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

[ZapperZ]

Dear ZapperZ,
There is a difference between the phrases: “a class” and “a member of a class”. I do not use terms out of context. You fail to read the word “member” in my text, which makes all the difference.
As for your remark: “…waiting for you to find for me the definition of a photon that involves a physical size…”, as I have tried to said in every one of my posts, the question is valid, while the provided answer makes the question obsolete. So, you keep asking the same question. I never said that there is a physical “size of a photon”, although I defend the legitimacy of the searching of a “size of photon” from the "ignorant".
I will provide later some questions about your article.

Let's review, shall we?

The purpose of the science of Physics is to provide answers to any question. It was also indicated to you that ‘asking “how big is a photon" is similar to asking "how painful is purple?” ’. The question “how big is a photon?” is a legitimate question which got its answer by the methodical work of many scientists. The scientific quest in finding the answer for the “size of a photon” eventually provided by the mathematical knowledge that: *a photon is actually anywhere at the same time, but we can not define its position nor can we predict its “space” !* Although this answer seems that it does not construct a specific “morphology” for a specific photon (I use the term “specific” with the conventional meaning, although it is not a proper scientific term for photons), it is the linguistic “expression” of a (mathematical) structure provided by science and constructed by scientists, which indeed provides the “morphology of the photon", which is just the answer that was asked by the specific question.

And the specific question here is how BIG is a photon, i.e. the size.

First, let me provide some answers to your requests. I am referring to the following scientists that worked methodically in order to provide the answer to the question of “the size of the photon”: "http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/The_Quantum_age_begins.html

I have already provided the ANSWER to this question, that is: *a photon is actually anywhere at the same time, but we can not define its position nor can we predict its “space” !* You can find the same ANSWER also in the following lecture: http://galileo.phys.virginia.edu/cla...principle.html , meaning that if you come to know the precise momentum of a photon its size would be infinite, since it could be located anywhere in space. At the same time the definite knowledge of the size of photon, for instance by experimental isolation of an individual photon through a hole, produces uncertainty in its momentum.

You just said that ITS SIZE WOULD BE INFINITE. Of course, you are referring a "class of photon", whatever that is physically.

And this is where I pointed out that you made a complete mess of the understanding of the HUP. You somehow think that POSITION is equivalent fo SIZE. The size of a single slit defines the uncertain in POSITION of the photon, NOT the size of the photon. But somehow, you read this differently and thought that it was relevant to the question of the SIZE, or else why did you enter this thread armed with that wrong "evidence"?

I'm not going to answer your questions till you answer mine.

Do you STILL claim that "...The question “how big is a photon?” is a legitimate question which got its answer by the methodical work of many scientists"? If you do, please provide peer-reviewed citations.

If you can't provide me a definitive answer on that, then do not expect a free physics lessons on here from me.

Zz.

 

[Schrodinger's Dog]

Clarifictaion of your understanding itself and then clarification of what and why it is your asking such a question. Define your reasoning citing why you believ what you believe and considering the points made in counter and then ask a valid question.

Can I have a free physics lesson ZZ?

And that volume you refer to Ben is there a website or link I could go to to find out more, I'd hate to have to pay out a lot of money to find out something that can be summed up in 5 pages of text.

 

[leandros_p]

Let's review, shall we?

And the specific question here is how BIG is a photon, i.e. the size.

You just said that ITS SIZE WOULD BE INFINITE. Of course, you are referring a "class of photon", whatever that is physically.
Zz.

Dear ZapperZ,

I can not understand why you constantly censor my texts, in your answers. By censoring my original phrase you take out of context the following certain part: “its size would be infinite”, and by that you are falsify my message. If you preserve the integrity of my phrase which is: "if you come to know the precise momentum of a photon its size would be infinite, since it could be located anywhere in space", then it is obvious that I am talking about the position of the photon, since the phrase "it could be located" means that the uncertainty in momentum means uncertainty in location/position.

Also you use the phrase "…class of photon…", although I used the term "…member of class…" and if you do not understand the difference, at least do not make omissions in transcription”.

One more time, for the last time, I say that I support the legitimacy of asking about the “size of a photon” coming from an inquirer/student of physics, while I support that the answer to this question turns the question upside down making the question “size of photon” to become groundless.

If I am allowed to use your pattern of attitude, I am getting tired of giving free English speaking lessons, considering that English is not my native language.

You were also constantly asking about the scientists that were searching for the “size of photon”. Please see the following article from the website of American Institute of Physics: ( http://www.aip.org/history/heisenberg/p07c.htm ) that cites a dialogue between Heisenberg and Einstein:

-Heisenberg: "We cannot observe electron orbits inside the atom...Now, since a good theory must be based on directly observable magnitudes, I thought it more fitting to restrict myself to these, treating them, as it were, as representatives of the electron orbits."

--Einstein protested: "But you don't seriously believe, that none but observable magnitudes must go into a physical theory?"

--Heisenberg asked in some surprise…: "Isn't that precisely what you have done with relativity?"

-Einstein : "Possibly I did use this kind of reasoning," Einstein admitted, "but it is nonsense all the same...In reality the very opposite happens. It is the theory which decides what we can observe."

Was Einstein’s question a meaningless question, just like the question about the “size of photon”? Einstein and Heisenberg were not talking about the size of photon, but Einstein’s question and reasoning is just the same meaningless in quantum physic's world as the question about "the size of photon". Was Einstein a fool, who was just babbling ? Surely the answers, provided by quantum physics, to Einstein’s question turn his question and his reasoning upside down, but he had the right to ask this question!

 

[Schrodinger's Dog]

You know you're right, you do make a different point from that portrayed. There are no stupid questions in science only stupid answers

 

[ZapperZ]

Dear ZapperZ,
I can not understand why you constantly censor my texts, in your answers. By censoring my original phrase you take out of context only the following certain part: “its size would be infinite”, and by that you are falsify my message. If you preserve the integrity of my phrase which is: "if you come to know the precise momentum of a photon its size would be infinite, since it could be located anywhere in space", then it is obvious that I am talking about the position of the photon, since the phrase "it could be located" means that the uncertainty in momentum means uncertainty in location/position.

And you ignored the point that I made that a LOCATION of an object does NOT signify the SIZE of the object. If you have understood this, why in the world are you entering a discussion on the SIZE of a photon, when the position of the photon give ZERO CLUE on its size?

You have entered this thread under the WRONG premise by confusing the HUP as an indication of the SIZE of a photon. And I QUOTED your entire paragraph before I highlighted that one phrase, so I did NOT take it out of context. To indicate that the size of a photon is INFINITE, whether one has made a measurement of its momentum or not is INCORRECT no matter the circumstances of the measurement! The size of a photon is NEVER INFINITE, especially when there is no such thing as a "size" of a photon. Your application of the HUP to this situation is what I called the bastardization of the HUP.

Also you use the phrase "…class of photon…", although I used the term "…member of class…" and if you do not understand the difference, at least make do not make omission in transcription”

And if you do not know enough physics, do not make a faulty application of it.

One more time, the last one, I say that I support the legitimacy of asking about the “size of a photon” coming from an inquirer/student of physics, while I support that the answer to this question turns the question upside down making the question “size of photon” to become groundless.

And how is this different than my original reply to indicate that asking for the size of a photon is the same as asking for how painful is purple? No where in here did I said that such a question SHOULDN'T be asked! I was illustrating WHY such a question is meaningless. So what is the problem with that? And for your benefit, I wasn't the one who made the suggestion to go look up QED, even though it is a valid suggestion.

If I am allowed to use your pattern of attitude, I am getting tired of giving free English speaking lessons, considering that English is not my native language.

That makes two of us. I am getting tired of having to dig up BASIC physics and pointing out how people simply use physics principles in ways it shouldn't be used and then have the gall to use it against me.

You also constantly were asking about the scientists that were searching for the “size of photon”. Please see the following article from the website of American Institute of Physics: http://www.aip.org/history/heisenberg/p07c.htm that cites a dialogue between Heisenberg and Einstein.
-Heisenberg: "We cannot observe electron orbits inside the atom...Now, since a good theory must be based on directly observable magnitudes, I thought it more fitting to restrict myself to these, treating them, as it were, as representatives of the electron orbits."
-"But you don't seriously believe," Einstein protested, "that none but observable magnitudes must go into a physical theory?"
-"Isn't that precisely what you have done with relativity?" I asked in some surprise…
-"Possibly I did use this kind of reasoning," Einstein admitted, "but it is nonsense all the same...In reality the very opposite happens. It is the theory which decides what we can observe."
Was Einstein’s question a meaningless question, just like the question about the “size of photon”? Einstein and Heisenberg were not talking about the size of photon, but Einstein’s question and reasoning is just the same meaningless in quantum physic's world as the question about "the size of photon". Was Einstein a fool, who was just babbling ? Surely the answers that quantum physics provides to Einstein’s question turn his question and his reasoning upside down, but he had the right to ask this question!

1. Point out to me where I prohibited the asking of such a question. It appears that it is YOU who misread what I wrote.

2. Are you implying that you cannot tell the difference in the situation here, and the situation you are quoting? HONESTLY?

3. An electron has the PRECEDENT to be considered as a classical particle. Classical E&M treated it that way. It was initially DEFINED to be that way. It is a legitimate question to ask "well, why I can't I treat this the same way in an atom?" And such questions HAVE been asked already MANY TIMES in PF. Go to the Atomic physics forum if you don't believe me. Did you see me responding with "how painful is purple?"? On the other hand, a photon WASN'T defined EVER as a classical particle. It isn't even a "particle" (Einstein called it corpuscular!) other than the fact that experimental observations were indicating it to have similar qualities - that is when the description of "particle" stuck! But nowhere in its definition is there a SIZE! THIS was my original response that you seem to have a problem with, and you took that and jump to the conclusion that this isn't a question that should be asked!

If that is how you understood things, no wonder you could make such horrible misinterpretation of the HUP.

Zz.

P.S. Where, again, is this methodical study of the size of the photon?

 

[Dense]

What a fun thread! Who knew that the humble photon could cause such vitriolic arguments, such ire, such conflict!

I wonder if other threads on this site are as entertaining.




PS -- this is my first post, so hello all! This place is so much better than other sites I've been to.

 

[Boffin]

And that volume you refer to Ben is there a website or link I could go to to find out more, I'd hate to have to pay out a lot of money to find out something that can be summed up in 5 pages of text.

Are you referring to my quotes from Brian Greene's book The Fabric of the Cosmos? Actually the quotes I used are really about all there is in the book on Quantum Mechanics that's relevant to this discussion. You can't read books like this on the Internet, but Google allows you to read pages in a book search. But while Brian Greene's book The Elegant Universe appears in a Google book search, The Fabric of the Cosmos does not, at least not yet.

 

[leandros_p]

I'm interested in various views on this issue. How far away from a hypothetical center point does the energy of a photon extent? Photons do travel at the speed of light in a vacuum. At an instant of time how far foreward and backwards does a photon interact in some way with the environment around it? Over a brief period of time how far to each side? Is it vibrating or is it a fixed point particle? If it is vibrating, are the vibrations spread out in space or zero width? One or more vibrations? If it is vibrating is it vibrating in reference to a background entity?

Dear Ben Wiens,

You can find an interesting analysis about the “size of photon and what does it look” in the pages of the book:

Physics, the Human Adventure: From Copernicus to Einstein and Beyond,
by http://www.fas.harvard.edu/~hsdept/faculty/holton/index.html"

Interestingly, page 401 of the book is web-published by Google under license at http://books.google.com/books?ie=UT...wave+dilemma&sig=7AAK90sPQDyTlbbBak84oHuwTkg" (just register to google and you will be given access to the page)

In this page, the authors of the book ask questions similar to your questions, and they present in the same page the framework that might help you understand the answers.

First, the questions: “…Indeed, the inconvenience of having to deal with photons is very real. They represent bundles of energy without having ordinary rest mass; in this they differ from the Newtonian corpuscles of light, leaving only a faint analogy between them, even through it is customary to refer to Einstein’s photon theory as the corpuscular or particulate theory of light. But our minds tend to insist on a good picture, and it requires great self control to visualize a quantum of energy without bringing in some matter to which it can be attached. It was a little easier to think of Maxwell’s light energy spread evenly through the “field” along a wave front.

Then, too, there are other questions: How large in volume and cross sectional area is the “spot” on the wave front where the photon is located? What could be the meaning of “wave-length” and “frequency” of light, which determine the energy content of the photon by E=hv, if the photon is, so to speak, only a dot on the wave front and not part of a whole wave train?

By what mechanism does the wave determine the path of the photon in such a wave phenomena as interference and polarization? How does an electron absorb a photon?...”

Then, the analysis: “In the past, one partial answer to such questions has been to hold concurrently two separate views on light – the wave and the photon model- and to apply one or the other as required by the problem. The more recent answers, which allows, in principle, a solution of every problem with physical meaning in this field, involves combining both views and assuming that the photons are distributed over the wave front in a statistical way, that is, non individually localized at a particular point. But at our level the more practical solution is, first of all, to realize that some of these questions, while possibly very disturbing, are asked on the basis of a mechanical view of atomic phenomena that may stem from an erroneous transfer of experience with large bodies obeying simple Newtonian laws. Thus the “size” of the photon is not a concept that we should expect to have the same meaning as the size of marbles and projectiles. Furthermore, photons (and atomic particles as well) differ from water waves, pebbles, and other large-scale entities in that one cannot make various experiments on the same subatomic entity. One can localize and measure and weight a stone, find its velocity, etc. and all the while it is the same unchanged stone. But a photon, after it has been sent into a Geiger counter or a photographic emulsion, is no more; two photons on which we impress different experimental conditions – for example, in searching for wave and for corpuscular properties – are not, strickly speaking, the same entities….”

Max Born’s remark at the end of the page continues to the next page, and it should be: “The ultimate origin lies in the fact (or philosophical principle) that we are compelled to use the words of common language when we wish to describe a phenomenon, not by logical or mathematical analysis, but by a picture appealing to our imagination. Common language has grown by everyday experience and can never surpass these limits. Classical physics has restricted itself to the use of concepts of this kind; by analyzing visible motions it has developed two ways of representing them by elementary processes: moving particle and waves. There is no other way of giving a pictorial description of motions – we have to apply it even in the region of atomic processes, where classical physics breaks down.

Every process can be interpreted either in terms of corpuscles or in terms of waves, but, on the other hand, it is beyond our power to produce proof that it is actually corpuscles or waves with which we are dealing, for we cannot simultaneously determine all the other properties which are distinctive of a corpuscle or of a wave, as the case may be. We can therefore say that the wave and the corpuscular description are only to be regarded as complementary ways of viewing one and the same objective process, a process which only in definite limiting cases admits of complete pictorial interpretation”

Max Born, 1946

Leandros

 

[Boffin]

Interestingly, page 401 of the book is web-published by Google under license.

Thanks, sometimes a person gets lucky.

 

[Boffin]

What theory does this thinking belong to?

There have been lot's of theories about the photon over the ages, Descartes ether theory, Newtons particle theory, Young's wave theory, Planck's quantum theory, Quantum Mechanics, Quantum Electrodynamics, Quantum Field Theory, String Theory, The Standard Model and other's inbetween. With all the theories that have been proposed over the ages, it's hard to get a handle on exactly what ideas belong to what theory, and how much of that theory is still retained in new theories. How much of this thinking below is still retained by Quantum Mechanics or Quantum Electrodynamics? How much of this is still valid?

Excerpt from http://hyperphysics.phy-astr.gsu.edu/hbase/mod2.html#c5
According to the Planck hypothesis, all electromagnetic radiation is quantized and occurs in finite "bundles" of energy which we call photons. The quantum of energy for a photon is not Planck's constant h itself, but the product of h and the frequency. The quantization implies that a photon of blue light of given frequency or wavelength will always have the same size quantum of energy. For example, a photon of blue light of wavelength 450 nm will always have 2.76 eV of energy. It occurs in quantized chunks of 2.76 eV, and you can't have half a photon of blue light - it always occurs in precisely the same sized energy chunks. But the frequency available is continuous and has no upper or lower bound, so there is no finite lower limit or upper limit on the possible energy of a photon. On the upper side, there are practical limits because you have limited mechanisms for creating really high energy photons. Low energy photons abound, but when you get below radio frequencies, the photon energies are so tiny compared to room temperature thermal energy that you really never see them as distinct quantized entities - they are swamped in the background. Another way to say it is that in the low frequency limits, things just blend in with the classical treatment of things and a quantum treatment is not necessary.

 

[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