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

[masudr]

Light was only properly described by 2 physical theories:

Classical electrodynamics (which is Maxwell's equations, formulated in the late 19th century) could explain the physical nature of light very well, to all non-atomic scales. It was also the first classical theory to be relativistically covariant, meaning it was valid in the relativistic regime. This is the theory of light waves; oscillating electric & magnetic fields etc. It was shown to be wrong when discussing the dynamics of atoms etc.

Quantum electrodynamics (formulated in the 1950s) is the quantum mechanics of the classical electrodynamic field (so it's still quantum mechanics, but in this case the system we're studying is the one that hopefully corresponds to the classical electromagnetic field in the classical limit). I'm fairly certain that this is the most accurate scientific theory we have (in the sense that theoretical predictions match the experimental values to the greatest level of accuracy).

I hope that clears things up a bit.

 

[Schrodinger's Dog]

Ben can you give me a link I can follow so I understand why light is bent by a difraction grating please, I'm not going to pay out 60 pounds for a tome if I don't have to. Thanks in advance for the link


And precisely a fun thread. But sometimes you just feel you're banging your head against a brick wall

 

[reilly]

For those of you who want to ask fundamental questions about photons, it makes sense, does it not, to learn as much as you can about what's already been done? Size of a photon? Depends on what you mean by size. As I already mentioned, much work has been done on localization of photons -- which can be argued to be related to "size." People have spent great effort to develop understanding of photons, and have done quite well.

This work may or may not agree with your notions, but surely your own ideas will become more crisp and focussed from analysing the work of others. And you will become more knowledgible, presumably a desired goal.

The plain fact is to get the real deal on photons, you'll have to contend with Mandel and Wolf, Optical Coherence and Quantum Optics, or equivalent. Virtually all the questions about photons that have been raised here, and in other threads as well, are answered in that book. Sometimes the answers are simple, and sometimes are couched in very sophisicated reasoning and math. But they are there.
Regards,
Reilly Atkinson

 

[quantumdude]

Sorry this is so late, but I feel that the following points must be responded to.

You asked a legitimate question and you got some unpleasant replies that turned your question into a personal conversation about your competence or your incompetence to understand the issue of quantum physics.

Not one single person said anything derogatory regarding Ben's competence with respect to understanding physics. In fact, by suggesting to him that he follow such-and-such a path to answer his own questions we have implicitly acknowledged his capacity to understand. If not, we wouldn't have wasted the time recommending it!

Now, according to my understanding which is conditioned by my temperament, the reply “GO STUDY QED” is a product of irrational discussion. It is a tautology, meaning “you will find the answer in the answer”, which makes no sense!

Then both your understanding and the temperament which conditioned it are wrong. Tautologies aren't statements, they are logical schemata. I think you meant to say that "GO STUDY QED" implies an analytic statement, not a tautology. If that's the case then you are still incorrect. Statements that are analytic to one reader can very well be synthetic to another, and that is in fact what we have here. If one doesn't already know that QED contains the answers that are sought after, then an admonition to look at that theory does in fact imply a synthetic statement, and there is nothing whatsoever irrational about making it.

As NBC says about their reruns, "If you haven't seen it, then it's new to you."

 

[leandros_p]

Tautologies aren't statements, they are logical schemata. "

Dear Tom Mattson,

http://dictionary.reference.com/search?q=tautology"
1. Needless repetition of the same sense in different words; redundancy.
An instance of such repetition
2. Logic. An empty or vacuous statement composed of simpler statements in a fashion that makes it logically true whether the simpler statements are factually true or false; for example, the statement "Either it will rain tomorrow or it will not rain tomorrow"

http://www.m-w.com/dictionary/tautology" =
1. a : needless repetition of an idea, statement, or word
b : an instance of tautology
2. a tautologous statement

http://dictionary.cambridge.org/define.asp?key=81495&dict=CALD" =
the unnecessary and usually unintentional use of two words to express one meaning

Tautology is a statement. It's in the dictionaries. I also know it because it is a Greek word, and Greek is my native language.

This is how I understand the conversation:

Ben Wiens: (I am studying QM and) I have these questions ...
Answer: Go study QM !

But I am afraid we are going off topic, on this.

 

[Schrodinger's Dog]

Ok since no one seems to know the answer to my question about the difraction grating thing I'm gona assume that no one knows, even if that is not the case Can anyone suggest a website where this is explained or was the first reply I got the correct one. There is no classsical model for this. And we don't know why.

 

[ZapperZ]

Ok since no one seems to know the answer to my question about the difraction grating thing I'm gona assume that no one knows, even if that is not the case

<sigh>

I already gave up on this thread, but silly me, I just can't let a statement like this alone.

This thing has been discussed MANY times already on here. Here's an example:

https://www.physicsforums.com/showthread.php?t=68917

I suggest you refrain from making such claims when you don't get anyone to address your question.

Zz.

 

[Schrodinger's Dog]

I apologise, maybe asking for an answer to this question was wrong. I can understand your frustration at being asked this but it probably comes up so often that you get fed up of it. Try to remember that this is the first time I had thought about it. I'm not here to provoke anyone I just want answers.

Thanks for the link.

OK interference patterns cause the light to be difracted, thank you.
I saw an experiment where light was detected as both a wave and a particle at the same time in a novel two slit experiment. Is this mentioned here or is this something different or am I missing the point because the two slit shows both anyway? Sorry to derail this thread. So a single photon won't be difracted then, but many will? I'm a bit shakey here how does all possible waveforms translate into difraction of a single photon?

 

[quantumdude]

Tautology is a statement. It's in the dictionaries. I also know it because it is a Greek word, and Greek is my native language.

I thought you were coming from a philosophical perspective. The word "tautology" has a specific, technical meaning that is not the same as the colloquial meaning that you find in the dictionary. From a technical perspective a tautology is a logical schema that is true under all possible interpretations. As I said, what you mean by tautology would be called an analytic statement.

But I suppose that is neither here nor there because the following is definitely not correct.

This is how I understand the conversation:
Ben Wiens: (I am studying QM and) I have these questions ...
Answer: Go study QM !

First, nothing in any of Ben's posts indicates that he is actually studying QM at any level higher than that of a freshman physics book. And second, QM and QED are two very different theories. Even if Ben were studying QM (which he isn't), directing him to QED is very much a non-trivial, useful piece of advice.

But I am afraid we are going off topic, on this.

I agree, but I can't just sit there while you call good advice "irrational".

 

[ZapperZ]

I apologise, maybe asking for an answer to this question was wrong. I can understand your frustration at being asked this but it probably comes up so often that you get fed up of it. Try to remember that this is the first time I had thought about it. I'm not here to provoke anyone I just want answers.

Maybe I'm strange, but denigrating the knowledge of the people you are asking answers from is NOT a very good way to seek help.

I saw an experiment where light was detected as both a wave and a particle at the same time in a novel two slit experiment. Is this mentioned here or is this something different or am I missing the point because the two slit shows both anyway?

I dislike something like this. There is no way anyone can possibly comment on this without GUESSING what exactly the info you got. You need to make a specific citation to whatever it is you want clarification on. If no one else here learn anything from this forum, just learn that you need to be AWARE of your sources, and remember the exact citation whenever you ask about them! Just mentioning "oh, I read that... or I heard that..." is useless. We have no way to know if you simply misread something, misinterpret something, or that the source you are seeing is plain crackpottery.

Zz.

 

[Schrodinger's Dog]

k sorry but what hapens with a single photon, I need clarity. Not dengrating anyones knowledge just questioning what's behind it. If it looks like I'm putting anyone down that's not my intent, I'm just prodding them to get some info.

 

[ZapperZ]

k sorry but what hapens with a single photon, I need clarity.

I don't understand. What exactly with a "single photon" that you want answer to?

The Marcella paper that I cited derived, in painful detail, ALL the interference effects, from single slit to multi slits, starting purely from quantum mechanical description without having to invoke any classical wave picture. And the properties of single-photon interference (as opposed to 2-photon, 3-photon, etc interference) has already been dealt with by Mandel[1]. Is there a specific question you have regarding a single photon?

Zz.

[1] L. Mandel, Rev. Mod. Phys. v.71, p.274 (1999).

 

[Schrodinger's Dog]

Can't acces that paper without money and I don't have any atm. But what I wanted to know(sorry for being vague) is what you mean by interferes with itself, this is a bit vague? Sorry how can a superposition of a single photon interfere with itself exactly? That superposition thing doesn't make much sense to me, so I asked a colleague and he said, that it's not true what you infer, no one is sure exactly why a single photon is defracted, whether in a classical or qm sense? Even Fennyman admitted he didn't know why? I got to say I've got to agree with my colleague, the interferes with itself answer seems a bit odd to me? Could you explain that further? bearing in mind I have no access to the Marcela paper.

 

[ZapperZ]

Can't acces that paper without money and I don't have any atm. But what I wanted to know(sorry for being vague) is what you mean by interferes with itself, this is a bit vague? Sorry how can a superposition of a single photon interfere with itself exactly?

The problem here is that you are focusing on the wrong "object". The issue here really isn't the photon. Rather it is a quantum object that has a superposition of paths, based on Feynman's path integral formulation.

What this means is that the quantum object, having a QM description, sees a number of possible paths that it can take. It is the superposition of such paths that produces the interference effects. It also means that if you try to detect which slit it passes, you have destroyed such superposition.

It also means that this concept is applicable to ANY quantum object with the right scale, not just photons! This is why you see intereference effects from electrons, protons, neutrons, even something as big as buckyballs! It isn't the object. If you can grab hold of any particle that can be described via QM, then it is the set-up that will allow it to make interference effects.

Zz.

 

[Schrodinger's Dog]

He says that sounds like a bohmian mechanics example of the effect and that that's not the conventional view. And that even Fennyman was puzzled by the reasoning 'till he died.

Physics World
September 2002 next article >>

The double-slit experiment
Editorial: September 2002

This article is an extended version of the article “The double-slit experiment” that appeared in the September 2002 issue of Physics World (p15). It has been further extended to include three letters about the history of the double-slit experiment with single electrons that were published in the May 2003 issue of the magazine.

What is the most beautiful experiment in physics? This is the question that Robert Crease asked Physics World readers in May - and more than 200 replied with suggestions as diverse as Schrödinger's cat and the Trinity nuclear test in 1945. The top five included classic experiments by Galileo, Millikan, Newton and Thomas Young. But uniquely among the top 10, the most beautiful experiment in physics - Young's double-slit experiment applied to the interference of single electrons - does not have a name associated with it.

Most discussions of double-slit experiments with particles refer to Feynman's quote in his lectures: "We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery." Feynman went on to add: "We should say right away that you should not try to set up this experiment. This experiment has never been done in just this way. The trouble is that the apparatus would have to be made on an impossibly small scale to show the effects we are interested in. We are doing a "thought experiment", which we have chosen because it is easy to think about. We know the results that would be obtained because there are many experiments that have been done, in which the scale and the proportions have been chosen to show the effects we shall describe".

It is not clear that Feynman was aware that the first double-slit experiment with electrons had been carried out in 1961, the year he started his lectures (which were published in 1963). More surprisingly, perhaps, Feynman did not stress that an interference pattern would build up even if there was just one electron in the apparatus at a time. (This lack of emphasis was unusual because in the same lecture Feynman describes the electron experiment - and other double-slit experiments with water waves and bullets - in considerable detail).

So who actually carried out the first double-slit experiment with single electrons? Not surprisingly many thought or gedanken experiments are named after theorists - such as the Aharonov-Bohm effect, Bell's inequality, the Casimir force, the Einstein-Podolsky-Rosen paradox, Schrödinger's cat and so on - and these names rightly remain even when the experiment has been performed by others in the laboratory. However, it seems remarkable that no name whatsoever is attached to the double-slit experiment with electrons. Standard reference books are silent on this question but a study of the literature reveals several unsung experimental heroes.

Quantum interference is one of the most challenging principles of quantum theory. Essentially, the concept states that elementary particles can not only be in more than one place at any given time (through superposition), but that an individual particle, such as a photon (light particles) can cross its own trajectory and interfere with the direction of its path. Debate over whether light is essentially particles or waves dates back over three hundred years. In the seventeenth century, Isaac Newton proclaimed that light consisted of a stream of particles; in the early nineteenth century, Thomas Young devised the double-slit experiment to prove that it consisted of waves. Although the implications of Young's experiment are difficult to accept, it has reliably yielded proof of quantum interference through repeated trials. The noted physicist Richard Feynman claimed that the essentials of quantum mechanics could be grasped from an exploration of the double slit experiment. For this variation of Young's experiment, a beam of light is aimed at a barrier with two vertical slits. The light passes through the slits and the resulting pattern is recorded on a photographic plate. If one slit is covered, the pattern is what would be expected: a single line of light, aligned with whichever slit is open. Intuitively, one would expect that if both slits are open, the pattern of light will reflect that fact: two lines of light, aligned with the slits. In fact, however, what happens is that the photographic plate is entirely separated into multiple lines of lightness and darkness in varying degrees. What is being illustrated by this result is that interference is taking place between the waves/particles going through the slits, in what, seemingly, should be two non-crossing trajectories.
We would expect that if the beam of photons is slowed enough to ensure that individual photons are hitting the plate, there could be no interference and the pattern of light would be two lines of light, aligned with the slits. In fact, however, the resulting pattern still indicates interference, which means that, somehow, the single particles are interfering with themselves. This seems impossible: we expect that a single photon will go through one slit or the other, and will end up in one of two possible light line areas. But that is not what happens. As Feynman concluded, each photon not only goes through both slits, but simultaneously traverses every possible trajectory en route to the target, not just in theory, but in fact.

In order to see how this might possibly occur, experiments have focused on tracking the paths of individual photons. What happens in this case is that the measurement in some way disrupts the photons' trajectories (in accordance with quantum theory's uncertainty principle), and somehow, the results of the experiment become what would be predicted by classical physics: two bright lines on the photographic plate, aligned with the slits in the barrier. Cease the attempt to measure, however, and the pattern will again become multiple lines in varying degrees of lightness and darkness.

Quantum interference research is being applied in a growing number of applications, such as the superconducting quantum interference device (SQUID), quantum cryptography, and quantum computing.

however Feynman wasn't happy with this model and 'til his dying day was still unsure.

So if one photon hits another why does it difract it is it a particle why do two waves interacting cause the light to be difracted. How can light interfere with itself when it is extremely difficult to get it to do so?

 

[ZapperZ]

Please do not cut-and-paste things like this. A link is sufficient. Furthermore, this article is something I've already referred to previously, so I do not need wholesale quotation out of this PhysicsWeb site.

Note that you wanted an "explanation", not a formalism, which you could have found in the Marcella paper. The proper formalism would have no such baggage.

And Feynman wasn't happy with A LOT of things by the time he died.

Zz.

 

[Schrodinger's Dog]

I cannot afford the Marcela paper as I already mentioned so that's kind of redundant. You seem to have a very confrontational way of adressing issues if I may say so. I realize a lot of people asking the same questions is annoying but there is no real need to get defensive or mildly offensive, a simple explanation without the implied criticism would be nice. However that said thank you for the information it has been very educational.

I was thinking of anyone reading this thread, having to trawl through all that information to find a small amount of info is quite time consuming: thus the information here, yes I could have posted a link but I didn't think of that at the time.

 

[inha]

You can get a hold of the paper by going to your nearest university's library. If there's a physics department there's also a collection of physics journals and they will have rev.mod.phys.

 

[ZapperZ]

I cannot afford the Marcela paper as I already mentioned so that's kind of redundant. You seem to have a very confrontational way of adressing issues if I may say so. I realize a lot of people asking the same questions is annoying but there is no real need to get defensive or mildly offensive, a simple explanation without the implied criticism would be nice. However that said thank you for the information it has been very educational.

Trust me, as you have seen, if I want to be "confrontational", it would have been VERY clear. I wouldn't have entertained your question the way I did.

I was thinking of anyone reading this thread, having to trawl through all that information to find a small amount of info is quite time consuming: thus the information here, yes I could have posted a link but I didn't think of that at the time.

I have read several of your posts, especially with regards to finding an "explanation" to this and that. At some point, you have to clarify what you mean by "explanation".

In many aspects (if not all) of physics, what one ends up with at the most fundamental level, is a DESCRIPTION. We can describe so-and-so phenomenon accurately via a theoretical description. This fundamental description EXPLAINS a higher level process. For example, the explanation on how superconductivity in conventional superconductors occur is based on the formation of condensates mediated by phonons, forming a "superfluid" that has long-range order. However, if you then look at the wavefunction of the condensates and ask for an explanation on why it works, then we have to stop because there's nothing beyond that. So the wavefunction produces a description, and that description in turn is the explanation of a higher level process.

This is why many physicists think that physics (and even science in general) can only provide an explanation of what Nature is at the most fundamental level. You can say "oh, photons mediate the EM interactions" as an explanation for the whole calss of EM phenomena. But if you then dig deeper at photons and electrons and quarks, you'll see that these are a set of descriptions. It doesn't mean these will stay that way because we continue to move the location of the "fundamental level" of our understanding - we continue to study the origin of charge, mass, and spin so that what is a description now will maybe become an explanation due to a more fundamental understanding.

So when you ask for an "explanation" of a photon passing through a slit, I have no clue on whether you want a complete description of such a phenomenon, or do you want a lower-level description of the process that may or may not exist. I certainly cannot explain why we can describe something with quantum superpostion of eigenstates, because a large part of it came out of the postulates of quantum mechanics.

Since you brought up how Feynman felt about things, he was also attributed to saying "shut up and calculate", which is often how I view things as an experimentalist. At some point, the "words" description of a phenomenon doesn't do justice to the exact formalism. Pop Science writers encouter this problem all the time. How does one put into words a complex mathematical description of a process? One then starts using analogies, and often, this comes up very short because one has to apply the mathematical description to a SPECIFIC case. It is why many physicists apt to claim that "no one understands quantum mechanics". It is not because we no nothing about it MATHEMATICALLY, but we don't understand it the way we understand classical physics. It doesn't fit into what we know and love. A physicist's level for saying "we understand something" is VERY high, much higher than what normal people take for granted as something they claim to understand. So often, quotes from Einstein and Feynman along this effect get misconstrued to mean we know nothing about QM.

So if you want a description of the diffraction process, there are plenty. I've even offered one based simply on the application of the HUP. If you want an "explanation" of the diffraction process, then you have to let us know at what fundamental level do you want it, because it may not exist, at least no in the way you want it.

Zz.

 

[reilly]

]Ben -- various comments on you comments:

If all the answers are plain as day in the textbooks, and the textbooks are totally right, and we should not question the theories, what is the point of having this website or even professors? There is absolutely nothing to discuss.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
RA
Your conditionals are way off -- physics is all about challenging theories; that's what professional physicists do. But, do note that sometimes challenges are met with hostile reations, and the challenge become very contentious. So, at time, physics is a contact sport. And, surprise, many professional physicists are arrogant -- I know more than you do kind of stuff -- so they tend to ignore beginners, with whom they are not kindly disposed to discuss basis. In a sense , it's little different than say, that the chances of the CFO of GM helping a low-level employee to answer an accounting question are slim to none.(Important people, are important, as many of them know.)

If you have the misfortune to make a mistake or two in your challenge, then you may be in for a very tough, antagonistic fight, in which personal attacks may well figure. The unwritten rule is: DO YOUR HOMEWORK. Know as much, if not more, about the subject than those challenging your ideas.
Worse yet is the fact that many folks with an interest in physics have a best a limited understanding of the field -- know little about the subject matter and of the culture of physics. But, that need not be quite so. DO YOUR HOMEWORK. One of the best features of this forum are the folks who will work with, spar with, advise you--if you are willing to listen and learn.

Indeed, some answers in some textbooks are right -- this is surprising? And, the texts will explain the "rightness' -- motion down a frictionless plane, RLC circuits, Rutherford's experiment scattering alpha particle off of nucleii, diffraction and optical interference, and ... Virtually every tme you turn on a light, a TV, ride in an airplane, use a magnet to post favorite photos on your fridge, you are testing theories of physics. It's a good thing that we can rely on the basics to be as reliable as sunsets and sunrises. Before you can enjoy a print novel, you must know how to read -- unless you can con someone to read to you. Before you can get a basic understanding of physics, or anything for that matter, you must pay your dues, as like, -- man, we say in the jazz scene -- don't get on the bandstand if you can't play at or above the level of the session. If you are old enough, and intellectually sophisticated enough to read and understand a textbook, then certainly you are capable of determing the correctness of what you are reading. DO YOUR HOMEWORK

>>>>>>>>>>>>>>>>>>>>>>>>>>>...
Ben
Empirically: relying on experience or observation alone often without due regard for system and theory.
Obviously we are interested in different issues, but why do you want to prevent someone from wanting to know something you don't happen to be interested in? I what to know how the model works, and you just want to know that it works. These are different issues.
,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
RA
I've always enjoyed reading about other people's ideas of what or how I think, and, in this case, in what I'm interested. In fact, I''ve written about "how a model works" and "how to know it works in practice", albeit in the 70's when I was Economist specializing in urban growth and housing. How you got the idea that you did about my empirical proclivitiesis and my atitudes toward models is quite beyond me. You didn't do any homework, so you got it wrong. That is, your characterization of me that's above is big-time wrong.

>>>>>>>>>>>>>>>>>
Ben

As Greene points out just because the model works, that doesn't mean the theory behind it is correct.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
RA
Tell us something new. Many of us here are at least a litle bit aware that theory and models don't always agree. And many of us have had to deal with such an issue in our professional lives.
>>>>>>>>>>>>>>>
Ben
But I know you don't like questioning here. I didn't make any negative comments about QM or QED in my last posts, I was only asking questions.
I just want to know how some of you interpret the basic model of QM And QED.
>>>>>>>>>>>>>>>>>
RA Really, how in the world can you make any statement about what I like or dislike? So far, for you, it's a train wreck, a bad day at the plate (0/2)

>>>>>>>
Ben

Surely there are slightly different interpretations? For example are you in the camp that thinks QM and QED is a perfect model, or are you in the camp that thinks that many of the ideas are just models. Surely that should be considered to be an acceptable enough type question here. Isn't Greene a peer reviewed published scientist? He is asking questions, so why can't I repeat the same question here? I don't see how I can get that out of a textbook, as I'll likely just get the authors viewpoint.

RA What else would you expect to get? The cure is: read several textbooks.In practice. ther'e just one interpretation which totally dominates day-to-day physics. I did my my physics in the 60s, including work on QED. The interpretation appropriate for my work is still the one in use -- it's just what I call Practical Born-Copenhagen. The square of the wave function (norm of the state vector if you prefer) is a probability density, which allows us to determine averages, cross sections, statistical properties and so on. That's about it. The physics is awesome -- see, for example, Pauling's General Chemistry, based on Quantum mechanics, show that all chemistry can be explained with the help of QM. Contrary to what you might think, many of Pualing's discussions are based on physical reasoning more than on mathematical manipulations. See Horowitz and Hill, The Art Of Electronics, who discuss semiconducters and transistors; quantum devices based on a very sophisticated model(,in this case the model = the theory.) They do so, with a bare minimum of algebra let alone calculus, with physical reasoning. The notion that QM is just a bunch of rules for computations is a very unenlightened view, a reaction shared by many working physicists. As I've suggested above, their are countless cases of QM giving the "why" as well as the how.

Again, God forbid I should suggest it, but some of this type of approach can be found in freshman physics books -- as best as I can figure, in part from personal experience, authors of textbooks in physics are seldom motivated by cash -- although Halliday and Resnick both became millionaires from their joint texts. Rather, the point is to help the new generation learn what's needed to learn physics, including how to master a subject without a teacher -- to pass on hundreds of years of knowledge -- and to help students become independent minded and critical thinkers. So, ask questions. But be prepared for clues rather than answers -- the plain fact is that almost all of the questions asked here indeed have been answered in the literature. So, again, at the risk of repetition, DO YOUR HOMEWORK.

Regards,Reilly Atkinson

 

[Schrodinger's Dog]

Thanks that was exactly the sort of answer I was looking for ZZ, well not exactly but pretty close.

Have you ever thought in an idle moment what it is that lies behind the interference, it would be easy to assume that lights behaviour as a particle is because it is neither a wave nor a particle(a warticle or a pave maybe) if it were in some way both then it would classicaly speaking have a solution but that is not really a very comfortable thought, or is it. If Photons have an extremely/almost infinitessemly small mass then they should be able to achieve light speed with enough energy surely? In that light speed would actually be the speed at which the particles mass cannot have enough energy to move any faster and if that is the case they could be literaly both a wave and a particle? I know it's speculation but it's interesting none the less? Could we at some point with sophisticated enough technology weigh light and prove or disprove once and for all that it is massless? If it is massless then the two slit experiment becomes even more bemusing because energy should not deflect energy like that or is there some other effect that we don't know about? Well I'll go off for a while and speculate, do carry on

Light having a very small mass does not destroy the theory, but it would explain difraction wouldn't it?

 

[ZapperZ]

Light having a very small mass does not destroy the theory, but it would explain difraction wouldn't it?

No it wouldn't. Where in diffraction theory is "mass" necessary?

Zz.

 

[Schrodinger's Dog]

Why wouldn't it. a single photon would not necessarily effect itself and would hit in a straight line. If you measure it, bounce light off it then it would deviate? The Energy of light itself would have too small a mass to deviate light
I didn't say it was necessary I just said it would help to explain difraction, photons would in fact bounce off each other. And nothing precludes this as far as I can see from being true.
I've even thought of an experiment to test the validity of this premise, you could use the fact that without mass light should be bent by gravity x amount if it has mass it will be bent by y amount y-x can be used to give the mass of light. If there is no difference then light is massless. Could we use this to test the masslessness of light too? The only thing stopping this from working is lights mass may just be too small to have any visible effect over distance. In which case back to square one.

 

[ZapperZ]

Why wouldn't it. a single photon would not necessarily effect itself and would hit in a straight line. If you measure it, bounce light off it then it would deviate? The Energy of light itself would have too small a mass to deviate light
I didn't say it was necessary I just said it would help to explain difraction, photons would in fact bounce off each other. And nothing precludes this as far as I can see from being true.
I've even thought of an experiment to test the validity of this premise, you could use the fact that without mass light should be bent by gravity x amount if it has mass it will be bent by y amount y-x can be used to give the mass of light. If there is no difference then light is massless. Could we use this to test the masslessness of light too? The only thing stopping this from working is lights mass may just be too small to have any visible effect over distance. In which case back to square one.

This is getting very confusing.

I don't need OTHER photons to produce a diffracton pattern. I can shoot ONE PHOTON AT A TIME at a slit. After I've done this a gazillion times, I REGAIN the diffraction pattern that I know and love from doing this the normal way.

So where is light boucing off each other, which is in itself a rather complicated effect, to produce such well-defined diffraction pattern?

Zz.

 

[Schrodinger's Dog]

Think of waves of water molecules in a tank, what pattern do the produce from bouncing off each other, do you see any inteference patterns that may be similar with light?

 

[ZapperZ]

Think of waves of water molecules in a tank, what pattern do the produce from bouncing off each other, do you see any inteference patterns that may be similar with light?

Interference pattern you see from water waves is NOT due to water molecules bouncing off each other! It is due to the interference of a COLLECTIVE effect known as WAVES!

Again, I brought up the issue of shooting one photon at a time and regaining the diffraction pattern. Where is the "collision" between photons there?

Photon-photon scattering cross-section is EXTREMELY SMALL! In fact, under QED, photon-photon scattering at optical range is practically non-existent! The photon-photon collider that is being envisioned is being designed for gamma range where the scattering cross-section is appreciable enough to be detected.

So I do NOT buy the explanation of photons bouncing off other photons as the explanation for diffraction. This makes zero sense if one just consider on how to explain why a smaller slit would cause a wider spread of the diffraction pattern, especially when such a change does not alter the photon density in the incoming light.

Zz.

 

[inha]

Think of waves of water molecules in a tank, what pattern do the produce from bouncing off each other, do you see any inteference patterns that may be similar with light?

Similiar? Perhaps. That does not imply any sort of connection between the two very very very different phenomena though.

 

[Schrodinger's Dog]

you have balls running together they pass through a slit due to gravity(they are in a vertical column with slits at the centre of the column and cups to catch the balls at the bottom, which slits difract the balls the most the larger ones or the smaller ones, if we have the same number of balls but vary the size of slit which slits effect the balls difraction the most? Now if we have a superposition of each ball does it effect the balls path? The light hits the detector in a straight unbroken line from a single photon unless we measure it then it difracts?

i'd try the experiment with 1,10,15,20,30 balls on varying slit sizes, if there was no change in difraction with the slit sizes with 10 balls i would be awfuly shocked, I would suspect the difraction would be the same regardless of how many balls you used too? Not sure having not done the experiment

 

[ZapperZ]

you have balls running together they pass through a slit due to gravity(they are in a vertical column with slits at the centre of the column and cups to catch the balls at the bottom, which slits difract the balls the most the larger ones or the smaller ones, if we have the same number of balls but vary the size of slit which slits effect the balls difraction the most? Now if we have a superposition of each ball does it effect the balls path? The light hits the detector in a straight unbroken line from a single photon unless we measure it then it difracts?

Will you please do the rest of us a favor and LOOK UP the diffraction effects? Figure out how the diffraction patterns changes with respect to the slit size, frequency of light, etc. You should know this already before you start requesting for an explanation, much less formulating one yourself.

And then, try, if you can, not to ignore my point that you CAN get diffraction patterns by shooting one photon at a time. In fact, you can get INTERFERENCE pattern by shooting one photon at a time! This nullifies as clearly as one can get your photon bouncing guesswork.

Zz.

 

[Schrodinger's Dog]

well that was quite patronising it's not the size of slit that counts it's the number of slits more slits means more interference. The same with balls in my gravity experiment. same thing would happen, increase the speed of the balls what happens. Also it says that single photons interfere with them selves because of super position can you tell me any reason why if they were both a wave and a particle at the same time this still wouldn't be the case?

 

[ZapperZ]

There is already an FAQ in the General Physics sub-forum on this question of Wave-Particle picture. This is on top of all the already discussed threads.

If anyone ever question on why I get rather annoyed by one of these things, please take a very close look at what has transpired here because it illustrates a very difficult and tedious task that one ALWAYS has to do in this situation. When you offer an answer to a question, you then end up having to explain the answer, and then end up explaining THAT answer. At some point, this goes on indefinitely and I don't care if you're a saint, it DOES get very tiring.

Do NOT be offended if someone tells you that you need to do some background studying/reading at some point! It means that your ability to comprehend the answer requires that you have some pre-requisite knowledge at some level. You have to do SOME of the work in putting an effort to understand something - it isn't just gimme, gimme, gimme. I shouldn't have to dig back down to now having to explain why wave-particle duality is only in NAME, not in principle, as far as QM is concerned. There is NO DUALITY! <shock rings through the forum>

So now, we went from photon "size" and property, to photon having mass, to photon going through a slit, to photon "wave-particle". Wouldn't it have been a lot more effective to start from ground zero and look at basic physics first?

If not, I give up!

Zz.

 

[Schrodinger's Dog]

Well the point I was trying to make is light could have mass, no reason why it can't and if it does it would mean that difraction would be because of interference on 2 levels at once, it also means the photon has a size, all I was saying was that you can't discount a photon having mass or otherwise and if it does have mass it might in part explain difraction as not just energy affecting energy but particles affecting particles. So we're agreed you have no idea whether a photon has mass since you can't prove it either way. And it's size could be very small so asserting it has no size is rather unproven? or was I reading a different thread? Or can you tell me why QM precludes a photon having mass? simple point? But obviously very hard to explain?

 

[inha]

Or can you tell me why QM precludes a photon having mass? simple point?

Very simple. Particles with any mass can not travel at the speed of light. Photons on the other hand do travel at the speed of light.

 

[Schrodinger's Dog]

Ok but the speed of light is defined by light nothing can go faster than light, not even light then, would that be so hard to accept? Light having no rest mass is not 100% certain is all I'm saying, so saying it has no size is not 100% certain either. That is the point. It's irrelevant to modern QM but none the less I think by logic you can see that light can have a size it's unlikley but not beyond the realms of QM theories; I don't personally believe it does but then I can't prove that.

 

[ZapperZ]

Well the point I was trying to make is light could have mass, no reason why it can't and if it does it would mean that difraction would be because of interference on 2 levels at once, it also means the photon has a size, all I was saying was that you can't discount a photon having mass or otherwise and if it does have mass it might in part explain difraction as not just energy affecting energy but particles affecting particles. So we're agreed you have no idea whether a photon has mass since you can't prove it either way. And it's size could be very small so asserting it has no size is rather unproven? or was I reading a different thread? Or can you tell me why QM precludes a photon having mass? simple point? But obviously very hard to explain?

I could have sworn we have gone through this already.

Try finding something in physics that you can PROVE! Asking me to prove that a photon has no mass is meaningless! This isn't mathematics where something can always be proven via a set of self-evident axioms! I can't prove Newton's Laws. I can show that it is VALID under the right cirumstances! There's a difference. Your insistance that I prove something is getting to be very annoying. I would like YOU to prove something in physics. Go on! Tell me something in physics that has been proven!

As far as the test for the postulates of SR, here you go if you're too lazy to dig this from my Journal

1. Severe Limits on Variations of the Speed of Light with Frequency;, B. Schaefer, PRL v.82, p.4964 (1999).

Also see Physics News Update report at http://www.aip.org/enews/physnews/19...t/pnu432-2.htm .
This is the most accurate measurement to-date that c is independent of frequency/wavelength. If photons have any mass, or if c isn't a constant, this would manifest itself as a variation in speed at different frequencies. So far, none has been detected.

2. http://www.aip.org/enews/physnews/20...t/pnu484-1.htm .

This is the most recent and accurate determination that the speed of light is independent of the speed of the source.

3. http://www.aip.org/enews/physnews/2002/split/590-1.html .

Again, this is the mostp recise test yet that the speed of light is independent of the direction of propagation.

4. Tests of Relativity Using a Cryogenic Optical Resonator;, C. Braxmaier et al., PRL v.88, p.010401 (2002).

Ether? What ether? This is the most precise determination to-date that the speed of light is independent of the velocity of the lab frame. The experiment used a version of the famous Morley-Michealson interferometer called the Kennedy-Thorndike test. You may read the Physics News Update report at http://www.aip.org/enews/physnews/2002/split/571-1.html .

5. Tests of Lorentz Invariance using a Microwave Resonator;, P. Wolf et al., PRL v.90, p.060403 (2003).

6. New Limit on Signals of Lorentz Violation in Electrodynamics;, J.A. Lipa et al., v.90, p.060403 (2003).

or read here http://physicsweb.org/article/news/7/2/12

In the 14th Feb. issue of Phys. Rev. Lett., there is not one, but TWO new experimental results that put a severe limit on any possible violation of the Lorentz transformation (which is built-in in Special Relativity). These two experiments present the most accurate result so far that c is velocity and earth-orientation independent. You may read the summary of one of this result at the AIP Physics News Update: http://www.aip.org/enews/physnews/2003/split/623-2.html or better yet, read the actual papers in PRL.

7. J. Luo et al., PRL v. 90, p.081801 (2003).

A report on a new measurement for the upper limit of a photon mass. In other words, if photons do have a mass, it can't be any larger than this, which is the current best resolution of our instruments to detect such a thing. This again put severe constraints on anyone claiming that photons do have a mass. http://www.aip.org/enews/physnews/2003/split/625-2.html

...

11. Muller et al., PRL v.91, p.020401 (2003).

This time, the evidence comes from the most accurate measurement to date of the uniformity of c using a modern version of the infamous Morley-Michealson experiment. Using cryogenic optical resonators, they measured for the possible anisotropy in the speed of light for over a year (as the Earth moves through space in its orbit around the sun and thus, changing its orientiation). The showed with unprecedented accuracy that the upper limit for any possible variation in c would have to be lower than 2.5 x 10^-15, which is 3 times more accurate than previous measurements.

12. M. Fullekrug, PRL v.93, p.043901 (2004).

We have another experimental evidence for the constancy of the speed of light - this time coming from very low frequency radio waves in the frequency range of 5 to 50 Hz. Again, this measurement places the upper limit on the photon rest mass (if any) at less than 4 x 10^-52 kg (yikes!).

13. This continues an earlier collection of experimental observations (listed here) that are consistent with the postulates of Special Relativity. This time the report comes from a workshop paper on the measurement of the isotropy of the speed of light using the cosmic microwave background radiation. This measurement claims the most stringent measurement on any possible anisotropy of c of our observable universe.

http://arxiv.org/abs/astro-ph/0410742

I will put it to you that each one of these indicates that there are MORE verification on the validity of SR's postulates than there are for "photons have mass".

Zz.

 

[Schrodinger's Dog]

I agree, but my point is prove it beyond reasonable doubt that was the only point I wanted to make. Therefore at the start of this thread when this question arose if you just said well we can't know 100% that light has no mass but it is extremely likely that it does and very unlikely that it doesn't if that is the case then etc,etc,etc. Might save yourself a lot of arguments.

 

[vanesch]

you have balls running together they pass through a slit due to gravity(they are in a vertical column with slits at the centre of the column and cups to catch the balls at the bottom, which slits difract the balls the most the larger ones or the smaller ones, if we have the same number of balls but vary the size of slit which slits effect the balls difraction the most? Now if we have a superposition of each ball does it effect the balls path? The light hits the detector in a straight unbroken line from a single photon unless we measure it then it difracts?

Entirely. The superposition of balls (especially the *vertical* superposition of balls) influences the diffraction pattern obtained by the slits. In fact, it is a not very well known fact, but exactly this argument was used by Einstein to try to invalidate quantum theory, and it was Bohr who answered him: the Sagnac effect of general relativity. Indeed, the Sagnac effect (also known under a slightly different angle, the Unruh effect) slightly changes the rate of the flow of the arrow of time in the gravitational field, and as such puts the eigentime of the different balls slightly out of phase, which result in the interference pattern of the balls with a coherence length equal to their inverse Compton wavelength. However, to avoid gravitational collapse, the no hair theorem states that there is a lower limit to the size of the balls that can diffract. Given the fact that photons must satisfy a similar relationship, this gives us already a lower limit on the size of the photon, and hence, through the Hawking-Galerkin theorem, a lower limit on the mass of the photon (well, the Majorana photon, of course, because the Dirac photon, through its Zitter motion, can annihilate its effective mass term in the jigsaw mechanism of the Higgs field).

However, the above reasoning is 1) not well known by most physicists, and 2) leads in practice to such a ridiculously low under bound, that it will be very hard if not impossible to determine either the size or the mass of the photon. As such, the approximation of the massless photon is usually taken to be exact, and this works for all practical purposes...


ZZ, I want my pin.

 

[ZapperZ]

I agree, but my point is prove it beyond reasonable doubt that was the only point I wanted to make. Therefore at the start of this thread when this question arose if you just said well we can't know 100% that light has no mass but it is extremely likely that it does and very unlikely that it doesn't if that is the case then etc,etc,etc. Might save yourself a lot of arguments.

Sorry, but this is OBVIOUS to anyone who has studied even elementary SR! It is why Einstein himself formulated that E = pc for photons! This is clearly in the introduction page of ANY lesson on modern physics, even online texts have such a thing! Refer to the hyperphysics page if you don't believe me! I didn't realize that I have to always qualify what is in the TEXTBOOKS as being "extremely likely"!

Reasonable doubt? Did you know that the most convincing evidence that the postulates of SR is correct is NOT from some esoteric experiements, but rather from the electronics that YOU are using? Every single second, you are validating a number of SR's postulates! How? The semiconductors that you are using in your solid-state transistors require RELATIVISTIC CORRECTIONS to account for the band structure. Without that, the theoretical band structure does NOT fit the experimental results, and we have no clue on why they work!

We regard SR with a high enough validity that we USE it for practical applications! A lot of people take this for granted or just ignorant about such facts, but yet, this is THE most convincing evidence that something is valid.

Zz.

 

[ZapperZ]

ZZ, I want my pin.

Not just yet, vanesch. There's a condition that has yet to be met!

Zz.

 

[Schrodinger's Dog]

I think the reson this argument went on for so long was because what you asserted to him you asserrted 100% to be true. It seems to me that he gave up posting when you explained some source material, but the confrontational nature of your posting made him continue to post long after you'd made the point. It does take a great deal of patience to explain some of th ideas in QM but if you say things like we know and site references your average student has no chance of accesing then your argument gets lost because we have no idea what your talking about you then say something like did you even read bla bla bal and we say no I can't get acces to it, and then we come up with something based on what we can actually read and you say my god din't I already explain this, then you say. Anyway the upshot is don't assert anything until you know the person has read the relevant material, I couldn't follow your arguments because I couldn't acces some of the stuff you'd referenced,when I made this clear you ignored it twice. No wonder you get into such lengthy arguments so often, your failing to acknowledge that not everyone has the source material you do. That on top of the fact that you seem to be saying(to most people anyway that you're right beyond reasonable doubt, not the case but it sounds that way) and you have a problem, which is why I suggested you start posting information people can actually access and not chastising them when they don't fully understand your argument because of it. I think I've made my point. And I think I should get that pin thankyou very much.

 

[inha]

I think the reson this argument went on for so long was because what you asserted to him you asserrted 100% to be true. It seems to me that he gave up posting when you explained some source material, but the confrontational nature of your posting made him continue to post long after you'd made the point. It does take a great deal of patience to explain some of th ideas in QM but if you say things like we know and site references your average student has no chance of accesing then your argument gets lost because we have no idea what your talking about you then say something like did you even read bla bla bal and we say no I can't get acces to it, and then we come up with something based on what we can actually read and you say my god din't I already explain this, then you say. Anyway the upshot is don't assert anything until you know the person has read the relevant material, I couldn't follow your arguments because I couldn't acces some of the stuff you'd referenced,when I made this clear you ignored it twice. No wonder you get into such lengthy arguments so often, your failing to acknowledge that not everyone has the source material you do. That on top of the fact that you seem to be saying(to most people anyway that you're right beyond reasonable doubt, not the case but it sounds that way) and you have a problem, which is why I suggested you start posting information people can actually access and not chastising them when they don't fully understand your argument because of it. I think I've made my point. And I think I should get that pin thankyou very much.

Any university student will have access to scientific journals. Even if one is not an university student but there's an university nearby can go to it's library. Someone not bothering to check the references provided is not the fault of the one who gave the references.

 

[ZapperZ]

I think the reson this argument went on for so long was because what you asserted to him you asserrted 100% to be true.

What exactly did I asserted to be 100% true? Where did I do this?

My incursion into this thread was to ILLUSTRATE why the question "how big is a photon" is similar to asking "how painful is purple?". Or did you forget that? How did I say what to be 100% true?

It seems to me that he gave up posting when you explained some source material, but the confrontational nature of your posting made him continue to post long after you'd made the point. It does take a great deal of patience to explain some of th ideas in QM but if you say things like we know and site references your average student has no chance of accesing then your argument gets lost because we have no idea what your talking about you then say something like did you even read bla bla bal and we say no I can't get acces to it, and then we come up with something based on what we can actually read and you say my god din't I already explain this, then you say. Anyway the upshot is don't assert anything until you know the person has read the relevant material, I couldn't follow your arguments because I couldn't acces some of the stuff you'd referenced,when I made this clear you ignored it twice. No wonder you get into such lengthy arguments so often, your failing to acknowledge that not everyone has the source material you do. That on top of the fact that you seem to be saying(to most people anyway that you're right beyond reasonable doubt, not the case but it sounds that way) and you have a problem, which is why I suggested you start posting information people can actually access and not chastising them when they don't fully understand your argument because of it. I think I've made my point. And I think I should get that pin thankyou very much.

Er.. as far as the "pin" is concerned, you have no clue what vanesh is up to, so try not to practice the same thing that you've been doing in here, which is jumping onto something that you barely know about.

Secondly, ignorance is not an excuse as a license to formulate things. That's like Bugs Bunny able to float in air because he said he never learned about gravity. When you ASKED me, not about SR's postulate, or the validity of SR's formulation, but its APPLICATION in the form of the observation of diffraction effects and "mass", I PRESUMED that you ALREADY know ALL about the postulates and what they mean! ANYONE can read up on this and you don't need special web access. Hyperphysics website has a wealth of info on this! But then you started asking for PROOFS! This is hysterical! What form of "proofs" do you want? How does one prove a physics theory or concept? If you want indications if they are valid, then I cite EXACT SOURCES. I don't just TELL you. I do not expect you to buy everything I said without backing it up. I tell everyone here the very same thing, that you need to be more discriminating when someone tells them something and to demand credible sources! That's what you got! I gave you exact citations on various papers in REPUTABLE JOURNALS. This is how we do physics! I treated you with enough respect that I EXPECT you to double check on my claims and the sources!

Instead, you complain that you can't get access to them! Oy vey!

Zz.

 

[Schrodinger's Dog]

The point is though if your trying to explain something to someone and they say I ahve no idea what your talking about because I can't read the sources, don't be surprised at why people then say something that goes against your sources, it's because they have no idea what you are talking about or how you came to that conclusion. It may be how the Physics world does things. But if a teacher walked into a classroom and said turn to page 40, And there were no books on the desks, don't be surprised when the students learn nothing. You also have a habit of missing the point a lot too.

 

[ZapperZ]

The point is though if your trying to explain something to someone and they say I ahve no idea what your talking about because I can't read the sources, don't be surprised at why people then say something that goes against your sources, it's because they have no idea what you are talking about or how you came to that conclusion. It may be how the Physics world does things. But if a teacher walked into a classroom and said turn to page 40, And there were no books on the desks, don't be surprised when the students learn nothing.

But it doesn't bother you that the book IS required in the class and the students don't have it? I would have asked "Why DON'T you have a book? Wasn't it listed as required to be in this class"?

The problem here is that you'd rather be told in having-waving forms to the answers you want, rather than specific technical details. I've been trying to ween people off this and make people want MORE details. Hand-waving arguments should no longer be tolerated as being the definitive answer, it is too ambiguous. Yet, look at this thread and see how "satisfied" some people are with such things. I can find unbelievable amount of loopholes in those explanations.

I have taught college level courses before. I have to assume that the students entering my class already POSSESS a certain level of knowledge. If not, I will NEVER be able to tackle the subject matter because I always have to backtrack and answer questions that they should have known already! This is what I had to do with you in this thread. It appears that almost at every turn, we are going back into the fundamentals of not only SR, but QM! At some point, such an exercise is no longer productive and gets nowhere fast. If you want to know about QM and SR's fundamentals, ask THOSE! But if you start asking about a higher-level question that makes use of those foundations, then do not be surprised if someone tells you to go look up those foundations yourself!

And talk about missing points. You missed MY question on where I asserted something to be 100% true. Where did I make such a statement? Or can you not back up things that you say?

Zz.

 

[reilly]

SDOG
[Anyway the upshot is don't assert anything until you know the person has read the relevant material, I couldn't follow your arguments because I couldn't acces some of the stuff you'd referenced,when I made this clear you ignored it twice.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
RA I'm quite baffled. I presume you are quite serious about requiring people to make sure you have found and understood sources and relevant material before they deign to help you. The nicest thing I can say about such a statement is that it is backwards and upside down. Nobody owes you nothin.'

Also, clarity, like beauty, is in the eye of the beholder.

If you cannot cut it, go back and study, and do homework problems -- unless, of course, the author of the problems is somehow disagreeable or unreliable.

>>>>>>>>>>>>>>>>>>>>>>>.


No wonder you get into such lengthy arguments so often, your failing to acknowledge that not everyone has the source material you do.
>>>>>>>>.. Everyone, i assume, can ask questions

That on top of the fact that you seem to be saying(to most people anyway that you're right beyond reasonable doubt, not the case but it sounds that way) and you have a problem, which is why I suggested you start posting information people can actually access and not chastising them when they don't fully understand your argument because of it. I think I've made my point. And I think I should get that pin thankyou very much.
>>>>>>>>>>>>>..
(RA)
You clearly have not taken the time to learn much about the field of
physics. If you did, then you would never say much of what you do above.And, is it not odd, that more often than not, many see this forum as a great opportunity to learn, and so they "listen' and learn, and enjoy the process.

With all due respect, you have just said what many of my college sudents said upon receiving a grade less than that expected. It was my fault, not theirs. Many more, however took a somewhat different tack.


I'm wondering -- Have you ever taught physics? or anything?

Regards,
Reilly Atkinson

 

[Boffin]

Photons do have mass, but

Or can you tell me why QM precludes a photon having mass?

Very simple. Particles with any mass can not travel at the speed of light. Photons on the other hand do travel at the speed of light.

I'm jumping in here with an answer, because none of the mentors have provided a decent one to this issue in my opinion. So if you don't like the answer I guess you'll just whack me over the head. So be it.

Photons do have mass! Relativistic mass (I originally wrote relative which of course is wrong) that is. But photons do not have rest mass. And the common definition of mass in science books basically is that when the word mass is used by itself then mass=rest mass. Ideally scientists should always be saying the photon does not have rest mass, because that is what they really mean. Then the issue would not cause confusion. All energy can be said to have relativistic mass (I originally wrote relative) directly proportional to the energy. I've studied the issue quite a bit and that is the best description by experts I can find.

 

[Dense]

In the two undergrad physics courses I took, there was no mention of QED, and relativity was only lightly touched on. So forgive my ignorance.

But what I'm getting from reading this thread is the following:

Q: How big is a photon?
A: It has no size. A photon has a location, but occupies no space.

Q: What does a photon look like?
A: It doesn't look like anything. It is a chunk of energy, not made of matter. It's hard to imagine energy by itself, because we tend to think of it in terms of its effect on matter, but that's what a photon is.

Q: What does a photon behave like?
A: A photon has some very strange and very cool behaviors. It has momentum but no mass. It only ever travels at the speed c; any slowing effect you observe is caused by absorbtion and emission of photons by matter. A single photon can be diffracted like a wave, and can behave as if that single photon passed through more than one slit at the same time and interfered with itself. There are even stranger and cooler behaviors.

Q: Why does a photon behave like that?
A: Quantum Electrodynamics is the best explanation we've got. It says a photon takes every possible path, at the same time, once it's been emitted (travelling in straight lines). Each path has a different probability, calculated with complex numbers, and the math gets funky. The weighted average of all the probabilities gives you the final observed path of the photon.

Q: How's that work?
A: This forum isn't the best place to get that answer. Here's some sources you might find helpful.


Am I sort of on target here?

 

[ZapperZ]

I'm jumping in here with an answer, because none of the mentors have provided a decent one to this issue in my opinion. So if you don't like the answer I guess you'll just whack me over the head. So be it.

OK, since you ask for it...

Photons do have mass! Relative mass that is. But photons do not have rest mass. And the common definition of mass in science books basically is that when the word mass is used by itself then mass=rest mass. Ideally scientists should always be saying the photon does not have rest mass, because that is what they really mean. Then the issue would not cause confusion. All energy can be said to have relative mass directly proportional to the energy. I've studied the issue quite a bit and that is the best description by experts I can find.

OK, NOW we're playing with words. When you say the mass of the Earth is so-and-so kg, or the mass of that object is so-and-so kg, do YOU ALWAYS have to clarify that this is the REST MASS and not the RELATIVISTIC (there's no "relative mass") MASS? Is this ALWAYS a required clarification, failure of which causes utter ambiguity? HONESTLY?

And I'll tell you what. I can play this game as well as the next person. I'll show you why EVEN the use the the term "relavistic mass" isn't kosher!

http://arxiv.org/abs/physics/0504110
http://arxiv.org/abs/physics/0504111

So take that!

Zz.

 

[ZapperZ]

In the two undergrad physics courses I took, there was no mention of QED, and relativity was only lightly touched on. So forgive my ignorance.
But what I'm getting from reading this thread is the following:
Q: How big is a photon?
A: It has no size. A photon has a location, but occupies no space.

Not quite. Recall there's a difference between an "empty set" and a "set of 0". Similarly, the question on the size of a photon doesn't have an answer "it has no size", because that means it's size is zero. In condensed matter physics, we deal with a "quantum dot" and can have several phenomena associated with such a thing. So even something with almost no size can be investigated.

No, the answer that *I* have given was that I can't answer that because "pain" was never defined with the color purple. I cannot say the answer to the question to how painful purple is that is it is not painful. That would be as meaningless as if I were to answer "yes, purple is VERY painful". Until we have a formulation regarding the SIZE of a photon and how that is defined, I have no way of answering that.

Q: How's that work?
A: This forum isn't the best place to get that answer. Here's some sources you might find helpful.
Am I sort of on target here?

From my observation on here, and in other forums, the BEST usage of a medium like this is as AN IMPETUS to study things in detail. What this means is that you ask a question, it turns out it is more complex than what you originally thought, people responding gave an idea of what is involved in the answer, and point to some references in which a clearer, more in-depth description can be found. This, to me, is the most effective usage of a forum such as this. Ultimately, the learning part has to done by the individual. You are the one who has to put in the effort. If it is tedious and difficult, then tough! If it were easy, EVERYONE would and could have done it!

Zz.

 

[Dense]

Thanks for the clarification.

So it's not even "a photon has no size," but rather "size isn't something that pertains to photons." Is that the distinction?