Re: "A Snapshot of a Photon"

Tom Roberts wrote:
John Kennaugh wrote:
Tom Roberts wrote:
The "length" of a photon is not well defined. Photons are QUANTUM OBJECTS and do not behave as you naively expect. In particular, a photon is NOT a short wave-train as you seem to think. The best definition I know of what a photon actually is: a specific type of term in the perturbative approximation to an amplitude in QED.

If you attempt to measure the "length" of a photon (e.g via self-interference), you will obtain a value that depends on precisely it was emitted, and your value will usually be the coherence length of the source. That can vary from microns for an incandescent bulb to many meters for a single-mode laboratory laser.
Surely the coherent length is related to photons 'on mass'.

As I said, the coherence length is related to properties of the SOURCE, not of the photons themselves.

If one assumes a photon has some property analogous to (or actually) 'spin' then the coherent length is the length of a burst containing photons 'spinning' 'in phase' with each other and can in theory be any length.

You must select another word than 'spin' -- photons do indeed have what is known as spin (anbgluar momentum). But I cannot see any relation between the photon's spin and what you say here. This seems like word salad to me.

Physics cannot keep switching between a wave and a particle whenever it suits. Somehow the two descriptions must be reconciled. Two bunches of photons constituting two coherent beams must in some way differ from two bunches of photons which do not constitute coherent beams. Each photon must possess therefore some property analogous to 'phase'. We know that each photon possesses something related to frequency (equivalent to energy). We also know that electromagnetic fields are somehow involved. All of these properties must in some way be embodied in each and every photon. It may be simplistic of me but I see a photon as consisting of a massless positive and negative charge rotating about a point which moves at c. They trace out a double helix in space. The rate of rotation giving frequency, the distance required to complete one revolution being wavelength, the angle between their axis of rotation and their direction of motion being related to polarisation. Two bunches of photons may or may not be coherent depending on whether they are rotating at the same rate and have a constant phase relationship.

As I say it may be simplistic but physics doesn't seem to have come up with a better model nor to have resolved the duality paradox. In fact we know little more about the nature of light than was known a century ago. This is evidence as far as I am concerned that Physics has taken a wrong turn. How can one possibly come up with a convincing model of a photon if one is forced to assume that its speed has nothing to do with the physical processes which create it - just because physics chose to back Einstein's attempt to retain a link with Maxwell's ether. Trying to square the circle has led to mysticism.

Any regular electrical waveform can be analysed using Fourier analysis [...] 

Yes, of course.


The question I am exploring is related to your statement:

Note that if you turn a source on and then off quickly enough, you will inherently introduce an uncertainty in the frequency of the pulse (Heisenberg always has the last word). For a conventional RF source you can certainly arrange to emit less than a full wavelength, which results in an enormous frequency spread.
The point I am wrestling with is this. OTOH the frequency spread is simply and a faithful conversion of electrical signal into RF signal. I can't see why, apart from the obvious difficulty of generating it, why it would not be possible to have short bursts of photons all with the same energy/frequency.

Do the Fourier analysis. That is, perform a Fourier integral of (say) a single cycle of a sinewave. 

I don't see a photon as a single cycle of a sinewave do you?

The result will be a large spread of frequencies, with a peak at the frequency of the sinewave. To get a delta-function at its frequency you need an INFINITELY-LONG sinewave.

We are trying to discuss photons and you keep slipping back into waves.
A sodium lamp does not produce an infinitely long pair of sinewaves yet it produces two precise frequencies. I do not think it is any better than a filament lamp when it comes to temporal coherence. This implies frequent changes of phase and phase changes would be expected to spread the spectrum as much if not more than amplitude changes.

If one considers a very, very dim source of light one can conceive of photons arriving one at a time. 

Yes.

Surely one photon constitutes an extremely short burst

No. See above. Photons are not "short pulse trains", and they are not like any ordinary object you have ever encountered. Thay are QUANTUM OBJECTS. 

Mysticism.

and we think of a single photon as having a single frequency f = h.nu rather than an enormous frequency spread 

Got my formula wrong - it was getting late


Wrong pronoun -- YOU may think of it like that, but physicists don't. Physicists know that the frequency of a photon is uncertain....

How can it be? If the energy of a photon = hf. The uncertainty of the frequency is related to the uncertainty of the energy.

"Question: Why are the wavelengths of the spectrum lines from a gas in a discharge tube so precisely defined? .
Answer: Because the permitted energies that electrons can assume within the atoms are precisely quantized.
Question: Oh - I thought it was the electron's angular momentum that was quantized?
Answer: That is also true. Both energy and angular momentum are precisely quantized.
Question: If that is so, then the position of an atomic electron must be precisely determined. How far is it from the nucleus?
Answer: We cannot tell you that, because of the Uncertainty Principle of Professor Heisenberg. We can only tell you where you are most likely to find it.
Question: So its energy and momentum are in fact not precisely determined?
Answer: That is so; they may take on any values within Heisenberg's limits.
Question: Then why are the spectral wavelengths, which you now say are dependent on indeterminate energy and momentum, themselves precisely defined?
Answer: Your questions pre-suppose that the atom has a mechanical structure. Our modern theory is a mathematical theory, not a mechanical theory. Hence the questions you ask are meaningless.
Question: But I thought you said the mathematical theory dealt with energy and angular momentum. Are these not ordinary mechanical quantities?
Answer: You are wasting my time. It is a matter of statistics. Look up the theory in any textbook.

You will have noticed the testiness of tone which arises characteristically at that point in the discussion." - A Heretics Guide to modern Physics - Dr Scott Murray

Suppose you have a monochromatic light beam which is only just too low in energy to make current flow in a photo electric experiment. Suppose you break up that beam by passing it through a rotating toothed wheel. Does that produce higher energy photons such that current now flows?

Only in a gedanken for which the monochromaticity is perfect and the "just too low" can be made vanishingly small. The energy uncertainty introduced by such a toothed wheel is ENORMOUSLY smaller than the energy scales for electrons. But yes, a pulsed light source cannot be truly monochromatic (see above -- do the Fourier integral).

You again have missed the point. If you are generating a radio burst of photons then the frequency spread is present in the electrical signal you feed to the aerial so the spread of energy of the photons produced will simply match it.

I am not saying it is possible to generate by any known means, but I see no reason in theory why a burst of photons, all with the same energy could not exist in nature. That would be inherently different to the burst created by feeding an already spread electrical signal to an aerial. You cannot do a Fourier analysis on a photon nor on a bunch of photons.

Note my point above relating to temporal coherence. I would suggest that what is happening in a normal light source (not a laser) is that a single spontaneously generated photon will precipitate a cascade of photons producing a burst of coherent photons. Such light I would suggest consists of overlapping bursts rather than continuous sinewaves and it is this which leads to short temporal coherence. The fact that this process does not result in frequency spread would indicate that the wave model breaks down. In theory it should because whether you model the bursts as a series of individual random overlapping bursts or whether you model it as a sinewave with phase discontinuities it should have frequency spread. This leads me to believe that a burst of photons all with the same energy as postulated above would not have a spread spectrum.

--
John Kennaugh
to email convert the number from hex to decimal
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