archive: SETI Sagan: Re: carlsagan-digest

SETI Sagan: Re: carlsagan-digest

Larry Klaes ( )
Wed, 30 Dec 1998 17:20:51 -0500

>X-Sender: waw@clarke
>Date: Wed, 30 Dec 1998 15:45:25 CST6CDT,3,-1,0,7200,10,-1,0,10800,3600
>X-OldDate: Wed, 30 Dec 1998 13:41:19 -0800 (PST)
>Sender: carlsagan-owner <>
>Reply-To: "Carl Sagan List" <>
>From: Bill Wheaton <>
>To: carlsagan <>
>Subject: Sagan: Re: carlsagan-digest
>Jayson & All,
>> Date: Tue, 29 Dec 1998 08:34:14 -0800
>> From:
>> Subject: Sagan: Relativism and Light Speed
>> But THAT is exactly why it is called a paradox. If it wasn't a
>> then everyone would say,"Oh! Okay! That makes sense! Cool!".
>> Answer me this though. In "Pale Blue Dot" CS writes that
regardless of
>> cost, we currently have the technology to build a ship to come CLOSE to
>> speed of light. Wouldn't/shouldn't this be tried out??? Hey! If you/we can
>> propel something the speed of light why the hell wouldn't you do it? Or
at least
>> attempt it?
>> Then we could see this so-called PARADOX in action!! Bring it on
>> PARADOX! I got your number ... or something ...
>> - Jayson
>I think the answer is in the "regardless of cost" caveat. In 1800 we
>could have built a Moon rocket, based on the black powder rocket
>technology of the day, but (if I recall the quote, from Willey Ley long
>ago) it would been have something like the size of Mt. Everest.
>We are roughly in the same situation w/r to interstellar travel today.
>There is no fundamental physical reason we cannot go at speeds that are a
>significant fraction of the speed of light, but the engineering problems
>are simply staggering, based on currently reasonable technology.
>At present the best bet for high speed travel (IMHO) is an ion drive; the
>first of these ever used for interplanetary propulsion is even now in test
>on the DS1 spacecraft, and working famously. In my opinion it will
>revolutionize the whole planetary exploration business once mission
>designers realize that it is really current technology; especially when
>used in combination with the gravity assist method which has already
>served us well. Just imagine almost any current mission (NEAR, Cassini,
>Mars Climate Orbiter) with an ion drive of the general DS1 type, and you
>will see what I mean.
>The problem for ion drives (or plasma) in general is power supply; because
>the power required is so high, and power supplies are massive, the
>acceleration is low. The DS1 demonstration mission has a max acceleration
>of around 2E-5 g's I believe. In a year that would get you to about 6
>km/sec, which is really quite respectable for an interplanetary mission,
>although DS1 does not actually carry enough Xe for a year's operation.
>For starship buffs, a good thing to remember is that 1 year at 1 g (both
>as measured by the passengers of the ship) gets you to c, the speed of
>light. (Relativistic effects only become important when you are really
>starting to go pretty fast, say more than 0.1c, and of course those
>intervene to prevent you from actually getting to c.)
>So then you look at power supplies, and first you think of nuclear
>fission. If all the engineering problems could be solved -- and I think
>this is reasonable in a time scale of maybe decades, less than a century
>-- based on the fission energy content of U and Pu alone, compared to its
>mass, which is about 0.1% of mc^2, the total rest energy -- a mission
>velocity of 0.05c, or 5% of c, is probably about the reasonable limit.
>That would get you to the nearest star (Alpha Centauri, about 4 lt years
>distant, a widely spaced triple system, with one component very much like
>the Sun) in 80 years; or actually twice that since you really do need to
>stop when you get there, and so can only afford go 2.5%c peak speed.
>Also, it takes some time to build up speed. If the engine and power
>supply can be light enough to get at least 0.001 g acceleration (which
>would certainly be a stretch but may be possible) then the cost in trip
>time is not too bad; if less than about 0.0003 g, the time to get up to
>speed kills you.
>So then you think of fusion, with about 1% of the mc^2 energy (ie, 10X
>more than fission) and that might get you to 15% of c, again assuming some
>kind of "rocket" -- an ion or plasma drive of some sort. Trip times go
>down by a corresponding factor of 3, but the acceleration problem becomes
>more troublesome -- now you have to get 0.003 g or more to really take
>advantage of your higher top speed in "short" trips like to Alpha Cen.
>Well, what about antimatter, everyone always says? Here the problems are
>much worse, because the energy mostly comes out in forms that are either
>very difficult to use (eg, high energy gamma rays) or simply impossible
>(neutrinos) by any means we know of today. And of course on top of that
>you have the problems in making and storing it. To my mind it is going to
>take such huge breakthroughs to solve these problems that it is not even
>very worthwhile to think about it too seriously; I'd put my energy
>elsewhere. Again, like trying to design project Apollo in 1800 -- just
>premature, more effective to understand physics and engineering better
>first. (Of course, anything can happen: if anyone figures out how to
>annihilate antimatter directly into nothing but a well-collimated beam of
>relativistic electrons and positrons, say, then that would change
>Not to be too gloomy, considering time scales of decades and centuries,
>anything can happen in fundamental physics, so it is silly to get too
>wrapped up in design detail. If we just knew a good way to stop, light-
>weight solar sails driven by laser beams from near Earth space can go very
>fast. In 30 years we are fairly likely to have telescopes in space that
>can give us views of earth-like planets, assuming any exist, around the
>nearer stars with the kind of resolution an astronaut has of Earth seen
>from the Moon by the naked eye. We should wait for that before we start
>cutting metal on a starship that will take 100 years to get there. Europa
>has taught us that Jupiter-like objects may have "planets" that could
>harbor life -- maybe you don't have to have a "star". But it is clear by
>now that there are almost surely more "L dwarfs" than all previously known
>stars, and probably more brown dwarfs and "interstellar Jupiter's" as
>well. It is therefore quite likely that there are interesting places to
>visit that are nearer than Alpha Centauri, may much nearer. Even if we
>never learn to go very fast, people have claimed that interstellar
>colonization would proceed at about 1/10 the max travel speed, which means
>the entire Galaxy can be colonized in less than one rotation (200,000,000
>y) even with fission-powered ion drives.
>Finally, progress in DNA, neurophysics, and computer research in
>artificial intelligence may get us to the point in a century where we
>really do not have to transfer matter from star to star at all, only
>information. If that happens, then the whole problem becomes almost
>ridiculously easy -- except we may not be quite human anymore.
>Re Jayson's other question, we actually can and do study "something",
>matter, moving at very close to the speed of light in particle
>accelerators. In a 100 GeV electron accelerator, the speed of the
>electrons is about 99.99999999875% of light speed, if I have counted my
>9's right. We find that the weird extreme relativistic behavior that
>special relativity predicts is confirmed essentially perfectly.
>Happy New Year!
>Bill W.
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