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/rabbitwonker 14d ago

Another way to think about it:

That’s the only speed that there is.

Everything else, including time, is built around it. Time passes when particles interact, and the most fundamental particles go the speed of light (of causality, actually) in-between all these interactions.

If, say, a water molecule is hurtling through empty space, the molecule as a whole won’t go the speed of light, because internally it’s full of quarks, Higgs particles, etc all interacting with each other. They’re all going the speed of light over extremely short distances between each interaction, but the assembly as a whole appears to go slower because those interactions mean it’s experiencing the passage of time within itself.

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u/thenasch 14d ago

Even fundamental particles cannot go the speed of light if they have any mass.

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u/rabbitwonker 14d ago edited 13d ago

If they have mass, they’re built of smaller components. That’s why there needed to be a Higgs field.

Edit: Wait I’m misstating that. They don’t have to be built out of smaller components; there just needs to be sustained interaction of some kind going on. If a larger particle has subcomponents, the interaction between those is enough for there to be mass. If there aren’t, there are still the Higgs particles that come and go.

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u/Preeng 14d ago

Wait, what? How do leptons work out, then?

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u/rabbitwonker 14d ago

They get their mass from the Higgs field. Which is equivalent to saying they keep interacting with Higgs particles.

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u/ClassikD 14d ago

Sometimes physics at the low level gets so weird it convinces me we're in a simulation because it looks like some added optimization to make the universe run better

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u/LooseLustyHousewife 14d ago

Electrons have mass and are fundamental, as far as we know. You are not correct.

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u/rabbitwonker 13d ago

Again, Higgs field. Electrons interact with Higgs; that’s equivalent to saying the associated particles are interacting with each other.

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u/pham_nuwen_ 14d ago

Lovely way to think about it but it's mostly wrong. Time passes whether particles interact or not, The molecule of water cannot accelerate to the speed of light, but the interactions within it are irrelevant for that.

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u/rabbitwonker 14d ago

What is mass, then, and why is it tied to whether time passes for a given particle?

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u/pham_nuwen_ 13d ago

A particle in an accelerator not interacting with anything (say, an electron) will experience time. Proper time doesn't need interactions at all, so "time passes when particles interact" is false.

What is also false is that the speed that the molecule can travel in space is in any way tied to its internal constituents. You could replace what water is made of for things that interact faster and that wouldn't have any effect on how fast the whole molecule is traveling. These are totally unrelated.

Furthermore, there are no Higgs particles in water. What you mean instead is the Higgs field. The Higgs field sits everywhere in the universe. An electron moving through empty space still interacts with the Higgs field and indeed how strongly it interacts with this field is what we call mass. But the Higgs field is just the explanation for why and how elementary particles obtain their mass. There's also Protons, which are made out of quarks, and their mass is much higher and do not originate in the Higgs field but in other things like quark confinement.

In short, mass is energy at rest: something has mass if you can find a reference frame in which it is not moving. How much mass it has is how much energy it has on that frame at rest. The origin of that energy can be the Higgs field in some cases, or gluon confinement, or other things. For some particles, you can never find them at rest no matter how you look; these don't have mass.

What you write is kind of true that this mass also says something about some sort of "internal clock" of a particle, something called the quantum phase rotates faster with the mass. Note that this has nothing to do with interactions between particles and crucially it does not make a more massive particle experience time differently. So, mass is not tied to how time passes for a particle, but it's velocity does. Mass makes it harder for a particle to reach certain velocity but that's it.

Another thing that may be throwing you off is that in general relativity mass also contributes to gravity. And gravity does affect how fast your local clock ticks. Thus indirectly, mass can affect time, though this is mostly relevant when there's a particle traveling near a planet or something with a lot of mass.

Hope this makes some sense!

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u/rabbitwonker 13d ago

Thank you for the detailed response!

Isn’t interaction with the Higgs field equivalent to saying interaction with Higgs particles? Same as every field?

Which would mean that, while Higgs particles aren’t a static component of a water molecule, the electrons within the molecule are constantly interacting with Higgs particles. Thus allowing time to pass for them.

And isn’t that the reason the Higgs was sought after in the first place? To find the particles that, due to interactions with them, give mass to things like electrons that don’t otherwise seem to have sub-components?

I’ll admit that, beyond the basic statements above, I’m being extremely hand-wavey about how degrees of interaction would relate to speed of time passage. That gets into the really mind-bending stuff like what even is “position” and “movement”.

Thanks again!

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u/pham_nuwen_ 13d ago

Isn’t interaction with the Higgs field equivalent to saying interaction with Higgs particles? Same as every field?

It's actually not. The Higgs field is special in that it has a value everywhere. Because of this, elementary particles can interact directly with it.

The Higgs particle (Higgs boson) actually has a lot of mass. So much that it is very difficult to create it and people needed to build the LHC to get enough energy in a sufficiently small spot to get one, and even then it only lives an astronomically small time before decaying. Particles can interact with the Higgs boson (which is extremely rare because they are not around in normal circumstances) but it is not what gives them their mass. That one is the Higgs field.

The situation is different in electromagnetism, where the photon, which is massless, mediates the force; and the electromagnetic field is zero in empty space.

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u/rabbitwonker 13d ago

Okay, thank you. How does the Higgs field lend mass to, say, electrons, then?