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

  1. Dec 19, 2005 #1
    I'm interested in variouis 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?
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  3. Dec 20, 2005 #2
    Well, from the point of view of the photon, at any given instant in time the photon is everywhere along its entire path of transmission from emission to absorbsion -- it all happens at once.

    So from the photon's perspective it can be a very very long and thin "thing." As long as the distance it covered, and as wide and as tall as the amplitudes of its electro- and magnetic wave components.
  4. Dec 21, 2005 #3
    Excuse me.But I can't understand. In experiments, such as the single-atom two-slits interference, the photon behaves like a partical,traveling through the slits and touching the screen.The retardation time of photon traveling can be measured in some other experiments.Why you consider them as long and thin "thing" ? By the way, the units of the amplitudes of electro- and magnetic wave components are far different from the space coordinates, how can they compare?
  5. Dec 22, 2005 #4


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    I wrote a post in this forum about the size of photons a long time ago. You may want to check it out. Link

    Ben, your questions are difficult to answer, mostly because photons are very hard to describe in non-mathematical terms.
  6. Dec 22, 2005 #5
    Fredrik, I do remember doing a search before posting the question, but thanks for the link, I don't seem to remember finding it before. Guess I should have mentioned that I'd seen some information on how big a photon was, but was hoping for some more detailed ideas on how the photon looks like and behaves.
  7. Dec 23, 2005 #6
    When you ask "what does a photon look like," it's difficult to present an accurate answer. It is just as accurate to say a photon doesn't look like anything, as it is to say it looks like everything. Because everything your eyes see is photons, and that's the only thing your eyes see. So you couldn't observe an individual photon unless it was the photon being absorbed by your retina, and even then you don't really see the photon itself, but rather the chemical reaction in your brain caused by the absorbtion of a particular photon by a particular receptor in your retina.

    But such technicalities aside, I think I see (ha) what you're asking. You want to know how one would describe the thing itself, as it WOULD appear if, say, there was something that could be reflected off a photon that our eyes could see, and we could magnify it sufficiently.

    The answer to THAT question is, nobody knows. There are a lot of plausible hypotheses, some of which seem to be incompatible.

    A photon is not a piece of matter. It is all energy, no mass. So you can think of a photon as a teeny tiny point of energy, flying across space very fast. This helps when you're measuring particle-like behavior of photons.

    A photon is what electromagnetic radiation is made of. Electromagnetic radiation is a kind of energy wave, which is actually 2 waves in one. There's the electro- wave, and the magnetic wave. They travel together, with the same wavelength period and amplitude, but oscillate in planes perpendicular to each other. So you could imagine a photon as a pair of sine waves (kinda). They have the same x-axis, but their y-axes are 90 degrees from each other. So if you looked at the path of the propagating wave end-on, it would look like a plus sign (+). This helps when you're measuring wave-like behavior of photons.

    There are other ways to imagine photons without resorting to math, but these two are the most easily imagined and commonsensical. (Other ways include thinking of photons and other phenomena as being not things in themselves, but simply intersections of fields, much like the intersections of ripples from two stones dropped in a pond, with c being the speed at which such intersections happen to travel.)

    To make it easier for yourself, it's probably best just to think of the electromagnetic wave image. When the photon is being measured as a particle, just think of the "particle" as being the shared x-axis of the two wavelengths. That is a straight line, is the average path of the photon, and can represent the "particle" quality of photons. This way, you only have to think of a photon in one way, with the added bonus of being accurate regardless of what you're measuring.

    Notice that I said "accurate," not "true." It's an explanation that works, that doesn't mean it's what's really going on. As I said, we're still trying to figure that out.
    Last edited: Dec 23, 2005
  8. Dec 23, 2005 #7
    IMHO using analogy in this kind of argument can be easily misleading...

    We should say "photon behaves in such and such ways in such and such experiments. So we concluded it has such properties... .etc. etc." No more, no less. Especially duality is impossible to explain with analogy, because we have no other "things" with similar properties around us. This is inevitable because we are trying to introduce new concepts in physics, many of which cannot be observed around us. And we use math to deduce the underlying theories.

    For example, as for duality, a correct explanation is "when a coherent photons pass through two slits and make interference stripes on a screen at a distance, photons behaves like a wave, but in a particle point of view, we should explain at each diffraction maxima, many photons are reached, while at each diffraction minima, much fewer photons are reached." it's impossible to bring analogous exmple for this experiment, so we must "remember" this experiment, and this remembering process is thought to be "understanding duality," IMHO. Analogy to our daily material is not impossible in this one.

    All we should know and give in physics is "under given circumstances, how photon (or other objects) is expected to be observed" including probability/uncertainty based on our theories (QM). So we don't always need analogy.....just my opinion.
    Last edited: Dec 23, 2005
  9. Dec 23, 2005 #8
    Atoms are unlike any physical objects around us, yet scientists have come up with pictures, complex descriptions of how they function, their properties, and what they are made up of. Somehow there is a resistance for giving photons a modern grown up description.

    We like to keep photons as mystical things and describe them like water waves or marbles. Can we not do better than this? Personally I think we have to get rid of this concept of duality. Photons don't behave like large objects and so we should not think of them as this. Is duality not an old worn out concept that should be discarded?

    I've said my thing, but what do you think?
    Last edited: Dec 23, 2005
  10. Dec 23, 2005 #9
    As far as possible, I think we should describe photon in analogy as with atoms for easy understandings. For example we can describe photon's wave-like behavior such as interference like water waves, while we can describe its particle like behavior as particle (as the name goes :smile: ) But in describing these two behavior combined as duality of photon, we cannot resort to analogy. We should just explain the thought experiments and its results, and explain photon (and at last any other things) is such a thing. We cannot use analogy here, because we cannot observe this duality in our daily life. Or can we take other ways? We have limited phenomena in our daily life, and particles have more complex properties than we can describe in analogy.

    There are a lot of such "interpretations" or "explanations" in physics, I think. In such cases, our "understanding" is only "rememberng the relationship of the experiment's conditions and the expected results by inducing from theories in physics"
    Last edited: Dec 23, 2005
  11. Dec 27, 2005 #10
    Personally I think trying to work out new concepts and describe them is a great process of physics,so I think the question "How big a photon is" is meaningful,although there is some difficulty to answer it.
    We don't always resort to analogy to understand new things,even in classical mechanics.For example,100 years ago,in a famous sci-fi 'a trip around the moon',the writer imagined that a dead dog flies beside a spaceship to the moon.It's our commen sense now that an astronaut can orbit the earth like a sitellite,but 100 years ago,it's a great imagination,because there was neither experence nor analoy in our life on earth.
    Can the same thing happen to the micro-world?I think yes.The concept of photon will jump out of the mathematical expressions,and be so popular with us,like the orbiting astronaut.
    To the photon, what does 'big' mean? The wavelength may be an option but it doesn't give more information to the partical nature of photon.If 'big ' can be measured by the number of electrons(or other) interacting with one photon at the same time,then maybe we can conclude that the photo is at last 'smaller' than the electron(or other).
    Here I think it's easier to ask the question'How big a electron is'.As far as I know,Samuel Chao Chung Ting worked on it for many years.I don't know more about it.
    By the way, I don't think a single-frequency photon covering the whole space exists.In fact, I don't think the picture is right.As the two sides of uncertainty principle,the photon cannot be located at one point,neither can it have a definit momentum.Because the definit momentum leads to a definit orbit(very short),which is a conflict to the infinit location uncertainty.
  12. Dec 27, 2005 #11
    Quantum mechanics taught us that energy doesn't have to be localised, so, for example, when you put any particle through even just 1 slit, the energy starts spreading and spreading across all space.

    To your other questions, firstly we must realise that photons are quanta of the electromagnetic field. There is a vastly complicated body of math that we use in "modern" physics to describe photons, and (the interesting bit) how they interact with electrons and other charged particles; it's called quantum field theory. That is the most "true" account we have of photonic behaviour, but it is by no means necessarily the last word on the issue (we may never approach that level of understanding, anyway).

    So if you really want to know, I would recommend you take an undergrad course in physics, then a PhD in the subject, taking QFT and QED as options. That may well give you a greater understanding than forum posts ever could.

    EDIT: Assuming you haven't taken these courses already.
  13. Dec 27, 2005 #12
    That's an "easy" one. Let me first tell you what a photon does NOT look like. A photon is NOT defined as a particle in the sense of "some physical entity with finite spatial boundaries, like for example a tennis ball". A photon is defined as a quantum of energy. The epitheton "quantum" in QM does not refer to particles but to little bits of energy. That is how QM was developed.

    So, you cannot ask what a photon "looks like" because we are not defining a photon in a spatial base but in an energy base. Besides, elementary particles cannot be distinguished from each other, but that's a whole other story...

    That's also an easy one : GO STUDY QED.

    If you have done this, you answer me this : "do photons mutually interact ?"

    Hell, i will even give you the answer

    Answer : NO in first order but they do interact indirectly in higher order.

    Do you get this ?

  14. Dec 27, 2005 #13
    If you ignore the (rather snide) exhortations to go study the subject yourself, you ought to be getting the concept that a photon doesn't "look like" anything. Because it isn't something that you could look at. It is energy, not matter.

    A photon is therefore not a "particle" as you're probably thinking of the term. It isn't a teeny tiny bit of stuff. It's not shaped like a little ball, or a little vibrating rubber band, or anything. Because it's not made of anything that has shape.

    Instead, a photon is a teeny tiny bit of energy. This teeny tiny bit of energy doesn't sit still, but travels through space at a given velocity, the constant c, which is simply the speed at which energy is transmitted through space.

    Okay, this should be clear by now. Everyone's answers should be making sense once you've got this concept.

    And don't feel dissed by the snide answers. People in the field (ha) have a very hard time explaining their understanding without resorting to symbolic math, or referring to abstract jargon like "Hilbert space" or "Lorentz covariant." Some would argue that inability to describe something in plain language to non-specialists indicates a lack of actual understanding, but I prefer to think that it is an unfortunate side effect of specialized study where everyone one deals with uses the same specialized vocabulary.
  15. Dec 27, 2005 #14
    Let me hazzard a snide reply here.:rolleyes:

    Energy is NOT transmitted at the speed of light. What you meant is this "EM-energy is transmitted at the speed of light". However, one cannot state this since the EM energy is basically everywhere because a photon has a definite momentum. This is why we use fields to decsribe the EM-interaction. Photons are NOT transmitted and they do NOT travel over a certain distance which is the implication of saying that "energy is transmitted". Photons arise due to fluctuations of the EM-fields which are "everywhere" thanks to the HUP and they are caracterized by local interactions.

  16. Dec 27, 2005 #15
    One cannot speak about "a photon's path of transmission" for the reasons i stated in my previous post.

    Huh ??? Are you talking about instantaneous events ?

    You cannot make statements on the shape of a photon in a spatial base (which you are trying to do here). The only thing you could do is refer to the magnitude of the fluctuation (wavelength) of the EM-field that yields a physical entity that behaves as a particle we called "photon"

  17. Dec 27, 2005 #16
    Nope, a photon corresponds to the energy dE associated to the transition of an EM-field from one configuration to another. Or, a photon corresponds to the fluctuations of the EM-field. This is not the same as saying that EM-fields are build out of photons because that implies you would need photons to build an EM-field. This is not the case and clearly contradicts with QED.

    Beware, that when engaging in the difficult task of simplifying/explaining physics, you do not bring over incorrect visions.

    Not intersections but fluctuations. Intersections would imply that you need more than one field. But there is only one EM-field that yields a photon.

    Sorry but this is not true. Besides saying that something is not "accurate" does not justify it being wrong.

    The particle wave duality needs to be understood like this. Photons arise as fluctuations of the EM-field. These fluctuations are described in terms of waves. This is quite straightforward if you think of ripples of water that arise when you throw a stone into the water. The particle-aspect comes from the fact that when going from one fluctuation to the other, this corresponds to a differential rise in energy (dE) expressed by the Einstein energy relationship. Since energy and mass are equivalent, we can look at this dE as representing a particle (IN AN ENERGY BASE) with certain energy and momentum, which can be calculated from the Einstein's energy relationship. In QFT, there are particles (that arise in the same way) that do not respect this E=mc² formula. These particles are called virtual particles.

  18. Dec 27, 2005 #17

    Tom Mattson

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    That's because, to the best of our knowledge (and I'm talking agreement with experiment to 10+ decimal places), photons are fundamental whereas atoms are composite systems. As far as we know photons are on the same level as the other fundamental particles of the Standard Model, the quarks, leptons, and (remaining) gauge bosons. That is one key difference between atoms and photons that you are missing.

    Heh. You drew a little flak in this thread, and this comment makes it easy to understand why. Physicists have done much better than what you describe in this quote, as has been explained by marlon. Instead of proclaiming what "we" like to do, it would be better to ask "we" what the current state of affairs is. "We" doesn't like it when you put words in its mouth. :biggrin:

    I would have used a different tone myself, but I would also have pointed Ben in the same direction. Exhortations to study should never be ignored, IMO. Courses are taught and books are written so that the time and energy of professors can be used as efficiently as possible. The people who study on their own are the ones who get the most benefit out of this forum.

    That said:

    Ben, if you are interested in a non technical description of QED then you should check out Richard Feynman's QED: The Strange Theory of Light and Matter. If you have some physics background then let us know, and we'll come up with a more suitable reference.
  19. Dec 27, 2005 #18
    Like the Pope proclaims just before the Urbi et Orbi Blessing :


    I don't really understand why several people here are bringing up "tone" to classify "go study QED" as being an impolite answer. If one asks "how photons are interacting", one is really asking about what QED is about. There is no simple, straightforeward and general answer to such a question. The answer covers an entire field of study. If one would ask me "how do quarks interact ?" , the answer really is covered by QCD completely. If i would just say, quarks interact via the strong force, which is caracterized by the principle of asymptotic freedom, would that really help ? Huuh ? I sure as hell leaves out a lot of important and essential aspects (like the role of virtual quark/anti quark pairs or gluons or gluonconfinement)

    Hooloovoo, I don't like the fact that you asses my answer to be "impolite" because of whatever reason, since i have really taken the effort to answer/correct and clarify several aspects that have been brought up in this thread. Besides, i would rather be a little too direct, yet very clear, in stead of providing others with incomplete, badly simplified or even wrong posts.

    ...err,...,for what it's worth...

    Last edited: Dec 27, 2005
  20. Dec 28, 2005 #19
    I guess I asked the question because most physicists, professors, and textbooks don't do a good job of describing the photon. I've also researched the topic quite a bit myself and there seems to be a wide range of opinions on the specifics. So I don't think that taking advanced status quo education is always the answer to such questions. What do you think now that I've spouted away on this issue?
  21. Dec 28, 2005 #20
    I like your deep understanding on this topic of physics explanations (Chuckle). I thought it was a conspiracy, but maybe your explanation is more grounded in reality (Smile). I think good explanations are necessary before the equations. I've seen so many people in my work churn out reams of equations as an answer, but the assumptions regarding the data were all wrong.
  22. Dec 28, 2005 #21


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    The ability to understand something is a two-way street. If I am explaining something to you, I have to be able to do that in a clear, and accurate fashion. However, you must also be CAPABLE in comprehending what I just explained. If I describe a rose, then using characteristics of "color=red", "texture=soft", etc. etc., are all characteristics that you are ALREADY FAMILIAR AND UNDERSTAND.

    Now, what if I describe a bcc lattice and tell you the reciprocal lattice is fcc and that one can verify that via bragg reflection from x-ray diffraction? What I've just said is comprehensible to someone with an appropriate background of solid state physics, but it is nonsense to someone who doesn't! This brings us back to the very question you asked in this thread. The very simple answer that can be given in such a way that you are CAPABLE to understand, is what you have seen, and what your texts and professors have given. Of course this isn't sufficient! It is why we have graduate level classes that delve into these things in more detail. But it is worthless to tell you the QED formulation of EM fields when you are not equipped to understand its description! It is why university courses have prerequisites!

    And while I'm at it, asking "how big is a photon" is similar to asking "how painful is purple?"

    I'm not trying to be cute. I am illustrating a point in which you are asking for a characteristic of something in which it wasn't defined with. A "purple" is a color, and can be defined with a range of frequency or wavelength of the visible spectrum. It was NEVER defined with "pain" or degree of pain, and thus, such a characteristics makes no sense when it is associated with "purple".

    The same can be said about "photon". It is a quanta of energy carrying a spin of 1. Nowhere in its definition, either now or when Einstein first proposed the corpuscular theory, is there any mention of its physical size! There is a length associated with a gazillion photons when they stream by, and that is a wavelength. But it would be dubious to assign this as the "size" of a photon when it isn't a "particle" in the conventional sense. It has no physical boundary in real space that we know of. It isn't part of its definition!

    So how salty is spring tension?

    Last edited: Dec 28, 2005
  23. Dec 28, 2005 #22
    While my education was in engineering, I've studied the basics of Richard Feyman's work on QED already. I think one of the issues with this forum is that it is supposed to be a status quo forum. We are supposed to be talking about the one right and true answer in Physics. Non of this speculative stuff is allowed here. If there was one right and true answer in Physics, there would not be the current dilema about Relativity and Quantum Mechanics not giving the same answer to certain questions. I personally don't agree with Richard Feyman's basic description of what light is and how it operates. He himself says it is strange. I could shoot holes in some of it based on engineering laws. Do all of you believe his theories? So just taking another advanced university course isn't going to necessarily help me because I want a wider range of explanations than are offered there. Presently status quo education does not provide a coherent set of ideas on this topic. There are lot's of competing models all bundled together. I like forum's for some questions because there can be a discussion of various answers instead of just a single interpretation by a single professor or author. So I appreciate everyone's input (Smile).
  24. Dec 28, 2005 #23

    Tom Mattson

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    Does that mean that you've read the pop-science book that I recommended? Or does it mean that you can do QED calculations?

    If the former then I would not agree that you've studied the basics of QED. If the latter than I would.

    Yes to the first sentece, no to the second.

    We stick to peer-reviewed, mainstream physics in the main sections of PF for a very good reason: it works. But no one here would say that there is one right and true answer in any science. Anyone who works in the field knows that science is essentially an a posteriori discipline.

    Material that has not been peer reviewed is permitted in the Independent Research Forum. That is written in our Guidelines, which you agreed to before posting here. I do hope you intend to honor that agreement.

    Since no one is working from the "one right and true answer" premise, this point is moot.

    He calls it strange because it is counterintuitive, not because it is wrong. QED is the most accurate scientific theory ever devised.

    No, you couldn't. I earned a BS degree in engineering, and when I went to grad school for physics I taught physics to engineers. Today I teach engineering courses. The typical engineering curriculum simply does not contain enough physics to be able to critique a theory such as QED. And if your curriculum was so atypical as to prepare you to discuss QED intelligently, you wouldn't have to ask us about it.

    QED has been verified to over 10 decimal places. What's not to believe?

    It is not possible to assess how a course will benefit you except from hindsight. You are being told by people who have that hindsight that you do need to study QED. Do with that advice what you will.

    And because you have not and will not study QED properly, your opinion on this matter has no merit whatsoever.

    No, as far as the photon goes there is a single, unified model: QED.
  25. Dec 28, 2005 #24
    While many others in this thread have said I basically asked a stupid question, or I should go to school, you've given a simple straight forward answer. Some people assumed because I asked a question, I don't know anything about the topic or didn't have a reasonable education. Actually I've researched the topic quite a bit, but that doesn't mean I don't want other people's view on it too. You're answer also agrees with Hans C. Ohanian's in his 1400 page textbook Physics First Edition 2nd volume pg 912.

    But there are all kinds of competing views of this topic in Physics today. The Ohanian answer obviously is completly different than the Quantum Mechanics answer which is based on particle probabilities. It is different than the Classical Physics view of light still taught largely today which is based on the sound wave model. Relativity did not have a view on the specific functioning of the photon. It isn't String Theory. It isn't QED. But Quantum Mechanics and QED is the accepted view for explanations on the quantum level. So even though Ohanian is a respected professor, is his understanding and what your described actually not the accepted viewpoint? What would you label the Ohanian view or theory of the photon?
    Last edited: Dec 28, 2005
  26. Dec 28, 2005 #25


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    But see, this is a clear example where you limited understanding of the physics causes you to "believe" in something that CLEARLY doesn't make sense. For example:

    Now think VERY carefully. The "amplitudes" of the E and B fields correspond the electric field STRENGTH and magnetic field STRENGTH. How are you able to believe this to relate to "size" in physical space? Why aren't you the least bit weirded out when you are told this? I could plot a sinusoidal "wave" of the E field, but the VERTICAL axis is in units of V/m, not a unit of length! Those "vectors" that we represent E and B fields in Ohanon's book is NOT to represent is physical real space size, but the AMPLITUDE of the vector corresponding in the appropriate units of that quantity!

    This is EXACTLY what I meant by learning being a 2-way street. I cannot explain something in terms of stuff that you do not comprehend, and if I try, you will take something and misinterpret it into something that you THINK is correct, but it isn't. Anyone with a good-enough understanding of EM fields would have immediately noticed something not kosher with translating E and B field amplitudes into "size". Anyone who do not understand EM fields as well, would see nothing wrong with it.

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