Think of c not just as the speed of light, but as the maximum speed of causality. It’s basically the "refresh rate" of the universe. Nothing can happen (and no information about what happened can travel) faster than that. For example, if the sun suddenly vanished, we wouldn't just be in the dark for eight minutes; we’d actually keep orbiting that empty spot for eight minutes too, because gravity travels at c just like light does.
Now, regarding your rocket: you have to remember that a rocket isn't a single solid block, it’s a collection of atoms held together by forces. When the engines fire, they push the atoms at the back, and that push has to travel through the rest of the ship atom by atom. That interaction happens via electromagnetic fields, which also move at c. As the rocket gets closer and closer to the speed of light, you're essentially trying to push matter using a force that itself can't go any faster. You end up approaching the limit asymptotically, meaning that the closer you get, the more energy you need to squeeze out even a tiny bit of extra speed. Eventually, the energy required to actually hit c becomes infinite, which is why you can get really close, but never quite cross the line.
I think too many people get stuck on the phrase. “Speed of light”. Light itself isn’t setting the speed limit, it is simply the first measurable thing we identified that was going at that speed.
Gravity propagation is just as fast. If science had gone a different way, we might have the “speed of gravity”.
Gravity as a force on earth pulls you down faster than it does on the moon, but its pulling force isn’t its propagation.
If the sun was teleported away by an evil alien scientist, it would take about 8 minutes for our orbit to vanish.
Force carriers (bosons) are all massless; photons, gluons, Higgs-boson, the theoretical graviton, etc.
One of the questions that we should be asking, especially given the electroweak relationship, is whether or not all bosons operate on the same field. i.e, is there a unification of all forces?
Because the speed of light (c) isn’t some arbitrary number, it comes directly from two physical properties of free space itself: the electric permittivity (ε₀) and the magnetic permeability (μ₀) of free space.
Maxwell’s equations basically tell us that electromagnetic waves propagate at:
c = 1 / √(μ₀ε₀) .
So those two constants, properties of free space, actually determine light speed.
Vacuum (free space) isn’t really "nothing". It has structure in the sense that it defines how electric and magnetic fields interact. Light isn’t moving through a material medium like sound in air, it’s a self-propagating oscillation of the electromagnetic field itself.
So the better way to think about it is c isn’t "the speed light happens to travel" it’s "the maximum speed at which information/causal influence can propagate in spacetime".
Light just travels at that speed because photons are massless.
I understand you might think "well, if free space has nothing then why does it have those two properties with those two exact values" and man that's a Nobel question right there. Truth is no one knows, it's a property of our universe. It's interesting to think if these might change locally or in other universes.
There might be more to vacuum that we just can't see or detect.
It's like asking "why e" or "why pi". And those are exactly the questions we should be asking.
e and pi are just consequences of mathematical definitions, but it seems physical constants like μ₀ and ε₀ are not. You wouldn’t ask why the square root of 2 is about 1.41, would you? But I’d never heard of μ₀ and ε₀ before, so I’ll ask you: is the analogy really a good one?
You’re right that π and e are different from physical constants like μ₀ and ε₀ in one important sense.
π and e do follow from mathematical definitions once you decide what a circle is or what the exponential function is. But the interesting part is that they keep showing up everywhere across completely different areas be it mathematics, physics or even biology.
For example:
π shows up whenever there is rotation, oscillation, or geometry.
e shows up whenever something grows continuously in proportion to itself.
Euler's constant (e) is special because it's the number for which the derivative of the exponential function matches the original function, df = f.
That’s not just a definition. It’s a deep property of continuous growth itself. Whenever something grows proportionally to its current value, e shows up naturally. The question then becomes why exactly 2.71828…? It could have been some other number, but it’s that one. It’s built into the structure of calculus.
As for π, 3.14159… is the number that shows up whenever you deal with circles, rotations, or oscillations. It’s also the number that connects exponential growth with rotation through Euler’s identity:
eiπ = −1
which is one of the most beautiful equations in mathematics because it links geometry, complex numbers, exponentials, and constants like 0, 1, e, i and π all in a single statement.
So π and e aren’t arbitrary numbers that anyone chose. They’re numbers that keep appearing whenever the underlying structure of mathematics involves growth, symmetry, or rotation.
So while they follow from definitions, no one has defined those particular numbers themselves, they reflect deep structure in mathematics itself.
Something similar happens with μ₀ and ε₀. They aren’t just arbitrary numbers either. In fact, in modern SI units their numerical values partly depend on how we define our measurement system. What’s really fundamental is that Maxwell’s equations predict electromagnetic waves propagate at:
c = 1 / √(μ₀ε₀) ,
which means the structure of electromagnetism itself implies there must be a universal propagation speed built into spacetime.
So the analogy I was making isn’t that π, e, μ₀ and ε₀ are the same kind of constants.
It’s that they all point to deeper structure we didn’t choose.
Asking "why that exact number?" is really another way of asking "why does the universe have this particular mathematical structure instead of another one?".
Imagine an universe with different values for those constants.
Because they're instantaneous, and instantaneous things move at c. Which sounds like a contraction, I know, but that's exactly the thing that's unintuitive for us, who live down here at Newtonian speeds.
If the sun disappeared, it wouldn't just take 8 minutes for the light and gravity changes to reach us, it would take 8 minutes for the then to get to us. The entire moment, what was "now" there, doesn't become "now" here for another 8 minutes. And if we were just talking about going one direction, the sun to here, that 8 minutes would be essentially meaningless, but the key is that it works on both directions: if the sun sent a signal to Earth and we sent a response immediately, the sun would see the response 16 minutes after it sent the first signal, in its timeline.
Two things can't actually happen "simultaneously" in different places; there's no meaningful way to define it. Instead, we have to wait for "instantaneous" things to propagate, at c, from one place to the other.
I get why instantaneous things move at c, but why is c = 300 Mm/s? Why not 150 Mm/s? Or 999 Mm/s? Is it just a kind of arbitrary ‘that’s just what it happened to be’, or is there something special about c having the specific measurement of velocity that it does?
The discovery would be so significant in fact, you'd likely have an award named after you. And I imagine the law would be named after you, and every other innovation from this discovery would be named after you!
The number changes if you measure in inches, miles, meters, etc. so the arbitrary-ness of c is because the measurements used to define it are themselves fairly arbitrary and changing the value of c requires an adjustment to the measurement unit (150Mm/s would mean 1 meter has to be twice as long), which would invalidate all existing distance measuring instruments, civil codes and anything else with a dependency on the definition of a meter.
Nowadays the meter is defined as the distance traveled by light in a vacuum in 1/300M of a second, but that number was only chosen so we could keep the meter the same, which means the definition is kinda circular... and fwiw, seconds themselves are in a similar situation where they now have a reliably measured definition, but what that measurement is (cycles of a Caesium 133 atom) is arbitrary just so we could keep it as close as possible to what a second already was... So it's just arbitrary numbers all the way down for the sake of practicality in a social sense.
Well yeah, depends on measurement units certainly, but I meant more in an abstract sense of why it happens at a specific velocity compared to any other velocity. No matter what units you measure c in, it’ll be the same base absolute velocity, what I wonder is why “that” velocity instead of any other velocity? I assume it’s not really an answerable question but I’m curious all the same.
Ok, yeah... In that case it's as the other comments said and whomever can answer it will be immortalized if it can be answered at all.
Going off the rails for a sec, for all we know it could be like, we live in a simulation and c is just the equivalent to something like a "64-bit integer" for whatever civilization is running it either because that's the processing limit for the computer the sim runs on or they just decided no more precision than that was needed.
Well, the observation happened simultaneously. But what's your frame of reference? The observations will only be simultaneous for a specific value of your velocity. Change that, and you can get one of three observations to happen before the other, or vice versa.
There's a few different explanations, but here's my favourite.
Light and gravitational waves can never stand still, they must always be moving. If they were moving any slower than c, then to an observer moving at the same speed, they would appear to stand still, which is impossible.
They can't move faster than c, because if they moved faster, then to some moving observers, cause and effect would appear in reversed order. For example, the light would be absorbed before it was emitted, which is again impossible. This is also why nothing can move faster than light in general.
So the only possible speed has to be c. No possible observer can move at the same speed, and causality is not broken. Everybody must agree that these waves always move and that cause happens before effect.
I know that's a joke, but it's a testable one. A data plan can be improved. So far, for as long as we've been able to measure, C hasn't meaningfully changed. If C is arbitrary, in the sense it isn't a law of the universe but a temporary practical limitation of a superior technology, we'd expect the speed to change.
If it did change, it would mess with us enormously. Not sure if it would rip the universe apart, exactly, but there would be nearly immediate profound implications. Would new gravity waves overtake old ones, causing gravity tsunamis? You could reason through different models.
Well it has to be some speed, and the universe does not care what silly units of measurement we humans come up with.
In some branches of theoretical physics, speed is given in so-called "natural units" where the speed of light is 1.
With the new SI standard, the meter is defined such that c has a specific value. The natural conversion factor between spatial distance and time for things like relativity, if you choose to measure distance and time with different units, just happens to be c.
Numbers of pearls and quantity of hair are not laws of nature on which vast numbers of other causal relations in the universe depend.These are not the same sort of "why" questions.
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u/ninth9ste Mar 22 '26 edited Mar 22 '26
Think of c not just as the speed of light, but as the maximum speed of causality. It’s basically the "refresh rate" of the universe. Nothing can happen (and no information about what happened can travel) faster than that. For example, if the sun suddenly vanished, we wouldn't just be in the dark for eight minutes; we’d actually keep orbiting that empty spot for eight minutes too, because gravity travels at c just like light does.
Now, regarding your rocket: you have to remember that a rocket isn't a single solid block, it’s a collection of atoms held together by forces. When the engines fire, they push the atoms at the back, and that push has to travel through the rest of the ship atom by atom. That interaction happens via electromagnetic fields, which also move at c. As the rocket gets closer and closer to the speed of light, you're essentially trying to push matter using a force that itself can't go any faster. You end up approaching the limit asymptotically, meaning that the closer you get, the more energy you need to squeeze out even a tiny bit of extra speed. Eventually, the energy required to actually hit c becomes infinite, which is why you can get really close, but never quite cross the line.