h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Actually, I agree with you about non-carbon-based life: I was attempting to recapitulate someone else's argument earlier.

However, here is a question -- supposing that a silica based lifeform (let's say) existed, it presumably would live at a much slower rate than us, since we don't see it moving or breathing or whatever. How would we know?

Similarly, if life exists to which the dust on my computer screen is a galaxy... how would we speak?

So while I am not sure I agree with the restriction to carbon-cased lifeforms, for as you say, there may in fact be other types out there we simply haven't recognized, for purposes of calculating the likelihood of other life trying to talk to us I think it is legitimate to limit the discussion to life which CAN talk to us....
since if that life is simply too alien to be recognized as life, no dialogue is going to result, regardless of good intentions (or not) on our part or on theirs...

Miztres, I vaguely remember the book, but can't dredge up the title out of my memory. Perhaps someone else will think of it.

This, of course, leades straight to the discussion as to what life actually is. How can we distinguish between a complex chemical reaction and life?
I, for one, have no answer...

There is a very clear sign of life: Reproduction. In my opinion it's necessary and sufficient for life.

::putting on my devil's advocate hat::

aren't mules sterile?

I would be more in favor of motion. But I seem to recall from some public broadcasting show that creatures with a very high metabolism
(how many times a minute does a hummingbird beat its wings?) may simply not perceive us.

I don't have a good answer either. I think that you can accept the framework of answers that makes most sense, but you have to be prepared to discard, review or patch it as necessary.

I think the reproduction thing is the only useful test for life. If certain twigs of the tree of life are dead ends that doesn't mean that they do not belong to it.

Actually it's not a new idea that reproduction is the prime consideration for life. Just recently, it was discussed that groups like ants colonies and bee hives could be considered alive as they can reproduce...but a single being like an ant or bee could not be as it could not survive long, little lone reproduce by itself.


It is a scary proposition when talking about human, because that would mean only couples would be considered alive, and then only couples without infertility problems. Singles, and those who can't have children, would not be considered 'alive' by this theory.

Speaking for us singles...I think we have more life in our than some couples with screaming children.

Well, actually I didn't say this.

'If certain twigs of the tree of life are dead ends that doesn't mean that they do not belong to it.' -- This implies that those ants and singles too are living beings.

Another possible definition of life may use this very obscure physical quantity called entropy. However, neither I've thought about that nor I've read about it so far.

A classic. Unfortunately that definition would include fire.
Perhaps fire is alive?

Fire doesn't reproduce itself.

A flame can create a new fire that sustains itself even after the original flame is removed. It really depends on what you mean by reproducing. In this case I let reproducing mean "capable of making a copy of itself".

A flame doesn't reproduce itself because there is no relationship between two parts of a fire. They look similar because physical laws don't allow for arbitrary types of flames, but not because there is a mother and a daughter. One flame may kick off another flame, but it doesn't produce it; the new flame is *only* the result of the substances burning.

Unless, of course, you mean the 'idea' that is copied. But this is philosophical, and I'm a mere physicist.

a few thoughts in random order

by entropy, I suppose you mean that life is organized and unlikely to otherwise occur. This would lead to the conclusion that cars, manufacturing systems and certainly artificial intelligence constitute life. I am not sure I disagree; remember the Star Trek episode where someone wanted to disassemble Data to see how he worked? However, most manufactured "life" isn't intelligent or autonomous.

the fire example is an example of the nomenclature problem. Yes, fire spreads. So does life; urban sprawl is an argument for this. Could I imagine that the sparks were seeds? Yes. Nonetheless. Can we talk to fire? Not enough in common....

My own suggestion of movement as a criteria is refuted by the entire plant kingdom, as I remembered at 2am, too late

Dana

Good point R about fire. Really, fire is a chemical reaction.... couldn't life be just the same?

I also like Elinruby's theory of motion, but then we would have to consider that not just this planet is alive, but the whole solar system as they move and change in relation to their environment. Again, not a new theory, but one that has more to do with ancient Roman and Greek gods than sterile science...I like it!

I found this searching through my files:

New Scientist 28th October 2000 'It's alive' by Kathryn Brown pp30-33

In this article it says that the usual way of determining if something is a life form is to look for DNA or at least RNA. This pretty much assumes that the life definition has to be for a carbon based entity as these chemicals are organic compounds.

The article is mostly about the idea that the structures that entities build for themselves to sustain their lives are an extention and part of the living entity. An exampe of this is a web weaving spider. These spider can not survive without their webs. The webs, catch their prey, help with the digestion of food and in other forms provide safe nests for eggs. Therefore, the web is the organism just as much as they creature that built it.

Interesting thought when once again applied to humans. Is a fisherman a unique species because of the specialised equipment he uses to 'catch his prey'. Is he a different form of life to that of a computer operator...?

Getting back to the idea of other life on distant planets...it can be assumed that we will see what looks to be the structures of life without seeing any life forms. Can I remind you of the face on Mars? For years considered to be proof that there had been intelligent life on earth, it is now of course been proved to be just an interesting shadow and some clever computer graphics work. Wouldn't it be interesting if we found structures of life on some distant planet and thought that was the lifeform itself?

Gosh wouldn't we look silly talking to a building or someones web.

A "mother" flame does produce a the "daughter" flame in the sense that without the original flame the new one would never be created. At least, that's how I see it.

BTW
My friend just handed me our biology book. It contains a definition of life as we know it on earth. These are 1) Life consists of cells 2) It can reproduce 3) Life needs "fuel" and has a metabolism 4)It reacts to its environment 5) Organisms can regulate their inner environment.

*NOTE* This only applies to life on earth, and even then it is certainly not conclusive.

And the new flame would not be created if the substance doesn't react with oxygen, if there is oxygen at all, if the temperature is not too low, ...

So, the old flame, the oxygen-reactivity, the oxygen itself, the warmth, and the non-existence-of-any-shoe-suffocating-it produce a new flame? Hardly.

A simple cause and effect relationship is obviously not enough that something *produces* something else.


As you've already said, those 5 point checklist is rather limiting. However, the reproduction thing I've proposed has weak points, too. E.g. it's possible that by mere coincidence molecules come together spontaneously and form a living being that is far away from the ability to reproduce. Mindbogglingly highly improbable, but possible.

I think the entropy thing is the way to go. But it's difficult.

Eureka.

The best test of life would be growth.

This applies to all life-forms I can think of except possibly my ex; however he isn't really human anyway so that just goes to show

I can't think of an exception either; well, your ex, of course.

However how do you define growth?

And how do you mean growth of your ex?!

How do you mean grow? Mountains "grow", well get a bit taller anyway.

A fire needs a specific type of fuel, a substance that reacts to oxygen. We also need special fuel, we can't eat rocks for example.
A fire needs oxygen, so do we. A fire needs heat, so do we. If someone stamps on us, we don't fare any better off than a flame that receives the same treatment.

Perhaps we're thinking of different things when we say produce? I mean that a set of events cause a distinct reaktion that wouldn't've happened without the events.

I'm not trying to argue that fire is life, just that I don't agree with the reproduction theory.