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Relativistic Spaceship

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Re: Relativistic Spaceship

#291

Post by wincy »

sega_sai wrote: post #280
dfee wrote: post #147

Earlier quoted context omitted.

I’d like to see a map of the known universe visualizing atomic density on a log scale. 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. And on the other hand, I’ve seen claims that “space is really big” as you mentioned; but that claim has always seemed dubious.

One nice illustration of "space is really big" is a fact that if you take the cube with the side equal to the distance from the Sun to the nearest star and fill it with water, then the mass of this cube will be roughly equal to the mass of the whole visible Universe.

Another good one for me is that a cubic light year or butter would immediately collapse into a black hole with a Schwarzchild radius larger than the observable universe.

Re: Relativistic Spaceship

#292

Post by WitCanStain »

sega_sai wrote: post #280
dfee wrote: post #147

Earlier quoted context omitted.

I’d like to see a map of the known universe visualizing atomic density on a log scale. 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. And on the other hand, I’ve seen claims that “space is really big” as you mentioned; but that claim has always seemed dubious.

One nice illustration of "space is really big" is a fact that if you take the cube with the side equal to the distance from the Sun to the nearest star and fill it with water, then the mass of this cube will be roughly equal to the mass of the whole visible Universe.

Source?

Re: Relativistic Spaceship

#293

Post by fyver »

wiml wrote: post #190
abhibeckert wrote: post #152

Earlier quoted context omitted.

> Every proton would have the energy of a baseball I wonder if you could capture that energy and use it to generate thrust. Most of the energy is coming from your thrust so it'd be a lossy process however if you're able to capture all of the energy then there won't be anything left to damage the ship.

The old Bussard ramjet concept was to capture these high velocity protons (with some kind of magnetic field), cause them to fuse, and use the fusion energy for propulsion. There are a few engineering difficulties with the idea but it makes for some good SF stories...

The spaceship in Poul Anderson's Tau Zero uses a Bussard ramjet.

Re: Relativistic Spaceship

#295

Post by mlyle »

TaylorAlexander wrote: post #269
MilStdJunkie wrote: post #26

Earlier quoted context omitted.

Every proton would have the energy of a baseball. It's bananas. Granted, space is really empty, but it's not that empty, somewhere around a hundred atoms per cubic meter. Your ship might look like a very, very long shooting star. Probably dialing the speed down a touch would be worth it for whatever your shielding material is, but who knows? We're talking miracle engines here. I think in the "Valkyrie" ship-on-a-stri…

What do you mean “the energy of a baseball”? A baseball at what speed?

They're saying a fastball pitch-- 340 joules or so of energy.

The thing is, you need to be going implausibly fast to have a proton have a fastball level of energy. Even at .99999999 C, a hydrogen atom still has less than a microjoule of energy. You need a lot of 9's to get up to 340 joules.

  1/sqrt(1-(0.99999999)^2) * (1 atomic mass unit * (speed of light)^2
  1.05529895 × 10-6 joules

Re: Relativistic Spaceship

#297

Post by The_Colonel »

jcul wrote: post #86
robertlagrant wrote: post #43

Earlier quoted context omitted.

I like that old Arthur C Clarke novel, The Songs of Distant Earth[0], where they travel with an ablative shield in front made of ice, and they can make new shield segments by finding planets with water. [0] https://en.wikipedia.org/wiki/The_Songs_of_Distant_Earth

I read project hail Mary over Christmas, and they deal with this problem too. Though I can't say much without giving away spoilers. A great read if you are a sci-fi fan!

I think Hail Mary (and Astronaut) is more suited for people who don't usually like sci-fi.

Re: Relativistic Spaceship

#298

Post by eru »

BHSPitMonkey wrote: post #240
eru wrote: post #199

Earlier quoted context omitted.

Planets are tiny compared to the vastness of space. So you would never accidentally warp inside a planet.

Sure, if your probability distribution looks at every point in the universe equally. But we're talking about introducing error in a situation where you're _trying_ to drop yourself right next to a planet, so the areas nearest to your target have a much greater probability.

The volume of space up to only geostationary orbit is about 177 times bigger than the volume of the earth.

Given that Star Trek's impulse drives are already traveling at up to around 0.9c, parking somewhere between the earth and the moon (which is about 1 light-second out), the ratio of space to volume of earth becomes 219,648.

That ratio growth with the cube of the distance to the planet.

Re: Relativistic Spaceship

#299

Post by idiotsecant »

TaylorAlexander wrote: post #269
MilStdJunkie wrote: post #26

Earlier quoted context omitted.

Every proton would have the energy of a baseball. It's bananas. Granted, space is really empty, but it's not that empty, somewhere around a hundred atoms per cubic meter. Your ship might look like a very, very long shooting star. Probably dialing the speed down a touch would be worth it for whatever your shielding material is, but who knows? We're talking miracle engines here. I think in the "Valkyrie" ship-on-a-stri…

What do you mean “the energy of a baseball”? A baseball at what speed?

I feel like regardless of the exact speed of the baseball, that's somewhat more energy than a typical proton.

Re: Relativistic Spaceship

#300

Post by eru »

StevePerkins wrote: post #258
cameron_b wrote: post #225

Earlier quoted context omitted.

If we’re talking about the known universe, the odds of your near-light navigation accidentally clipping a not-mapped-yet planet are certainly not zero. Our gaze into the heavens is much better at spotting stars than their dark orbiting bodies, and we have fingers left over from one hand counting the number of observation platforms observing deep space from above our shimmering atmosphere.

Indeed. We can't even agree on whether or not there's an extra Neptune-sized gas giant out beyond the orbit of Pluto, and that's right here in our OWN solar system!

So? Even ten extra Jupiters or thousand extra suns would take up only a tiny amount of space compared to the size of the solar system out to Pluto:

The distance from the sun to Pluto is about 5.9 billion km. The radius of the sun is about 696,340 km. The ratio of radii is about 8,473. Cube that to get the ratio of volumes, and you get 608,263,848,559 for the ratio of volume in a sphere out to Pluto vs volume of the sun.

(Doing the numbers, I'm actually surprised: I had expected the ratio of radii to be bigger than 8,473. But I'm not surprised that the sun barely takes up any space.)

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