Relativity, speed of light and stuff

[Dale]

How is the rest frame of the air determined in this case?

You can use Doppler radar, a pitot tube, or a anemometer. Or if your instrumentation budget is really low you can always use the "finger in the wind" method.

 

[atyy]

You can use Doppler radar, a pitot tube, or a anemometer. Or if your instrumentation budget is really low you can always use the "finger in the wind" method.

So a rest frame is simply a frame in which we move at zero velocity relative to the wave? Isn't that just the rest frame of the wave?

 

[atyy]

You can use Doppler radar, a pitot tube, or a anemometer. Or if your instrumentation budget is really low you can always use the "finger in the wind" method.

BTW, what I'm trying to understand is why spacetime is not considered a medium for light. JesseM suggested that that was because spacetime had no rest frame of its own, which I thought I understood a few days ago as there being no sense in which spacetime can move relative to something else.

 

[Dale]

So a rest frame is simply a frame in which we move at zero velocity relative to the wave? Isn't that just the rest frame of the wave?

I thought you were trying to find the rest frame of the air, not the rest frame of the sound wave. The rest frame of the air would be the frame where any of the above airspeed measurement devices read 0.

BTW, what I'm trying to understand is why spacetime is not considered a medium for light. JesseM suggested that that was because spacetime had no rest frame of its own, which I thought I understood a few days ago as there being no sense in which spacetime can move relative to something else.

I agree with JesseM. Air has a velocity, measureable by any of the means I mentioned. To my knowledge there is no such velocity measurement for empty spacetime.

 

[atyy]

I thought you were trying to find the rest frame of the air, not the rest frame of the sound wave. The rest frame of the air would be the frame where any of the above airspeed measurement devices read 0.

Yes, but then I could just be stationary relative to the sound wave. If the wind blows, and I travel at the same speed as the wind, there is no wind. That doesn't mean the wind isn't blowing.

I agree with JesseM. Air has a velocity, measureable by any of the means I mentioned. To my knowledge there is no such velocity measurement for empty spacetime.

OK, at least it seems I understood that part correctly. From that example, I also understood from that the vacuum state photon field is also not a medium, because it fills all of space, and cannot be moved relative to space - the only way it can move is to be excited into a wave. That makes sense to me.

So if the air is enclosed in a box, and the box is the universe, then the air in the box cannot be moved relative to the box. All possible motions of the air in the box are some sort of sound wave. In this case, it seems that sound waves can also be thought of as not having a medium (which I'd be happy to accept).

 

[MeJennifer]

Yes, but then I could just be stationary relative to the sound wave. If the wind blows, and I travel at the same speed as the wind, there is no wind. That doesn't mean the wind isn't blowing.

This is basic relativity, there is no absolute means on detecting whether the wind is blowing or the Earth is moving. The only thing we can say is that the air and the Earth are in motion.

 

[atyy]

This is basic relativity, there is no absolute means on detecting whether the wind is blowing or the Earth is moving. The only thing we can say is that the air and the Earth are in motion.

If the air is in a box, and the box is the universe, then the air cannot move relative to the box. Any wind that I feel is a long wavelength sound wave. If I don't feel any wind, it could either be that there is no sound wave and the air is at absolute rest, or there is a sound wave and I am traveling at the same speed as the wave. So if a medium is something that has a rest frame of its own (ie. can be moved without waves relative to space), it seems like sound sometimes has no medium.

What I am puzzled by is that there is a quantum description of sound waves that looks very much like the quantum description of light waves. In this sense, light is made of "stuff" (that is not controversial, the only question is what to call the "stuff"). The question then is how this "stuff" is different from the "aether" or a "medium". Given that light does not have a "medium" but is made of "stuff", and given the similarity of sound and light, it seems that I should be able to say (under some circumstances) that sound is made of "stuff" but does not have a "medium".

 

[JesseM]

Hi JesseM, another question to make sure I understand this definition correctly:

I enclose air in a rigid box, and declare the box the entire universe. I am an observer in the box who only has low energy tools, so air is a continuum to me, and all excitations of the air are sound waves. Let's assume Galilean relativity. This means I can travel at a speed that makes a sound wave stationary relative to me. But I guess from our earlier conversations that this only means the wave has a rest frame of its own, and isn't equivalent to the medium having no rest frame of its own. My previous understanding of air having a rest frame of its own was that air without sound waves could itself move relative to space. But that doesn't seem to work here, since the box is the entire universe. How is the rest frame of the air determined in this case? (I have a rigid ruler and protractor)

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.

 

[MeJennifer]

If the air is in a box, and the box is the universe, then the air cannot move relative to the box.

You clearly do not understand the principle of relativity.

 

[Dale]

If the air is in a box, and the box is the universe, then the air cannot move relative to the box. Any wind that I feel is a long wavelength sound wave.

I don't see what you are saying here. What does a box and the universe have to do with wind and sound?

Anyway, a sound wave, regardless of the frequency, does not involve bulk displacement of the medium. I.e. it is energy transport without net mass transport. In a box you can certainly have eddies, vortices, and all sorts of other "winds" that are not sounds because they involve bulk displacement of the air and net mass transport through a given region.

In this case, it seems that sound waves can also be thought of as not having a medium (which I'd be happy to accept).

You are really not making any sense here. Can you explain your thought process better? I have no idea why you think that putting a big box around a medium makes the medium cease to exist.

 

[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?