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Comment by MilStdJunkie | Hacker News Reader

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-string concept, they had some sort of magnetic thingummy that generated more power as more stuff smacked into it, then as they decelerated they let out this sort of gas mist to go ahead of the ship and smack into things.

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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 instead. A kiloton of TNT is 4GJ, so it’s also like a 500 kiloton bomb going off every second.

Dropping 5e5kg of rock into the worlds biggest dumpster truck at 10m/s yields 25MJ of chaos so it’s also like a parking lot of 80,000 of those being filled with a continuous stream of rubble. That’s probably the best analogy given that we’re talking about machinery — your spaceship needs to have the build resilience of tens of thousands of dumpster trucks but condensed into the cross sectional area of a dinner table.

> 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 numbers are big but they don’t seem “big big” like Sagittarius A* is big or the distance to Andromeda is big.

Distance to Andromeda isn't big.

Galaxy filaments are big.

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 everything down to WW2 levels. 7.62x5X, .50 BMG, 46 cm/45 Type 94 capital ship weapons. All except for exceptional plot items, like lightsabers and doomsday weapons, which behave . . well, they're magic. Sensors are likewise pretty primitive, enhanced with canon exotics like Kronau radiation, so engagement ranges are (relatively) piddling, hostiles zip by each other all the time. Electronics and technology in general must be barely understood by literally anyone, with the powerful assemblies - hyperdrive cores, droid motivators, repulsorlift "sand" - being exotics, possibly xenotechnology from the deep past mined by xenoarchaeologists[2], but hooked together by varying degrees of "competent 1950s electrician".

In short, it's a fantasy setting with guns that players get excited about. And a community of worldbuilders that is, let's not dice words here, insane. That second point is huge if you're not 24 or otherwise gifted with a combination of hubris and spare time.

[1] All the kids today with their streamlined narrative-focused games! Seriously, though, I get it. The physics simulationist in me that brings me to HERO System says more about me as a person than my players.

[2] Archaeology a much more valuable degree in the Star Wars universe.

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.

> How do I get an idea for what 20 billion baseballs (2TJ) feels like?

In the 90s, the show Special Relativity’s Funniest Home Videos would often have guys getting hit in the crotch 20B times with a baseball. Honestly, it never gets old.

They all started to look staged after the first billion though.

There we go again, Americans using anything but the metric system :) scnr

>> baseball

How large is one of those, even? I much prefer to work with perfectly spherical Olympic swimming pools.

Would that be the original British Imperial SOP or the closely-related but not-quite-the-same American SOP?

Actually, the American SOP is the original because the British changed the definition of the gallon in 1824, which you will notice is long after the Revolutionary War. No one in America cared, so we kept on using our customary units.

I’m British with, funnily enough, a Duke’s County cricket ball on my desk.

My nephew is Californian and I gave him one for his sixth birthday. Mine felt a bit lighter so we did an experiment over Zoom together to measure the density. He’s certainly a bit young for that — “it’s the same size but yours is heavier” — but there’s no harm in influencing them from an early age.

Of relevance to your comment: I made a point of doing it in grams and millimetres.

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 of new physics might make all of this possible, but - by definition - we have no idea what that might be.

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 for why the universe seems to lifeless seem to rely on elaborate hypotheticals like us being an early civilisation, us being extremely lucky/improbable in other ways, or that alien life is anti-social. But it always seemed to me that the best explanation is probably just that such things are not possible.

I mean there's a chance there's some new physics out there, but you'd think if there was star wars level tech out there (warp drives, etc) then something out there would have built one already and would rather quickly spread outwards...

It seems reasonable, although we may still be able to send messages with radio, lasers etc. Though even that isn't easy.

Sending humans across thousands of light years seems almost impossible , but sending von Neumann probes throughout the galaxy should be possible with some reasonable improvement to our technology.

Yes. That's my assumption: a sufficiently advanced race won't send members of its own species into the inhospitable furthest reaches of space, but rather probes that can report data back to the home-world. That was always my issue with the Kardashev Scale: isn't the technological level of a species better dictated by how little energy they use to accomplish some goal?

Von Neumann probes don’t just send information back, they reproduce themselves. Anything that can manufacture a copy of itself from materials scavenged in unexplored territory can probably build anything else you want as well. A Kardashev II civilization would build a Dyson swarm around it’s own star (or one near by if they are cautious) and use a fraction of its power to send self–replicating Von Neumann probes to a few hundred or thousand nearby stars and galaxies. Those probes would build not just copies of themselves but new Dyson swarms to launch them with. Once the Dyson swarm is built there is plenty of energy available to do all kinds of things, like moving planets around, terraforming them, and seeding them with life.

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.

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.

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."

Well, a Bussard ramjet has been shown to have a maximum possible velocity of .12c. No idea if a buzzard version can do better. :)

It's been literal decades since I saw it, but the premise of K-Pax always seemed neat: aliens move their consciousness around through some sort of superliminal signal[1], but it looks an awful lot like madness to us humans.

Until he starts solving cosmological problems . . I sort of felt like the truth of the matter should have been more ambiguous than was presented. More Shining, less Friday the 13th.

I remember the premise being far superior to the film, but maybe the novel is worth a look.

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.

Would make an effective weapon.

Worse, it has deleterious effects on the spacetime inside the bubble too. The leading edge of the bubble has exactly the same curvature as a white hole, and the back edge is exactly like a black hole. The result is a beam of Hawking radiation between them. Expect temperatures on the order of 10³⁵K if you manage to stabilize the bubble, but note that this amount of radiation is much more than enough on its own to destabilize it. <https://en.wikipedia.org/wiki/Alcubierre_drive#Survivability...>

So not only do you need “exotic” matter with negative mass to bend spacetime into a warp bubble, the bubble itself creates so much radiation that spacetime is flattened back out again.

Could you repeatedly send disposable ‘dozer’ drone ships towards your destination to help clear the way, and potentially devise a means of keeping the clearing free of stray atoms?

No, interstellar atoms (or other particles) aren't static, they aren't just hanging in one place. Everything is moving. Some particles are moving very fast.

I like that old Arthur C Clarke novel, The Songs of Distant Earth[0], where they travel with an ablative shield in front made of ice, and they can make new shield segments by finding planets with water.

[0] https://en.wikipedia.org/wiki/The_Songs_of_Distant_Earth

Alistair Reynolds also used this in his ship designs.

Pushing ice… a brilliant book - my first foray into hard sci-fi

I read project hail Mary over Christmas, and they deal with this problem too.

Though I can't say much without giving away spoilers.

A great read if you are a sci-fi fan!

My second favorite sci-fi book of all time. Three body being the first.

Indeed. Have you read the rest of the series? It just gets crazier and crazier.

The rest of the hail mary series?? Or Three Body?

I have read two of the three body books, but I wasn't aware of a sequel to project hail mary, would be very excited if that's the case!

The rest of the three body problem series.

Ah got you, I think I still have one more to read.

Yes, the trilogy is really a unit for me. Books 1 and 2 are really a single story. Book 3 is different but amazingly great.

I recommend the audiobook in particular for this book.

I think Hail Mary (and Astronaut) is more suited for people who don't usually like sci-fi.

I like sci-fi, and enjoyed Hail Mary very much

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).

* spoiler alert *

yes, the writing was annoying in parts (especially the Earth flashbacks). What I liked about it was 1) if you don't know anything about it (I didn't even read the back cover) the surprises evolve nicely; and most importantly, 2) it was a story of humans cooperating with another species rather than either subjugating or resisting subjugation

Yeah I totally agree, the surprises were great. I knew literally nothing except for the title and the author and that it could have something to do with space, as he wrote the Martian.

I hadn't read any Andy Weir before, but someone recommended it to me so just started reading.

I would say anything by Greg Egan!

My favorite is probably Permutation City, followed by Diaspora. His stuff can get heavy though.

Thank you for the recommendation! Started reading yesterday and I'm loving it.

Glad to hear, enjoy!

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.

100 atoms per cubic meter is on the lower end of typical densities in the interstellar medium. It can get many orders of magnitude denser than that.

Outside galaxies you have better chances of surviving high speeds. The intergalactic medium is only 1-10 particles per cubic meter in the web of gas we call the warm–hot intergalactic medium, and possibly less outside of that.

So would probably better to get out of the galaxy disc, travel -fast- to the other end and dive back into rather that go straight through the galaxy?

Except we can't get to edge of the galaxy quickly.

I guess Hitchhikers guide to the galaxy building space highways destroying everything in the way could be more than a gag after all.

Go up and over, that's only a few hundred light years. To the edge of the disk is 30,000.

> 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 all the stuff on your ship that's not fuel, you'd probably need quite a bit of water for survival needs.

So you would make your spaceship relatively long and thin (to maximize internal volume for a given frontal area), and you would store your fuel (and water) in front of you to serve as exactly that shield.

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.

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 the reaction mass for your ion drive. (I haven't done the numbers to see how the required mass per second for your ion drive compares to the helium mass per second a nuclear fusion reactor would spit out.)

Because it's a free shield, you don't really get to complain about your shield being gradually consumed.

Of course, you can have some extra shielding further inside. You would keep your water forward of your people, but behind your fuel. So your water would not bear the brunt.

Hydrogen doesn't really get all that radioactive: you can use chemical means to remove any helium or so you might accidentally produce; and hydrogen's isotopes are both pretty short lived and relatively easy to separate. (At least much easier than eg enriching uranium.)

Your water and food is also only a very small fraction of the overall mass of your rocket: as always, the vast majority is made up of fuel.

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 impossible to isolate tritium out of water. So if any tritium forms (which is an extremely common by-product of any fusion reactions which might happen, and the most common decay product of heavier hydrogen isotopes), it will render your water quite poisonous for human consumption, virtually irrevocably.

> 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 impossible to isolate tritium out of water.

On the contrary, it's quite easy to do if there is any significant fraction of tritium present. The proportional mass difference of ³H vs. ¹H is x3, which alters the chemistry enough to make separation easy. You can use fractional distillation or electrolysis even for ²H — even mere hobbyists involved in the DIY fusion reactor scene sometimes extract deuterium from water this way, tritium would be easier.

Oops, looks like reading the first Google result I could find wasn't the best way to research this topic... Sorry and thank you for the correction.

Happens to us all, thank you for being gracious about it :)

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.

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

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?

One nice illustration of "space is really big" is a fact that if you take the cube with the side equal to the distance from the Sun to the nearest star and fill it with water, then the mass of this cube will be roughly equal to the mass of the whole visible Universe.

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, about 100x smaller than the radius of the known universe.

Wasn’t one of the great puzzles of the 20th century exactly this — will the universe collapse back into a singularity or not?

> 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.

I like it! Not sure it works though. We observe the expansion of the universe accelerating, with gravity too weak to counter it. Reversing that would mean it's collapsing faster than the gravitational attraction of it's contents can account for. So either way, gravity isn't enough to explain what we observe.

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+mass+of+the+universe

https://www.wolframalpha.com/input?i=radius+of+universe

https://www.wolframalpha.com/input?i2d=true&i=+schwarzschild +radius+of+2.96*Power%5B10%2C53%5D+kg

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.

> 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>.

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.

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

The mass of ordinary matter in the universe is 2×10^53 kilograms, which would have a Schwarzchild radius of 31.39 billion light years. The explanation from popular science communicators on this topic have never satisfied me.

Your maths is correct for one cubic light year of butter. Proxima Centauri is 4.247 light years away, and that gives such a cube of water[0] a mass of 6.468×10^52 kilograms[1], which would have a Schwarzchild radius of 10.15 billion light years.

[0] At STP, which isn't realistic at all

[1] Close enough; I think it was Brian Cox who once joked that in cosmology it is standard practice to approximate π as 1.

The schwarzchild radius of the observable universe is indeed roughly the size of the observable universe.

Now, since the universe doesn't appear to be a blackhole, we assume there's an equal amount of stuff outside of it pulling it back into flatness.

The hoop conjecture doesn't apply in a small region of a very homogenous universe

This sounded completely false to me initially. When I think of taking mass and turning it into black holes, it involves squashing the mass into a strictly smaller volume.

Thus I would've expected the radius of the black hole to be necessarily smaller than the dimensions of the butter cube.

However, I now realize my mistake - in examples like squashing mount Everest or earth or a neutron star into a black hole, we're starting with masses that are stable/in equilibrium. This would not be the case for a cubic light year of butter!

Further, it looks like the radius is directly proportional to the mass. Given that mass grows cubic with respect to dimension, it's expected the radius of the black hole would eventually outgrow the cube of butter if made sufficiently large...

Source?

https://www.wolframalpha.com/input?i2d=true&i=Divide%5B%5C%2...

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.

Even if you managed to avoid matter, at some point, photons will start to become a problem. As you continue to accelerate, eventually, the cosmic microwave background itself will become deadly X-rays.

Would a layer of water in the forward hull positions mitigate this?

To a point, sure. Lead would be better. Some joke that this is the reason why Klingons have these thick foreheads.

X-ray sources would turn to gamma rays. Not that it’s any better that X-rays. Other comments suggested lead plates. It would quickly get irradiated and would probably need to get shed as soon as you got to a destination.

No accounting for particles yet, which you'll also keep hitting, making your ship's materials radioactive and causing lots of secondary particle showers, bremsstrahlung and the likes.

First the particles will act like radiation, then they'll start causing matter-antimatter pair creation with your hull, then you'll get some exotic heavy quarks popping into existence, then you'll get some Higgs particles forming and at some point questions like "what is the mass of my ship" stop making sense.

No--x-rays will not make anything radioactive. Gamma rays can only do so when they're in the 2GeV range or higher (indirectly, through pair production.)

It's particles that make things radioactive, mostly neutrons as they aren't repelled by the nucleus and thus have a much easier time getting in. And while it would be hard to construct a shield out of it helium is effectively immune to becoming radioactive under neutron bombardment. And while lead isn't immune the reaction sequence produces nothing that won't be contained by the lead and it self-regenerates, enough bombardment returns it to where it started.

These regenerative braking schemes are getting out of hand.

> somewhere around a hundred atoms per cubic meter.

Wikipedia says that intergalactic space contains less than one hydrogen atom per cubic meter; and that most of the baryonic matter in intergalactic space consists of hydrogen and helium atoms. If I've understood it correctly...

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

Yes, but that's intergalactic space not interstellar space. We've not even begun exploring the latter :-)

intergalactic != interstellar

Well, I don't think the discussion was restricted to interstellar space; for example there's been quite a bit of chat about how long it would take, at 1G (on the astronaut's watch) to reach the edge of the (known) universe.

> 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.

Isn't this kinda like mounting a fan on your car's roof to charge the battery? You have accelerated your spaceship to 0.995C (or whatever) and now you are encountering space dust at a phenomenal rate. Some of that dust is moving away from you, some of it toward you, some of it is at rest. On average it's all just sitting there unaware that your vessel is about to smack into it. The energy is in the difference between your speed and the particle's. If you try to harness it, you slow down.

(I'm asking genuinely here. My analogy might be wrong because it's too classical!)

I think in principle you should be able to generate thrust greater than drag orthogonal to the particle flow. That's what wings do.

This is a bit like driving into a brick wall, and then asking if you could use the released energy to go faster ...

Imagine you drive into a wall of tnt, break through it, and as you exit it explodes and gives you an extra boost. Yes, you’ve lost some speed at the beginning but you’ve gained much more

The tnt has potential energy. The only reason those protons have energy is because you’re smacking through them with energy you’ve already found.

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.

Also all Stat Trek Federation ships

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.

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.

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 or Plutonium) then you have an unlimited source of energy...

So maybe sort of? Running into things slows you down, but then you capture that mass and release the energy out of it to go faster... because of the nature of e=mc^2 you'll usually get more energy out of something if you convert its mass than what you lose by running into that mass.

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.

What do you mean “the energy of a baseball”? A baseball at what speed?

I assumed they meant a baseball at baseball speed.

Rather than relativistic speeds, which would be bad:

https://what-if.xkcd.com/1/

They're saying a fastball pitch-- 340 joules or so of energy.

The thing is, you need to be going implausibly fast to have a proton have a fastball level of energy. Even at .99999999 C, a hydrogen atom still has less than a microjoule of energy. You need a lot of 9's to get up to 340 joules.

  1/sqrt(1-(0.99999999)^2) * (1 atomic mass unit * (speed of light)^2
  1.05529895 × 10-6 joules

I feel like regardless of the exact speed of the baseball, that's somewhat more energy than a typical proton.

A baseball at football speed, of course

A magnetic Ramjet could help (1)

1. https://www.sciencedirect.com/science/article/pii/S009457652...

Already patented?

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)

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

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.

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.

It's more stable (and structurally leaner, I think) to instead have the engines at the front in tractor configuration, as in the interstellar ships in Avatar movies.

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