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Through what medium does EM propagate in empty space?

  1. Aug 28, 2005 #1

    turbo

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    Through what medium does EM propagate in "empty" space?

    Einstein stated in his Leyden address (1926, I think) that an EM ether was mandatory for the transmission of EM waves through "empty" space. He was unable to reconcile this with the dynamical gravitational ether that had to exist to make GR work, so he proposed an EM ether that had NO sensible properties. In other words, it could not possibly be subject to polarization, densification, or any other measurable variation, unlike all the other fields known to exist. This concept seems a little silly, since fields are known to exhibit variations and the variations must be explainable via physical laws.

    Is there anybody here that is willing to contemplate that the quantum vacuum might be this EM field - the palette upon which the Universe is written?
     
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  3. Aug 28, 2005 #2

    SpaceTiger

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    You mean that the quantum vacuum would be the ether? This sounds more like a question for the Quantum Physics forum. As far as I know, the universe is still etherless, but someone more knowledgable in QFT might be able to give you a better answer.
     
  4. Aug 29, 2005 #3

    Chronos

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    In my opinion, Einstein was pointing out the futility of positing an 'ether' with any physically quantifiable qualities. I interpret that as meaning it's irrelevant. I also think it's OK to disagree on that.
     
  5. Aug 29, 2005 #4

    turbo

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    Yes. The quantum vacuum consists of a field of virtual particles that pop in and out of existence continuously and exist for such brief periods of time that it completely satisfies Einstein's requirement that it have no sensible properties, such as motion.

    From this paper on the ether:
    http://redshift.vif.com/JournalFiles/V08NO3PDF/V08N3GRF.PDF

     
  6. Aug 29, 2005 #5

    selfAdjoint

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    These quotations show that Einstein and Poincre thought that what extends and supports radiation was the spacetime geometry, sometimes called the metric field. Neither man had anything to do with the quantum vacuum. Not that it isn't correct that the quantum vacuum supports radiation! But it isn't an ether in the nineteenth century sense, a medium in which the light waves wave.
     
  7. Aug 29, 2005 #6
    Yes. The quantum vacuum has "wave-like" properties, itself. So by the 19th century idea it too would require some sort of ether.
     
  8. Aug 29, 2005 #7

    turbo

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    Perhaps not by that name, but what we have come to understand as the quantum vacuum serves the purpose admirably. Einstein himself said that without a transmissive medium ("ether" in the parlance of the day) light could not propagate through space. Lest we multiply entities unnecessarily, it would behoove us to examine the vacuum's role in the transmission of EM. As a neutral sea of virtual particles, it provides a baseline against which fluctuations can be expressed - a medium through which waves can be transmitted.

    Why not? And if not, how can we describe the propagation of light through a vacuum? Must we resort to some corpuscular theory that reduces photons to little points of energy hurtling through "empty" space, heedless of the sea of virtual particles all around them?

    What is the concordance view on the transmission of EM through the vacuum?
     
  9. Aug 29, 2005 #8

    selfAdjoint

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    Virtual Photons, taking zero time en route as experienced by themselves and therefore unable to interact. Photons are not corpuscles.
     
    Last edited by a moderator: Sep 2, 2005
  10. Aug 29, 2005 #9

    turbo

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    I don't think it's irrelevant. I think that the quantum vacuum (consisting of particle/antiparticle pairs that last for minimal times, in accordance with the Heisenberg uncertainty principle) can provide the Machian ether that Einstein envisioned without being "sensible" as a fixed background.

    In later years (early 1950s) Dirac also made a case for the existence of an ether.

    http://home.tiscali.nl/physis/HistoricPaper/Dirac/Dirac1951b.pdf
     
  11. Aug 29, 2005 #10

    turbo

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    Now I'm confused. If EM propagates as waves, not corpuscular entities traveling through empty space, how does EM propagate? Without a transmissive medium, waves go nowhere.
     
  12. Aug 30, 2005 #11

    Chronos

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    Light waves do not obey the rules of classical mechanics. Maxwell's equations work perfectly without ascribing any properties to a 'transmissive media'. I think it is fair to say a media devoid of any physically quantifiable properties is the same as saying it is nonexistent [an occams razor thing]. I also think this is the point of Einstein's Leiden address. He was trying to put the issue in perspective and coax his audience into relinquishing their traditionally cherised concepts.
     
  13. Aug 30, 2005 #12

    turbo

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    Please read the address carefully. Einstein was not denouncing the ether - he was announcing his embrace of it and defining its role in physics, with historical references to its role in earlier physical models. He had come to realize that GR demanded the existence of an ether, else the properties of rotation and acceleration could not exist, except as Machian action-at-a-distance, which he would not accept. GR demanded that rotation and acceleration be expressed as changes relative to a LOCAL frame - the ether.

    http://www.geocities.com/antonioferrigno/ether.html

    As for my original question, regarding the nature of the transmissive medium through which EM propagates in "empty" space :

    Einstein did not, as many imagine, kill the notion of an all-pervasive ether. To the contrary, he embraced the concept, stripped it to its bare essentials, and attempted to use it to unify gravitation and electromagnetism. He failed to accomplish this, but I firmly believe he was on the right track.

    To get back to the original question: what is the nature of the transmissive medium through which EM propagates?
     
  14. Aug 30, 2005 #13

    selfAdjoint

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    Yes, and his ether was the dynamic metric field of spacetime gemoetry. He spent the rest of his life trying to tweak that metric field so it would support EM, but he died before fully achieving that. I don't see what his opinion forces on us today.

    It is clear that EM by itself, while perfectly usable within its proper energy/scale envelope, cannot predict the detailed spectra or behavior of charged particles such as electrons. There are further effective theories: Dirac's. QED. and the Standard Model, which do that much more accurately. So trying to unify "light waves" with gravity at this late date is like putting elliptical epicycles into Ptolemy.
     
    Last edited: Aug 30, 2005
  15. Aug 30, 2005 #14

    turbo

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    His opinion forces nothing on us; however it might be wise to reconsider his opinion periodically, especially in the light of things that came to be better-understood later.

    The properties of the vacuum were poorly understood when Einstein was at his most productive, and Einstein was not too fond of quantum theory anyway. Today (and for some time actually) we have demonstrable proof that the quantum vacuum exists (Casimir effect, Lamb effect, etc). It is the baseline of our universe, and as such, it is practically insensible. It is pervasive and universal, and Sakharov believed that interaction of matter with this vacuum field endowed the matter with mass, inertia, and gravitation. Does not this sound suspiciously like the Machian ether described by Einstein in his Leyden address?

    Again, does the quantum vacuum act as a transmissive medium for EM waves? If not, what is the nature of the field upon which EM waves travel in "empty" space?
     
  16. Aug 30, 2005 #15

    selfAdjoint

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    First Einstein and now Sakharov! You can't build physics by cherry-picking quotations from famous dead men. The quantum vacuum as it exists in actual physics is the ground state of a quantum field. In order to talk about gravitation and quantum vacuum meaningfully you have to quantize gravitation. When you have done that come back and we'll talk about whether the quantum vacuum you derive from your quantized gravity theory does or does not support EM in such a way as to be sensibly called an ether.
     
  17. Aug 30, 2005 #16

    turbo

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    I think you would find it very difficult to do real physics without studying the problems that have stumped the great minds. Understanding and breaking down these problems lead to paradigm shifts. It is called epistemology, and Einstein was a great fan of that process. Case in point: Mach's concept that inertia arose from a body's acceleration relative to the entirety of the universe. Einstein's distaste for this action-at-a-distance concept was central to his view that the ether is real and local.
    I think that it is a bit much to demand that I develop a full-blown theory of quantum gravity before you will condescend to answer a simple question:

    Through what transmissive medium does EM propagate in "empty" space?

    Let's start there.
     
  18. Aug 30, 2005 #17

    selfAdjoint

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    Assuming that the teachings of QED and the electroweak theory survive the quantization of gravity, light is carried by real photons, which do not interact with the quantum vacuum. The photons are not corpuscles as you well know, and it is disingenuous to suggest they are. They may on occasion be observed as plane waves when travelling from one star to another. But those are not waves that change the state of any circumambient medium.
     
  19. Aug 31, 2005 #18

    Chronos

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    I think the point is lost in semantics. The quantum vacuum does have properties, specifically, it produces measurable effects in the presence of matter [e.g., casimir effect]. But it does not exhibit properties consistent with those of a transmissive media in classical mechanics. There is nothing suggesting the quantum vacuum interacts with EM fields. Thus the contextual meaning of this statement by Einstein:

    "...we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether..."

    But, the more important point Einstein made is, IMO:

    "... But this ether may not be thought of as endowed with the quality characteristic of ponderable media..."

    Which is to say, IMO, it is not endowed with any of the properties typical of a classical transmissive media [e.g., viscosity, mass, rigidity, etc.] Does that make any sense?

    Hence, the central argument of the great ether controversy is, does empty space behave consistent with classical mechanics? Einstein's answer is no.
     
  20. Aug 31, 2005 #19

    wolram

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    I think people do give the medium properties, i remember Marcus told me that
    ST rings like a bell, How else can gravity waves propagate?
     
  21. Aug 31, 2005 #20

    selfAdjoint

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    Casimir, at least, thought of the virtual particles as having wave lengths; that was how he worked out his "effect".
     
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