
Stephen Hawking: "Will Extraterrestrial Life Be Carbon Based?" - toni
http://www.dailygalaxy.com/my_weblog/2009/07/stephen-hawking-asks-will-alien-life-be-carbon-based.html
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araneae
I actually think that alien life won't be very alien at all.

Given that the laws of physics are constant throughout the universe/s in
question, I think they're probably very similar. Why? Well, DNA and RNA are
self replicators that, to the best of our knowledge, arose independently.
Given that the emergence of self-replicators is improbable to begin with, it's
even more improbable that two different types of self-replicators arose that
are so similar to each . This probably means that there is some about the way
natural laws are shaped that means that certain types of self-replicators are
more probable than others; and those that use heterocyclic aromatic organic
compounds as code fall under this category.

What about other aspects of life? All large forms of life are modular; that
is, composed of many smaller but similar types. It's likely that alien life
will follow this basic structure as well, because it's much easier to evolve
that way. Evolving a larger body without modularity requires fundamental
changes in structural composition which may be difficult to achieve with
incremental changes.

The temptation with exploratory thinking is to deliberately think as much
outside the box as you can. Doing so is good, but it's not always correct.

~~~
ubernostrum

      Given that the laws of physics are constant throughout the universe/s in question
    

That actually turns out to be a pretty big given, and there's some evidence
that the "laws" as we know them are contingent and could be subject to
variation across different regions of the universe. Of course, those regions
are in principle unobservable by us, but if it can vary on a large scale it'd
be rash to rule out variation on a smaller scale.

~~~
rglovejoy
Not really. What you are describing might happen on a bad episode of Star Trek
TNG (Q changes the gravitational constant, for example), but in reality stuff
that happens on a planet orbiting a distant star are not going to be all that
different than what happens here on Earth. Chemical bonds work the same way in
Sunnyvale as they do in the Oort Cloud or in a galaxy billions of light years
away. We know this by studying the spectra of the light coming from these
places.

At the chemical level, I do not anticipate that extraterrestrial life would be
very different than that found here. Water is a terrific solvent and is liquid
at a wide range of temperature. Unlike methane and ammonia, its molecules are
dipoles (one end is more positive and the other more negative), which makes it
easy for it to form hydrogen bonds and a strong surface tension at the
surface.

Carbon and Oxygen are both fairly abundant. They are formed during the carbon-
cycle of aging stars (3 He -> C, 4 He -> O). I have read SF stories that tell
of creatures that breathe Florine or are Silicon-based, but these are pretty
far-fetched. Florine is a lot rarer than Oxygen, and though it is more
electro-negative than O, because of this it is likely to stay locked up in
rocks than remain in the atmosphere. Silicon has the same number of valence
electrons as Carbon does, but the bonds that it forms are at a higher energy
state (n=3, vs. n=2 for C). This means that it is hard for Si to form double
or triple covalent bonds; this is something C can do very easily. For a more
detailed discussion on this subject, see:
[http://en.wikipedia.org/wiki/Hypothetical_types_of_biochemis...](http://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Silicon_biochemistry)

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mhartl
_Sulfur is capably [sic] of forming long-chain molecules like carbon. Some
terrestrial bacteria have already been discovered to survive on sulfur rather
than oxygen, by reducing sulfur to hydrogen sulfide._

Terrestrial bacteria use H2S an an _energy_ source, not as the basis of life.
Extraterrestrial life will almost certainly be carbon-based---or rather, its
ancestors will be. ET, if we find it, will almost surely be a machine
intelligence.

~~~
rriepe
Machines have a lot of disadvantages that aren't readily apparent. Machines
have to build collection systems or mine energy sources to thrive. We live on
stuff that grows out of dirt-- that's pretty handy.

Of course, machines could become more and more advanced if they didn't go
extinct in those first pivotal years, but I think it'd be to the end of
looking organic. And if you're going to life out of something, make sure it's
low on the periodic table. We're carbon-based because that's what works.

~~~
rue
Well, _we_ have to build collection systems for those things growing out of
the dirt!

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10ren
down for me. Cache:
[http://google.com/search?q=cache%3Ahttp%3A%2F%2Fwww.dailygal...](http://google.com/search?q=cache%3Ahttp%3A%2F%2Fwww.dailygalaxy.com%2Fmy_weblog%2F2009%2F07%2Fstephen-
hawking-asks-will-alien-life-be-carbon-based.html)

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Torn
Not down -- the author has renamed the article (presumably for SEO)

[http://www.dailygalaxy.com/my_weblog/2009/07/stephen-
hawking...](http://www.dailygalaxy.com/my_weblog/2009/07/stephen-hawking-on-
the-possibility-of-noncarbonbased-extraterrestrial-life.html)

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pmichaud
Who's to say that aliens will even be matter-based?

It doesn't seem to imporbable to me that life forms, for example, on the
surface of stars, where energy is abundant. Self contained, self replicating
energy patterns are just as much "life" as bacteria, and I don't see why they
can't evolve intelligence either.

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biohacker42
_Somehow, Hawking observes, "some of these atoms came to be arranged in the
form of molecules of DNA. One possibility is that the formation of something
like DNA, which could reproduce itself, is extremely unlikely._

Or it could be quite likely. There are theories that amino acids can form
without life. Just get something resembling our planet's crust and they'll
form and then they can form peptide chains and and so on. This means that any
planet similar to earth could end up not just with carbon based life but with
DNA based life.

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TheSOB88
Carbon don't enter into it. Just because something has a lot of carbon in it
doesn't mean it acts like we do. There are plenty of other things to do with
carbon than the ways organisms on Earth use it. DNA? Forget it, that was
evolved. Alien biology will be completely alien.

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derefr
"Completely alien" is a misnomer—it's like saying alien computers won't be
Turing machines, just because they're "so alien." DNA is simply a way of
sticking polymers together to form a storage device and self-stable
electrochemical API for accessing it. Organisms that want to reproduce,
especially sexually, _need_ storage devices. Likewise, oxygen is the simplest
and most robust carrier of electrochemical capacitance. Aliens _will_ have
these things—not because they evolved in the same way we did, but because
those are the most fit mutations in the early chemical soup. Sure, given an
incredibly constrained soup containing lots of sulphur and no oxygen, sulphur-
dependent life might evolve, but by the time it has evolved enough that it
meets up with us after some good ol' intergalactic travel, it will probably
realize that oxygen is a lot easier to manage and rewrite its genetics to use
it instead. Or, to take that concept further, it'll be a post-Singularity
machine intelligence, as mhartl said.

~~~
TheSOB88
Well, you're probably right about the oxygen thing. I just meant that the
chemical mechanisms behind their functioning will be completely different than
ours. I mean, some Earth organisms have completely different RNA from ours. I
do not expect aliens to have guanine, adenine, etc. as their storage device,
but that's not to say they won't have one.

