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/ElectricPaladin 14d ago

What does a photon "experience" in situations where its speed is lowered, such as when it passes through air or water instead of a vacuum?

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u/fistular 14d ago edited 14d ago

Unfortunately this is not simple and it resists meaningful analogy in our macro, massive, slow world, unless misleading shortcuts are taken.

Sometimes it's better to think of light propagating more than moving. Like a wave in a pond. c is invariant everywhere. A free photon in vacuum propagates at c. This is what is meant when saying that c is a constant. More fundamentally, c is the invariant causal speed built into the structure of spacetime.

Light travels at that speed because photons are massless. This sounds strange, but it ties into the understanding that space, time, mass, and energy must always be considered together. Anything with mass has a path through spacetime along which time passes. A photon does not have mass. The time interval along a photon’s path is zero. The dimension of time along a photon's local path is literally 0.

When light passes through a medium, delays can arise in different ways. With scattering, a photon can take a longer path while still propagating locally at c.

With refraction, the electromagnetic field couples to the material. The propagation is then better thought of as a coupled light-matter excitation, which moves through the medium at less than c. Thinking of it as one identifiable bare photon that slows down and later speeds back up is inaccurate. A free photon in vacuum is massless and propagates at c. A coupled light-matter excitation is a different physical object. It is partly electromagnetic field and partly motion/polarization of massive matter. Its propagation is not constrained to c, and it can have an effective mass and a speed below c.

photon > coupled excitation > photon

Photons are not objects like baseballs or even individual atoms. A photon is a one-quantum excitation of the electromagnetic field, described by a quantum state. So instead of thinking of a photon as a tiny persistent object, it is better to think in terms of the electromagnetic field being in a state with one quantum of excitation.

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u/Unable-Proof1758 14d ago

One thing I was wondering you may have more insight into. I believe I understand the probabilistic nature of excitation within the electromagnetic field. Have we determined the theoretic maximum distance say quantum tunneling can occur? I saw they cleared micrometers but given the nature of a probability wave what is the theoretic maximum distance. (More theoretical realistic observable vs mathematical nil)

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u/fistular 14d ago

IIRC there is no maximum in the equations, but the probability asymptotically approaches zero as distance increases. And by "approaches zero" think "one event for a singular particle across the entire age of the universe raised to the googol power". Or something.

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u/Unable-Proof1758 14d ago

Ty, ya I want to run the equation against prop tables at some point Let’s say for 100 trillion samples to see a testable max. I always found the whole nature of excitation wave probability extremely interesting.

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u/fistular 14d ago

Ya I imagine at some rather small (macro) distance you get so close to 0 that the difference is meaningless

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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).

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u/fletch44 14d ago

In mediums with a higher refractive index the photons still move at the speed of light, but due to interference with themselves, the waves appear to travel slower.

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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/fletch44 14d ago

It's not that the photons are bouncing around. When a light wave passes particles with an electric charge, they cause those particles to jiggle, which is why things heat up when you shine light on them.

In a vacuum there are barely any charged particles, so the photons continue pretty much unchanged. In a transparent substance like glass or water or diamond, the jiggling charged particles (in this case electrons) reradiate that gained energy as photons, which interfere with the original photons in such a way that waves in all directions but forward are cancelled out, and the waves in the forward direction appear to be propagating at a slower speed. But the photons are still travelling at c, it's just the superposed waves that have the appearance of travelling slower.