How big is a photon and what does it look and behave like?

[quantumdude]

I admit that Ohanian is confusing to me because he reveals multiple theories in his book so it's hard for me to say exactly how he interprets things.

Ohanian used to be a professor at my school, RPI. He left before I arrived, but my professors knew him well. He has a reputation of being a razor-sharp thinker. I doubt that there is any ambiguity in his remarks. If you post direct quotes here rather than paraphrases, I think you will make more progress.

And it's not clear what is mainstream physics.

It is to those who have studied it. Do you see a pattern emerging here?

It's probablly better to discuss this just on the ideas rather.

The theories are the ideas. Everything else is an analogy that merely approximates the ideas.

You said for a photon of frequency ... I take the word "a" to mean a single photon. If the photon is not a vibrating entity, then if one is talking about a single photon, what does the frequency relate to?

The photon frequency $\nu$ is the frequency of oscillation of the EM field that would result if there were a large number of those photons.

 

[Sherlock]

That said, there is no reason why someone who understands what they're talking about can't explain the concept in ordinary language. In the event that one or two specialized definitions are required, then state them and get on with it.

Your efforts are appreciated, even if somewhat misguided. All of us would like to have some picture of what it is that corresponds to a photon out there in the 3D physical space between emitters and detectors.

Unfortunately, there is no such picture. The concept of a photon is a mathematical concept with no analogy to ordinary experience, and therefore can't be communicated using ordinary language.

The only unambiguous, working definition of a photon is the QED one -- and wrt this definition, as pointed out by the mentors et al., the OP's question is a nonsensical one.

Whether an unambiguous, ordinary language understanding of light quanta is even, eventually, possible given the principles of quantum theory, I don't know. I like to think that after a century, or a few, and thousands more various experiments involving photons, that some day there will be a picture of sorts and a way to talk about photons to people not well-versed in the math of qm. But (and maybe the mentors can offer something on this) that could be nothing more than just wishful thinking.

 

[marlon]

The photon frequency $\nu$ is the frequency of oscillation of the EM field that would result if there were a large number of those photons.

Tom,

This is not entirely correct though. It is indeed true that the frequency associated with a photon corresponds to the frequency of the EM field. This statement is very general but the point should be clear. However, it's the "if there were a large number of those photons"-part that is not really correct.

Suppose you have a classical mode of the EM field with frequency f. After quantization of the EM-field, one can couple this f to the energy of the QM EM field via $$E=h(n+ \frac{1}{2})f$$. This formula expresses that one classical state is expressed by n QM states. The n indeed denotes the number of photons. My point is that n can be chose as 1. In the QM state we have one photon which corresponds to one classical mode of the EM field with frequency f. In general, if there are m classical modes, then the set of integers $$n_1...n_m$$ fully describes the QM state. Each integer $$n_i$$ corresponds to the number of photons. You see ?

Another thing. Your post makes it look like you want to say that many photons make up the EM field. I don't know if it is your message, but i hope not because it is wrong. It is very dangerous (not to say incorrect)to say this for several reasons:

1) the only direct connection between the classical EM field and the quantummechanical concept "photon" is the one i gave above, nothing else. What you say about "the number of present photons" is irrelevant.

2) the most complete picture : QED, clearly states how ONE single photon arises from the quantisized EM-field. I have pointed this out many times in this thread. A photon arises due to fluctuations of the quantisized EM-field. So, if you want to be talking about the relation between "fields" and photons, you should have said this.

Hope this clarifies

regards
marlon

 

[Boffin]

That's an "easy" one. Let me first tell you what a photon does NOT look like. A photon is NOT defined as a particle in the sense of "some physical entity with finite spatial boundaries, like for example a tennis ball". A photon is defined as a quantum of energy. The epitheton "quantum" in QM does not refer to particles but to little bits of energy. That is how QM was developed. So, you cannot ask what a photon "looks like" because we are not defining a photon in a spatial base but in an energy base. Besides, elementary particles cannot be distinguished from each other, but that's a whole other story... (how does a photon) behave ..GO STUDY QED.

I do read the messages in this thread. I've just read them from start to finish a few times again. It's tough to keep up, there are different flavors, and I'm just following your flavor Marlon from the start. To understand any concept one must start from big picture to small.

I take it you are saying that QED is the correct view of how electromagnetic energy behaves. Unfortunately most Physics textbooks and undergraduate physics courses don't get into QED in any major way, or not at all, they present electromagnetic theory in classical or semi classical terms. I'd read many articles that mention QED as a very speculative theory in some aspects, so how was I to know that even mainstream physics views this as the correct model? I really do have some questions about QED being a totally accurate theory in every way, but how much of typical textbook undergraduate electromagnetic theory descriptions are actually replaced by QED, making those typical textbook undergraduate electromagnetic theory descriptions obsolete, or largely obsolete, and historical?

I wish textbooks had sidebars with descriptions such as "this is now a historical theory, or this is mostly historical, or this is just a convenient model, or this is only historical background to the newer theories".

 

[Hooloovoo]

You say that your analogy is "a better way". A 6 year old might like your analogy because it is written in terms of things he can understand. A physics professor would flunk you if you wrote that on an exam. "Better" according to whom? And why should that person's point of view be preferred? What a "good" explanation is is relative to the audience (again, learning is a 2-way street).
This is Physics Forums. When someone asks about electromagnetic fields or QED or relativity, we talk about physics, not jello. If that means that the questioner must spend some time hitting the books, then so be it. Kabish?
We operated this way long before you joined us last month, and we will continue to operate that way in the future. It is what makes PF the best scientific discussion site on the internet.

My goodness, but someone is touchy today. As I said, my little jello analogy was merely an example of how one might make an explanation to someone who clearly DOESN'T have the background, and I pretty clearly said it was NOT a better actual explanation of the phenomenon (or even a good one).

It is obvious from the question that was posed that the concept wasn't understood. The proper answer is to say "your question reveals some confusion, let me clear it up for you" rather than "you don't know what you're talking about, go somewhere else and read something before you come back here and ask questions again."

That said, I think it has been beaten to death now that "how big is a photon" and "what does it look like" are meaningless questions. However, the obvious thing these questions were asking is for a basic description of what a photon is, and how it behaves. This can be done, and has been done by some very fine minds, in plain language. At least for the basic summary information. I think the original poster probably would have been satisfied with "a photon has no size, doesn't look like anything, and behaves like this:..." without needing any explanation of how one would derive such conclusions.

This is indeed a fantastic forum, and everyone here is interested in gaining understanding. This justifies your snarky reactions how, exactly?

 

[Physics101]

I've recently read QED by Richard Feynman and was utterly shocked by how strange the "reality" is. Although I took Physics courses while in college (some 20 years ago -- ancient in Physics terms), I'm willing to bet that not many of today's college graduates (non-Physics majors) are even aware of anything having to do with QED. The same goes for many other modern-day physics topics, including the String Theory, Brane Theory and the QM in general.

As a layperson, I am concerned about how Physics and other disciplines of Science are losing touch with the general public. A great chasm exists between those in-the-know and the public with diminishing possibility of connecting the two. The only tool appears to be the Math which appeals to 0.001% of the population, if at that. I think Feynman did a fantastic job of trying to explain the unexplainable and I'm afraid we need more of these individuals if Science is going to attract young minds (and the hearts) of future scientists.

I see nothing wrong with describing the light/photon with an imperfect model as long as it retains the flavor of what they are and if a caveat alerts the reader as such. Bohr's model of atom is still a useful visualization tool so long as one recognizes the intentional simplicity behind it. Also, any given theory can be superceded by another, more elegant one should such come along. QED does explain all the classic behaviors of light/photon and, if I remember correctly, even avoids requiring the uncertainty principle of QM in certain cases. It does not, however, mean that it is the way nature works. It simply means that it's the best we have (or one of the leading theories) to explain the way nature behaves when it comes to light/photon.

P.S. This was a great topic and I really enjoyed reading the exchange of messages. Learning something new everyday.

 

[marlon]

Bohr's model of atom is still a useful visualization tool so long as one recognizes the intentional simplicity behind it.

Hey Physics,

One does not need to recognize the simplicity behind Bohr's Model, one needs to recgonize that it is plain wrong because it does not respect the Heisenberg Uncertainty. This model was a first, ingenious, attempt to solve the fact that stable atoms cannot exist using only Coulomb's Law to describe the nucleus + electrons interactions.

QED does explain all the classic behaviors of light/photon

What you want to say is that QED reduces to classical EM within the correct energy/distance scale. A photon is a QM concept that therefore has no "classical" behaviour.

and, if I remember correctly, even avoids requiring the uncertainty principle of QM in certain cases.

This is impossible. Keep in mind what the Q in QED stands for.
QED has no competitors when it comes to physical models that describe the EM interaction. It's accuracy is up to 10 decimals ! Besides, there are no experimental/theoretical problems with QED because there are no contradictions with any experiment what so ever AND QED is renormalizable.


regards
marlon

 

[marlon]

I'd read many articles that mention QED as a very speculative theory in some aspects,

Look,
QED has no conflicts with any experiment what so ever. Also on the theory side there is no problem because QED is completely renormalizable.

If you want to mention these articles than i would like to ask you to quote them. Give references. You cannot just say "i ready many articles..." and therefore conclude that there is something wrong with QED. This is not how science works.

Don't take this the wrong way, i don't want to insult you here but it really needs to be stated that making loose statements cannot be taken seriously because you have no proof. If you have no proof you have nothing.

regards
marlon

 

[quantumdude]

Another thing. Your post makes it look like you want to say that many photons make up the EM field. I don't know if it is your message, but i hope not because it is wrong.

I don't understand why you say that the number of photons is irrelevant. Considering only monochromatic photons for simplicity, their creation and annihilation operators are, respectively, given by $\hat{a}^{\dagger}$ and $\hat{a}$. As you know they satisfy the commutation relation $[\hat{a},\hat{a}^{\dagger}]=1$. The "1" is what puts the "Q" in "QED". In the high-$n$ limit the 1 is negligible and we have approximately $[\hat{a},\hat{a}^{\dagger}]=0$. In other words, in the high-$n$ limit we recover classical electrodynamcs.

All I did was state the correspondence principle in my own words. I don't understand why you object to it.

 

[quantumdude]

My goodness, but someone is touchy today.

Yeah, and that someone is obviously you.

As I said, my little jello analogy was merely an example of how one might make an explanation to someone who clearly DOESN'T have the background, and I pretty clearly said it was NOT a better actual explanation of the phenomenon (or even a good one).

Well, now I'm confused because you clearly did say that your explanation was better than mathematical explanations that are incomprehensible to novices. My point here is that rather than dumb down the explanation, we should help smarten up the novice.

It is obvious from the question that was posed that the concept wasn't understood. The proper answer is to say "your question reveals some confusion, let me clear it up for you" rather than "you don't know what you're talking about, go somewhere else and read something before you come back here and ask questions again."

I've been helping people here for 4 years, and marlon has been for about 1.5 years. You've been here less than two months. Rather than lecture us on what a proper response is, how about you watch and learn for a little while?

First of all, marlon did give a perfectly fine answer to Ben's question in Post #12. For some reason you decided to ignore that part of the post and instead focus on one line: "GO STUDY QED."

And second, when there is such an enormous gap in knowledge between what one (in this case, Ben) has studied and what he is asking about, it becomes necessary for someone to sit down and teach him the subject in order to fully answer his question. Since that is too much to ask of anyone on a forum, we (that is, Mentors, Science Advisors, and Homework Helpers) frequently point people in the right direction for self-study. Marlon did it with his "go study" comment, and I attempted to engage Ben by referring him to a popular account of QED and asking him where his last physics course left off. There is nothing wrong taking people who are seeking help, and advising them on how to help themselves. I don't understand why you find that idea so distasteful, but I sincerely believe that you are wrong for holding that opinion.

Third, if anything your comments in this thread only served to deepen the confusion, not to help clear it up. That is what makes your admonition about a "proper answer" particularly irritating.

That said, I think it has been beaten to death now that "how big is a photon" and "what does it look like" are meaningless questions.

Another thing that has been beaten to death is the point you were trying to make. You have been answered thoroughly and lucidly by myself and others. I am astounded that you are still pushing the point.

However, the obvious thing these questions were asking is for a basic description of what a photon is, and how it behaves. This can be done, and has been done by some very fine minds, in plain language. At least for the basic summary information. I think the original poster probably would have been satisfied with "a photon has no size, doesn't look like anything, and behaves like this:..." without needing any explanation of how one would derive such conclusions.

He was given those answers, and the thread is still going!

This is indeed a fantastic forum, and everyone here is interested in gaining understanding.

Glad you like it.

This justifies your snarky reactions how, exactly?

I haven't made any snarky reactions. I've been perfectly reasonable about explaining to you how we do things here. That is what moderators do. Kindly lose the defensive posture.

 

[Boffin]

Third, if anything your (Hooloovoo) comments in this thread only served to deepen the confusion, not to help clear it up. That is what makes your admonition about a "proper answer" particularly irritating.

He was given those answers, and the thread is still going!

Ohanian used to be a professor at my school, RPI. He left before I arrived, but my professors knew him well. He has a reputation of being a razor-sharp thinker. I doubt that there is any ambiguity in his remarks. If you post direct quotes here rather than paraphrases, I think you will make more progress.

Tom, there are many reasons why I and some other people don't just behave or learn like you want us to:

1. As Physics101 pointed out not many people with a university science degree are even aware that QED is the only accepted model of electromagnetic interactions. So why would a person listen when the first person says so, while some of our teachers and professors have either told us otherwise or not mentioned it.

2. Some people are actually confused. You said for example that you doubt if there was any ambiguity in Ohanian's textbooks. But he is describing light and radio waves with three different models, continuous localized plane waves, wavicles as individual vibrating entities (it appears), and in passing that there is also a theory called QED. I know I'm paraphrasing but the actual text is so long. Since these are all competing theories, which one is correct, he doesn't say. And the section on QED is very short. He doesn't even include QED in the single volume edition which is about 1400 pages long. I happen to have a two volume copy which is even longer that also included QED.

3. I don't just run to start studying QED in detail because I'm not a believer yet. Why should I? It's all new to me that it's the only theory I should believe. Would you start studying something before you are convinced it's worth studying?

4. QED is hard to believe. That is a likely reason why I and maybe some others are still going with this thread, now that we have been made more aware of this QED theory. Preachers shouldn't bash people into believing, they should convince them. You mentioned that QED is accurate to 10 decimal places. I don't have an issue with the predictive aspect of QED, why of course it is accurate like you said, but that doesn't mean that many different models can't predict exactly the same thing. Such logic isn't convincing to me. It's the basis of inductive vs deductive thinking.

I'll quote directly from Richard Feyman's book QED The Strange Theory of Light and Matter pg 10 1985 version which I'm re reading. "I'd like to talk a little bit about understanding. When we have a lecture, there are many reasons why you might not understand the speaker ... language is bad ... uses funny words ... you won't understand why nature works that way ... Finally there is the possibility: after I tell you something, you just can't believe it. You can't accept it. You don't like it. A little screen comes on and you don't listen any more. The theory of quantum electrodynamics describes Nature as absurd from the point of view of common sense".

 

[quantumdude]

Tom, there are many reasons why I and some other people don't just behave or learn like you want us to:

All I attempted to do is point you in the right direction and get you to do some work for yourself. What good reason could anyone possibly have for objecting to that?

1. As Physics101 pointed out not many people with a university science degree are even aware that QED is the only accepted model of electromagnetic interactions. So why would a person listen when the first person says so, while some of our teachers and professors have either told us otherwise or not mentioned it.

All physicists are aware of it, and you asked your questions on a message board populated by physicists, and they have answered you. Even if you were wary at first, surely now you must realize that QED is what you should be looking at.

2. Some people are actually confused. You said for example that you doubt if there was any ambiguity in Ohanian's textbooks. But he is describing light and radio waves with three different theories,

(snip)

Since I don't have the book I can't comment on this part.

3. I don't just run to start studying QED in detail because I'm not a believers yet. Why should I? It's all new to me that it's the only theory I should believe. Would you start studying something before you are convinced it's worth studying.

What exactly are you waiting for, some magical post that suddenly makes the clouds part and the angels sing? It simply doesn't work like that. You have heard from a number of people who have studied physics at the graduate level (marlon and myself) and a working physicist with a PhD (ZapperZ). All we can do is point you in the right direction, and we have done so. You aren't going to get the answers you are looking for without doing some work. And if you don't believe that physicists can point you in the right direction, then why bother asking us?

4. QED is hard to believe. Convince people of the basics,

It has been repeatedly mentioned that QED agrees with experiment to over 10 decimal places, thus making it the most accurate scientific theory ever devised. That is as basic as it gets, and if you aren't convinced by such spectacular agreement with experiment then you are beyond the reach of reason. It really is just that simple.

don't bash them over the head.

Speaking for myself: I never did. All I did was tell you the truth: That you are not going to achieve the understanding you are looking for without rolling up your sleeves and doing some work for yourself.

That is a likely reason why some of us are still going with this thread. Preachers shouldn't bash people into believing, they should convince them.

I am not preaching, and the remark I made regarding QED's track record with experiment would be enough to convince any sincere seeker of knowledge.

Are you sincere? It remains to be seen.

I'll quote directly from Richard Feyman's book QED The Strange Theory of Light and Matter pg 10 1985 version which I'm re reading. "I'd like to talk a little bit about understanding. When we have a lecture, there are many reasons why you might not understand the speaker ... language is bad ... uses funny words ... you won't understand why nature works that way ... Finally there is the possibility: after I tell you something, you just can't believe it. You can't accept it. You don't like it. A little screen comes on and you don't listen any more. The theory of quantum electrodynamics describes Nature as absurd from the point of view of common sense".

I hope you don't use that as an excuse to stop trying to understand the subject.

 

[quantumdude]

I either missed this part or you edited it in as I was typing my response.

You mentioned that QED is accurate to 10 decimal places. I don't have an issue with the predictive aspect of QED, why of course it is accurate like you said, but that doesn't mean that many different models can't predict exactly the same thing.

Those models don't exist. We can't very well point you in the direction of models that don't exist, can we?

Such logic isn't convincing to me. It's the basis of inductive vs deductive thinking.

Inductive logic is indelibly etched into scientific methodology. So much so in fact that discarding inductive reasoning is tantamount to discarding science itself.

And even if there did exist another model that makes the exact same predictions as QED, what basis do you expect it to have? Deductive or inductive? Of course, it will be inductive and you will find yourself in the exact same boat. So to get yourself out of this dilemma you are going to have to start accepting strong inductive arguments, such as the one that supports QED.

 

[ZapperZ]

Tom, there are many reasons why I and some other people don't just behave or learn like you want us to:
1. As Physics101 pointed out not many people with a university science degree are even aware that QED is the only accepted model of electromagnetic interactions. So why would a person listen when the first person says so, while some of our teachers and professors have either told us otherwise or not mentioned it.

I cannot believe you actually said this. This has got to be THE single worst excuse I've heard in a very long time. You actually used YOUR IGNORANCE as an excused to trivialized QED!

Think about it! You have already made up your mind about something that you have not even begin to understand! If you apply this type of reasoning to social decision, we call that bigotry. Ignorance and laziness are NEVER a good excuse. Try using that when you're in trouble with the law!

3. I don't just run to start studying QED in detail because I'm not a believer yet. Why should I? It's all new to me that it's the only theory I should believe. Would you start studying something before you are convinced it's worth studying?

YES! Or else, how are you able to decide if it should be taken seriously? I don't buy what you are selling here. Yet, how would you like it if I criticize what you have to say without reading and attempting to understand what you wrote? That makes no sense.

If you don't wish to study QED until you "believe it", then you also have no ability to criticize it until you study it. It works both ways. By not bothering to understand it, you have forfieted your ability to do both. If not, you are simply espousing an opinion based on ignorance. If you do not mind doing that, then go right ahead, but not in this forum.

Zz.

 

[ZapperZ]

This type of thread appears every now and then. This is where someone is convinced that (i) one can have a "conceptual" understanding of things such as QM (or in this case, QED) FIRST; (ii) that a superficial idea one gets about it from pop-sci texts is sufficient. All of these done without first understanding the mathematical formulation.

I wrote my essay "Why Is Quantum Mechanics So Difficult" in my journal (and Greg has also posted it on the Physics Post website) because I have seen things like this often. If you have read that essay, you would have seen all the symptoms and signs exhibited in this thread. People expect to be able to understand QM, QED, QCD, etc. conceptually FIRST, and this is a fallacy. There is a discontinuity in the concepts presented in those field with what we are familiar with. The ONLY savior of what is familiar is the mathematics! Without understanding that first, the concepts of QM etc. will appear as if the come out of nowhere, dangling in mid-air.

So it is a self-perpetuating and self-fulfilling fate. The refusal to dive into the mathematical formalism is the very reason why one cannot "believe" QED.

Zz.

 

[marlon]

All I did was state the correspondence principle in my own words. I don't understand why you object to it.

Tom,

After some consideration of my own words...Well...Just forget about my remark. Let's be clear, you are right in saying that large quantumnumbers should reduce QED to classical EM. Which ofcourse, it does. I do realize that that was your point.

The only thing i wanted to make sure is that others don't interprete your words as "many photons make up an EM field". This is a popular misconception that has occurred many times in the nuclei and particles subforum.

Just to be clear, it was not my intention to oppose to the content of your post.

regards
marlon

 

[Physics101]

This [not requiring the uncertainty principle] is impossible. Keep in mind what the Q in QED stands for.

While I understand that the Uncertainty Principle is a fundamental fact of nature in QM, I was merely referring to what Richard Feynman noted in one of his footnotes which I'm quoting below. Please read the following and see where I might have misunderstood.

Page 55-56 of QED: footnote 3

This is an example of the "uncertainty principle": there is a kind of "complentarity" between knowledge of where the light goes between the blocks and where it goes afterwards -- precise knowledge of both is impossible. I would like to put the uncertainty principle in its historical place: When the revolutionary ideas of quantum physics were first coming out, people still tried to understand them in terms of old-fashioned ideas (such as, light goes in straight lines). But at a certain point the old-fashioned ideas would begin to fail, so a warning was developed that said, in effect, "Your old-fashioned ideas are no damn good when..." If you get rid of all the old-fashioned ideas and instead use the ideas that I'm explaining in these lectures -- adding arrows for all the ways an event can happen -- there is no need for an uncertainty principle.

 

[quantumdude]

Marlon,

I was sloppy with my terminology. I thought that it would be clear from the context, but evidently it was not. Let me explain.

Hoovooloo said that photons could be thought of as a pair of sine waves. Since plane wave solutions to the classical EM wave equation are sine waves, the claim on the table strongly suggested to me that photons could be pictured classically. Ben seemed to buy into this claim, and that is what I was opposing. When I said, "EM field" I meant "classical EM field". I cut the term short because I thought that Hoovooloo's comment put us in that mode of thinking.

So my comments should be read as follows: The photon cannot be viewed as a pair of sine waves (that is, as a classical EM wave) because classical electrodynamics only applies in the limit of large $n$, as per the correspondence principle.

I was sloppy by leaving off the adjective "classical". You are correct to point out that just because the EM field intensity reduces to a single photon, it doesn't mean that the field ceases to be an EM field. I wasn't trying to contradict that fact, which I see you now realize.

 

[quantumdude]

If you get rid of all the old-fashioned ideas and instead use the ideas that I'm explaining in these lectures -- adding arrows for all the ways an event can happen -- there is no need for an uncertainty principle.

I think I understand why he said that. There was a time when all the ideas of quantum theory were still forming and the uncertainty principle seemed to stand alone as a physical postulate. As quantum theory developed it was found that by disposing of all the old ideas of how particles and light behave and introducing new, revolutionary postulates, it was no longer necessary to postulate an uncertainty principle.

But the uncertainty principle is still deducible from the newer postulates. So while it is not a necessary first principle, it remains a necessary consequence of first principles, and hence is still with us.

 

[ZapperZ]

While I understand that the Uncertainty Principle is a fundamental fact of nature in QM, I was merely referring to what Richard Feynman noted in one of his footnotes which I'm quoting below. Please read the following and see where I might have misunderstood.
Page 55-56 of QED: footnote 3
This is an example of the "uncertainty principle": there is a kind of "complentarity" between knowledge of where the light goes between the blocks and where it goes afterwards -- precise knowledge of both is impossible. I would like to put the uncertainty principle in its historical place: When the revolutionary ideas of quantum physics were first coming out, people still tried to understand them in terms of old-fashioned ideas (such as, light goes in straight lines). But at a certain point the old-fashioned ideas would begin to fail, so a warning was developed that said, in effect, "Your old-fashioned ideas are no damn good when..." If you get rid of all the old-fashioned ideas and instead use the ideas that I'm explaining in these lectures -- adding arrows for all the ways an event can happen -- there is no need for an uncertainty principle.

To add to what Tom said, the HUP is really not a "principle" in the same vein as the principle of conservation of momentum, etc. It isn't the starting point. Most peole who have not studied QM does not know that, i.e. they don't that the HUP is a consequence, not the source. What is more important here is the commutation relation of 2 observables, i.e. [A,B], which in some cases is called the First Quantization.

It is based on this mathematical formulation that you get the HUP. The HUP falls right out of the mathematics. It is not inserted by hand and not some ad hoc introduction into the QM description. Most of us don't even deal with it when we use QM under normal working conditions, because we know it will be taken care of by the formulation.

Zz.

 

[marlon]

This is an example of the "uncertainty principle": there is a kind of "complentarity" between knowledge of where the light goes between the blocks and where it goes afterwards -- precise knowledge of both is impossible. I would like to put the uncertainty principle in its historical place: When the revolutionary ideas of quantum physics were first coming out, people still tried to understand them in terms of old-fashioned ideas (such as, light goes in straight lines). But at a certain point the old-fashioned ideas would begin to fail, so a warning was developed that said, in effect, "Your old-fashioned ideas are no damn good when..." If you get rid of all the old-fashioned ideas and instead use the ideas that I'm explaining in these lectures -- adding arrows for all the ways an event can happen -- there is no need for an uncertainty principle.

Well Tom, explained Feynman's point in post 54 so i don't have anything to add. But do keep in mind that Feynman is NOT saying that even when starting from the "new revolutionary QM ideas" one cannot retrieve the HUP from these new ideas. Basically he is saying that when trying to describe QM phenomena in terms of classical physics (as far as that were possible ofcourse), you will be making mistakes and errors. In other words, classical physics does not work. That's were the HUP comes in. The HUP just expresses that classical physics should not be used to describe QM stuff.

marlon

marlon

 

[Rade]

2) the most complete picture : QED, clearly states how ONE single photon arises from the quantisized EM-field. I have pointed this out many times in this thread. A photon arises due to fluctuations of the quantisized EM-field.

I have a question about your comments above, something that is unclear to me. When you say that a photon "arises" from the EM-field, are you then saying that the EM-field is a thing that exists independent of the observation of the photon arising ? I know, perhaps more philosophic than physics, but I really do have a hard time grasping the nature of this EM-field, e.g., its essence. A photon I can "sort of" get a handle on (e.g. a quantized packet of pure energy), but it is the EM-field that is the "cause" of the photon "effect" that I just cannot grasp mentally. Or, is it a case as discussed by Zapper that one really cannot grasp EM-field as a "mental concept", that it can only be grasped using "mental mathematics" ? Thanks for any comments, and a very happy new year 2006 to all.

 

[Sherlock]

I have a question about your comments above, something that is unclear to me. When you say that a photon "arises" from the EM-field, are you then saying that the EM-field is a thing that exists independent of the observation of the photon arising ? I know, perhaps more philosophic than physics, but I really do have a hard time grasping the nature of this EM-field, e.g., its essence. A photon I can "sort of" get a handle on (e.g. a quantized packet of pure energy), but it is the EM-field that is the "cause" of the photon "effect" that I just cannot grasp mentally. Or, is it a case as discussed by Zapper that one really cannot grasp EM-field as a "mental concept", that it can only be grasped using "mental mathematics" ? Thanks for any comments, and a very happy new year 2006 to all.

In lieu of Marlon replying to your query, you can do a Google book search ... typing in, "rodney loudon the quantum theory of light". Loudon discusses 'the photon' right at the beginning, and somewhere later on (I don't remember exactly where) he talks about the essence of the quantized em field, I think. You can get some good references there for further study also.

Eventually, I suspect you will come to the conclusion on your own that, as some mentors have noted, ZapperZ especially, trying to envision the physical reality of what a photon corresponds to independent of its operational definition (ie., independent of its existence as a measurement result and mathematical construction pertaining to photon measurement) in terms of some set of classical analogies will be inadequate (ie., there isn't any classically analogous picture that can be generally, unambiguously applied).

Trying to envision what an em field corresponds to in 3D physical space might be even more problematic. I don't know enough to comment definitively, but so far I don't have any sort of working physical picture of either photons or fields. I think that part of what the mentors' want to communicate about this is that as we students learn more (and become more fluent in the mathematical methods), then the reasons for the absence of a visualizable understanding (in terms of analogies with our ordinary experience) will become clearer and the need for such visualizations will diminish.

Nevertheless, the attempt to develop a picture of the deep nature of light doesn't seem to me to be an entirely metaphysical one ... and it seems likely that there will continue to be efforts to build such a picture from the data.

Do Bell's Theorem and the results of Bell-type tests tell us that such efforts are doomed from the start? Well, given the extant data and experimental capabilities, yes ... but, who knows what the future holds.

By the way, also coming to mind are discussions from the archives of sci.physics.research (and sci.physics ??). One had to do with the 'length of a wavetrain of a photon', and the other was called 'photon schmoton' or something like that.

 

[member 11137]

... as some mentors have noted, ZapperZ especially, trying to envision the physical reality of what a photon corresponds to independent of its operational definition (ie., independent of its existence as a measurement result and mathematical construction pertaining to photon measurement) in terms of some set of classical analogies will be inadequate (ie., there isn't any classically analogous picture that can be generally, unambiguously applied).
Trying to envision what an em field corresponds to in 3D physical space might be even more problematic. I don't know enough to comment definitively, but so far I don't have any sort of working physical picture of either photons or fields. I think that part of what the mentors' want to communicate about this is that as we students learn more (and become more fluent in the mathematical methods), then the reasons for the absence of a visualizable understanding (in terms of analogies with our ordinary experience) will become clearer and the need for such visualizations will diminish.
Nevertheless, the attempt to develop a picture of the deep nature of light doesn't seem to me to be an entirely metaphysical one ... and it seems likely that there will continue to be efforts to build such a picture from the data...

What a beautiful and exact commentary... Even if I didn't took part to your present conversation, I continue to read the discussions on these forums and I feel concerned by this discussion about the HUP and the Feynman's approach. Not only because of my own approach on the independant research forum where I personnaly try to explore the existence of other mathematical procedures able to give back this HUP. I recently discovered some stuff to understand the Feynman's paths integrals and the photonic crystals that are indeed concrete applications of these metaphysical "Überlegungen" "pensées" "thoughts". Happy new year 2006 to the team and to every body here.

 

[jtbell]

By the way, also coming to mind are discussions from the archives of sci.physics.research (and sci.physics ??). One had to do with the 'length of a wavetrain of a photon', and the other was called 'photon schmoton' or something like that.

I turned up those threads myself a couple of weeks ago when I was searching for material on the "nature" of photons. They have lots of interesting material, and I highly recommend them to anyone with plenty of time on their hands. Be prepared to do a lot of digging, because Google Groups doesn't collect all the postings from one thread together. I think it's because some people changed the subject line along the way, which confuses Google. As is usually the case with Usenet, some posters are more "trustworthy" than others, so you need to evaluate people's comments and apparent expertise.

 

[Nereid]

A most interesting thread!

There was a comment in marlon's first post (in this thread) that I feel merits further discussion:

[snip]

If you have done this, you answer me this : "do photons mutually interact ?"
Hell, i will even give you the answer
Answer : NO in first order but they do interact indirectly in higher order.

What does this mean? That photons can 'collide'? That a photon can split into two?

More generally, we've discussed the 'how big?' and 'what does it look like?' parts of the OP; I feel we've somewhat neglected the 'how does it behave?' part.

The many different two-slit experiments are good examples of the 'behaviour' of photons (and electrons, and atoms, and ...) - the 'photon' behaves just like a (QED) photon in these experiments (it does NOT behave 'like' a classical wave, or a classical particle, when you attempt to account for the experimental results in their totality).

What are some of the other notable (quantum) behaviours of photons? In particular, what are some 'extreme' behaviours, some of the most counter-intuitive behaviours, predicted by theory, later observed?

 

[ZapperZ]

I do not know how accurate it is but here is what I was taught about what a Photon may look like and why it acts like both a particle and a wave function.

But this is shown in CLASSICAL E&M! You get something like this by solving the Maxwell Equation. Such a picture that you were "taught" didn't come out of nowhere. It came from a classical wave picture. So where's the "particle" behavior? And not only that, this is valid only for a plane-polarized light.

And please don't pursue the line of discussion that you started that was locked earlier.

Zz.

 

[quantumdude]

There was a comment in marlon's first post (in this thread) that I feel merits further discussion:What does this mean? That photons can 'collide'? That a photon can split into two?

No, photons can only couple to matter currents. What happens is that two photons can momentarily be converted to $e^+e^-$ pairs, and those can collide. By default this is a higher-order process since it requires multiple vertices in the Feynman diagram. If you have Jackson's Classical Electrodynamics, check out the introduction. You'll find a Feynman diagram for this process with the caption, "Scattering of Light by Light" or some such.

 

[Intuitive]

But this is shown in CLASSICAL E&M! You get something like this by solving the Maxwell Equation. Such a picture that you were "taught" didn't come out of nowhere. It came from a classical wave picture. So where's the "particle" behavior? And not only that, this is valid only for a plane-polarized light.
And please don't pursue the line of discussion that you started that was locked earlier.
Zz.

Kindly disregard the posting that I deleted about Classical EM.

P.S. It is much more difficult to draw a Photon(QED), Classical was easier.
I have been studying Java applets that represent Photon(QED) with various slits and Wave Guides, It's very fasinating indeed, I love the
thought of Photon Duality and Multiple Universes, It's keen.

 

[Nereid]

No, photons can only couple to matter currents. What happens is that two photons can momentarily be converted to $e^+e^-$ pairs, and those can collide. By default this is a higher-order process since it requires multiple vertices in the Feynman diagram. If you have Jackson's Classical Electrodynamics, check out the introduction. You'll find a Feynman diagram for this process with the caption, "Scattering of Light by Light" or some such.

Thanks Tom.

It sounds like a thoroughly non-classical thing - has it been confirmed by (direct) observation?

I don't have the book to hand, does anyone know of a brief overview that's on the net?

 

[masudr]

The classic experimental observation that one talks about when discussing QED is the magnetic moment of an electron. The theoretical prediction and observed experimental result agree to unprecedented levels (to one part in $10^{11},$ I think).

EDIT: The prediction for this value involves higher-order corrections, which is when a photon becomes a $e^+e^-$ pair etc.

 

[Boffin]

I don't see a reason for the amount of opposition QM receives from some people. At times this forum seems to be a place for desperate attempts to disprove QM rather than a way to learn about it. https://www.physicsforums.com/showpost.php?p=501143&postcount=12

Hmm, OK I read that?

Take comfort in Feynman's quote, which sums up all of the problems with QM: Nobody understands quantum mechanics. https://www.physicsforums.com/showpost.php?p=501089&postcount=8

But somehow I think we can do a little tiny bit better than this. I haven't been adding to this thread because I've been reading several books and articles on this issue. I did some major reading of the Physics Forum past threads also. Here are some of the highlights of related threads. It'll give some of you something to do.

Electromagnetic wave in quantum physics? https://www.physicsforums.com/showthread.php?t=91205
Light, Wave or Particle? https://www.physicsforums.com/showthread.php?t=101692
Light and photon, confused https://www.physicsforums.com/showthread.php?t=63802
Picturing wave/particle duality https://www.physicsforums.com/showthread.php?t=57911
Wave/particle duality https://www.physicsforums.com/showthread.php?t=56561
What is light? https://www.physicsforums.com/showthread.php?t=83859
Particles or Waves? https://www.physicsforums.com/showthread.php?t=77457
Light = Particle or Wave? https://www.physicsforums.com/showthread.php?t=35854
What's Wrong with QM? https://www.physicsforums.com/showthread.php?t=68265

But after reading all these messages I still didn't even come close to answering one basic question. There seems to be a disagreement on the following issue on this forum even among the following mentors as to whether a wave in Quantum Mechanics can be a single photon or does it need to be composed of multiple photons?

The photon frequency $\nu$ is the frequency of oscillation of the EM field that would result if there were a large number of those photons. https://www.physicsforums.com/showpost.php?p=868561&postcount=36

QED, clearly states how ONE single photon arises from the quantisized EM-field. I have pointed this out many times in this thread. A photon arises due to fluctuations of the quantisized EM-field. https://www.physicsforums.com/showpost.php?p=868582&postcount=38

No, the single photon can manifest as an EM plane wave, for example. Wave particle duality is serious, not just a statistical phenomenon of little bullets "doing the wave". https://www.physicsforums.com/showpost.php?p=766621&postcount=2

Here is what is puzzling to me. A single photon in Quantum Mechanics is able to interference with itself. Isn't then a single photon wavelike and have a frequency? Normal light is incoherent light. There is no regular spacing between photons to arrive at any frequency? Does not a prism separate light photons by their individual wavelengths or frequency? Am I being confused with definitions or imperfect models? Please enlighten me!

 

[quantumdude]

But after reading all these messages I still didn't even come close to answering one basic question. There seems to be a disagreement on the following issue on this forum even among the following mentors as to whether a wave in Quantum Mechanics can be a single photon or does it need to be composed of multiple photons?

There's no disagreement at all. We all agree that classical EM fields (that includes EM plane waves) aren't manifested by a single photon. This was clarified by the exchange between marlon and myself.

Here is what is puzzling to me. A single photon in Quantum Mechanics is able to interference with itself. Isn't then a single photon wavelike and have a frequency?

A single photon does have a frequency, but it is not a classical EM wave. See the following website on single photon interference.

http://www.fas.harvard.edu/~scdiroff/lds/QuantumRelativity/SinglePhotonInterference/SinglePhotonInterference.html

The familiar interference pattern emerges only after many photons have hit the screen. But each individual photon hits the screen in only one location.

 

[vanesch]

There's no disagreement at all. We all agree that classical EM fields (that includes EM plane waves) aren't manifested by a single photon. This was clarified by the exchange between marlon and myself.
A single photon does have a frequency, but it is not a classical EM wave.

The relationship between the classical EM description and the quantum mechanical description is rather subtle, and depending on what aspect you want to look at, things can be formulated differently and sometimes, at first sight, even contradictory. But when you understand the relationship well enough, you can often see the veracity of the different apparently contradictory statements.

Here are a few.
The classical EM description is of course a 4-vector field over spacetime:
A(x,y,z,t), from which the more well-known description in terms of E and B fields can be derived: E(x,y,z,t) and B(x,y,z,t).

The quantum description of the free EM field consists of Fock space, which has the basis:
|0> the vacuum
|k1,e1>, |k2,e2>, ... the 1-photon states
|k1,e1,k2,e2>,|k3,e3,k4,e4> ... the 2-photon states
...

Each (k,e) pair corresponds to a momentum vector (3-dim) and a polarization vector e which can take on 2 different values.

The above basis is the basis of a hilbert space called "Fock space" and all possible linear combinations of the above states are the possible quantum states of the free EM field.
But only very special combinations correspond to classical EM waves ; these are called "coherent states". Coherent states are special superpositions of the vacuum, 1-photon, 2 - photon ... 28621-photon... states which are eigenvectors of the so-called destruction operator. There is a coherent state that corresponds to each possible state of the classical EM field, but in Fock space there are many, many more possible states.

However, the 1-photon state |k,e> behaves *in certain respects* as the classical EM wave that corresponds to a plane wave with wave vector k and polarization e. It is not the equivalent (that would have been the coherent state, which CONTAINS |k,e> as a term, but has many others in them too) of the classical EM wave, but for certain aspects, one can *pretend* it to be like the classical EM wave. For instance, the probability of detection on a screen of the one-photon state is proportional to the intensity of the classical EM wave with wavevector k and polarization e. So IF YOU KNOW WHAT YOU'RE DOING, you can jump back and forth sometimes between a 1-photon state and a classical EM wave, because things come out the same for certain quantities.
And then people get sometimes a bit sloppy, and say that "the 1-photon state interferes with itself like the classical EM wave" and things like that. In the proper context, this is an operationally correct statement.
And then others say that 1-photon states are totally different from a classical EM wave which needs many photons to be correctly described. This is ALSO correct (the coherent state contains n-photon states).

cheers,
Patrick.

 

[jtbell]

the probability of detection on a screen of the one-photon state is proportional to the intensity of the classical EM wave with wavevector k and polarization e.

The electric portion of a classical EM plane wave is

$$\vec E = \vec E_0 \cos (\vec k \cdot \vec r - \omega t + \phi_0)$$.

By "intensity" do you mean ${|\vec E|^2}$ or ${|\vec E_0|}^2$? I suspect the second one, because in the absence of interference I don't expect the probability of detecting a photon to vary with the wave's oscillation.