buckley@reed.UUCP (Ken Buckley) (08/04/90)
To all those who responded to my posting ("Can unrelated twins exist" on
sci.bio), thanks.
The reason I posted was a pair of ideas in science fiction stories I
read recently: one in a P.J. Farmer *Riverworld* story and one in Asimov's
*Prelude to Foundation*; the answers to my posting allow a bit of fanciful
speculation.
The Farmer *Riverworld* series takes place on a planet where some being
has reincarnated all the people who have ever lived on earth -- about
10 trillion, by Farmer's estimate. The story in question concerns Tom Mix's
adventures on this world, together with a Jew from Jesus's day and an
Egyptian woman (I think.) Almost as a just-for-fun afterthought, Farmer
throws in the fact that Tom Mix and his Jewish companion look precisely
identical -- this is ascribed to random genetic happenstance. In a population
of only ten trillion, this seemed quite unlikely to me, but I chalked it
up to Farmer's strange imagination.
Then I read *Prelude to Foundation*. Not having read any of the *Empire*
series, I was struck by the huge size of Asimov's universe: 25,000,000
_worlds_. The largest, Trantor, has 40 billion people; a small backwater
world has around 500 million. Taking an average world to have around
1 billion people, Asimov has conjured up _25x10^15_ -- yes, that's
quintillion -- people. Now here, it seemed, we had a sufficient population
for the implausible Farmer plot twist to occur. I am not a biologist, so
I posted my question to sci.bio to see if my hunch was right.
To my intense surprise, the answer is a definitive no! Using some of the
responses I got, a friend and I made the following conservative(!)
estimate: approximately 10^166 humans would be needed to make this an event
with even a remote chance of occurring.
Thus, it seems that Hari Seldon is safe from meeting his twin, while Farmer
is as usual off in space. Another interesting (?!) question poses itself,
though. Asimov's empire spans only our own galaxy. Assuming one Earth-type
planet per star, are there enough stars in the galaxy to support
10^166 humans? And can anyone write a science fiction story set in such a
universe? :^)
--Ken Buckleyrim@csadfa.cs.adfa.oz.au (Bob McKay) (08/06/90)
From article <15289@reed.UUCP>, by buckley@reed.UUCP (Ken Buckley): > To my intense surprise, the answer is a definitive no! Using some of the > responses I got, a friend and I made the following conservative(!) > estimate: approximately 10^166 humans would be needed to make this an event (unrelated twins arising by chance) > with even a remote chance of occurring. Could we have more details of the computations involved? The reason I'm asking is that I'm guessing you needed to assume independence of distribution of individual genes - presumably on the basis that unrelated humans would have independently distributed genes. But when we are talking about such astronomical numbers, it's probably not a valid assumption: for example, there may be synergistic effects on the evolutionary fitness arising from particular combinations of genes. Normally, we can afford to ignore these because the effects are likely to be small. But when we're dealing with numbers like 10^166, all bets are off. Bob McKay Phone: +61 6 268 8169 fax: +61 6 268 8581 Dept. Computer Science ACSNET,CSNET: rim@csadfa.cs.adfa.oz Aust. Defence Force Academy UUCP: ...!uunet!munnari!csadfa.cs.adfa.oz!rim Canberra ACT 2600 AUSTRALIA ARPA: rim%csadfa.cs.adfa.oz@uunet.uu.net
sbishop@desire.wright.edu (08/06/90)
Concerning the question about identical twins, I have a first cousin who is my physical double. We look exactly alike, allowing for the fact that she is about ten years older than I. We even have the same voice and mannerisms, even though we haven't seen each other in nearly twenty years. (She lives on the other side of the country.) Evidently we look so much alike that her sister (whom I saw a couple of years ago) is so uncomfortable about it that she wouldn't even stay in the same room with me. Which I thought was pretty silly but she seemed to think it was too wierd to stand. My own sister looks NOTHING like me..... My cousin's father is my mother's brother. And there is about 14 years difference in their age.
shaw@iti.org (Howard Shaw) (08/06/90)
In article <15289@reed.UUCP> buckley@reed.UUCP (Ken Buckley) writes: >series, I was struck by the huge size of Asimov's universe: 25,000,000 >_worlds_. The largest, Trantor, has 40 billion people; a small backwater >planet per star, are there enough stars in the galaxy to support >10^166 humans? And can anyone write a science fiction story set in such a The usual estimate of the number of elementary particles in this universe is 10^80, so the answer is "no". Maybe in Asimov's universe....
urjlew@uncecs.edu (Rostyk Lewyckyj) (08/07/90)
Perhaps a significant proportion of the chromosome and gene rearrangements which you considered as possible are not in fact possible thus the probability of a doppelganger may be significantly higher than the 1 in 10^166 which you naively calculate. Also you can increase the number of people available for matching by some small factor, since there is no reason to restrict the doppelganager to be alive at the same time. At the very least you should allow the case where their lifetimes only intersect for a moment. Of course in the Riverworld everything that ever lived is supposed to be alive concurrently. Happy theorizing/fantacizing :-)