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Relativistic Spaceship

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Re: Relativistic Spaceship

#101

Post by throwaway11460 »

qayxc wrote: post #91
eek2121 wrote: post #84

Earlier quoted context omitted.

Well the obvious solution would be to figure out a mechanism to capture the particles that bombard your ship and figure out a way to turn those particles into thrust so you don’t need to take a bunch of fuel with you, which I get it, is another hard problem to solve, but maybe not impossible. We have done some incredible things before.

The good old Bussard Ramjet. Problem is, recent calculations showed that it's not viable - even for a Kardashev Type II civilisation [1] [1] https://www.sciencedirect.com/science/article/pii/S009457652...

It's not viable to use it to visit the galactic center during a lifetime, but it still flies, if I understand that correctly?

Re: Relativistic Spaceship

#103

Post by cypherpunks01 »

ryandrake wrote: post #69
cypherpunks01 wrote: post #62

Earlier quoted context omitted.

While traveling near c, I believe you'd be receiving Earth transmissions from approximately the same time you left, Earth clocks would appear to slow down to a crawl. But if you land on another planet and change reference frame to be more "stationary" again relative to Earth, I think a massive amount of Earth time would suddenly appear to have elapsed very quickly during the process of slowing/landing because of the…

Pretty sure only those 10 ship years worth of transmissions would suddenly appear. You're still 20K light years away so the rest of those years' transmissions are still in flight.

Hmm I suppose you're right. I guess you'd see very few transmissions during the journey due to time dilation, and then those 10 years would arrive relatively quickly when slowing/stopping?

I was thinking of this being the first half of the twin paradox, but perhaps for the apparent "time gap" in the spacetime diagram to appear, it's necessary for the traveling twin to turn and head back towards Earth quickly to shift simultaneity plane back in the other direction.

Re: Relativistic Spaceship

#104

Post by perardi »

throwaway11460 wrote: post #96
perardi wrote: post #28

Earlier quoted context omitted.

Unless you could somehow make an Alcubierre warp drive. Of course…even if that was possible, it’s conjectured that the colonists already at your destination won’t appreciate you boiling the atmosphere when you hit them with blue shifted radiation. https://www.universetoday.com/93882/warp-drives-may-come-wit...

Ah, so that's why they never warp near planets in Star Trek :)

[pushes up glasses]

They implied they avoided that because warping into a gravity well would cause some vague catastrophe.

Of course, the real reason was far more sinister: it’s way more dramatic to slowly creep up on the planet while listening to the captain’s log monologue to start the episode.

Re: Relativistic Spaceship

#105

Post by carpdiem »

acchow wrote: post #95
ryandrake wrote: post #78

Earlier quoted context omitted.

> Instead you have to worry about the actual speed you are traveling at That's the other big punchline in relativity: There's no one "actual speed" because that implies that there is a single "important" frame of reference wherein those atoms are floating around waiting to be hit by a spaceship.

What if you define the "important" reference frame as the speed of light's (in the same direction you are going). Since c is universally constant, it seems like a reasonable privileged reference.

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 really define a reference frame with a speed "the same as the speed of light". And if you try, you'll run into nasty infinities in all your equations that will cause them to blow up and stop being useful.

Re: Relativistic Spaceship

#106

Post by timschmidt »

notfed wrote: post #92
MagicMoonlight wrote: post #44

Earlier quoted context omitted.

You just turn around at the half way point

At which point shielding gets even more complicated.

Less complicated. The rocket exhaust literally forms a plasma shield in front of the decelerating rocket.

Re: Relativistic Spaceship

#107

Post by eh_why_not »

Beautiful!

For the comments saying we "just" need to maintain a 1G acceleration, should point out that as the ship approaches the speed of light, its mass increases [0].

And as the mass increases, so does the thrust required to maintain that acceleration. So that engine better be able to tap into some magical energy source, otherwise it can't maintain the acceleration at the higher speeds. :)

[0] https://en.wikipedia.org/wiki/Mass_in_special_relativity#Rel...

Edit: this interpretation may not be entirely accurate (comments below).

Re: Relativistic Spaceship

#109

Post by LegionMammal978 »

sgt wrote: post #67
ortusdux wrote: post #7

Earlier quoted context omitted.

Acceleration vs speed. Adjust the acceleration until the "The ship is flying at X% the speed of light" is almost zero.

I did... after approaching near 100% speed of light, I decelerated, but it refuses to budge below around 45%. I accelerated both axes.

The acceleration/deceleration always points forward. If you've turned in other directions, then you can end up with some lateral velocity. You can cancel it out by decelerating until your speed hits a minimum, turning roughly 90 degrees, and decelerating along that axis as well. (If you still have residual speed, just repeat with more roughly-perpendicular axes.)

Re: Relativistic Spaceship

#110

Post by randallsquared »

eh_why_not wrote: post #107

Beautiful! For the comments saying we "just" need to maintain a 1G acceleration, should point out that as the ship approaches the speed of light, its mass increases [0]. And as the mass increases, so does the thrust required to maintain that acceleration. So that engine better be able to tap into some magical energy source, otherwise it can't maintain the acceleration at the higher speeds. :) [0] https://en.wikipedia…

Is the ship gaining mass (and shortening, and having its time slowed) in its own frame of reference? No, right? So wouldn't it be that the 1G acceleration from the viewpoint of the ship remains from the internal perspective, even though near the speed of light, more and more of that energy goes into higher mass and slower time?
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