When I discussed the idea of Alien AI I met two points of view:
1) AI is impossible.
2) EY: AI will travel with almost the speed of light
I think it worth consider the intermediate point of view, that is 0.5 с as the total speed of colonization.
Reasons:
Colonization speed is lower than the speed of travel of the von Neumann Probes, as they need some time to replicate.
It is lower than cruise speed of the probe, as it needs some time to accelerate and decelerate. Declaration of the high-speed probe is especially difficult.
Closer to с speeds require much more energy because of the special relativity, so energy investments will produce diminishing returns in the speed of colonization.
If a civilization is remote, the universe acceleration will also work against it perceived speed of colonization.
High speed creates a lot of problems with space dust, which require heavier protection shields.
So, 0.5-0.7с speed are a realistic estimate for the maximum speed of space colonization.
And though it mentions 0.5c for fission engines, most “realistic” probe designs will be much faster, because you can use more exotic things like bussard ramjets to decelerate (using them to accelerate, the way they were originally designed, is harder that it seems, but they are perfect to decelerate). Also Eric Drexlers’s been working on some interesting dust shielding designs. So the paper is very much “conservative” in its assumptions.
It is possible that total colonization wave speed is much higher, closer to с, but I suggest just to put some weight to the hypothesis that is around 0.5-0.7c because of some unknown tradeoffs. I would estimate such outcome as 30 per cent. Only in that case observing alien astroengineering and SETI is possible.
Do you have a nice reference (speculative feasibility study) for non-rigid coil-guns for acceleration?
Obvious idea would be to have a swarm of satellites with a coil, spread out over the solar system. Outgoing probe would pass through a series of such coils, each adding some impulse to the probe (and doing minor course corrections). Obviously needs very finely tuned trajectory.
Advantage over rigid coil-gun: acceleration spread out (unevenly) over longer length (almost entire solar system). This is good for heat dissipation (no coupling is perfect), and maintaining mega-scale rigid objects appears difficult. Satellites can take their time to regain position (solar sail / solar powered ion thruster / gravity assist). Does not help with g-forces.
Disadvantage: Need a large number of satellites in order to get enough launch windows. But if we are talking dyson swarm anyway, this does not matter.
How much do we gain compared to laser acceleration? Main question is probably: How does the required amount of heat dissipation compare?
I have not seen any papers about it, but did look around a bit while writing the paper.
However, a colleague and me analysed laser acceleration and it looks even better. Especially since one can do non-rigid lens systems to enable longer boosting. We developed the idea a fair bit but have not written it up yet.
Do you have a non-paywalled link, for posterity? I use sci-hub, but paywalls are a disgrace to science.
Also, do you have a nice reference for the bussard ramjet/ramscoop deceleration?
Obvious advantage: A priori you don’t need nuclear fusion at all. You use a big em-field for cross-section and use, ultimately, drag against the interstellar medium for both deceleration and energy generation. No deceleration needed in (thinner) intergalactic medium. Entropy gain should be large enough to run mighty heat-pumps (for maintaining high field superconductors and radiating excess heat). No need to carry fuel or manage fusion; your kinetic energy at relativistic speeds has almost as much energy as antimatter. Antimatter sucks because production, containment, and difficulty of not frying yourself with the resulting radiation (light probe cannot shield against gamma), and probably a couple more reasons.
Disadvantage: not obvious whether this works. I would appreciate an actual engineer doing the computation. (I am just a mathematician, and have not seen a study of this deceleration design because I suck at searching the literature)
Probably at least three problems:
(1) How much impulse at what speeds? Determined by cross-section of collecting EM-field over required mass of collector.
(2) Might be good for decelerating from 0.9c to 0.05c over maybe 10k years (pulling numbers out of my ass). Would still need secondary system for the remaining deceleration, until slow enough for gravity assists. Could collect propellant over the long deceleration, but then would need to dissipate a shitload of heat; unclear whether net gain.
(3) Heat dissipation.
I agree that deceleration is the thing to care about; beat the rocket equation on deceleration by clever designs using the interstellar medium, and on acceleration by big machines.
Question: I suppose it’s implicitly assumed that FTL travel is physically impossible even for extremely advanced civilizations, are there reasons to feel relatively secure in our belief that aliens will not have discovered some sort of exotic or unknown physics that allows them to achieve that?
Very interesting! But I feel that this is very improbable:
When I discussed the idea of Alien AI I met two points of view:
1) AI is impossible. 2) EY: AI will travel with almost the speed of light
I think it worth consider the intermediate point of view, that is 0.5 с as the total speed of colonization. Reasons:
Colonization speed is lower than the speed of travel of the von Neumann Probes, as they need some time to replicate.
It is lower than cruise speed of the probe, as it needs some time to accelerate and decelerate. Declaration of the high-speed probe is especially difficult.
Closer to с speeds require much more energy because of the special relativity, so energy investments will produce diminishing returns in the speed of colonization.
If a civilization is remote, the universe acceleration will also work against it perceived speed of colonization.
High speed creates a lot of problems with space dust, which require heavier protection shields.
So, 0.5-0.7с speed are a realistic estimate for the maximum speed of space colonization.
I have a paper on that :-)
http://www.sciencedirect.com/science/article/pii/S0094576513001148
And though it mentions 0.5c for fission engines, most “realistic” probe designs will be much faster, because you can use more exotic things like bussard ramjets to decelerate (using them to accelerate, the way they were originally designed, is harder that it seems, but they are perfect to decelerate). Also Eric Drexlers’s been working on some interesting dust shielding designs. So the paper is very much “conservative” in its assumptions.
Thanks for interesting link.
It is possible that total colonization wave speed is much higher, closer to с, but I suggest just to put some weight to the hypothesis that is around 0.5-0.7c because of some unknown tradeoffs. I would estimate such outcome as 30 per cent. Only in that case observing alien astroengineering and SETI is possible.
Second question:
Do you have a nice reference (speculative feasibility study) for non-rigid coil-guns for acceleration?
Obvious idea would be to have a swarm of satellites with a coil, spread out over the solar system. Outgoing probe would pass through a series of such coils, each adding some impulse to the probe (and doing minor course corrections). Obviously needs very finely tuned trajectory.
Advantage over rigid coil-gun: acceleration spread out (unevenly) over longer length (almost entire solar system). This is good for heat dissipation (no coupling is perfect), and maintaining mega-scale rigid objects appears difficult. Satellites can take their time to regain position (solar sail / solar powered ion thruster / gravity assist). Does not help with g-forces.
Disadvantage: Need a large number of satellites in order to get enough launch windows. But if we are talking dyson swarm anyway, this does not matter.
How much do we gain compared to laser acceleration? Main question is probably: How does the required amount of heat dissipation compare?
I have not seen any papers about it, but did look around a bit while writing the paper.
However, a colleague and me analysed laser acceleration and it looks even better. Especially since one can do non-rigid lens systems to enable longer boosting. We developed the idea a fair bit but have not written it up yet.
I would suspect laser is the way to go.
Interesting idea. No, I don’t have any references, sorry!
Do you have a non-paywalled link, for posterity? I use sci-hub, but paywalls are a disgrace to science.
Also, do you have a nice reference for the bussard ramjet/ramscoop deceleration?
Obvious advantage: A priori you don’t need nuclear fusion at all. You use a big em-field for cross-section and use, ultimately, drag against the interstellar medium for both deceleration and energy generation. No deceleration needed in (thinner) intergalactic medium. Entropy gain should be large enough to run mighty heat-pumps (for maintaining high field superconductors and radiating excess heat). No need to carry fuel or manage fusion; your kinetic energy at relativistic speeds has almost as much energy as antimatter. Antimatter sucks because production, containment, and difficulty of not frying yourself with the resulting radiation (light probe cannot shield against gamma), and probably a couple more reasons.
Disadvantage: not obvious whether this works. I would appreciate an actual engineer doing the computation. (I am just a mathematician, and have not seen a study of this deceleration design because I suck at searching the literature)
Probably at least three problems:
(1) How much impulse at what speeds? Determined by cross-section of collecting EM-field over required mass of collector.
(2) Might be good for decelerating from 0.9c to 0.05c over maybe 10k years (pulling numbers out of my ass). Would still need secondary system for the remaining deceleration, until slow enough for gravity assists. Could collect propellant over the long deceleration, but then would need to dissipate a shitload of heat; unclear whether net gain.
(3) Heat dissipation.
I agree that deceleration is the thing to care about; beat the rocket equation on deceleration by clever designs using the interstellar medium, and on acceleration by big machines.
Use Google Scholar to find fulltexts like https://pdfs.semanticscholar.org/847d/8dabb12f67124868af0876c77538e4fd1c60.pdf
Question: I suppose it’s implicitly assumed that FTL travel is physically impossible even for extremely advanced civilizations, are there reasons to feel relatively secure in our belief that aliens will not have discovered some sort of exotic or unknown physics that allows them to achieve that?
Yes, I assume it. If not true, they must be here.
No.
Exactly. FTL is allowed even by relativistic mechanics (who know what allows quantum gravity?), although time travel paradoxes ensue.