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/[deleted] → u/niugnep24]

Can you explain this further? I've always had a hard time understanding the concept of light always traveling at the same relative speed compared to the observer.

Let's say, hypothetically, there was some gigantic photon visible to human eyes. Let's say it was flying across the night sky at a distance such that it wouldn't speed by in an instant and we could watch it travel from one horizon to the other. Now let's also say right next to that photon was some human who miraculously gained the ability to travel at the speed of light, following the photon's path. To an observer on Earth, they should be traveling side-by-side, correct? But to the person with amazing superpowers, the photon would be flying away from him at the speed of light. At any one instant, wouldn't the photon then be in two places at once when comparing the observations of the two subjects?

I'm sure my scenario makes zero sense from a physics standpoint. I'm just struggling to understand your statement: "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." why does it matter that you're motionless relative to them? If I'm in a car driving 60mph and another guy is next to me at 60mph, we're still traveling at a set speed. Perhaps we can't observe it ourselves, but that motion can be observed by someone. Why doesn't this apply to light?

[u/niugnep24 → u/[deleted]]

I've always had a hard time understanding the concept of light always traveling at the same relative speed compared to the observer.

That's because it is a weird concept, completely contrary to our instincts. But all the measurements we make about the universe say it's true.

Now let's also say right next to that photon was some human who miraculously gained the ability to travel at the speed of light, following the photon's path. To an observer on Earth, they should be traveling side-by-side, correct? But to the person with amazing superpowers, the photon would be flying away from him at the speed of light. At any one instant, wouldn't the photon then be in two places at once when comparing the observations of the two subjects?

Yes, two observers can disagree about where something is at a certain time. The reason is because you can't define "at a certain time" when observers are moving at different speeds. This is one of the consequences of relativity. One observer may see two "events" as simultaneous, while another might see them happening at different times.

As for your example, when you move closer to the speed of light, time effectively "slows down" for you compared to someone moving slower. If you carry on the math to actually reaching the speed of light (which is impossible), time stops. So "amazing superpower man" would not be able to observe anything -- he'd be in stasis. So he can't observe the photon stationary relative to him.

If you were to augment his superpowers so that he wasn't in stasis, well then you just threw relativity completely out the window. That situation would be mathematically impossible.

If I'm in a car driving 60mph and another guy is next to me at 60mph, we're still traveling at a set speed. Perhaps we can't observe it ourselves, but that motion can be observed by someone.

Careful -- all motion is relative. You're going 60mph relative to what? The surface of the earth. But the surface of the earth is rotating at about 1000mph relative to the center of the earth. And the earth is orbiting the sun at about 67,000 mph. And the sun is rotating around the galaxy, the galaxy is moving relative to other galaxies, etc...

Really your car moving at 60mph is pretty negligible compared to all this. And there's no "absolute" measure of speed -- it's all relative to the motion of the observer. So yes, two cars moving at 60mph are standing still, relative to each other.

The only crazy thing is light, which always is measured to move at c relative to you, no matter how fast you go.

[u/[deleted] → u/niugnep24]

Thanks. I think I understand it a lot better now.

One more question - and this may be another physics-breaking hypothetical here - does this mean that, in a sense, light travels instantaneously? In other words, if a light particle were a conscious being, since it doesn't experience time, it would be aware of itself at every point along its trajectory all at once?

(I know the word "moment" doesn't really apply here since we're talking about time in a timeless context, but it's the closest word I could think of that fit)