Can someone explain me the ether drag hypothesis?
Ether drag was a concept invented to help explain the failure of the the M&M experiment to measure any movement of the Earth with respect to the ether.
The idea was that massive objects drug a "bubble" of ether with them as they moved. Thus the local ether is moving with the Earth and we wouldn't be able to detedt any motion with respect to the ether as a whole with experiments close to the Earth's suface.
This hypothesis fails due to the fact that we see stellar aberration that we do. If we drug a bubble of ether with us, the amount of stellar aberration we measure would be much less or absent all together.
While you are on the topic of a controversial ether I think I should mention that in 1920 A. Einstein accepted the idea that there is an ether. Please check the following link for more on this.
Oh lord, not that old canard again!
Short answer: what Einstein meant, as is clear from the whole quote, is that his curved, active spacetime in general relativity had taken the place of the old ether as a "something" extended through space (it WAS space!) that supported physics. He did not accept any form of the luminiferous ether that was thought up in the nineteenth century.
Don't forget that the ether proposal was a way to explain the transmission of light waves. Of course it did not make sense because the ether had to be very dense and rigid in order to account for the high velocity of light waves.
I can see where people could think that the dragging of spacetime looks a lot like the ether proposal unless you consider why it was made.
polyb - It is not necessary to regard the ether as rigid to get a value for c that complies with its experimental value. If you take the average energy density rho of the universe you get about 10^-26 kgm/m^3. If that energy exists in the form of spatial stress, then since c = (p/rho)^1/2 you can arrive at numbers that are in the range of c depending upon the assumptions made about pressure and/or modulus
Self adjoint - your slant upon Einstein's later views re the ether are not born out by an unbiased reading of what was said. Einstein left no doubt that the concept of an ether (what he frequently referred to as space) was substantive - not just in his 1920 address at Leyden, but in many other confirmations thereafter - in particular his speach honoring Faraday. The only thing dismissive about the ether was his statement that the idea of motion could not be applied to it.
That is just plain wrong, yogi. Einstein flatly rejected any notion of 'aether'. SA is not taking a 'slant', he is merely being factual, as was polyb.
Chronos - your are either uniformed as to Einstein's writings - or you don't understand the plain meaning of his words.
Could you expand, yogi - and explain the apparent contradiction in the fact that motion can't be applied to Einstein's "aether"? That is, affter all, one of the most important features of the classical "aether." Also, did Einstein consider this "aether" a medium on which light propagated? And finally, how does all of this jive with Relativity?
In short, yogi, I ask you to explain how that all fits (or doesn't fit) with the conventional interpretation (and it is the conventional interpretation) of Einstein's position that this new "aether" is wholly different from the classical aether believed-in previously.
Russ - I have posted excerpts from Einstein's addresses previously - if you want I will repost them. But I think you are equally familiar with them. Einstein only said the notion of an ether is superfluous to the derivation of the LT(s). So its properties, whatever they are, do not figure into SR. But the conditioning of space by matter as per GR, and Einsteins statements regarding the reciprocity of reactionary force being consequent to either 1) acceleration of mass wrt space or 2) acceleration of space wrt mass, would certainly support the idea of a substantive medium. To my knowledge he never said that the propagation of em waves involved the ether or that a medium was necessary for light waves - that comes from Maxwell. I am not sure what your referring to when you say "New" ether - did Einstein change his trajectory between 1905 and 1920 as to the ether? - it would appear that he did - but again, contrary to the many text books that assert SR did away with the concept of an ether, it did not. SR is simply silent on the subject. So it could be argued that Einstein really hadn't changed his mind, he simply wanted to stress the symmetry of the Lorentz transforms as being self referential as between relatively moving inertial frames. - no ether being required.
As for whether a classical ether exists in the sense of a medium for light propagation, it is still unresolved. Space as a capacity per unit length and an inductance per unit length and those two factors determine the velocity of em waves just as they do in a transmission line. You wouldn't argue that the inducance and capacitance of a transmission line do not determine is impedance or its propagation properties - so why would it make sense to ignor these same factors when dealing with space.
Neither MMx nor any of the other over and back experiments proved anything about the one way velocity of light, ergo they do not disprove the existence of a medium. The invariance of the round trip velocity of light is not inconsistent with an ether because time dilation provides the exact amount of correction to make the round trip velocity appear to be isotropic.
So let me get this right. You're saying that there is nothing that can distinguish between this "ether" and "no ether"? That all the MM, Kennedy-Thorndyke, etc. type experiments are consistent to BOTH? That there is this "thing", but you can't distinguish any measurement from the scenario that says that it doesn't exist?
I am not quite sure what is being imputed. As for Kennedy-Thorndyke, MMx and the like, these experiments are consistent with "no ether." But they are also consistent with several ether theories such as MLET, LR, and the Inertial Transforms developed by Selleri. Kennedy Thorndike did eliminate one class of ether theories based upon length contraction alone.
I am not saying there is nothing to distinquish between the existence of some spatial medium (call it what ever you want) and the idea of a vacuum devoid of both characteristics and energy. The question is whether the measured properties of space are a lose collection of unrelated attributes, or do they define the characteristics of a continuum of some sort.
I'm not "imputing" anything. I just wanted to make sure I am clear about what you were trying to say, that ALL of these experiments so far cannot distinguish between "there is ether" and "there is no ether", in whatever form of ether that you defined.
Yes - I would say that SO FAR the experiments are inconclusive. But to examine this answer further - what is it that they are inconclusive about? - if the properties of space are sui generis we don't have much of anything to base a conclusion on - physics is basically a study of relationships - For example, we don't know what an electron is in terms of something else - we know it repels other electrons, has mass and contains exactly one unit of charge - but what is charge? ..We can ask a similar question with regard to the nature of the photon ...the point being that at a fundamental level we don't have a good model of many things - so when we say there is no ether - what does that mean? This is why the question of aetheral reality is not easily answered.
But if we go by your argument, then you are implying that the existence of the "electron" and "photon" are also in doubt. I'd say that this is going a bit too far to say that we don't have a good model of many things.
An electron and a photon are defined by a set of properties. We TEST for those properties. When we get positive results, we conclude that there is a very high probability that electrons and photons exist. The nature of those properties are something else that will require a separate set of study, which is why people are looking for the Higgs, study spin-physics at RHIC, etc, etc.
The issue here is if there is a similar set of properties of the ether. I see different people defining it differently. The classical ether as pre 1900 clearly made several predictions. When one tries to measures these predictions and get null results, one draws one own conclusions. However, it seems nowadays that it is "fashionable" to try to redefine the ether in some other ways. So far, the ONLY consequences that I have seen coming out of such new definitions is that one cannot distinguish its presence from experiements that indicate it not being there. I don't know about you, but I find this highly dubious.
At some point, there HAS to be tangible deviations from "ether exists" and "ether does not exist". Anyone claiming that it does exist MUST make such predictions that are measureable.
I have double checked a number of citations that I have listed as verifying the postulate of SR and that claim the non-existent of the classical ether. These include:
C. Braxmaier et al., PRL v.88, p.010401 (2002);
P. Wolf et al., PRL v.90, p.060403 (2003);
Muller et al., PRL v.91, p.020401 (2003);
M. Fullekrug, PRL v.93, p.043901 (2004)
H. Muller, Phys. Rev. D 71, 045004 (2005).
In NONE of these, were there ANY challenges, rebuttals, or comments submitted to dispute or contradict the claims made. No one submitted anything saying to the effect that "oh, there's still ether. You're just not measuring its effect in this case because so-and-so".
When the properties of this new "ether" is itself rather etherial, it is impossible to pin down what properties it will have that will cause it to shout to the rest of the universe and says "I'm here!". Till then, I will go by the standard textbooks, because obviously, its presence so far has made zero impact on physics.
I have a question for you, and not in an argumentative sort of way. Your posts indicate that your are quite familiar with ether theories (both old and new). I am not. My question is probably a simple one for you then. Do any of these "define" the ether simply as the energy-momentum of space-time itself?
The reason I ask is there is an obvious and direct relationship between EFE's (in GR) and the classical equations of hydrodynamics. Schrodinger's "Space-Time Structure" does a nice job of making that point, in case you're interested.
Zapper, Reality Patrol. Granted there are numerous authorities that claim there is no ether - and by their definition of ether, that is propably true. Einstein during his years of contemplation following 1905, left little doubt of his convictions on the subject as per his 1920 address. From my perspective, it is not within the spirit of science to make outright assertions that certain things exist or do not exist. It is true we can define many of the properties of electrons and photons - but are they a substantive chunk of something?... a particle is again only described by its relationship with other particles and perhaps with all other particles if you take a holistic view. I have always been fond of Einstein's comment in his letter to his lifelong friend Besso near the end of his life: "All these years of conscious brooding about the photon have brought me no closer to the truth. Nowdays every Tom, Dick and Harry thinks he knows the answer, but he is wrong."
If you believe there is more to space than a vacuum you are in good company - Einstein and Dirac will do for a start. Consider the following:
1) Inertia - why is it that acceleration wrt space produces a reactionary force.
2) EM wave velocity - determined by the product of the inductance/lenght and capacitance/length of free space as per Maxwell
3) The Free space Impedance (377 ohoms) determined by the ratio of the same quantities.
4) The stretching of space as per Robertson's generally accepted explanation of cosmological red shift
5) The conditioning of space by matter as per GR
6) The expansion of space under conditions of negative pressure as per inflationary theories of energy creation
7) The attraction of two closely spaced parallel plates as per ____________
AS to RP's question, there is a website by a group that contain a collection of ether theories. Don't have the link available now - if you are curious I can find it. It contains about 50 or so papers some good, some not so good.
As to your question PR regarding space defined in terms of energy-momentum, you have hit very close to home as to my own personal opinion on the subject. Some years ago I derived a set of relationships that tied the spatial energy density to expansion .. for this to work, space must be considered to be under tension (a false vacuum as per ongoing inflation). The energy density turns out to be 1/R where R is the Hubble radius and the total energy turns out to be proportional to the area of the Hubble manifold. Whether these ideas are correct I cannot say - but it was gratifying that the predicted spatial energy density turned out to be precisely what is required to close the universe.
You actually did not address at all the point I made in my last posting. Why aren't there ANY challenges to those papers and their conclusions in the form of rebuttals, comments, etc.? And please don't point to me some website that tries to do this. Websites challenging established physics are a dime a dozen (or maybe even cheaper than that).
If there are legitimate rebuttals regarding the existence of the ether, then based on the non-existence of any rebuttals in legitimate journals, all I can say is that these people cannot put their money where their mouths are. If all they can manage are simply to whine about it in internet forums and some personal webpages, this then lends even less credence to any validity of their claims.
Please note that nowhere in here am I making any argument for "yes, there is ether" or "no there is no ether".
ZZ, this to me is the key (and thanks for picking up where I left off):
1. Classical aether theorists predicted xxx properties and these properties were not found. Therefore, there is no evidence of the classical aether.
2. Einstein's aether does not have the same properties as the classical aether. If it doesn't look like a duck, quack like a duck, or walk like a duck, its not a duck. Call it "Einstein's duck" if you want, yogi, but its not the same as the "classical duck".
I have noticed some either theorists (and I'm not saying that's you, yogi - I'm not really sure what you believe) using point 2 to make an end-run around point 1. Ie, if we can convince people there is still an aether, maybe we can convince them its still the classical aether.
I too have thought of the space/time field as being best defined by an energy/momentum field structure. I would also add that it is probably necessary to add a vector spin field to this formulation. These formulations should be dynamic and not static.
Various structures and symmetries within this basic field idea might also be used to define all the fields and matter structures.
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