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Relativistic Spaceship

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Re: Relativistic Spaceship

#431

Post by dsign »

kypro wrote: post #402
TheOtherHobbes wrote: post #374

Earlier quoted context omitted.

It's clearly not a solvable problem with current ideas about technology. No amount of ice, rock, magnetism, or Magic Unobtanium is going to make this practical. I'm not even sure about warp drives. Bending spacetime one way means it has to squish back the other way. The energy released might not affect the ship - possibly - but I'd be surprised if it didn't affect the spacetime it had just passed through. Some kind o…

Am I wrong to think it seems probable that such things aren't realistically possible given the fact that the universe seems to be so lifeless? If the practical limits of rocket technology don't allow life to much beyond their own solar system then given the vastness of space that would be a good reason why we don't see any evidence of intergalactic civilisations or large feats of engineering. All other explanations f…

There is so much we don't know. But I would happily engage in speculation:

- Without needing to make it to the closest star, we have big problems here. If we solve those problems before we leave our solar system, we may be changed beyond recognition. We may not be biological any longer, for example, or at least not forcibly so, and traveling as solid matter may seem silly to our future descendants.

- We don't understand well enough the nature of reality. For all we know, our machines and organisms made of atoms and molecules may be, by far, more inefficient and wasteful than an equivalent process at some other layer or scale. Like somebody who discovers themselves living inside a match box in a forgotten attic, we may decide to move to the more spacious main floor of the castle.

- A variation of the above: maybe space-time itself is something we use inefficiently. It could be that a way to stop being troubled by the slow speed of light is by lowering our own "life" speed, increasing our volume to span entire solar systems, and decrease our density so much that your ancestors would confuse us with sparse interstellar matter. Or, at the opposite end, it could be that we find a way to move our entire future civilization to a cubic centimeter of space and a few microseconds that feel like eons.

Re: Relativistic Spaceship

#432

Post by MilStdJunkie »

hnuser123456 wrote: post #409
lloeki wrote: post #325

Earlier quoted context omitted.

> 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 a…

schwarzschild radius of mass of the universe (3.4 * 10^54 kg) = 5.05 * 10^24 km radius of universe = 4.4 * 10^23 km (47b ly) mass required to have schwarzschild radius of 4.4 * 10^23 km = 2.96 * 10^53 kg Surely we are still discovering the implications of these numbers being so close. I like your time-reversed black hole framing, thanks for that. https://www.wolframalpha.com/input?i2d=true&i=+schwarzschild +radius+of…

IANAP, but possibly because you're measuring different attributes of the same thing - i.e., mass - but we as a species don't really understand the fundamentals of what mass actually is.

Before ANGRY KEYBOARD SOUNDS commence, I'm not saying we don't know what mass is, I'm saying fundamentals. I.e., why is mass. What causes it to come into being? Bulk entanglement, i.e., a function of probability or "mass as destiny"? Tiny signals? Lots of rubber sheeting? Etc.

Re: Relativistic Spaceship

#433

Post by MilStdJunkie »

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…

Bussard ramjets will have problems at higher relativistic speeds. The fundamental issue[1] is that from the perspective of a near light speed system, everything else is near-frozen - which includes things like neutron capture and electromagnetism.

It's sort of funny, but dozens or hundreds of orders of magnitude below, the same sort of dynamics are at work in air-breathing ramjets. The impact velocity of the medium is starting to tell, and the exhaust velocity isn't particularly more energetic than what's threatening to ionize the air around your leading edges.

[1] Well, aside from the fact you're exceeding the average velocity of your exhaust mass

Re: Relativistic Spaceship

#434

Post by ninkendo »

ben_w wrote: post #166
ninkendo wrote: post #148

Earlier quoted context omitted.

> which is kinda like "seconds at 1g" if the fuel is a very small fraction of the total mass, which it really won't be in a practical rocket. I think you mean “a very large fraction of the total mass”… generally the best efficiency comes if your fuel mass fraction is high , as it means there is little overhead of things in your spacecraft of things that are not fuel (the mass of the engine, etc)

No, I mean small, because I'm talking about the quality of the approximation not the way to maximise delta-v. If you have 1 gram of fuel and a 1 ton payload and that fuel has 1e6 seconds of Isp, you can accelerate the ship for 1 second at 9.8m/s/s. If you have 1 ton of fuel and a 1 gram payload and the same fuel and burn at 1 gram/second, the first second is mostly spent accelerating the fuel, which means you're no l…

Makes sense, I misunderstood your initial post. Thanks for clarifying!

Re: Relativistic Spaceship

#435

Post by MilStdJunkie »

wiml wrote: post #190
abhibeckert wrote: post #152

Earlier quoted context omitted.

> Every proton would have the energy of a baseball I wonder if you could capture that energy and use it to generate thrust. Most of the energy is coming from your thrust so it'd be a lossy process however if you're able to capture all of the energy then there won't be anything left to damage the ship.

The old Bussard ramjet concept was to capture these high velocity protons (with some kind of magnetic field), cause them to fuse, and use the fusion energy for propulsion. There are a few engineering difficulties with the idea but it makes for some good SF stories...

Relativity effects will start to bite. The proton's going to stop being interested in your magnetic field[1], and you're approaching the velocity of the exhaust mass of the fusion reaction.

[1] The momentum vector just completely flattens almost any other physical characteristic. I'm not sure there's even enough time for nuclear fusion to take place.

Re: Relativistic Spaceship

#436

Post by rootusrootus »

fifticon wrote: post #428
qayxc wrote: post #97

Earlier quoted context omitted.

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...

I have a confused question here. I asked chatGPT what the approx terawatt output of the sun is. It claims it is 384.6 terawatts. I find that hard to reconcile with the earth having 12TW production. Given that your number is probably OK, that would suggest the sun output calc is way off. I wonder if any non-AI contributors have better ballpark estimates for the sun's energy output.. Hmm, I found a non-chatGPT source,…

This says we use about 1/10000 of the solar energy that hits the Earth at any given moment.

https://phys.org/news/2011-10-vast-amounts-solar-energy-eart...

Re: Relativistic Spaceship

#437

Post by neaden »

mplewis9z wrote: post #327
fredthedeadhead wrote: post #323

Earlier quoted context omitted.

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 co…

Unfortunately for cool space battles, no this still doesn't work for the reasons outlined.

First off you have to remember that when a ship turns it's engine on it's going to be visible effectively to the entire solar system unless there is a planet or something in the way. We already have the capability that we would notice a ship moving in the asteroid belt from Earth and in the Expanse it's going to be even higher. So everyone already will have known when you turned your engines on and can run the calculations to see where you are while you are coasting, so even within the confines of the fiction that shouldn't work.

But let's ignore that and say that for some reason no one was looking when you turned your engines on. Assuming your ship is somehow running with all systems off it's going to be roughly 300 C hotter then the vacuum around it, even with a heatsink like you mention (and a heatsink that can store that heat for months is also unrealistic) you're going to be sticking out like a sore thumb to any infa-red. You're just too hot.

Re: Relativistic Spaceship

#438

Post by ben_w »

MilStdJunkie wrote: post #429
kaashif wrote: post #315

Earlier quoted context omitted.

> A kiloton of TNT is 4GJ, so it’s also like a 500 kiloton bomb going off every second. Funnily, my first reaction to this comparison is that it makes it seem more plausible to me that this is possible. After all, a Star Destroyer can take gigaton level hits! But the problem with that is Star Destroyers are fictional. I've been spending too much time on spacebattles.com.

The canon energy levels given for Star Wars weaponry are so inconsistent with the presentation that one either ignores canon or accepts that the Star Wars galaxy has fundamentally different physics. Extreme geek caution: I've been running a Star Wars pen and paper RPG since . . oh dear. . 2015, in an ancient simulationist[1] game system, and the only way I've been able to make anything consistent is dialing everythin…

Your description reminds me of Star Fleet Battles. As a young teen I made the mistake of treating SFB like Star Trek, while also making the mistake of not realising that Star Trek itself was just doing space combat as ${year of filming}-era naval battles with latex and lightbulbs.

SFB is 1900-1945 naval warfare, but themed with every plot point from TOS and TAS and probably some novels too, so has the Kzinti, Tholian webs, Klingon stasis field generators, and two distinct weapons where the TV series uses just photon torpedoes.

Re: Relativistic Spaceship

#439

Post by AdamN »

jcul wrote: post #380
ozim wrote: post #362

Earlier quoted context omitted.

It is not a bad book but I'd say for sure over-hyped. As much as I enjoyed reading it, I would not recommend it as "must have" if someone was into "The Martian". It is not a "must have" more like it is OK for general public and for hardcore sci-fi fans I would say that I can see how it could be disappointing. But yeah no one can easily appease hardcore fans anyway :)

I read it without any expectations and without knowing anything about the plot. It was just recommended to me by someone, but I'm usually into sci-fi, stuff like Greg Egan. I really enjoyed it, but maybe I would have been disappointed if I was expecting more of a mind melter!

Any recommendations on a mind melter?!?!? Something that is logically consistent is my only criterion (not much of a mind melter if it's incosistent within its own world) ... and the writing not being at a 10th grade level (distracting).

Re: Relativistic Spaceship

#440

Post by ijustlovemath »

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…

Where'd you get 100atoms/m^3? I remember in astrophysics we learned it was closer to 1 proton/m^3, but maybe that was the universe average (which includes the vast and desolate intergalactic medium)
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