[pja]

So you're suggesting two almost completely undeveloped technologies (light sails and extremely high-powered lasers in space) and telling us that interstellar propulsion is a solved problem? That's a tad optimistic :)

Btw, if you'd read 'Accelerando' you'd know that Charlie is fully aware of the possibilities offered by laser powered light sails.

[adastra]

Stross and cletus both say we will never leave the solar system and travel to another star. Never!

I'm trying to show that notion is dead wrong. I didn't say it was a solved problem, if you mean in the sense of the engineering is basically done and we're launching something tomorrow. But certainly it is very reasonable to think that within with 150 years we will have both the technological and economic advancement to do a mission to another star. We know how we'd do it, and it uses real physics and engineering, materials, etc. that already exist.

The main reason Stross is wrong though, is he says we'd need free energy to do it. He's right that the limiting factor is the cost of energy, but he's overlooking what you get for 150 years of economic growth. World GDP has grown at an average annual rate of 3.5% over the last 100 years. World GDP is currently at about $80 trillion. If it grows at 3.5% for another 150 years, it will be $14,000 trillion. You can buy a lot of energy with that. And a lot of spaceships.

[pja]

50 years ago it was very reasonable to think that we'd have worked out how to make a fusion reactor too.

Just because something uses real physics and engineering materials that already exist, doesn't mean that it's actually possible in the real world & getting to the stars requires that we solve three or four very hard problems.

Impossible? Of course not. Much, much harder than the 'whee, we're all going to space!' crowd likes to think? I'm afraid so.

NB. GDP is not a good proxy for available energy for hopefully obvious reasons: Just because you have a high GDP doesn't mean that you have a lot of energy available, it may mean that you use the energy you have very effectively.

[lmm]

>50 years ago it was very reasonable to think that we'd have worked out how to make a fusion reactor too.

Most of the reason we haven't is economic; modern fission reactor designs are far more efficient than even the most optimistic estimates of 50 years ago.

[pja]

Most of the reason we haven't is because it turns out to be really, really hard to contain a high energy plasma long enough to get useful levels of fusion out of it. We still haven't effectively solved the 'how do we efficiently extract the generated energy' problem either.

Perhaps these problems will be solved in the future, but so far we've spent billions and billions of $ with (relatively) little to show for it in terms of output.

[sesqu]

  World GDP has grown at an average annual rate of 3.5% over the last 100 years.

That's not actually very relevant. According to DeLong[1], the rate of growth has been anything but constant over the last 100 years, nor has it always been positive. You may well expect the industrial revolution to carry us forward for another 150 years, but I'm not sure if I do.

[1] http://img641.imageshack.us/img641/1946/gwp.png

edit: corrected the 2000-2011 segment for inflation with CPI data (not int$).