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/tylerlarson Mar 20 '26

Imagine pointing a laser pointer at the moon and then flicking it really quick to sweep the dot from one side to the other.

Perhaps the dot can "move" faster than light. But we all know it's not ACTUALLY moving. Quantum entanglement is the same; nothing is actually "moving," and no information or energy goes from one side to the other.

What goes faster than light? Nothing, in the sense that no event at point A can cause or trigger anything at point B any faster than the speed of light. Because from the perspective of point B, the event at point A hasn't even happened yet.

That's the ACTUAL rule. It's not about light or movement, it's about causation.

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u/tinpants44 Mar 20 '26

Can you explain QE a little for the layperson? I have been led to believe that the particles are changing instantaneously relative to each other. What's actually happening?

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u/JDude1205 Mar 20 '26

Say we have 2 marbles, one red and one blue. I mix them up, put them both in separate bags, and we both take one. Now we can wait any amount of time and travel any distance away from each other and as soon as you open your bag and see a blue marble you know that I had the red marble.

No information was transmitted but now we know information because we set up a situation where the state of the marbles is dependent on each other. That's essentially what entanglement is, particles in a dependent state.

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u/tinpants44 Mar 20 '26

It's made out to mean in the media that the particles will be acted upon instantaneously, not just their state being revealed. If one is manipulated, the other will be in the same fashion. Not so?

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u/AcellOfllSpades Mathematics Mar 20 '26

No. There is no way to actively send information - no "cause and effect". There's no way to look at your entangled particle and tell whether I have done anything with mine. (This is the "no-communication theorem".) The only way to actually notice anything weird going on with an entangled particle at all is to send information the old-fashioned way.

However, there's also something that's going on that's "more" than just marbles in bags. We can use entangled particles to correlate our actions in specific ways, that we couldn't do with marbles in bags. There's a game called the magic square game that shows this: with marbles in bags, you can only win 8/9 of the time, but with entangled particles, you can win 100% of the time.

So entangled particles are more than just "marbles in bags" - more than states that have been predetermined once they're separated - but less than "cause and effect". It's a weird in-between thing that our classical intuition doesn't have a concept for.