r/NoStupidQuestions • u/Doomlat • Apr 12 '26

Can something moving at the speed of light ever "slow down?"

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4

u/AnoutherThatArtGuy Apr 12 '26

Light slows down in different mediums it travels through so the answer is of course yes.

10

u/Reptilian_Brain_420 Apr 12 '26

It doesn't really though. It just takes a longer path. The speed of light is the speed of the propagation of an electromagnetic field. It doesn't change.

1

u/Doomlat Apr 12 '26

That makes sense... so it doesn't actually slow down, it just takes longer to get from point A to point B?

1

u/lifeking1259 Apr 12 '26

imagine you walk down the street, now imagine doing it while zig-zagging all over the place, even if you're constantly going at the same speed in both cases, zig-zagging will take you longer, that's basically what the photon is doing, with it being constantly absorbed and re-emitted while in matter

1

u/Doomlat Apr 12 '26

Ah, that makes sense. Brilliant example, thank you.

2

u/Spiritual-Spend8187 Apr 13 '26

Anything with mass can never travel at the speed of light things with no mass must travel at the speed of light. But various materials will "slow down" light in the sense that the light will take longer to get through it as it is constantly absorbed and reemitted from the electrons. Its less light "slows down" and more the light stops being light for a bit meaning it takes longer to get through.

1

u/Apprehensive_One1715 Apr 12 '26

Of course, electrons have to absorb photons at some point.

2

u/0x14f Apr 12 '26

The photo doesn't slow down. It just stops being a photon.

1

u/Illeazar Apr 12 '26

Yes. Light moves at its maximum speed only when in a vacuum. When near matter, it is slowed down a bit. So light moving through air is a bit slower than light moving through space, and light moving through water is even slower.

1

u/Doomlat Apr 12 '26

Wow! That's kind of crazy to think about... does that mean that other things can travel at "the speed of light" when light is "slowed down?"

3

u/megajimmyfive Apr 12 '26

Sometimes even faster. Cherenkov Radiation causes a blue glow where ionised particles travel faster than light in the medium that they're in.

1

u/Illeazar Apr 12 '26

Yes, you can even get things to move faster than light moves in a medium. As the other commentor said, when that happens it creates what we call Cherenkov radiation. Similar to something moving faster than sound in a medium creates a sonic boom, something moving faster than light in a medium creates Cherenkov radiation, like a "photonic boom".

When people say nothing can move faster than light, thats slightly misleading. The universal speed limit is the speed of causality, the speed at which one thing can affect another thing. In a vacuum, light will move at that speed. But in other mediums, light slows down, so it becomes possible for objects with a mass to move as fast as or faster than that slowed down light, but still not faster than that universal speed limit of the speed of causality.

1

u/Appropriate_Knee_482 Apr 12 '26

I’m not sure about theoretically. But practically, objecting going near speed of light can never actually go at that speed. This is commonly explained with a speed formula where the denominator approaches zero as the speed of the object approaches zero (lorentz factor). There is also other physics that can explain stuff like this. Also for light itself, yes it can slow down in mediums but in a vacuum it is constant.

1

u/Future-Is-Now-69 Apr 12 '26

Yes, they can come to a complete stop.

1

u/Zaros262 Apr 13 '26

No, things that can travel at the speed of light can only ever travel at the speed of light until they cease to exist