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I Absolute Relativity (who needs virtual photons anyhow?)

  1. Aug 11, 2016 #1
    I've seen, in the description of a black hole's photon sphere, a mention of a "light-like ds." I was happy to see it because i've been thinking about a photon's physical environment; at the speed of light, it would experience no radial dimension & no time dimension. Might it make more sense to understand the photon in this context, rather than imagining a field of virtual photons?

    Also, when it comes to the collapse of the wave function; might that have to do with incoherence: as long as the coexistences are not contradictory, they will continue?
  2. jcsd
  3. Aug 11, 2016 #2


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    For approximately the 8,978th time on this forum, there is no such thing as the point of view of a photon. It does not "experience" anything so saying that it would not experience time is a meaningless statement.

    Clearly, this having to be debunked so frequently means that you are hardly the only one with this misconception.
  4. Aug 11, 2016 #3
    I didn't say "point of view," i said "experience" because the photon is presumably an actual thing: things have trajectories. It's worth considering the functionality of a photon relative to itself, for a little while at least.

    How does a photon present *it's* trajectory in various contexts? In the universal context? Presumably, the universe is a sum of its parts. Why keep something so obviously made up as virtual photons?

    I don't suppose you'd be inclined to speak to the "incoherent entanglement=>collapse" point at all.

    & why is it ok for some physicist to say "light-like ds" but, if i make a similar point i'm saddled with misconception?
  5. Aug 11, 2016 #4
    In physics it's "the same". To experience time you have to have "point of view", that is a refrence frame connected with you, so that you are at rest in it. There's no reference frame in which photons are at rest, so saing that "photons don't experience time" is meaningless.
  6. Aug 11, 2016 #5


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    "Actual thing" is not defined clearly enough to have a sensible discussion about whether a photon is an "actual thing" - but whatever photons are, they are not very small objects that move at the speed of light and they do not have trajectories.

    Of course, this just invites the question: If a photon isn't what you're thinking it is, then what is it? You'll find a pretty good answer in the link in the first post of the recent What is a photon? thread. If you want something that's a bit less math-intensive (that reference is appropriate for an I-level thread, but just barely) there's is Richard Feynman's excellent and math-free "QED: The strange theory of light and matter".
    When we speak of a light-like interval, we're saying something about the geometric relationship between two points in spacetime - the interval between them is light-like, as opposed to timelike or spacelike. The misconception comes in if we try to leverage that geometric statement into something about the experience of a flash of light traversing that interval. The problem ihere is that it implicitly assumes a frame in which (by analogy with the way in a person on a moving train can choose coordinates in which the train is sitting at the origin while the ground is moving backwards) the flash of light is not moving while the rest of the universe is moving backwards at speed c. Although such a frame is mathematically impossible, the temptation to assume that one exists is so irresistable that we have a FAQ on this question: https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/
  7. Aug 11, 2016 #6


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    Virtual particles are one of the most widely misunderstood and misdescribed things in all of modern physics. (I've said before, with tongue only slightly in cheek, that nothing you hear about vrtual particles is right unless it came from a quantum field theory texbook).

    At a very oversimplified level, they're just a useful mathematical concept. They're like negative Euros when we're trying to answer the question "What is the total value of my assets?" and I have a one Euro coin in my pocket but I owe you 1.50 Euros - you'll never find a negative half-Euro coin but the answer to the question is still "negative .5 Euro".

    For more on virtual particles, you could try two of our Insights articles: https://www.physicsforums.com/insights/physics-virtual-particles/ and https://www.physicsforums.com/insights/misconceptions-virtual-particles/
  8. Aug 11, 2016 #7
    Not an actual thing... but, wavelength though... hmmm
    Thanks, i'll spend some time with this.
    That makes sense, thanks again.
  9. Aug 11, 2016 #8
    It sounds like the question has been mostly answered, but I don't see a direct answer to this, so I will try to clarify.

    A light like ds is a space-time interval. This is invariant for all inertial FoR. I can measure the ds between light being emitted and then absorbed somewhere else from my inertial (sort of) FoR. The problem, as others have noted, is trying to make sense of the FoR of light. That is not a valid idea despite the fact that light's space-time interval can be measured from other frames.
  10. Aug 11, 2016 #9
    A line along a light-cone(it's been a while) is a light-like ds? something to do with not violating causality, if i remember approximately correctly?
    I think the link does a good job of that; if we want to talk relativity, light goes 3x10^8m/s by definition. ... Still, there's the virtual photon thing... I have to read the quantum field stuff.
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