What is the anthropic principle and how does it relate to defining life?

[Max Faust]

As far as I know, there isn't any definition of "life" that holds up to a physics standard.

Sure enough, we can talk about (bio) chemistry and the specific arrangements of certain molecules, but what I'm missing is a simple, physical definition of life. I know that Erwin Schroedinger once stated that "life is a system that shows negative entropy" (and considering his bizarre cat thought-experiment, one would assume that this was a bit tongue-in-cheek), but I am unaware of any following-up on this subject.

Enlightenment, please?

 

[shungun]

Well, isn't biology and chemistry technically part of physics?
what kind of definition are you looking for?

 

[Max Faust]

A simple, matter-of-factly statement which defines life (in no uncertain terms) such that it won't need any "special" or "magical" ingredient to emerge.

 

[shungun]

Ha, now that would be hard... =P
i'd like to know too, till then i'll throw around schrodinger's definition it definitely is interesting and not to mention =P :D

 

[Max Faust]

Well, while I am holding my breath waiting for that, I have chosen to think that "life" equals existence. That is to say, *everything* is alive - and our specialised niche here on Earth is but one of many quadrillions of possible permutations when it comes to the question of how to organise basic quantums into larger systems.

 

[dx]

Unfortunately, life is not that simple!

 

[Max Faust]

Why not? We think of "gravity" as simple, so why not "life"?

 

[sophiecentaur]

That's all very well but, if you were somewhere, elsewhere in the Universe and you bumped into some Gravity, you would recognise it and could test for it. Could you be sure of recognising 'life' if it wasn't quite "as we know it"'

 

[Max Faust]

That's what my question is about!
What, by a phycisists terms, is life?

 

[dx]

Physics is not about inventing final definitions for everything. Our current understanding of life is simply not enough to characterize it in one or two sentences.

 

[mgb_phys]

The trouble is you are trying to come up with a simple definition that encompasses a whole range of processes but only when they occur together.

Try defining something simple like a star. Does you definition cover pulsars, neutron stars, brown dwarfs, should it include Jupiters and black holes?

My favorite defn of life is the maths/computer science one - it's anything with inherited characteristics.
It skips over the whole physics/chemistry phenomena stuff.

 

[Max Faust]

Try defining something simple like a star.

I'd love to - except I don't see it as simple.
To me, a star is alive - and able to shapeshift into other forms (over time).

 

[DrMik]

Self replication.

 

[Max Faust]

Self replication.

I like that. A local state of negative entropy which increases the overall volume of (nonlocal) positive entropy in the universe. The question is; does "life" require mass?

 

[Vanadium 50]

Self replication.

Are crystals alive? What about flames? You might argue this isn't self replication, but living things also require externalities like food.

If the biologists can't draw a sharp line between living and non-living, why do you think physicists can?

 

[Max Faust]

living things also require externalities like food

Which essentially boils down to "life" being a (self-contained, replicating and local) process which interacts with its surroundings by reconfigurating (parts of) them. Right?

 

[bp_psy]

Our current understanding of life is simply not enough to characterize it in one or two sentences.

I think its actually the other way around. Our present knowledge and understanding about the complexities of life is to great to permit the formulation of a 2 sentence complete definition. The good thing is that we do not need one. A definition is a kind of tool that can be useful in some contexts but not always.In physics most of the time knowing the definition does not necessarily mean that we understand anything about the thing we are studying.The understanding comes by looking at how the thing behaves under different circumstances.The same is true about life. Observing the characteristics of life does give an understanding and helps generate new knowledge. A definition is useless.

 

[mgb_phys]

Are crystals alive? What about flames? You might argue this isn't self replication, but living things also require externalities like food.

That's why it's inherited characterstics not reproduction. A fire doesn't contain any information about the spark that made it.
You could argue that crystals that set a certain arrangement dependent on the seed crystal are inheriting characteristics - but early life could have been as simple as self catalysing crystals/proteins.

 

[Q_Goest]

It might be too obvious but all life we know has one thing in common; DNA. In fact, DNA seems to be a natural language of sorts, and the only language we know of that produces organisms that are alive.

 

[mgb_phys]

In fact, DNA seems to be a natural language of sorts, and the only language we know of that produces organisms that are alive.

Rather circular argument though.
If aliens appeared and had a different molecule would they be alive?
There were also probably earlier forms of life on Earth that had a different molecule by lost out to the DNA gang.

There's also the extra question of whether RNA viruses are life.

 

[Max Faust]

all life we know has one thing in common; DNA

I would say that this only describes a very specialised "niche" of all possible life-forms.
However, it is the anthropocentric one; i.e. the one which is about "things that are like us". What I miss is a comon denominator which covers all possible manifestations of "life", with or without DNA (which seems to be little but a database for the construction of a specific organism which can only exist under specific conditions). I am missing the basic vector, so to speak, the force (or field?) that makes molecules seek together and form proteins (as it were), and I am not at all happy with any religious approach of "faith".

 

[Q_Goest]

Rather circular argument though.
If aliens appeared and had a different molecule would they be alive?
There were also probably earlier forms of life on Earth that had a different molecule by lost out to the DNA gang.

There's also the extra question of whether RNA viruses are life.

If is a good question. Are there any others? Maybe... For one thing, it seems DNA is a language with just one interpretation. Would DNA for a human be the same anywhere in the universe, not just earth? Or might we find human DNA on planet X producing an aquatic life form with roots like kelp because it was interpreted differently? I don't think anyone knows the answer to that. Point being, we can speculate about alien life forms but we have nothing to base that speculation on.

Same answer @ max faust.

 

[mgb_phys]

Would DNA for a human be the same anywhere in the universe, not just earth?

Unlikely, there's nothing special about DNA.
It's likely that another genetic coding molecule would be a double strand just because you can't get much simpler than that, although there's no reason not to have a triple or more.
There's nothing particularly optimal about 4base pairs and of course the actual molecule structure is pure chance.

 

[Q_Goest]

Unlikely, there's nothing special about DNA.
It's likely that another genetic coding molecule would be a double strand just because you can't get much simpler than that, although there's no reason not to have a triple or more.
There's nothing particularly optimal about 4base pairs and of course the actual molecule structure is pure chance.

Yea, that sounds good. But I'm not convinced. In fact, I've heard others argue just the opposite.

Regardless, the fact remains that DNA seems to be a natural language or molecular code as opposed to one that was invented.

I've been meaning to start a thread on this in the bio forum... maybe I should. What do you think?

 

[Studiot]

I know you asked for a physicists definition but how about an Aristotelian mathematics approach?

Define non-life.

Everything that is not non-life is defined as life.

 

[HaydenB]

If life were defined as existence and not so much on the idea of physically being alive then everything in the universe would fall under the definition of life. Leaving open room for any future scientific discoveries or even an alien visit as someone mentioned before.

Would anyone consider a planet itself to be alive?

 

[DaveC426913]

I know you asked for a physicists definition but how about an Aristotelian mathematics approach?

Define non-life.

Everything that is not non-life is defined as life.

Doesn't matter. Ultimately, you are still attempting to define the boundary beteen the two. It matters not from which side you look at it.

 

[vanesch]

Are crystals alive? What about flames? You might argue this isn't self replication, but living things also require externalities like food.

The more you think about it, the more you have to consider a flame as a kind of a life form, indeed: it adapts to its environment, it has a metabolism, and it propagates in as much as it can in its ecological niche
Maybe one should add "complexity" as a requirement...

 

[Galap]

A process which due to the nature of its construction transforms its surroundings into itself, and is a region of negative entropy.

Not the be-all, end-all, but IMO pretty good...

 

[DaveC426913]

The more you think about it, the more you have to consider a flame as a kind of a life form...

No, you don't. It isn't.

But fire is an excellent example of non-life against which some too-loose definitions of life can be tested and found to fail.

 

[Studiot]

Doesn't matter. Ultimately, you are still attempting to define the boundary beteen the two. It matters not from which side you look at it.

Actually it does matter, Van Vogt made many $ out of this question with science fiction novels.

But the boundary comment is very perceptive.
I think so far the discussion has been limited to whole entities and ignored damage.
Select something we all agee is life and take something away.

Take a daisy

Pluck it and stand it is water: Is it still life? : Is it still a daisy?
Remove one petal: Is it still life? : Is it still a daisy?
Remove another: Is it still life? : Is it still a daisy?
.
.
Until there are no petals left: Is it still life? : Is it still a daisy?

At what point is an autumn leaf dead?

The boundary is not an infinitesimal line.

 

[Max Faust]

Is it still a daisy?

You could argue that the material remains a daisy as long as it is DNA-coded to be (or become) a daisy.

At this point, we run into a corollary of the first problem: We don't have any good definition for death (or non-life) either. This is why I have chosen to think, for the time being and until somebody can present a better theory, that "life" is equal to "existence" - i.e. all things that exist are alive (which I suppose is a sort of animism) - but not all things are "active" all the time. Thus, "dead matter" isn't relly dead, it's simply in a resting state. At any given moment, it can be picked up and utilised by a dynamic system which happens to be "alive".

Does this even make any sense?

 

[Studiot]

I don't pretend to have all the answers.
I was just pointing out that your question is not as clear cut as some have made out.

Hospital staff every day ahve to make decisions about when the body being kept 'alive' by machines has 'died'. What is death? Heart death or Brain death?

The issue, you mention, of resting or dormant is interesting as well.
What is the status of say a dried up seed taken from a Pyramid in Egypt.
Such seeds have shown no signs of life for the past several thousand years, but have successfully been rehydrated and grown into plants.

 

[mgb_phys]

The issue, you mention, of resting or dormant is interesting as well.
What is the status of say a dried up seed taken from a Pyramid in Egypt.
Such seeds have shown no signs of life for the past several thousand years, but have successfully been rehydrated and grown into plants.

The point isn't the difference between a living and dead object, that's pretty easy to define thermodynamically.

It's how you say a plant is life and the pyramid isn't

 

[Studiot]

The point isn't the difference between a living and dead object, that's pretty easy to define thermodynamically.

It's how you say a plant is life and the pyramid isn't

Which just strengthens my point.

I don't think there are many here who would argue that a pyramid is alive.

However let us hear your thermodynamic arguments about seed number 29 if I take 100 such seeds as previously described. Given that following rehydration approximately 20% will grow into plants.

How do you test the seed prior to rehydration for inclusion in the 20% or 80% category ? As far as I know all seeds appear the same.