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Relativistic Spaceship

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Re: Relativistic Spaceship

#131

Post by schiffern »

whimsicalism wrote: post #120
MilStdJunkie wrote: post #26

Earlier quoted context omitted.

Every proton would have the energy of a baseball. It's bananas. Granted, space is really empty, but it's not that empty, somewhere around a hundred atoms per cubic meter. Your ship might look like a very, very long shooting star. Probably dialing the speed down a touch would be worth it for whatever your shielding material is, but who knows? We're talking miracle engines here. I think in the "Valkyrie" ship-on-a-stri…

why are protons more likely to be closer to rest relative to earth than relative to the fast spaceship?

"Although the density of atoms in the interstellar medium is usually far below that in the best laboratory vacuums, the mean free path between collisions is short compared to typical interstellar lengths, so on these scales the ISM behaves as a gas (more precisely, as a plasma: it is everywhere at least slightly ionized), responding to pressure forces, and not as a collection of non-interacting particles."

https://en.wikipedia.org/wiki/Interstellar_medium

Re: Relativistic Spaceship

#132

Post by zabzonk »

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?

not so much protons (which would be a problem, but could at least notionally be deflected), but atoms.

Re: Relativistic Spaceship

#133

Post by jvanderbot »

LoganDark wrote: post #52
jvanderbot wrote: post #48

Earlier quoted context omitted.

Well that depends on the ejection velocity. If you could shoot it out at close to speed of light, you'd need much less.

If you were to shoot it out at close to the speed of light, you'd knock the Earth out of its orbit.

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.

Re: Relativistic Spaceship

#134

Post by LoganDark »

jvanderbot wrote: post #133
LoganDark wrote: post #52

Earlier quoted context omitted.

If you were to shoot it out at close to the speed of light, you'd knock the Earth out of its orbit.

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.

Re: Relativistic Spaceship

#135

Post by eh_why_not »

randallsquared wrote: post #110
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?

I'm reading up again to refresh my relativity knowledge. Though:

> even though near the speed of light, more and more of that energy goes into higher mass and slower time?

This to me does not sound very different from saying the (relativistic) mass to be accelerated is larger, and is what's stopping a 1G (or any constant) acceleration to be maintained to bring the ship to the speed of light.

The concept may have been an overly simplified pedagogical tool though that I need to upgrade from. (I'll keep the comment chain intact since it's educational to me).

Re: Relativistic Spaceship

#136

Post by actionfromafar »

whimsicalism wrote: post #120
MilStdJunkie wrote: post #26

Earlier quoted context omitted.

Every proton would have the energy of a baseball. It's bananas. Granted, space is really empty, but it's not that empty, somewhere around a hundred atoms per cubic meter. Your ship might look like a very, very long shooting star. Probably dialing the speed down a touch would be worth it for whatever your shielding material is, but who knows? We're talking miracle engines here. I think in the "Valkyrie" ship-on-a-stri…

why are protons more likely to be closer to rest relative to earth than relative to the fast spaceship?

Because the entire Milky Way is mostly at rest relative to itself.

If you are charging full speed ahead into its center, you are going against just about everything, including energetic particles no doubt coming the from the crowded center of the galaxy.

Re: Relativistic Spaceship

#137

Post by actionfromafar »

perardi wrote: post #104
throwaway11460 wrote: post #96

Earlier quoted context omitted.

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.

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. ;-)

Re: Relativistic Spaceship

#138

Post by tshaddox »

bee_rider wrote: post #113
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.

I’m not sure I’d call that a punchline of relativity; the idea of frame of reference is also part of the classical model.

Relativity (the fact that the laws of motion are the same in all inertial frames of reference) is itself a crucial part of Newtonian mechanics, except that Einstein's theories of relativity were such a big deal that we now tend to reserve the word "relativity" for his stuff, and call the old thing "Galilean invariance" or similar.

Re: Relativistic Spaceship

#139

Post by Koshkin »

darreninthenet wrote: post #94
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.

Curiously, general-relativistic effects needed to be accounted for as well.

Quoting Wikipedia, GPS “must account for the gravitational redshift in its timing system, and physicists have analyzed timing data from the GPS to confirm other tests. When the first satellite was launched, some engineers resisted the prediction that a noticeable gravitational time dilation would occur, so the first satellite was launched without the clock adjustment that was later built into subsequent satellites. It showed the predicted shift of 38 microseconds per day. This rate of discrepancy is sufficient to substantially impair function of GPS within hours if not accounted for.”

Re: Relativistic Spaceship

#140

Post by actionfromafar »

pantalaimon wrote: post #124
ryandrake wrote: post #27

Earlier quoted context omitted.

Sure, there are plenty of practical problems that would make the trip impossible. The fuel mass alone that would have to be shot out the back to make that trip would be something on the order of 800 million times the mass of the payload (you). So a 100kg person sitting in a 100kg spaceship would require something like 160 billion kg of fuel, assuming zero energy loss in burning the fuel. Relativistic rocket calculato…

Imagine a generation ship, consuming the energy of an entire star for the trip.

We have one of those (stars)!

Just need to build a parabolic reflector on one side of it and point it towards the opposite direction where we want to go. When the reflector shoots to far from the same, tilt the reflectors, drop down closer to the Sun by gravity, tilt them back, do it again.

We could be going places in a few billion years!

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