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

Hanbury Brown and Twiss

  1. May 8, 2015 #1

    Apparently the first measurement of the diameter of a star other than the Sun was done using quantum mechanics -- specifically, the Hanbury Brown and Twiss effect. If one has two detectors, then there are distinguishable and indistinguishable interactions with photons from the star. My question: Is it supposed to be obvious on the face of it that one is distinguishable and the other not? Or does the answer lie on some other effect deep within the apparatus/experiment?

    Tech details:
    Consider photons coming from the star; some from the "left" side of the star, and some from the "right" side. If two left photons hit the detectors, they are distinguishable. Similarly, if two right photons hit the detectors, they are distinguishable. However, if a left and right photon hit the detectors, they are indistinguishable: we don't know if the left photon hit the left detector and the right photon hit the right detector, or the left photon hit the right detector and the right photon hit the left detector (and, in fact, must be both at once).

    Why is one distinguishable and the other not? Do people just look at this and get it, or is more information required?

  2. jcsd
  3. May 11, 2015 #2
    Can you post a link to where you read this way of explaining the HBT experiment?

    I just took a quick look at
    And it explains the effect without specifically saying that the "both right" or "both left" pairs are distinguishable.
    Last edited: May 11, 2015
  4. May 11, 2015 #3
    Hi Swamp Thing,

    Thanks for your response. It is problem 70.9 from Exercises for the Feynman Lectures On Physics, The New Millennium Edition. I looked around the web, but could not find the problem in e-form (not surprisingly, it's probably copyrighted). The answer in the back refers to some paths being distinguishable and others not. It seems impossible to me to determine that from the information given in the problem.

    I agree that the Wikipedia article doesn't explicitly refer to the problem as I stated it, but the last section, Quantum Interpretation, and the figure with red and green arrows may hint at it: "Consider two points a and b [....] A joint detection [occurs for the arrows that are red or green...] If the photons are indistinguishable [(presumably what is meant by "joint" in the preceding)], the two amplitudes interfere constructively to give a joint detection probability greater than that for two independent events [(the Wikipedia article makes no mention of what these independent events might be, but my other source says they're two photons from a, or two photons from b)].

    I suppose I'm wondering whether to keep working through Exercises for the Feynman Lectures on Physics or toss it.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook