Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Isotropic and anisotropic propagation of light

  1. Feb 20, 2006 #1
    Do you aggree that there is an inertial reference frame in which light in free space propagates isotropically whereas in all other inertial reference frames its propagation is anisotropic?
  2. jcsd
  3. Feb 20, 2006 #2


    User Avatar
    Science Advisor

    Not if you use the Einstein clock synchronization convention when defining your coordinate system. Also, if you have two observers in windowless boxes which are both moving inertially at different velocities, then if each observer measures the velocity of light in the same way within their box, they will both get the same answer.
  4. Feb 20, 2006 #3


    User Avatar
    Staff Emeritus
    Science Advisor

    No, light propagates isotropially in all inertial frames. This is fundamental to relativity.
  5. Feb 21, 2006 #4


    User Avatar
    Science Advisor
    Gold Member

    The question is a little ill defined.

    Does it not depend on how the light originated? i.e. The CMB was emitted by the Surface of Last Scattering (SLS). An observer co-moving with that SLS sees the CMB isotropically whereas all others, including ourselves moving relative to the SLS at about 0.1%c, sees a dipole anisotropy of 10-3.

  6. Feb 21, 2006 #5


    User Avatar
    Science Advisor
    Gold Member

    Observing the CMB as isotropic is not the same thing as observing light propagation to be isotropic. Observers moving relative to the SLS will observe anisotropy in the CMB, but they will still observe light propagation (i.e. speed) as isotropic.
  7. Feb 21, 2006 #6
    Yes, I fully agree with this.

    Others that agree are:

    1991 Roland DeWitte (Ether wind detected using an electrical one-way test).

    1988, Gagnon, Torr, Kolen and Chang (Guided-wave measurement of the one-way speed of light. Although they reported, "Our results have not yielded a measurable direction-dependent variation of the one-way speed of light. A clear null result is obtained for a hypothesis in which anisotropy of the cosmic background radiation is used to define a preferred reference frame", Harold Aspden's considers their work important, as their experimental data clearly shows an eastward motion effect. And so it is possible to sense the speed of a test device using optical speed-of-light sensing wholly confined within the enclosure housing the apparatus).

    In 1986, E W Silvertooth claimed to have measured the 378 km/s cosmic motion using an optical sensor that measures the spacing between standing wave nodes. Although the experiment has not been confirmed.

    Dayton Miller's ether drift experiments (similar to Michelson-Morley type experiments but more sensitive) A review of his work by James DeMeo shows indisputable evidence that data collected by Miller was affected by the sidereal period and this is clear proof of a cosmological ether drift effect.

    It will be interesting to see if the newer more accurate clocks in space detect anisotropy in the one-way speed of light. My bet is they will.
  8. Feb 21, 2006 #7


    User Avatar
    Science Advisor
    Gold Member

    In which case I have read the question wrongly, I was confused as to this use of the word "isotropic" in regard to radiation.

    I see what Bernhard means now and I have to say that I disagree with his statement.

  9. Feb 21, 2006 #8
  10. Feb 21, 2006 #9


    User Avatar
    Science Advisor
    Gold Member

    Okay, my slip of the tongue/finger, :blushing: your question was, "Do you agree..." I was simply saying (IMHO) that I didn't.

    Last edited: Feb 21, 2006
  11. Feb 21, 2006 #10
    This is the assumption of the test theories of Robertson-Mansouri and Sexl.
    It has been disproved by experiment multiple times.
  12. Feb 21, 2006 #11
    The above is wrong and misleading. Given your views about relativity is within character. You have been told repeatedly (by several people) that:

    1. One way light speed experiments have proven the isotropy
    2. Quoting scandal writers like DeMeo is not proof, moreover Dayton Miller has been completely refuted by the modern reenactments of his experiment.
    3. De Witte experiment is a hoax
    4. Silvertooth experiment could not be duplicated by anybody (including himself)
  13. Feb 24, 2006 #12
    Yes, I do, Bernhard. You are absolutely right!
    One way light speed is relative. It is equal to c in the reference system K, that is at RELATIVE rest, and is different in different directions in the systems, which are moving RELATIVE to the system K.
    I shake your hand!!!
    Einstein absolutized the rest state of the own systems of the different observers. Einsteins observers have fear of moving. They think, as the people on Earth before Copernicus, that they can ONLY rest and NEVER move.
    The people, who write that “One way light speed experiments have proven the isotropy”, do not read good books. Nobody never measured one way speed of light. Einstein understood it and wrote that it was impossible. But the people don’t read Einstein. They read only the interpreters of Einstein.
    Einstein understood the problem with one way speed of light, but he did not understand that one way speed of light is relative. You do!
    Do I right understand you?
  14. Feb 24, 2006 #13

    Nonsense, Vadim

    First off, you did not understand Bernhard's question so you twisted it into your own question, exactly as "wisp" tried it.

    Secondly, contrary to what you think, one way speed of light has been measured repeatedly to be equal to c INDEPENDENT of the RELATIVE movement of the observer and the source. Just go take a class in relativity before posting all this nonsense.
    Or read paragreaph 3.2 here:

    http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#one-way tests

    There are many more experiments like this.
    Last edited: Feb 24, 2006
  15. Feb 24, 2006 #14


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    I will caution you to re-read (since you are emphasizing about reading here) our Guidelines that you have explicitly agreed to. If you believe that there are evidence contrary to the standard interpretation of physics, then it is your burden to provide valid citations to reputable work.

    Things have CHANGED since the last time you posted here.

    Last edited: Feb 24, 2006
  16. Feb 25, 2006 #15
    I respect your point of view but i do not aggree with it
  17. Apr 7, 2006 #16
    I've just come across this paper (published yesterday) on another forum and I believe it answers Bernhard's question.

    http://arxiv.org/PS_cache/astro-ph/pdf/0604/0604145.pdf [Broken]

    It supports my view that there is a preferred reference frame and observers moving through this frame measure anisotropy in light's oneway speed.

    Quoted from the paper: -
    "The main objective of this paper is to present experimental results on an “one-way light path” laser diffraction experiment mounted in the shell of the TUPI muon telescope [7] and that shows clearly that the speed of light depends on the propagation direction."


    "The analysis of The Global Positioning System (GPS) carried out by Hatch [10] provides also strong indirect evidence for the presence of an ether-drift velocity."
    Last edited by a moderator: May 2, 2017
  18. Apr 7, 2006 #17
    While the link is certainly interesting, you forgot to mention that:

    The Hatch paper was published in...Galilean Electrodynamics, a well known venue for crank publications. This in itself makes the paper you just quoted very questionable.

    As to the link above, why don't you wait until there is :

    1. independent experimental confirmation (this is the standard)

    2. publication in a refereed journal?

    As an aside, it looks as if the authors have "forgotten" that the speed of Earth exhibits a sine dependency. It may be very well that they are measuring this effect (remember the Dayton-Miller - Maurice Allais fiasco?) instead of measuring the one way light speed anisotropy. Contrary to what the authors show, the SRT prediction is not a horizontal line stuck on 0 but....a sine wave (because the Earth's speed is variable).
    Give it a year or so, if the paper is still unpublished, then you have your answer.
    Last edited by a moderator: May 2, 2017
  19. Apr 14, 2006 #18
    Last edited by a moderator: May 2, 2017
  20. Apr 14, 2006 #19


    User Avatar
    Gold Member

    I asked you here to show "Where exactly in the paper you cited do the author's claim that they are measuring the isotropy of the one-way speed of light as opposed to the isotropy of the two-way speed of light?", and you didn't respond. Now you're repeating this apparently false claim here, so I'll ask you again: Where exactly in these two papers you cited do the author's claim that they are measuring the isotropy of the one-way speed of light as opposed to the isotropy of the two-way speed of light?

    This appears to be a proposal for a future experiment rather than a report on the results of an already completed experiment. I'll read the paper and comment further after you have answered my question.
    Last edited by a moderator: May 2, 2017
  21. Apr 14, 2006 #20
    "Test of the isotropy of the speed of light using a continuously rotating optical resonator"
    Last edited by a moderator: Apr 22, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook