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I Inverse square law and ensembles of photons

  1. Apr 1, 2016 #1
    I'm trying to visualize the effect of the inverse square law, not on a direct source of light, but on scattered light carrying visual data, such as that responsible for our everyday sight of things as well as our images of earth from satellites.

    It seems to me that it should be true that, while the photons spread out in space the farther and farther the scattered light travels, making the photons less dense, the photons are always locked in ensemble. So that a thousand eyes on a wall at a distance from a light scattering object each receive, not a proportion of the photons giving a piece of the scene like the piece of a jigsaw puzzle, but a near copy of the same meaningful ensemble of photons, i.e., an ordered collection of photons providing an image of nearly the same scene. In other words, that ensembles of photons essentially don't breakup as a function of distance.

    Is this true?

  2. jcsd
  3. Apr 1, 2016 #2


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    Easy answer: No. But I suppose that doesn't help you very much.

    Perhaps you want to follow these Feynman lectures (the link came from Simon Bridge) -- Richard Feynman talks about photons
  4. Apr 1, 2016 #3
    Thanks, but if you know how the original statement is incorrect, then why not say it? You yourself posit the unhelpfulness of your answer...but isn't the point to be helpful? Perhaps I'm a little annoyed at having spent the last hour and twenty minutes watching a lecture on my cell phone to find virtually nothing in it relating to my question.
    Last edited: Apr 1, 2016
  5. Apr 1, 2016 #4


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    Where did you get the idea that 'what seems to you' has any relation with reality ? Can you underpin this with anything at all ?
  6. Apr 1, 2016 #5
    I got the idea from the fact I myself am a part of reality, which also happens to possess the means of seeing reality. And if I see an object at one distance, it becomes fainter as the distance is increased. This is because my eyes are receiving a lesser and lesser portion of the photons coming from the object, that is, as distance between the latter and myself is increased. But never do I see, as this distance is increased, a lesser and lesser portion of the object, spacially speaking. Which leads me to conclude that, at varying distances, while I may be receiving into my eyes varying portions of the photons describing an object, the photons received are always in ensembles describing the whole object or scene.
  7. Apr 1, 2016 #6


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    The problem here is that photons aren't what you're thinking they are (and at the risk of putting words in BvU's mouth, he may have been trying to steer you in that direction when he pointed you at the Feynman lectures).

    Light is not a stream of photons moving through space the way a river is a stream of water molecules moving by. Instead, you should think of the light as ordinary classical electromagnetic waves propagating through space and losing intensity according to the inverse square law as it spreads out.

    Photons only come into the picture when this radiation interacts with matter, such as the light-sensitive cells in your eyes. It turns out that the waves always deliver their energy and momentum in discrete amounts at a single point, and whenever that happens we say that "a photon landed there". The probability of a photon landing at any given point at any given moment depends on the intensity of the electromagnetic wave at that point.

    Thus the weakening of the light with distance doesn't mean that you only get parts of the image. You still get photons from the entire image, just fewer of them per unit time so the image is dimmer.

    (This model will start to break down when the light intensity is really seriously dim, but this doesn't happen until we reach intensities many orders of magnitude smaller than you eyes can detect).
  8. Apr 2, 2016 #7


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    I would say that your intuition have gone wrong somewhere. What allows us to get information about the whole object is ability of light to reach our eye from any point despite all the light that is crossing it's path. It's called superposition principle. So it's more that light originating (scattered) from different surface points of object propagate independently and is not locked in ensemble.
  9. Apr 2, 2016 #8
    Thanks, Nugatory and Zonde!

    Is it true to say that, if scattered light-waves or photons are not locked in ensemble, but propagate independently, then they allow a true picture of reality for no other reason than that the light-waves or photons move in perfectly straight lines, hence preserving their relation to each other after leaving the surfaces of things?
  10. Apr 2, 2016 #9


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    (nit: as mentioned in the previous posts, photons don't move the way youi're imagining).

    "True picture of reality" is not an especially meaningful concept, but it is certainly not the case that our ability to describe and understand the universe depends on light travelling in straight lines. At most, sometimes it's easier to make sense of what we see when light travels in a straight line, but not always - microscopes and telescopes depend on light not moving in straight lines.
  11. Apr 2, 2016 #10
    So the question is, how much do I have to pay you to tell me the way photons do move?

    What I'm trying to understand is what it is about light-waves scattered from objects that cause them to travel and arrive in a person's eye in an orderly manner, that is, in relation to each other retaining that information that allows the eye-brain complex to make out a generally true picture of the objects. For example, that the table in front of me is hard-edged and not furry.

    It's hard to tell if you're speaking in hyperbole. "Yes, it's more or less the geometry of it, but there's exceptions." Is that what you're saying?
  12. Apr 2, 2016 #11
    Nugatory told you the right way to think about "how photons move" in the previous post. Viz.: don't.

    If you're not familiar with classical electromagnetism, it can be explained, and studied. If you think in terms of photons instead of waves (prior to interaction with matter) it can lead to the type of confusion your original post is about. To give you some feel for the non-intuitive aspect of it, the location of a photon is entirely unknown after it leaves the emitter (like a table-top) and before it hits your eye. We can even say it doesn't have a position (I'm not sure exactly how it should be phrased, but you get the idea). Furthermore we can't even speak of "the same" photon from emitter to eye - unless there's only one photon involved - because they're indistinguishable. Really, the only way to avoid a lot of useless confusion is to take Nugatory's advice.

    I'd say the simple, not entirely correct answer to your concern is "yes, light (waves) moves in straight lines". The exceptions are very important but, perhaps, merely cloud the issue here.
    Last edited: Apr 2, 2016
  13. Apr 2, 2016 #12


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    The location of a photon is not only unknown but doesn't exist. There's no proper position operator for massless particles of spin ##\geq 1##.
  14. Apr 2, 2016 #13
  15. Apr 2, 2016 #14
    I don't understand how the way I spoke of "photons" moving, instead of "waves" moving was relevant to the question I asked. In my last post I summarized my question without the use of the concept of photons at all. For my question was essentially about the relation between the units of scattered light (whether photons or waves), that is, how they generally arrive, and hence travel, preserving the data that they collectively carry.

    Anyway, I didn't ignore Nugatory's advice to "think of the light as ordinary classical electromagnetic waves propagating through space and losing intensity according to the inverse square law as it spreads out." I did try to think of light like that, and still am. It didn't have much bearing on the question though.

    Right. But isn't it more than just, "Yes, light waves move in straight lines?" Isn't it more like, "The fact that light waves move in straight lines is generally responsible for light waves (scattered off an object) reaching the eye in an orderly manner, i.e. collectively giving intact data about the object looked at?"
  16. Apr 2, 2016 #15
    I hope it's clear now that to talk of photons moving opens a can of worms best left closed. My habit is to read what the person means, not what he actually says - which can get me in trouble. These physicists don't do that, which is admirable; but they tell you what's wrong with the question, and won't answer until you fix it. Nugatory is an exception, he corrects you and also answers. That's why when you asked him "how much do I have to pay" I jumped in - believe me, you're picking on the wrong guy

    - True. I didn't notice that

    I'll sign up to that in a heartbeat! Whether those who really know will, is another question :-)
  17. Apr 2, 2016 #16
    As a matter of fact, now that I look back, I never spoke of photons moving in my early posts. Only in post #8 did I mention "light-waves or photons moving in straight lines."

    Hmm...maybe photons should be thought of as simply the start- and end-points of what moves as waves. We know it's at the start-point because it's only as a photon that a unit of light can be properly conceived as being emitted from matter, and we know it's at the end-point because it is detected precisely as a photon wherever the wave lands. But even though "start- and end-point" represent two things, they're really one thing for the simple reason that the photon is a generic property of the unit of light, it's existence as "substance" at its two ends. In a sense, perhaps a photon can be thought of as the interface-substance form of what moves as a wave, i.e. a unit of light. The question now, of course, is whether they'll mail me my nobel prize.
    Last edited: Apr 2, 2016
  18. Apr 2, 2016 #17
    not important

    Sure. Planck's original idea was that EM energy was "taken up" and emitted by matter (conceived of as a collection of harmonic oscillators) in quantized units (1900, re. thermal radiation). Later Einstein treated the energy itself as consisting of quanta (1905, re. photoelectric effect) and Gilbert Lewis called it a photon (1926). But, to come full circle, David Bohm showed you didn't have to say the energy itself came in photons (The Undivided Universe, 1993); Planck's original view was still valid. It's very convenient to accept Einstein's view but when it gets confusing, drop it.

    There are very many situations like this in physics, mathematics, and thought in general. There can be multiple ways of expressing the same underlying fact. At first blush this seems bad: confusing. But once their equivalence is established it's good: means you can choose the representation which makes a given problem easiest. Examples are Heisenberg (matrix) & Schroedinger (wave) mechanics; Coulomb and Lorentz gauge in electrodynamics; Einstein and Lorentz interpretations of special relativity; glass half full & glass half empty; etc.

    So you're on the right track now, except unfortunately Planck stole your Nobel
    Last edited: Apr 2, 2016
  19. Apr 2, 2016 #18
    Not to blow your mind too much further, but the light that interacts with your retina doesn't come from the table or chair; the mean free path of light (how far it gets after emission before getting absorbed) in the atmosphere at the Earth's surface is measured in centimeters... light from the table or chair is being emitted and then absorbed a short ways away, then a subsequent emission and absorption... a long series of them... the last emission occurring immediately in front of the eye or very likely even inside the eye itself... because the density of the aqueous and vitreous humour shortens the light's mean free path even more.
  20. Apr 2, 2016 #19
    @secur To be accurate, that statement of mine doesn't necessitate movement of photons since it implies the possibility of the photons merely being detected at increasing distances. But you're right, it's not important...especially since I was conceiving of photons moving. But not any more.

    Very interesting. But what do you mean, "Later Einstein treated the energy itself as consisting of quanta?" Why wouldn't it? Doesn't that mean just a specific type, quantity, or wavelength of energy? And what do you mean that Bohm showed you didn't "have to say" that energy itself came in photons? What was it that he actually showed or proved?

    Yeah, but why does it have to be "multiple ways of expressing the same underlying thing?" It seems to me now that what we have in reality is one thing, light, or electromagnetic radiation, that actually manifests in two forms, as a particle and a wave. So that instead of it being "two ways of seeing the same thing," it's one thing that in reality transforms from one form into another (i.e. from a photon into a wave) and then back again into the first.
  21. Apr 2, 2016 #20
    E. explained photoelectric effect by saying the electron in metal absorbed one quanta of EM energy, no more no less. On the face of it that can't work, if light is a wave. For one thing it's not clear how the electron would be in resonant vibration, also it has to happen in about 10^-8 seconds. Very natural to think of a little packet of energy arriving as a unit, hitting electron, and being absorbed (via still-unspecified mechanism). Planck, working with thermodynamic equilibrium of harmonic oscillators, had no need for that hypothesis. His oscillators could absorb energy by resonant vibration, and how long it took didn't matter.

    The reason "why it wouldn't", sometimes, be right to treat the energy as photonic is pretty well shown by this thread. For "how light gets from one place to another", it's more natural to go back to classical thinking.

    Bohm first developed his pilot wave theory for one fermion, where it worked well, but when it came to bosons, that approach was just no good. He was forced to drop the idea of particles of bosons (e.g. photon). He showed the energy could get "swept up" quickly enough; it's complicated and I don't remember details. (Get the book, I'm sure you'd like it.) But given that his theory, and his work, is accepted as an alternative to regular QM, it can be taken as correct. This is another example of the general principle mentioned above. Once it's established that pilot-wave is an equivalent interpretation (actually there's still some question, but ignore that) then if something is shown using it - like, EM not necessarily photon-ized - then it's true in any other interpretation, strange as it might seem from that point of view.

    We've agreed that's the best way to look at it for your question; but it's not the only way. You're making the typical mistake: one view looks good, so you "get married" to it. One can actually - if one is perverse - insist that it transits as photons and is emitted / absorbed as waves!

    Given that no one really knows what's going on in these tiny obscure processes, don't insist on one interpretation. People often do that with the first one they "get". You're in very high-powered company: Deutsch and Carroll (et al)'s fanatical proselytizing of MWI is a fine example of this mistake.

    Anyway, my advice, read science and philosophy, continue thinking about it ... but don't worry about it.

    Not to blow your mind even further, but every time each photon is absorbed / re-emitted by an air molecule, it could happen in different ways (for one thing it has an amplitude for reflection instead) so according to MWI, for each, an entire universe splits off, with slightly different copies of you. Multiply that by approximately infinite numbers of photons out there, doing it every fraction of a nanosecond ... and we haven't even gotten to matter yet! Almost makes you think MWI is ridiculous, doesn't it? Just the same it's a useful tool for some problems; it's clearly equivalent to sensible interpretations, so can be used as appropriate
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