Understanding Photon Propagation: Entity or Quanta?

[DiracPool]

What is the most accurate way to think of a photon moving through space?

1) Should we think of it as an entity that travels as an integrated corpuscle that parts the "quark sea" of empty space and maintains it's internal integrity as it moves from point A to point B, or..

2) Should we think of it more as the propagation of a "quanta" or measured value of energy through an essentially unmoving, rigid medium, kind of like a domino effect where a perturbation in one part of the medium in turn perturbates the adjacent part, and so on down the line?

In latter instance, the medium doesn't move but for a simple oscillation around a center point, and nothing is "cutting through" the medium as we would think of, say, a thrown baseball cutting through the medium of the atmosphere. Whereas in the former case, we have an integrated, autonomous object cutting through some medium.

 

[bhobba]

What is the most accurate way to think of a photon moving through space?

Not to ascribe it any properties independent of observation.

A photon is an excitation of a quantum field - with that being detailed in the theory.

The closest you can get is it's like the harmonic oscillator:
https://en.wikipedia.org/wiki/Quantum_harmonic_oscillator

The ground state represents no photons, the one particle state is the creation operator applied to the ground state, the two particle state the creation operator applied to the one particle state etc etc. Quantum fields are a superposition of such states so even the number of particles is not fixed.

And this applies to the momentum representation - so thinking they have an actual position is incorrect.

Thanks
Bill

 

[bahamagreen]

Isn't photon propagation always a counterfactual definiteness?

 

[bhobba]

Isn't photon propagation always a counterfactual definiteness?

The concepts are not in any way related - at least as far as I can discern.

Thanks
Bill

 

[bahamagreen]

Wiki says, "...counterfactual definiteness (CFD) is the ability to speak meaningfully of the definiteness of the results of measurements that have not been performed (i.e. the ability to assume the existence of objects, and properties of objects, even when they have not been measured)."
but,
StackExchange says, "There is no quantum mechanics of a photon, only a quantum field theory of electromagnetic radiation. The reason is that photons are never non-relativistic and they can be freely emitted and absorbed, hence no photon number conservation."

Is that how you mean?

 

[DiracPool]

Not to ascribe it any properties independent of observation.

A photon is an excitation of a quantum field - with that being detailed in the theory.

The closest you can get is it's like the harmonic oscillator:
https://en.wikipedia.org/wiki/Quantum_harmonic_oscillator

The ground state represents no photons, the one particle state is the creation operator applied to the ground state, the two particle state the creation operator applied to the one particle state etc etc. Quantum fields are a superposition of such states so even the number of particles is not fixed.

And this applies to the momentum representation - so thinking they have an actual position is incorrect.

Ok, let me rephrase the question more fundamentally; what is the difference between a photon moving through space and a small massive object moving through space? Or is there a difference? Again, as I stated in my initial post, is it that a massive object "cuts" through space but that a photon doesn't? Or alternatively, do we have to eschew a classical picture altogether in reverence to quantum creation and annihilation operators and quantum "wave packets" with their equally confusing properties of "anomalous dispersion" as they propagate through a medium?

 

[bhobba]

Ok, let me rephrase the question more fundamentally; what is the difference between a photon moving through space and a small massive object moving through space?

Photons do not have the property of moving through space.

Thanks
Bill

 

[bhobba]

Is that how you mean?

Good enough.

Words are generally inadequate for this stuff.

Thanks
Bill

 

[DiracPool]

Photons do not have the property of moving through space.

Ok, so when Hippolyte Fizeau was shining light through the teeths of a wheel at a mirror on a hill 8 miles away in order to determine how fast the light propagated from his hill to that hill and back, what was he measuring? Was he measuring some property of photons that do not move through space?

 

[bhobba]

Ok, so when Hippolyte Fizeau was shining light through the teeths of a wheel at a mirror on a hill 8 miles away in order to determine how fast the light propagated from his hill to that hill and back, what was he measuring? Was he measuring some property of photons that do not move through space?

He was measuring the behaviour of a large number of photons which is described accurately by Maxwell's equations that does have that property.

Remember Ehrenfest's Theorem:
http://www.physics.ohio-state.edu/~jay/631/ehrenfest.pdf

Thanks
Bill

 

[Nugatory]

Ok, so when Hippolyte Fizeau was shining light through the teeths of a wheel at a mirror on a hill 8 miles away in order to determine how fast the light propagated from his hill to that hill and back, what was he measuring? Was he measuring some property of photons that do not move through space?

He was measuring the speed of electromagnetic waves, which do travel through space. Of course he didn't know that's what he was doing, as this was before the discovery that light is electromagnetic radiation.

If you want to understand what photons are, and why you will only confuse yourself if you imagine that they have positions and velocities, you might give Feynman's (layman-friendly) book "QED: The strange theory of light and matter" a try.

It's also generally good practice to think of light as an electromagnetic wave except when you're working on a problem that involves the quantized exchange of energy or momentum between the electromagnetic wave and some form of matter. Only then does the concept of photon make sense.

 

[Buzz Bloom]

Hi @Nugatory and @bhobba:

Is there a meaningful way to describe a relationship between the motion of an electromagnetic wave and the motion of the quantum wave function of amplitudes?

Regards,
Buzz

 

[bhobba]

Is there a meaningful way to describe a relationship between the motion of an electromagnetic wave and the motion of the quantum wave function of amplitudes?

Check out:
http://www.cft.edu.pl/~birula/publ/CQO7.pdf

Thanks
Bill

 

[Buzz Bloom]

Hi Bill:

Thanks for the link. It will take me some time to grasp what I want to learn from it, but I am sure the knowledge is there somewhere.

Regards,
Buzz

 

[zincshow]

Photons do not have the property of moving through space.

Thanks
Bill

How do they get from one location to another? Maybe they don't have a location either?

 

[Nugatory]

How do they get from one location to another? Maybe they don't have a location either?

You're right, they don't. I've already recommended Feynman's book once in this thread.

 

[bhobba]

How do they get from one location to another? Maybe they don't have a location either?

They don't have any property until observed to have it.

For photons its really bad because even the number of particles is unknown.

Thanks
Bill

 

[zincshow]

@Nugatory You often see people saying things like "photons takes about 8 minutes to get from the sun to the earth" or "photons trapped in a reflective cavity". You say this is wrong? Should this terminology be avoided? How else can a person express these ideas to other people?

 

[zincshow]

even the number of particles is unknown.

I'm not sure what you mean by this. If someone talks about a 25 femtosecond burst of 1.97 eVolt photons, if you know the total energy, you will know the exact number of photons.

Also, people talk about sending single photons in a 2-slit experiment where it is unknown which slit it goes through are certainly dealing with an exact number of photons (ie. one at a time) that take specific time to arrive at the detector from the emitter.

 

[Nugatory]

@Nugatory You often see people saying things like "photons takes about 8 minutes to get from the sun to the earth" or "photons trapped in a reflective cavity". You say this is wrong? Should this terminology be avoided? How else can a person express these ideas to other people?

The easiest way is to say something like "light takes about eight minutes to get from the sun to the earth" because light does move in the usual sense of the the word.

Often when people say "photon" when they mean "light" they're thinking that a flash of light is a bunch of photons moving through space together the same way that a splash of water is a bunch of water molecules moving through space together. You can get away with this when there are no quantum mechanical effects involved - we do it all the time over in the relativity forum - but it's something that you have to unlearn before you can take on quantum mechanics.

 

[bhobba]

I'm not sure what you mean by this.

What I mean is that in QFT even the number of particles is a superposition. It's weirder than ordinary QFT and admits to pictorial visualisations like moving through space to an even less degree.

Thanks
Bill

 

[zincshow]

...A photon is an excitation of a quantum field - with that being detailed in the theory.

The closest you can get is it's like the harmonic oscillator:
https://en.wikipedia.org/wiki/Quantum_harmonic_oscillator

...

Can we assign a location to "an excitation of a quantum field"? Can we say that "an excitation of a quantum field" has moved from one location to another location via a straight line at the speed of light?

 

[Nugatory]

I'm not sure what you mean by this. If someone talks about a 25 femtosecond burst of 1.97 eVolt photons, if you know the total energy, you will know the exact number of photons.

You don't have a 25 femtosecond burst of 1.97 eV photons here, you have a short (very short! 25 femtoseconds is only about 10 wavelengths) pulse of 630 nanometer electromagnetic radiation. It will be described by a superposition of quantum states (google for "Fock state") containing different numbers of photons with different frequencies and energies.

 

[Nugatory]

Can we assign a location to "an excitation of a quantum field"?

We can assign a location to where this quantized excitation interacts with matter, but...

Can we say that "an excitation of a quantum field" has moved from one location to another location via a straight line at the speed of light?

We cannot. We can say that matter interacts with and transfers energy to the electromagnetic field at one location; and one nanosecond later the electromagnetic field interacts with and transfers energy to some other matter at another location thirty centimeters away; and we can say that the electromagnetic radiation propagated from one point to another. But when we look at the quantum electrodynamic treatment of this interaction we don't get a photon moving from one location to another, we get a statement of the probability of a photon being created at the source and of the probability of a photon being destroyed at the destination. That calculation explicitly rejects the possibility of the photon having any specific location in between (or even that it is the same photon at both ends).

 

[zincshow]

You don't have a 25 femtosecond burst of 1.97 eV photons here, you have a short (very short! 25 femtoseconds is only about 10 wavelengths) pulse of 630 nanometer electromagnetic radiation. It will be described by a superposition of quantum states (google for "Fock state") containing different numbers of photons with different frequencies and energies.

I like the wording they use in this recent study, they are using "A pulsed 775 nm-wavelength Ti:Sapphire picosecond mode-locked laser", I assume each pulse is a bunch of 775 nanometer wavelength (1.6 eVolt) photons.

In the same paper, they talk about tracking locations and interactions with the photons "After the polarizer, the photons are coupled back into a single-mode fiber and sent to superconducting nanowire single-photon detectors"

 

[zonde]

That calculation explicitly rejects the possibility of the photon having any specific location in between (or even that it is the same photon at both ends).

Phrases "calculation rejects" and "experiments falsify" are rather different. Aren't you too categorical with your statement?
... and we could consider pilot wave theory as counterargument to your statement.

 

[Nugatory]

Phrases "calculation rejects" and "experiments falsify" are rather different. Aren't you too categorical with your statement?
... and we could consider pilot wave theory as counterargument to your statement.

Is there a pilot-wave formulation of quantum electrodynamics? We're talking about photons here.

The QED sum of all paths calculation rejects the notion of a single path, and even if we find an alternative, I don't expect it will do anything to make it possible to think of photons as little grains of light moving through space like bullets... And that's what this B-level thread seems to be about.

 

[zonde]

The QED sum of all paths calculation rejects the notion of a single path

It's a calculation not a physical model. Feynman in his book (that you mentioned in this thread) states this quite clearly.

and even if we find an alternative, I don't expect it will do anything to make it possible to think of photons as little grains of light moving through space like bullets

I understand that from pedagogical perspective it would be easier if we could state that there is only one right model and all other conceivable models are wrong. And if one teaches engineers that might be the right approach. But it's sin against science to state things like that just the same.

 

[bhobba]

It's a calculation not a physical model.

A calculation using a physical model is part of that model, and obviously so.

I suspect you are reading Feynman incorrectly.

To be clear - in physics there is no such thing as a math model. All physical theories are mathematical models describing reality without going into what reality is because philosophers can't agree on that. Personally I take it to be what our theories describe which of course is circular unless you break it by saying at the start reality is what our theories describe.

Thanks
Bill

 

[bhobba]

Can we assign a location to "an excitation of a quantum field"? Can we say that "an excitation of a quantum field" has moved from one location to another location via a straight line at the speed of light?

Its rather complex and really only understandable using the math.

When you take the Fourier transform of a quantum field you get the momentum representation and that's where creation an annihilation operators show up. They create particles with a certain momentum so, via the uncertainty principle, they have an unknown position.

Thanks
Bill