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Relativistic Spaceship

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Re: Relativistic Spaceship

#391

Post by taneq »

MilStdJunkie wrote: post #165
knodi123 wrote: post #51

Earlier quoted context omitted.

In the book series The Expanse, that was pretty much the only unrealistic technology that the humans had, which allowed them to colonize the solar system. Sure, there was some might-as-well-be-magic alien stuff later in the book, but by page 1, humans have colonized the solar system, purely with a hand-wavy engine that can generate sustained thrust without needing tons of reaction mass and without turning your exhaus…

I'm going to embarrass myself horribly here, but I've only seen the series, so I'm not a hundred percent clear on how much acceleration they're pulling in cruise[1]. If it's something like .3 g then they're fairly close[2] to the limits of something like a fusion pulse drive (tiny nodules of deuterium or whatever, ejected out the back of the ship, then lasered into fusing, then the reaction pushes on the whatever - m…

Didn’t the guy who invented the drive essentially squish himself flat (too many G’s to lift his hand and reach the ‘off’ button, anyway) with his first prototype being efficient enough that it was still going during the story?

Re: Relativistic Spaceship

#392

Post by simonh »

lloeki wrote: post #325
jemfinch wrote: post #305

Earlier quoted context omitted.

It's impossible for your fact and the fact you're replying to both be true. Water is denser than butter, and the nearest star to the sun is about 4.3ly away; if your fact were true, the universe would be a black hole. A cubic lightyear is about 8.468e+50 liters, and butter weighs 911 g/L, giving the mass of a cubic lightyear of butter to be 7.714348e+50, whose Schwarzchild radius is about 121,103,293 lightyears, abou…

> if your fact were true, the universe would be a black hole ... maybe it is? Hear my pet theory out. Extrapolating backwards from the expansion of our universe, the Big Bang model posits a hyperdense state that exceeds black hole levels originating from a singularity, yet it's thought that somehow it did not collapse back, handwaving it as "physics as we know it did not apply". But maybe physics as we know it does a…

I like it! Not sure it works though. We observe the expansion of the universe accelerating, with gravity too weak to counter it. Reversing that would mean it's collapsing faster than the gravitational attraction of it's contents can account for. So either way, gravity isn't enough to explain what we observe.

Re: Relativistic Spaceship

#393

Post by gbin »

lp4vn wrote: post #352
huytersd wrote: post #262

Earlier quoted context omitted.

Sure, and we would probably be able to tell when they were from based on the approaching crafts. It’s going to be amazing to be able to step 50,000 years into the future to see how humanity has turned out. I’d give up anything to have a chance at that.

If you stop to think, a trip to the future is a trip to immortality because you can go to a time where the technology to live forever will be available.

Unfortunately humanity is constantly on the verge of self destruction so I would expect to come back to a radioactive smoldering place with some survivors roaming around still trying to finish one another with sticks and stones.

Re: Relativistic Spaceship

#394

Post by ben_w »

jemfinch wrote: post #305
wincy wrote: post #291

Earlier quoted context omitted.

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.

It's impossible for your fact and the fact you're replying to both be true. Water is denser than butter, and the nearest star to the sun is about 4.3ly away; if your fact were true, the universe would be a black hole. A cubic lightyear is about 8.468e+50 liters, and butter weighs 911 g/L, giving the mass of a cubic lightyear of butter to be 7.714348e+50, whose Schwarzchild radius is about 121,103,293 lightyears, abou…

> if your fact were true, the universe would be a black hole.

The mass of ordinary matter in the universe is 2×10^53 kilograms, which would have a Schwarzchild radius of 31.39 billion light years. The explanation from popular science communicators on this topic have never satisfied me.

Your maths is correct for one cubic light year of butter. Proxima Centauri is 4.247 light years away, and that gives such a cube of water[0] a mass of 6.468×10^52 kilograms[1], which would have a Schwarzchild radius of 10.15 billion light years.

[0] At STP, which isn't realistic at all

[1] Close enough; I think it was Brian Cox who once joked that in cosmology it is standard practice to approximate π as 1.

Re: Relativistic Spaceship

#395

Post by arethuza »

moosedev wrote: post #276
TaylorAlexander wrote: post #269

Earlier quoted context omitted.

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

I assumed they meant a baseball at baseball speed.

Rather than relativistic speeds, which would be bad:

https://what-if.xkcd.com/1/

Re: Relativistic Spaceship

#396

Post by mattmaroon »

khazhoux wrote: post #347
heads wrote: post #313

Earlier quoted context omitted.

If you’re doing 2e8 m/s, your ship is long and thin with a 1m3 nose cone, and space has 100 baseballs per m3 then you’re being hit by 2e10 baseballs a second. How do I get an idea for what 20 billion baseballs (2TJ) feels like? Apparently bullets are about an order of magnitude more energetic than baseballs (800J vs 80J) so I guess I could try to build my intuition based on being shot ~2 billion times a second instea…

> How do I get an idea for what 20 billion baseballs (2TJ) feels like? In the 90s, the show Special Relativity’s Funniest Home Videos would often have guys getting hit in the crotch 20B times with a baseball. Honestly, it never gets old.

They all started to look staged after the first billion though.

Re: Relativistic Spaceship

#397

Post by ben_w »

tsimionescu wrote: post #384
eru wrote: post #335

Earlier quoted context omitted.

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 t…

Your fuel still gets consumed, so you still can't rely on your fuel as the main form of shielding. Towards the end of your journey, your rocket is approximately 0% fuel. And at the point of highest speed, before you start decelerating, it is roughly 50% fuel. And you are entirely wrong about the isotopes of hydrogen. Tritium is highly radioactive, with a half life of ~12 years. And it is not just hard, but virtually…

> And you are entirely wrong about the isotopes of hydrogen. Tritium is highly radioactive, with a half life of ~12 years. And it is not just hard, but virtually impossible to isolate tritium out of water.

On the contrary, it's quite easy to do if there is any significant fraction of tritium present. The proportional mass difference of ³H vs. ¹H is x3, which alters the chemistry enough to make separation easy. You can use fractional distillation or electrolysis even for ²H — even mere hobbyists involved in the DIY fusion reactor scene sometimes extract deuterium from water this way, tritium would be easier.

Re: Relativistic Spaceship

#398

Post by ben_w »

micw wrote: post #307
wongarsu wrote: post #180

Earlier quoted context omitted.

100 atoms per cubic meter is on the lower end of typical densities in the interstellar medium. It can get many orders of magnitude denser than that. Outside galaxies you have better chances of surviving high speeds. The intergalactic medium is only 1-10 particles per cubic meter in the web of gas we call the warm–hot intergalactic medium, and possibly less outside of that.

So would probably better to get out of the galaxy disc, travel -fast- to the other end and dive back into rather that go straight through the galaxy?

Go up and over, that's only a few hundred light years. To the edge of the disk is 30,000.

Re: Relativistic Spaceship

#399

Post by gattr »

defrost wrote: post #322
tomtomistaken wrote: post #321

Earlier quoted context omitted.

So such ships would look "aerodynamic"?

More like bridges and skyscrapers, perhaps with an ablating sheild at the lead end, maybe an electromagnetic field to divert particles away. If the thrust is high enough, a tenth of a G to 1.5 maybe, the ship has to "stand up" on the thrusting engine in the same manner as a building must stand up over it's footprint, supporting itself against the force of (artifical) gravity. If it's higher thrust (as the human meats…

It's more stable (and structurally leaner, I think) to instead have the engines at the front in tractor configuration, as in the interstellar ships in Avatar movies.

Re: Relativistic Spaceship

#400

Post by ben_w »

wnkrshm wrote: post #378
xattt wrote: post #370

Earlier quoted context omitted.

X-ray sources would turn to gamma rays. Not that it’s any better that X-rays. Other comments suggested lead plates. It would quickly get irradiated and would probably need to get shed as soon as you got to a destination.

No accounting for particles yet, which you'll also keep hitting, making your ship's materials radioactive and causing lots of secondary particle showers, bremsstrahlung and the likes.

First the particles will act like radiation, then they'll start causing matter-antimatter pair creation with your hull, then you'll get some exotic heavy quarks popping into existence, then you'll get some Higgs particles forming and at some point questions like "what is the mass of my ship" stop making sense.
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