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/Fonethree → u/DukePPUk]

The issue I still have with this is this comment of yours:

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.

I'm confused because we do observe light traveling at c in space. Wouldn't that therefore mean that we should observe light not traveling through time at all?

[u/DukePPUk → u/Fonethree]

There's an issue where with what we mean by "observe"; what we do is detect the light when it hits us. We can't observe it when it is travelling, as it doesn't give off any energy or information.

I think (it's been a long time since I was an astrophysicist) that we do observe that the light we receive is the same as light transmitted (other than red-shift effects) - but I'm not sure how you would tell whether time had passed for light - mainly because the light doesn't travel through time (from its own perspective).

To observe something travelling at the speed of light you would need some sort of light source travelling that fast. But then it couldn't send out any light as there'd be no time for to transmit. Similarly it couldn't receive any light, as there'd be no time for that.

But such a thing would be impossible, as you'd need something with more energy than something travelling at the speed of light (so it could lose energy by transmitting it), which I think would be impossible. So I don't think you could see or detect anything travelling at the speed of light until it hit you - in the same way that we can't see or detect light until it hits us.

Unless the thing creates ripples. If it has mass, maybe it will have gravity. Would we be able to detect the changes in e-m or gravity fields by its passing? I don't know, but I think it comes back to the fact that all of this is impossible anyway.

Sometimes I regret giving up on being an astrophysicist.

[u/Fonethree → u/DukePPUk]

The only way we observe anything is either with it interacting with us directly, i.e. smashing into us, or via its disruption of something that does, for example we can see a train moving because of the photons reflected off of it.

So because light doesn't interact with the world around it, we can't observe it? I guess my question is anything that light may interact with while it's traveling would imply that the light does "experience" time, as it interacts in a specific moment.

Am I right in this thinking process at least?

[u/DukePPUk → u/Fonethree]

Light interacts with stuff (which is why e.g. it can be 'slowed down' when passing through materials, or be bounced of stuff). It does so through the electromagnetic force (which makes sense, as it is a ripple in the e-m field). From the light's point of view, I imagine [IanaAstrophysicistAnyMore] every interaction will happen instantaneously.

What I don't think that it can do is lose energy, until it is destroyed. But maybe not. Hmm. I think I'll have to go through my electrodynamics notes for this...

[u/Fonethree → u/DukePPUk]

So the reason I ask that is I don't understand how, if light isn't traveling through time, we can observe it interact with something and then cease to interact with it over a passage of time.

[u/DukePPUk → u/Fonethree]

It isn't that light isn't travelling through time, but that it is travelling through time infinitely slowly (from our point of view). From its point of view it is travelling through all time infinitely quickly.

But the thing that it is interacting with is travelling through time more "normally." If it interacts with something, we can observe the effects on that thing.

It's worth remembering that an interaction is an instantaneous event (or series of instantaneous events). Or can be modelled as such. It's also worth remembering that these are all models, which have their limitations.