I found the email debate between Smolin and Susskind
to be quite interesting. Unfortunately, it mixes several issues.
The Anthropic Principle (AP) gets mixed up with their other agendas.
Smolin advocates his CNS, and less explicitly loop quantum gravity.
Susskind is an advocate of eternal inflation and string theory. These
biases are completely natural, but in the process the purported question
of the value of the AP gets somewhat lost in the shuffle. I would
have liked more discussion of the AP directly.
Smolin's view is that the AP should be discarded on the basis of
being incapable of being falsified. In addition, he claims that potential
support for the AP is because it usually applied in the context of
other more valid observations, getting a free ride so to speak.
Susskind argues that the AP has some value, but seems to stop short
of saying just how much. He uses Weinberg's discussion of the cosmological
constant as an example, but it begs the question of how much predictive
value does the AP have?
Put another way, Smolin says it has zero value, Susskind holds that
it has some positive value. Why not ask how much it has?
Given that we exist, it is not surprising that the universe has physical
properties that are consistent with our existence. Consistency of
this sort is to be expected. It is surely true that physical theories
(or parameters in a landscape of theories) that are inconsistent
with our existence would have a big problem — sort of like the old
Ricky Ricardo line "Lucy, you got some 'splainin to do!".
There is nothing special there — as physicists we require any theory
to have predictions consistent with experiment and observation.
The question of the AP hinges on whether one can invert this proposition
and say because we (intelligent life) exist, that therefore puts
a constraint on physical theories (or parameters to physical theories),
at least for our neck of the woods.
I'll just use "theories" as a shorthand for either a choice
among a landscape of theories, or a set of parameters to a physical
theory — all within a given pocket universe. At the level of this
discussion they serve the same purpose.
Making this inversion generally means you slide along the following
slippery slope starting with the most skeptical and progressing onward:
The skeptics answer is that you cannot invert the proposition.
Consistency is nice, but it only works in one direction. There
at least three common sub points:
We have no experience of pocket universes except our own
— in which we manifestly do exist. Our
is based on
one data point — the pocket universe we live in — and
thus we cannot generalize with any confidence.
(b) Our existence is only one of millions of observational
facts about the universe — why single out intelligent life
as being special?
It is not necessary because many of these other observations
(c) Indeed we have reason not to single life out. We have
only the weakest possible understanding of the causal chain
of events between
fundamental physics at the cosmological level and the evolution
of intelligent life. So, while our physics is based on one
the situation with our biology is even worse. We have only
carbon based life, only on one planet, with only one intelligent
Until and unless we understand the chain of biology better,
with more examples we have climbed out on a very precarious
2. The first step toward the AP is to argue that there is
some landscape or ensemble of possible physical theories
in a set of pocket universes.
Many aspects of physical theory make this seem like a sensible
question to ask, because no intrinsic mechanism sets the
parameters or properties
of the theories. If there is no hard reason to prefer one
theory, we naturally can ask about probability distribution
of those theories.
Assuming that there is a range of theories consistent with
life, we can try to estimate something about the
based on our one data point. This requires making
a generalization from that one data point — usually by
a further assumption
are a "typical" observer in some probabilistic sense. The "mediocrity
principle" is one example, but there are many
other examples of similar reasoning — i.e. the
so called doomsday paradox, and
related paradoxes in probability theory. The details
of the assumption are
notoriously critical. Given the assumption about
how to generalize from the one data point, we can
infer something about the
distribution of physical theories. This is at its
essence a derived observation,
based on an assumption — not a predictive theory.
(b) A different direction is to postulate further that the
probability distribution is biased in favor of the range
of theories that is
consistent with intelligent life. This is usually called
the Weak Anthropic Principle or WAP. It assumes that there
is an intrinsic
physical process, of unknown origin, that makes a pocket
universe more likely. Note that it is different than 2(a)
above. 2(a) says
that one can derive observational bounds on theories based
on our existence (plus an assumption). 2(b) says that there
is a real physical
effect, without justifying why.
3. The third step is to make the leap to causality. This
comes in a couple stages.
Intelligent life is a requirement for physical theories.
Pocket universes are constrained to have physical theories
lead to intelligent life. This is
usually called the Strong Anthropic Principle or SAP.
(b) Intelligence and information processing must come into
existence in the universe, and once it exists will persist.
This is called
the Final Anthropic Principle.
Undoubtedly this summary of the various stages is not complete,
and may enrage a fan of the AP, but it covers much of the
I think that Smolin would say that his position is best covered
by 1(a) — since we have only one data point, how can we falsify,
1(b) — most seemingly valid invocations of AP rely on other
data. Smolin does not seem to argue 1(c) in these postings,
is the most common criticism of AP in other circles.
Smolin instead argues that his CNS is a concrete suggestion
of a physical process that would bias the distribution of
— but not because of life, but rather because it stimulates
black hole formation. Coincidentally, this is similar to
Susskind does not really say where he is on this scale. He
finds the AP useful. He seems to be positive about Vilenkin's
principle. This is a good example of 2(a). However, he does
not go further to say whether he is in favor of 2(b), or
the stronger forms
in 3. I would be curious as to what he says about them.
In particular, what is the explanatory
value of 2(a) ? The Garriga , Tanaka
ends with the
also argue that observers should not be surprised to find themselves
living at the time when curvature is about to dominate".
OK, but that begs the question
of what value there is in not being
surprised. Consistency is not that
and thus has little predictive
Technically speaking, the issue here is whether the generalizing
assumption required in 2(a) — such as the mediocrity principle
— leads to any additional information about the physics.
2(a) is about
deriving something from an observation.
Conversely, if a 2(a) derivation
did yield a surprise, then it might
Or it might not.
If there was
focus then would shift on questioning
the generalizing assumption. Critics
of the doomsday
paradoxes will point out that in
most cases unusual results stem
problems with "reasonable" generalizing
Now, one could imagine that, for some other reason, the density
of states in the landscape of possible string theories is
the region consistent with our existence. If so, then even
a small amount of information about the distribution might
information. So, for example if there were only a small number
of viable string theories — in some discrete spectrum, then
it may not
take much of an observation to allow us to decide between
them. However, this seems to be unlikely based on current
2(b) would seem to be Susskind's best hope for a modestly
useful AP as a physical theory. Weinberg's argument, which
is a good example of it. However, the problem here is that
2(b) is not really a principle or an argument — it is an
ansatz — a hunch
that needs to be confirmed.
However, one could equally ask
(and Smolin almost does, but not
quite) what is the
or 2(b) with
about the observable universe substituted
for "intelligent life"?
Why is intelligent life so important?
We are calling this the Anthropic principle not the "universe
as we see it" principle.
Rather than to assert that AP has zero value, why not ask
this — does it have more value to discuss intelligent
life, than to discuss
say the fluctuations in the
microwave background, the distribution
of matter in galaxies?
In actual fact, the cosmological physics that impact this
debate are either gross scale things like condensation of
matter into galaxies,
expansion of the universe etc., or very fine scale things
like the properties of carbon chemistry that depend on the
constant and similar quantum parameters. None of these one
is very directly tied to intelligent life. So why do we go
This to me is the crux of the issue. Why is intelligent life
If it is special, in the sense
of having more explanatory power
with respect to
cosmological physics, then
the name "anthropic" is
justified and so is the debate. Conversely, if intelligent life is
not special with respect to cosmology, then we should shut up about "anthropic" aspect.
Note that this is orthogonal to the point discussed by Susskind
and Smolin — regardless of whether the AP can be falsified,
tell me why
I should care.
By 1(c) above one can doubt the special-ness of intelligent
life on many grounds. We have no real evidence of life outside
and no evidence of intelligent life here other than ourselves.
These are testable.
One can easily imagine both of
these being empirically tested
in our lifetimes.
over a Martian life form — or if
not there then perhaps a later
to Europa or elsewhere. Computer
scientists may someday present
us with a second
may find a
communiqué from intelligent
life elsewhere. So, the situation
is far from
at the moment we know next to nothing
about them. Until the happy day
when we do know something about
either life, or intelligent life
as an explanation
Other observations surely constrain
our choices of fundamental physical
on the fluctuations
in the microwave
background have revolutionized
cosmology, weeding out many previously "un-testable" theories.
Unless there is powerful additional explanatory value in
invoking intelligent life, why go there?
When one gets to the extremes of 3(a) and (b), or even 2(b)
when posed in strongly terms of intelligent life, it seems
to be little
more than mysticism.
I wonder if Smolin and Susskind would agree?