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Comment by dylan604 | Hacker News Reader

does this take into consideration that you would need to start slowing down halfway there?

It wouldn't need to be halfway. At 3gs acceleration, it would take about 115 days to accelerate up to the speed of light. You'd then travel ballistically for the majority of the journey before having to decelerate for another 115 days.

Given the total journey would be ~40,000 days the 230 days of acceleration probably isn't going to impact things too much. Even if you brought it back to 1g acceleration, it's still only around 700days out of 40,000.

I just read somewhere in this thread that 'ship-time' at the speed up time is zero. So that would mean 230 consecutive days of acceleration.

No, it assumes you start and end at whatever that maximum velocity is that we've been able to achieve.

That'd be fun to add, though. I'm not really sure what human-safe acceleration is - people here assume it's in the 1g-3g range. (That seems like a lot to me though, particularly for a long period of time - I think anything more than a fraction of g would be wildly uncomfortable.)

1G would be great as it would remove any unexpected physiological impacts of living in zero or low gravity.

Having said that, the distance and duration means that only a small portion of this journey would be under acceleration, the rest would be in zero g.

Spending a century in zero g would probably have a bunch of strange side effects and almost certainly mean that the people alive when they arrived wouldn't be able to stand the planets gravity.

This exact topic is what about 80% of The Expanse is about.

Not quite true. You'd spend half of the travel time under 1g accelerating, then the ship does a 180 degree flip in 0g, and then you spend another half of the travel time under 1g decelerating.

Edit: Of course, at constant 1g acceleration, at day number 354 we reach 1c. Don't know if we should start coasting before we reach 1c or just the hell with it and see what happens if we keep accelerating.

So many people in this thread are acting like constant acceleration is possible. Unfortunately the faster you get, the more energy it takes to maintain constant acceleration.

> Don't know if we should start coasting before we reach 1c...

Don't worry about it. Your rocket would need to expend an infinite amount of energy to accelerate to c. The energy stored in your finite-sized rocket is finite. At a certain point it becomes futile to keep trying to accelerate -- though I suspect that the impacts of space dust will eventually cause a significant amount of drag

http://math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/r...

"Unfortunately the faster you get, the more energy it takes to maintain constant acceleration."

I'm not sure what you mean by this. You could also say "the slower you get, the more energy it takes to maintain constant acceleration". Constant means ongoing, so as long as it is constant, you are going to have to expend more energy.

Unless you mean that somehow, you could determine your absolute speed by how much you accelerate for a given expenditure of energy, but wouldn't that violate relativity?

Can you alternate accelerating and decelerating? I mean that's a heck of a lot more energy, but at least you're not risking reaching light speeds.

Wouldn't 1g be perfect? I've been enjoying it for a few years now on this planet.

I took it as 1g of acceleration. I don't know. Wouldn't plain old 1g mean no acceleration at all?

Edit: Oh... I guess not. :) I got confused with the extra g's we'd need to escape earth, but that's so short-term that it can be ignored. Yeah, I think 1g would be perfect.

When you sit in a car and it accelerates forward you can feel your back being pushed into the seat. When you lie in bed gravity will also push you into the bed. In both cases it's a force acting on your body. So being pushed in space in a ship accelerating at 1g will feel like normal gravity with the 'ground' being the back of the ship.

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