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Feynman says photons ARE particles

  1. Feb 2, 2011 #1
    but isn't a particle just an idea? like the point in geometry
    i guess what i'm having trouble understanding is the divide between theory and reality
    is explaining how a system behaves the same as saying what the system is?
  2. jcsd
  3. Feb 3, 2011 #2


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    "particle" is a word, it describes lots of things, including photons …

    do "membrane" "neuron" "bicycle" and similar words describe what a thing is, or do they describe how it behaves?

    what does that distinction have to do with whether something is real? :confused:
  4. Feb 3, 2011 #3

    Andrew Mason

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    You are correct that the concept of "particle" is really a model to help poor humans picture things at the quantum level.

    Some mass objects are more like particles (sand grains, atomic nuclei) than others (electrons, quarks). At the atomic level, an atom is mostly empty space: a tiny massive nucleus surrounded by a large region of space occupied by a few electron specs. At the nuclear level, protons and neutrons are made up of almost empty space occupied by 3 dimensionless quarks.

    Photons exhibit some of the behaviour that macroscopic particles exhibit. But they also exhibit wave-like behaviour. The question: are photons particles? begs the question: what is a particle?.

  5. Feb 3, 2011 #4

    A. Neumaier

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    Almost. For example, H.B. Callen says in his famous textbook on equilibrium thermodynamics,
    H.B. Callen.
    Thermodynamics and an introduction to thermostatistics,
    2nd. ed. Wiley, New York, 1985.
    on p.15: ''Operationally, a system is in an equilibrium state if its properties are consistently described by thermodynamic theory.'' This quote can also be found at the end of Section 2 of the article
    http://www.polyphys.mat.ethz.ch/education/lec_thermo/callen_article.pdf [Broken]

    We need the theory already to define precisely what it is that we observe.

    On the other hand, the theory must be crafted in such a way that it actually applies to reality - otherwise the observed properties cannot match the theoretical description.

    As a result, theoretical concepts and experimental techniques complement each other in a way that, if a theory is reaching maturity, it has developed its concepts to the point where they are a good match to reality. We then say that something in real life ''is'' an instance of the theoretical concept if it matches the theoretical description to our satisfaction.

    It is not difficult to check that this holds not only in physics but everywhere where we have clear concepts about some aspect of reality.
    Last edited by a moderator: May 5, 2017
  6. Feb 3, 2011 #5


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    So basically speaking, I am made of nothing, mostly (and some dots).

    And I thought I wasn't a cartoon... :-D
  7. Feb 3, 2011 #6

    A. Neumaier

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    At the atomic level, an atom is a tiny massive nucleus surrounded by a large region of space occupied by a delocalized electron field. At the nuclear level, protons and neutrons are made up of 3 tiny quarks surrounded by a large region of space occupied by a delocalized meson and gluon field.

    These fields are transparent to certain kinds of rays (alpha rays in case of atoms, very energetic electrons in case of protons and neutrons, and hence look (to these) like not there.

    But we don't think glass doesn't occupy space because it is transparent for light, or that only the bones of our bodies occupy space because the remainder is transparent for X-rays.

    So why should we think of the electronic field surrounding nuclei not to occupy space simply because it is transparent to alpha rays, or of the meson and gluon field in which the quarks are embedded not to occupy space simply because it is transparent to fast leptons?

    Glass is hard because it is occupied by a matter field that resists other matter (though not photons).
    Atoms are even harder because it is occupied by a matter field that resists other matter (though not alpha rays).
    Protons and neutrons are even harder because they are occupied by a matter field that resists other matter (though not fast leptons).

    So we are not made of almost nothing, but we get our volume and tangibility from the electron field, and our weight from the embedded nuclei.
  8. Feb 3, 2011 #7
    Electrons are only points exactly at the instant of a idealized position measurement.

    Electrons are referred to as point particles because the set of all "point states" |x> at all possible positions x in R^3 furnish a complete basis of the electron quantum Hilbert state space (spin must also be included of course, but that doesn't change the reasoning).

    On the contrary, the Hilbert space describing the quantum states of a closed string in R^3 is not spanned by the set of such point states |x>. It is instead spanned by the set of all possible closed curves in R^3 (in addition to vibrational states of course).

    Ok, I guess this is probably too technical but I'm posting it anyway since I've already written it. :redface:
  9. Feb 3, 2011 #8


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    Here we see yet another "What is it really?" question.
    You can only 'really' say how something behaves and not what it 'is'. If you insist on having photons as tiny bullets, then how do you square this with the fact that they also have to be parts of Long Wave radio broadcasts - with a wavelength of 1500m and there must be swarms of them because the energy of each LF photon is vanishingly small (=hf)? I don't think we should read too much into the Feynman diagram, either; there has to be a temporary 'squiggle' joining the two interacting particles together but he doesn't actually say how 'big' this squiggle is.
  10. Feb 3, 2011 #9
    Could the problem with particle-ontology be that point-dots are conceptualized separately from the fields they anchor? It seems like whenever we think of a field interacting with a particle, the particle it interacts with is conceptualized as being directly acted upon by field-force instead of having its own field(s) engaged by the stronger field. So if photons are viewed as points separated in space, wouldn't the EM field-force that connects/binds them together in the wave be ignored?
  11. Feb 3, 2011 #10

    A. Neumaier

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    Yes. The problems solely stem from trying to interpret particles as little dots or balls even when they are delocalized (such as inside a bound state, or after they pass through a narrow slit or beam splitter).

    But the particle picture is appropriate only in the limit where geometric optics is applicable.
  12. Feb 3, 2011 #11
    In the Douglas Robb Memorial Lectures, Feyman will correct himself when he believes he's given the impression that light comes in the form of point-like particles.

    http://www.vega.org.uk/video/subseries/8" [Broken]
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  13. Feb 3, 2011 #12
    What about in the difference between relatively short and long EM waves? Do the EM fields of photons expand and contract depending on the wavelength, or do the fields simply neutralize their outermost regions at shorter wavelengths? I wonder about something very similar regarding what happens to the magnetic fields of two iron magnets when their south poles are pushed together. Do the magnetic fields get compressed or do they just cancel each other out where they intersect?
  14. Feb 3, 2011 #13
    When describing quarks as "dimensionless" does this mean that they have no dimensions, or that any dimensions they may have cannot be defined?
  15. Feb 3, 2011 #14

    A. Neumaier

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    The geometric optics approximation is adequate when the wavelength is short compared to the relevant length scales of the equipment, and _only_ then.
    http://en.wikipedia.org/wiki/Geometric_optics: ''when initial conditions oscillate much faster than the coefficients of the differential equation, solutions will be highly oscillatory, and transported along rays.''

    By deBroglie's relation, this is the case only when the momentum is sufficiently large. This makes macroscopic bodies (bullets) behave particle-like, while microscopic massive systems (electron fields) behave particle-like only when they are very fast, and a microscopic massless system (light) behaves particle-like only at high frequencies.

    At narrow slits, the size of the slit determines the relevant length scale, and only light with a wavelength much smaller than the slit behaves particle-like. In the standard double slit experiment, this is not the case, and one sees a wave-like behavior.
  16. Feb 3, 2011 #15

    A. Neumaier

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    Does the video play in the future???
  17. Feb 9, 2011 #16
    the reason for making this thread was because i watched these videos :)
    thanks for all the responses
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  18. Feb 10, 2011 #17
    Let me admit my relative ignorance before I set out into any words. My name is Tommy, I am twenty years old, living in the hills of Kentucky, and this reply is my first post here. Being expelled from high school, I lack even the most basic knowledge of mathematics, and have never completed a course in algebra. After my expulsion, I was allowed to return and make up some lost work, but my defiance in school was a burden that the administration needed rid of, and at age 16 they offered me the chance to get my GED early and drop out - I took the test, and at 17 I took had my GED. Attempts to learn in community college were of no result but failing grades and drugs.

    I have always been interested in all of space and physics, which have served as an escape in rigid school environments. I have now started a medication for ADHD and am finding school much easier to bear. I wanted to let this go before I perhaps made a fool of myself in a lack of good understanding. I hope you will all bear with me as I learn and try to contribute.

    Light can exert pressure on the object of it's focus, and can affect the rotation of asteroids, and in my belief it may be responsible for the same on the atomic level.

    If light is "massless" or without a mass, would it's trajectory be altered by such intense gravitational fields as are seen in black holes? We can witness a change in form from, say, Gas to liquid, solid to liquid and such may be the case with light if it were of a particulate nature.

    I've often wondered whether light might work to make new expressions of its energy as in its responsibility for the development of biological organisms. If light were able to direct the motions within an atom by the exertion of its gentle pressure, it could eventually a create a world that represented it's motion. Just as the trees reach in praise of the Sun's light, the finest ordering of our universe may do the same.

    I am technically and terminologically lacking, so take this as rambling if it is.

    Maybe light is the enabler of mass for the sole reason that light needed form. We are all originated from a blazing star, and what is a star but light, and what is light but the energy that moves at such a blistering pace that it creates heat, life, and all that is in turn. we can travel the universe and in the most remote regions there is no gas, or mass, no heat. . . all there would be to experience is the brilliance of the distant stars and death without their closeness.
  19. Feb 10, 2011 #18

    A. Neumaier

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    But you _need_ to learn if you want to understand physics in a bit more depth - if not in school then on your own, using internet resources (starting with wikipedia) and online books (or books from a public library). You'll soon realize that you also need some understanding of mathematics to progress in physics....

    The pressure is caused by the momentum that light (though massless) possesses. Associated with the momentum is a corresponding energy, which (according to Einstein) is equivalent to mass in the sense that it also responds to gravitation. Thus light is slightly bent when passing close to the sun. This was the first experimental verification of Einstein's general relativity.
  20. Feb 10, 2011 #19
    Thanks for the reply!

    I've been studying as much as I can and drawing from as many sources as are available. Most of my finds have been outdated but are many.
    I'm looking for more schools, and have found an astrophysics program at a nearby college.

    In response. . .

    If electromagnetic waves are said to be without Mass and yet possessing momentum why is their not an independent term for that phenomena?

    If Einstein said that this energy is equivalent mass, couldn't we just conclude that mass is equivalent to Energy?

    Who is to say that what we measure as having Mass is not just an expression of Electromagnetism?
  21. Feb 10, 2011 #20

    Andrew Mason

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    There is. It is the subject of quantum physics. But you have to walk before you can run. I think we all would encourage you to read as much as you can and take some physics courses if they are available. PF can always help!

    That is what "equivalent" means.

    It is unlikely to be an expression of electromagnetism since some particles that have mass do not interact with electomagnetic phenomena (neutrinos for example). No one has been able to satisfactorily explain inertia, yet.

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