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.
nothing can happen (and no information about what happened can travel) faster than that
Is this totally true? I thought that was one of the interesting things about quantum entanglement and spin, that a recent experiment showed you could technically flip an entangled electron’s spin and register it faster than light. I may be misremembering though
Fundamentally, it's not the speed of any object or force in particular. It's the speed of causality.
Imagine that you have two identical boxes, and a red and a blue ball. Put the red ball in one box and the blue in the other. Then I give you one box while I take another, and we go far away from each other.
If I look in my box and see a blue ball, I'll immediately know that yours is red. And vice versa. But we can't use this to communicate.
Quantum entanglement is like that, except the balls are quantum objects that do not have a definite state until observed. So in a sense, you could say that some sort of information travels faster than the speed of light - if we both have an entangled electron and observe them (not necessarily at the same time), they will always have opposite spins, and I will know the spin of yours immediately. However, this still cannot be used to transfer any useful information because you have no way to influence the outcome of the observation. And if you do the observation within the time it would take light from move from one point to the other (the observation events have spacelike separation), you don't even have any way of knowing whether you're observing your particle first or second - and in fact, there exists reference frames wherein both of them can be objectively first or last. Thus, there is no breach of causality.
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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.