EM - when I said "So a Windows 7 computer found executing on a moon of Neptune would be a sign of life there? You are kidding, surely", I don't think you got the joke.

Martin A

Sorry not to laugh, but a lot of thought has gone into the problem of defining life.

You are not thinking it through.

The negative entropy criterion applies not just to the organism itself, but to the things it makes.

A bird's nest has a lower entropy than the twigs from which it was made. From it you can infer the existance of the bird.

A computer has a lower entropy than the materials of which it is made. Finding a computer on Triton would allow you to infer the existance of an engineer. If you have the wherewithal to reach Neptune there is nothing to preclude another human preceding you and leaving their computer behind.

Suppose you had found a von Neumman machine busily building copies of itself; would you regard that as alive?

There is anotherb even more general definition of life:-

Any system undergoing Darwinian evolution.

TheBigYinJames

"0.2C/decade?"

Rounding slightly.

GISTEMP from 1970 to present gives a linear trend of 1.8C+/-0.09

TheBigYinJame

Finger trouble. Make that 0.18C+/-0.09.

Just to recap, EM, defining life was not something I was gagging to do. I was nitpicking about RR's statement that proving life exists is simple. It seemd to me that defining it, and then recognising it when it passed under your nose, were not necessarily 'simple'.

In making a joke about Windows 7, I probably should have said intelligent life.

Suppose you had found a von Neumann machine busily building copies of itself; would you regard that as alive?

What do you mean by a von Neuman machine EM ? Simply a computer that uses the same memory for both code and data? Like my laptop?

Does it have to be an actual hardware machine? If my laptop started making copies of itself that would certainly be a bit queer, even if it had a milling machine connected to it via its RS-232 port.

But if it were a software emulation of a computer running on a real hardware machine and it started cloning copies of itself and setting them running... I don't think I'd regard it as alive at all. Can a program make a copy of itself? Don't see why not. But I vaguely remember that something about it no being as simple as that.

There is anotherb even more general definition of life:-

Any system undergoing Darwinian evolution.

So by that definition, memes are alive as I think I suggested to RR. Or evolution simlulated on a computer. I don't think many people would agree that was alive in any ordinary sense of the word.

Turing, EM?

Von Neumann and Turing both worked on concepts of computer architecture.

A Von Neumann machine was a sconcept for a space probe which would arrive in a solar system and build copies of itself to send onwards. Since copying errors would make it subject to Darwinian evolution it would be meet both the entropic and evolutionary definitions of life, but not all of the biological criteria.

As a biologist/science fiction fan my definition of life is pretty broad, but I could understand those who think in terms of proteins and DNA.

Now back to the main topic.