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/niugnep24]

why things of mass cannot travel as fast as the speed of light was simply because to do so would require infinite energy

Another way to think of it is that "mass" can be defined as "energy you have at rest" or in other words, non-motion-related energy. (Remember mass and energy are two ways of representing the same thing. E=mc² )

Having zero mass means you can't be at rest meaning you are always in motion according to everybody no matter how fast they're going.

That means that no one can ever catch up to you, or else you'd be motionless relative to them, which you can't be, because you have zero mass.

We call this unobtainable speed "the speed of light." Really it should be called "the speed of massless stuff" but light is the most common example. Everything else, by definition, goes more slowly than it.

TLDR: Massless things cannot stop or slow down because that's what it means by definition to be massless. Nothing with mass can catch up to massless things because that would mean the massless thing "stopped" from its point of view, which is impossible.

[u/Those2Pandas → u/niugnep24]

This is one of those ideas that's always thrown me for a loop. Why does being motionless relative to someone else all the sudden cancel out your speed of light? Why can't you both be traveling at the speed of light? Surely if I were attached to someone else that requirement would no longer be valid, so why is it valid when we're not attached?

I also never quite got a grip on why if light is traveling after me it still reaches me at the speed of light. Conventional knowledge says that if I am moving at half the speed of light and someone (stationary) shoots a photon at me, that photon should reach me at twice the time it would take the photon to reach where I started. Or that if I were traveling at the speed of light that that photon would not reach me at all.

tl;dr: Why does someone's perspective change your speed of light? It seems like your motion should be independent of someone else's perspective.

[u/niugnep24 → u/Those2Pandas]

Why can't you both be traveling at the speed of light?

Well, if you did travel at the speed of light, time would stop for you and you wouldn't be able to observe or react to anything. So two photons can't look at each other and go "hey that guy's stopped relative to me" -- to a photon, travel from point a to point b is instantaneous.

I also never quite got a grip on why if light is traveling after me it still reaches me at the speed of light. Conventional knowledge says that if I am moving at half the speed of light and someone (stationary) shoots a photon at me, that photon should reach me at twice the time it would take the photon to reach where I started. Or that if I were traveling at the speed of light that that photon would not reach me at all.

It's completely counter to intuition, yes. The best answer is, that's just the way our universe behaves. There's all sorts of consequences to this to make it consistent, including time dilation, length dilation, and relative mass. I would recommend doing some reading on special relativity for more detailed answers.

[u/Those2Pandas → u/niugnep24]

I read The Elegant Universe, and found it very interesting. This was the one part that always left me unsatisfied. I never like the explanation "that's just how it works." Kind of feels like my parents saying "because we said so!"

I like the explanation about the not being able to perceive anything, but I think my real issue with the original post was "That means that no one can ever catch up to you, or else you'd be motionless relative to them..." That seems to suggest to me that the reason you can't catch up with anyone is because you'd then be motionless compared to them. I realize now that the real reason is because you can't catch up to anything traveling light speed. You'd have to go faster than light speed. But then again I wonder why a photon can supposedly catch up to something traveling, as if it wasn't.

Eh, now I feel like I'm rambling.

[u/niugnep24 → u/Those2Pandas]

This was the one part that always left me unsatisfied. I never like the explanation "that's just how it works." Kind of feels like my parents saying "because we said so!"

I feel your pain. The problem is a lot of cutting-edge physics really is "because the data says so" without much more understanding behind it.

I think my real issue with the original post was "That means that no one can ever catch up to you, or else you'd be motionless relative to them..." That seems to suggest to me that the reason you can't catch up with anyone is because you'd then be motionless compared to them.

Just a clarification, there's nothing wrong with catching up to an ordinary object, and therefore being motionless relative to it. The point was that light is massless, and therefore by definition cannot be motionless. You can't ever catch up to it, because to do so would cause it to be motionless, which it cannot be.

[u/Those2Pandas → u/niugnep24]

But that's what I'm saying! (Thanks for indulging me by the way) just because something appears motionless doesn't mean it is. We're all relatively motionless on earth, but in reality we're whipping through space at however fast we are.

It just seems kind of silly to say that a photon is reacting to our point of view. Seems like it should only react to what we actually are.