# 'Photons only exist at the moment they are emitted or absorbed' (!)

1. Nov 7, 2008

### DrGreg

This remarkable claim was made over in the "Special & General Relativity" forum of this site, in post #3 of this thread, and then subsequently discussed in posts #9, #11, #16, and all posts from #19 onwards in that thread. Please read those posts and then respond here.

To my way of thinking, this is absurd use of language. But sometimes absurd things happen in quantum theory so I'm posing the question here to see what experts think.

As I understand it, the claim is based on the premise that photons can be measured only by emission or absorption. I can accept that premise, and photons are considered to take all possible routes between those events so it is impossible to determine the position of a photon between emission and absorption. But to claim that because we choose not to measure a photon's position then the photon "does not exist" seems to me to be an abuse of the English language. The claim seems to suggest that the lifetime of a photon is:

1. Photon emitted
2. Photon immediately ceases to exist
3. Some time later, photon comes back into existence
4. Photon is immediately absorbed.

That just sounds like nonsense to me. Just because we choose not to detect a photon between two events A and B does not mean we could not have detected the photon had we tried to do so. If we tried, we would succeed, but then event B would not occur because our detection would have absorbed the photon.

Am I talking sense, or is the original quote in this post actually meaningful?

(My knowledge of quantum theory isn't too deep, but I understand the basics.)

2. Nov 7, 2008

### Primordial

DrGreg: I hope I'm not out of step by asking about this concept, but in what direction does the initial and final EM field make its change during emission and absorption of the photon, or is the dirction known.

3. Nov 7, 2008

### Cthugha

I am a bit puzzled. How are weak or qnd measurements like the one quoted below explained in a framework, in which there is no photon between emission and detection?

See for example:
"Progressive field-state collapse and quantum non-demolition photon counting"
Nature 448, 889-893 (23 August 2007)

4. Nov 7, 2008

### enotstrebor

This is a copenhaiganist world view. The fact is that all observed behavior is the interaction. Thus one can always take the view this is all one can know and therefore there is no proof'' of existence. This gets extended to things blinking in an out of reality, particles no longer have a physical path, etc.

It is more than an absurd use of language, I find it an absurd way of thinking. Don't try to understand it or debate about it as there is no fundamental common ground if you believe in a continuous consistent fundamental reality outside oneself.

5. Nov 7, 2008

### Jonathan Scott

I'd agree that saying a photon "doesn't exist" in flight is just silly. However, I think I heard that in situations involving interference or multiple coherent photons, as in a laser, the photons being absorbed can often be considered as combinations (loosely speaking "sums and differences") of those emitted rather than each one arising from a specific emitted photon, so the wave in flight may represents a number of mixed-up photons rather than a collection of independent particles. That might be rather loose terminology. Can anyone clarify the official situation, please?

6. Nov 7, 2008

### Fredrik

Staff Emeritus
Do photons exist? Since photons are defined by QED, the question can only be answered by QED (if it can be answered at all). Experiments won't tell us the answer. Experiments can only tell us how accurately a theory predicts the results of experiments.

So does QED say that photons exist? I can't really answer that without a definition of what it means to "exist", but I'm tempted to answer "yes", just because we're dealing with a Hilbert space that contains photon states.

7. Nov 7, 2008

### Mentz114

Well, I've been called 'silly' a few times above so I have to respond. I persist in believing that photons, have no particulate existence in flight. Obviously the energy of the photon, and its momentum and polarization etc still exist. But it is only localizable at the time of creation and absorption.

DrGreg,
your unprecedented attack is based on a misrepresentation,
That is not what I said ! And I strongly object to this misrepresentation.

Your error is in step 3. A photon does not spontaneously come into being. An atom absorbs and that process CREATES a photon for as long as it takes to be absorbed.

This what I said

1. Photon emitted by atom
2. Photon's energy momentum etc join field
3. Some time later, an atom absorbs a quantum
4. Photon exist only while absorption is happening.

All the dynamics are in the creation and absorption events.
There are no free photons !

8. Nov 7, 2008

### lightarrow

In what exactly is Physics different from other kinds of philosophies? In the fact that physical objects exists only if you can measure them. Of what you cannot measure, you can have theories, and however, these theories must be proven experimentally, one day or the other, otherwise they are of little usefulness (for Physics). Do you agree with it?

9. Nov 7, 2008

### lightarrow

Why you can't describe that in terms of an EM field interacting with the detector in the place you put it?

10. Nov 7, 2008

### lightarrow

We can always discuss a situation where only a single photon is present from source and detector, if we make the time interval between two consecutive detection be greater than L/c, L = distance source-detector.

11. Nov 7, 2008

### CaptainQuasar

I have a far more humble measure of understanding of quantum phenomena than the other people here but what Mentz114 is saying makes sense to me. It seems to me a violation of scientific rigor to insist that we're certain that the photon still exists as a "probability cloud" or something like that.

I should think that the best we can really say is that that there is some causal connection between the emission of the photo and its absorption. Making concrete statements like "between emission and absorption the photon is in flight between locations" appears to exceed our purview. Whether the unseen underlying mechanism were Mentz114's analogy of the cup of water joining the ocean or something outré and science-fictiony, like "reality is just an information matrix and the emission is simply an event that causes a cosmic reality accountant somewhere to note down that they have to manifest an absorption event at some point" it appears that there are a variety of possibilities other than the photon existing in flight as a definite entity discrete from the rest of the universe.

Now that's definitely the way we'd intuitively expect the universe to work, based upon things we learned as infants. But the rigor of science does not permit us the luxury of assuming that the photon still exists when it rolls behind the couch.

12. Nov 7, 2008

### Fredrik

Staff Emeritus
I don't agree with this. First of all, you don't "prove" a theory. All you can do is to find out how accurately it predicts the results of experiments. Second, the phrase "of what you cannot measure, you can have theories" suggests that you can have something more than a theory about those things you can measure. You can't. In physics there's nothing better than a theory.

That phrase also suggests that you can have theories about just about anything that can't be measured. You probably didn't mean to suggest that, but I'm still going to point out that a "theory" that contains unmeasurable quantities that don't directly influence the results of experiments shouldn't be called a theory. I'd call it a "collection of thoughts" or something like that.

13. Nov 7, 2008

### f95toli

I can see why this point of view makes sense to some. However, I guess it also depends on what you mean by "exist" (I guess this is why this thread is in the philosphy forum).

There are various ways of indirectly "sensing" photons without destroying them. One way is to use a cavity which contains a nonlinear medium and create a number state at a resonance with frequency $$f_0$$. Due to the nonlinear medium (as far as I remember it is enough to have a Kerr-like term in the Hamiltonian) it is now possible to count the number of photons with frequency $$f_0$$ in the cavity probing it at a frequency $$2f_0$$, i.e. without neither creating nor destroying photons at the fundamental $$f_0$$.

However, of course one could argue that we are not really "seeing" the photons at $$f_0$$. since this is a indirect measurement but that argument could be applied to everything (no one has actually seen an electron either) so it is not very productive.

14. Nov 7, 2008

### Proton Soup

can't you change the path or polarization of the photon in flight without absorbing it? that would indicate that it existed somewhere between here and there.

15. Nov 7, 2008

### Mentz114

Do I exist in between postings to the PF ?

Well, we've been bounced to the Philosophy sub-forum. I suppose that is just, because it matters not one bit if one person believes that photons have independent existence and another believes the opposite. There is nothing to be vehement about, nothing to justify the use of unkind exclamation points. To DrGreg I say, peace to you bro, and let others hold differing views, even if they irritate you. In the end experiment will guide us to such truths as we are capable of understanding.

f95toli:
Thanks for that. The cavity/cold atom laboratory probably is where these things can be settled.

This thread has some good stuff

Last edited: Nov 7, 2008
16. Nov 7, 2008

### lightarrow

Ok, maybe I'm not so good in expressing concepts, then I let you find a better way of saying that, between the theory "light rays coming from stars are not seen curved by a massive object as the sun" and General Relativity, you would choose the second instead of the first.
About this I agree with you: saying that photons always exist from source to detector, it's not even a theory, just an idea...

17. Nov 7, 2008

### Fredrik

Staff Emeritus
That's not at all what I said. It's almost the opposite of what I said. The photons described by the theory do interact with matter, so they do affect the result of experiments. That's why they are a valid concept in a theory of physics.

To really discuss if something exists or not, you really need a definition of what it means to exist. I have some ideas about how that can be done, and I'll post them later, but I don't have time right now. For now, I'll just say that I'm surprised that some of you support the view that photons don't exist when they're not being measured. You usually hear claims like that from people who don't understand what a theory is.

I have a few questions for you guys. I'd like to understand what you're really saying.

1. Are you talking about the photons defined by QED, or about the objects in the real world that correspond to the theoretical photons? If it's the latter, then how do you define that concept?
2. I assume that photons are not the only things that you would claim don't exist. What exactly is the set of "things" that you think only exist when they are being measured?
3. What makes you think that the chair you're sitting on exists?

18. Nov 7, 2008

### CaptainQuasar

"Exists when being measured" is only one particular subset of "exists when emitted or absorbed" in this context, I think.

19. Nov 8, 2008

### lightarrow

What interacts with matter is the light emitted by a source. The energy of this interaction is quantized. This what you know. Then, you can describe the process as due to flying corpuscoles, but this, at the moment, is just a description and nothing more, until you prove it.
Exactly. Isn't this an important subject? Shouldn't physicists discuss about what they are talking about at all, before talking about all the rest?
For what concerns me, the latter.
Because it doesn't disappear after measuring it.

20. Nov 8, 2008

### Fredrik

Staff Emeritus
This is very very very wrong. You can't prove a theory. As I said before, the only thing you can do is find out how accurately it predicts the results of experiments. And there's no such thing as "more than a theory".

You didn't say how you define that concept, but based on the first three sentences of your post I'd guess that you agree that this is a reasonable definition: "A real-world photon is what makes a photomultiplier click".

Neither does a photon. So what makes a chair different from a photon in this discussion?

Last edited: Nov 8, 2008