4 ms·

> 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 misconcepti

> 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
- blamestross 6y ago
  I agree with everything except this: > 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 Rest vs relativistic mass only exists from the outside observers perspective. The traveler doesn't see any change in it's own mass. On a ship traveling near the speed of light, people walking around won't have to spend extra energy to move in the direction of ships motion. Relativistic mass just accounts for the energy being spent to accelerate that doesn't result in an increase in velocity (and thus momentum) from an external perspective. > 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. This is a really good point! This means the Andromeda galaxy could be only 30 years of travel away. The primary point I am trying to make is that "FTL" isn't important for interstellar travel if a reactionless drive gives us most of the benefits we need (getting to other stars within human lifespans). Honestly I don't think either reactionless or FTL drives are actually possible, so generation ships or ships full of human popsicles coasting most of the way are most likely how we will get to other stars.

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

blamestross 6y ago
I agree with everything except this: > 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 Rest vs relativistic mass only exists from the outside observers perspective. The traveler doesn't see any change in it's own mass. On a ship traveling near the speed of light, people walking around won't have to spend extra energy to move in the direction of ships motion. Relativistic mass just accounts for the energy being spent to accelerate that doesn't result in an increase in velocity (and thus momentum) from an external perspective. > 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. This is a really good point! This means the Andromeda galaxy could be only 30 years of travel away. The primary point I am trying to make is that "FTL" isn't important for interstellar travel if a reactionless drive gives us most of the benefits we need (getting to other stars within human lifespans). Honestly I don't think either reactionless or FTL drives are actually possible, so generation ships or ships full of human popsicles coasting most of the way are most likely how we will get to other stars.