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

[lightarrow]

As I pointed out above: There are experiments where we can e.g. "sense" the presence of photons in one mode of a cavity by how they change the properties of photons in another mode.

Don't know this effect and so I don't know to what extent it disproves the idea that photons have an effect only when they are being emitted/detected.

 

[WaveJumper]

Someone correct me if i am wrong, but isn't the nature of movement in the quantum world simply "skipping" from one collapsed state at Planck time scales to the next superpositioned state. If so, how do we define exist, time and space?

 

[out of whack]

I still don't know what it means for something to "exist".

To exist is to matter. What matters exists. What exists matters. The words are equivalent.

 

[CaptainQuasar]

I would say that in this case to ask whether the photon exists is to ask whether it is an entity discrete from the rest of the universe. Whether a being with an omniscient viewpoint could discriminate between the photon as a thing and other things, or whether the cause of the absorption event would be something indistinguishable from a greater whole, like Mentz114's "cup of water and the ocean" analogy.

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[WaveJumper]

To exist is to matter. What matters exists. What exists matters. The words are equivalent.

My dreams matter but do they exist? My imagination also matters, it's a vital part of my consciousness, but if it exists we have to provide a new definition of "exist".

 

[out of whack]

My dreams matter but do they exist? My imagination also matters, it's a vital part of my consciousness, but if it exists we have to provide a new definition of "exist".

Both your dreams and your imagination exist of course. Mental processes are real. They may not be exactly what you think they are, but they exist nonetheless. Likewise, illusions exist. I've seen them. They were not what I thought they were, but they definitely mattered.

We must not confuse what exists with what it is. These are separate questions.

 

[CaptainQuasar]

We must not confuse what exists with what it is. These are separate questions.

Especially to Bill Clinton during the impeachment hearings!

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[out of whack]

Especially to Bill Clinton during the impeachment hearings!

It depends what you mean by "the".

 

[f95toli]

Don't know this effect and so I don't know to what extent it disproves the idea that photons have an effect only when they are being emitted/detected.

This effect is more or less the basis for the whole fied of nonlinear quantum optics. I can't find a good reference now, but e.g. this should give you some idea of what it is about

http://arxiv.org/abs/quant-ph/0310066

Note that the Fock state is not affected (i.e. no photons are destroyed) when the probe field picks up the phase shift.

 

[Primordial]

They must exist between the moment emitted and the moment absorbed, however different their reference system.

 

[CaptainQuasar]

They must exist between the moment emitted and the moment absorbed, however different their reference system.

Note DaleSpam's point:

Out of curiosity, suppose two quanta of EM energy (E1 and E2) are released and later absorbed. Are they always absorbed as E1 and E2, or could they be absorbed as some other E3 and E4 such that E1+E2=E3+E4?

You're saying that simply cannot be so, that if E1 is an emission event it must be the exact same thing E1 which is absorbed? Or would E1 being emitted and E3 absorbed qualify as merely a different reference system?

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[Primordial]

Captain Quasar : Because the emitted photon event occurs in an inertial system and the absorption of the photon also occurs in an inertial system, these inertial reference events may not be the only system that can effect the transfer, an example we know of, is the expansion of the universe, one of the primary causes of redshift between large star systems.

 

[Fredrik]

And doesn't it worry you? If we pretend physics to be different from other philosophies, we should find a better answer to that question.

(The question we're talking about is "What does it mean for something to 'exist'?").

Physics isn't a philosophy, it's a science. I have tried to summarize, as succinctly as I can, what I consider to be the definition of science:

1. A theory is a consistent set of statements that can be used to predict the probability of each possible result of any member of a set of experiments that's specified by the theory.
2. Experiments can tell us how accurately a theory predicts the results of experiments, and nothing more than that.
3. Some, but not all, experimental methods are reliable in the sense that they yield the same results regardless of who uses them. Those methods are called scientific methods.
4. Science is the ongoing process of finding new theories and using scientific methods to find out how accurately the existing theories predict the probabilities mentioned in 1.

Note that I'm saying "predict the probability of each possible result..." rather than "predict the result of...". That choice of words is necessary to make sure that quantum mechanics is considered a theory. What I mean by a "possible result" is a result that we would be able to detect if it happened. For example, a rock falling upwards is a "possible result" of an experiment in which I drop a rock to test the predictions of some theory of gravity.

Also note that I don't require that a theory or any of the mathematical models used in it actually describes reality. As long as we don't have a final theory of everything, a mathematical model is at best a description of a fictional universe that resembles our own. And even if we had a "final" theory that's an exact description of our universe, it would be impossible to prove that it is. It could be equivalent to another theory that uses a very different mathematical model to predict the same probabilities, and there would be no way of knowing that the first theory is the one that describes reality.

Now, where exactly does the notion of "existence" fit into the framework of science defined above? I'm not at all convinced that it's a meaningful concept in science. I want to define "existence" by saying that something exists if it's a concept defined by a mathematical model that's an exact description of the universe, but that doesn't make sense because of what I said in the previous paragraph. So in my opinion, "existence" is an unscientific concept because a) "description" is an unscientific concept, and b) we don't have a final theory yet.

To exist is to matter. What matters exists. What exists matters. The words are equivalent.

I don't think that definition works. Take the gravitational force for example. It certainly matters in Newton's theory of gravity, but in general relativity gravity isn't a force. So the gravitational force only exists in the inferior theory. It seems that your definition only tells us that every concept that's mentioned by a theory exists in that theory. It doesn't tell us anything about what exists in the real world.

Note DaleSpam's point:
You're saying that simply cannot be so, that if E1 is an emission event it must be the exact same thing E1 which is absorbed? Or would E1 being emitted and E3 absorbed qualify as merely a different reference system?

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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.

 

[lightarrow]

Both your dreams and your imagination exist of course. Mental processes are real. They may not be exactly what you think they are, but they exist nonetheless. Likewise, illusions exist. I've seen them. They were not what I thought they were, but they definitely mattered.

Don't know if you noticed, this is a forum of physics, so the term "exist" means "in physics". Furthermore, to avoid confusion, I expressely referred to "physical meaning", "physical object", "in physics" and so on.

 

[WaveJumper]

On the previous page i tried to 'steer' the discussion towards finding a definition of whether there is movement at all in the quantum realm. I believe QM says there is none(at least not in the sense of extrapolating our notion of continuous movement from SR to QM) and i don't think this discussion can move forward without delving into the true nature of "motion" in the quantum world. Here is a good summary of my thoughts, by the the famous physicist, father of the atomic bomb - Robert Oppenheimer":

"If we ask, for instance, whether the position of the electron remains the same, we must say 'no'; if we ask whether the electron's position changes with time, we must say 'no'; if we ask whether the electron is at rest, we must say 'no'; if we ask whether it is in motion, we must say 'no'.


This discussion landed in the Philosophy department for a good reason.

 

[lightarrow]

(The question we're talking about is "What does it mean for something to 'exist'?").

Physics isn't a philosophy, it's a science.

Exactly. If we pretend it to be a science and not a phylosophy, we should give scientific meanings to what it describes. Now the problem is: what is a "scientific meaning"?
To begin, I would propose a question. Which is the difference from:
1. A blue angel with rose wings is released every time light is emitted and disappears every time you try to detect it.
2. There are no blue angels in that context.
?
How would a scientist do to show why he would prefer the second?

Note DaleSpam's point:
You're saying that simply cannot be so, that if E1 is an emission event it must be the exact same thing E1 which is absorbed? Or would E1 being emitted and E3 absorbed qualify as merely a different reference system?

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.

It's not so difficult: the photon's energy depends on the frame of reference. If it has energy E1 in your laboratory at emession event, it has energy E2 different from E1 in a moving laboratory at detection event.

E2 = E1*Sqrt[(c+v)/(c-v)]

if the moving laboratory approaches the photon with speed v (if v > 0, E2 > E1; v is negative if it recedes the photon and in that case E2 < E1).

 

[CaptainQuasar]

Now, where exactly does the notion of "existence" fit into the framework of science defined above?

If you're going to say that existence is not even within the purview of science, then you're of course declaring that science definitely cannot assert that the photon exists between the emission and absorption events. (Perhaps this is what you intended; I can't entirely tell. But declaring the question to be unscientific looks suspiciously like a bit of sophistry to avoid addressing it.)

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.

Right... so... if you don't know the answer to that question, doesn't that mean you don't know whether the entity comprising E1 exists any more at the point of the corresponding absorption event, or whether the absorption event might involve a completely different entity E3? It seemed to me that was the point of his question, to ask if that cannot be.

Because if that can be, if nothing in our body of collected evidence is available to contradict that, it means that we don't know for sure that the photon exists between the emission and absorption events...

Sorry to be asking straightforward philosophical questions about this that aren't diverted by discussing the details of calculations, but you should blame it on whoever decided this ought to be a matter of philosophy rather than in one of the science forums.

P.S. out of whack's definition of "existence" doesn't quite look workable to me either, more because it looks a bit like circular reasoning than anything else. Which is why I presented my own definition, of course.
P.P.S. And science actually is a form of philosophy, anyways - it's an epistemological method, a particular formulation of Empiricism.

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[Fredrik]

On the previous page i tried to 'steer' the discussion towards finding a definition of whether there is movement at all in the quantum realm. I believe QM says there is none(at least not in the sense of extrapolating our notion of continuous movement from SR to QM) and i don't think this discussion can move forward without delving into the true nature of "motion" in the quantum world. Here is a good summary of my thoughts, by the the famous physicist, father of the atomic bomb - Robert Oppenheimer":

"If we ask, for instance, whether the position of the electron remains the same, we must say 'no'; if we ask whether the electron's position changes with time, we must say 'no'; if we ask whether the electron is at rest, we must say 'no'; if we ask whether it is in motion, we must say 'no'.

It's very likely that he was just trying to emphasize that a particle doesn't have a position in the classical sense. The answer to the question "where is the particle right now?" isn't three real numbers, it's a ray in a Hilbert space. That ray also happens to be the answer to any question you can ask about the particle's properties right now.

Since particles don't have positions in the classical sense, they don't move in the classical sense, but that doesn't mean that there's no movement. The closest we can get to classical movement is to have a wave function with a sharp peak at a location that changes with time.

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

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

we should give scientific meanings to what it describes. Now the problem is: what is a "scientific meaning"?

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

To begin, I would propose a question. Which is the difference from:
1. A blue angel with rose wings is released every time light is emitted and disappears every time you try to detect it.
2. There are no blue angels in that context.
?
How would a scientist do to show why he would prefer the second?

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.

It's not so difficult: the photon's energy depends on the frame of reference.

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.

If you're going to say that existence is not even within the purview of science, then you're of course declaring that science definitely cannot assert that the photon exists between the emission and absorption events. (Perhaps this is what you intended; I can't entirely tell. But declaring the question to be unscientific looks suspiciously like a bit of sophistry to avoid addressing it.)

So far I'm the only one in this thread who has made a serious attempt to address the real issue (the meaning of "existence"). My conclusion is that if we use my definition of "existence", the original question is unscientific. That may not be the best definition, but I haven't seen a better one yet.

Right... so... if you don't know the answer to that question, doesn't that mean you don't know whether the entity comprising E1 exists any more at the point of the corresponding absorption event, or whether the absorption event might involve a completely different entity E3?

I don't know how to make sense of that question since I still haven't seen a definition of "exists" that's consistent with science. I also don't think the answer to DaleSpam's question is going to shed any light on the question of whether photons exist. I think the answer to his question is that E3 and E4 are arbitrary except for the constraint E1+E2=E3+E4, but to me that answer doesn't imply anything about "existence".

Which is why I presented my own definition, of course.

I actually didn't see that one until now. Your idea might be a good start, but it's clearly incomplete, and it obviously needs to be stated without references to omniscient beings.

 

[CaptainQuasar]

I actually didn't see that one until now. Your idea might be a good start, but it's clearly incomplete, and it obviously needs to be stated without references to omniscient beings.

Heh heh, okay, sure: either just completely cut out the clause mentioning an omniscient being (which I thought pretty obviously is just a device talking about underlying reality; the involvement of an actual omniscient being isn't a dependency of that definition), or replace it with "Whether a future scientist able through some method to more closely monitor the events occurring in the course of the experiment could discriminate between the photon as a thing and other things, or whether the cause of the absorption event would be something indistinguishable from a greater whole."

This definition is what I'd meant in a previous comment by talking about whether the photon is "a definite entity discrete from the rest of the universe." (Which I realize you may not have seen either.)

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[out of whack]

To exist is to matter. What matters exists. What exists matters. The words are equivalent.

I don't think that definition works. Take the gravitational force for example. It certainly matters in Newton's theory of gravity, but in general relativity gravity isn't a force.

You think there is a problem because you are making a conversion error. First you speak of gravitational force, which matters and exists according to Newton's theory. Then you omit the force and speak of gravity, which matters and exists according to relativity. 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.

So the gravitational force only exists in the inferior theory.

Or more correctly, the gravitational force is said to exist (and to matter) according to this theory.

It seems that your definition only tells us that every concept that's mentioned by a theory exists in that theory. It doesn't tell us anything about what exists in the real world.

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.

I underline the equivalence between "to exist" and "to matter" in light of the original post of this thread where it is noted that abuse of the English language can be a problem. I see this frequently on this board, words being used in a debate until someone eventually asks "what do you mean by that word?" A greater sin of bad communication is to use a word while at the same time claiming that we don't even know what the word actually means!

Langauge does not arise by creating random words and then trying to attach meaning to them. On the contrary, first we conceive of something that we wish to communicate; then we create a word for it. This must also be true for the word "exist". Why would the word even be created if not for something that matters in some way?

Both your dreams and your imagination exist of course. Mental processes are real. They may not be exactly what you think they are, but they exist nonetheless. Likewise, illusions exist. I've seen them. They were not what I thought they were, but they definitely mattered.

Don't know if you noticed, this is a forum of physics, so the term "exist" means "in physics". Furthermore, to avoid confusion, I expressely referred to "physical meaning", "physical object", "in physics" and so on.

Uh, I did notice, thank you. Everything that exists is subject to physical study. Mental processes are not excluded from this. There is no confusion here. If any item of a physical nature exists then this physical item matters from a physical point of view, and vice versa. There is no basis to claim that a physical item exists and yet has no physical relevance to anything. Neither is there any basis to claim that a phycical item is relevant when it does not even exist.

 

[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.
...
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?