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The big issue is energy density, IMHO. Joel Send a noteboard - 30/09/2011 02:53:39 AM
And, yes, this thread is eerily reminiscent of when Isaac and I threadjacked Camillas posting of the article suggesting advanced ETs might "punish" us for Global Warming (frankly, that sounds like someone who watched the remake of "The Day the Earth Stood Still" too many times. :P)

Anyway, like I say, energy density seems like the biggest problem, in terms of mass as well as volume. As Isaac pointed out, burning the Amazon would provide enough energy to get Apollo 11s Lunar Lander to the Moon many times over, and whether it would give an object enough energy to escape the suns gravity well is as much a function of the objects mass as the energy available. The real questions are twofold:

1) Can the more than adequate energy be supplied at a great enough RATE to overcome the diminishing but constant pull of planetary and stellar gravity and

2) Is the energy source itself so massive that IT makes the answer to the first question "no"?

The Apollo vessel itself weighed a little over 43 tons (including fuel for use to and from the lunar service and back to Earth.) The Saturn V that launched it weighed about that much more than 3000 tons, and was near the limit of its lunar capacity of a 45 ton payload. It took that much just to get to the Moon and back, which is not even (technically) out of the Earths gravity well (else the Moon would not stay in orbit.) All 3039 tons of it was burned in just under 19 minutes. Wood could also do it--but it would take ~11,000 tons of wood burned in that same 19 minutes and focused in one direction on an object about the size of a bus, preferably without disintegrating it and any passengers. Just to get to the Moon, not Mars, Jupiter, Centauri Prime or Gliese 581g.

It gets a little complicated because the Saturn Vs first stage burned highly refined kerosene while the later ones burned liquid hydrogen. The reason is the problem I alluded to earlier: Liquid hydrogen has a much lower density than other conventional fuels, despite having a much higher energy density per unit mass. At low altitudes, the kerosene is more efficient, even though it has less energy per unit mass, because it takes up less space (among other reasons; it is also more stable and requires less energy to pressurize and cool.)

The problem is not getting the energy from the fuel, but doing so quickly enough to launch the craft and without the sheer mass and size of the fuel itself making any launch impossible. That is the kind of problem that led us to fossil fuels at the dawn of industrialization in the first place. As Isaac notes, a steam engine can be run as easily with wood as with anything else (the first ones did, after all.) Make it large enough and magnetic, then wrap a coil of wire around it, and it generates electricity. Unfortunately, it is impractical to run a large city power grid that way, so we generally use more compact energy sources like coal, oil and nuclear, though even garbage (which has less than half woods energy density) is used to a limited degree in some places. As Isaac noted in our earlier discussion, cars can run on wood, if one does not mind filling their back seat and trunk with logs.

The irony here is that not only does space travel require more compact energy sources like fossil fuels, the industrial era gave humanity the incentive to discover and develop them as well as the means to do so. There are others as good as and better than fossil fuels; fission, fusion and anti-matter come to mind, in ascending order of energy density. Yet the fact humanity has only harnessed the least energetic one of those testifies to how critical industrialization is for that purpose. Advanced extra-terrestrials do not need fossil fuels for interstellar travel, but DO need an equivalent to fuel the industrial and technological development that opens the door to interstellar travel. If an equivalent can be found and harnessed fossil fuels become irrelevant, but that seems likely to be a very big "if."
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Link to our long discussion of whether interstellar travel REQUIRES fossil fuels.
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An interesting thought I had that may be worth discussion. - 29/09/2011 07:22:01 PM 741 Views
Well, Fermi Paradox discussions usually end with ???? - 29/09/2011 07:51:59 PM 418 Views
you do have to hope there's intelligent life somewhere out there - 29/09/2011 07:56:48 PM 474 Views
I definitely agree with the first part. - 29/09/2011 08:32:33 PM 569 Views
why do you think only those options would be available? - 29/09/2011 08:35:53 PM 421 Views
Could be cultural domination. - 29/09/2011 09:32:18 PM 438 Views
*shrug* Humans aren't that bad - 29/09/2011 09:51:32 PM 407 Views
Oh yes we are. - 29/09/2011 10:27:52 PM 431 Views
Unless their advancement is so great they are VERY ethically enlightened, her fears seem valid. - 30/09/2011 03:04:40 AM 486 Views
hmmm - 30/09/2011 02:34:47 PM 415 Views
"And hey. I'll eat most anything once." - 30/09/2011 07:22:01 PM 396 Views
I think I've met a few aliens... - 30/09/2011 01:31:55 AM 428 Views
Why do fossil fuels have to be involved? - 29/09/2011 08:33:26 PM 400 Views
Going into the fossil fuel vs wood thing - 29/09/2011 08:51:07 PM 497 Views
The big issue is energy density, IMHO. - 30/09/2011 02:53:39 AM 508 Views
That's so perfect. - 30/09/2011 06:43:31 AM 446 Views
We really have no idea how rare advance technologies societies are - 30/09/2011 02:00:56 PM 546 Views
That's a really good point. - 30/09/2011 04:34:28 PM 458 Views
The doomsdays options don't really hold up well though - 30/09/2011 05:22:57 PM 475 Views
I think we have different interpretations of "silence" - 30/09/2011 07:47:32 PM 577 Views
It kind of comes down to whether FTL is possible - 30/09/2011 09:12:32 PM 540 Views
hmmm - 30/09/2011 07:51:42 PM 332 Views
Yeah but that's just a variant of "Highly advanced aliens who for some reason are totally stupid" - 30/09/2011 10:21:34 PM 553 Views
I didn't mean to imply that were not talking to us because they looked down on us - 01/10/2011 01:07:02 AM 499 Views
Re: I didn't mean to imply that were not talking to us because they looked down on us - 01/10/2011 02:50:42 AM 495 Views
thanks for the detailed answer - 01/10/2011 03:08:48 PM 485 Views
No prob, hopefully it's not all inaccurate nonsense - 01/10/2011 03:18:05 PM 521 Views
not to mention that the universe is HUGE - 30/09/2011 07:53:33 PM 411 Views
True - 30/09/2011 10:37:58 PM 374 Views
of course I can't say, that was my point - 30/09/2011 07:42:55 PM 424 Views
I was totally agreeing with you until I wasn't. - 30/09/2011 08:09:08 PM 558 Views
That's pessimistic, though. The "blaze of glory" is ongoing. - 01/10/2011 04:25:28 AM 401 Views
another thing for consideration - 01/10/2011 11:55:12 AM 409 Views

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