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Relativistic Spaceship

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Re: Relativistic Spaceship

#221

Post by eru »

whimsicalism wrote: post #118
bee_rider wrote: post #49

Earlier quoted context omitted.

You can’t “get to” light speed, that’s one of the big punchlines in relativity. If you pick an acceleration equal to the acceleration we experience on Earth (aka 1g, aka ~9.8m/s^2), you hit relativistic speeds (speeds at which you need to take into account the effects of relativity to do anything) surprisingly fast. On the order of hundreds of days. So, it is not really a matter of safe acceleration on a long space t…

dumb question - why does going faster make it more likely you encounter fast protons? couldn’t protons in any reference frame be going quite fast relative to you?

> couldn’t protons in any reference frame be going quite fast relative to you?

By the laws of physics, yes. But most of the stuff in our universe, and definitely in our galaxy, tends to move roughly at the same speed.

See https://en.wikipedia.org/wiki/Comoving_and_proper_distances#...

Re: Relativistic Spaceship

#222

Post by gosub100 »

naikrovek wrote: post #158
carpdiem wrote: post #105

Earlier quoted context omitted.

Doesn't really work this way. A lot of the wonkiness in SR is tied to the fact that the speed of light is the same, measured in _any_ reference frame. So, say you're on earth and you measure the speed of light... you find that it's c (~3x10^8 m/s). Now you get on a spaceship and accelerate to 0.5c with respect to earth, and you measure the speed of light relative to your spaceship... still c! In this way, you can't r…

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…

> so c relative to what?

it's either "relative to any observer." or "relative to any inertial reference frame". no matter where you go (on the ship, on a planet you pass by, on another ship) you will never see the apple travel as fast as the photons coming out of your flashlight. Depending on where the observer is, they will see the apple accelerate to 0.5c (if they are aboard the ship) or they will see it gain mass (or rather, see you throw it more slowly as if it had gained mass), contract in the direction it's thrown, and slow down (due to time dilation...relative to the moving frame).

The case I don't know how to answer is two apples thrown at each other, each with a speed greater than 0.5c.

Re: Relativistic Spaceship

#223

Post by eru »

MilStdJunkie wrote: post #176
jandrese wrote: post #79

Earlier quoted context omitted.

There is also the question of how you get the lasers to your destination in the first place.

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?

Re: Relativistic Spaceship

#224

Post by jvanderbot »

LoganDark wrote: post #134
jvanderbot wrote: post #133

Earlier quoted context omitted.

That's silly. My flashlight shoots stuff at speed of light all the time. Mass matters. Shooting just enough mass at very high velocity is not much different than shooting a lot more mass at lower velocity, in terms of force.

> Mass matters. Exactly. You try accelerating 200kg up to anywhere close to the speed of light (say 80%). That is a lot of force. Technically even photons can exert force on objects, but they have such a small mass that it's a difficult effect to observe.

This is getting overly pedantic.

Opening comment said that you'd need absurd amounts of mass to accelerate a person to near light speed.

I said that the velocity of ejecting that mass mattered. That if you could push out mass at speed of light, you'd need a lot less mass.

Not that you'd push out the same mass at speed of light. Or that you could arbitrarily push things out at speed of light. Sheesh.

Re: Relativistic Spaceship

#225

Post by cameron_b »

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.

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.

Re: Relativistic Spaceship

#226

Post by agent281 »

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…

That's basically the plot of the Forever War. When you drop out of hyper drive (or whatever they called it) you never know if you are going to be battling a ship from your future or your past. The very society that you fight for is completely alien. It's incredibly alienating.

Re: Relativistic Spaceship

#227

Post by e40 »

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!

My second favorite sci-fi book of all time. Three body being the first.

Re: Relativistic Spaceship

#228

Post by JumpCrisscross »

PaulDavisThe1st wrote: post #209
JumpCrisscross wrote: post #182

Earlier quoted context omitted.

> doable for "normal" speeds, not one where you accelerate 1G for twenty years It's still a lot of energy you can bleed off, particularly if you're aiming for a system with gas giants. I'm not suggesting one only rely on passive deceleration. But especially given it's fuel saved at the very end of the journey, fuel you no longer need to accelerate and decelerate for the entire duration of the trip, the savings could…

the top of this thread is filled with a discussion of the almost unimaginably catastrophic consequences of a ship moving at 0.9C hitting atoms in interstellar space. trying to decelerate by "braking" anywhere close to a gravitionally significant mass sounds like a guarantee to total destruction from the impact of "stuff" (even individual photons).

> a ship moving at 0.9C

You decelerate from 0.9C to 50 km/s conventionally, more if you can aerobrake or line up multiple slingshots, and that last 0.00001% with gravity assist.

It saves you more than that in fuel, because the fuel you'd have used on that last bit of deceleration needed to be accelerated and decelerated the entire way from 0 to 0.9C back to close to zero.

Re: Relativistic Spaceship

#230

Post by caust1c »

knodi123 wrote: post #195
moron4hire wrote: post #112

Earlier quoted context omitted.

No, the exhaust port definitely was an unimaginable hell cannon. The characters spoke several times about slagging a ship or a surface base in their drive plume.

Oh, I missed that! Seems like those drives ought to have been more carefully regulated then.

Cool detail about that in the Nauvoo launch sequence, slagging the scaffolds / gantries / whatever the construction exoskeleton is.

https://youtu.be/0KZRFIdbJzc?t=147

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