September 4, 2004 12:13 PM   Subscribe

Smolin vs. Susskind on the anthropic principle. For those keeping score: Stephen Hawking is for it. Brian Greene is not.
posted by kliuless (10 comments total) 3 users marked this as a favorite
in a review of hawking's book the universe in a nutshell, joseph silk sez:
The anthropic principle is one of the more remarkable swindles in physics. Indeed it is metaphysics, and that is the essence of the problem for most physicists in accepting it. The anthropic logic is either immensely subtle, by arguing that we, via our mere existence, control the cosmos, or unabashedly naive, by setting aside any physics explanations that any ultimate theory of physics might reasonably be expected to deliver. Metaphysics lacks predictive power, the very core of physics. The anthropic principle is an extreme expression of our ignorance.
while roger penrose sorta equivocates:
In my opinion, the strong anthropic principle has a somewhat dubious character, and it tends to be invoked by theorists whenever they do not have a good enough theory to explain the observed facts (i.e. in theories of particle physics, where the masses of particles are unexplained and it is argued that if they had different values from the ones observed, then life would presumably be impossible, etc.). The weak anthropic principle, on the other hand, seems to me to be unexceptionable, provided that one is very careful about how it is used.
and on this note, a slashdot commentator remarks:
It's trivially true (at least in its less extreme varieties). If you want to reason probabilistically, say, about the universe, you have to make sure you condition everything on the fact that you are there to reason about it. There's a nice example that goes back to Boltzmann. Given a disordered set of particles (call them a mini 'universe') you can write an expression for their entropy. If you pick a 'typical' state it will have high entropy and if you let the system evolve chances are the entropy will stay high. Let the system evolve for long enough and you expect the entropy to dip down low occasionally through chance. Suppose the entropy needs to be very low for life to evolve in the system. Then typically any organism that evolves in this system is going to get a skewed view: they're going to find that they live in a universe with unusually low entropy. Their scientists might spend ages trying to figure out just why they live in a low entropy universe but from an 'observer' outside their universe there's nothing surprising - their universe is a long expanse of extereme boredom with occasional low entropy islands, some of which contain life.
also see: not even wrong, scientific alternatives to the anthropic principle ("cosmological natural selection") and environment as a witness (via not exactly :) cheers!
posted by kliuless at 12:25 PM on September 4, 2004

interesting. It's really amazing (but not terribly surprising) to me how cosmology becomes philosophy with pretty mathmatics to make it appear scientificly proven. It looks like in order to come up with a unified theory we'd also have to come up with a unified philosophy...which to me makes the whole damn thing impossible because we all have our own philsophical opinions which barely seem to be related to logic.

So if all of science is based on the concept that the universe is rational (one thing follows from another, cause and effect), can it actually be proven with science that rationality holds or is the whole argument question begging as your trying to use rational logic to prove rational logic exists?

ok, my brains melted now...back to the real world where i don't have to prove i have two hands in order to use them.
posted by NGnerd at 12:48 PM on September 4, 2004

I take the anthropic principle one step further. I believe that we live in a universe every detail of which has been calibrated to produce... plastic. We, the human race, are merely a way station in the evolution of this material, which (I understand) will still be around, long after our species has become extinct.
posted by Faze at 1:13 PM on September 4, 2004

The anthropic principle doesn't seem that problematic to me. I like Susskind's summation:
1'. The universe is big—about 15 billion light years in radius.

2'. The expansion of the universe led to a huge number of condensed astronomical objects — at minimum 10[23] solar systems.

3'. The laws of gravity, nuclear physics, atomic physics, chemistry thermodynamics allow a very diverse set of possible environments, from the frozen cold of interstellar space to the ferocious heat of stellar interiors, with planets, moons, asteroids and comets somewhere in between. Even among planets the diversity is huge—from Mercury to Pluto.

4'. The universe is filled with these diverse environments, most of which are lethal. But the universe is so big, that statistically speaking, it is very likely that one or more habitable planets exists.

I don't think anyone questions these points. But what is it that decides which kind of environment we live in—the temperature, chemistry and so on? In particular what determines the fact that the temperature of our planet is between freezing and boiling? The answer is that nothing does. There are environments with temperatures ranging from almost absolute zero to trillions of degrees. Nothing, determines the nature of our environment—except for the fact that we are here to ask the question! The temperature is between freezing and boiling because life (at least our kind) requires liquid water. That's it. That's all. There is no other explanation. [1]

This rather pedestrian, common sense logic is sometimes called "The Anthropic Principle." Note that I mean something relatively modest by the A.P. I certainly don't mean that everything about the laws of physics can be determined from the condition that life exists— just those things that turn out to be features of the local environment and are needed to support life.

Let's imagine that the earth was totally cloud bound or that we lived at the bottom of the sea. Some philosopher who didn't like these ideas, might object that our hypotheses 1'—4' are un-falsifiable. He might say that since there is no way to observe these other regions with their hostile environments—not without penetrating the impenetrable veil of clouds—the theory is un-falsifiable. That, according to him, is the worst sin a scientist can commit. He will say, "Science means falsifiability. If a hypothesis can't be proved false it is not science." He might even quote Karl Popper as an authority.

From our perspective we would probably laugh at the poor deluded fellow. The correctness of the idea is obvious and who cares if they can falsify it.

Even worse, he wouldn't even be correct about the falsifiability. Here is a way that the anthropic reasoning might be proved false without penetrating the veil of clouds: Suppose an incredibly accurate measurement of the average temperature of the earth gave the answer (in centigrade) T=50.0000000000000000000000000000
degrees. In other words the temperature was found to be exactly midway between freezing and boiling, to an accuracy of one hundred decimal places. I think we would be justified in thinking that there is something beyond the anthropic principle at work. There is no reason, based on the existence of life, for the temperature to be so symmetrically located between boiling and freezing. So discovering such a temperature would pretty convincingly mean that the existence of life is not the real reason why the temperature is between 0 and 100 degrees.
posted by Tlogmer at 3:33 PM on September 4, 2004

I generally agree with truisms.
posted by Pretty_Generic at 4:17 PM on September 4, 2004

So discovering such a temperature would pretty convincingly mean that the existence of life is not the real reason why the temperature is between 0 and 100 degrees.
But the whole point is that the existence of life is not the cause of anything, but the effect of a sum of variables. My understanding is that all the AP says is that life requires that certain conditions be met, and that this universe is an example of one where life is possible. This is, as P_G points out, a truism, and thus unfalsifiable. It's pretty, but it's pretty useless.

Great post; that was a lot of fun to read!
posted by mote at 6:37 PM on September 4, 2004

I don't think susskind would disagree with you, except on the uselessness point: the AP allows us to understand, for example, that the solar system having a planet exactly 93 million miles from the sun is pure chance, not a from-the-very-beginning inevitability. It's just a logical shorthand.
posted by Tlogmer at 9:52 PM on September 4, 2004

The universe is filled with these diverse environments, most of which are lethal.

You said a mouthful there, brother.
posted by semmi at 11:30 PM on September 4, 2004

the AP allows us to understand, for example, that the solar system having a planet exactly 93 million miles from the sun is pure chance, not a from-the-very-beginning inevitability.

Not to mention the Sun has to be a certain size at a certain point in it's lifecycle, consisting of a certain mass of atoms to be fused in such a way that make it relatively non-volatile for the said planet, some 93 million miles away to eventually produce life.

There is also the case that small, rocky planets with relatively thin atmospheres, orbiting close in to the home star "need" large gas giants some ways out from the center to "lure" cometary fragments away from the inner "habitable zone".

But see, there are those words: "lure" and "need"

The anthropic principle at work I say. I'm no physicist, but I'll tell ya, if a meteor fell on my head thirty minutes ago, I doubt I'd be writing this now. Same with Brian Greene. If a pebble sized interloper tunneled a pea-sized hole through his brain a decade or so ago, I very much doubt that I would have enjoyed "The Elegant Universe" as much as I did.

Just because we don't "control" the physics of the Universe now, doesn't mean we won't in the future. Which kinda makes the whole argument extraneous. Wouldn't it be "funny" if Susskind and Smolin agreed to a duel "Zell Miller style" and one of them was killed? Surely some would lament the passing of a great scientist and thinker. Others of course would simply move on. Sans Brian Greene's "elegant universe" because he'd of course, been hit by a pebble traveling at 100X the speed of sound some time ago.

The question of the Anthropic Principle was impossible a thousand years ago. In another 1000 years, I predict, this will all be water under the bridge just like witches made of wood.

This argument between these two, is Douglas Adams territory.
posted by crasspastor at 12:28 AM on September 5, 2004

This was a very good read. Thanks kliuless!

I do agree that the AP is useful as a way of thinking about observational bias. Some astronomical variables may be uniquely determined and others may be merely contingent.

I dont see why so many people are so against it. As for the objection that it is metaphysics, many of the questions being tackled today by modern cosmology were once thought to be in the realm of metaphysics.

I see how it can be overused as a guiding principle but I dont think we are anywhere near a point where great thinkers like Hawking or Susskind or perhaps even Witten are in danger of being led astray by it.
posted by vacapinta at 1:57 PM on September 5, 2004

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