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

[DrGreg]

"Photons" only exist at the moment they are emitted or absorbed i.e. when they interact with matter. There is no evidence ( nor any way of getting any ) that photons exist in the EM field when it is not interacting with matter.

Speculations about 'free' photons usually lead to apparent contradictions, as evidenced by your question.

This remarkable claim was made over in the "Special & General Relativity" forum of this site, in post #3 of this thread[/color], 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.)

 

[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.

 

[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)

 

[enotstrebor]

This remarkable claim was made over in the "Special & General Relativity" forum of this site, in post #3 of this thread[/color], 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.

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.

 

[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?

 

[Fredrik]

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.

 

[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,

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

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 !

 

[lightarrow]

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.

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?

 

[lightarrow]

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.

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

 

[lightarrow]

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?

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.

 

[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. http://www.runemasterstudios.com/graemlins/images/whaat.gif

⚛​

 

[Fredrik]

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?

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.

 

[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.

 

[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.

 

[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

https://www.physicsforums.com/showthread.php?t=130365&page=2

 

[lightarrow]

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.

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.

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.

About this I agree with you: saying that photons always exist from source to detector, it's not even a theory, just an idea...

 

[Fredrik]

About this I agree with you: saying that photons always exist from source to detector, it's not even a theory, just an idea...

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?

 

[CaptainQuasar]

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

⚛​

 

[lightarrow]

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.

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.

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.

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?

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?

For what concerns me, the latter.

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?

Because it doesn't disappear after measuring it.

 

[Fredrik]

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.

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".

For what concerns me, the latter.

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".

Because it doesn't disappear after measuring it.

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

 

[Borek]

I wonder - if they don't exist, how can they be bent by gravitational field?

 

[Jonathan Scott]

I wonder - if they don't exist, how can they be bent by gravitational field?

If it's not photons, it's electromagnetic waves.

No-one is claiming that nothing travels from where a photon is emitted to where it is absorbed. Clearly there is an electromagnetic wave carrying some energy. In most cases, the clumps of energy can be seen to correspond to single photons, and contain quantum properties that were associated with the emission event. You can also "bounce" photons off things as if they were particles (as in the Compton effect).

However, what I've heard (probably from a simplified version of QED) is that although electromagnetic energy interacts as quantized photons, in flight it is not strictly divided up into individual photons, so if two or more photons are involved in some interference or similar, although the number being absorbed may overall be the same as the number emitted, a given absorbed photon may not necessarily correspond to a single emitted photon, but may instead be effectively be a sum of components derived from more than one photon.

 

[lightarrow]

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.

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".

Ok, but I asked you to find, for me, a better definition of the concept I expressed in my post N.16

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".

Maybe: "is THE photomultiplier click"

Because it doesn't disappear after measuring it.

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

Have you ever seen a photon? Sent light to it which has then bounced back? How would you define the Physical Object "photon"? Where in the definition is written that you can localize it in space between source and detector? Does a position operator exist for a photon? Which is a photon's shape? And its dimensions?

 

[Fredrik]

Ok, but I asked you to find, for me, a better definition of the concept I expressed in my post N.16

How about just saying that we always prefer the theory that does a better job of predicting the results of experiments?

Maybe: "is THE photomultiplier click"

Hm...to me that's the "detection event", not the "real-world photon". The actual "click" even occurs some time after the absorption of the photon, but at least it's part of an interaction that involves a photon. A bigger problem with your definition is that if you define a real-world photon to be the detection event, then the claim that a real-world photon doesn't exist before detection doesn't contain any information. It's true by definition of the word.

Have you ever seen a photon? Sent light to it which has then bounced back? How would you define the Physical Object "photon"? Where in the definition is written that you can localize it in space between source and detector? Does a position operator exist for a photon? Which is a photon's shape? And its dimensions?

Those are all ways in which a photon is different from a chair. But I'm not so sure that any of them is a reason to say that chairs exist and photons don't. It seems so much more relevant that there's a theory involving photons (the standard model) that predicts the results of a very wide range of experiments better than any theory that doesn't involve photons. It even explains chairs. (It explains why it's possible for chairs to exist, not why they do exist).

 

[lightarrow]

How about just saying that we always prefer the theory that does a better job of predicting the results of experiments?

Ok. Then, given the experimental fact that you have never measured a photon between source A and detector B, which of the following theories would you prefer:

1. Photon exist between A and B
2. Photon doesn't exist between A and B

Hm...to me that's the "detection event", not the "real-world photon". The actual "click" even occurs some time after the absorption of the photon, but at least it's part of an interaction that involves a photon. A bigger problem with your definition is that if you define a real-world photon to be the detection event, then the claim that a real-world photon doesn't exist before detection doesn't contain any information. It's true by definition of the word.

Certainly. But, IMO, physicists should give precise physical meanings to the objects they discuss, or, at least, specify the classes of objects for which such precise physical meanings are given and those for which such precise physical meanings are not given...
It's not so easy to find a book (and I personally don't have seen any) where it's written which is the precise physical meaning of "photon", that's what makes me angry. I don't say to have the answer, but that my "invented answer" is better than no answer at all. To me, it's quite unconceivable that people works with photons from almost a century without having a precise physical meaning of them!

 

[out of whack]

To exist means to be significant is some way, it means to matter to anyone or to anything, anywhere at any time under any condition. If it matters then it exists. If it does not exist then it does not matter.

The equivalence of "what exists" and "what matters" is underlined by two observations based on the following tautology: we are concerned with what matters and what matters concerns us. First, it would be meaningless to say that something matters but is said not to exist because this case would be indistinguishable to our concerns from something that matters and actually exists. Second, it is also meaningless to say that something does not matter but is said to exist because this case is indistinguishable from something that does not matter and does not actually exist. The equivalence is natural as long as we are concerned with what matters and unconcerned with what doesn't matter; clearly, any other stance is useless for all purposes.

So the question is this: in what way does a photon matter between its emission and its absorption? If this can be described then photons exist in transit. If we can show that a photon isn't relevant to anything between these events then photons do not exist in transit.

 

[f95toli]

So the question is this: in what way does a photon matter between its emission and its absorption?

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.
Hence, the idea that photons can only have an effect when they are being emitted/detected is simply incorrect.

 

[Dale]

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?

 

[Fredrik]

Ok. Then, given the experimental fact that you have never measured a photon between source A and detector B, which of the following theories would you prefer:

1. Photon exist between A and B
2. Photon doesn't exist between A and B

I don't know because I don't know what those phrases mean. I still don't know what it means for something to "exist".

 

[lightarrow]

I don't know because I don't know what those phrases mean. I still don't know what it means for something to "exist".

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.

 

[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.

⚛​

 

[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!

⚛​

 

[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?

⚛​

 

[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?

​

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.