How big is a photon - Interesting video

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BvU
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TL;DR Summary
Interesting video about interpreting 'single photon' interference experiments
Bumped into this video after admiring the falling cat by @A.T.
As an experimental physicist, indeed experienced a little of the 'hole in the brain' phenomenon!

 
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  • #2
DennisN
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Summary:: Interesting video about interpreting 'single photon' interference experiments

Sounds interesting, but I don't see any link or video. Or am I missing something?
 
  • #3
BvU
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Sounds interesting, but I don't see any link or video. Or am I missing something?
Oops, my bad o:) . Thanks for the hint -- and @Drakkith for fixing it :smile:

##\ ##
 
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  • #4
WernerQH
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Very nice demonstration! It brings home the fact that the "photon" notion is relevant not for propagation, but for the conversion of light (emission and absorption). And it demonstrates that a photon is not created in an instant! The light source needs to have a fairly long coherence time of at least a nanosecond.

I prefer to think of this as an exchange of actually two photons: one photon emitted by the laser and hitting the screen, and an anti-photon leaving the screen and going backwards in time to the laser. (I know that the photon is identical to its anti-particle. In this context it seems appropriate to give a photon going backwards in time a separate name, aka as "advanced wave"). Since the anti-photon can take a different route, the emission and absorption events should last long enough to allow for the possible time delay. Only then can photon and anti-photon complete the transaction. After the transaction a quantum of energy ## \hbar \omega ## has definitely passed from the laser to the screen.
 
  • #5
dextercioby
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[...]

I prefer to think of this as an exchange of actually two photons: one photon emitted by the laser and hitting the screen, and an anti-photon leaving the screen and going backwards in time to the laser. (I know that the photon is identical to its anti-particle. In this context it seems appropriate to give a photon going backwards in time a separate name, aka as "advanced wave"). Since the anti-photon can take a different route, the emission and absorption events should last long enough to allow for the possible time delay. Only then can photon and anti-photon complete the transaction. After the transaction a quantum of energy ## \hbar \omega ## has definitely passed from the laser to the screen.

Is this part only your interpretation, or is there some back-up in the literature?
 
  • #6
WernerQH
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Is this part only your interpretation, or is there some back-up in the literature?
I'm actually surprised about this question. Nobody seems to know about the transactional interpretation of quantum mechanics any more:
J.G.Cramer, Rev.Mod.Phys, 58, 647 (1986)

Julian Schwinger had worked out a "complete time-path" formalism even earlier:
J.Schwinger, J.Math.Phys. 2,407 (1961)
This has grown into the Keldysh formalism that plays an important role in the quantum field theory of non-equilibrium systems.
 
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  • #7
dextercioby
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I'm actually surprised about this question. Nobody seems to know about the transactional interpretation of quantum mechanics any more:
J.G.Cramer, Rev.Mod.Phys, 58, 647 (1986)

Julian Schwinger had worked out a "complete time-path" formalism even earlier:
J.Schwinger, J.Math.Phys. 2,407 (1961)
This has grown into the Keldysh formalism that plays an important role in the quantum field theory of non-equilibrium systems.
Sorry, don't feel offended. But I was literally curious if that part was your reasoning (as in an originality issue), or you were just taking someone else's opinion/ideas and propagating it/them further.
Thank you for the two references, I will try to get hold of them.
 
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  • #8
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I'm actually surprised about this question. Nobody seems to know about the transactional interpretation of quantum mechanics any more:
Whether people know about it or not, if you are using it, you should explicitly say so. You should not expect other people to just know automatically what intepretation you are using.

Also, interpretation-dependent claims and discussions belong in the QM interpretations forum, not here. Any claims in this forum should be based on the basic math of QM only, with no particular intepretation adopted.
 
  • #9
Cthugha
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Is this part only your interpretation, or is there some back-up in the literature?
Interestingly, it is quite common to adopt such an approach for entangled photons in the so-called Klyshko picture, where one investigates the advanced wave from one of the detectors involved on its way to the SPDC crystal an onwards to the other detector. It is popular for SPDC stuff and for ghost imaging.

https://royalsocietypublishing.org/doi/10.1098/rsta.2016.0233

http://www.jetp.ac.ru/cgi-bin/e/index/e/78/3/p259?a=list

However, it is not adopted because the intepretation seems especially reasonable, but because the math and physics of two-photon correlations become so intuitive that one can essentially teach it to a dog.
 
  • #10
PAllen
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Just thinking about the original video, I note that everything said about light and photons would true for electrons or cold buckyballs. Some the explanations given that make sense for EM waves are problematic for these other cases. Quantum mechanics is just strange, period.
 
  • #11
Ivan Seeking
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In an arm-waving qualitative sense, I am comfortable with a quantized unit of energy going to non-quantized fields, but I don't see how we get back to the quantized detection of the same amount of energy as the original. Did I miss something or did he kick the can down the road and avoid that problem?
 
  • #12
Strilanc
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I liked the video. A lot.

One question, though. The video claims that you wouldn't see the same "interference across different arm lengths" for photons that come from spontaneous emission. Is that actually true? What is it about spontaneous emission that breaks it? The video says something like spontaneous emission is "statistical instead of coherent", but I don't understand how "it's statistical" can possibly translate down into the underlying quantum mechanics.
 
  • #13
gentzen
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Is that actually true? What is it about spontaneous emission that breaks it? The video says something like spontaneous emission is "statistical instead of coherent",
The coherence length of his laser was huge, much longer than his delay line. The coherence length of light from spontaneous emission is much shorter, and especially shorter than his delay line.

You could imagine the spontaneous emission as producing many wave packets of length roughly corresponding to the coherence length. If the delay line is too long for single wave packets to interfere with themselves, then no interference will be visible in the end. "statistical instead of coherent" might refer to the fact that interference between different wave packets will average out statistically in the visible result.
 
  • #14
Jarek 31
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