Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Relativistic Spaceship

dmytry.github.io

Post reply on HN
Print view View original

331–340 of 500 posts

Re: Relativistic Spaceship

#331

Post by h45x1 »

[Show HN] Admittedly much less visually engaging :), but some time back I was interested in the twin paradox, so I made a website where you fly a toyish spaceship between three stars and see how the various clocks and the space itself change. It's flat spacetime, so relatively easy to simulate and everything just runs in JS: https://twinparadox.org

The original motivation was, can one make a game with relativistic spaceships, conceptually speaking? The issue is that if a player calls another player IRL it'll be like instantaneous communication... In flat spacetime, though contradictory to its postulates, one can choose a preferred frame of reference, say the planets, and allow instantaneous communication within that plane, not leading to any inconsistencies in a game, I think. Curved spacetime doesn't admit a notion of simultaneity, so that's completely out, no black holes in a game :( Never really got to the point of modelling multiple ships though, the life caught up. So it's just a single ship on the website for the moment.

Re: Relativistic Spaceship

#332

Post by marmaduke »

MilStdJunkie wrote: post #31
AlexAndScripts wrote: post #13

I had no idea you could achieve relevant amounts of time dilation so "easily" with relatively small (1m/s) of constant thrust - such that interstellar travel becomes somewhat viable provided you can continuously accelerate for the entire trip. Googling it, it would take approximately three years of observed time for a traveller to get to alpha centauri at 1g. Could any somewhat-plausible technologies (fusion etc) get…

One gee over the course of months is insane . Even if you converted the entire "fuel" reaction mass directly into momentum, you're still carrying tens or hundreds of thousands of tons of fuel for any remotely habitable spacecraft. And no engine has perfect mass conversion. Matter-Antimatter has been simulated to be maybe sixty percent efficient, fusion is around 17%, nuclear pulse propulsion a surprising 7%, fission…

What do you think about the gravity well "drive" in three body problem trilogy? I thought it was fairly clever, especially given the stories describing it beforehand (maelstrom iirc?)

Re: Relativistic Spaceship

#333

Post by dcanelhas »

In case you're interested in these kinds of relativistic effects, there is a toolkit called OpenRelativity http://gamelab.mit.edu/research/openrelativity/ that you can use, for your simulated near-luminal speed graphics.

It's used by the (free) game "A Slower Speed of Light". http://gamelab.mit.edu/games/a-slower-speed-of-light/

Re: Relativistic Spaceship

#335

Post by eru »

tsimionescu wrote: post #324
eru wrote: post #198

Earlier quoted context omitted.

> I’d never seen “a hundred atoms per cubic meter”, but it’s always been my intuition that, without some quite interesting shielding, you couldn’t make it anywhere near the speed of light. It's not that bad. With currently known science, your fuel would most likely be hydrogen so you can run a fusion reactor. The rocket equation tells you that most of your starship by mass would be fuel, if you want to go fast. Of al…

Wouldn't your water and food then become highly radioactive over time? Even if no, it would mean your shield will be gradually consumed. Not sure this is the best idea.

Your fuel is the outermost layer of your shields (modulo whatever is necessary to keep the fuel in place. But you might use magnetic fields perhaps).

That's basically free shielding: you have to carry the fuel around anyway, so you might as well put it to good use. If you run a nuclear fusion reactor, you won't really lose much of the mass of your fuel, unless you want to. Eg you could use the helium you produce as the reaction mass for your ion drive. (I haven't done the numbers to see how the required mass per second for your ion drive compares to the helium mass per second a nuclear fusion reactor would spit out.)

Because it's a free shield, you don't really get to complain about your shield being gradually consumed.

Of course, you can have some extra shielding further inside. You would keep your water forward of your people, but behind your fuel. So your water would not bear the brunt.

Hydrogen doesn't really get all that radioactive: you can use chemical means to remove any helium or so you might accidentally produce; and hydrogen's isotopes are both pretty short lived and relatively easy to separate. (At least much easier than eg enriching uranium.)

Your water and food is also only a very small fraction of the overall mass of your rocket: as always, the vast majority is made up of fuel.

Re: Relativistic Spaceship

#336

Post by PeterisP »

notfed wrote: post #92
MagicMoonlight wrote: post #44

Earlier quoted context omitted.

You just turn around at the half way point

At which point shielding gets even more complicated.

This is already an unrealistic thought experiment, since continuous 1g acceleration for 20 years is so ridiculous that it requires a magic engine (i.e. consuming most of the ship mass in a matter-antimatter reaction) so if we're at it, we can also assume magic shielding which will also shield the crew from all that antimatter and its annihilation.

It's not a coincidence that the original post relativistic spaceship doesn't even bother considering larger accelerations than 0.1g, since achieving even that is a wild assumption.

Re: Relativistic Spaceship

#337

Post by stavros »

tempestn wrote: post #238
stavros wrote: post #187

Earlier quoted context omitted.

> You can accelerate to 0.5c and then toss an apple out the window and say you’re moving at the speed of an apple tossed out of the window, relative to the apple. You have all of c available as headroom again? You can accelerate up to 0.5c again, relative to the apple you tossed out the window? Yes you can. You can even do it with 0.6c for both those speeds.

But critically, having done that, you still won't be going >=1.0C relative to the road.

Yep, exactly.

Re: Relativistic Spaceship

#338

Post by c12 »

AnimalMuppet wrote: post #50
MilStdJunkie wrote: post #31

Earlier quoted context omitted.

One gee over the course of months is insane . Even if you converted the entire "fuel" reaction mass directly into momentum, you're still carrying tens or hundreds of thousands of tons of fuel for any remotely habitable spacecraft. And no engine has perfect mass conversion. Matter-Antimatter has been simulated to be maybe sixty percent efficient, fusion is around 17%, nuclear pulse propulsion a surprising 7%, fission…

Can you explain why matter-antimatter only has 60% efficiency?

While there may be a 100% anilation with matter-antimatter reactions the resulting thrust is not perfectly efficient due to energy consumed as the rest mass of charged and uncharged pions, energy consumed as the kinetic energy of the uncharged pions (which can't be deflected for thrust); and energy consumed as neutrinos and gamma rays.

I'm unsure if the resulting efficienty is 60% however, not all of the energy produced in the anilation is converted to "stuff" that is useful for thrust.

Re: Relativistic Spaceship

#339

Post by anonyfox »

PerryCox wrote: post #312
goblinux wrote: post #188

Earlier quoted context omitted.

Isn’t that the plot of Skyrim?

In Skyrim they didn't send Alduin into the end of time, just into the future (also there is going to be an Elder Scrolls VI so V can't be the end of time, but will likely be the end of the fifth era). It was one of the reasons that the Felldir the Old was not onboard with the plan to use the Elder Scroll on Alduin. They didn't know how far into the future he would be sent. Most people on Nirn at the time of Skyrim di…

the unrealistic part of the whole narrative is: in that distant future he returns, another "hero" might step up and just kill him again, so he can't just "destroy all existence", or even Tamriel after some hundreds of years has modern/advanced military tech and will just obliterate a spawning dragon completely.

If something can be "killed" by a dude with an axe, it ain't a world-ending superpower for me. I get that Alduin could harass the known world by eating people or burn down villages if left unchecked, but "end of the world" for me is kinda different scale.

Re: Relativistic Spaceship

#340

Post by PeterisP »

cypherpunks01 wrote: post #70
Teever wrote: post #40

Earlier quoted context omitted.

What if you didn't carry the fuel? Use stationary lasers from your starting point to power your journey half way until you lasers from your destination to slow your craft down.

I think the perceived output from stationary lasers on Earth would appear to decrease as your speed increases, since you're accelerating away from the laser photon packets. Because of that behavior, I'm not sure it's a feasible way to get up to relativistic speed? I think the lasers would also need ~10,000 years of fuel, as opposed to ~10 years of fuel if you could somehow figure out how to carry it. But the energy t…

Time dilation also has an effect, since you're receiving 10000 Earth-years of laser energy over 10 ship-years.
Post reply on HN