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Comment by amelius | Hacker News Reader

And how much time will it take a person to reach that speed at human-safe acceleration? And how much energy would it take?

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It will take longer that people here have calculated. As you approach c the energy required to accelerate quickly rises. To reach c you need infinite energy.

Maintaining 1g of acceleration for a useful amount of time would require an extraordinary amount of propellant.

All of those estimates are tongue-in-cheek and are accurate if your energy expenditure is actually unlimited.

Just under a year at 1g (according to google. I'm on mobile and would have to derive from F = ma on paper to solve). Energy is variable. I'd assume energy isn't relevant, as it's currently impossible, so we're taking that for granted.

Not really that long.

Not a physicist, but I think it might not be that simple. From our point of view, a ship with constant 1g acceleration increases its speed by 9.8 m/s every second, but people on a ship moving at a significant fraction of c take more than one of our seconds to experience one of their seconds. During the time they experience one of their seconds, the speed increases more than 9.8 m/s, so they must experience greater than 1g acceleration. I haven't done the math but it would be interesting to work out how the acceleration in our reference frame needs to reduce to ensure constant acceleration in the ship's reference frame, and how long it would really take to reach .99c without squashing the passengers.

Isn't a second now officially defined as a single period of atomic decay of a certain atom (I think Cessium?)? So this theoretical near-lightspeed craft could adjust it's accelerators based on an onboard atomic clock that would not be based on seconds as we tend to perceive them, but on SI seconds. I assume these would be getting longer as velocity approached 1c. Unless radioactive decay is also affected by relativity (obviously I am not a scientist), in which case of course that wouldn't work.

One of the consequences of the theory of relativity is that there is no "absolute time". The atomic clock will perceive time just as you will–that is, slower than an inertial reference frame.

Err, that isn't a correct understanding of physics, I believe. The ship wouldn't need to reduce its output as it gets faster relative to some other object in the universe. It can happily keep accelerating at 9.8 m/s for as long as it likes (or has fuel). No passengers would get squashed.

If the ship's means of propulsion is set to impart a constant force, then I understand that the passengers could experience a constant acceleration indefinitely, but that constant acceleration would be with respect to the ship's reference frame, not ours. A constant acceleration of 1g in our reference frame would mean by definition that the speed reaches 2c in two years, which can't be right. Subject again to the disclaimer that I'm not a physicist and haven't done the math, the only outcome I can picture is that in our reference frame the speed asymptotically approaches c in some interesting way.

Oh I see! 1G from our reference frame! Fascinating! That’s a perspective I haven’t seen before, I didn’t realize that that’s what you meant. Indeed, it isn’t possible. We’d see the ship slowly approach but never reach the speed of light, no matter how hard or long they step on the accelerator.

Length contraction is something outside observers would see; the people on the ship would see themselves as normal (and hence not be "squashed".)

This paper has some interesting ideas. They want to manufacture really tiny black holes (radii of "a few attometers") and use Hawking radiation (which is inversely related to mass) to drive the vehicle. The black hole serves as both engine and fuel tank.

https://arxiv.org/abs/0908.1803

I calculate it to take about a year, with 1 g acceleration, but don't take my word for it, am curious to compare with a more authoritative answer.

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