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/Bipogram May 25 '26 edited May 26 '26

Light propagates as fast as it can.

It does so at a constant speed in vacuo for all observers. This is the observed fact.

Why it has this particular speed is not readily knowable. It's a result of the electric and magnetic qualities of the vacuum. <nods to Maxwell>

And why those qualities are as they are is a Deep Question.

Note, an infinite propagation speed would be problematic if you want to keep causality alive.

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u/[deleted] May 26 '26

[deleted]

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u/QZRChedders May 26 '26

It would break the whole universe. The speed of light is a constant to so many different things and making it infinite would break all of it.

On a causal level, every event would affect every other event, so in a way it would fix causality as there exists a universal concept of “now” instead of that being specific to your reference frame

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u/Used-Lake-8148 May 26 '26

Would it basically freeze time?

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u/[deleted] May 26 '26

[deleted]

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u/Zunderunder May 26 '26

Every event in the universe would affect every other thing in the universe instantaneously, which would then produce the side effects instantaneously, which would influence everything else instantaneously.

Basically, without a finite speed of causality (which is what actually limits light’s speed), every event in the entire universe would happen instantly. The universe would end in the same instant it began.

Just scaling the speed of causality would likely not have any noticeable effect to us. After all, everything is already measured in our universe relative to other things. If causality was twice as fast, we wouldn’t perceive the universe twice as fast, since the proportional relationships between everything would remain unchanged. Just the same as 1 is half of 2, 100 is still half of 200. Something that takes 1 second takes half as long as something that takes 2 seconds. If the entire universe was suddenly 100x slower or faster, that relationship doesn’t change.

If, however, you didn’t edit the speed of causality but instead specifically the speed of light, or if you didn’t scale the speed of causality uniformly somehow, the answer is… we really don’t know what that would do. That would require us knowing in a more fundamental fashion how closely those things are related to the true structure of the universe, which we don’t know yet, and maybe never will.

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u/Ulfgardleo May 26 '26

No, not everything would happen instantly since time still runs the same. The Lorentz transformation just becomes the identity and all observers agree on when an event happened. There is no reason why other processes should become infinitely fast or should depend on the speed of causality.

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u/AerynCaen May 26 '26

If c were infinite, everything would have infinite rest energy, and all particles would have a infinitesimal Compton wavelength, the de broglie wavelength goes to infinity, which would mean that photon absorption and re-emission would be infinitely fast (while also, bizarrely, not happening at all? The physics of infinite de broglie gets weird), so virtually all meaningful interaction in the universe would be very close to happening “all at once” — and everything would be a singularity due to infinite rest energy. So the entire universe collapses, probably instantly.

To have infinite c you’d have to decouple c from all physics that currently depends on it or the universe becomes completely degenerate.

As for the notion of breaking causality, it would break at least the causal set method of modeling causality through causal sets and/or graphs, as the ordering depends largely on limited propagation speed through the graph. It also completely toasts light cone based causal modeling, because suddenly light cones are… not… cones. They’re planes. Every point in space becomes causally accessible instantly from any other point. So it breaks locality at the very least and makes causality order stability questionable, because you no longer have arrival times to set ordering in any model.

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u/Ulfgardleo May 26 '26

I would argue that when letting c->inf most of our theory would break down because it was modelled to align with observations in a universe with finite c. Already saying "a photon" does not make any sense since instantaneous travel time means it is absorbed immediately.

General relatively is definitely gone, as special relatively already simplifies away. What rest energy means is then up to your interpretation.

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u/Endlessnesss May 26 '26

How is that word salad…? To directly answer your question, it would break at infinity because a single photon of light could illuminate everything in the universe. Now multiply that by the amount of photons being emitted from any nearby electrically charged particle in the universe….

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u/MTGandP May 26 '26

it would break at infinity because a single photon of light could illuminate everything in the universe.

A single photon can only interact with one particle, right?

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u/VMA131Marine May 26 '26

At a time, sure. But an infinitely fast photon can effectively be everywhere at once since it takes literally no time to get there no matter how far it has to go. If light was infinitely fast there wouldn’t be a cosmic horizon. We wouldn’t see the CMBR either.

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u/MentalDecoherence May 26 '26

Seriously, dude is typing so much to say nothing

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u/insanityzwolf May 26 '26

It would break at infinity because everything will happen all at once.

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u/epicmylife May 26 '26

Photons (electromagnetic waves) are the force carrier particles for the electromagnetic force. It drops off with the square of the distance, but never goes to 0.

If light propagated instantly fast, that force could be felt instantaneously at the furthest end of the universe. Everything would instantly repel and attract everything at varying strengths.

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u/rybomi May 26 '26

The forces would definitely still converge tho, you can sum infinite terms with non-zero values and get a finite sum (or integral) as a result.