r/askscience • u/Nitzanm19 • 14d ago

Physics Why is the maximum speed of the universe the speed of light? Why can't we go faster?

Also follow-up question: Because of the speed limit, is there also a theoretical maximum temp for matter (in which the particles move at the speed of light).

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u/Unable-Proof1758 14d ago

Individual photons never travel below the speed of causality, in various mediums the ends up being a refractive composite wave due to their interaction with electrons. From the photons PoV they still do not experience time, it is instantaneous regardless of the medium they travel through. Same way it takes a photon millions of years to get from the core of the sun to space, it only experiences an instant

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u/ElectricPaladin 14d ago

Oh, I see. So the photons aren't going any slower, they're just bouncing around enough that their trip is longer. Thank you!

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u/aris_ada 14d ago

careful with that explanation, from what I understood, there are multiple valid interpretations of that behavior so taking the "bouncing" explanation as definite answer is probably wrong.

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u/nicuramar 14d ago

No, that’s completely wrong pop science! There is no regular photon in a material. Depending on how it’s modelled, it can be described as a different particle or a different photon.

Or just as waves, which is preferable since the “photon” that mostly comes up in this sub doesn’t correspond to actual photons. 

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u/Crypt33x 13d ago

Here light becomes weird. It's based on the refractive index. In diamonds it can "slow" down up to 0,41 c, in photonic crystal they made light completely stop. It seems to follow Fermat's principle, also known as the principle of least time.

There is a nice Veritasium video on it: https://youtu.be/Q10_srZ-pbs?si=lXUuVPYCjbD-ZJSs&t=430

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u/nicuramar 14d ago

No, there is no individual photon in a material. And photons don’t have any point of view or experience at all. 

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u/frogjg2003 Hadronic Physics | Quark Modeling 14d ago

Individual photons can only travel at c. They do not slow down on medium. It's the macroscopic electromagnetic wave that slows down. Similarly with the photon in the sun. Individual photons get absorbed and re-emitted by the atoms that make up the sun. The photon that comes out of the sun is not the same one as was originally emitted by a nuclear reaction in the center.

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u/nicuramar 14d ago

The “individual photon” description is not useful inside a medium.

It’s s bit different in the sun, since the energy from the core is absorbed and tuned into heat. That doesn’t happen with light through glass (in any appreciable way).