In the grand body of the universe and even our own solar system the speed of light is incredibly slow on the cosmic scale. Why does it have this speed limit ? It is theoretically possible to go faster than light ? Or is light just the fastest thing we have observed thus far ?
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Sometimes I think that, although we know so much about the universe and its behavior, there is so much we still need to know that many of the things we believe to be true today will fall apart in the future.
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As you travel faster, your experience of time slows for you and you become more massive – meaning you need even more energy to go faster. You need more and more energy as you get closer and closer to this ‘maximum speed’ where you need infinite energy and will be infinitely massive. So effectively you cannot get to that ‘maximum speed’.
The exception to this is if you have no mass – like light. Light *only* travels at that speed and cannot travel any slower of faster. It also experiences no time. So a particle of light can spend billions of years crossing the universe from our perspective but from its perspective it is instantaneous.
All of this strangeness comes from a deeper rule that says everyone must experience the same laws of physics regardless of their speed relative to each other. This was the rule Einstein applied and created the Theory of Relativity. Before his theory everyone was really puzzled as to why the speed of light seemed to be the same regardless if we were travelling toward the light or away from it.
The speed of light has nothing to do with light itself. It is the speed of causality. It is the rate at which one event can lead to another. In other words, it is the universal clock ticking.
Light, because it has no mass, travels at this speed because it experiences no forces opposing it. A particle with mass requires more and more energy to accelerate as it speeds up, and in order to reach the speed of light, it would require infinite energy. As this is not possible, massive particles can never reach that speed.
But time, space, and causality are all functions of the universe, and are not concepts outside of it. Or, to put it more simply, the universe created these concepts. Outside of the universe, these things do not exist. This is the reason there is one thing faster than the speed of light.
The only thing faster than the speed of light was the expansion of the universe in the moments after the Big Bang. The universe went from a quantum scale to something on the scale of the current universe in a fraction of a second, which is faster than light. But within that universe, all causality happens at C (which is the constant referencing the speed of light/causality).
The idea that this is “incredibly slow” as you stated isn’t really accurate.
Try watching this video: https://youtu.be/qdycfWfAtsM
Time and space are not separate. If they were, then time would be unchanged no matter how fast you moved through it, and you could accelerate to any speed without limit except for fuel. But they are actually two aspects of one thing, called spacetime. So when you change how you move in one (space) then the way you move in the other (time) also changes to match, keeping overall spacetime the same.
Think of a horizontal line and a vertical line that cross in the middle, drawn on a piece of paper. The two lines are clearly distinct, so they seem separate in that sense, like time and space. But if you rotate the piece of paper, the two lines both change away from horizontal and vertical. And if you rotate them far enough, then they swap places: the line that was vertical becomes the horizontal one and vice versa. But the piece of paper stays exactly how it was, just rotated 90 degrees.
The same is true of space and time, but there’s a wrinkle: the geometry of spacetime is a bit different from the piece of paper. On the piece of paper, you can calculate the distance between two points by giving them X and Y coordinates and then use the Pythagorean theorem. Spacetime geometry is governed by an equation called the Minkowski metric, which actually is just a version of the Pythagorean theorem – but one where you subtract the square of the time dimension instead of adding it, and that changes the math. Accelerating in space is equivalent to a ‘hyperbolic rotation’ in spacetime. We’re familiar with circular rotation, where you come back around to the same orientation after 360 degrees. But in spacetime, you can rotate a line infinitely in one direction and it never comes back to facing the same direction. (Again, the video helps make sense of this.) This is why the speed of light is a limit: no matter how much you accelerate (‘rotating toward’ the speed of light) away from where you started, you never reach it: it’s always infinitely far away, in that sense.
The Minkowski metric divides spacetime into two domains: one where the distance in time is greater than the distance in space (this is called being timelike), and another where the distance in space is greater than the distance in time (called spacelike). The speed of light is the dividing line between the two: It’s the line where the distance in space and the distance in time are exactly equal. Because of the math of hyperbolic rotations, any observer who starts one one side of that line stays on that side, permanently: they can never accelerate enough to cross it, because they would have to do so for an infinite amount.
There’s also an interesting theory that this explains quantum mechanics: that QM is where our timelike world view comes into contact with spacelike phenomena. In this theory, tachyons (faster than light particles) are real and mediate thing like quantum entanglement, and in fact all the weirdness of quantum physics is down to the way ordinary phenomena like causality look utterly transformed across the divide created by the speed of light.
That was a lot. I hope it helps.
If you look at all the top answers, all of them dodge the question and instead answer a different question that they made up in their head.
The answer is we have no idea why light can’t travel at 400 000km/s or 1 million km/s.
In ELI5 terms:
* Light has no mass. Imagine hitting a ball with a baseball with a fixed strength. If it’s a bowling ball, it won’t move much. If it’s a baseball ball, it’ll move some far away. If it’s a golf ball (lighter, smaller), it’ll move much farther. So the lighter it is, the further it goes. How far does it go if it has 0 weight? That’s light, the 0-weight ball that you hit.
* But it doesn’t go at infinite speed. In space (vacuum), it looks pretty empty but it’s still not empty. There’s what is called “vacuum energy”, think of it as a very, very light version of air. Only, it doesn’t have resistance in the classic sense, it doesn’t “slow down over time” the ball, but it’s still the key to understand this.
* So the light speed can be calculated through two variables that are intrinsically related to this “vacuum energy”, the electric constant ε_0 and the magnetic constant μ_0. They are like how dense air is, but for space. It’s a constant through the galaxy, and it determines the fixed speed of light.
You ask: *why is does light only travel 300,000 km/sec ? Why does it have this speed limit?*
The speed of light in a vacuum, commonly referred to as “c,” can be derived from fundamental constants of nature. Specifically it can be derived from the vacuum permittivity (ε₀) and the vacuum permeability (μ₀) according to:
c = 1 / √(ε₀ * μ₀)
where ε₀ is the vacuum permittivity, which represents the electric constant and describes how electric fields interact with matter in a vacuum and μ₀ is the vacuum permeability, which represents the magnetic constant and describes how magnetic fields interact with matter in a vacuum. The values of ε₀ and μ₀ are defined constants, and their product is exactly known. So, c, in a vacuum is also a precisely defined constant. The point: the speed of light can be derived from fundamental constants and its value is integrally related to the fabric of the universe. So, the speed of light is what it is for sort of the same reason that π is π and *e* is *e.*
Your follow up question: *Is it theoretically possible to go faster than light ?*
No, because as a the velocity of a mass approaches *c,* it’s mass also increases as described by the equations of special relativity, approaching infinite mass as its velocity approached *c*. It would require an infinite amount of energy to continue accelerating, an impossibility.
The implication is only massless objects, like waves and photons, can travel at *c*, the speed of light.
Anything without mass moves at c2 (speed of light in a vacuum). Nothing with ANY mass at all can go c2 because the faster an object with mass goes, the more energy required to increase speed. At c2, the energy required is literally infinite, which makes it not possible to accelerate to or past c2.
Fun fact, from the point of view of anything going c2, time doesn’t really exist. From the moment c2 is reached until its destination, to that object, no time has passed at all. It’s literally instantaneous.
There are theoretical particles that flow backward through time. In a way, they travel faster than light, since speed and time are intrinsically connected the way we measure them. Only way to go faster than instantaneous is to literally arrive at a destination before you left.
Isn’t the ‘speed of light’ just the maximum we have perceived particles to move at?
It’s only our smugness that has this as the absolute limit.
Everything we ‘know’ should always have an asterisk to say ‘at this time’.
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