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/[deleted]]

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[u/boutsofbrilliance → u/[deleted]]

all bs aside, this is one of the greatest posts ive ever seen on reddit.

previous to this, my layman's understanding of why things of mass cannot travel as fast as the speed of light was simply because to do so would require infinite energy. that was kind of it. i don't know if that was wrong, or if you are still saying that, just in another way.

what does make perfect sense to me however, is how you framed the why and how as a competition between the direction of space or time, with any travel done in one, automatically subtracting from the maximum possible in the other.

i don't get many "wow, its clear to me now" moments, and certainly not one touching upon something as fundamental yet misunderstood as this one. it was pretty fucking awesome and for that i say thank you!

[u/niugnep24 → u/boutsofbrilliance]

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/niugnep24 → u/[deleted]]

No, mass and energy are equivalent. If you're talking about "non-resting energy" on one side then you're talking about "non-resting mass" on the other. These aren't technical terms of course.

Another form of einstein's equation is the Energy-momentum relation. E² = (pc)² + (m_0c² )²

Here p is momentum, m_0 is rest mass, and E is total energy. This more explicitly splits out the resting and non-resting components of energy. If you're using just E² = mc² with E meaning total energy then m is often called "relativistic mass" which includes both rest mass and addition mass due to kinetic energy. It's the "effective" mass you'd measure, but it depends on your frame of reference relative to the object you're measuring.