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Relativistic Spaceship

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Re: Relativistic Spaceship

#301

Post by eru »

BHSPitMonkey wrote: post #243
eru wrote: post #211

Earlier quoted context omitted.

You are right. One addendum: many humans would be bothered by one-way trips, but humanity is large, and in absolute numbers there are still plenty of volunteers for one-way trips to the planets and stars.

And how many in that group are actually fit to spend many years living that lifestyle without losing their mind and getting into deadly fights with one another? And who's covering the great expense of building these generational colony ships and training their only inhabitants, only to have them zip away never to be heard from again? (With no benefit other than believing there's a slight chance we've succeeded in mak…

As humanity becomes more numerous and richer, the fraction of humanity you need that have such strange ideas and desire keeps shrinking.

(Btw, I share your intuition that the number of people willing to sign up for a one-way trip to the stars, or even just Mars, is fairly low. However, if you already look at people who are dedicated enough to become astronauts, I suspect the additional filter of asking for one-way-trip volunteers for a mission to the stars isn't all that severe. My purely speculative guess is that at least 10-20% of current astronauts would be willing to sign up.)

Re: Relativistic Spaceship

#302

Post by sveolon »

amelius wrote: post #184
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.

This is a bit like driving into a brick wall, and then asking if you could use the released energy to go faster ...

Imagine you drive into a wall of tnt, break through it, and as you exit it explodes and gives you an extra boost. Yes, you’ve lost some speed at the beginning but you’ve gained much more

Re: Relativistic Spaceship

#303

Post by insane_dreamer »

The_Colonel wrote: post #297
jcul wrote: post #86

Earlier quoted context omitted.

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.

I like sci-fi, and enjoyed Hail Mary very much

Re: Relativistic Spaceship

#304

Post by penteract »

WitCanStain wrote: post #292
sega_sai wrote: post #280

Earlier quoted context omitted.

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?

https://www.wolframalpha.com/input?i2d=true&i=Divide%5B%5C%2...

Re: Relativistic Spaceship

#305

Post by jemfinch »

wincy wrote: post #291
sega_sai wrote: post #280

Earlier quoted context omitted.

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.

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, about 100x smaller than the radius of the known universe.

Re: Relativistic Spaceship

#306

Post by phire »

galangalalgol wrote: post #289
phire wrote: post #286

Earlier quoted context omitted.

Things get more insane the further you go. After an observed 12 years of constant 1g acceleration, you will be 113,000 lightyears from your point of departure, which is enough distance to cross from earth to the opposite side of our galaxy. Of course, you are now traveling at 99.999999996% of the speed of light. If you actually wanted to stop at the other side of the galaxy to take a look around, you would need to tu…

Is there any relativistic bonus that makes deceleration easier? Does it take less energy to go from 0.999C to 0.99C than to go from 0.009C to 0C?

Acceleration and Deceleration are symmetrical. It takes the same amount of delta-v to accelerate between up to 0.999c as it takes to decelerate all the way back down to 0. Though as you burn fuel your ship gets lighter and the actual energy usage to maintain that constant delta-v of 1G will go down.

As for the energy used to accelerate from 0C to 0.009C compared to 0.99C to 0.999C, I'm not sure. I know the time taken (from an external reference frame) changes, but part of me suspects the total energy stays the same and the difference in time taken is caused entirely by time dilation. However, I suspect I might be messing up reference frames, I don't actually know the equations.

Re: Relativistic Spaceship

#307

Post by micw »

wongarsu wrote: post #180
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.

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?

Re: Relativistic Spaceship

#308

Post by Tostino »

daedalus_j wrote: post #177
jcadam wrote: post #125

Earlier quoted context omitted.

Could use such a ship as a time machine (well, that only goes one way).

The Forever War novels have this as a key plot point, if you're interested in that sort of thing.

The whole Ender's game series also uses this as a plot device.

Re: Relativistic Spaceship

#310

Post by zwaps »

fyver wrote: post #293
wiml wrote: post #190

Earlier quoted context omitted.

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.

Also all Stat Trek Federation ships
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