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Relativistic Spaceship

dmytry.github.io

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Re: Relativistic Spaceship

#481

Post by eru »

MilStdJunkie wrote: post #433
eru wrote: post #198

Earlier quoted context omitted.

> 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 i…

You are right. But I wasn't talking about a ramjet. I was talking about a rocket that brings all the fuel and propellant it needs.

I'm not sure what you mean by electromagnetism being near-frozen? Doesn't electromagnetism also travel at the speed of light?

Re: Relativistic Spaceship

#482

Post by eru »

tsimionescu wrote: post #384
eru wrote: post #335

Earlier quoted context omitted.

Your fuel is the outermost layer of your shields (modulo whatever is necessary to keep the fuel in place. But you might use magnetic fields perhaps). That's basically free shielding: you have to carry the fuel around anyway, so you might as well put it to good use. If you run a nuclear fusion reactor, you won't really lose much of the mass of your fuel, unless you want to. Eg you could use the helium you produce as t…

Your fuel still gets consumed, so you still can't rely on your fuel as the main form of shielding. Towards the end of your journey, your rocket is approximately 0% fuel. And at the point of highest speed, before you start decelerating, it is roughly 50% fuel. And you are entirely wrong about the isotopes of hydrogen. Tritium is highly radioactive, with a half life of ~12 years. And it is not just hard, but virtually…

With a half-life of 12 years and the time interstellar voyages take, you can just let your tritium sit around for a while. (In addition to what the other comment said about separation actually being relatively easy.)

Depending on your fusion reactor, you might actually highly value any tritium produced, instead of seeing it as a nuisance.

> Your fuel still gets consumed, so you still can't rely on your fuel as the main form of shielding. Towards the end of your journey, your rocket is approximately 0% fuel. And at the point of highest speed, before you start decelerating, it is roughly 50% fuel.

With a nuclear engine you make a difference between fuel and propellant. When you fuse your hydrogen into helium, you still have the helium afterwards.

I haven't run the numbers to see how your fuel consumption would compare to your propellant consumption for a reasonable fusion powered rocket. Though I suspect that you also need oodles of propellant, given how the rocket equation works.

For the first part of your journey you could rely on passive shielding via your propellant. For the latter part, you could use more costly active shielding like a big magnetic field or ablative shields in front of your main rocket etc.

Basically, you would still want to use your propellant as free shielding as much as possible.

Re: Relativistic Spaceship

#483

Post by noduerme »

mihaaly wrote: post #422
noduerme wrote: post #382

Earlier quoted context omitted.

>> the diagonal of the two can never go beyond c Sure, from the observer's system (home planet "stationary" the speed is measured from) it can't go faster than c... but that's where I don't think it's accurate. From the flyer's perspective, if you're close to c (as viewed from home) and suddenly accelerate at a 90deg angle, that shouldn't bleed off your speed forward, or be limited by your speed forward. Only from th…

The near c component of the velocity remains near c with applying 90 deg acceleration, hence that the velocity vector will remain towards this near c regardless of 90 deg acceleration characteristics. For a significant change one need to apply acceleration that has very significant deceleration component to this near c velocity vector (significant amount of >90deg acceleration applied, best to be close to 180deg).

yeah, and that's the way it would appear to a motionless observer. Buy not how it would appear to you. Remember, once you have achieved .99c velocity, no longer accelerating, you are standing still. From your perspective, any new acceleration at a 90deg angle should begin accumulating light from that direction.

Re: Relativistic Spaceship

#484

Post by tEMporality7 »

CobrastanJorji wrote: post #169
A_D_E_P_T wrote: post #39

~850 years of ship time at max acceleration and the universe is 10^30 years old. At this point, only white dwarfs, neutron stars, and various stellar remnants remain. Galaxies have dissipated, and stars like our Sun are a distant memory. (If any still exist to remember them. Which is, I'll add, not out of the question. Life can cling to structures built around black holes, white dwarfs, etc., and extract energy from…

You could imagine a technically advanced society using it as a sort of penal death sentence. "You were not safe around our people, so we are sending you to the end of time."

The Master did that in Doctor Who

Re: Relativistic Spaceship

#485

Post by sliken »

dfee wrote: post #147
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…

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.

From quora:

The vacuum in solar space is around 10^7 atoms per cubic metre; the vacuum of interstellar space is around 10^6 atoms per cubic metre; and the vacuum of intergalactic space is around 1 atom per cubic metre.

Re: Relativistic Spaceship

#486

Post by sliken »

lloeki wrote: post #325
jemfinch wrote: post #305

Earlier quoted context omitted.

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 a…

The way it was explained to me is that at the big bang space itself was expanding at faster than the speed of light. So over blackhole density could evolve into less than blackhole density.

Re: Relativistic Spaceship

#487

Post by denton-scratch »

eru wrote: post #479
denton-scratch wrote: post #372

Earlier quoted context omitted.

> 10-20% of current astronauts would be willing to sign up. Astronauts seem to me to have a higher propensity for kookiness than the general population. You may be right.

Yes. There's currently way more people who want to be astronauts, than people who can be astronauts. So you get some extreme selection effects; and those tend to bring out weird people.

I was really thinking of astronauts who have actually been to space; I wonder whether the experience disposes people to kookiness. I suspect that trainee/aspiring astronauts might suppress their kookiness.

Re: Relativistic Spaceship

#488

Post by fifticon »

qayxc wrote: post #460
fifticon wrote: post #428

Earlier quoted context omitted.

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,…

I'm really not trying to sound condescending here, but your maths is WAY off. Where did you get 384.6 TW from? Your actual source says The power output or luminosity of the Sun is 3.8 x 10^26 W For reference that's 3.8 x 10^14 TW - so your little AI friend is a cool 12 orders of magnitude off here. Bottom line: do NOT rely on figures provided by generative models. The chance of them being completely wrong seems to be…

thank you, you are indeed right. To wit (for people like me) - 3.8 x 10^26 W 3.8 x 10^23 kW 3.8 x 10^20 MW 3.8 x 10^17 GW 3.8 x 10^14 TW

Re: Relativistic Spaceship

#490

Post by LorenPechtel »

trenchgun wrote: post #343
CobrastanJorji wrote: post #169

Earlier quoted context omitted.

You could imagine a technically advanced society using it as a sort of penal death sentence. "You were not safe around our people, so we are sending you to the end of time."

Vernor Vinge, Across the Realtime trilogy

No. People were sent to the future as a method of effectively murder, but most of them were voluntary travelers.

The one time it was used as a punishment it was not sending them to the end of time, but leaving them in the now while everyone else jumped ahead.

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