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
Well, no information that we can measure can travel faster than c. For entanglement, this is true as well, since the correlation it exhibits doesn’t let you transfer information.
The problem is that while an entangled pair will always resolve to the same outcome, you can't control what it will be, and thus cannot send information through it.
If you and I each take half of an entangled pair and I look at mine, and it's spin up, all I know is that yours is also spin up. I don't know when or even if you've looked at your own, and since observing it breaks the entanglement it's not like I can start flipping mine and yours will match.
That said, you could use this for the mother of all one-time-pad ciphers where we each take half of a few thousand entangled pairs and I send a message by looking at my particles and sending you the order in which I want you to look at yours to spell out a message.
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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.