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Any reactionless drive might as well be FTL. The limit to traversing the universe isn't the speed of light it is the rocket equation. As long as you don't mind

Any reactionless drive might as well be FTL. The limit to traversing the universe isn't the speed of light it is the rocket equation. As long as you don't mind rather intense time dilation, you can get anywhere as fast as you want with enough fuel and ability to tolerate sustained acceleration. Remember: you can go any speed you can accelerate to, it is everything else that can't go faster than light.

progman32 6y ago
Well, another implicit barrier is human lifetime. Though I suppose if we don't mind massive time dilation, we might not mind thousands of generations passing by during transit.
- xenadu02 6y ago
  That's not correct. If you could accelerate to a significant fraction of c then you can traverse the Milky Way well within a human lifetime from your point of view. Hundreds of thousands of years will pass on earth and your destination while you travel, but it's not a problem for the passengers.
  - edgyquant 6y ago
    Right if which is almost certainly not possible for a human to do
  - Dylan16807 6y ago
    Depends on your acceleration and whether "traverse" means you get to skip around a bunch of destinations or only make a single direct trip. If your acceleration is somewhere between .5G and 2G then every single hop costs a significant chunk of your lifetime, even if those hops can reach extremely far.
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"

progman32 6y ago

Well, another implicit barrier is human lifetime. Though I suppose if we don't mind massive time dilation, we might not mind thousands of generations passing by during transit.

xenadu02 6y ago
That's not correct. If you could accelerate to a significant fraction of c then you can traverse the Milky Way well within a human lifetime from your point of view. Hundreds of thousands of years will pass on earth and your destination while you travel, but it's not a problem for the passengers.
- edgyquant 6y ago
  Right if which is almost certainly not possible for a human to do
- Dylan16807 6y ago
  Depends on your acceleration and whether "traverse" means you get to skip around a bunch of destinations or only make a single direct trip. If your acceleration is somewhere between .5G and 2G then every single hop costs a significant chunk of your lifetime, even if those hops can reach extremely far.

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"