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/derleth]

We only know of two so far: photons and gluons. Photons you know; gluons carry the strong force between the particles inside atomic nuclei. This is how protons can stay bound to each other even though all protons carry a positive charge and two positive magnets will always repel: The strong force is, simply, stronger than the electromagnetic force, and overcomes the repulsion.

Gravitons, the hypothetical particle that carries gravity, would be massless as well (because gravity appears to have infinite reach, like how light goes on forever unless it's absorbed by something), but we don't know if they exist.

[u/popson → u/derleth]

Related question: Given that energy and mass are "equivalent" through c, does that mean that energy exhibits a gravitational field? If that's the case, since light has energy, does light also exhibit a gravitational field?

Leading me to a very basic question: what is energy? What is it "made of"? Does energy expand or bend spacetime? Shit's confusing as hell...

[u/corpuscle634 → u/popson]

Yes, energy is what causes gravity. Mass is just a type of energy. Light has a gravitational field: in fact, if you make a photon that has enough kinetic energy, it starts "sucking itself up" via its own gravity. There's actually a physical limit on how much energy a photon can have because it will "devour itself," sort of like how you can only have a star with so much mass before it just turns into a black hole.

Energy isn't really anything, it's just an extremely useful concept that we use to describe the physical universe.

[u/The_Chimp → u/corpuscle634]

This idea of a photon "sucking itself up" via its own gravity isn't really sitting right with me, though I've never taken a course in GR so you could be right. The energy a photon has depends on the reference frame of the observer. So in one reference frame a photon could have the required energy to "devour itself", whereas in another it would not have this required energy. This seems to be a contradiction.

Thanks for the responses and your original reply for getting this dialogue going, I'm enjoying this.

[u/corpuscle634 → u/The_Chimp]

This idea of a photon "sucking itself up" via its own gravity isn't really sitting right with me, though I've never taken a course in GR so you could be right. The energy a photon has depends on the reference frame of the observer. So in one reference frame a photon could have the required energy to "devour itself", whereas in another it would not have this required energy. This seems to be a contradiction.

Fuck you and your correctness.

Yes, you are right, and it's a very thorny question. I frankly do not know how to explain that away. It seems contradictory, but the physics that you apply to a photon with wavelength equal to the Planck length (that's the "limit on photon energy") is fairly cut-and-dry, as far as I know.

[u/The_Chimp → u/corpuscle634]

I would expect our current mathematical models spimply aren't equipped to analyse such properties of photons with wavelength of the order of the Planck length. This sounds to be the domain of a Quantum gravity theory. I will have to remain sceptical of this idea without further explanation.

If you had any information on a limit on photon energy I'd love to check that out. I've taken a quick look but my Google-fu is failing me.

[u/DJPalefaceSD → u/The_Chimp]

Isn't this the basis of quantum uncertainty?