Mine? Yes. And maybe you’ve wondered just how improbable your life is. But that’s not really the question today…
Here at the Centre for Quantum Technologies, Dagomir Kaszlikowski asked me to give a talk on this paper:
• John Baez, Is life improbable?, Foundations of Physics 19 (1989), 91-95.
This was the second paper I wrote, right after my undergraduate thesis. Nobody ever seemed to care about it, so it’s strange—but nice—to finally be giving a talk on it.
My paper does not try to settle the question its title asks. Rather, it tries to refute the argument here:
• Eugene P. Wigner, The probability of the existence of a self-reproducing unit, Symmetries and Reflections, Indiana University Press, Bloomington, 1967, pp. 200-208.
According Wigner, his argument
purports to show that, according to standard quantum mechanical theory, the probability is zero for the existence of self-reproducing states, i.e., organisms.
Given how famous Eugene Wigner is (he won a Nobel prize, after all) and how earth-shattering his result would be if true, it’s surprising how little criticism his paper has received. David Bohm mentioned it approvingly in 1969. In 1974 Hubert Yockey cited it saying
for all physics has to offer, life should never have appeared and if it ever did it would soon die out.
As you’d expect, there are some websites mentioning Wigner’s argument as evidence that some supernatural phenomenon is required to keep life going. Wigner himself believed it was impossible to formulate quantum theory in a fully consistent way without referring to consciousness. Since I don’t believe either of these claims, I think it’s good to understand the flaw in Wigner’s argument.
So, let me start by explaining his argument. Very roughly, it purports to show that if there are many more ways a chunk of matter can be ‘dead’ than ‘living’, the chance is zero that we can choose some definition of ‘living’ and a suitable ‘nutrient’ state such that every ‘living’ chunk of matter can interact with this ‘nutrient’ state to produce two ‘living’ chunks.
In making this precise, Wigner considers more than just two chunks of matter: he also allows there to be an ‘environment’. So, he considers a quantum system made of three parts, and described by a Hilbert space
Here the first corresponds to a chunk of matter. The second corresponds to another chunk of matter. The space corresponds to the ‘environment’. Suppose we wait for a certain amount of time and see what the system does; this will be described by some unitary operator
Wigner asks: if we pick this operator in a random way, what’s the probability that there’s some -dimensional subspace of ‘living organism’ states in , and some ‘nutrient plus environment’ state in , such that the time evolution sends any living organism together with the nutrient plus environment to two living organisms and some state of the environment?
A bit more precisely: suppose we pick in a random way. Then what’s the probability that there exists an -dimensional subspace
and a state
such that maps every vector in to a vector in ? Here means the 1-dimensional subspace spanned by the vector .
And his answer is: if
then this probability is zero.
You may need to reread the last few paragraphs a couple times to understand Wigner’s question, and his answer. In case you’re still confused, I should say that is what I’m calling the space of ‘living organism’ states of our chunk of matter, while is the ‘nutrient plus environment’ state.
Now, Wigner did not give a rigorous proof of his claim, nor did he say exactly what he meant by ‘probability’: he didn’t specify a probability measure on the space of unitary operators on . But if we use the obvious choice (called ‘normalized Haar measure’) his argument can most likely be turned into a proof.
So, I don’t want to argue with his math. I want to argue with his interpretation of the math. He concludes that
the chances are nil for the existence of a set of ‘living’ states for which one can find a nutrient of such nature that interaction always leads to multiplication.
The problem is that he fixed the decomposition of the Hilbert space as a tensor product
before choosing the time evolution operator . There is no good reason to do that. It only makes sense split up a physical into parts this way after we have some idea of what the dynamics is. An abstract Hilbert space doesn’t come with a favored decomposition as a tensor product into three parts!
If we let ourselves pick this decomposition after picking the operator , the story changes completely. My paper shows:
Theorem 1. Let , and be finite-dimensional Hilbert spaces with . Suppose is any unitary operator, suppose is any subspace of , and suppose is any unit vector in Then there is a unitary isomorphism
such that if we identify with using , the operator maps into .
In other words, if we allow ourselves to pick the decomposition after picking , we can always find a ‘living organism’ subspace of any dimension we like, together with a ‘nutrient plus environment’ state that allows our living organism to reproduce.
However, if you look at the proof in my paper, you’ll see it’s based on a kind of cheap trick (as I forthrightly admit). Namely, I pick the ‘nutrient plus environment’ state to lie in , so the nutrient actually consists of another organism!
This goes to show that you have to be very careful about theorems like this. To prove that life is improbable, you need to find some necessary conditions for what counts as life, and show that these are improbable (in some sense, and of course it matters a lot what that sense is). Refuting such an argument does not prove that life is probable: for that you need some sufficient conditions for what counts as life. And either way, if you prove a theorem using a ‘cheap trick’, it probably hasn’t gotten to grips with the real issues.
I also show that as the dimension of approaches infinity, the probability approaches 1 that we can get reproduction with a 1-dimensional ‘living organism’ subspace and a ‘nutrient plus environment’ state that lies in orthogonal complement of . In other words, the ‘nutrient’ is not just another organism sitting there all ready to go!
Theorem 2. Let , and be finite-dimensional Hilbert spaces with . Let be the set of unitary operators with the following property: there’s a unit vector , a unit vector , and a unitary isomorphism
such that if we identify with using , the operator maps into . Then the normalized Haar measure of approaches 1 as .
Here is the orthogonal complement of ; that is, the space of all vectors perpendicular to .
I won’t include the proofs of these theorems, since you can see them in my paper.
Just to be clear: I certainly don’t think these theorems prove that life is probable! You can’t have theorems without definitions, and I think that coming up with a good general definition of ‘life’, or even supposedly simpler concepts like ‘entity’ and ‘reproduction’, is extremely tough. The formalism discussed here is oversimplified for dozens of reasons, a few of which are listed at the end of my paper. So far we’re only in the first fumbling stages of addressing some very hard questions.
All my theorems do is point out that Wigner’s argument has a major flaw: he’s choosing a way to divide the world into chunks of matter and the environment before choosing his laws of physics. This doesn’t make much sense, and reversing the order dramatically changes the conclusions.
By the way: I just started looking for post-1989 discussions of Wigner’s paper. So far I haven’t found any interesting ones. Here’s a more recent paper that’s somewhat related, which doesn’t mention Wigner’s work:
• Indranil Chakrabarty and Prashant, Non existence of quantum mechanical self replicating machine, 2005.
The considerations here seem more closely related to the Wooters–Zurek no-cloning theorem.