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Relativistic Spaceship

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Re: Relativistic Spaceship

#231

Post by MilStdJunkie »

eru wrote: post #223
MilStdJunkie wrote: post #176

Earlier quoted context omitted.

Send a mirror ahead, bounce the beam back toward the ship to push it backwards. Loses a lot of efficiency, though. Laser propulsion, a lot of the thrust is coming from ablation of the surface.

How do you 'send a mirror ahead'? Do you want it to stop, or just keep flying away indefinitely?

Yep, throwaway mirror, it goes off to wherever once it's done bouncing light back at your ship. Again, much less efficient, so when you get where you want to go, you'd want to put in a laser station there too.

Re: Relativistic Spaceship

#232

Post by JumpCrisscross »

antihipocrat wrote: post #202
JumpCrisscross wrote: post #183

Earlier quoted context omitted.

> space is known to be full of atmospheres to use to slow down The places people talk about travelling to tend to have atmospheres and gravity, yes.

How many G's of deceleration are we talking here? I imagine even 2Gs of force wouldn't be tolerable for humans for more than an hour

> even 2Gs of force wouldn't be tolerable for humans for more than an hour

You can tolerate 2G for hours, particularly if everyone's oriented eyeballs in. That said, both aerobraking and gravity assists are intermittent accleerations.

Re: Relativistic Spaceship

#233

Post by eru »

cameron_b wrote: post #225
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.

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.

It doesn't matter whether we can see those dark orbiting bodies: we observe minuscule gravitational impact on the stars, and that places a very sharp upper limit on the amount of mass that's outside of the star in a solar system.

The sun contains roughly 99.8% of the mass of the solar system, and is by far the largest object in it. But you wouldn't hit the sun randomly either. Space is just so damn large.

Re: Relativistic Spaceship

#234

Post by steffenfrost »

AlexAndScripts wrote: post #15
ryandrake wrote: post #12

Great visualization of the unintuitive nature of special relativity. It's kind of mind blowing to realize that if we could accelerate our spaceship at a mere 1G continuously, we could visit the center of the Milky Way in under 20 years (spaceship time), totally do-able in a human lifespan. Of course 27900 years would have passed on Earth, so you wouldn't be able to tell anyone about your vacation.

I suppose you would also have to worry about surviving the trip - every atom in the interstellar medium would damage your ship, likely penetrating the entire way through, and electromagnetic repulsion would only work with charged particles.

Couldn't you shoot an electron bream ahead of the ship to ionize the atoms and the deflect them with a magnetic field?

Re: Relativistic Spaceship

#235

Post by tempestn »

huytersd wrote: post #214
ryandrake wrote: post #12

Great visualization of the unintuitive nature of special relativity. It's kind of mind blowing to realize that if we could accelerate our spaceship at a mere 1G continuously, we could visit the center of the Milky Way in under 20 years (spaceship time), totally do-able in a human lifespan. Of course 27900 years would have passed on Earth, so you wouldn't be able to tell anyone about your vacation.

Time travel seems so easy and we could do it with the tools we have now for the most part. Maybe we’ll never go into the past but relatively contemporary humans are going to be all over the future in all kinds of places. After the first time it happens people in the future can start to expect a human from the past to keep visiting every so often.

If that did start happening, it's not like the people from the past would just pop up suddenly. We would be aware of their journey. It's not like a time machine where someone just disappears from the past and appears in the future.

Re: Relativistic Spaceship

#236

Post by bhickey »

steffenfrost wrote: post #234
AlexAndScripts wrote: post #15

Earlier quoted context omitted.

I suppose you would also have to worry about surviving the trip - every atom in the interstellar medium would damage your ship, likely penetrating the entire way through, and electromagnetic repulsion would only work with charged particles.

Couldn't you shoot an electron bream ahead of the ship to ionize the atoms and the deflect them with a magnetic field?

Briefly: No.

I read a paper on the topic a few years back. My recollection is that once you go faster than about 0.3c it becomes impossible to shed heat faster than you gain it from collisions with Helium. I'll try to dig it up.

Re: Relativistic Spaceship

#237

Post by mattclarkdotnet »

ben_w wrote: post #56
JumpCrisscross wrote: post #21

Earlier quoted context omitted.

> Could any somewhat-plausible technologies (fusion etc) get us to that point? Nuclear thermal or nuclear pulse propulsion. No fusion needed.

For practical values, nuclear pulse propulsion would only get you up to a few percent of the speed of light. The best nuclear thermal, gas core, would give you 7000 seconds[0]. A "dusty plasma" rocket (suspend the nuclear fuel in dust form in a magnetic field) might get 100,000 seconds, or just over a day of accelerating at 1g. [0] for the benefit of non-space nerds: the unit of measure is "lb-force seconds per lb-ma…

g-seconds then? That seems like a very handy unit

Re: Relativistic Spaceship

#238

Post by tempestn »

stavros wrote: post #187
naikrovek wrote: post #158

Earlier quoted context omitted.

So depending on how you measure, you’re always stationary or moving near light speed, or somewhere in between, depending on your measurement reference (the thing you’re moving relative to)? How is there a speed limit at all, if that’s the case? 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 o…

> 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.

Re: Relativistic Spaceship

#239

Post by BoiledCabbage »

sophacles wrote: post #115
ryandrake wrote: post #41

Earlier quoted context omitted.

> As an SF writer, it's pretty interesting, though, to think of the weird shapes a society would take when your astronauts are outliving entire civilizations. I guess this all depends on how you define simultaneity. Say I got on a relativistic rocket traveling near c and colonized Planet X in 10 years ship time and say 20,000 years Earth time. I put up my TV antenna as soon as I land, and I'd still be receiving Earth…

That's not how it works. If the planet is N light years away from earth, the transmissions you receive would be those sent 20000-n years ago. along the way you'd get most of the messages sent during the 20k years as very red-shifted light, and extremely rapidly from your pov.

I think you may have that wrong (If I'm understanding you correctly).

If you traveled at the speed of light then you'd be at the planet in 0 time your time, and you would arrive with the first of the broadcasts over those 20k years. So once on the planet you'd get to watch all 20k years of broadcasts.

If you traveled at 1/2 the speed of light (not focusing on your dilation for the moment), then you'd still beat 50% of the transmissions and have 10k earth years of broadcasts to watch.

I think the question is what % of the speed of light is a gamma factor of 20k/10 = 2000. That's something like 99.9999999% of the speed of light.

Meaning you would get there before 99.99999% of the broadcasts had arrived and you'd be able to watch just about all of them in real time over the next (just less than) 20k years.

Assuming those two planets are roughly in the same reference frame, the only way 20k years could pass on earth w.r.t. your arrival is if the destination planet is 20k light years away.

If the destination planet is moving relativistically away from earth at an appreciable percentage of the speed of light then I couldn't say what the math would be. Maybe still the same, maybe not.

Re: Relativistic Spaceship

#240

Post by BHSPitMonkey »

eru wrote: post #199
actionfromafar wrote: post #137

Earlier quoted context omitted.

If nothing else, if you miss just a little dropping out of warp inside a planet, must be a Very Bad Thing in-universe. The ramifications would be seen even out-of-universe. ;-)

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.
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