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Comment by jp57 | Hacker News Reader

I don't have references in front of me, (EDIT: I do!) but IIRC it takes about a kilogram of mass-energy to accelerate a kilogram mass to about 0.85c. But that kilogram would have to be carrying another kilogram of matter/anti-matter fuel to decelerate again. So there is a kind of relativistic Tsiolkovsky equation for mass-energy propulsion vehicles that carry their own fuel.

Zipping around the galaxy at 0.95c, stopping at destinations and then zipping off again will require carrying a lot of antimatter with you.

EDIT: Thanks to Wolfram Alpha I was able to see that it the kinetic energy of 1 kg at 0.87c is very close to the mass energy of 1kg of matter.

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Even at .99c, everything interesting is years, decades, centuries, and millennia apart. All interstellar solutions include maintenance over millennia. Once you're doing that, relativistic velocity is just a hazard, not a boon. If we do conquer interstellar travel it'll probably be at 1% c. The factor of 10-20x scale won't matter to such long lived civilizations.

From the perspective of someone on the ground, yes. But due to time dilation, it could be just a few days for the people on the ship.

At .99c, the Lorentz factor is only about 7, so 1 year of ship time for 7 years of earth time.

.99c only dilates time by a factor of 7, so one year of time passed on a vehicle would yield ~7 light years.

Heck, you have to get better than 5 nines to even compress a year into a day, .9999963c, which would take a freakish amount of energy to accelerate a KG to (3.3 × 10^19 J).

On top of this at 1g it takes ~1 year accelerating to that speed and another year decelerating. So even the nearest star is going to be a multi year subjective journey or a really unpleasant trip.

This is assuming that you have to be conscious during the trip. Antimatter storage is a lot more scifi than human hibernation.

The problem with human hibernation is that it is almost impossible to ethically test.

What medical ethics board is going to approve a research project "put healthy experimental subject in coma for 5 years, observe what they are like when they wake up?"

Reminds me of https://en.wikipedia.org/wiki/Deep_sleep_therapy and https://en.wikipedia.org/wiki/Chelmsford_Royal_Commission

I think this is an example of a technology which, if it is ever developed, is most likely to be developed by some kind of totalitarian regime which has no ethical qualms about human experimentation.

That's not really how hibernation works in mammals, they have to raise their body temperature and come out of hibernation every couple weeks, the leading theory is this is to (ironically) catch up on sleep.

Inducing torpor would have major medical uses in surgical and emergency medicine, it's not just useful for passing the time.

Heinlein story time! Door into Summer https://en.wikipedia.org/wiki/The_Door_into_Summer Protagonist signs their life away for a 'cold sleep' that will allow them to wake up many years later. Tech is iffy. They do it because they are desperate, and the interest gained over time should make them rich (doesn't happen of course).

People with incurable diseases or just strange individuals can volunteer for such experiments. After that, if they write about their experiences, it could be featured first on Hacker News.

I think the opposite is true.

We do know how to inefficiently create and store antimatter, we don’t have any idea how to cause human hibernation.

To me getting two dozen of orders of magnitude better at something is clearly hard, but that still beats trying to do something we don’t have a clue how to start. For human hibernation step 1 is probably serious genetic engineering and there’s going to be other steps.

Inefficiently in the context of antimatter storage makes every industrial accident since the dawn of time combined seem like a kid spilling their juice.

The inefficiencies around storage are a practical not a safety concern. Once cooled we can contain the particles just fine the issue is only 0.1% get trapped and the apparatus is vastly larger than the mass of antimatter stored.

To put that into number that people can understand, to accelerate 1kg of matter to 94% the speed of light you need 1kg of antimatter. That is the equivalent of the tsar bomb. So the storage facility would need to withstand that type of explosion 1000 times over for every kg of antimatter it produces.

World ending doesn't begin to describe what that looks like.

That assumes the energy is released in a fraction of a second which would preclude cooling the antimatter.

Do the same thing over a year in many different devices and you’re basically dealing with a normal nuclear power plant’s output.

I imagine you will want to send more than 1kg worth of payload.

I’d expect people would need a more efficient system, but it’s also the work of years and multiple sets of space based equipment working in parallel.

If you look at particle accelerators, the anti-matter storage is still very much experimental with anti-hydrogen storage holding it for less than 20s at most. We can trap individual particles (e.g. protons) for a long time but things get exponentially more complicated as you increase the size of the system.

Hell, we can't even store hydrogen without leakage issues and with anti-hydrogen any leakage is very bad.

We can freeze and reanimate living things only five or so orders of magnitude removed from humans; if we're just talking hibernation, it's as little as two.

How many orders of magnitude more complicated is a human mind than a cell? Infinity, I'd guess.

Everybody forgets about tardigrades, but they will be here when we are long passed from all memory.

Maybe, but there are mammals that can survive deep hibernation and near-total suspension. In pure biological/biochemical terms I don't see any reason to think we're much more than 100 times more complex than a rodent, or 1000 times more complex than the various fish and amphibian species who can survive their bodies reaching near-zero temperatures.

They've put pigs into suspended animation for an hour or two, I'm not sure what the upper bound is on that or how much further it's possible to go with it. Ditto organ deep-cooling. So while it's not feasible now, it feels like we need "only" 100x or so improvement to make it viable.

Unfortunately, it only takes one thing to make it not work.

Damn, c is too big for traveling with compressed enough time for cheap, yet too small to make communication within the earth be pretty much instantaneous (like getting 1ms roundtrip latency everywhere).

Right? I’ve often thought this. Light is both frustratingly fast and frustratingly slow at the same time.

It’s not that light is too fast it’s that space is too big.

The problem is that we don't live long enough. We're fireflies.

Idk, given how things are going I'll take 45yrs over 10^6yrs

Isn't this a pointless goal anyways since any spaceship we'd have capable of getting to any meaningful percentage of c would get destroyed by any small amounts of matter or gas it bumps into on the way.

Seems reasonable might have some way of dealing with that when they’re able to accelerate sizable objects to multiple percents c.

My understanding is that space is so big and empty that the odds of you actually hitting anything is quite small.

I'd imagine that simply interacting with stray hydrogen atoms faffing about in interstellar space would cause significant damage over time?

Sounds about right.

In the interstellar medium, matter is primarily in molecular form and reaches number densities of 10^12 molecules (mostly composed of) hydrogen, then helium, oxygen, nitrogen) per cubic meter (1 trillion molecules per cubic meter).[1]

So, what, about a picogram per cubic meter.

If your ship has a cross-sectional area of one square meter, and Alpha Centauri is 40,000,000,000,000 meters away [2] (and you thought it was a long way to the shop if you want a Chiko Roll), you’ll have to manage with 2.62^23 tonnes of mostly hydrogen in the way.

The interstellar medium is also about 1% dust, so about 2.6^21 tonnes of solid matter.

Someone check my mass maths.

1. https://en.m.wikipedia.org/wiki/Interstellar_medium

2. Google Search AI Overview

I wonder if it would be possible to push it out of the way with some kind of charged field ahead of the space craft. But even if that's the case it would act like air and cause deceleration.

ChatGPT’s math says that a picogram at 0.999c delivers about the energy of a strong human punch. Aside from issues of metal fatigue that wouldn’t seem like the thing to be most concerned with regardless of distance - the effect to me would seem closer to space rain (assuming you could dissipate the heat which seems like the bigger problem).

Dust is a concern but again the typical dust particle size would be about the power of a gun shot of a small caliber rifle. A problem to deal with but a rarer event still. Of course another challenge would be larger dust particles which while improbable are still possible which largely rules out humans in these craft. But that goes without saying since the acceleration to get to that speed would be unlikely something humans could withstand anyway.

Right, but a .357 round delivers roughly the same kinetic energy as nunchucks and particle accelerators are also a thing, which is to say delivery of a human punch to a cross section of area the size of a hydrogen atom is going to have permanent negative impacts to whatever is on the receiving end. I'm imagining the front of the craft ablating at the atomic level and throwing off some wild-ass radiation in the process?

Could they just send lots of ships on the same path and specifically design them to break apart outward if destroyed, eg spin them at very high rates, to basically sweep the path clean, or would most of the particles in the path only be there because they happened to move into the path momentarily? I don’t know much about the velocities of interstellar particles.

At these velocities, even a spec of sand is a major hazard. And the distances covered in these shorts of journeys would pretty much guarantee a collision of that sort. (Random search showed a 13mg grain of sand at .999c is equivalent to 1700 tons of TNT).

Even interstellar/stellar wind would have enough molecules of gas to cause some crazy erosion/damage.

Isn't some of that matter/gas moving at really high speeds anyway?

Deflector shields of course

That's why we usually end up starting with projectiles instead of ships. Once the drive is ready, someone will take it from you for a few decades to shoot at things.

Impact seems to have a timelined meaning, starting with destruction, then following up with conquering, and exploration coming somewhere before constructive applications at the far end.

As apes we just can't keep holding back till we can build houses or something with it, when we also could just throw that stone at something. Especially before someone else does :)

With a ship that accelerates at a constant 1G, you can go pretty much anywhere in the universe in less than 50 (subjective) years [1]. And when I say anywhere, I mean anywhere.

[1] https://www.reddit.com/r/dataisbeautiful/comments/s4tbry/oc_...

Maybe I'm mistaken, but that graph would be for passing by places, as it doesn't include the time it would take to slow down from that constant acceleration, if you want to actually visit any other places than your zipping spaceship.

The title of the graph says it is for accelerating half way, then decelerating the other half

This comes up every time this sort of thing is discussed on HN. Sustained 1 g acceleration takes a staggering amount of fuel, and carrying enough to slow down again makes it even worse.

It's definitely sci-fi for now, but we can always imagine some far out way to store energy much more densely than antimatter. A huge amount of photons orbiting a micro black hole on its Schwarzschild radius, would only weigh as much as the black hole and let you store a potentially infinite amount of energy.

No, they would have as much mass as the black hole plus the energy of the photons (they still distort spacetime and exert a gravitational force, and have inertial mass) and you still have to move that mass with you. You may have increased your fuel density, but you haven’t reduced its mass compared to antimattter, and you still have not dodged the relativistic rocket equation.

That's exactly the point. The limiting factor to space travel is how to achieve acceleration, and not the acceleration limits of the human body.

The CMB becomes deadly well before these speeds (the rear facing light burns you; the forward facing light nukes you). That's not even accounting for objects in space, or the light from stars, which will be amplified according to the Lorentz factor.

EDIT: here's a small discussion: https://physics.stackexchange.com/a/196171 There's a whole PBS SpaceTime episode about this, but I can't find it, rn.

A mere engineering problem.

And at high enough speeds, you don’t want to be carrying anything metal with you, eddy currents as you pass through magnetic fields will melt your wedding ring.

So it's only worthwhile to do big excursions if participants self-fund it or we missed something about relativity and faster-than-light travel. Funding it would be like the Roman empire telling us a small boat will land in Genoa in 2025 that spent the last two millennia traveling the world and collecting data with the best tools of the first century.

Yep. Interstellar space is pretty famously big.

You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.

Space is big. Space is dark. It’s never hard to find a place to park

- Burma Shave

This is why there’s ideas like the Bussard ramjet, which may not work but try to work around this problem by using in situ mass-energy.

Now I have read that it may be possible to use a powerful magnetic field to assist with slowing down by braking against the interstellar medium, which helps.

The Avatar films have (in spite of very derivative plots) fairly realistic (at least physics wise) interstellar ships. They accelerate using beamed laser propulsion from the Sun and use antimatter rockets to decelerate, then repeat this in reverse to come home. One assumes they somehow recharge at their destination but this is not shown. Too bad all that cool tech is in service to humans who decided to be the bad guys from War of the Worlds.

Still traveling that close to c brings up tons of other problems. Collision with a micrometeorite would be like an atomic explosion, and blue shifting of incident and cosmic background radiation would blast you in the head with x-rays and gamma rays. Those problems would demand more mass for active or passive shielding, and you’re already mass constrained.

All things considered it’s way more practical to go slower — which could still be insanely fast e.g. 0.25c — and figure out how to cryosleep or become an AI that can just turn yourself off for the trip. Cryosleep for humans is a brutally hard biomedical problem but way easier than trying to approach the speed of light. There are other multicellular animals that can do it, albeit much simpler ones, so it’s probably possible.

0.25c allowing for acceleration and deceleration gets you to Centauri in around 25 years and to further star systems with promising exoplanets in hundreds of years.

Then there’s generation ships, but that’s the kind of thing Mormons would do. :)

I'd imagine interstellar travelers who have mastered D+D fusion could break the journey down into hops of maybe 10,000 or 100,000 AU looking for large comets or plutoid objects which they could use to replenish their supplies.

I'd imagine it would take them 10,000 years or so to make it to the next star system but they might not care if they can live a comfortable lifestyle in the great dark.

Something I didn’t think about: there is debate over how empty the space between stars is or whether there are a lot of rogue planets, comets, asteroids, maybe even exotic objects like asteroid or planet mass primordial black holes (these are a dark matter candidate). If the space between stars is not as empty as we imagine it opens other possibilities like flybys and refueling.

Interstellar space can be filled with junk yet still be effectively empty because it's so mind bogglingly large. Comets and rogue planets also aren't helpful unless 1) you can see/chart them ahead of time and 2) they're on the way to your destination.

The first problem is a big challenge as they're cold, dark, and small. Just finding them to begin with is a giant problem. Accurately charting them is another order of magnitude increase in difficulty. Even tiny error bars in the measure of their proper motion means your spaceship can miss them by millions of miles. Even missing them by a dozen miles is the difference between life and death.

Even with a huge catalog of extremely accurate interstellar fuel-capable objects they don't do you any good if they're not on the way to where you want to go. A meandering route to a destination in order to visit refueling stops adds tons of extra complexity and points of failure.

Might I humbly recommend the disaster of a TV show "The Starlost".

I saw the draft script Phoenix without ashes in an anthology. I ought to look up the show as I'm a fan of trashy SF. (Just read Pohl's notorious Plauge of Pythons)

At sufficiently relativistic speeds, it wouldn’t just be “like” an explosion - you would have instant nuclear fusion at impact.

Which is to say nothing of the mass of whatever containment equipment would be necessary.

For stop and go; we'd be looking at something that could deploy a dyson swarm, generate and contain its own antimatter and then go again right?

I should play universal paperclips again.

The energy budget for any kind of interstellar travel is large, almost incomprehensibly large. Like many orders of magnitude greater than what our entire planet produces and consumes. The numbers you're quoting seem reasonably accurate but they also assume perfect mass-to-energy conversion, which we'd never get.

It's why some kind of generation ship, that is basically a colony, is really the only conceivable method of traveling between stars.

Also remember that whatever energy is produced by antimatter, you need more than that to produce the antimatter to begin with. Where are you getting that energy? I believe it's from solar power from a Dyson Swarm.

We just have to get creative, or discover some new physics :-)

In addition to the ark-ship colony, or the cryosleep slow ship:

1. Assuming it's a stream of robotic probes doing flybys, without decelerating, we have the Breakthrough Starshot approach. Maybe there's a way to use the target system's sun for solar sail braking? Send smart enough robots that have agency, that can do the exploring for us.

2. For human travel - it could just be a bunch of frozen embryos with a robotic nursemaid, accelerated via external propulsion and decelerated via nukes / high-g aerobraking... (Raised by Wolves had a cool introduction like this in the first episode - then went quickly downhill)

3. ...

"we need new physics" basically means "I hope this isn't true". I get it. I think many of us would like to wander the stars in a reasonable timeframe but there's simply no evidence the Universe works this way.

As for cryosleep, this curently seems unlikely but not impossible. For one thing, the decay of radioactive elements in your body (primarily Carbon-14) would give you about a lethal dose of radiation after about a century. Some organisms have natural antifreeze and other means of surviving low temperatures. We do not. Freezing water tears our cells to shreds.

Cryobabies and artifical wombs are another vector. This is nontrivial too but also, woould you trust the automation? Some AI might have to raise humans hundreds or thousands of years in the future without any context of what's happened in that time. We might be able to communicate with such a ship and update it but should it trust such updates?

You're also creating a whole bunch of people who haven't consented to never see Earth. Generation ships have this problem too to some degree. That has questionable ethics.

As for using the target star to decelerate, that's entirely possible. It's just a solar sail. And that might be the only way we could do interstellar travel anyway because of the reaction mass problem. But solar sails can only accelerate so fast. Travel too fast and you might not have time to decelerate as well. So you're still looking at hundreds of years most likely.

It really seems like we need radical life extension while maintaining quality of life to make these time frames reasonable (relatively). That actually does seem doable.

I agree with most of this, but the ethics question intrigues me: did any human ever have a chance to consent to the circumstances they were born into?

If you could avoid accelerating so much and go a little slower, you could get better lypkg.

Sure, and if you want to go at non-relativistic speeds you hardly need to carry much fuel at all.

> there is a kind of relativistic Tsiolkovsky equation for mass-energy propulsion vehicles that carry their own fuel

Yes, it's called the relativistic rocket equation.

https://math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/...

Yup the problem with antimatter propulsion is it’s still subject to the tyranny of the rocket equation. The only way to escape that is to leverage energy external to the vehicle, like laser pushed light sales or relativistic railguns. You just can’t be carrying your own fuel around for an interstellar trip.

would be real nice to have a small device in your pocket that works at nano scale and converts input matter into output antimatter. Like feed it rock powder and you get antimatter rock powder. Would be real nice such a device could be made

Hopefully Von Neumann probes will be much lighter than a Kg and we can construct receiving stations on the other end and transport the information we care about back and forth directly at light speed.

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