4 ms·

> Remember: you can go any speed you can accelerate to, it is everything else that can't go faster than light. You cannot accelerate masses to the speed of lig

by qayxc 6y ago

> Remember: you can go any speed you can accelerate to, it is everything else that can't go faster than light. You cannot accelerate masses to the speed of light, because if General Relativity holds, you'd require infinite energy to do so. In addition, every bit of radiation arriving from the direction of travel will be turned into high energy gamma radiation and cook you something fierce real quick; not to mention dust or tiny rocks that would release the energy equivalent of a thermonuclear explosion upon impact. Since these effects alone will create a practical upper limit to your interstellar travel speed, there's still a huge difference between FTL and reactionless drives. An FTL drive might get you to Proxima Centauri in a month, whereas even the most optimistic subluminal option will take at least a decade.

jandrese 6y ago
The FTL drive might get you to Proxmia Centauri in a couple of seconds. It's purely fictional so why set artificial limits? You are right that the reactionless drive will be limited by the speed of light, but thanks to time dilation that 10 years might only feel like a couple of months to the passengers. It's cold comfort for their aging friends and relatives back home, but for the passengers it's not that much difference.
- qayxc 6y ago
  > The FTL drive might get you to Proxmia Centauri in a couple of seconds. It's purely fictional so why set artificial limits? It was just an arbitrary example. If we're in the realm of the purely fictional we can just as well forego FTL entirely and use teleportation instead - instant travel to anywhere, why not?
- Dylan16807 6y ago
  At that point you're performing ridiculous contortions of physics mostly just to avoid engineering a better cryosleep?
blamestross 6y ago
> You cannot accelerate masses to the speed of light, because if General Relativity holds, you'd require infinite energy to do so. This is a common misconception. There is exactly 1 mass you can accelerate to any speed you want. YOU. Time dilation canceling out acceleration from the external perspective is why it looks like externally that it takes infinite energy to reach C. Light travels at C and thus experiences zero travel time. Things going slower than C will experience "some" travel time. Not 1 year per lightyear. On the ship it is locally Newtonian the entire way. Sure your destination and origin are doing weird gymnastics out the view screen, and the radiation is getting a bit energetic, but if you have the fuel and willingness to take Gs, you can travel any distance in any subjective amount of time. There exists a fuel budget (a lot) (at overwhelming Gs) to reach another star in 5 minutes. > In addition, every bit of radiation arriving from the direction of travel will be turned into high energy gamma radiation and cook you something fierce real quick; not to mention dust or tiny rocks that would release the energy equivalent of a thermonuclear explosion upon impact. This is a very real problem I offer no solution to other than adding shielding budget to the tyranny of the rocket equation. "warp drives" might offer a solution to this problem but all flavors of reactionless drives are currently SciFi. "If you have enough fuel" here likely means "a couple Jupiters worth of hydrogen that you fuse at perfect e=mc^2 efficiency"
- qayxc 6y ago
  > There is exactly 1 mass you can accelerate to any speed you want. YOU. Still not true. YOU have rest mass ant therefore the faster you go, the more of the energy used for momentum transfer will be converted to relativistic mass, thus preventing you from ever approaching the speed of light in any frame of reference. Local time has nothing do with that. Sure, you once you exceed ~70.7% c time in your local frame of reference will tick so slow that you perceive less time than an observer on Earth would see light take to get to your destination. But in any frame of reference that's more or less "at rest" relative to you, time will tick faster so any message you send once you get to say Alpha Cen, will only be received about ~8.74 years after you left Earth. An FTL vessel, however, would allow you to go to Alpha Cen, take a picture, send it to Earth, return and arrive back on Earth before the message containing the picture arrives. THAT's the difference. > you can travel any distance in any subjective amount of time Even that doesn't seem to be true [1] as the CMB seems to slow relativistic spacecraft down even in intergalactic space. > "a couple Jupiters worth of hydrogen that you fuse at perfect e=mc^2 efficiency" You don't fuse hydrogen to get that kind of efficiency. Only matter-antimatter reactions are capable of coming close to that. While antimatter is hard to come by, it's the ultimate rocket fuel. Ultimately, even a reactionless drive would be limited by the human physique - it takes about 3½ months to accelerate to 30% light speed at 1g constant acceleration. So unless inertia negation devices are invented, that sets the practical lower limit for any interstellar travel time. [1] https://arxiv.org/pdf/1503.05845v1.pdf https://arxiv.org/pdf/1503.05845v1.pdf
mrfusion 6y ago
That’s why you don’t travel without a deflector dish.

by qayxc 6y ago

jandrese 6y ago

The FTL drive might get you to Proxmia Centauri in a couple of seconds. It's purely fictional so why set artificial limits? You are right that the reactionless drive will be limited by the speed of light, but thanks to time dilation that 10 years might only feel like a couple of months to the passengers. It's cold comfort for their aging friends and relatives back home, but for the passengers it's not that much difference.

qayxc 6y ago
> The FTL drive might get you to Proxmia Centauri in a couple of seconds. It's purely fictional so why set artificial limits? It was just an arbitrary example. If we're in the realm of the purely fictional we can just as well forego FTL entirely and use teleportation instead - instant travel to anywhere, why not?
Dylan16807 6y ago
At that point you're performing ridiculous contortions of physics mostly just to avoid engineering a better cryosleep?

blamestross 6y ago

> You cannot accelerate masses to the speed of light, because if General Relativity holds, you'd require infinite energy to do so. This is a common misconception. There is exactly 1 mass you can accelerate to any speed you want. YOU. Time dilation canceling out acceleration from the external perspective is why it looks like externally that it takes infinite energy to reach C. Light travels at C and thus experiences zero travel time. Things going slower than C will experience "some" travel time. Not 1 year per lightyear. On the ship it is locally Newtonian the entire way. Sure your destination and origin are doing weird gymnastics out the view screen, and the radiation is getting a bit energetic, but if you have the fuel and willingness to take Gs, you can travel any distance in any subjective amount of time. There exists a fuel budget (a lot) (at overwhelming Gs) to reach another star in 5 minutes. > In addition, every bit of radiation arriving from the direction of travel will be turned into high energy gamma radiation and cook you something fierce real quick; not to mention dust or tiny rocks that would release the energy equivalent of a thermonuclear explosion upon impact. This is a very real problem I offer no solution to other than adding shielding budget to the tyranny of the rocket equation. "warp drives" might offer a solution to this problem but all flavors of reactionless drives are currently SciFi. "If you have enough fuel" here likely means "a couple Jupiters worth of hydrogen that you fuse at perfect e=mc^2 efficiency"

qayxc 6y ago
> There is exactly 1 mass you can accelerate to any speed you want. YOU. Still not true. YOU have rest mass ant therefore the faster you go, the more of the energy used for momentum transfer will be converted to relativistic mass, thus preventing you from ever approaching the speed of light in any frame of reference. Local time has nothing do with that. Sure, you once you exceed ~70.7% c time in your local frame of reference will tick so slow that you perceive less time than an observer on Earth would see light take to get to your destination. But in any frame of reference that's more or less "at rest" relative to you, time will tick faster so any message you send once you get to say Alpha Cen, will only be received about ~8.74 years after you left Earth. An FTL vessel, however, would allow you to go to Alpha Cen, take a picture, send it to Earth, return and arrive back on Earth before the message containing the picture arrives. THAT's the difference. > you can travel any distance in any subjective amount of time Even that doesn't seem to be true [1] as the CMB seems to slow relativistic spacecraft down even in intergalactic space. > "a couple Jupiters worth of hydrogen that you fuse at perfect e=mc^2 efficiency" You don't fuse hydrogen to get that kind of efficiency. Only matter-antimatter reactions are capable of coming close to that. While antimatter is hard to come by, it's the ultimate rocket fuel. Ultimately, even a reactionless drive would be limited by the human physique - it takes about 3½ months to accelerate to 30% light speed at 1g constant acceleration. So unless inertia negation devices are invented, that sets the practical lower limit for any interstellar travel time. [1] https://arxiv.org/pdf/1503.05845v1.pdf https://arxiv.org/pdf/1503.05845v1.pdf

mrfusion 6y ago

That’s why you don’t travel without a deflector dish.