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Relativistic Spaceship

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Re: Relativistic Spaceship

#321

Post by tomtomistaken »

MilStdJunkie wrote: post #26
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.

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…

So such ships would look "aerodynamic"?

Re: Relativistic Spaceship

#322

Post by defrost »

tomtomistaken wrote: post #321
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…

So such ships would look "aerodynamic"?

More like bridges and skyscrapers, perhaps with an ablating sheild at the lead end, maybe an electromagnetic field to divert particles away.

If the thrust is high enough, a tenth of a G to 1.5 maybe, the ship has to "stand up" on the thrusting engine in the same manner as a building must stand up over it's footprint, supporting itself against the force of (artifical) gravity.

If it's higher thrust (as the human meatsacks are suspended in a fluid they've also swallowed ??) then the ship has to look even more like a heavy load brutilist building.

Re: Relativistic Spaceship

#323

Post by fredthedeadhead »

mplewis9z wrote: post #155
neaden wrote: post #74

Earlier quoted context omitted.

Eh, there is a ton of unrealistic technology in book 1, the stealth ship for instance is impossible. By the authors own words they don't think of the books as being Hard Sci-Fi, harder then Star Trek sure.

What makes you think the stealth ship is impossible? They explain the concept pretty well in the first book: extensive cooling systems to soak up waste heat, large liquid tanks to store that heat internally for a while, and radar-absorbing coatings. We literally have all of those pieces today, it’d just cost unfathomable amounts of money to build with our current technology. As a bonus, they even talk about the radar…

Project Rho gives several compelling reasons why there ain't no stealth in space

https://www.projectrho.com/public_html/rocket/spacewardetect...

Re: Relativistic Spaceship

#324

Post by tsimionescu »

eru wrote: post #198
dfee wrote: post #147

Earlier quoted context omitted.

I’d like to see a map of the known universe visualizing atomic density on a log scale. I’d never seen “a hundred atoms per cubic meter”, but it’s always been my intuition that, without some quite interesting shielding, you couldn’t make it anywhere near the speed of light. And on the other hand, I’ve seen claims that “space is really big” as you mentioned; but that claim has always seemed dubious.

> I’d never seen “a hundred atoms per cubic meter”, but it’s always been my intuition that, without some quite interesting shielding, you couldn’t make it anywhere near the speed of light. It's not that bad. With currently known science, your fuel would most likely be hydrogen so you can run a fusion reactor. The rocket equation tells you that most of your starship by mass would be fuel, if you want to go fast. Of al…

Wouldn't your water and food then become highly radioactive over time?

Even if no, it would mean your shield will be gradually consumed. Not sure this is the best idea.

Re: Relativistic Spaceship

#325

Post by lloeki »

jemfinch wrote: post #305
wincy wrote: post #291

Earlier quoted context omitted.

Another good one for me is that a cubic light year or butter would immediately collapse into a black hole with a Schwarzchild radius larger than the observable universe.

It's impossible for your fact and the fact you're replying to both be true. Water is denser than butter, and the nearest star to the sun is about 4.3ly away; if your fact were true, the universe would be a black hole. A cubic lightyear is about 8.468e+50 liters, and butter weighs 911 g/L, giving the mass of a cubic lightyear of butter to be 7.714348e+50, whose Schwarzchild radius is about 121,103,293 lightyears, abou…

> if your fact were true, the universe would be a black hole

... maybe it is? Hear my pet theory out.

Extrapolating backwards from the expansion of our universe, the Big Bang model posits a hyperdense state that exceeds black hole levels originating from a singularity, yet it's thought that somehow it did not collapse back, handwaving it as "physics as we know it did not apply".

But maybe physics as we know it does apply. Notably physics as we know it does not imply a specific direction for the arrow of time.

So our universe might very well be a black hole, but we have time backwards compared to the usual way we think of black holes: what we think of as the origin of time and space is what we think of as the irremediable end of time and space in a black hole.

Re: Relativistic Spaceship

#326

Post by tzs »

It's the aberration effects that were the most unintuitive for me. Specifically the difference when you are looking straight ahead in your direction of travel in how the stars appear to move when you are moving at constant speed and when you are accelerating.

At constant speed when a distant star first appears it moves directly away from the center point. When accelerating distant stars first move toward the center point then move away.

If you constantly increase the acceleration you can hold a star that is not at the center point stationary for a while, which is totally not something I would have expected.

I'd like to see something like this except instead of a seemingly random assortment of stars the stars are on a regular grid, connected by glowing filaments along the grid lines. That would make it easier to see what the heck is going on.

Re: Relativistic Spaceship

#327

Post by mplewis9z »

fredthedeadhead wrote: post #323
mplewis9z wrote: post #155

Earlier quoted context omitted.

What makes you think the stealth ship is impossible? They explain the concept pretty well in the first book: extensive cooling systems to soak up waste heat, large liquid tanks to store that heat internally for a while, and radar-absorbing coatings. We literally have all of those pieces today, it’d just cost unfathomable amounts of money to build with our current technology. As a bonus, they even talk about the radar…

Project Rho gives several compelling reasons why there ain't no stealth in space https://www.projectrho.com/public_html/rocket/spacewardetect...

I don't know when that was written, but it definitely feels like The Expanse authors read that exact page and built their stealth system to address those points - the stealth ships in the books aren't permanently stealthy (they have to radiate their heat from the internal heatsinks after some amount of time, seemingly on the order of months), and are immediately detected when they light up their engines (and hence coast until they reach their destination).

The main characters also manage to find a stealth ship after coming across coordinates to it - it was parked and completely offline (and unmanned) in the orbit of a small asteroid, which would further obscure any signature it might give off.

Re: Relativistic Spaceship

#328

Post by hwillis »

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.

no, the earth is actually big. 47 trillion times heavier than that mass. At 90% c that 160 billion kg would still be a small fraction of the planets (moving 10,000x slower, at 30 km/s) kinetic energy. 500,000x less.

Of course that .06 m/s velocity change would be near instant, so bad things would happen. Probably a humanity-ending but not life-ending disaster. Global tsunamis and incredibly large tectonic changes, for sure. Imagine the entire water column of the marianas trench jumping up in the air and slamming into the ground below.

If the energy was transferred at a single point, it'll be the worst extinction event ever (4000x worse than chicxulub) but I'd bet single-celled and maybe even some multicellular life would survive. Anything bigger than a mouse is fucked, though.

Re: Relativistic Spaceship

#329

Post by tsimionescu »

api wrote: post #123
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...

> 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. There's a general principle known as Jon's Law (not sure where the name comes from) that any powerful space drive is by definition a weapon of mass destruction. You see this in The Expanse when incredibly powerful fusion torchship rockets (a major part of the Exp…

I think that law is basically Newton's second law of motion. F=ma essentially tells you that anything that decelerates a lot, such as a very fast spaceship crashing into the rock of your planet, pushes with an awful lot of force before it stops.

Edit: to be slightly more pedantic, the right form that still remains true with relativity is F = dp/dt, i.e. the force the spaceship would exert is equal to its change in momentum.

Re: Relativistic Spaceship

#330

Post by tsimionescu »

eru wrote: post #201
hotpotamus wrote: post #144

Earlier quoted context omitted.

From memory, photons are massless, otherwise they could not move at light speed. They do have momentum though.

Photons have mass, but no rest mass. (Or something like that.)

Typically momentum is thought as mass times velocity, and since photons do have momentum, there was a desire to give them some kind of "relativistic mass".

In more recent times, it has been seen as easier to use just one concept of mass, and to redefine momentum entirely. So, photons have a mass of 0, and we don't need to specify "rest mass". But they do have momentum.

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