How do waves travel in space?

[Dustin Kesselberg]

Hello all..I've searched this using the search button and on google and have found nothing.

My question is.How can radio/light/other waves travel in space while some waves such as sound can not?

Does it have to do with the fact they are electromagnetic waves?
Why don't they need a meduim?

How does it work?




Thanks...

[Pragmatist]

Originally posted by Dustin
Hello all..I've searched this using the search button and on google and have found nothing.

My question is.How can radio/light/other waves travel in space while some waves such as sound can not?

Does it have to do with the fact they are electromagnetic waves?
Why don't they need a meduim?

How does it work?

Thanks...

Electromagnetic "waves" are not really waves as such, they are actually photons which are rather like self-contained particles. Photons (and in fact all particles) appear to have some sort of internal periodic motion (i.e. repetitive motion). Mathematically this periodic motion can be modelled as a wave and that is one reason why we still refer to "waves" when talking about such things. Of course the periodic motion in question is not necessarily a literal "wave" as such.

Electromagnetic theory was originally developed in the 19th century before quantization had been discovered (even the electron was unknown). James Clerk Maxwell developed a mathematical formal theory of electromagnetism to explain Michael Faraday's experimental results. Maxwell assumed that he was dealing with waves in some sort of medium (the aether), and he developed the mathematics based on that assumption. Perhaps surprisingly, the mathematics itself is correct regardless of whether the aether exists or not - and in the early 20th century, experiments (like the Michaelson Morley experiment) showed that no evidence of the aether could be found. Planck and others also found that light and other electromagnetic phenomena seemed to occur in discrete jumps called quanta. So putting the two together led to the rejection of the aether hypothesis and the acceptance of the idea that light etc., were better modelled as discrete self-contained entities (photons) with some sort of associated periodic motion.

To give a simple analogy:

Firstly imagine a wave in water - that's easy to do.

Now take a drop of water and imagine it suspended in the air. Now imagine that there are waves in that drop.

Obviously the waves are in the drop, but are not in the air surrounding the drop. So the drop in the air is rather like a "self-contained" object in which waves occur.

So now imagine that a photon is like a drop of water in empty space, moving with some sort of internal wavelike motion. Of course the photon is not water - we don't know what the "substance" of the photon is.

Does that help?

[Ziggurat]

Originally posted by Dustin
Does it have to do with the fact they are electromagnetic waves?
Why don't they need a meduim?

Yes, it's because they're electromagnetic waves. They don't need a medium for exactly the same reason that an electric and magnetic fields don't need a medium, and can exist perfectly well in a vacuum.

[Bruce]

Originally posted by Pragmatist

Now take a drop of water and imagine it suspended in the air. Now imagine that there are waves in that drop.


Interesting concept. I've always pictured it as a pulsating sphere, rather like a heart, with the pulses corresponding the wave mechanics.

[CurtC]

Originally posted by Dustin
Why don't they need a meduim?
How does it work?Like the others have said, we don't really know. We have it modelled mathematically very well, but understanding the "why's" is a different matter.

[Soapy Sam]

Sound waves are just air molecules in motion, passing kinetic energy pulses to each other. The pulses of energy, detected by the ear are experienced as sound.

Pump the air out of a bell jar with a bell inside. Ring the bell. The clapper moves normally, but the energy can't escape as compression pulses between molecules, so you don't hear anything. The energy goes into vibration of the bell itself- and, (I suppose) ultimately is radiated as heat (Infra red), which is electromagnetic (ie carried by photons) and can escape through the vacuum.

The sound "barrier"is a feature of our atmosphere and will be different on other worlds. The light barrier is a fundamental feature of our universe. There may be universes, or hyperspaces, where the value of c is higher. If we can get there, travel and get back, we might be able to travel FTL, effectively.

But that, so far as we know, is science fiction.

[davefoc]

My own cut at this is the aether does exist. I realize that physics books put forth the idea that aether doesn't exist and the michelson-morley experiments proved it.

They also are big on the idea that there is no unigue frame of reference.

My suspicion is that there is a unigue frame of reference that is at rest with respect to the aether.

Now I realize that it is incredibly pretentious of me to think that I would have any insight into this given the vastly more sophisticated and nuanced views of the universe that even your average physics phd has than me. OK, I accept that and freely admit that if I knew more or understood better I might change my mind on this.

A unigue frame of reference that is at rest with respect to a hypothetical aether suggests an explanation for is the transmission of electromagnetic waves. It is an observed fact that regardless of the velocicty of a photon's source the photon itself travels so as to neither gain nor lose distance to photons fired in the same direction from another source traveling at a completely different speed. This sounds exactly like there is a transmission media for the photons to me.

It turns out this theory is not unigue to me and there is a guy who has a web site that has worked out the mathematics so as to demonstrate that this view is as valid as a view without a preferred frame of reference at explaining observed phenomena. The biggest problem that I see with the theory is that it makes no predictions that I am aware of that are different than predictions made for a universe without any unigue frames of reference. But it does offer a kind of explanation for electromagnetic wave propagation and special relativity has no such explanation.

[Soapy Sam]

davefoc- I'm glad I'm not the only one. I'd have to say what I feel is a "preference" rather than a belief. I would not try to defend it for a moment.
We could start a "wierd physics we would like but don't actually believe in "society.

[Bodhi Dharma Zen]

My head hurts! ;) I find the topic really fascinating. I wish I had a way to contribute, but this is really complex.

[Bruce]

Without the ability to measure or experiment, theories can reach a point were they are indistinguishable from beliefs. You can only extrapolate so much from experimentally supported theories. The more assumptions you have to make, the farther from reality it takes you.

[pgwenthold]

Originally posted by Pragmatist
Electromagnetic "waves" are not really waves as such, they are actually photons which are rather like self-contained particles. Photons (and in fact all particles) appear to have some sort of internal periodic motion (i.e. repetitive motion). Mathematically this periodic motion can be modelled as a wave and that is one reason why we still refer to "waves" when talking about such things. Of course the periodic motion in question is not necessarily a literal "wave" as such.


Of course, the concept of electromagnetic waves that traversed space without the need of the aether predates wave/particle duality, and the concept of photons.

Maxwell's equations describe electromagnetic waves just fine, and they were around before guys like deBroglie came on the scene.

[Pragmatist]

Originally posted by pgwenthold
Of course, the concept of electromagnetic waves that traversed space without the need of the aether predates wave/particle duality, and the concept of photons.

Maxwell's equations describe electromagnetic waves just fine, and they were around before guys like deBroglie came on the scene.

I'm not sure I'd agree with that, but it all depends on what sense you refer to with "concept".

Maxwell developed his equations based on the fluid mechanics of a hypothetical aether. Therefore, the concept of the waves without the medium did not apparently exist when the equations were developed.

There was resistance to the idea of waves without a medium when Einstein brought it up - and, the idea of quantization had already been developed by that time by Planck and Einstein himself IIRC.

It is remarkable that Maxwell's analysis was so good, that it could be used in abstract without consideration of the medium, but that doesn't imply that it was developed that way. It was Einstein who first seemed to realise that.

I'd have to check, but I thought that Einstein had already developed the photon concept with the photoelectric effect, before he came up with relativity and light without a medium, but I may be mistaken about that.

[pgwenthold]

Originally posted by Pragmatist
I'm not sure I'd agree with that, but it all depends on what sense you refer to with "concept".

Maxwell developed his equations based on the fluid mechanics of a hypothetical aether. Therefore, the concept of the waves without the medium did not apparently exist when the equations were developed.

There was resistance to the idea of waves without a medium when Einstein brought it up - and, the idea of quantization had already been developed by that time by Planck and Einstein himself IIRC.

It is remarkable that Maxwell's analysis was so good, that it could be used in abstract without consideration of the medium, but that doesn't imply that it was developed that way. It was Einstein who first seemed to realise that.

I'd have to check, but I thought that Einstein had already developed the photon concept with the photoelectric effect, before he came up with relativity and light without a medium, but I may be mistaken about that.

Sorry, didn't mean to necessarily imply that it was the basis for them or anything, just that Maxwell's equations describe everything perfectly fine, even without a medium, and they were developed before the concept of particle/wave duality. Therefore, you don't need to resort to photonic behavior to explain how waves travel in space, because Maxwell's equations do just fine.

Mickelson-Morley had shown in 1877 that there wasn't an aether, but it was also true that Maxwell's equations (1864) still worked.

I do know that Einstein spent a lot of time thinking about Maxwell's relations. But I don't think the concepts of relativity and the absence of a medium are inherently linked. Relativity works just fine for light traveling through a medium.

[drkitten]

Originally posted by davefoc
My own cut at this is the aether does exist. I realize that physics books put forth the idea that aether doesn't exist and the michelson-morley experiments proved it.

They also are big on the idea that there is no unigue frame of reference.

My suspicion is that there is a unigue frame of reference that is at rest with respect to the aether.

The biggest problem that I see with the theory is that it makes no predictions that I am aware of that are different than predictions made for a universe without any unigue frames of reference. But it does offer a kind of explanation for electromagnetic wave propagation and special relativity has no such explanation.

It does seem that the Michaelson/Morley experiments would contradict your theory. Unless the Earth were constantly at rest with respect to the transmission medium -- which it would not be at all points in its orbit except one, and which any given point on the Earth's surface could not be except once per day -- then the ether "wind" should be detectable.

[davefoc]

new drkitten,
The Michelson-Morley experiment doesn't contradict the possibility of an aether or a unigue frame of reference.

I realize that it is often presented that way, but that is, I think, not an accurate representation of the situation.

Just because time and length contraction occur that make all frames of reference seem the same does not preclude the possibility that there is actually a special frame of reference at rest with respect to the aether.

I like the idea of the aether because it suggests an explanation for the observed facts. Relativity makes predictions based on the observed facts which time has shown to be true. As such Einstein's relativity theories are seen as great intellectual achievements. However the theories don't suggest a physical explanation for how light propagates or even what it is.

My thought and now it turns out that it is also the thought of other people who seem to know more about it than me is that in fact there might be an aether that serves as the propagation medium for electromagnetic waves. The idea came to me when I was trying to understand this strange notion that no matter what the velocity of emitting source of an electromagnetic wave the emitted wave would neither gain on fall farther behind a wave emitted from a source with a completely different velocity. That suggests to me that there is a medium that that the wave propagates through.

Another thing that comes out of this idea is the possibility that everything we know of as matter and energy are just perturbations in this aether. That explains why what we think of as matter can change to other forms of matter or electromagnetic energy. What is happening is one kind of peturbation in the aether has just changed to a different kind of pertubation in the aether.

As an aside I believe it was reported that Michelson believed that the aether existed until the day he died.

edited to add link to site where this idea is discussed with some mathematical rigor:
http://peterallport.com/

[drkitten]

Originally posted by davefoc

edited to add link to site where this idea is discussed with some mathematical rigor:
http://peterallport.com/

Unfortunately, Mr. Allport's understanding of the Theory of Relativity is deeply, deeply, flawed -- failure to understand the resolution of the "Twin Paradox" is only the tip of the iceberg. Similarly, the four questions to which orthodox theory supposedly cannot provide an answer all have well-understood and well-supported theoretical and experimental answers.

There are also a few easy experimental disproofs of the theory as presented in the papers -- the chief one being that two objects flying in parallel courses will not observe doppler shifts w.r.t. each other under conventional SR, but will observe doppler shifts w.r.t. each other depending upon the motion of the underlying ether, depending upon whether they are both flying "upstream" or "downstream." (Figure B in "QM View 101" illustrates this nicely.) I should even be able to produce a measurable doppler effect, that varies with apparatus orientation and with the Earth's "proper motion", by bouncing a signal off a mirror. Needless to say, I am unfamiliar with any experimental support for this claim.


The theory itself is inconsistent as well. For example (again, from "QM View 101"), "why should observers in B observe a "relativistic" foreshortening of the diamond formation in A when no foreshortening exists? And why should observers in B observe that the clocks in A are running at only .6 times the rate of clocks in B when just the opposite is true? Several factors combine to cause the virtual length contraction and time dilation determined by the observers in B. These factors include the shortened units of distance in B, the slowness of clocks in B, the observers' lack of awareness of the actual distance and time units kept by their measuring instruments, and the observers' belief that the speed of light is constant in their diamond formation." (For the benefit of the lurkers, objects in A are "at rest" w.r.t. the hypothetical ether, objects in B are not.

Unfortunately, his reasoning is exactly backwards. If observers in situation B are using clocks that are OBJECTIVELY slowed and meters that are OBJECTIVELY shortened relative to situation A, then observers in B will notice that A is experiencing exactly the opposite situation than relativity predicts. In fact, observers in B will notice a much worse situation than actually holds.

Again, I believe that the Michaelson/Morley situation provides a direct experimental disproof of this theory. It's greatly to Mr. Allport's discredit that he doesn't even appear to discuss the implications of M/M.

[Ziggurat]

Originally posted by davefoc
new drkitten,
The Michelson-Morley experiment doesn't contradict the possibility of an aether or a unigue frame of reference.

In a sense, this is correct. But what it DOES do is prove that no aether or unique frame is necessary to describe the propagation of light. And in the absence of any need for such an aether, why would anyone want to introduce it?

I like the idea of the aether because it suggests an explanation for the observed facts. Relativity makes predictions based on the observed facts which time has shown to be true. As such Einstein's relativity theories are seen as great intellectual achievements. However the theories don't suggest a physical explanation for how light propagates or even what it is.

An aether doesn't help you out here either. It just shifts the question from "what are electric and magnetic fields" to "what is the aether". That's not an improvement. It's an added, and unnecessary, complication.

My thought and now it turns out that it is also the thought of other people who seem to know more about it than me is that in fact there might be an aether that serves as the propagation medium for electromagnetic waves. The idea came to me when I was trying to understand this strange notion that no matter what the velocity of emitting source of an electromagnetic wave the emitted wave would neither gain on fall farther behind a wave emitted from a source with a completely different velocity. That suggests to me that there is a medium that that the wave propagates through.

It wouldn't suggest that if you really understood relativity. In the hyperbolic geometry of relativity, such "strange" behavior is completely natural and expected. It's really quite simple indeed. All you need is the hyperbolic space-time metric (s^2 = x^2 + y^2 + z^2 - (ct)^2 ), and everything else falls out naturally. Electromagnetism becomes self-consistent and matches with experiments with NO need for any medium, the slowing of clocks in motion falls out (also experimentally observed), c as an upper limit on velocity falls out, it's all internally consistent AND matches with experiment. Just from that ONE little equation. There is no aether theory that can match experiment with anywhere near this simplicity. That, ultimately, is why nobody serious pays any more attention to the aether. It's simply unnecessary. Occam's razor, and all that.

[davefoc]

new drkitten,
Thank you for your response. I, frankly, am not knowledgeable enough on this subject to respond directly to your criticism of allport. I do plan to spend some time with your post and attempt to understand your points better, however.

you wrote:Again, I believe that the Michaelson/Morley situation provides a direct experimental disproof of this theory.

Which theory does the michelson/morley experiment disprove?

Is it your view that it disproved the possibility of a unigue frame of reference?

I believe that it found that it wasn't possible to distinguish one frame of reference from another based on the speed of light. Not being able to distinguish one frame of reference from another frame of reference is different than determining that there is no unigue frame of reference.

[drkitten]

Originally posted by davefoc

you wrote:

Which theory does the michelson/morley experiment disprove?

Is it your view that it disproved the possibility of a unigue frame of reference?



It is my view that it disproved the possibility of a stationary aether through which EM waves propagate and that the EM Doppler effect is caused by source or observer motion through this aether.

Basically, Mr. Allport's theory.

The simplest interpretation of the M/M experiments is that, IF an aether exists, the Earth is at rest relative to it. There may indeed be another theory that postulates a unique frame of reference that is compatible with M/M. (A number of scientists, for example, experimented with the idea of "aether drag," where the Earth's mass produces a local effect on the aether where it is at rest. This is compatible with M/M, but contradicted by other experiments.) But Allport's theory does not appear to be compatible with M/M or with the observations of SR.

[davefoc]

ziggurat, I basically agree with everything you said including your suggestion (veiled a bit) that I might not understand relativity.

The only thing where I quibble is that relativity provides any kind of physical explanation for what is going on. Relativity starts with an assumption of the constancy of the speed of light and builds a mathematical world that makes other predictions. We think this mathematical world is a good description of what is going on because it makes predicitons that turn out to be correct.

But the theory doesn't provide any idea of what the underlying physical mechanism is. You say that it doesn't need an aether since it can make predicitons without the aether so the aether is an unncessary add on. You also say that hypothesizing aether doesn't get you any further as to knowing what is going on because you don't know what causes the aether or much about its nature. All of this I basically agree with to the degree I understand the issues.

Still, when one is looking for an explanation for how electromagnetic waves propagate or what is their actual nature the existence of this mathematical model that makes all these cool predicitons doesn't provide any insight into an actual mechanism.

As a slight digression this discussion reminds me a bit of the discussion on what is the point of hypothesizing a god. It just moves the answer to basic questions of how we came to be one step further away since it provides no answer to how did god come to be. I think you are saying that the idea about aether is sort of like the idea of a god. For reasons I can't defend (at least right now) I see the situation as different but fully acknowlege that people who know a lot more about this don't agree with me.

I am afraid I don't have time to look this up right now but I believe that Lorentz who derived the equations for time and length contractions based on an assumption that there was an aether.

[CurtC]

davefoc wrote:
Still, when one is looking for an explanation for how electromagnetic waves propagate or what is their actual nature the existence of this mathematical model that makes all these cool predicitons doesn't provide any insight into an actual mechanism.Yes, that's true, but it appears to be the world we live in. Our intuitive understanding of how the world works evolved/developed in an environment where quantum and relativistic effects can be ignored. It's not surprising to me that when when we then try to apply our intuition to situations outside this normal range, our intuition doesn't apply and we can't really imagine how it works.

It just has to be enough that we have mathematical models describing how the world works - an intuitive explanation is probably not going to happen.

[Ziggurat]

Originally posted by davefoc
ziggurat, I basically agree with everything you said including your suggestion (veiled a bit) that I might not understand relativity.


Not many people really do understand it. The frustrating part is that it's not actually that complicated, the biggest problem is getting over what are very deep-seated and completely reasonable assumptions pretty much everyone has that are just wrong.

But the theory doesn't provide any idea of what the underlying physical mechanism is.

What I'm trying to get at is looking for a "mechanism" isn't even the right way to approach it. Relativity is a description of what the geometry of space-time looks like. Nothing about light is special here: gravitational waves will also travel at c (and aether theories don't help you out there either). ANY perturbations of a massless field will travel at c. But you never hear aether theory proponents talking about how gravitational waves get carried by the aether.

Still, when one is looking for an explanation for how electromagnetic waves propagate or what is their actual nature the existence of this mathematical model that makes all these cool predicitons doesn't provide any insight into an actual mechanism.

But neither do aether theories. They're even MORE of a mathematical abstraction. You can measure electric fields and magnetic fields, even if you're not confident of the "mechanism" for how a field extends through a vacuum. But you cannot measure an aether. All it gives you is "waves travel in a medium, and the aether is the medium of light". But that then just becomes dogma, not physics - we're used to waves traveling in a medium, but why must this be so? It's worse that just putting the mechanism question one step back, it actually obscures the question and makes it harder to figure things out.

I am afraid I don't have time to look this up right now but I believe that Lorentz who derived the equations for time and length contractions based on an assumption that there was an aether.

Yes indeed. That's one of the ironies of the development of relativity. But Lorenz was originally trying to fix an awkward theory with kludges - he had no mechanism for his length contraction, he just needed some tweak of the theory to try to fit the experimental results, and that's what worked. But it's a simple consequence of geometry in relativity, not some adjustment to make things fit.

[Bodhi Dharma Zen]

Originally posted by Ziggurat
gravitational waves will also travel at c

If a graviton is discovered, does that affects the notion of mass distorting space/time? (and distortion as the cause for the "force" of attraction) or the gravitons would be taken as the cause for the distortion?

[gnome]

Originally posted by davefoc
I believe that it found that it wasn't possible to distinguish one frame of reference from another based on the speed of light. Not being able to distinguish one frame of reference from another frame of reference is different than determining that there is no unigue frame of reference.

If one frame of reference cannot be distinguished from another, how can uniqueness exist? To me the two are logically equivalent.

ehh, it's all bollocks anyway. The Technocracy destroyed the aether by making everyone disbelieve it earlier this century. It was an attack on the Sons of Ether.

[Ziggurat]

Originally posted by Bodhi Dharma Zen
If a graviton is discovered, does that affects the notion of mass distorting space/time? (and distortion as the cause for the "force" of attraction) or the gravitons would be taken as the cause for the distortion?

A graviton is a quantized gravitational wave. Gravitational waves appear very naturally in general relativity, and happen any time you have an oscillating mass distribution (such as two stars orbiting each other), and in fact the whole thing would fall apart if you DIDN'T have gravitational waves. But once you start worrying about how, or if, those waves are quantized (as light is quantized electromagnetic waves), you get into the intersection of general relativity and quantum mechanics, where things get messy, and nobody is really sure what goes on.

But the distortion of space/time around massive bodies, as predicted by general relativity, is an observed fact - we know it happens because we can see it happening. Calculating the correct precession of Mercury's orbit was one of the early triumphs of general relativity. Right now they're performing an experiment called Gravity Probe B, which will measure something called frame drag, which is an effect in general relativity where a massive body (in this case the earth) not only warps space by its mass, but also in a sense "drags" space around it as it rotates.
http://einstein.stanford.edu/

[drkitten]

Originally posted by gnome
If one frame of reference cannot be distinguished from another, how can uniqueness exist? To me the two are logically equivalent.



The original quote was "it wasn't possible to distinguish one frame of reference from another based on the speed of light." This raises the possibility of being able to distinguish one frame from another by other methods. (The original quote is also a misunderstandng of the M/M experiment, but we'll let that pass).

[davefoc]

Michelson/Morley as I understand it:

Before Michelson/Morley there was a view that an aether existed that served as a transmission medium for electromagnetic waves in a similar way that air serves as a transmission medium for sound waves.

The idea was that the earth was traveling through this medicum and as such the speed of light would be different depending on the direction of the light that the speed was measured.

In order to test this idea Michelson/Morley set up an experiment whereby the phase of two light beams arriving at 90 degree angles with respect to each other could be compared using an interferometer.

A phase difference was not detected thereby suggesting that the light beams were traveling at the same speed.

This suggested that the idea that the aether as it was understood prior to that experiment didn't exist.

If this isn't correct any corrections will be appreciated.

Did the result of this experiment preclude the possibility of an aether? As I understand the results, it didn't. The implications of the experimental result was that time and distance were not the fixed quantities that had previously been thought and that they varied depending on the relative speeds of different frames of reference. The experiment did not preclude the possibility of a unique frame of reference but it did determine that the technique for finding the unique frame of reference that had been hypothesized wouldn't work.

I have carefully read Ziggurat's responses and I take them seriously in that he not only sounds knowledgeable his views are consistent with the views of others that understand seem to have an understanding of relativity.

I appreciate dr newkitten's comments and would like to understand her criticism of allport's views a little better. I hope to get a chance to look at this a little better this weekend.

[drkitten]

Originally posted by davefoc

[The M/M experiment] suggested that the idea that the aether as it was understood prior to that experiment didn't exist.

If this isn't correct any corrections will be appreciated.

Did the result of this experiment preclude the possibility of an aether?

No, of course not. No experiment can possibly do that if you are allowed to shift the definition of the aether. And, in particular, if I am allowed to define the aether as some sort of omnipresent mist that has no physical consequences whatsoever, then no experiment can possible disprove such an aether. But such a definition is scientifically useless, because no evidence can ever be gathered to support such an aether, either.



The implications of the experimental result was that time and distance were not the fixed quantities that had previously been thought and that they varied depending on the relative speeds of different frames of reference.


Well, that's one interpretation of the implications -- but not a common one by any stretch of the imagination. (I believe that only Lorentz and Einstein took such implications seriously, and they ran into a lot of resistance). A much more common implication was that the aether, as commonly envisioned, didn't exist -- and so there were some other aetheric properties that needed to be identified. (See the experiments on aether drag, for example.)

As thirty more years of experimental evidence mounted, it became more and more apparent that there were no obvious aetheric properties to be found.

Unfortunately, Allport's theories predict -- in fact, rely on -- several aetheric properties. As far as I can tell, a number of them have been experimentally looked for, and not found.

There may in fact be a viable theory of the aether. But Allport doesn't have it. And as far as I can tell, neither do you.

[Ziggurat]

Originally posted by davefoc
This suggested that the idea that the aether as it was understood prior to that experiment didn't exist.

Yes.

The experiment did not preclude the possibility of a unique frame of reference but it did determine that the technique for finding the unique frame of reference that had been hypothesized wouldn't work.

Yes.

Strangely enough (and you might get a kick out of this), general relativity does allow for a unique reference frame, a sort of "rest frame" for the universe. In general relativity, this unique reference frame is sort of a topological concept: it's only when you try to map out a coordinate system for the entire universe that the preference emerges, if you only focus on small patches then all your physics is still exactly the same regardless of reference frame. But this unique reference frame is actually experimentally observable: it's the frame in which the cosmic background radiation is uniform. If you change to a reference frame moving with respect to the unique reference frame, then the background radiation will be blue-shifted on the "forward" side and red-shifted on the "backward" side, and you can determine the direction and speed of your velocity relative to this unique reference frame. But this is a sort of "global" test, since you're basically observing the entire visible universe when you examine the background radiation. Any local test of physics will still be unable to determine any unique reference frames.

[Bodhi Dharma Zen]

Originally posted by Ziggurat
But the distortion of space/time around massive bodies, as predicted by general relativity, is an observed fact

Thats exactly what I asked. Does gravitons will be "the cause" for that distortion instead of mass? Im confused.

[drkitten]

Originally posted by Bodhi Dharma Zen
Thats exactly what I asked. Does gravitons will be "the cause" for that distortion instead of mass? Im confused.

I'm not sure the question is well-phrased. That's kind of like asking whether photons are "the cause" of visible brightness instead of a flashlight. Different scale, different description, same phenomenon.

[Ziggurat]

Originally posted by Bodhi Dharma Zen
Thats exactly what I asked. Does gravitons will be "the cause" for that distortion instead of mass? Im confused.

No. Gravitational waves are ripples in the distortion of gravitational fields, they are not the cause of those fields. Similarly, light is not "the cause" of magnetic and electric fields, but ripples in those fields.

[Bodhi Dharma Zen]

new drkitten

You are right, it is not well phrased.

Ziggurat

Nevertheless, this was the answer I was looking for.

So, one thing is the curvature of space/time, and another the generated gravitational waves. The graviton would be the "particle" (quantized wave) of those waves, and both the result of space/time distortion. Do I finally get it?

[davefoc]

One thing I meant to add because I find it interesting not because it is relevant to what I was talking about is that Einstein said that he might not have been aware of the Michelson/Morley experiment when he first devised special relativity.

That really amazes me since it implies that even before experimental evidence was available he was conceiving of a universe where the speed of light was constant and the consequences of that fact.

Ziggurat, that is interesting. I didn't understand everything you said but I think I did regarding the fact that the CMB provides a kind of unique frame of reference. I vaguely remember thinking about that once before and if I can't find anything interesting on the radio on the way to this old building I'm restoring today I think I'll think about that for awhile.

[Ziggurat]

Originally posted by Bodhi Dharma Zen
So, one thing is the curvature of space/time, and another the generated gravitational waves. The graviton would be the "particle" (quantized wave) of those waves, and both the result of space/time distortion. Do I finally get it?

Yes.

[Ziggurat]

Originally posted by davefoc
One thing I meant to add because I find it interesting not because it is relevant to what I was talking about is that Einstein said that he might not have been aware of the Michelson/Morley experiment when he first devised special relativity.

That's certainly possible, and you run into similar questions about the meaning of electromagnetism regardless of the M/M theory. For example, imagine you've got a magnet, and you throw a charged particle past it. A moving charged particle passing through a magnetic field feels a force. But now imagine the same problem from the particle's point of view: it's not moving at all, so there can be no force on it from the magnetic field. However, the magnet is moving, so it produces a time-varying magnetic field, which induces an electric field, and in the particle's reference frame, it's this induced electric field that produces the force. But it's the same force. How could you have force from a magnetic field in one frame, but from an electric field in the other? The assumption was that there was some "correct" reference frame in which to do these calculations, but a happy coincidence that you could get the right answer (the force) even in the wrong frame. Einstein didn't believe it was a coincidence. And indeed, under relativity, it's not, because electricity and magnetism are part of a single electromagnetic field (typically expressed as an antisymmetric 4x4 matrix, which has 6 free parameters, the same as two separate 3-dimensional vectors), which transforms smoothly as you go from one reference frame to another, and which no longer has any different behavior in different reference frames. The Lorenz transformations which convert positions in space-time between reference frames are exactly the same ones that convert the electrodynamic matrix, and the change from a magnetic field only in the above problem to a combined magnetic and electric field is a straightforward result. Maxwell's equations are then Lorenz-invariant, meaning they really are the same for all reference frames, and there's no longer a problem of worrying about which reference frame was really the "correct" one to do the calculation in, since they're all the same.

So in short, yes, Einstein had reason to not be satisfied with classical E&M even before the Michelson/Morley experiment.

[69dodge]

Feynman Lectures on Physics, volume 2, section 1--5, "What are the fields":

We now make a few remarks on our way of looking at this subject. You may be saying: "All this business of fluxes and circulations is pretty abstract. There are electric fields at every point in space; then there are these 'laws.' But what is actually happening? Why can't you explain it, for instance, by whatever it is that goes between the charges." Well, it depends on your prejudices. Many physicists used to say that direct action with nothing in between was inconceivable. (How could they find an idea inconceivable when it had already been conceived?) They would say: "Look, the only forces we know are the direct action of one piece of matter on another. It is impossible that there can be a force with nothing to transmit it." But what really happens when we study the "direct action" of one piece of matter right against another? We discover that it is not one piece right against the other; they are slightly separated, and there are electrical forces acting on a tiny scale. Thus we find that we are going to explain so-called direct-contact action in terms of the picture for electrical forces. It is certainly not sensible to try to insist that an electrical force has to look like the old, familiar, muscular push or pull, when it will turn out that the muscular pushes and pulls are going to be interpreted as electrical forces! The only sensible question is what is the most convenient way to look at electrical effects.

[...]

The best way is to use the abstract field idea. That it is abstract is unfortunate, but necessary. The attempts to try to represent the electric field as the motion of some kind of gear wheels, or in terms of lines, or of stresses in some kind of material have used up more effort of physicists than it would have taken simply to get the right answers about electrodynamics. It is interesting that the correct equations for the behavior of light in crystals were worked out by McCullough in 1843. But people said to him: "Yes, but there is no real material whose mechanical properties could possibly satisfy those equations, and since light is an oscillation that must vibrate in something, we cannot believe this abstract equation business." If people had been more open-minded, they might have believed in the right equations for the behavior of light a lot earlier than they did.

In the case of the magnetic field we can make the following point: Suppose that you finally succeeded in making up a picture of the magnetic field in terms of some kind of lines or of gear wheels running through space. Then you try to expalin what happens to two charges moving in space, both at the same speed and parallel to each other. Because they are moving, they will behave like two currents and will have a magnetic field associated with them (like the currents in the wires of Fig. 1--8). An observer who was riding along with the two charges, however, would see both charges as stationary, and would say that there is no magnetic field. The "gear wheels" or "lines" disappear when you ride along with the object! All we have done is to invent a new problem. How can the gear wheels disappear?! The people who draw field lines are in a similar difficulty. Not only is it not possible to say whether the field lines move or do not move with charges---they may disappear completely in certain coordinate frames.

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section 20--3, "Scientific imagination":

I have asked you to imagine these electric and magnetic fields. What do you do? Do you know how? How do I imagine the electric and magnetic field? What do I actually see? What are the demands of scientific imagination? Is it any different from trying to imagine that the room is full of invisible angels? No, it is not like imagining invisible angels. It requires a much higher degree of imagination to understand the electromagnetic field than to understand invisible angels. Why? Because to make invisible angels understandable, all I have to do is to alter their properties a little bit---I make them slightly visible, and then I can see the shapes of their wings, and bodies, and halos. Once I succeed in imagining a visible angel, the abstraction required---which is to take almost invisible angels and imagine them completely invisible---is relatively easy. So you say, "Professor, please give me an approximate description of the electromagnetic waves, even though it may be slightly inaccurate, so that I too can see them as well as I can see almost invisible angels. Then I will modify the picture to the necessary abstraction."

I'm sorry I can't do that for you. I don't know how. I have no picture of this electromagnetic field that is in any sense accurate. I have known about the electromagnetic field a long time---I was in the same position 25 years ago that you are now, and I have had 25 years more of experience thinking about these wiggling waves. When I start describing the magnetic field moving through space, I speak of the E and B fields [E is the electric field; B is the magnetic field. --69dodge] and wave my arms and you may imagine that I can see them. I'll tell you what I see. I see some kind of vague shadowy, wiggling lines---here and there is an E and B written on them somehow, and perhaps some of the lines have arrows on them---an arrow here or there which disappears when I look too closely at it. When I talk about the fields swishing through space, I have a terrible confusion between the symbols I use to describe the objects and the objects themselves. I cannot really make a picture that is even nearly like the true waves. So if you have some difficulty in making such a picture, you should not be worried that your difficulty is unusual.

Our science makes terrific demands on the imagination. The degree of imagination that is required is much more extreme than that required for some of the ancient ideas. The modern ideas are much harder to imagine. We use lots of tools, though. We use mathematical equations and rules, and make a lot of pictures. What I realize now is that when I talk about the electromagnetic field in space, I see some kind of a superposition of all of the diagrams which I've ever seen drawn about them. I don't see little bundles of field lines running about because it worries me that if I ran at a different speed the bundles would disappear. I don't even always see the electric and magnetic fields because sometimes I think I should have made a picture with the vector potential and the scalar potential, for those were perhaps the more physically significant things that were wiggling. [The combination of vector potential and scalar potential is another way of representing the same electromagnetic field that can be represented by the combination of electric field and magnetic field. --69dodge]

Perhaps the only hope, you say, is to take a mathematical view. Now what is a mathematical view? From a mathematical view, there is an electric field vector and a magnetic field vector at every point in space; that is, there are six numbers associated with every point. Can you imagine six numbers associated with each point in space? That's too hard. Can you imagine even one number associated with every point? I cannot! I can imagine such a thing as the temperature at every point in space. That seems to be understandable. There is a hotness and coldness that varies from place to place. But I honestly do not understand the idea of a number at every point.

So perhaps we should put the question: Can we represent the electric field by something more like a temperature, say like the displacement of a piece of jello? Suppose that we were to begin by imagining that the world was filled with thin jello and that the fields represented some distortion---say a stretching or twisting---of the jello. Then we could visualize the field. After we "see" what it is like we could abstract the jello away. For many years that's what people tried to do. Maxwell, Ampere, Faraday, and others tried to understand electromagnetism this way. (Sometimes they called the abstract jello "ether.") But it turned out that the attempt to imagine the electromagnetic field in that way was really standing in the way of progress. We are unfortunately limited to abstractions, to using instruments to detect the field, to using mathematical symbols to describe the field, etc. But nevertheless, in some sense the fields are real, because after we are all finished fiddling around with mathematical equations---with or without making pictures and drawings or trying to visualize the thing---we can still make the instruments detect the signals from Mariner II and find out about galaxies a billion miles away, and so on.

The whole question of imagination in science is often misunderstood by people in other disciplines. They try to test our imagination in the following way. They say, "Here is a picture of some people in a situation. What do you imagine will happen next?" When we say, "I can't imagine," they may think we have a weak imagination. They overlook the fact that whatever we are allowed to imagine in science must be consistent with everything else we know: that the electric fields and the waves we talk about are not just some happy thoughts which we are free to make as we wish, but ideas which must be consistent with all the laws of physics we know. We can't allow ourselves to seriously imagine things which are obviously in contradiction to the known laws of nature. And so our kind of imagination is quite a difficult game. One has to have the imagination to think of something that has never been seen before, never been heard of before. At the same time the thoughts are restricted in a strait jacket, so to speak, limited by the conditions that come from our knowledge of the way nature really is. The problem of creating something which is new, but which is consistent with everything which has been seen before, is one of extreme difficulty.

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It's interesting to note that Feynman takes it for granted that there isn't really any ether; the only purpose that the concept of ether might serve is to help us visualize the electromagnetic field, but once we've done that we are supposed to abstract it away. And then he dismisses it as not being useful even for that limited purpose.

Compare that modern view to the one expressed by Sir William Thomson (Lord Kelvin) in 1884 (excerpted from http://www.bartleby.com/30/15.html):

I move through this “luminiferous ether� as if it were nothing. But were there vibrations with such frequency in a medium of steel or brass, they would be measured by millions and millions and millions of tons’ action on a square inch of matter. There are no such forces in our air. Comets make a disturbance in the air, and perhaps the luminiferous ether is split up by the motion of a comet through it. So when we explain the nature of electricity, we explain it by a motion of the luminiferous ether. We cannot say that it is electricity. What can this luminiferous ether be? It is something that the planets move through with the greatest ease. It permeates our air; it is nearly in the same condition, so far as our means of judging are concerned, in our air and in the inter-planetary space. The air disturbs it but little; you may reduce air by air-pumps to the hundred thousandth of its density, and you make little effect in the transmission of light through it. The luminiferous ether is an elastic solid, for which the nearest analogy I can give you is this jelly which you see [here he exhibits a large bowl of clear jelly with a small red wooden ball embedded in the surface near the centre], and the nearest analogy to the waves of light is the motion, which you can imagine, of this elastic jelly, with a ball of wood floating in the middle of it. Look there, when with my hand I vibrate the little red ball up and down, or when I turn it quickly round the vertical diameter, alternately in opposite directions;---that is the nearest representation I can give you of the vibrations of luminiferous ether.

Another illustration is Scottish shoemakers’ wax or Burgundy pitch, but I know Scottish shoemakers’ wax better. It is heavier than water, and absolutely answers my purpose. I take a large slab of the wax, place it in a glass jar filled with water, place a number of corks on the lower side and bullets on the upper side. It is brittle like the Trinidad pitch or Burgundy pitch which I have in my hand---you can see how hard it is---but when left to itself it flows like a fluid. The shoemakers’ wax breaks with a brittle fracture, but it is viscous and gradually yields.

What we know of the luminiferous ether is that it has the rigidity of a solid and gradually yields. Whether or not it is brittle and cracks we cannot yet tell, but I believe the discoveries in electricity and the motions of comets and the marvellous spurts of light from them, tend to show cracks in the luminiferous ether—show a correspondence between the electric flash and the aurora borealis and cracks in the luminiferous ether. Do not take this as an assertion, it is hardly more than a vague scientific dream: but you may regard the existence of the luminiferous ether as a reality of science; that is, we have an all-pervading medium, an elastic solid, with a great degree of rigidity—an rigidity so prodigious in proportion to its density that the vibrations of light in it have the frequencies I have mentioned, with the wave-lengths I have mentioned. The fundamental question as to whether or not luminiferous ether has gravity has not been answered. We have no knowledge that the luminiferous ether is attracted by gravity; it is sometimes called imponderable because some people vainly imagine that it has no weight; I call it matter with the same kind of rigidity that this elastic jelly has.

[...]

Now what is the luminiferous ether? It is matter prodigiously less dense than air---millions and millions and millions of times less dense than air. We can form some sort of idea of its limitations. We believe it is a real thing, with great rigidity in comparison with its density: it may be made to vibrate 400 million million times per second; and yet be of such density as not to produce the slightest resistance to any body going through it.

Going back to the illustration of the shoemakers’ wax; if a cork will, in the course of a year, push its way up through a plate of that wax when placed under water, and if a lead bullet will penetrate downwards to the bottom, what is the law of the resistance? It clearly depends on time. The cork slowly in the course of a year works its way up through two inches of that substance; give it one or two thousand years to do it and the resistance will be enormously less; thus the motion of a cork or bullet, at the rate of one inch in 2,000 years, may be compared with that of the earth, moving at the rate of six times ninety-three million miles a year, or nineteen miles per second, through the luminiferous ether; but when we can have actually before us a thing elastic like jelly and yielding like pitch, surely we have a large and solid ground for our faith in the speculative hypothesis of an elastic luminiferous ether, which constitutes the wave theory of light.