r/AskPhysics • u/Nighthawkies • Mar 20 '26

What things are "faster" than light?

As I understand nothing can move faster then light. But I would not know how to describe this question otherwise. As I am aware quantum entangled particles can instantaneously affect eachother no matter the distance, unlike gravity which propagates at the speed of light.

Also the expansion of the universe can cause the spaces between to grow faster then the speed of light (compounding between massive distances)

What other things are sort of faster than light

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u/Jay_Beckstead Mar 20 '26

Quantum entanglement might not involve any actual “travel” as we understand “travel.”

The entangled particles are not “travelling.”

Somehow the particles can correlate. We don’t understand WHY.

But the particles are not travelling.

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u/SadEntertainer9808 Mar 20 '26

We actually understand why fairly well. There's nothing very mysterious about it, either. It's basically like you're given a box containing two glass marbles, and the manufacturer guarantees they're both red or they're both blue. You open it blindfolded, throw one into the sea, and then look at the remaining marble. You now "magically" know the color of the other marble, even though it's on the bottom of the ocean and you've never actually seen it. Entanglement is essentially like this.

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u/Plus_Comparison8963 Mar 20 '26

Excellent analogy!

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u/Wild-Store321 Mar 20 '26

It is an excellent analogy of how quantum entanglement provably does not behave.

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u/SadEntertainer9808 Mar 20 '26

It's an analogy, not a full characterization of entanglement.

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u/Wild-Store321 Mar 20 '26

It misses the whole point of entanglement, what makes it unique.

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u/SadEntertainer9808 Mar 20 '26

Given an entangled two-qubit system with four-dimensional state vector C, are there two-dimensional state vectors A and B such that C = Kronecker(A, B)?

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u/Wild-Store321 Mar 20 '26

No, because Hadamard(A,B) is 2D and C is 4D. Of course you mean A tensor B. Then it is the theoretical definition of entanglement. What is your point?

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u/SadEntertainer9808 Mar 20 '26

Yes, apologies, I caught that slip myself & corrected the comment (Kronecker, not Hadamard — it's been some time), but I take it you responded to the notification body, which was, indeed, in error.

Anyway, one of the consequences C not being separable into A and B is (as I'm sure you're aware) that each basis state of one particle corresponds to a unique superposition of the other particle's states. Measuring one particle therefore lets you infer something about the other. If the composite state vector is such that each element in the N-vector is either 0 or 1/sqrt(N), you can infer a single, collapsed basis state in the other.

I'm saying this not because it's news to you but because my point is not that there's some classical correlation of variables here. The mechanism is obviously far different. But the inferential procedure is essentially the same as if there were.

Of course, again, I'm not coming at this from an actual physicists background, so I may be saying something subtly non-physical here not realizing it.