ryan_m_b said:
With regard to your Project Orion claim I refer you to my above post.
IIRC Solar sails are potentially good for either supplementing normal propulsion on a probe or good for very tiny masses. The effect you allude to whereby the sail must receive less energy than it can radiate is why solar sails would be terrible mechanism for transporting anything above a
starwisp and even then there are horrendous engineering difficulties. Quoting from the wiki article you linked the most efficient solar sail design yet comes from Drexler whose thesis states his design would only be 50-80 times better than existing, not a huge difference when you consider that simply enlarging the sail would produce the same effect.
Laser powered sail probes may be good when we want to send a few grams to the next system at both horrific expense and severe danger (a gigawatt laser with interstellar range pointed at your town is enough to ruin anyone's day) but they are not a practical solution to manned space exploration.
No problem
it's a pet peeve of mine. I find it fascinating to discuss space travel more so when we stick to reality than when we start with the proposition "if we had magic technology X". It clashes with my other pet peeve, Ray Kurzweil and his exponential change arguments. It really is disturbing when intelligent people propose that we will have an extraordinary technology by X year and base their reasoning on, what is essentially, Moore's law. It really is staggering.
As for the Fermi paradox it always struck me that there are three conclusions;
1)We lack the capability to detect interstellar civlisations
2)No other tool using species has evolved in our galaxy
3)There is something prohibitive about interstellar travel
Option one is impossible to falsify without discovering a civlisation so we can leave it out, option two is interesting because it points to the rarity of such phenomenon. Option three is the only one we can actually investigate and it really is fascinating. Aside from the huge engineering hurdles to overcome there's also the little matter of the devastating potential of an interstellar war fought with relativistic weapons...
I agree with you about the looney Orion Propulsion idea, although it resulted in a damn good book written by Freeman Dyson's tree-house living, canoe-building son.
Solar sails are a very tough engineering problem, but at least they would get a constant acceleration from a reasonably non-divergent light beam. I think the heat problem would be insignificant as long as it doesn't melt the sail. The heat would radiate out in both directions, so the net momentum changes would cancel out.
What I envision is a space based extremely powerful EM rail gun for initial propulsion, with the solar sails unfolding and used after that. Like you, I don't think it is possible, with today's technology, to use it with anything but very small, unmanned probes.
Enlarging the sail would require more (and longer) tether lines, so there would be diminishing returns from that. If the ship has a supplementary on board propulsion system that would require the tether lines to be non-compressible as well as having good tensile properties.
That's also something that would be required by a "space elevator" cable. You won't get that using carbon nano-tubes as the cable raw material, which seems to be the consensus material of choice by "space elevator" believers.
Your 3 conclusions are all negatives and negatives cannot be proven, but are probably all true, but I would suggest that:
1) We probably can detect an alien civilization, but only if that alien species is purposely trying to contact us by sending a signal they know we will detect.
2) There may be intelligent species in our solar system that don't require tools and/or could be so different from us that we don't even recognize them as living beings.
3) Probably true, but that may only apply to
manned interstellar travel.
There is one more possible way to achieve manned interstellar travel, that I've thought of:
Use spacecraft made of huge chunks of ice that feed water into a nuclear reactor heat exchanger, heating the water to insane temperatures, then letting it blow out of a rocket-like nozzle.
In order to reduce the amount of nuclear fuel you would need for the manned craft, you could first launch many similar, but much smaller unmanned craft with a nuclear propulsion module, giving them an initial velocity using space based EM rail guns.
You have them programed to burn just enough fuel to get to a particular velocity, then shut everything down. The unmanned crafts would have no radiation shielding and just enough electronics to control the reactor and navigation system.
After all the unmanned crafts have launched, launch a manned craft using the same rail guns to get the thing going initially, but unlike the unmanned craft, at accelerations that humans can survive.
Then, the manned craft "catches up" with each unmanned craft one by one, with the unmanned craft accelerating to make the manned craft's velocity. Then the manned vehicle rendezvouses with the unmanned craft and scavenges the remaining fuel and ice from them - or maybe just switch reactor modules.
It would still have to go significantly slower than the speed of light to keep the crew alive.
The crew would have to be in suspended animation for most of the voyage.
There have been some animal studies that suggest you can do that using H2S, IIRC.
I agree with you about Ray Kurzweil, although he seems to be right about a lot of things.
He has been quoted as saying that if he can live 50 more years, he expects aging to be completely eliminated, resulting in nearly limited life-spans.
(We can't get past that "heat death" thing, though, without moving to another universe.)
