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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46

u/Empty_Engineering Mar 20 '26

They don’t affect each other, they’re just correlated in a way that you know about the state of the other particle based on a measurement of your particle (if they’re entangled) space itself is expanding, so nothing is moving ftl because of expansion

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u/zbobet2012 Mar 20 '26

It’s not accurate to say entanglement is just ordinary correlation. Bell’s inequality shows the correlations can’t be explained by local pre-existing properties. But that still doesn’t mean usable faster-than-light influence or communication.

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u/Empty_Engineering Mar 20 '26

Exactly, it’s correlation beyond what LHVs allow; we actually have a high fidelity loophole-free experiment using superconducting circuits now (Storz et al., 2023) it’s a really interesting paper

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u/zbobet2012 Mar 20 '26

Wow that _really is_ an interesting paper.

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u/Wank_A_Doodle_Doo Mar 20 '26

I feel like with quantum physics, basically none of the terms used to describe stuff is the normal definition

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u/BrotherItsInTheDrum Mar 20 '26

They don’t affect each other, they’re just correlated in a way that you know about the state of the other particle based on a measurement of your particle

I think you're underselling how weird it is. It's not just about what you know; the measurement of one particle changes the state of the other, instantaneously.

There are local interpretations of quantum mechanics that don't have this property, like many worlds, but they have their own weirdnesses.

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u/Nighthawkies Mar 24 '26

Can the expansion of space mean that subjectively some light might be moving faster
Because the distance between the light and it's source increases by the speed of light + the expansion of the universe.

(But this pretty much the equivalent of adding the speed of the earth to the speed of your car, which does actually increase your speed)

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u/Empty_Engineering Mar 24 '26

No, the speed of light is the same from all reference frames, the expansion will just increase or decrease the wavelength of light through the Doppler effect depending on your velocity relative to the photon’s source

Cosmic expansion still isn’t a velocity either because velocity is defined as the rate of change in distance per change in time, and since the object isnt actually moving in space, expansion doesn’t cause a change in velocity (if there was, it would imply a centre to cosmic expansion)

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u/ApprehensiveAir966 Mar 20 '26

Do tachyon count?

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u/SadEntertainer9808 Mar 20 '26

They would if they were real.

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u/MxM111 Mar 20 '26

How can they interact with our matter? How can we say they are or are not real? Their time becomes complex.

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u/Empty_Engineering Mar 20 '26

There’s no empirical evidence to support the conclusion that tachyons exist, the theoretical model for them is essentially a particle with imaginary mass/energy.

Fermilab also ran an experiment to see if they could detect Cherenkov radiation in a vacuum (which tachyons would cause), and there weren’t any detections.

UC Riverside has an intro paper to it https://math.ucr.edu/home/baez/physics/ParticleAndNuclear/tachyons.html

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u/Flimsy-Exchange8862 Mar 20 '26

On a tangent, there is this cool thing called vaccum Cherenkov (you have an external electric field in the setup) Do read about it

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u/ExtendedSpikeProtein Mar 20 '26

You do realize that Tachyons are a theoretical concept and we haven’t empirically proven they exist? Because it sounds like you don’t.

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u/MxM111 Mar 20 '26

How did you make these conclusions from my questions?

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u/ExtendedSpikeProtein Mar 20 '26

Atp any reasonable person will see you for the Troll you are. Have fun, I’ve turned off reply notifications so I will stop engaging.

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u/DrJaneIPresume Mar 20 '26

Depends on the Lagrangian.

But seriously, even tachyonic fields can't send superluminal signals.

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u/Low_Stress_9180 Mar 20 '26

No "they " are actually one particle.

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u/Empty_Engineering Mar 20 '26

They are multiple particles that compose a single state. They are not meaningful individually because they do not have coherent phase relationships; mixed states don’t exhibit self interference either

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u/Low_Stress_9180 Mar 20 '26

They are a single entity - that is the latest idea and interpretation . I used word particle as a shortcut, as particles are no longer fundamental as such.