Talk:Hanbury-Brown and Twiss effect
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Note that many of the first radio interferometers in the 1940s were intensity interferometers. Also note that all astronomical interferometers use light which is not temporally coherent, so the lack of temporal coherence for the light in the first optical intensity interferometers was not unusual. Rnt20 13:26, 27 September 2005 (UTC)
[edit] Photons and HBT
The page at it currently stands gives me the impression that there is anti-correlation between the signals at the two detectors. The whole point of the Hanbury Brown - Twiss experiment, with light from a high-pressure mercury arc lamp is that the signals at the detectors are correlated, not anti-correlated. This is only practically measurable, as I understand it, with narrowband incoherent light. It is hard or impossible to detect any correlation with a laser. Robert Hanbury Brown, in his 1972 book "The Intensity Interferometer" rejects the particle notion of light. I think he uses "photon" to mean the generation and deposition of (implicitly unquantized) electromagnetic radiation (emr) by matter, not to indicate that light can be regarded as a stream of "photon" particles. On page 7 he writes: "These difficulties about photons troubled physicists who had been brought up on particles and had not fully appreciated that the concept of a photon is not a complete picture of light. Thus many people are reluctant to accept the notion that a particular photon cannot be regarded as having identity from emission to absorption."
On page 6 he refers to the work of Forrester, Gudmundsen and Johnston (FGJ), also in 1955, (http://prola.aps.org/abstract/PR/v99/i6/p1691_1) who used the interference between two closely spaced wavelengths of a Zeeman split emission line to generate an electrical beat frequency on a photo-cathode. This seems to be a very clear instance of two emr signals summing to produce a modulated total signal. The FGJ experiment creates a modulated electromagnetic wave at the photo-cathode, leading to an electrical signal, at about the 10GHz resonance of their 3cm cavity. This is not explicable by two separate beams of "photons", with different frequencies, since such photons in the pure particle sense are not, or were not, regarded as being capable of interfering with each other - the two beams should both produce a continuous current independently.
It seems to me that the Hanbury Brown - Twiss effect, which is correlation between the signal of photo-detectors receiving a split version of a narrowband incoherent light source, can't be explained with conventional "photons", and can only be explained with the emr being a classical wave. Both the FGJ experiment and the Hanbury Brown - Twiss experiment constitute arguments against the conventional view of emr being quantitized as "photons". I think that the current state of this page does not reflect this, and by explaining the effect solely in terms of particles (photons, which are bosons) hides the fact that there is a debate and presents one view as if there was no debate.
Robin Whittle
You are right to complain that the article did not make explicit that Hanbury-Brown and Twiss themselves observed a correlation. Is the current revision better? A positive correlation was also observed in the Mercury experiment you descibed (referenced in the page as Morgan and Mandel 1966). However, the experiment pictured does in fact result in an anticorrelation, as described in the 1986 paper. This result can be intuitively understood as simply demonstrating the fact that a single photon can only be observed at a single detector. Anticorrelation, unlike the correlation found by Hanbury-Brown and Twiss, cannot be understood classically.
There is perhaps an interesting discussion of the history of Dirac's famous statement that a photon interferes only with itself. Although the development of quantum field theory has made it clear that photons can interfere with each other, it was at one time widely believed that this was not the case. Hence the trouble introduced by the simple interference effects (FGJ) you describe. I'm not particularly well versed in this history - maybe there should be another page for it. If you accept that photons are able to interfere with one another, HBT and FGJ are suddenly much more accessable from the quantum point of view. You might imagine that two nearby photons, being bosons, tend to interfere constructively, causing their wave packets to be amplified in the place where they overlap. For this reason, two photons which are close to each other tend to be detected right next to each other, since that's where their amplitudes are greatest. Similarly, you might imagine that fermions tend to destructively interfere, so that the amplitudes of their wave packets are smallest where they overlap. This is just the Pauli exclusion principle. That's all very informal, but I hope it helps conceptually.
I think the section titled "Wave Mechanics" does a good job explaining the effect purely as a classical one. I don't think there's a need to emphasis it anymore. Finally, I think I ought to emphasis that there is no debate concerning the existance of photons. They are not some construct whose existance is controversial in any way, they are established physics. In fact, a careful consideration of the century old photoelectric effect should essentially convince you that the energy of light must be delivered in a corpuscular manner, and that each corpuscle must have energy hf. Particularly note the fact that increasing the intensity of the incident light does not increase the energy of the emitted electrons, but increasing the frequency does. Since the classical energy of light is given only by its intensity and not its frequency, its hard to believe that this can be explained classically without heavy modification. Combined with Planck's solution for the blackbody radiation curve and the anticorrelation effect described in the 1986 paper, I hope you are convinced.
--Hyandat 08:44, 4 March 2006 (UTC)
I agree there is no debate in the field about the existence of photons. However, there was much debate about whether the photoelectric effect could be described by the interaction of classical waves with quantized atoms (i.e. with energy levels). For instance, see the writings of Marlan Scully and Lamb. The conclusion is that the photoelectric effect does not require the existence of photons, although usually in undergrad it is still given as proof of their existence. The Hanbury, Brown, Twiss interferometer performed with single photons at the input (or possibly the Lamb shift) is the true conclusive proof of photons. BTW: Loved the article - especially the history.
--J S Lundeen 20:19, 21 March 2006 (UTC)
[edit] Please amend the heading
The page: Hanbury-Brown and Twiss effect (and associated pages)
The problem: Hanbury Brown is not hyphenated, but I don't know how to fix the heading of the article. Starrylady 23:54, 27 January 2007 (UTC)
