• Entropius@lemmy.world
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      1 year ago

      Acceleration and Velocity are vectors. Changes in a velocity vector are an acceleration. Therefore when photons change direction technically it’s a form of acceleration.

      • metallic_z3r0@infosec.pub
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        1 year ago

        I thought photons are always moving in straight lines from their perspective, and it’s space that’s bent. Unless it’s through a medium, then they just get absorbed and re-emitted, sort of.

        • Entropius@lemmy.world
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          1 year ago

          Space bending is a general relativity thing, which isn’t really related much to how mirrors work.

          Regarding the medium bit, photons being absorbed and remitted can’t explain how light moves slower in glass. This is just an extremely popular myth. Photons are only absorbed by atoms at very specific frequencies. Also, the entire reason glass is transparent to begin with is that it’s not absorbing the photons (requires too much energy to bump the electron’s energy level so the photon isn’t absorbed and it keeps on trucking). Also photon absorption and remission is stochastic so there’s no way to control the direction it happens in or how quickly it happens. Random directions of remitted light would make glass translucent, not transparent. So for a few reasons, that’s not how it works.

      • ziggurism@lemmy.world
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        1 year ago

        Ok but photons don’t change direction either. Treating photon scattering as an individual particle accelerating due to an applied force, well that’s just not a correct description of how perturbative QED models photon interactions.

      • Ook the Librarian@lemmy.world
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        1 year ago

        Since photons are indistinguishable, it’s hard to say too much concretely, but it some sense a diffracted photon is different photon. In order for a photon to interact with say, a diffraction grating, the interaction is done with “virtual photons”.

        So for a photon to change course, aka accelerate, it does it by absorbing a virtual photon and emitting another. Whether that is the “same photon” after the interaction is kinda more philosophy than physics, at least to me.

        Feynman diagrams are surprisingly accessible for how much information they contain. It’s one way to think about photon (and other particle) reactions.

        • ziggurism@lemmy.world
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          1 year ago

          There is no tree level photon-photon interaction. Photons scatter off electrons (or any other charged particle), not off neutral photons.

            • ziggurism@lemmy.world
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              1 year ago

              The electromagnetic field does have a force carrier. It is the photon.

              The photon mediates the force between electrically charged particles. It cannot mediate any reaction between two neutral photons.

              • Ook the Librarian@lemmy.world
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                1 year ago

                Ah, I see. Sorry for the snark. I was thinking more in line with the Compton effect, and thought you were talking about something like that too. (Even though it’s clear that you were explicitly not. I thought you were denying photon-virtual photon interaction because I was talking about it in a funny way.)

                I would still say it’s still philosophical whether photons experience acceleration, but I concede that photon-photon interaction is not done by virtual photon exchange.

                • ziggurism@lemmy.world
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                  1 year ago

                  I am indeed denying the existence of photons interacting with virtual photons. I am also saying there is no tree level photon-photon interaction of on shell photons. Neither Compton scattering nor Bhabha nor pair production nor pair annihilation involves a photon-photon interaction. There is no photon-photon vertex in QED. There is no tree level Feynman diagram that you can look at and say “this is, at least philosophically, a photon changing its momentum”.

                  There is a 1 loop diagram that represents photon-photon scattering. But even that doesn’t have any photon-photon vertices, instead it is mediated by electron-positron pair.

                  Non-abelian gauge bosons (gluons) couple to themselves. So does gravity (gravitons). Abelian ones (photons) do not.

                  Photons don’t accelerate. They are emitted or absorbed. That’s their only interaction.

  • Tb0n3@sh.itjust.works
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    1 year ago

    Does a photon actually accelerate? Sure seems like it always goes at light speed through whatever medium from its creation.

    • Vilian@lemmy.ca
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      1 year ago

      well, if it get reflected and change direction it going to be at light speed, so it can be interpreted (probably incorrectly lol) that it “accelerated instantly to the other direction after the reflection”?

      • Kogasa@programming.dev
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        1 year ago

        This is an interesting question. Instant acceleration is mathematically implausible, but I don’t know if there’s a better physical interpretation for what happens to a bouncing photon. I’m guessing this is one of those “less particle, more wave” situations where the instantaneous velocity of the photon is undefined.

        According to some random internet sources, reflection is the not-quite-instantaneous process of the photon being absorbed and then emitted by the electrons in the mirror.

        • trash80@lemmy.dbzer0.com
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          1 year ago

          Light doesn’t travel the same speed in water or glass as in a vacuum.

          In a medium, light usually does not propagate at a speed equal to c; further, different types of light wave will travel at different speeds. The speed at which the individual crests and troughs of a plane wave (a wave filling the whole space, with only one frequency) propagate is called the phase velocity vp. A physical signal with a finite extent (a pulse of light) travels at a different speed. The overall envelope of the pulse travels at the group velocity vg, and its earliest part travels at the front velocity vf.

          https://en.wikipedia.org/wiki/Speed_of_light#In_a_medium

          • Neato@kbin.social
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            1 year ago

            That’s light as an aggregate wave. Photons, actual light, always travel at c. What’s happening in a medium is the rapid absorption and readmission of photons. The probability of admission is based on structure of material causing things like lens or mirrors to work.

            You can think of it as the photons having to jump between platforms before the can continue running at c.

          • there1snospoon@ttrpg.network
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            1 year ago

            But doesn’t relativity explicitly state that c is the speed of light in a vacuum, and travelling through other mediums explicitly changes and is explained by relativity?

            I am 100% a layman and do not know the answer.

            • sushibowl@feddit.nl
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              1 year ago

              Not really no. Special relativity explains the relationship between space and time. General relativity expands on this to account for gravitation.

              One of the postulates (i.e. assumptions) of relativity is that the speed of light in vacuum is the same for all observers. But the theory doesn’t actually require any particular value for c, it only needs it to be constant. And it doesn’t explain the behavior of light in a medium at all.

              In fact, relativity doesn’t explain the mechanism by which light interacts at all, that is the domain of Quantum Electro Dynamics.

              • there1snospoon@ttrpg.network
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                1 year ago

                Wow that is so interesting. So am I understanding that relativity explains space, time and gravity’s interactions with one another, while quantum science explains interactions with much smaller objects like matter?

              • ziggurism@lemmy.world
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                1 year ago

                the speed of light expressed in units of distance per time, is a dimensionful quantity so it probably doesn’t mean anything to say some theory does or does not predict a value for it. The value is entirely determined by how big you choose your yardsticks and sundials to be, which is arbitrary convention.

                It is only meaningful to talk about theoretical predictions of the values of constants if they are dimensionless, like the fine structure constant.

                However relativity does suggest as a natural point of view that space and time are just orthogonal directions in a unified spacetime. In this point of view, relativity gives you the option of measuring your timelike and spacelike coordinates with the same yardstick (which you may still choose arbitrarily). And then relativity does predict its value. It’s 1. No units.

      • marcos@lemmy.world
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        1 year ago

        No, they don’t. They can get absorbed and re-emitted, and the space they are moving though can compress sideways. But they can’t make curves at all.

          • marcos@lemmy.world
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            1 year ago

            That’s basically all that refraction is. A dead giveaway is that light doesn’t move at the speed of light in them.

          • Neato@kbin.social
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            1 year ago

            Yes.

            Don’t think about individual photons. Think about billions of them with destructive and constructive interference. The probabilities of all the sitting l additive waves of light.