Make something heavier and it's tougher to move it.

The more mass something has, the harder it is to keep on making it go faster.

Anything with no mass moves as quickly as anything is able to move.

Light has no mass.

It's more that there is a universal speed limit and light travels at it. because it has no mass.

We know that on the surface of the Earth, there is a maximum distance you can get from any other object. We could have confirmed this empirically by placing transmitters at points around the world and seeing how far away they were, but we instead we proved it conclusively by showing that the Earth is a sphere, and given the way that distances work, it doesn't make sense for something to be more than 20,000 km away along Earth's surface. If you try to, the object will seem to appear closer behind you, though from the outside we know it just moved across the point opposite you on the Earth's surface.

There's an analogous fact about spacetime. Minkowski space, which seems to be what we live in, does not geometrically allow for something to accelerate above the speed of light. It's not simply something we haven't observed, it's more like trying to get more than 20,000 km away on Earth's surface - the way velocities work in this geometry make that physical action kind of nonsensical. We 'proved' things can't go faster than light by experimentally confirming various effects that we would expect in a Minkowski space like length contraction and time dilation.

It may be that things can go faster than light, but if so it will require new physics, and will mean we don't really live in a Minkowski space, but something that usually behaves similarly but something is different. This is very probably the case, since we know that general relativity is an incomplete theory, but so far no extensions allow for faster-than-light travel either.

Basically it goes like this:

Our current understanding of physics says there's a maximum speed that anything can travel. This is the speed of causality, which we call c.

One result of the equations we've discovered that gave us this information is that the more mass something has, the harder it is to accelerate, but also that that acceleration gets even harder as it gets going faster and faster. A corollary of this is that if something has no mass, it's super easy to accelerate. In fact, so easy that something without mass literally cannot be still, and it can only travel at one speed: the speed of causality, c.

So we know a) there's a maximum speed limit in the universe, b) anything without mass must always travel at that speed, and c) light has no mass. From that we conclude that light must also travel at that speed, which is why we often call it the speed of light.

Light is one of the fastest things. In fact, there are other things that move precisely at the speed of light. These are gluons (tiny particles that hold protons and neutrons together) and potentially gravitons (the particles that would carry the gravitational force if we can merge quantum mechanics with General relativity).

The speed of light isn't actually just the speed of light. It's the speed of anything that does not have mass. Photons are the thing that we often see that doesn't have mass, so it was given the name "speed of light" rather than "speed of gluons".

But the key is that photons are massless. We cannot have things go faster than the thing with 0 mass. Because mass inherently slows things down.

So unless you can find something with negative mass. (We have no real expectation that such a thing exists), everything is moving either at the speed of 0 mass particles, or slower.

I recommend reading this reply and the next few in this thread. I find they explained it pretty well.

It's the fastest thing that we know of. That's the beauty of science, we can accept new information as it becomes available but for the time engaged all the calculations indicate the light is the fastest known thing.

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We don't.

What we do know is this:

1. There are ways that are inherently unable to get you to the speed of light. For example, the acceleration of an object having mass will never get you there, because the closer you get the greater your mass becomes. To get to the speed of light you need infinite energy, so getting faster would require more than infinite energy.
2. FTL inherently allows you to set up situations in which you can go back in time and change the past. This is anathema to many people, including physicists.
3. There are things faster than light. Spacetime can expand at a rate faster than light, for example.

However, we do not know for certain that FTL isn't possible for, say, that object having mass mentioned before. What we do know is that it cannot *accelerate* to such a velocity and that it could violate causality if it found another way to get to that velocity.

How fast does gravity travel?

This is one of the best actual ELI5s I've seen before.

The effects of gravity propagate at the same speed

https://en.wikipedia.org/wiki/Speed_of_gravity

Same speed, c.

At c as well. For example, if the sun suddenly disappeared we wouldn't know about it in any way for 8 minutes or so. Then the lights would go out and the Earth's path through space would change.

As far as I understand we do not know it, we think that the speed of light is a maximum possible speed. And we know that all of our post observations and consequences of observations and various old and new observations confirm this fact, but.. its still a theory.

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But it does not really answer the question: why can light not be even faster? Why are there not other massless things that are faster than light?

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This is wrong and it makes no sense

There could be. But all our physics* implies that any such thing would be unable too interact with us slower-than-light beings. Like, at all, not even in the weakest sense. Then it boils down to the question of what "exists" means; can things "exist" that we cannot, will never, nor could ever in any way detect?

The material/scientific version says no, because either Occam's razor or it not being testable makes it pointless for it. Quite a few religions allow such concepts, though. Same with Science-Fantasy, with parallel universes and all.

*: Which could be completely wrong, we have just found it to be accurate to a very high degree and probability.

It’s a bit more well known than that. There is a maximum speed at which anything in the Universe can travel based on the constraints of General Relativity. Things with no mass must travel that speed. Anything with mass is slower. So the only way something could be faster, is if it had negative mass, which does not exist.

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That we know of.

So it does-- all things with no mass travel at the same speed.

The deeper question is why is that speed this value: 299,792,458 m/s. This speed appears to be a fundamental "limit" of the universe we live in

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This is a bit above eli5 but a simplfied version is because of relativity the faster you go the slower time happens. At c time stands still, if you were to go faster than c time would happen backwards. And with our current understanding of physics we dont have the slightes clue what time going backwards would look like. Also even though most people know c as the speed of light its actually not, light has diffrent speeds depending on the medium it travels through, the fastest is through a vacum which is c or the speed of causality.

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We have a very accurate mathematical model of the universe and it does not have anything in it that requires negative mass.

Well we dont know right? There Is this Quasar jet which dissapear for a bit And Is thought to be Faster than c, So we can't see it. You have to understand that heat, visible light, UV, radiowaves, your WiFi, tv signal And more is still "light" you just can't see most of it. Let have this box which can be shielded from everything in universe, but Is still in our universe, if you turn on a flash light in there, surprise you Will see light. But if this box Is not part of our universe, there Will be no light, cuz the energy can't travel thru Space like a wave. There Is no particle in photon, it is energetic Peaks in Space which sometimes act like a particle. if you swoosh a long straight rope, the wave Will travel along the rope, it Is the same with light, the rope Is Space And the wave Is photon.

The invariance of the speed of light with respect to frame of reference is a special property. If something is faster than the thing observed to be invariant, which happens to be light, the geometry of special relativity breaks down, predicting spacial and temporal inversions. This isn't just limited to breaking relativity. Other areas of physics would break too. For example, this would also be a violation of the second law of thermodynamics. It doesn't stop there. Much of accepted physics would have be be rolled back.

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