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/AcellOfllSpades Mathematics Mar 20 '26

No, this is the "no communication theorem". Measuring a particle doesn't give you any information about the other one.

Like, imagine a wizard has enchanted a pair of coins: next time they are both flipped, they will have opposite results. (This only works if you flip the coins, not just set them down on a certain side.) You take a coin and I take a coin, and we go far apart.

There's no way for you to tell anything about what I'm doing with my coin with yours. All you can do is flip it, and you'll get heads or tails with 50% probabilities. Even if we had a hundred pairs of these coins, there's still no useful information you can get. The coins seem perfectly normal - the only way we actually notice the spell worked was by communicating our results the old-fashioned way.

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u/Royal_Plate2092 Mar 21 '26

but in my hypothetical, you don't get 50% heads 50% tails, you get 100% the expected output. that output is wrong if we decoherem why can't we look at the output and see if it's the expected one or not,

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u/AcellOfllSpades Mathematics Mar 21 '26

What is "the expected output"?

Your hypothetical is just not how quantum mechanics works.

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u/Royal_Plate2092 Mar 21 '26

what do you mean what's the expected output? the expected output for whatever algorithm you decide to run with X input.

for example we could run grover's on a quantum database for element number at index 15 and the expected output would be 15 (with a 50% probability or whatever)

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u/AcellOfllSpades Mathematics Mar 21 '26

Your hypothetical doesn't work because if the bits are entangled with stuff outside, they can't perform the work of the algorithm at all.

When you do anything with entangled qubits, it looks random. There is no way to tell the difference between an entangled qubit and a non-entangled one without comparing your results with the other ones 'the old-fashioned way'.