The Delayed Choice experiment: Is Photon speed important?

In the Delayd Choice experiment, it seems as if the photon kan "knows" what is going to happen before it happens, at least thats one interpretation of the results. Is maybe the fact that the photon, traveling at speed C, maybe relevant? It does not "experience" time? It would be nice to hear your thoughts about this mind-bending experiment.

rolnor said:

The photon does not experience time?

It is not even meaningful to talk about a photon "experiencing" time. This is a common misconception. So common that we have a FAQ about it.
rest frame of a photon

Thats why I use qoutationmarks, I think you know that no one knows what a photon "experience"

rolnor said:

Thats why I use qoutationmarks, I think you know that no one knows what a photon "experience"

You asked a question

the photon does not experience time?

and I answered it.

I think its clear that something traveling at C has a different relation to time than something traveling slower than C.

rolnor said:

I think its clear that something traveling at C has a different relation to time than something traveling slower than C.

Well, in at least one important sense, it does not. The time something takes to get from one place to another is based on only two things --- how far is it traveling and how fast is it going. This applies to photons just as much as it does to massive objects.

phinds said:

Well, in at least one important sense, it does not. The time something takes to get from one place to another is based on only two things --- how far is it traveling and how fast is it going. This applies to photons just as much as it does to massive objects.

If you discuss things apart from the actuall topic you miss what I am trying to discuss. What you say is relevant but its not on-topic. I am trying to put forward an idea that can shed light on this experiment, is this idea meaningful? If it is not, please explain why and also suggest some other explanation for the results of the experiment if you can, please

rolnor said:

What you say is relevant but its not on-topic.

Well, I would argue that you had a two questions. One about photon time and the other about the experiment, but you are correct that I have only addressed the one about photon/time and not the experiment.

I don't believe that the speed of the photon is the crux of the delayed choice experiment, I think it's more about the difference between the concept that waves and particles are different (Newtonian Mechanics) and quantum objects (Quantum Mechanics) which are neither one nor the other until measured. HOWEVER ... I'm not expert in this so perhaps one of our more knowledgeable members can jump in.

I can agree. Its not easy to understand what happens at the speed of light, but we do know that time slows down for objects moving compared to objects moving slower. Somehow the photon is outside the framework of time as we know it. If we could repeat the expriment with electrons and get the same result, then my idea is not relevant since the electron is not moving at the speed of light. I dont know if that is technicaly possible

rolnor said:

TL;DR Summary: Is the photons speed C important in the Delayed Choice experiment? The photon does not experience time?

In the Delayd Choice experiment, it seems as if the photon kan "knows" what is going to happen before it happens, at least thats one interpretation of the results. Is maybe the fact that the photon, traveling at speed C, maybe relevant? It does not "experience" time? It would be nice to hear your thoughts about this mind-bending experiment.

View attachment 337590

Photons are used in these experiments because they are relatively easy to generate in large numbers. And, in particular that there are techniques for generating a pair of entangled photons. These experiments could also be done with pairs of enangled electrons/positrons, but those are harder to generate.

The outcome of the experiment, in terms of interference or not, would be the same with other particles. It's certainly not dependent on the speed of light.

But you dont know that without doing the experiment? Thats why we have experiments. This is not the usual double-slit experiment, its more complicated. But it could be so, that all QM objects create this results if we could make half-mirrors for them and send them through the apparatus. The photons in this experiment are not entangled as I understand I dont think there are any half-mirrors for electrons but maybe its possible to make them.
https://en.wikipedia.org/wiki/Wheeler's_delayed-choice_experiment

rolnor said:

But you dont know that without doing the experiment?

That's an interesting question. Let's take the example of conservation of momentum. We drop a rubber ball on a hard floor and it bounces back up, essentially reversing its momentum. This is reconciled by asserting that momentum is transferred to the Earth. If we measure the momentum of the ball and the Earth, then we would find conservation of momentum.

Has this experiment been done? Well, of course not. For many reasons it's impossible to confirm the change in the Earth's momentum on this scale.

Do we, therefore, doubt Newton's laws of motion? Until this experiment has been done?

rolnor said:

But you dont know that without doing the experiment? Thats why we have experiments. …

Yes, and the one you cited is just one of the important delayed choice experiments in the literature. All of them share the important point that photon speed is not really a critical variable in the quantum prediction. And all of them match standard quantum theory, with or without regard to relativistic considerations.

For example: you can entangle photons remotely. Not only can you do this action at distances that are greater than a photon can travel in that time; you can also perform a delayed choice version. In that version, the choice to entangle the photons is made after they are detected. So speed is not even a factor. See middle of page 5:

https://arxiv.org/abs/quant-ph/0201134

phinds said:

Well, in at least one important sense, it does not. The time something takes to get from one place to another is based on only two things --- how far is it traveling and how fast is it going. This applies to photons just as much as it does to massive objects.

With the only problem that photons can in no sense be thought of as classical massless particles. They simply do not have a position observable, and it's impossible to localize them in a clear way particularly in the direction of their momenta.

The meaning of "the speed of light" is entirely as in classical electrodynamics, i.e., it's the phase velocity of electromagnetic waves. The photon is a quantum state of the quantized free electromagnetic field, i.e., generically quantum. It can neither be described as a classical massless point particle in any way nor entirely as a classical electromagnetic wave, although the latter intuition is almost always working right.

rolnor said:

I am trying to put forward an idea that can shed light on this experiment, is this idea meaningful? If it is not, please explain why and also suggest some other explanation for the results of the experiment if you can, ple

No, the idea is not meaningful.
The delayed choice experiment relates various events (photon emission, first beam splittetr, insertion or not of second beam splitter, detection) without regard to any speed. We could do this experiment with massive particles moving at speeds less than the speed of light; we don't because it is impractical to manipulate these particles the way we manipulate photons.

rolnor said:

Its not easy to understand what happens at the speed of light, but we do know that time slows down for objects moving compared to objects moving slower.

As a wise man once said, “It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so". The statement that "time slows down" doesn't mean what you think it does, and your understanding of it is flat-out incorrect. There are many threads over in the relativity forum discussing what it does mean and you may want to follow up there.

As this thread is based on a misconception, it is closed.

As with all such thread closures, if there is more to say on the subject it can be reopened.