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Relativistic Spaceship

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Re: Relativistic Spaceship

#91

Post by qayxc »

eek2121 wrote: post #84
ianburrell wrote: post #25

Earlier quoted context omitted.

It isn't possible without a magic reactionless drive; it might even require a zero-energy drive since the kinetic energy is huge and needs to come from somewhere. The rocket equation kills you. A reasonable fusion rocket would require an enormous (like an entire planet worth) to continuously accelerate to relativistic speeds. Antimatter drives are the only option and still require large amounts of reaction mass since…

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...

Re: Relativistic Spaceship

#93

Post by caturopath »

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.

Why not just increase power to the main deflector?

Re: Relativistic Spaceship

#94

Post by darreninthenet »

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…

Even the GPS satellites have to take into special relativity - they have to correct for a tiny amount of time per day due to time dilation causing the moving clocks on the satellites to tick slightly slower than the stationary ones on Earth.

Re: Relativistic Spaceship

#95

Post by acchow »

ryandrake wrote: post #78
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…

> 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.

Re: Relativistic Spaceship

#96

Post by throwaway11460 »

perardi wrote: post #28
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.

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 :)

Re: Relativistic Spaceship

#97

Post by qayxc »

floxy wrote: post #38
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…

"Roundtrip Interstellar Travel Using Laser Pushed Lightsails" https://ia600704.us.archive.org/view_archive.php?archive=/24... "A one-way interstellar flyby probe mission uses a 1000 kg (1-metric-ton), 3.6-km-diam. lightsail accelerated at 0.36 m/s2 by a 65-GW laser system to 11% of the speed of light (0.11 c), flying by a Centauri after 40 years of travel. " "...The third mission uses a three-stage sail for a roundtr…

Holy cow! I just read the energy requirements of that third mission. Total electricity generation capacity of the entire world is about 12TW today [1], whereas that 3rd mission would require a "formidable" - as the author admits - 45,000TW at least. A mere three orders of magnitude more :)

[1] https://www.statista.com/statistics/267358/world-installed-p...

Re: Relativistic Spaceship

#99

Post by nabla9 »

ianburrell wrote: post #25
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…

It isn't possible without a magic reactionless drive; it might even require a zero-energy drive since the kinetic energy is huge and needs to come from somewhere. The rocket equation kills you. A reasonable fusion rocket would require an enormous (like an entire planet worth) to continuously accelerate to relativistic speeds. Antimatter drives are the only option and still require large amounts of reaction mass since…

You need to get around the rocket equation.

> we estimate the neutral hydrogen density in the unperturbed local interstellar medium of 0.195 ± 0.033 cm−3 https://iopscience.iop.org/article/10.3847/1538-4357/abb80a

You need a hydrogen 'scoop' and fusion. If you can accelerate to 0.01c, you can collect enough hydrogen for 1000 MW fusion power from 32 km2 (6 km diameter) scoop area. Faster you fly, more you get. You can accelerate until interstellar particles start to do real damage.

Re: Relativistic Spaceship

#100

Post by richardw »

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.

Not a physicist, but my impression is that you're always going 0 percent of the speed of light (in all directions) from your own frame of reference. All you notice is that our solar system is moving away, faster.

I guess you'd notice a change in light frequency based on the light in front/behind. Redder behind, bluer in front.

Edit: supposedly we can measure our speed compared to the cosmic microwave background which is fairly uniform, which gives us a value of 400 to 800km/s relative to the CMB: https://www.researchgate.net/post/How_can_we_practically_mea...

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