When is a photon a photon?

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When is the term "photon" valid?

This question seems to have taken on different forms in a separate post:
a) Do photons exist in empty space (vacuum)?
b) Do photons exist in the absence of interaction (with matter)?
c) Are photons called photons only when they are exhibiting particle-like behavior?
d) Are electromagnetic waves photons?
e) ...

There may be subtleties to these questions, but lets try to cover the straightforward answers first.

Please review marlon's and ZapperZ's FAQ topic "Is light a particle or a wave"
 

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  • #2
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My take is that photons exist continuously between emission and absorption/annihilation regardless of interaction with matter, else statements like "photons travel (in a vacuum) at the speed of light, c" would have no meaning.
 
  • #3
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My take is that photons exist continuously between emission and absorption/annihilation regardless of interaction with matter, else statements like "photons travel (in a vacuum) at the speed of light, c" would have no meaning.
Furthermore, quantum electrodynamics, quantizing the electromagnetic field, assumes photons exist whenever the em field exists, so, by definition, even between source and detector. However, since, in my opinion, we should always talk about measurable things, in physics, then I have doubts about the physical meaning of photons between source and detector, but it's only my opinion of course.
 
  • #4
ZapperZ
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d) Are electromagnetic waves photons?

Are water waves H2O molecules?

Zz.
 
  • #5
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measurement alters what's being observed

I have doubts about the physical meaning of photons between source and detector.
Cool. What if I turn the statement around and have doubts about the physical meaning of photons at the source and detector instead?

As an analogy, nitroglycerin might really be described with a calm image of an oily liquid. However the imagery and measurements that we get upon arrival at the (clumsy) detector is completely different because the original item is no longer nitro, but dramatically transforms into spent fuel because of the detector.

If we look at the endpoints, aren't we really often measuring the destruction of a photon rather than the photon itself?
 
  • #6
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Cool. What if I turn the statement around and have doubts about the physical meaning of photons at the source and detector instead?

As an analogy, nitroglycerin might really be described with a calm image of an oily liquid. However the imagery and measurements that we get upon arrival at the (clumsy) detector is completely different because the original item is no longer nitro, but dramatically transforms into spent fuel because of the detector.

If we look at the endpoints, aren't we really often measuring the destruction of a photon rather than the photon itself?

You need to look at the issue surrounding Bell-type experiments and "local realism" (or lack thereof) within the context of QM.

Furthermore, everything that you know of, really, is based on what you measured/detected/observed. It isn't restricted only to "photons".

Zz.
 
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Are water waves H2O molecules?
QUOTE]

Thanks, how about:
Photons aren't the electromagnetic waves themselves, but photons are responsible for electromagnetic phenomena.​
 
  • #8
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Help?
You need to look at the issue surrounding Bell-type experiments and "local realism" (or lack thereof) within the context of QM.
Are you suggesting to ponder a sort of "undefined-ness" before detection as opposed to unknown?

Furthermore, everything that you know of, really, is based on what you measured/detected/observed. It isn't restricted only to "photons".
Okay, knowledge vs theory.
Our typical observations of a grain of sand are gentle enough not to alter the grain all that much. On the other hand if we are in a dark room detecting nitro with a swinging hammer we're more likely to detect an explosion than to detect an oily liquid. The hammer, when it doesn't detect nitro tells us something, about say the limited size of the nitro in its stable state. When the hammer does detect nitro, it gives us a very different image that we shouldn't extrapolate backwards as the interpretation of the stable state.
 
  • #9
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Cool. What if I turn the statement around and have doubts about the physical meaning of photons at the source and detector instead?
The photon represents the quantized interaction between EM field and matter, so it's at least present at source and detector by definition.
As an analogy, nitroglycerin might really be described with a calm image of an oily liquid. However the imagery and measurements that we get upon arrival at the (clumsy) detector is completely different because the original item is no longer nitro, but dramatically transforms into spent fuel because of the detector.

If we look at the endpoints, aren't we really often measuring the destruction of a photon rather than the photon itself?
But you *are* able to detect nitroglicerin oil with the appropriate instruments; with photons it's all another story because you are'nt even theoretically able to detect "the original item". Can you see the difference? :smile:
 
  • #10
ZapperZ
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Okay, knowledge vs theory.

No such thing. This isn't about knowledge versus theory, because "theory and knowledge", at least valid ones, are based on what we can verify emprically!

Our typical observations of a grain of sand are gentle enough not to alter the grain all that much. On the other hand if we are in a dark room detecting nitro with a swinging hammer we're more likely to detect an explosion than to detect an oily liquid. The hammer, when it doesn't detect nitro tells us something, about say the limited size of the nitro in its stable state. When the hammer does detect nitro, it gives us a very different image that we shouldn't extrapolate backwards as the interpretation of the stable state.

You have missed the point. Everything that you think you know is based on your knowledge of a set of properties and characteristics of that entity. Think about it. These properties and characteristics are based on what you have measured/observed/detected of these entities. It has nothing to do with what you just described here.

Zz.
 
  • #11
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The photon represents the quantized interaction between EM field and matter, so it's at least present at source and detector by definition.

I'm a little confused. The above statement on its own would lead me to think that the photon is defined only during the interaction. I don't think that's what you are trying to say though.
 
  • #12
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But you *are* able to detect nitroglicerin oil with the appropriate instruments; with photons it's all another story because you are'nt even theoretically able to detect "the original item". Can you see the difference? :smile:

I'm not sure I'm catching the nuances of your statement. In some sense, it's like all of our instruments are like hammers to the photon, no?

If we aren't able to even theoretically detect the original item, which is my point too, then aren't we on the same page when I try to represent that the "pure" thing really only exists when we, or matter itself, aren't tampering/interacting with it? I'm pretty sure I'm not representing your thoughts, but I'm not sure what's different still.
 
  • #13
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You have missed the point. Everything that you think you know is based on your knowledge of a set of properties and characteristics of that entity.

I'm not sure how I missed the point Zz. I hope it didn't sound sarcastic.

From where I stand, a theory isn't just a list of measured facts, but it's reasoning that fills in the blanks to explanation of facts. There can be multiple good and competing theories supporting the same verifiable facts. In this sense, knowlege and fact are different than theory, no?

I thought you and lightarrow were emphasizing that technically, the only facts we have are measurements, and whatever else goes on between the measurements was sort of in a black box that we could only technically theorize about.

Thinking that I got your point, I started to draw out that the measurement of the facts was a two-edged sword--it alters the thing we are measuring and it can be difficult to subract out the effects of the measurement in order to create a theory of what happens in the black box.

Not sure where the disconnect is.
 
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  • #14
ZapperZ
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I'm not sure how I missed the point Zz. I hope it didn't sound sarcastic.

From where I stand, a theory isn't just a list of measured facts, but it's reasoning that fills in the blanks to explanation of facts. There can be multiple good and competing theories supporting the same verifiable facts. In this sense, knowlege and fact are different than theory, no?

Illustrate this with an example. For instance, how are Maxwell equations a "reasoning"?

Zz.
 
  • #15
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Illustrate this with an example. For instance, how are Maxwell equations a "reasoning"?

How about this: Maxwell made mathematical reasoning to modify Ampere's law. Maxwell's equations and Newton's mechanics both agreed with observations made at the time. Maxwell's reasoning, and not necessarily measured observation, filled in some blanks that were unobservable at the time. The blend of facts and reasoning made predicitions whose facts could only be measured later.

Sound okay?
 
  • #16
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How about this: Maxwell made mathematical reasoning to modify Ampere's law. Maxwell's equations and Newton's mechanics both agreed with observations made at the time. Maxwell's reasoning, and not necessarily measured observation, filled in some blanks that were unobservable at the time. The blend of facts and reasoning made predicitions whose facts could only be measured later.

Sound okay?

No, because none of what you said can be backed up. Maxwell equations really are nothing more than a set of mathematical description. It "reasoned" nothing! In fact, one can easily describe them as phenomenological, which means that to put it crudely, it is nothing more than a well-defined description of empirical observations. It says nothing on why the E and B fields are that way, i.e. no reason!

Again, show a theory that actually satisfy what you are claiming that it can do.

Zz.
 
  • #17
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No, because none of what you said can be backed up. Maxwell equations really are nothing more than a set of mathematical description. It "reasoned" nothing! In fact, one can easily describe them as phenomenological, which means that to put it crudely, it is nothing more than a well-defined description of empirical observations. It says nothing on why the E and B fields are that way, i.e. no reason!

Again, show a theory that actually satisfy what you are claiming that it can do.

Zz.

I thought your point was that we know nothing more than what we observe.

The mathematical descriptions are not the same as empirical observations. Empirical observations wouldn't generally use an "equals" sign, but would put bounds on the experimental error. As we get better measuring techniques, we often recheck basic theories for any undiscovered news that the mathematical descriptions didn't show.

We can take mathematical descriptions, combine them and conjure up physical situations that have never before been observed. In this case, it isn't the reason why things happen, but it is still reasoning.

Maybe an example could be that a ...body stays in motion... We haven't actually observed this "law" to be true--nobody has observed a body for an infinitely long time, but it seems reasonable enough to use in our equations.

Are we still on different pages? I do want to understand the photon better, so...
 
  • #18
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Our set of observations allows us to formulate (i.e. put in mathematics) how a system will behave under the same condition wihtout having to redo the whole experiment. What's what a phenomenology is - to describe the behavior of a system. It "reasoned" nothing the way you claim a theory would do.

Newton's first law is a description of a behavior of an object under (or without) a force. I could say that no one as verified that it will work for all forces over an infinite amount of time, yet, we know the description has worked so far (your house was built using that premise). But did this "reasoned" anythiing? Look at the original premise that you made that I objected to.

If you want to understand photons better, learn QM and QED, not by claiming something a theory isn't.

Zz.
 
  • #19
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[...]It "reasoned" nothing the way you claim a theory would do.
[...] But did this "reasoned" anythiing? Look at the original premise that you made that I objected to.

If you want to understand photons better, learn QM and QED, not by claiming something a theory isn't.

Nobody here was trying to make claims about the definition of the word "theory" as a tool for understanding photons. That doesn't make sense to me. Additionally, statements like "It has nothing to do with ...", and "...none of what you said..., and "...nothing the way you..." are a bit suspiscious.

I'll try to stick with the dictionary definition of theory and hope you will too. Note that there are differing degrees of certainty to the word theory ranging from simple conjecture to scientific theory whose statements are well-backed by data and which are generally accepted. Many definitions will use the word "explain" in the definition. I may have used the word "reason". I'd almost bet that we agree that there are limits to what is meant by "explain" or "reason" in this context.

Not sure what premise you were talking about. Was it that measurement interferes with the subject being measured? I thought we agreed on that.
 
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  • #20
ZapperZ
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Nobody here was trying to make claims about the definition of the word "theory" as a tool for understanding photons. That doesn't make sense to me. Additionally, statements like "It has nothing to do with ...", and "...none of what you said..., and "...nothing the way you..." are a bit suspiscious.

I'll try to stick with the dictionary definition of theory and hope you will too. Note that there are differing degrees of certainty to the word theory ranging from simple conjecture to scientific theory whose statements are well-backed by data and which are generally accepted. Many definitions will use the word "explain" in the definition. I'd almost bet that we agree that there are limits to what is meant by "explain".

Here's what you said earlier:

From where I stand, a theory isn't just a list of measured facts, but it's reasoning that fills in the blanks to explanation of facts. There can be multiple good and competing theories supporting the same verifiable facts. In this sense, knowlege and fact are different than theory, no?

You have continued to neglect to illustrate where such "reasoning" occurs. A valid theory does a lot of describing. I've given Maxwell Equations as an example, which you appears to have completely misunderstood. Did Gauss's law "reasoned" something? Or does it simply "describe" how the E-field looks like, given a charge distribution. Tell me where it "reasoned" it. The same could be said with Quantum Mechanics. Does it "reasoned" with you which outcome you would measure in a superposition of states? Or does it simply tell you all the possible outcome you would measure?

Your idea of what "knowledge" is is also strange. A set of facts or observation does not make a knowledge. That's stamp-collecting. It is the theory that provides a "frame" to understand how these facts and observations are connected together. What separates phenomenological theory versus a well-defined theory is that the latter could derive the former via First Principles. So a valid, well-defined theory IS, for all practical purposes, knowledge!

This thread is diverging into philosophy and anymore of this will probably push it into that forum.

Zz.
 
  • #21
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I found two paragraphs on wikipedia that I hope leads to mutual understanding:

According to Stephen Hawking in A Brief History of Time, "a theory is a good theory if it satisfies two requirements: It must accurately describe a large class of observations on the basis of a model which contains only a few arbitrary elements, and it must make definite predictions about the results of future observations". He goes on to state, "any physical theory is always provisional, in the sense that it is only a hypothesis; you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory. On the other hand, you can disprove a theory by finding even a single observation which disagrees with the predictions of the theory".​


An example of how theories are models can be seen from theories on the planetary system. The Greeks formulated theories which were recorded by the astronomer Ptolemy. In Ptolemy's planetary model, the earth was at the center, the planets and the sun made circular orbits around the earth, and the stars were on a sphere outside of the orbits of the planet and the earth. Retrograde motion of the planets was explained by smaller circular orbits of individual planets. This could be illustrated as a model, and could even be built into a literal model. Mathematical calculations could be made which predicted, to a great degree of accuracy, where the planets would be. His model of the planetary system survived for over 1500 years until the time of Copernicus. So one can see that a theory is a "model of reality," one which explains certain scientific facts; yet the theory may not be a satisfactory picture of reality. Another, more acceptable, theory can later replace the previous model, as when the Copernican theory replaced the Ptolemaic theory. Or a new theory can be used to modify an older theory as when Einstein modified Newtonian mechanics (which is still used for designing bridges and gasoline engines) with his theories of relativity.​
 
  • #22
ZapperZ
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Why would anything of Wikipedia be any better? I mean, I could come in and edit that to suit my needs and interpretation.

Long-time members of this forum know better than to use Wikipedia when having a discussion with me. I'm never impressed by such source.

Zz.
 
  • #23
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Why would anything of Wikipedia be any better?
Honestly, I think you are intentionally twisting words. Quoting wikipedia stands a chance that the authors have had time to avoid verbal landmines.

As an example, my paraphrasing of the definition of the word "theory" included the word "reason" where other definitions may have used the word "explain". You seem to have relentlessly pounced on that even after I invited correction by refering to the dictionary and specifically downplayed the word "reason" in your behalf.
As an other example, you criticized the strangeness of my definition of the word "knowledge" when all I said was that it differed from theory. That isn't strange.
I mean, I could come in and edit that to suit my needs and interpretation.
Agreed, but that's not the issue here, it's the verbal landmine. I'm not giving you a perfect score on the usage of words either, but I hope there are future possibilities of communication. I enjoyed your FAQ entries and hope some of those ideas in your head can make it past the language barrier.
 
  • #24
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Back to physics?
I thought I sensed a reluctance by some of the posters to call a photon a photon outside of the points of emission and absorption. It seemed that the only description given between these points was an e/m wave. It almost seems as though some, but not all, posters didn't consider it as quantized energy between these points.

Here's the question: is there any difference between how we should be envisioning the photon at the emission/absorption point as opposed to the photon in empty space, or should we envision it as being exactly the same throughout its entire existence? The FAQ suggests to me that we envision it as exactly the same throughout, and that we should probably abandon the vision of switching between a traditional particle and wave, but its own thing.
 
  • #25
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Back to physics?
I thought I sensed a reluctance by some of the posters to call a photon a photon outside of the points of emission and absorption. It seemed that the only description given between these points was an e/m wave. It almost seems as though some, but not all, posters didn't consider it as quantized energy between these points.

Here's the question: is there any difference between how we should be envisioning the photon at the emission/absorption point as opposed to the photon in empty space, or should we envision it as being exactly the same throughout its entire existence? The FAQ suggests to me that we envision it as exactly the same throughout, and that we should probably abandon the vision of switching between a traditional particle and wave, but its own thing.
How do you define "photon"? If we don't agree on it we can't discuss about it. Please note that we are talking about physics, so your definition should refer to a *physical* entity.
 
  • #26
I never quite understood the concept of photons "travelling" through space...or more accurately, existing at any point between emission and absorption.

We see a beam of light because the medium that exists between emission and absorption "scatters" some of the photons causing the absorption point to be our eyes instead of the object at the end of the beam for those few photons, giving us the impression that there is a torrent of energy coursing through space. We place lenses/prisms/etc... in the beam and watch the beam change its shape and/or direction, enforcing that impression. Those lenses/prisms/etc... may even get hot because the atoms they are made of absorb some of the photons, even further enforcing the impression that some powerful beam of energy is coursing through it.

But relativity states that for anything travelling at the "speed" of light, space would have to contract to a singularity in the direction of travel. I don't pretend to know what it's like to experience the knowable universe as a singularity, as I assume a photon must since it obviously "travels" at the speed of light, but I assume that in a singularity, all possible points along the path of the thing doing the travelling must overlap, that is, in some way, every place a photon can go (its knowable universe) exists in one place to the photon (even the term "place" would be meaningless in a singularity). I also assume that from relativity we can deduce that from the photons perspective, it's "journey" would be instantaneous (time dilation), which makes sense since if all possible point along its path overlap, it shouldn't take any time for it to get where it's going. So does it necessarily have to actually exist between emission and absorption? Is it possible that what we interpret as the wave-function is actually just an interpretation we make because we don't quite get what it's like inside a singularity. Could it be that the concept of existing in spacetime to a photon is as meaningless as the concept of existing in a singularity is to us?

I'm just having fun (I like to bend my mind around crap like this), but when thinking about relativity, I can't help but intuit that the photon's target is actually chosen the instant it is emitted...that an atom absorbing a photon has as much a causal effect on the atom emitting it (no matter how far back in time) as the emitting atom has on the absorber. Putting things like lenses in the path certainly affects the probabilities of where the photon can be absorbed, but who knows how a lens can be said to affect a photon when it exists as a part of a singularity? Does the photon actually have to exist between emission and absorption?
 
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  • #27
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...I also assume that from relativity we can deduce that from the photons perspective, it's "journey" would be instantaneous (time dilation)
...Is it possible that what we interpret as the wave-function is actually just an interpretation we make because we don't quite get what it's like inside a singularity...
...I'm just having fun (I like to bend my mind around crap like this)

I'm relating to this bending thing. Seems like we have to let our imaginations explore a lot of ground to help gel facts together. Have others had success with this "out"?
 
  • #28
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How do you define "photon"? If we don't agree on it we can't discuss about it. Please note that we are talking about physics, so your definition should refer to a *physical* entity.

Good point lightarrow. The American Heritage Science Dictionary:
The subatomic particle that carries the electromagnetic force and is the quantum of electromagnetic radiation. The photon has a rest mass of zero, but has measurable momentum, exhibits deflection by a gravitational field, and can exert a force. It has no electric charge, has an indefinitely long lifetime, and is its own antiparticle.​

The word "particle" in this definition seems to trip a lot of people up including myself, which is one reason why I wanted to involve the FAQ's commentary on that word.
Please note that we are talking about physics, so your definition should refer to a *physical* entity.

Physics speaks of energy, waves, fields, force, etc., not just nuggets of matter as I think you'll agree, so I'm not sure what you meant by restricting to a physical entity. According to the above, the photon "is" the quantum of electromagnetic radiation.
 
  • #29
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I think I'm finding genneth's comment helpful.

As a rule: physics can't ever say what something *is*. It can only say what something *does*. However, we all like to have consistent mental models which does what the maths say, and a photon is one such model.​

"When is a photon a photon?" is a different type of question than "When does a photon cause certain effects?" The second question seems more obviously a question of physics, but the first I think is also explored by saying that the photon has an indefinitely long lifetime. Yes, no?
 
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  • #30
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I believe that photons are universal in that they are created or absorbed by all observations - and that otherwise, physics involves photons as mass-energy, information, measurement, geodesics, quanta or light.
 
  • #31
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The American Heritage Science Dictionary:
The subatomic particle that carries the electromagnetic force and is the quantum of electromagnetic radiation. The photon has a rest mass of zero, but has measurable momentum, exhibits deflection by a gravitational field, and can exert a force. It has no electric charge, has an indefinitely long lifetime, and is its own antiparticle.​
The word "particle" in this definition seems to trip a lot of people up including myself, which is one reason why I wanted to involve the FAQ's commentary on that word.
Infact the word "particle" doesn't necessarily imply "moving corpuscle", in my opinion. Furthermore, the fact a photon "exhibits deflection by a gravitational field" it's just a speculation, made on the fact that light is really gravitationally deflected but on the assumption that light is made of moving corpuscles which should be the photons.
Physics speaks of energy, waves, fields, force, etc., not just nuggets of matter as I think you'll agree, so I'm not sure what you meant by restricting to a physical entity. According to the above, the photon "is" the quantum of electromagnetic radiation.
I intended to refer to the fact that physics studies measurable entities (directly or through their properties) and not something else. So, this given, what does "the quantum of electromagnetic radiation" mean, in your opinion?
 
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  • #32
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the word "particle" doesn't necessarily imply "moving corpuscle", in my opinion.
Okay. Among the options are propogation and some other fun ideas. Do we agree though, that physicists speak of photons at least changing locations at speed c? This may be tricky since it'd be hard to prove sameness, but maybe that's where the statement about indefinitely long lifetime comes in. I think sameness from point A to point B is assumed. Am I on your wavelength?
what does "the quantum of electromagnetic radiation" mean, in your opinion?
Uhm, errh... I think it refers to the idea of the e/m , or its energy, being sort of packaged, and not loosely continuous, giving an all-or-nothing proposition--you get the whole quantity of energy or none of it. Beyond that, for me, there seems to be freedom in meaning, but without mass.
 
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  • #33
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the word "particle" doesn't necessarily imply "moving corpuscle", in my opinion.
Okay. Among the options are propogation and some other fun ideas. Do we agree though, that physicists speak of photons at least changing locations at speed c? This may be tricky since it'd be hard to prove sameness, but maybe that's where the statement about indefinitely long lifetime comes in. I think sameness from point A to point B is assumed. Am I on your wavelength?
It indeed seems physicists talk about photons in those terms, yes. For what I understand, however, we can only say: "a photon was generated in A, after some time a photon is detected in B"; what happens in between is not clear to me.
what does "the quantum of electromagnetic radiation" mean, in your opinion?
Uhm, errh... I think it refers to the idea of the e/m , or its energy, being sort of packaged, and not loosely continuous, giving an all-or-nothing proposition--you get the whole quantity of energy or none of it. Beyond that, for me, there seems to be freedom in meaning, but without mass.
Yes, but how do you relate this concept to something measurable? Which is the quantization that you can measure? That of the EM field or that of the interaction between EM field and matter?
 
  • #34
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It indeed seems physicists talk about photons in those terms, yes. For what I understand, however, we can only say: "a photon was generated in A, after some time a photon is detected in B"; what happens in between is not clear to me.?
Okay, it sounds like physicists speak in terms of a sort of travel, but you are leaving the possibilities between A and B wide open. Is that the jist?
Yes, but how do you relate this concept to something measurable? Which is the quantization that you can measure? That of the EM field or that of the interaction between EM field and matter?
I think your point is that we need an interaction with something to give us information. I'll go along with that.
I might point out that the interaction can also be with the photon's antiparticle, and it can be an interaction followed by a prolonged period of time "reacting" with empty space. So we at least have some clues/measurements that go beyond interaction with traditional matter alone.
I'm thinking I'm getting and accepting your point. I guess you're emphasizing that we only know what we measure, and I'm thinking that the measurements are only clues to what the pure thing is between the measurements. I don't necessarily see a conflict in the ideas so far.
 
  • #35
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It indeed seems physicists talk about photons in those terms, yes. For what I understand, however, we can only say: "a photon was generated in A, after some time a photon is detected in B"; what happens in between is not clear to me.
Okay, it sounds like physicists speak in terms of a sort of travel, but you are leaving the possibilities between A and B wide open. Is that the jist?
Sorry, don't understand "jist".
I think your point is that we need an interaction with something to give us information. I'll go along with that.
I might point out that the interaction can also be with the photon's antiparticle, and it can be an interaction followed by a prolonged period of time "reacting" with empty space. So we at least have some clues/measurements that go beyond interaction with traditional matter alone.
I'm lost, here.
I'm thinking I'm getting and accepting your point. I guess you're emphasizing that we only know what we measure, and I'm thinking that the measurements are only clues to what the pure thing is between the measurements. I don't necessarily see a conflict in the ideas so far.
Mmmh, I would talk about "clues to what the pure thing is between the measurements" if we could analyze the "pure thing" properties through another kind of measurement, otherwise, how would you prove that such a "pure thing" really exist and it's not a mere speculation?
 

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