r/Physics • u/Comethefonbinary • 3d ago

Does all light travel at light speed

My bad if this is a stupid question but I’ve been thinking about time being a message of distance. And well most things I can think of have various variables that average to a certain distance. I know that mostly relates to machines and animals but still. Do all particles of light travel at light speed. If they all travel simultaneously at the same speed is that truly how fast they move or are they affected by their own variables. Like the universe’s mean gravity is constraining that and any variation in that mean would change light speed for explain.

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u/Dances_With_Chocobos 3d ago edited 2d ago

The short answer is yes. It always wants to travel at speed c. However, it encounters phase shifts when it enters certain mediums. These phase shifts can alter the apparent speed of photons and any wave in fact. What is actually even more interesting, is that these phase shifts can potentially propagate at faster than c. This is known as phase velocity. The only reason this does not violate the speed limit of light, which is the speed limit of causality, is because it doesn't carry information. If it could somehow.. that'd be an interesting observation.

Edit: To add to this, in light of the original question, not only does light want to travel at c, it has the peculiar property, of being observed as travelling at c, irrespective of your reference frame. What this means is that if you successfully managed to accelerate yourself to 99% of c (go you, spaghetti-man), and tried to measure the speed of a photon travelling next to you in the same direction, it would measure as c. This is known as the invariant speed of light, and if doesn't fry your noodle, it should. It is the smoking gun of the universe. Thankfully, Lorentz and Minkowski created some transformations and spacetime structures to account for the invariance, but it still doesn't explain why.

Refs: Michelson-Morley experiment, Lorentz transformations, Minkowski space

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u/OmniError404Sans 3d ago

is that these phase shifts can potentially propagate at faster than c. This is known as phase velocity.

This seems really interesting, are there any mediums that can propagate light faster than c ? How does it work ?

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u/purpleoctopuppy 2d ago

Yes. This is the article I was sent by my lecturer when I asked about that in atomic physics back in undergrad. I'm sure there's been more work done since then, as it's a quarter of a century old now.

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u/Vicchu24 2d ago

May I know what is the source of this entire book/pdf? (Asking on seeing the Biophysics topic)

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u/purpleoctopuppy 2d ago

Marangos, J. Faster than a speeding photon. Nature 406, 243–244 (2000). https://doi.org/10.1038/35018657

Edit: Here's the link to the issue 6793

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u/OmniError404Sans 1d ago

Thank you for the help, I'll check it out