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

[Fredrik]

These two different models make different claims about what exists (and what matters). Each model is still concerned about what exists in terms of how it matters to its predictions.
...
Or more correctly, the gravitational force is said to exist (and to matter) according to this theory.
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It is not the definition that tell us what exists in the real world, it is the theory. When a theory states that X exists, it says so because X is needed as part of the explanation and therefore X matters. If X didn't matter then it would not be part of the theory and we would not assume that X even exists.

So you're interpreting each theory as a claim that the concepts they define exist. But what does it mean for something to actually exist? Isn't that what we're talking about?

 

[CaptainQuasar]

out of whack, I think that many of the objections raised against your definition are invalid, but it does seem to me that the one WaveJumper brought up is significant. There are many immaterial things that could be said to matter - WaveJumper's imagination, the Flying Spaghetti Monster, justice, et cetera - that wouldn't be said to exist in the same way we're talking about it here.

Whether something "matters" (i.e. whether it's material, very astute of you to point out the dual meanings) is also a somewhat relative and hence malleable assertion, one which might be said to relate to whether humans care about it. But here we're talking about "existence" in a sense that must be independent of humans, correct? I just wouldn't want to give the impression that we're talking about anything like the "consciousness causes collapse" type mystical interpretations of quantum physics since it seems to me that this is a much more substantial question. (Heh heh, "substance" ↔ "substantial", there's another one.)

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[Dale]

I'm pretty sure that the question mark at the end of his post meant that he isn't making a statement about what can or can't be. He's just asking a question. Unfortunately I don't know the answer. I haven't done an actual calculation of this sort of things in ten years.

Yes, I was asking a question to which I don't know the answer.

If a collection of photons is always absorbed in the exact same quanta as they were emitted then my inclination would be to say that they do exist as independent entities in the interim. If a collection of photons can sometimes be absorbed in a different combination of quanta than they were emitted then my inclination would be to say that only the field exists in the interim.

I know that is not a rigorous analysis of the deeper physical and meta-physical issues regarding the meaning of the word "exist", but a practical working definition is sufficient for me.

 

[out of whack]

So you're interpreting each theory as a claim that the concepts they define exist.

In case we disagree on the meaning of the word "exist", for the purpose of answering your question I will say that I interpret each theory as a claim that the concepts they define matter. I also observe that the word "exist" is redundant in scientific theories. Science describes concepts that matter and disregard concepts that don't matter.

But what does it mean for something to actually exist? Isn't that what we're talking about?

I was indeed talking about what the word means inasmuch as we assume it has any meaning at all. I also suggest that we refrain from using ill-defined words. Statements that contain undefined terms are rather meaningless. But try this as an exercise: assume that the verb "to exist" is no longer available. Substitute the verb "to matter" in its place. While we're at it, replace the adjective "real" with "relevant". I think you will find that you can still express the same coherent scientific ideas just fine, perhaps with enhanced meaning.

For my part, when I say that something exists, I am saying that it matters. I find this definition clear because I have yet to see anything that is said to matter but that doesn't exist.

There are many immaterial things that could be said to matter - WaveJumper's imagination, the Flying Spaghetti Monster, justice, et cetera - that wouldn't be said to exist in the same way we're talking about it here.

These are thoughts. When you say that one thing doesn't exist "in the same way" as a different thing, what you do is describe the different nature of two things that yet both exist and matter. It is so certain that your thoughts exist that they are in fact one of the rare few philosophical certainties. It is impossible for you to deny the existence of your own thoughts (Descartes's "I think therefore I am"). And of course they matter, at least to you.

Note: The correspondence between the verb "to matter" and the "material things" physicists are interested in is coincidental. I would rather use the term "to be relevant" instead but I find it too long and unwieldy.

But here we're talking about "existence" in a sense that must be independent of humans, correct?

When I say that something matters, it isn't restricted to what matters to you personally in the sense that you consciously care about it. It's not restricted to what humans have a deliberate interest in it. Photons matter even to squirrels despite their lack of awareness or interest in them.

 

[CaptainQuasar]

When I say that something matters, it isn't restricted to what matters to you personally in the sense that you consciously care about it. It's not restricted to what humans have a deliberate interest in it. Photons matter even to squirrels despite their lack of awareness or interest in them.

Well... yeah, but I think that what WaveJumper was trying to get at is something along the lines of, the Flying Spaghetti Monster can be proven to exist via your definition but, I guess, only insofar as it exists in the form of a certain pattern of electrical impulses firing in diverse human brains.

But the sense in which we're asking whether photons exist in certain circumstances is inquiring about existence in a greater sense, or at least a different sense, than them being imagined flying through space between two points in the mind of the experimental scientist.

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[out of whack]

Well... yeah, but I think that what WaveJumper was trying to get at is something along the lines of, the Flying Spaghetti Monster can be proven to exist via your definition but, I guess, only insofar as it exists in the form of a certain pattern of electrical impulses firing in diverse human brains.

Exactly. This is why clear language matters and one reason why people will argue for a long time when they actually agree. The concept of the FSM exists, the thought of the FSM exists, these things matter. But the FSM doesn't exist, it doesn't matter to anything, anywhere at any time (as far as we know). Likewise, a perfect circle doesn't exist even though the concept does.

But the sense in which we're asking whether photons exist in certain circumstances is inquiring about existence in a greater sense, or at least a different sense, than them being imagined flying through space between two points in the mind of the experimental scientist.

Consider hypothetical photon T said to be in transit between a source and a receiver. We assert that there is a difference between the claims "T exists" and "T doesn't exist". Given the assertion of a difference we must conclude that it somehow matters whether there is a T or not, and not merely as a mental concept in this specific case.

 

[lightarrow]

If a collection of photons is always absorbed in the exact same quanta as they were emitted then my inclination would be to say that they do exist as independent entities in the interim.

For what I know, the number of photons cannot be completely determined, in the same way as you cannot have a completely determined value of position or momentum; phase and number of photons are coniugate variables as position and momentum.

 

[lightarrow]

Exactly. If we pretend it to be a science and not a phylosophy

"Pretend"? Are you saying that physics isn't a science?

Probably my choice of the english word was very bad... maybe I should have written "demand", "want".

I don't understand what you're trying to say here. Are you disagreeing with my definition of "science".

In my opinion, we should add something more to your definition. We should add the operational (hope it's the correct term) procedures with which we define the objects it describes, in the same way as we define the physical quantities lenght, time, mass, ecc.

A set of statements that includes your #1 isn't a theory in my opinion, since the statement about the blue angels doesn't say anything about the result of any experiments. (I'm going to have to find a way to include that requirement explicitly in my definition). Do you see the difference between photons and blue angels? Photons do affect the results of experiments.

I assumed as given, the fact that the angels interacted with detectors in the same way as light does.

DaleSpam's question was about the total energy of two photons. Reference frames don't really have anything to do with it. Your answer is probably a good one, but to a completely different question.

Ah, yes, now I realize, thank you.

 

[Fredrik]

In my opinion, we should add something more to your definition. We should add the operational (hope it's the correct term) procedures with which we define the objects it describes, in the same way as we define the physical quantities lenght, time, mass, ecc.

In my opinion, that's already included in the definition. I required that a theory must be able to make predictions about the results of experiments. A mathematical model alone can't do that. For example, in special relativity we define "proper time" as the integral of a certain quantity along a curve. Then we postulate that the numbers displayed by a clock is the proper time of the curve in Minkowski space that represents its motion. That postulate must be a part of the theory. Otherwise it wouldn't be a theory according to my definition. Minkowski space is the mathematical model of spacetime used by SR, but the theory of physics that we call SR consists of a set of postulates that identify things in the real world with things in the model. The "things in the real world" must be defined operationally.

I assumed as given, the fact that the angels interacted with detectors in the same way as light does.

That means that you have either abandoned quantum mechanics (in favor of what?), or that the angels are represented mathematically by the QED Lagrangian. If it's the former, you don't have a theory. If it's the latter, your theory is still QED. You have just replaced the word "photons" with "angels". I don't have a problem with that.

 

[Jonathan Scott]

What do people mean by photon?

Suppose the word was "drip", as in drip of water (an extension of the "cupful" idea earlier in this thread). If I have a tap dripping into a bowl, and the bowl is overflowing with drips falling from the side, then do the drips exist from when they enter the bowl to when they leave it? I'd say that the water from which they were formed continues to exist, but the one drip ends when it hits the water, and another one forms when it falls from the side.

Similarly, if a photon is a quantized transfer into or out of a field, then the energy and other properties persist in the field, and what comes out of the field equals what goes in. Calling the traveling energy a collection of photons may be useful in some contexts, but is it like calling the bowl of water a collection of drips?

 

[Fredrik]

In case we disagree on the meaning of the word "exist", for the purpose of answering your question I will say that I interpret each theory as a claim that the concepts they define matter. I also observe that the word "exist" is redundant in scientific theories. Science describes concepts that matter and disregard concepts that don't matter.

I don't think that changing the word from "exists" to "matters" solves any problems. I agree that a definition of "exists" should be such that the FSM doesn't exist, but I don't agree about perfect circles. They aren't physical objects, but they certainly matter. What you said about circles can be said about anything in mathematics, so we'd be forced to conclude that no mathematical definition matters. But we know that mathematics is extremely useful, so it seems very strange to say that mathematical concepts don't matter.

It seems to me that what you're really going for is the difference between a physical object and an abstract concept. So in order to define what "exists" or "matters" means, you'd have to define "physical". Here's one idea about how to do it: Anything that can be defined operationally is "physical" (and therefore "exists"). (An operational definition is done by specifying what sort of device measures the concept we're defining. Example: "Real-world time" is what you measure with a clock).

Photons can be defined operationally: "A photon is what makes a photomultiplier click". This means that they are physical objects and should be said to exist according to this definition (my version of your definition).

 

[CaptainQuasar]

Do you guys really think we have to get all that deeply into the semantics here? I mean, we all understand what the question is, don't we? It's whether there's some localized thing, no matter be it particle / corpuscle like or diffused / wave like, that transits through space approximately between the locations of emission and absorption.

It's certainly good to be careful with terminology - clear, precise, and well-defined phrasing is a virtue of a good question - but it does not seem to me that we really need to get into what "exists" means or what "physics" or "physical" means to grasp this question nor to attempt to answer it.

It seems to me a straight-forward empirical matter: if we have some evidence for a distinct photon-entity existing between the emission and absorption events we can make some sort of specific statement that defines a scope for what may or may not exist between those points besides a causal relationship; if emission and absorption events are all we've got, we can't definitively say that the relationship between them is due to the spatial transit of a localized entity.

Now the only thing anyone has said that really seems to have bearing is f95toli's talk of ways to indirectly sense photons. I don't have the scientific knowledge to imagine or understand what he's talking about.

But a discrepancy seems to be that f95stoli talked about counting photons and now lightarrow just said that number of photons is one of a pair of conjugate variables in terms of Heisenberg-uncertainty-type-stuff. Which one is the case and does anyone have more information on f95stoli's indirect-photon-measuring method? And also, f95stoli was talking about a "cavity" involved in the measurement technique and in my limited science experience the word "cavity" always seemed to refer to a Gedankenexperiment hypothetical construct, not an actual piece of equipment.

Cripes, this is like herding cats, you wacky physicist types you.

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[out of whack]

I don't think that changing the word from "exists" to "matters" solves any problems.

This wasn't meant to solve any problem other than to elucidate how the word "exist" is being used.

I agree that a definition of "exists" should be such that the FSM doesn't exist,

I assume you also agree as I do that the FSM doesn't matter since it doesn't exist. It's only the concept that exists, as a useful discussion instrument.

but I don't agree about perfect circles. They aren't physical objects, but they certainly matter.

So you DO agree: what I said is that the concept of a perfect circle exists even though no perfect circle does. Concepts matter, concepts are thoughts and thoughts exist. But you cannot find a perfect circle anywhere in nature, they are all imperfect or grainy at some level. So again: perfect circles do not exist but the concept of a perfect circle does.

What you said about circles can be said about anything in mathematics, so we'd be forced to conclude that no mathematical definition matters. But we know that mathematics is extremely useful, so it seems very strange to say that mathematical concepts don't matter.

I hope you see after re-reading me that this does not follow.

It seems to me that what you're really going for is the difference between a physical object and an abstract concept.

Not at all. I am trying to explain that if something exists then it must matter and that if something matters then it must exist. This goes for concepts as well as for physical objects. The remaining of your post follows from this incorrect interpretation so I will stop here.

 

[out of whack]

Do you guys really think we have to get all that deeply into the semantics here?

Well, this thread "exists" because someone elsewhere stated that "Photons only exist at the moment they are emitted or absorbed" and DrGreg indicated that "To my way of thinking, this is absurd use of language." It seems entirely pertinent to discuss to great length what it means to exist if we are to answer the question.

 

[Cthugha]

But a discrepancy seems to be that f95stoli talked about counting photons and now lightarrow just said that number of photons is one of a pair of conjugate variables in terms of Heisenberg-uncertainty-type-stuff.

Well, the relation is similar to a Heisenberg-like uncertainty relation. However, the fact, that the phase is not an observable in qm limits the analogy a bit. Still, there are states with well defined photon number, the so called Fock states. Their phase is of course pretty undefined.

Which one is the case and does anyone have more information on f95stoli's indirect-photon-measuring method? And also, f95stoli was talking about a "cavity" involved in the measurement technique and in my limited science experience the word "cavity" always seemed to refer to a Gedankenexperiment hypothetical construct, not an actual piece of equipment.

These experiments and cavities are very real and not just Gedankenexperiments. Here is a free arxiv version of the paper about nondestructive photon counting and progressive field collapse I quoted in post number 3:

http://arxiv.org/pdf/0707.3880

 

[CaptainQuasar]

These experiments and cavities are very real and not just Gedankenexperiments. Here is a free arxiv version of the paper about nondestructive photon counting and progressive field collapse I quoted in post number 3:

http://arxiv.org/pdf/0707.3880

I'm sorry I missed that, it seems to be written very readably. Though I'm kicking myself for not bothering to figure out bra-ket notation during the three semesters of college physics I took (the professor didn't assign it or test for it, so, y'know.)

So let me test my understanding here: photons are trapped in one of these Fabry-Pérot cavity things confined in some way so that they can only coexist in a finite number of states, the way that electromagnetic force traps electrons around an atomic nucleus where electrons can only exist in a finite number of states. There are also these rubidium Rydberg atom thingies in the cavity and each atom is coupled in some way to exactly one photon. (Or are they coupled to each other en masse? I wasn't clear on that.)

The coupling causes the atoms as a group to emit some signal while the photon is still present in the cavity, attenuating as each photon is absorbed into the cavity walls. The experimenters are recording this signal and it's the signal's staircase-like behavior graphed out on page 4 of the paper that confirms the experiment's theoretical underpinnings.

So, if I've got that correct, these are the questions I've got: what does the signal actually represent? Is it this "light shift" property of the Rydberg atoms mentioned on page 1? And do you guys think that the signal represents something that would definitely only be connected to the qualities of a group of distinct and individual entities, or could this be the output a single thing (perhaps a single thing that extends throughout the entire universe) that is influenced by the experimental environment to produce this staircase-like signal?

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[Fredrik]

It seems to me that what you're really going for is the difference between a physical object and an abstract concept.

Not at all. I am trying to explain that if something exists then it must matter and that if something matters then it must exist.

In that case I don't see how your "definition" makes any sense at all, even efter reading your recent posts again. You have just replaced one word with another.

 

[Fredrik]

Do you guys really think we have to get all that deeply into the semantics here?

I don't think we're going deep enough.

I mean, we all understand what the question is, don't we?

Not at all. I certainly don't.

It's whether there's some localized thing, no matter be it particle / corpuscle like or diffused / wave like, that transits through space approximately between the locations of emission and absorption.

So it needs to be localized now. Why?

 

[Fredrik]

I have another question for those of you who support the view that photons don't exist between emission and detection:

Would you say that quantum states exist between measurements?

 

[DrGreg]

Since I started this thread last Friday, I have been offline and unable to respond until now.

First of all, to anyone who was offended by the tone of my original post, and especially to Mentz114, I apologise. I meant no disrespect; I was merely trying to understand a statement that I found incomprehensible.

And I originally posted this in the Quantum Physics forum because of my lack of expertise in that subject. I wanted to find out if the view expressed by Mentz114 had some theoretical justification behind it.

If we are talking about the emission and reception of a single photon, what is the entity responsible for the transfer of energy-momentum from the emission event to the reception event? To me, it seems entirely reasonable to describe that entity as "a photon" even if it doesn't much resemble a classical particle. I side with Fredrik here in that I don't really make any distinction between the mathematical theory that models the propagation and the propagation itself.

When we consider multiple photons, some contributors have suggested that the number of photons emitted might not equal the number of photons received. (Ignore the possibility of "photons escaping to infinity" for the purposes of this argument.) I don't know enough about quantum theory to know whether that is possible or not. But if it is, can we not talk of "two photons coalescing into one" (or something similar) for example? Even if we can't pinpoint an event in spacetime where the merger occurred? (Actually a 2-to-1 merger sounds impossible to me on classical energy-momentum conservation grounds, but perhaps my classical intuition is wrong here?)

 

[Mentz114]

There are strong sounding arguments that suggest that the EM field retains its discreteness when its not interacting with matter. I still think this is an undecidable ( by experiment ) question.

arguments against discreteness
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I'm not an expert but in quantum optics it's hard to 'pin down' the photon. The photon number operator can have vacuum fluctuations which are large compared to the average photon number. There's no conservation of photon number, obviously.

The first quantization wave equation of the photon lacks localizability, which may be interpreted as saying that the photons are everywhere at the same time.

In quantum field theory ( second quantisation ) photons are just notches on a stick, ie Fock space states.

How can one define the 'size' ( spatial boundaries) of a photon ?

[end of arguments]

Whether one believes that EM fields are a lot of photons flying about, or a non-local energy field, makes little difference in practice. The fact is that in current mainstream theories, there are no photons per se, although the word is used a lot.

If we define a photon as 'a quantum of the EM field' and leave it there, we should get along fine.

I would like to amend my original claim to "we may not be able to determine if the EM field is discrete (quantised) other than when interacting with matter".

 

[CaptainQuasar]

So it needs to be localized now. Why?

"Needs to be"? What do you mean by "needs to be"? It seems like you're doing something other than trying to understand the question under discussion. As if you're engaging in some sort of legalism, though for what purpose I can't guess.

So anyways, if you look at what the people in the thread have said who seem to have been able to grasp the question immediately without any semantic hangups, they speak of the photon being "in flight" - transitioning between points in space. If a photon was completely diffuse and omnipresent, if it was everywhere at once, that would be inconsistent with the impression of it moving through space between points - so clearly the idea of a photon which they've somehow arrived at together is not an omnipresent entity but a localized one.

It's obvious to me that being localized is a characteristic that people are associating with photons from the way they're talking about it. And I think the only reason you wouldn't see that is if you were trying on purpose to ignore it. It's as though you're being didactic or something, like you think it's your job to teach the rest of us a lesson about articulating questions.

You don't need to pretend as though I am not describing various aspects of an intelligible question that is at least tangentially related to what people have been talking about here, or act as though the points I'm making are bizarre and incoherent or something. It's fine if you think that there's some additional question that can be formulated out of the discussion here that's radically different from what I'm describing. Sure, go ahead and go off and play in semantic-land. But seriously, cut the crap, you don't need to deny my formulation of the question to make your own.

I have another question for those of you who support the view that photons don't exist between emission and detection:

Would you say that quantum states exist between measurements?

See, now, there you go. You don't think we have explored the meanings of the terms "exist" and "physical" deeply enough, but you're going to toss off in a single sentence an equivalent to a question that DrGreg used six paragraphs, a bulleted list, and a request to read most of a separate thread to ask - which you're claiming was inadequately articulated?

I think that, as I said above, you, I, and everyone else understand the question under discussion just fine. At this point I'm beginning to think that your quibbles about semantics are actually some sort of underhanded, circumspect way of making indirect points about what you think photons are.

I wanted to find out if the view expressed by Mentz114 had some theoretical justification behind it.

My impression is not that Mentz114 was saying that he's working off of any particular comprehensive theory that definitely says that photons do not exist between absorption and emission, but making the point that there are models like his "cup of water and the ocean" analogy that would not counterfactualize the experimental results we have pertaining to the emission and absorption of electromagnetic energy.

He's simply pointing out that, although thinking of photons as distinct and individual localized entities that transit between emission and absorption locations is a handy model to work on and certainly a natural way to think of it, to date we don't have evidence to definitively say that it's accurate. As has often happened in the history of science a model that has served us well up to a point may be pulled back to reveal something that is much more complex or exotically different from what we'd thought.

Some simple models seem to work and persist just fine - like (I think, correct me if I'm wrong) conservation of energy and momentum or symmetry, but elsewhere you get... well, you guys undoubtedly know better than I how weird things can get.

To take something more my speed, consider the 19th century concept of aether as a medium for light - of course light's waves have a medium, there has to be something for light to be a wave in! But Michelson-Morely showed that no, light behaves as a wave without a medium. (That we know of, so far.)

If we are talking about the emission and reception of a single photon, what is the entity responsible for the transfer of energy-momentum from the emission event to the reception event? To me, it seems entirely reasonable to describe that entity as "a photon" even if it doesn't much resemble a classical particle. I side with Fredrik here in that I don't really make any distinction between the mathematical theory that models the propagation and the propagation itself.

Like I said, it's reasonable and handy to think of an individual, localized packet of energy transitioning through space between the locations of the emission and absorption events - but do we really know incontrovertibly the case? Couldn't some sort of cosmic accounting mechanism, that "knows" there must be a future absorption event in a location determined by a bunch of geometric rules, also explain everything we have observed?

The mathematical theory doesn't require individual localized energy packets transitioning through space, does it? It specifies methods of calculating the amounts of energy and momentum involved, et cetera, and the geometry and all that - but is there anything that would make localized moving energy packets necessary for things to work properly?

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[lightarrow]

I have another question for those of you who support the view that photons don't exist between emission and detection:

Would you say that quantum states exist between measurements?

It depends on the quantum state and on how we define "exist".

 

[lightarrow]

In my opinion, that's already included in the definition. I required that a theory must be able to make predictions about the results of experiments. A mathematical model alone can't do that. For example, in special relativity we define "proper time" as the integral of a certain quantity along a curve. Then we postulate that the numbers displayed by a clock is the proper time of the curve in Minkowski space that represents its motion. That postulate must be a part of the theory. Otherwise it wouldn't be a theory according to my definition. Minkowski space is the mathematical model of spacetime used by SR, but the theory of physics that we call SR consists of a set of postulates that identify things in the real world with things in the model. The "things in the real world" must be defined operationally.

Which is the operational definition of "photon"? That is, what stays for the clock of your example?

 

[Dale]

I think we need to pin down the term "photon" more than the term "exists".

 

[Cthugha]

So let me test my understanding here: photons are trapped in one of these Fabry-Pérot cavity things confined in some way so that they can only coexist in a finite number of states, the way that electromagnetic force traps electrons around an atomic nucleus where electrons can only exist in a finite number of states. There are also these rubidium Rydberg atom thingies in the cavity and each atom is coupled in some way to exactly one photon. (Or are they coupled to each other en masse? I wasn't clear on that.)

In a nutshell, there is a cavity with a certain number of photons inside. Now you prepare one atom in a specific and well defined spin state and let it evolve and fall through the cavity with the photons. The spin will precede and will have some specific value after it fell through the cavity. Now loose speaking the precession of the spin depends on the em field around, in particular on the photon number. So the final spin state will depend on the photon number inside the cavity. Of course - due to uncertainty restrictions - in most cases it is not possible to have an unambiguos relation between photon number and spin state, so you just get conditional probabilities, that there are n photons present. But to resolve this problem you just take another atom and let it fall through the cavity and another one and so one. This method is nondestructive and after a (rather short) while, the conditional probability to have one certain photon number inside the cavity converges to 1.

arguments against discreteness
------------------------------
I'm not an expert but in quantum optics it's hard to 'pin down' the photon. The photon number operator can have vacuum fluctuations which are large compared to the average photon number. There's no conservation of photon number, obviously.

You are right here, although mostly the fluctuations are due to the emission process. In a laser for example the photon number can be treated as the emission of an ensemble of statistically independent emitters, which gives a Poisson distribution for the photon number statistics, where the fluctuations of the mean photon number are always on the order of the mean photon number itself. The fluctuations due to vacuum fluctuations are most prominent in the emission process as well, mostly as the reason for spontaneous emission. Once the emission is on its way to the detector, there are usually no additional vacuum fluctuations altering the emission.

How can one define the 'size' ( spatial boundaries) of a photon ?

Well, the best guess is the coherence volume, I suppose, which in turn relates to coherence time and therefore to the uncertainty of the exact point of time, when a photon was emitted.

If we define a photon as 'a quantum of the EM field' and leave it there, we should get along fine.

Definitely.

 

[CaptainQuasar]

In a nutshell, there is a cavity with a certain number of photons inside. Now you prepare one atom in a specific and well defined spin state and let it evolve and fall through the cavity with the photons. The spin will precede and will have some specific value after it fell through the cavity. Now loose speaking the precession of the spin depends on the em field around, in particular on the photon number. So the final spin state will depend on the photon number inside the cavity. Of course - due to uncertainty restrictions - in most cases it is not possible to have an unambiguos relation between photon number and spin state, so you just get conditional probabilities, that there are n photons present. But to resolve this problem you just take another atom and let it fall through the cavity and another one and so one. This method is nondestructive and after a (rather short) while, the conditional probability to have one certain photon number inside the cavity converges to 1.

Okay, I think I understand that... yeah, I'm understanding the graphs on page 3 better. So the y-axis of the graphs at the bottom of page 4 of that report, which is labeled "n" but is clearly a continuous value instead of an integer value because it's all spikey - is that something like an averaged or summed measurement of something related to the spin states of the atoms after they've fallen out of the cavity?

Also - another thing I wasn't clear on earlier - it looks like they were running this experiment many, many times and the data in the charts page 3, for example, is grouping the data for all the 1st atoms in every run, 2nd atoms in every run, 3rd atoms in every run, on up to the 50th or 110th atoms in every run, right?

Thank you for patiently explaining this to me.

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[Cthugha]

Okay, I think I understand that... yeah, I'm understanding the graphs on page 3 better. So the y-axis of the graphs at the bottom of page 4 of that report, which is labeled "n" but is clearly a continuous value instead of an integer value because it's all spikey - is that something like an averaged or summed measurement of something related to the spin states of the atoms after they've fallen out of the cavity?

Do you mean the graphs showing plateaus with some spikes? The y-axis labeled <n> gives the mean photon number. If you have a look at graph 2c for example, you will notice, that there are situations, where just one photon number has maximal probabilities and all others have almost 0 probability and there are situations, where several photon numbers have not too small probability amplitudes simultaneously. These second situations lead to non integer values of <n>.

Also - another thing I wasn't clear on earlier - it looks like they were running this experiment many, many times and the data in the charts page 3, for example, is grouping the data for all the 1st atoms in every run, 2nd atoms in every run, 3rd atoms in every run, on up to the 50th or 110th atoms in every run, right?

The charts on page 3 are just showing one run of 110 atoms each. The field starts in a state, where each possible photon number has the same probability and then progressively collapses towards one of them. So it is just the probability distribution after 1, 2, n atoms have fallen through the cavity. Summing over whole runs would not give any good results because each run is independent and the final photon number inside the cavity will be different every time.

 

[CaptainQuasar]

Okay, along with another re-read of the paper that clarifies things even more. Understanding how the probability distributions are constructed might be beyond me, but I'm still uncertain as to what measurements are being taken as inputs to the process of constructing the probability distributions.

Measuring the spin along O–u is performed by submitting it, after cavity exit, to a pulse R₂ whose phase is set to map O–u onto O–z (Fig. 1b).This rotation is followed by the measurement of the atom’s energy, equivalent to a spin detection along O–z. The combination of R₁ and R₂ is a Ramsey interferometer.

So, is the action of the Ramsey interferometer both preparing the atoms before they enter the cavity and measuring them subsequent to their exit from the cavity? And is a Ramsey interferometer a kind of http://en.wikipedia.org/wiki/Atom_interferometer" ? What are these pulses composed of - light / EM radiation or something else?

⚛​

 

[ThomasT]

Interesting thread. I've read quite a few, but not all yet, of the posts. Just thought I'd offer my two cents on this.

... is the original quote in this post actually meaningful?

I think so. To say that photons exist independent of the means used to detect them (ie., independent of detection) in some level of physical reality underlying, and inaccessible to, our sensory perception is physically meaningless.

The word 'photon' refers to experimental materials and instruments, the preparations and observable behaviors of those materials and instruments, and the mathematical shorthands, the models and theories (as quanta of the electromagnetic field), which represent, and quantitatively relate and communicate all experimental productions of photons.

Other than this, the word 'photon' has no physical meaning.

Quantum theory is about the behavior of emitters and detectors. What happens apart from that behavior is a matter of metaphysical speculation. The behavior of an underlying quantum reality might in some ways correspond to the evolution of a wave equation model or operations in an imaginary space, but there's no way to know. It's for just this reason that Heisenberg developed and offered his matrix mechanics representation which makes no pretense to having anything to do with any sort of hidden, underlying quantum reality.

The only unambiguous, current definition of 'photon' is, via QED representation, as a quantum of the electromagnetic field. Again, what aspects of QED might or might not correspond to an underlying quantum reality is a matter of speculation.

My own metaphysical view is that there is an underlying quantum reality and that it is essentially wavelike. Particulate structures and media, and hence macroscopic particulate detection phenomena, arise via the interactions of a hierarchy of bounded wave complexes which is not comprehensibly represented in any theory.

So, of course, whatever it is precisely that's happening to produce photon detection events has something to do with what's happening between emission and detection in an underlying quantum reality. Unfortunately, due to the existence of a fundamental quantum of action, that quantum scale reality is untrackable, and there's no way of talking about it precisely enough to make any difference regarding what's experimentally observed.

 

[wawenspop]

My own metaphysical view is that there is an underlying quantum reality and that it is essentially wavelike.

If you were a photon, so to speak, then no time would pass from emission to absorption for you (as a photon). Indeed, you could travel to the other side of the Universe (eg as a snapshot image of you falling off a chair - for illustrative example) and after 90 billion years of travel, exactly no time would have passed for you at all (remember, you are a photon now).
Conversely, the distance between emission and absorbtion is zero (for you as a photon - because the journey took no time).

So as a photon its not surprising that you only exist at the points of observation - and even that is only a realignment of states. So from the photon's point of view it really does not exist between observations - how could it?

 

[ThomasT]

If you were a photon, so to speak, then no time would pass from emission to absorption for you (as a photon). Indeed, you could travel to the other side of the Universe (eg as a snapshot image of you falling off a chair - for illustrative example) and after 90 billion years of travel, exactly no time would have passed for you at all (remember, you are a photon now).
Conversely, the distance between emission and absorbtion is zero (for you as a photon - because the journey took no time).

So as a photon its not surprising that you only exist at the points of observation - and even that is only a realignment of states. So from the photon's point of view it really does not exist between observations - how could it?

Unfortunately, (or maybe fortunately?) that point of view isn't available to us.

 

[wawenspop]

Also, an electron 'does not exist' going from point A to Point B unless something interacts with it to reveal a state (eg position) at some time, t.
By existence what do we mean? It only ever has a probability of being at some place (existing?) at some time - so in that sense there is not a point particle at all, only the probability of observing one - (ie its 'position state').

Even then, its only a realignment of states - its not as if it suddenly *pops into existence* - it is observed at an instant in time - the 'collapse of the wave function' is misleading as it kind of gives the impression of probabilities ending and particles becoming *real*. No, its a realignment of states that allows observation. Same for the photon. You could also say it must entangle itself with something else to be observed - but that is another bag of philosophical worms - if there is such a thing as a philosophical worm.






I had a philosophy girlfriend once, but she kept trying to prove I did not exist.

 

[lightarrow]

I had a philosophy girlfriend once, but she kept trying to prove I did not exist.

My last girlfriend hadn't studied philosophy, but was trying to prove the same. I've left her

 

[Mentz114]

Posted by atakor in this thread - it's a great read.

https://www.physicsforums.com/showthread.php?t=269121&goto=newpost

Hi,
I recommand the reading of What is a Photon?

(http://stochastix.files.wordpress.com/2008/05/what-is-a-photon.pdf) (PDF - 1.30 MB) by the Optical Society of America (OSA).

 

[WaveJumper]

Just thought of quantum tunneling and the nature of movement of photons. It seems my classical way of thinking is overriding the little possibility there is that i could ever picture how elementary particles move.

When in flight, photons exist as waves. Or more like a probability wave with different amplitudes for places where that wave might be. Now, quantum tunneling says that partciles are described by their wave-function which represents the probability amplitude of finding that particle in a certain location. And sometimes that location happens to be on the other side of the wall.

Now, it seems that according to our classical idea of movement the particle would have to have gone through the wall. But we know it didn't happen, so it leaves us with only one possibility - that movement at the quantum level has nothing in common with movement at the macro level, and hence the photon in question in the OP exists only at emission and absorbtion.

But then, photons are said to move at the speed of light and quantum tunneling happens FTL as in:

"There have been various reports in the popular press of experiments on faster-than-light transmission in optics — most often in the context of a kind of quantum tunneling phenomenon. Usually, such reports deal with a phase velocity or group velocity faster than the vacuum velocity of light. But, recall from above, that a superluminal phase velocity cannot be used for faster-than-light transmission of information. There has sometimes been confusion concerning the latter point."

http://en.wikipedia.org/wiki/Faster_than_light



How do you explain this "movement" through barriers in a FTL fashion with our human classical way of thinking?
What is the nature of movement at the quantum level - skipping from one decohered state of superpositions to the next at Planck time units(thus skipping to a position that is on the other side of the wall)? What if the particle is not observed/measured? How does it "move"? Do all eigenstates skip/hop with the emitted photon to the absorbtion event and then only the right trajectory photon is absorbed(the one that arrived first through the shortest route - whether that's in straight line or via quantum tunneling over a hill)?

 

[ZapperZ]

There is nothing exotic here if you also understand classical optics. It is well-known that the phase velocity in classical optics/classical E&M that the phase velocity can be larger than c under certain circumstances. This isn't new and one does not need "tunneling" to observe something like this.

Zz.

 

[bassplayer142]

Just thinking. Doesn't it coincide with our views that if a photon were to disappear that it would have a certain probability to reappear in another stop. This would be inversely proportional to distance so that the farther away the less the probability. If you were to compare this to a source of light like the sun and how at a distance X number of photons strike yet the farther away you get the number declines.

I don't see how this theory would differ in appearance to the currently accepted theory.

 

[wawenspop]

Just thought of quantum tunneling and the nature of movement of photons. It seems my classical way of thinking is overriding the little possibility there is that i could ever picture how elementary particles move.

When in flight, photons exist as waves. Or more like a probability wave with different amplitudes for places where that wave might be. Now, quantum tunneling says that partciles are described by their wave-function which represents the probability amplitude of finding that particle in a certain location. And sometimes that location happens to be on the other side of the wall.

Now, it seems that according to our classical idea of movement the particle would have to have gone through the wall. But we know it didn't happen, so it leaves us with only one possibility - that movement at the quantum level has nothing in common with movement at the macro level, and hence the photon in question in the OP exists only at emission and absorbtion.

But then, photons are said to move at the speed of light and quantum tunneling happens FTL as in:

"There have been various reports in the popular press of experiments on faster-than-light transmission in optics — most often in the context of a kind of quantum tunneling phenomenon. Usually, such reports deal with a phase velocity or group velocity faster than the vacuum velocity of light. But, recall from above, that a superluminal phase velocity cannot be used for faster-than-light transmission of information. There has sometimes been confusion concerning the latter point."

http://en.wikipedia.org/wiki/Faster_than_light



How do you explain this "movement" through barriers in a FTL fashion with our human classical way of thinking?
What is the nature of movement at the quantum level - skipping from one decohered state of superpositions to the next at Planck time units(thus skipping to a position that is on the other side of the wall)? What if the particle is not observed/measured? How does it "move"? Do all eigenstates skip/hop with the emitted photon to the absorbtion event and then only the right trajectory photon is absorbed(the one that arrived first through the shortest route - whether that's in straight line or via quantum tunneling over a hill)?

The quantum tunnel can be considered as a local region of distorted space-time:
http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html

Consider a photon that is passing along multiple paths (using beam splitters, for example),
the wave is passing down, say, 100 optical fibres to interfere at a screen to produce the observable. This shows that the 'wave' *was* in 100 optical fibres. Now if we decide to observe a photon particle, then that particle is found to exist in only one on the optical fibres. i.e. it is not in the other 99.
This is a sort of distributed wave packet - distributed into 100 optical fibres.

So, at the instant a particle photon is observed in one fibre - the wave function in the 99 other fibres must instantly vanish. That 'vanishing' would be instant. Then we ask which way did the photon go, to 'describe' the motion of the particle between observations? We get two answers depending whether we think of a particle or a wave.

Did the photon 'exist' between observations? Well, a wave packet is not a photon, only the probability of one (likewise a wave). So, the question can be rephrased as 'does the wave function' exist between observations. A wave function only 'reveals' a photon if observed (as a wave or particle). So, in this sense the photon, does not exist because it has nothing to define what exactly is exisiting between observations. A wave packet is neither a photon or a wave - its a probability of them having observable states only.
So we cannot really ask the question does a photon exist between observations because its a wave packet that exists - not a real particle or a real wave.

 

[WaveJumper]

Did the photon 'exist' between observations? Well, a wave packet is not a photon, only the probability of one (likewise a wave). So, the question can be rephrased as 'does the wave function' exist between observations. A wave function only 'reveals' a photon if observed (as a wave or particle). So, in this sense the photon, does not exist because it has nothing to define what exactly is exisiting between observations. A wave packet is neither a photon or a wave - its a probability of them having observable states only.
So we cannot really ask the question does a photon exist between observations because its a wave packet that exists - not a real particle or a real wave.

Even if we introduce a wave-packet we are still stuck with "exist". Most people here agree that to exist is to move, so the wave packet has to be moving through space. But how would a probability wave move? I think this question can be answered by a theory of everything. Perhaps a string theorist can tell us how a wave packet is supposed to move through space from A to B.

 

[Primordial]

I think photons only exist in the dimension of time(both proper and relativistic) while in transit.