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Relativistic Spaceship

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Re: Relativistic Spaceship

#441

Post by ijustlovemath »

TheOtherHobbes wrote: post #374
heads wrote: post #313

Earlier quoted context omitted.

If you’re doing 2e8 m/s, your ship is long and thin with a 1m3 nose cone, and space has 100 baseballs per m3 then you’re being hit by 2e10 baseballs a second. How do I get an idea for what 20 billion baseballs (2TJ) feels like? Apparently bullets are about an order of magnitude more energetic than baseballs (800J vs 80J) so I guess I could try to build my intuition based on being shot ~2 billion times a second instea…

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…

The theoretical Alcubierre drive basically collects all of the material you would intersect with in a gravitational well ahead of the vehicle as you travel, which is great!

...until you stop, releasing all of that mass as a gargantuan amount of energy at your landing site.

Re: Relativistic Spaceship

#442

Post by foobarbecue »

bilsbie wrote: post #405
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…

Hydrogen is diamagnetic (1) so you’d just need the right level of magnetism to repel it (or slightly redirect it) Or if we shape the field into a buzzard ramjet then you actually take advantage of the interstellar material for fuel. (1) https://sciencing.com/magnetic-properties-hydrogen-7648446.h...

I imagine you got "autocorrected" but to slow the error propagation I'll point it that it's a "Bussard" ramjet, not "buzzard."

Re: Relativistic Spaceship

#443

Post by sudhirj »

_factor wrote: post #353
sudhirj wrote: post #288

Earlier quoted context omitted.

There's two different kinds of energy here - the kinetic energy of a moving thing hitting you in opposition is a problem. No way to capture that as far as I know - it's like running into a wall and asking how it can help you go faster. But then there's e=mc^2, so if the stuff you're running into is the fuel source for your fusion engine (could be a fission engine, but unlikely you'll run into heavy atoms like Uranium…

I’m imagining a ship with a hole in it and a piece of fuel at the backside where the particles hit. It would slow you down first since you’re tethered to the particles, then the explosion would push you forward. The issue is that the energy for that explosion comes from the slowing down of your ship, so it doesn’t work.

The energy that comes from a mass-energy conversion greatly exceeds kinetic energy losses.

For the same reason that the power of an atomic bomb does not depend on how fast you smash the sections together - it depends on how much mass is converted to energy in the resulting reaction.

Re: Relativistic Spaceship

#444

Post by ausp »

MilStdJunkie wrote: post #31
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…

One gee over the course of months is insane . Even if you converted the entire "fuel" reaction mass directly into momentum, you're still carrying tens or hundreds of thousands of tons of fuel for any remotely habitable spacecraft. And no engine has perfect mass conversion. Matter-Antimatter has been simulated to be maybe sixty percent efficient, fusion is around 17%, nuclear pulse propulsion a surprising 7%, fission…

> As an SF writer, it's pretty interesting, though, to think of the weird shapes a society would take when your astronauts are outliving entire civilizations.

Spin, a novel by Robert Charles Wilson, touches on this from a planetary perspective. Quite the interesting read, though I won't say more here for fear of spoilers.

Re: Relativistic Spaceship

#445

Post by raattgift »

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…

> Hear [me] out

Ok.

> a black hole, but we have time backwards compared to the usual way we think of black holes

Observations of our universe are straightforwardly understood -- and predicted -- by laying matter fields on an expanding Robertson-Walker metric. The same observations are not at all easy to understand by laying matter fields on a time-reversed Oppenheimer-Snyder-like black hole metric.

The first thing you run into is that at the largest scales (i.e., where the solid angles subtended by galaxy clusters are small for observers like us) visible matter is arranged roughly isotropically and roughly homogeneously: we detect typical spiral galaxies (and more importantly various atomic line transitions associated with them, like the https://en.wikipedia.org/wiki/Lyman-alpha_forest>) at all sorts of redshifts.

Your homework would be to generate lightlike geodesics that can reproduce these observations at any time in a black-hole-like metric. If you can do that at for a single spacelike slice of your black hole, you then would want to work on evolving that slice using e.g. the https://en.wikipedia.org/wiki/Initial_value_formulation_(gen...>.

Just scratching the surface of how you would go about doing that would be an interesting research project for a layperson. Among other things, you would end up learning a lot more about what's in your second paragraph, and likely develop an idea about how much work is involved in writing down even a simple "pet theory" of physical cosomology that accords with observational data. Or at least you'd have a better idea of what observational data there is that needs to be accounted for. You'd also confront all sorts of open questions about the interiors of black holes where there is significant matter; that would be timely given the recent preprint by Roy Kerr at https://arxiv.org/abs/2312.00841>.

Re: Relativistic Spaceship

#446

Post by crispyambulance »

I think it's perhaps better to abandon ideas about going at high relativistic speeds to bridge large distances.

There's a more practical idea in nature: become a seed. It's how living things traverse vast distances and overcome intractable resource constraints during transit.

Compact oneself into an inert crystalized form, aim rocket, launch, wait. All you've gotta do is prevent chemical/structural breakdown for a few millennia or eons. Upon arrival to destination, have a machine turn-on and reconstitute the body and voila, you're there, all refreshed and ready to explore and virtually no time has passed.

Of course to do that one would have had to master biology, or maybe skip biology entirely and opt to copy oneself into a machine format, but that is likely easier than travelling at a sizeable fraction of c?

Re: Relativistic Spaceship

#447

Post by api »

denton-scratch wrote: post #367
api wrote: post #123

Earlier quoted context omitted.

> 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…

> This has been studied at least on paper by militaries. I thought these weapons were real and deployed. Specifically, I understand that hypersonic missiles don't really need an explosive warhead; a hypersonic tungsten rod would make a bigger explosion than any conventional warhead.

Hypersonic missiles are different. I think the idea with those is whether they have a warhead or not they come in so fast you can't possibly shoot them down. The US, Russia, and China are all either confirmed or rumored to have hypersonic delivery systems like this.

The "rods from God" concept is the idea of creating an artificial meteorite as a weapon that comes in from space. These may or may not already exist, but if they do they'd be secret and would probably violate some treaties.

Re: Relativistic Spaceship

#449

Post by bejd »

fyver wrote: post #293
wiml wrote: post #190

Earlier quoted context omitted.

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...

The spaceship in Poul Anderson's Tau Zero uses a Bussard ramjet.

Fantastic book. I read it for the first time about a year ago but I still think about it once every week or two. Thought about it as soon as the "spaceship" started to accelerate towards light speed.

Re: Relativistic Spaceship

#450

Post by hetman »

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…

I don't think you need such extreme relativistic speeds to obtain the desired result though. The above assertion that every proton would have the energy of a baseball puts us at a rather incredible speed. A quick Google gives the energy of a baseball pitched at 90 mph to be around 117.4 J. For a proton to have the equivalent relativistic kinetic energy would put it at 99.999999999999999999999918% the speed of light!!

Now let's take a much more reasonable speed like 10% the speed of light. Assuming the 100 protons per cubic meter figure above, each square metre of ship now only needs to dissipate 2.27 mW of energy. 10% the speed of light is enough time to reach Alpha Centauri within a single lifetime (42 years). And fast enough to visit every part of the galaxy in less than a million years. We could even imagine generational ships travelling at 1% the speed of light (now the energy dissipation demands are 2.25 μW per square metre of ship surface). That's still under 10 million years to colonise the entire galaxy.

If intelligent life is abundant in the galaxy then I don't think the speed of spaceships at least offers a fundamentally insurmountable technical challenge for that life to spread everywhere.

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