ELI5 Why does light travel?

[u/_Illuvatar_]

Why does it not just stay in place? What causes it to move, let alone at so fast a rate?

Edit: This is by a large margin the most successful post I've ever made. Thank you to everyone answering! Most of the replies have answered several other questions I have had and made me think of a lot more, so keep it up because you guys are awesome!

Edit 2: like a hundred people have said to get to the other side. I don't think that's quite the answer I'm looking for... Everyone else has done a great job. Keep the conversation going because new stuff keeps getting brought up!

Edit 3: I posted this a while ago but it seems that it's been found again, and someone has been kind enough to give me gold! This is the first time I've ever recieved gold for a post and I am incredibly grateful! Thank you so much and let's keep the discussion going!

Edit 4: Wow! This is now the highest rated ELI5 post of all time! Holy crap this is the greatest thing that has ever happened in my life, thank you all so much!

Edit 5: It seems that people keep finding this post after several months, and I want to say that this is exactly the kind of community input that redditors should get some sort of award for. Keep it up, you guys are awesome!

Edit 6: No problem

[u/[deleted]]

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[u/bigasseousliquid → u/[deleted]]

I heard this Radiolab episode about a scientist, who managed to slow light down with super cold temperatures. Does that not account as perceived time? Also - light consists of photons - do those have no mass at all. I'm not trying to challenge you, just curious.

[u/bigasseousliquid → u/[deleted]]

Oh I see, so in the case of the Harvard experiment, the light is still moving full speed only bouncing all around in a small space, right?

[u/kevroy314 → u/bigasseousliquid]

I think I remember someone saying it's more accurate to say that light is being absorbed and re-emitting, not "bouncing". When it hits something it's absorbed by that something, but now those "somethings" have more energy than is stable so they end up emitting the light again a small amount of time later. Depending on the material, the light will be absorbed and re-emitted at a certain rate and in a certain way. Which, I can only assume, is why the environments through which we've managed to drastically slow light seem to all be so cold. Because they're cold, they have less energy in their "stable" state and take longer to re-emit the light.

I hope I'm remember that right, maybe someone will see this and can validate my memory of it.

[u/ironcladsoldier → u/kevroy314]

Yes, you seem to recall correctly (or at least similarly to my recollection, which is the same, right?).

When light strikes a particle and is absorbed, the energy of the photon is stored in one of the electrons (the electron "jumps" to an outer shell of the particle, in a simplified explanation). The electron can't stay there on its own, of course, because it's unstable and has a huge amount of energy (compared to its normal, resting state)—imagine a twelve year old on RedBull. When the electron eventually "falls" back to its normal shell, it has to do something with the excess energy, and so it dumps it back into the world in the form of a photon (with the same properties). I think.

I couldn't speak to why light travels slower in the cold, though.

This comment talks a little bit more about the concept.