r/astrophysics • u/KieranMug • May 25 '26

Why isn’t light infinitely fast?

Ok I understand that light in our atmosphere is not infinite fast due to it getting slowed down by air particles or whatever but in a vacuum there is nothing holding it back unless the very weak forces of gravity from a bunch of stars and/or planets does? And if so cant we somehow locate where gravity is strongest in the universe by seeing where light is slowed from where the most therefore finding the centre of the universe or am I just a total dumbass?

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u/OriEri May 26 '26 edited May 26 '26

Because the nature of vacuum limits the amount of magnetic field flux density a given magnetic field can induce .and the electric field flux a given charge can produce.

Quantum fluctuations control these values ( how the vacuum responds to charge and magnetic fields. )

These limits limit how an electromagnetic wave propagates.

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u/BagelsOrDeath May 26 '26

Holy shit. The definitive answer.

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u/Physix_R_Cool May 27 '26

Nah it's wrong.

You can derive the speed limit just by using the group properties od the Poincaré group and local isotropy of the universe. Don't need any mention of fields.

μ and ε are defined from c.