Comment on Speediest little fella.

• trash80@lemmy.dbzer0.com ⁨11⁩ ⁨months⁩ 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.

  en.wikipedia.org/wiki/Speed_of_light#In_a_medium

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  • Neato@kbin.social ⁨11⁩ ⁨months⁩ 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.

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    • Entropius@lemmy.world ⁨11⁩ ⁨months⁩ ago

      What’s happening in a medium is the rapid absorption and readmission of photons. […]

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

      That’s an intuitive model, but unfortunately it doesn’t have the advantage of actually being correct. Photons are not being absorbed and reemitted. See here for why: lemmy.world/comment/5444224

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      • Neato@kbin.social ⁨11⁩ ⁨months⁩ ago

        That is wrong. Stochastic yes. Photons emission is probabilistic. Destructive interference causes emission to overwhelming follow classical wave theory. Here's a better explanation with a neat graphic.

        https://physics.stackexchange.com/questions/466/what-is-the-mechanism-behind-the-slowdown-of-light-photons-in-a-transparent-medi

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    • trash80@lemmy.dbzer0.com ⁨11⁩ ⁨months⁩ ago

      Now I’m not sure how reflective telescopes work.

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      • TonyTonyChopper@mander.xyz ⁨11⁩ ⁨months⁩ ago

        reject reflectors return to long tubes Image

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      • Neato@kbin.social ⁨11⁩ ⁨months⁩ ago

        Interference in matters structure causes classical wave like behavior.

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  • there1snospoon@ttrpg.network ⁨11⁩ ⁨months⁩ 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?

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    • sushibowl@feddit.nl ⁨11⁩ ⁨months⁩ 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.

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      • ziggurism@lemmy.world ⁨11⁩ ⁨months⁩ 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.

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      • there1snospoon@ttrpg.network ⁨11⁩ ⁨months⁩ 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?

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    • trash80@lemmy.dbzer0.com ⁨11⁩ ⁨months⁩ ago

      I don’t know. I thought I used to know.

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      • wildginger@lemmy.myserv.one ⁨11⁩ ⁨months⁩ ago

        This is how I feel every time I touch any non-basal physics topic.

        I swear this made sense once upon a time…

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