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Comment by MilStdJunkie | Hacker News Reader

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 nuclear gas rockets might max out at 12% but those numbers are dodgy as we don't have sharp numbers on gas core reactors.

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. I imagine a sort of neo-Polynesian culture, with travellers never quite knowing how the place will look if they ever come back that way.

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> 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.

I guess this all depends on how you define simultaneity. Say I got on a relativistic rocket traveling near c and colonized Planet X in 10 years ship time and say 20,000 years Earth time. I put up my TV antenna as soon as I land, and I'd still be receiving Earth transmissions from approximately 10 years after I left, no? So even though civilization is long gone from Earth's point of view, from my point of view, I can keep up with the latest news as if I were still living there. So from my point of view I did not outlive anything.

If I were to go back, yes, everything would be gone.

That's not how it works. If the planet is N light years away from earth, the transmissions you receive would be those sent 20000-n years ago. along the way you'd get most of the messages sent during the 20k years as very red-shifted light, and extremely rapidly from your pov.

I think you may have that wrong (If I'm understanding you correctly).

If you traveled at the speed of light then you'd be at the planet in 0 time your time, and you would arrive with the first of the broadcasts over those 20k years. So once on the planet you'd get to watch all 20k years of broadcasts.

If you traveled at 1/2 the speed of light (not focusing on your dilation for the moment), then you'd still beat 50% of the transmissions and have 10k earth years of broadcasts to watch.

I think the question is what % of the speed of light is a gamma factor of 20k/10 = 2000. That's something like 99.9999999% of the speed of light.

Meaning you would get there before 99.99999% of the broadcasts had arrived and you'd be able to watch just about all of them in real time over the next (just less than) 20k years.

Assuming those two planets are roughly in the same reference frame, the only way 20k years could pass on earth w.r.t. your arrival is if the destination planet is 20k light years away.

If the destination planet is moving relativistically away from earth at an appreciable percentage of the speed of light then I couldn't say what the math would be. Maybe still the same, maybe not.

Are you planning to decelerate by crashing into the planet at relativistic speed?

While traveling near c, I believe you'd be receiving Earth transmissions from approximately the same time you left, Earth clocks would appear to slow down to a crawl. But if you land on another planet and change reference frame to be more "stationary" again relative to Earth, I think a massive amount of Earth time would suddenly appear to have elapsed very quickly during the process of slowing/landing because of the huge shift in simultaneity.

Pretty sure only those 10 ship years worth of transmissions would suddenly appear. You're still 20K light years away so the rest of those years' transmissions are still in flight.

Hmm I suppose you're right. I guess you'd see very few transmissions during the journey due to time dilation, and then those 10 years would arrive relatively quickly when slowing/stopping?

I was thinking of this being the first half of the twin paradox, but perhaps for the apparent "time gap" in the spacetime diagram to appear, it's necessary for the traveling twin to turn and head back towards Earth quickly to shift simultaneity plane back in the other direction.

There is no payback day in time dilation.

In the book series The Expanse, that was pretty much the only unrealistic technology that the humans had, which allowed them to colonize the solar system. Sure, there was some might-as-well-be-magic alien stuff later in the book, but by page 1, humans have colonized the solar system, purely with a hand-wavy engine that can generate sustained thrust without needing tons of reaction mass and without turning your exhaust port into an unimaginable hell cannon.

No, the exhaust port definitely was an unimaginable hell cannon. The characters spoke several times about slagging a ship or a surface base in their drive plume.

Oh, I missed that! Seems like those drives ought to have been more carefully regulated then.

Cool detail about that in the Nauvoo launch sequence, slagging the scaffolds / gantries / whatever the construction exoskeleton is.

https://youtu.be/0KZRFIdbJzc?t=147

I'm going to embarrass myself horribly here, but I've only seen the series, so I'm not a hundred percent clear on how much acceleration they're pulling in cruise[1]. If it's something like .3 g then they're fairly close[2] to the limits of something like a fusion pulse drive (tiny nodules of deuterium or whatever, ejected out the back of the ship, then lasered into fusing, then the reaction pushes on the whatever - magnetic field, pusher planet, etc). That would make the solar system something more like what the Atlantic Ocean was in the days of sail.

They'd need refuels every stop though. Like with early coal fired steamers back in the day.

I got a funny feeling that without much better genetic muckety muck, the hard limit will be the human organism itself. Someone will get to the stars = if we don't screw everything up - but the someone won't be human, or maybe nothing like human.

[1] I know in emergencies they pull mad g, but it doesn't seem like something they keep up for long. Would still burn through their deuterium in no time though.

[2] Like, within an order of magnitude or two.

The ships cruise anywhere from 0.3 to 1G, the lower end mainly for comfort of people who grew up in space. But, the fictional drive is so efficient it could sustain 10s of G for extended periods (weeks, months).

Didn’t the guy who invented the drive essentially squish himself flat (too many G’s to lift his hand and reach the ‘off’ button, anyway) with his first prototype being efficient enough that it was still going during the story?

There's a scene about the discovery: He disables voice control (or switches it to Chinese) so at high-G he cannot hit the stop button, nor command it to stop.

Eh, there is a ton of unrealistic technology in book 1, the stealth ship for instance is impossible. By the authors own words they don't think of the books as being Hard Sci-Fi, harder then Star Trek sure.

What makes you think the stealth ship is impossible? They explain the concept pretty well in the first book: extensive cooling systems to soak up waste heat, large liquid tanks to store that heat internally for a while, and radar-absorbing coatings. We literally have all of those pieces today, it’d just cost unfathomable amounts of money to build with our current technology.

As a bonus, they even talk about the radar-absorbing coating not being perfect, and being able to detect the stealth ship as an object a few Kelvin above the background radiation when they pumped their radar into it.

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 coast until they reach their destination).

The main characters also manage to find a stealth ship after coming across coordinates to it - it was parked and completely offline (and unmanned) in the orbit of a small asteroid, which would further obscure any signature it might give off.

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.

That's basically the plot of the Forever War. When you drop out of hyper drive (or whatever they called it) you never know if you are going to be battling a ship from your future or your past. The very society that you fight for is completely alien. It's incredibly alienating.

Have you read Tao Zero?

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

Classic hard sci-fi. A colony ship’s Bussard ramjet has a glitch, and it cannot stop accelerating—and basically forever, given time dilation.

> I imagine a sort of neo-Polynesian culture

This makes me think of the Quen Ho in Vernoe Vinge's "Zones of Thought" universe.

What if you didn't carry the fuel? Use stationary lasers from your starting point to power your journey half way until you lasers from your destination to slow your craft down.

I think the perceived output from stationary lasers on Earth would appear to decrease as your speed increases, since you're accelerating away from the laser photon packets. Because of that behavior, I'm not sure it's a feasible way to get up to relativistic speed?

I think the lasers would also need ~10,000 years of fuel, as opposed to ~10 years of fuel if you could somehow figure out how to carry it. But the energy to sustain in either case is probably measured in Dyson spheres, or some fraction of all energy in the universe pretty quickly too.

Time dilation also has an effect, since you're receiving 10000 Earth-years of laser energy over 10 ship-years.

That's the drive of the eponymous generation ship in Kim Stanley Robinson's Aurora. It sends a mirror ahead of itself, then the Sol-Mercury lasers bounce off the mirror, back on the ship to slow it down.

I could see the light spacecraft with their matter-antimatter engines laying down "lightways", like railroads in the old west. Once a lightway is established, then it's much less of a big deal to get from system to system. Apart from the tens of thousands of years in subjective time, of course.

Of all the dangers associated with spaceflight it's a pretty interesting concept to add "lasers at the origin shut off for whatever reason" and "lasers at the destination never turn on / are misaligned / turn on too soon / turn on too late" to the list. Not sure I'd want to be in a spacecraft careening through the cosmos with no realistic way to slow it down.

There is also the question of how you get the lasers to your destination in the first place.

Send a mirror ahead, bounce the beam back toward the ship to push it backwards. Loses a lot of efficiency, though. Laser propulsion, a lot of the thrust is coming from ablation of the surface.

How do you 'send a mirror ahead'? Do you want it to stop, or just keep flying away indefinitely?

Yep, throwaway mirror, it goes off to wherever once it's done bouncing light back at your ship. Again, much less efficient, so when you get where you want to go, you'd want to put in a laser station there too.

Use lasers for the first half of the journey to send a rocket that can decelerate and build the laser station at the destination.

The farther you get, the dimmer the laser gets. The faster you get, the less energy each photon has.

Note that the 2nd statement only applies because of the Doppler effect - energy of the photon is a function of its frequency.

> 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.

What do you think about the gravity well "drive" in three body problem trilogy? I thought it was fairly clever, especially given the stories describing it beforehand (maelstrom iirc?)

Can you explain why matter-antimatter only has 60% efficiency?

> This is the primary reason for selecting the annihilation of a proton (p+) and antiproton (p–-); the products include neutral and charged pions (πo, π+, π-), and the charged pions can be trapped and directed by magnetic fields to produce thrust. However, the pions produced in the annihilation reaction do possess (rest) mass (about 22% of the initial protonantiproton annihilation pair rest mass for charged pions, 14% for the neutral pions), so not all of the protonantiproton mass is converted into energy. This results in an energy density of the proton-antiproton reaction of "only" 64% of the ideal limit, or 5.8x1016 J/kg.

I love that someone wrote and published a serious paper where the end design involved first stage thrust of 550 million ft-lbs and the total ship weighs something like 17 million metric tons and if I'm reading page 26 correctly, the payload on the ship has to be 7,500 kilometers from the ignition source to survive.

https://www.relativitycalculator.com/images/relativistic_pho...

> Can you explain why matter-antimatter only has 60% efficiency?

We don’t know how to use all the reaction products for thrust [1].

[1] https://en.m.wikipedia.org/wiki/Electron–positron_annihilati...

While there may be a 100% anilation with matter-antimatter reactions the resulting thrust is not perfectly efficient due to energy consumed as the rest mass of charged and uncharged pions, energy consumed as the kinetic energy of the uncharged pions (which can't be deflected for thrust); and energy consumed as neutrinos and gamma rays.

I'm unsure if the resulting efficienty is 60% however, not all of the energy produced in the anilation is converted to "stuff" that is useful for thrust.

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