Does the Photon Exist? Exploring Its Necessity

[hutchphd]

What does "directly" mean?
This is not a new thought and does not seem to be the way things work.

 

[sophiecentaur]

Sure: the fact that whenever you detect sufficiently faint light, you detect it as discrete particles

That's ok as long as all participants of this thread are aware that the word "particle`" doesn't imply that it's like other particles that have mass and that have a properly defined position and extent. Between being produced and being detected, the photon is no way like a little bullet (or even a big one). The photon is a quantum of energy or momentum.

The "wave / particle duality" idea is often dismissed, in a superior way, on PF but it's a very good way into an understanding about what's happening, even if there are better pictures of it. It's a good expression which prepared a newcomer to the fact that things are going to get harder for the student.

 

[neobaud]

Let me ask it like this. If I were able to track all of the energy of all the matter in the universe, would it decrease over time because it is radiated away by photons? Have there been experiments like this that are more practical?

 

[PeroK]

If we are straying into cosmology, look up the CMBR.

 

[Dale]

I am wondering why not transfer the energy/momentum between the two particles directly?

That is not feasible.

One particle loses momentum and energy, then some time later a different particle gains momentum and energy. During that intervening time where is the momentum and energy?

It is in the photon. If you get rid of the photon then you get rid of momentum and energy conservation. They change, and then change back. That is not conservation. You simply cannot get rid of photons and keep momentum and energy conservation.

 

[Hill]

That is not feasible.

One particle loses momentum and energy, then some time later a different particle gains momentum and energy. During that intervening time where is the momentum and energy?

It is in the photon. If you get rid of the photon then you get rid of momentum and energy conservation. They change, and then change back. That is not conservation. You simply cannot get rid of photons and keep momentum and energy conservation.

Good to see that the comment #40 closes the loop with the comment #2

 

[neobaud]

That is not feasible.

One particle loses momentum and energy, then some time later a different particle gains momentum and energy. During that intervening time where is the momentum and energy?

It is in the photon. If you get rid of the photon then you get rid of momentum and energy conservation. They change, and then change back. That is not conservation. You simply cannot get rid of photons and keep momentum and energy conservation.

Thanks Dale. This is the one response that actually addresses my question.

 

[Dale]

Several posts have been removed. Please keep things polite and professional

 

[vanhees71]

That's ok as long as all participants of this thread are aware that the word "particle`" doesn't imply that it's like other particles that have mass and that have a properly defined position and extent. Between being produced and being detected, the photon is no way like a little bullet (or even a big one). The photon is a quantum of energy or momentum.

The meaning of the word "particle" in the relativistic + quantum realm is pretty subtle indeed. If you insist on an intuitive picture of a photon you are almost always better off to think in terms of an electromagnetic wave. Only when it's detected the specific nature of this single-quantum Fock state becomes important, i.e., it can be either detected at the position of the detector as a whole, or it isn't detected at all.

The "wave / particle duality" idea is often dismissed, in a superior way, on PF but it's a very good way into an understanding about what's happening, even if there are better pictures of it. It's a good expression which prepared a newcomer to the fact that things are going to get harder for the student.

It's the worst idea to impose wrong/outdated ideas in the very beginning of teaching a new subject. What's learnt first about a subject will be rembered best, and if you best remember a misconception, it's not a good thing. To get rid of such misconceptions is very difficult. There are some narratives, which should be completely avoided. In introductory QM it's "wave-particle duality" and the "Bohr-Sommerfeld model of atoms". If you want to start in a historical way, then the story should start with Born, Jordan, Schrödinger, and Dirac in 1925.

 

[PeroK]

That is not feasible.

One particle loses momentum and energy, then some time later a different particle gains momentum and energy. During that intervening time where is the momentum and energy?

It is in the photon. If you get rid of the photon then you get rid of momentum and energy conservation. They change, and then change back. That is not conservation. You simply cannot get rid of photons and keep momentum and energy conservation.

This sounds more like the description of a virtual photon. Although, it could also describe spectroscopy.

The deeper question of why charged particles are coupled to an electromagnetic field takes us to local gauge invariance.

 

[vanhees71]

Instead of "photon" say "electromagnetic field". Then everything becomes clear.

 

[Dale]

This sounds more like the description of a virtual photon.

I certainly had not intended it to sound like that.

If an atom at rest in an excited state drops to an unexcited state it emits a real photon, loses internal energy, gains momentum, and gains kinetic energy (but less than the lost internal energy). If you ignore the real photon, both energy and momentum are not conserved.

 

[vanhees71]

That's indeed a very good example since the desribed "spontaneous emission" if a really generically quantum-field theoretical phenomenon, and that's why QFT was rediscovered by Dirac in 1928 particularly because he was looking for a proper way to describe spontaneous emission. It was discovered by Jordan in 1926, but at this time nobody appreciated it.

 

[Blurf]

It propagates with a final speed, and it transfers energy and momentum.

What is the source of the photon's energy and momentum.

 

[PeterDonis]

What is the source of the photon's energy and momentum.

Whatever emitted the photon.

 

[Vanadium 50]

This seems to be going pretty far afield.

Photons can be created and the creation describe with well understood questions.
Photons can be absorbed, again with well understood consequences/effects on the environment.
Photons can be counted.

What would be a better definition of "exist"?

 

[Dale]

What would be a better definition of "exist"?

Philosophers have been debating that for centuries. As far as I can tell, they don't have a better definition.

 

[phinds]

Philosophers have been debating that for centuries. As far as I can tell, they don't have a better definition.

Doesn't that make this about the right time to close this thread?

 

[neobaud]

This seems to be going pretty far afield.

Photons can be created and the creation describe with well understood questions.
Photons can be absorbed, again with well understood consequences/effects on the environment.
Photons can be counted.

What would be a better definition of "exist"?

Ya I realize that I didn't do a very good job of framing the question.

 

[neobaud]

This seems to be going pretty far afield.

Photons can be created and the creation describe with well understood questions.
Photons can be absorbed, again with well understood consequences/effects on the environment.
Photons can be counted.

What would be a better definition of "exist"?

I think Dale's answer makes sense for an observer watching the transfer of momentum/energy through the em field. If there was no Photon, they would see energy disappear and then reappear at some later time. This is not allowed.

I was wondering about the perspective of the photon where this problem with the "energy glitch" would not happen. The photon itself never "sees" the disappearance of energy so the photon is really unnecessary. Even if the interaction happened across vast distances of space and time. Seems from one point of view you could say that photons, light and other massless particles aren't involved with events in the universe (events = transfer of momentum and energy).

 

[PeterDonis]

I was wondering about the perspective of the photon where this problem with the "energy glitch" would not happen.

Yes, it would. See below.

from one point of view you could say that photons, light and other massless particles aren't involved with events in the universe

No, you cannot say that. It's wrong. The fact that the arc length along a photon's worldline is zero does not mean that "photons don't experience time" or that there are not distinct spacetime events along a photon's worldline or that a photon's worldline is not part of the universe. If the photon did not carry energy and momentum from source to emitter, there would be a "gap" in spacetime between them that would cause local non-conservation, even though the arc length along the photon's worldline is zero.

 

[neobaud]

Yes, it would. See below.No, you cannot say that. It's wrong. The fact that the arc length along a photon's worldline is zero does not mean that "photons don't experience time" or that there are not distinct spacetime events along a photon's worldline or that a photon's worldline is not part of the universe. If the photon did not carry energy and momentum from source to emitter, there would be a "gap" in spacetime between them that would cause local non-conservation, even though the arc length along the photon's worldline is zero.

Isn't spacetime an emergent property of the universe?

I maintain that massless particles don't experience time. They are incapable of having internal clocks. I don't know why you would argue this point.

 

[Dale]

I was wondering about the perspective of the photon

There is no "perspective of the photon". Photons don't even have a position operator.

Seems from one point of view you could say that photons, light and other massless particles aren't involved with events in the universe

That “point of view” doesn’t exist.

 

[Vanadium 50]

Philosophers have been debating that for centuries

I pay them as little mind as I can.

To me, the key is countability. "They don't really exist but this box contains four of them" seems nonsensical.

 

[Dale]

I pay them as little mind as I can.

Always wise, in my opinion. (Which I realize is itself philosophical, but I don't have to get into a debate about it).

 

[Vanadium 50]

that massless particles don't experience time

Does an electron? After all, an electron that is 1000 years old acts identically to one that was just created.

This seems to be a) philosophical and not something subject to test via measurement and b) a set of properties unnecessary for understanding how matter behaves. Does an inclined plane experience time? How do you know, and what difference does it make?

 

[PeterDonis]

Isn't spacetime an emergent property of the universe?

What does this even mean?

I maintain that massless particles don't experience time. They are incapable of having internal clocks.

If by this you mean that the concepts of "experienced time" and "internal clocks" are not even well-defined for massless particles, that is correct. I covered this way back in post #3. However, that does not justify the further claims you made.

I don't know why you would argue this point.

I am not. I am simply pointing out, as above, that the correct point that I am not arguing does not justify the further claims you made that I did argue.

 

[PeterDonis]

Thread closed for moderation.

 

[PeterDonis]

After moderator review, the thread will remain closed.

 

[bhobba]

I know the tread is closed, but ages ago, I found an excellent paper that can be understood after a first course in QM (Susskind would be enough with persistence) that explains why a photon must exist. It was historically first worked out by Dirac. The issue is spontaneous emission. Einstein explained it in one of his famous papers that high school students these days learn about. But Dirac used the principles of QM to explain those rules. It was the first 'taste' of Quantum Field Theory, the foundation of the modern standard model. Without further ado:

https://www.physics.usu.edu/torre/QFT/Lectures/QFT_text.pdf

Thanks
Bill