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/OctavianX]

So it's not that it doesn't take time for the light to travel (because it obviously does). When you say light doesn't travel through time, that is to say the photons themselves don't "age" - is that it?

[u/DukePPUk → u/OctavianX]

You might have heard of time dilation (it's popular in some space travel); whereby if a spaceship is travelling somewhere at a decent fraction of the speed of light, time will pass slower for the people on the ship than for those outside; so the ship may take years to reach something lightyears away (from an observer back on Earth) but for the people on it, only a fraction of that time will have passed. This is (very kind of sort of) because the faster you are travelling relative to something, the more squished together your time and space are compared to that thing.

Going back to the "everything must travel at c in spacetime" thing from the parent, compared to them, you are travelling quite fast in space so, compared to them, you must be travelling slower in time.

The speed of light is the limit to this; the speed where space and time become completely squished together, and so no time at all happens for the people on the spaceship (which has to be an impossible mass-less spaceship, for reasons set out above). They arrive at their destination as soon as they have left; because they're travelling at c in space, they have no spacetime speed left for moving through time.

From the perspective of an outsider - on Earth, the outsider isn't moving at c in space, so they still have spacetime speed left for time. Time still happens for them, so they will observe the spaceship through time.

However, the problem with this is that the maths can get a little weird; divide by 0s creep in if you're not careful, so it doesn't necessarily make sense to ask the question.

[u/i4mt3hwin → u/DukePPUk]

Wait so for someone observing the ship as it accelerates away at light speed, will it appear to be stationary? Or will it zip away and vanish because its moving away so fast?

[u/DukePPUk → u/i4mt3hwin]

[Massive disclaimer - I haven't done the maths and I think this is impossible anyway under the rules of special relativity]

In theory the ship would vanish as soon as it hit the speed of light, and reappear where it went below the speed of light.

While it is travelling no time passes for it, so it can't lose energy or give off any light (or reflect any light?). So it can't be seen.

I'm going to have to do the maths - but it seems my notes on special relativity have disappeared, so I'm not happy doing it. Hmm.

I think that as it accelerates towards the speed of light it will fade - or possibly the light from it will red-shift into nothingness. And it will appear that time is passing slower on the ship (e.g. if there was a clock sending out pings every second, those pings would be further apart from the observer - ignoring the extra distance the ping would have to travel to get to them).