "sound requires a medium", I suggest he usually only has a vague idea of what he means. It could mean:
a) sound carries energy from one place to another, so it travels in something
b) sound travels in air, and air is made of atoms
c) sound defines a preferred reference frame (I'm sure this is usually the furthest from his mind).
The problem is that for sound, all three meanings are true and thus confused with each other!

8. For the corresponding definitions for light I would say:
a) This is called spacetime, and we already have a name for it, there is no need for a "medium" that is distinct from spacetime.
b) Light travels directly on spacetime in classical theory, but in quantum field theory, it is an excitation of a "medium" called the photon field, just as electrons are excitations of a "medium" called the electron field. We know we need the "medium", and not just its excitations, because of the Casimir effect.
c) Light defines a preferred class of reference frames, but rather than attributing this to an isotropic homogeneous medium, we attribute it to a symmetry of space, just as we would do for sound waves in a universe that was a solid box.

9. In an earlier post, I somehow felt that we only really need the idea of a medium when there are two media, and still do, but I don't quite know how it fits in the above points.

 

[gmax137]

Hurkyl, the reason I am not talking about historical definitions of a "medium" is I want to know why people *now* ask questions about light not needing a "medium". What do they mean? What is the source of their confusion, or inability to accept the statement that "light does not need a medium to propagate". I believe that some of the confusion has to do with physics, some has to do with semantics. What is the best way to make current physics intuitive to them?

Seems to me the confusion is simply this: "How can something be a wave, if nothing is waving?" It's no deeper than that, really.

edit - wow, sorry I just realized the last post was almost three months gone.