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How does VLBI work?

  1. Apr 14, 2015 #1
    The descriptions I have seen of VLBI seem to imply that a photon is detected at more than one detector (impossible), or that two photons interfere with one another (possible but extremely unlikely).
    Of course, an interference pattern can be built up from many individual photons over time, but then why is it important for the detectors to communicate time information to get the result?
     
  2. jcsd
  3. Apr 14, 2015 #2
    I should imagine that the time delay between the arrival at different detectors would tell you something about the direction of the source.
     
  4. Apr 14, 2015 #3

    Nugatory

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    Which ones?
    How about "neither of the above"? The object being studied isn't a single-photon source, and the detectors aren't photon detectors - this is ordinary radio-frequency electromagnetic radiation that we're looking at.

    We're comparing the phase differences between the signal received at various locations. That comes down to knowing the amplitude as a function of time at each location, and the observations from each location can only be compared if all the locations are using a common time standard.
     
  5. Apr 14, 2015 #4
    EM radiation that is not photons does not seem ordinary to me.
     
  6. Apr 14, 2015 #5
    Do you mean that this only works with coherent radiation?
     
  7. Apr 14, 2015 #6

    davenn

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    you misunderstand what Nurgatory is telling you

    The VLBI isn't detecting photons, its detecting radio waves, trying to think of radio waves as individual photons is not correct and will lead you to much confusion
     
  8. Apr 14, 2015 #7
    https://en.wikipedia.org/wiki/Near_and_far_field
    "In the quantum view of electromagnetic interactions, far-field effects are manifestations of real photons"
    Perhaps this thread belongs in the Quantum Physics forum.
     
  9. Apr 14, 2015 #8

    davenn

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    maybe, maybe not

    When I do radio astronomy, I am receiving radio signal from distant objects
    individual photons are irrelevant ....
    The principle idea behind any large baseline interferometer is to synthesise a radio telescope of the same size
    to improve the resolution of the object being studied
     
  10. Apr 14, 2015 #9
    Obviously it works, and works well. I just have trouble seeing how it works, in QM terms.
     
  11. Apr 14, 2015 #10

    Nugatory

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    It does not, which is why it was moved here after you started it there. VLBI radio telescopes are not quantum mechanical devices and they don't depend on quantum mechanical effects. We're using antennas to detect oscillations in the electromagnetic field, just as with any other classical radio receiver; the only remarkable thing here is the weakness of the signal and the technical challenges that creates.
     
  12. Apr 14, 2015 #11
    True, but I don't think they can contradict QM either.
     
    Last edited: Apr 14, 2015
  13. Apr 14, 2015 #12

    ZapperZ

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    They don't, but you are trying to kill a mosquito with a bazooka, which doesn't make any sense.

    QM is useful when the quantum mechanical effect is obvious, or when classical picture is insufficient. Trying to solve what can clearly and EASILY be described by classical wave by using a more complicated QM picture is irrational. That is like asking someone who builds houses to use Special Relativity.

    Zz.
     
  14. Apr 14, 2015 #13
    Do you think that VLBI would, in principle, work the same, if individual photons arrived at one-second intervals, or say one-year intervals? Or one-femtosecond intervals?
     
  15. Apr 14, 2015 #14

    ZapperZ

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    It wasn't designed to detect "single photons" of that wavelength.

    This is where your understanding of detectors are lacking. We design detectors based on what we intend to detect. No detectors can operate over all range possible, and this one works as a "pick-up antenna" so to speak.

    You've been told repeatedly why the picture you want to use isn't feasible here. I don't know what else anyone can say to you to get through.

    Zz.
     
  16. Apr 14, 2015 #15
    No one has ever seen an individual photon.
    No one has ever seen a carbon footprint either, but that doesn't mean they don't exist.
     
  17. Apr 14, 2015 #16

    ZapperZ

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    This is getting silly. I never said that we don't detect individual photons. I said THIS particular detector was never designed for such a purpose! READ CAREFULLY!

    Zz.
     
  18. Apr 14, 2015 #17
    http://astrogeo.org/vlbi_development/gravity/gravity.html
    "Measurements of differential retardation of photons' travel time caused by the gravitational field of the Sun [1], [2] and the Jupiter [3] allowed to get precise estimates of the Post-Newtonian parameter gamma. Currently, VLBI provides the most accurate estimate of this parameter: 1.00058 -+ 0.00014 (un-scaled formal uncertainty). This is an important contribution of VLBI to fundamental physics."

    http://arxiv.org/pdf/1012.2267
    "... it would be interesting to estimate gravitational bounds for the
    photon mass by considering the most recent measurements of the solar gravitational
    deflection of radio waves obtained by means of the VLBI."
     
    Last edited: Apr 14, 2015
  19. Apr 14, 2015 #18

    Drakkith

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    That is irrelevant, as it has nothing to do with how the detector operates. It doesn't change the fact that the detector is not detecting individual photons at a time. The energy of a single photon in the radio or microwave frequencies is far too low for this detector to detect. In fact there are no detectors capable of detecting single photons at these frequencies. Instead they are detecting the contribution from a great many photons that are absorbed by the detector over a short period of time.

    The fact that the detector isn't detecting single photons doesn't mean that the EM radiation isn't made up of photons. It just means that the EM radiation can be accurately described by classical physics for this particular purpose.
     
  20. Apr 14, 2015 #19
    I am not sure which particular detector you are talking about. My question was about VBLI instrumentation generally.

    In Principle, you could get better and better clocks so that you could match near-simultaneous detection events at the detectors. But I cannot see how one photon can be detected by two different antennas, and two photons at two antennas would be two events at slightly different times.
     
  21. Apr 14, 2015 #20

    Drakkith

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    The antennas directly detect the phase of the incoming EM wave. With accurate timing at each location, the phase difference between each site can be determined, which can then be used for interferometry.
     
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