[Steven Weinberg] And talk about each of
our big bangs as a sub-universe
but very often people refer to each big bang
-as a universe and then they use there term "multiverse"
-Multiverse, yes.
for the whole thing. I think its up to you
what language you use.
The multiverse idea has some very attractive features.
I mean it arose out of thinking of theories of inflation
but it has had interesting by-products because
if this idea is correct, and we don't yet know, its at this point
it's just a speculation, but if this idea is correct
there is every reason to expect that in
the different big bangs that occur
you have different conditions and values for what we call
the fundamental constants.
And so that the fact that the constants
of nature are suitable for life
which is clearly true we observe,
may not be a universal fact. It may just be an accident
just as the fact that the temperature of the earth
is suitable for life, is not true of planets in general.
[Richard Dawkins] Obviously we have to be 
on a planet, which is...
[Weinberg] We have to be on a planet in
which the temperature is suitable
I don't know exactly what the range is.
Most people think water has to be liquid.
-We could argue about that.
-It doesn't leave very much range.
[Weinberg] Yeah no. Perhaps life could arise in liquid methane
but clearly there are some limits. I don't think life can arise
at 1 degree kelvin and I don't think it
can arise at 100,000 degrees kelvin
so there is some range of temperatures in which
life can arise and its only, as you say, 
it's only on the planets that happen
to be in that fortunate position.
[Dawkins] Or in the universes that happen to be.
[Weinberg] And then this is, uh, carried over by analogy
into our universe, that's it only in those big bangs
where the values of the constants
are suitable where life can arise.
In other words, if there was, I don't think there's really
any evidence for a very precise fine tuning
of the constants of nature...
[Dawkins] That's interesting. Some physicists 
seem to think there is extreme fine tuning.
[Weinberg] I know. I've argued about that.
[Dawkins] Yes.
Uh, one of the examples that is often quoted is
the certain energy level of the carbon nucleus.
If it was 10% higher
or 10% lower then the nuclear reactions that build up
oxygen from carbon in stars wouldn't work.
[Dawkins] This is Fred Hoyle's argument, wasn't it?
[Weinberg] Yes, well, Fred Hoyle was the one who
realized there had to be this energy level and that 
nuclear synthesis wouldn't work well without it.
Well, the fact that there is such an energy level
at just that energy does not require
a fine tuning, I think, and I've argued with people
about this. I don't think it requires
a fine tuning of the constants of nature,
because that state of the carbon nucleus
is essentially a bound state of a beryllium 8,
I'm getting very technical,
a beryllium 8 nucleus and a helium nucleus
and that's just the condition that you need
in order to allow nuclear synthesis to occur.
So even if you change the constants of nature
the value of the energy of this state
-would change, but it would still be a bound state
-Yes I see, I understand, I understand. Yes.
of a beryllium 8 plus an alpha particle
plus a helium nucleus.
And so it wouldn't make much difference
as far as nuclear synthesis.
There is one constant
that seems to be fantastically fine tuned,
but again we are not sure,
and that is the constant called the dark energy.
The, or vacuum energy or the cosmological constant.
This is the energy in space itself, 
not associated with any particles or radiation.
But just an energy, so many calories per quart of space
everywhere in the universe, whether 
there is matter there or not.
The amount of this has been measured.
It has been observed that it is not zero.
It has a small finite value.
To give an idea of the value, its about the amount of...
in a volume the size of the earth, whether the earth
is there or not, just that volume of space,
the amount of of energy is the energy in
a few hundred barrels of petroleum.
Not a lot for a volume the size of the earth.
That energy is detected by its effect
on the expansion of the universe.
Its causing the expansion of the universe to accelerate.
That's something we discovered 10 years ago
by two different pairs, two teams of astronomers
studying the speed and distance of distant galaxies.
We try to calculate what this energy is
from first principles.
There are various reasons why we can't calculate
but we can calculate certain contributions to it.
For example we can say:
Fluctuations in the electromagnetic field
just due to the quantum nature of radiation
not down to arbitrarily small wavelengths,
because we don't understand anything
at very short distances,
but just down to the shortest distance
at which we think we understand physics,
which is roughly 1/100 the size of an atomic nucleus.
Those are the shortest distances that have been probed
in our accelerators.
So, counting the fluctuations in the electromagnetic field
or the gravitational field or any other field,
down to the shortest distances that we have probed
the energy, we can calculate what energy that gives space
and it comes out to about 56 orders of magnitude larger
than the observed values.
That is a one with 56 zeros.
Now , well you just have to shrug your shoulder and say
well, there are the other contributions we can't calculate
like the contributions from fluctuations with even shorter
wavelengths. They clearly cancel.
[Dawkins] Its a pretty accurate cancellation.
[Weinberg] It's a cancellation that's accurate to
56 decimal places and that has us disturbed.
Its possible that that will be explained in a way
that has nothing to do with the multiverse.
It may be that for some fundamental physics reason
that we don't know, the universe
is evolving towards a state where that vacuum energy
that dark energy, is exactly zero.
And it's small now just because the universe is old
and its not far from that final state.
No one has constructed a theory in which that is true.
I mean, its not only a speculation, the theory would be
speculative, but we don't have a theory in which
that speculation is mathematically realized.
But its a possibility.
But the only other explanation,
well it's not even an explanation because
we don't have a candidate theory, but
the only explanation that seems to work
is that this is just one of those things
that varies from subuniverse to subuniverse,
from big bang to big bang.
In most of the big bangs it is very large.
Its much larger than what we observe
and in those big bangs they go through,
because this energy drives the expansion of the universe,
depending on if its positive or negative,
the universe either blows up so rapidly there's
no time for galaxies or stars to form,
or it crunches, recollapses so rapidly,
[Weinberg] again there is not time for life to form.
[Dawkins] No time, yes.
So it has to be small for life to exist, and
its about as small as, in fact
that's interesting, its not much smaller than it would
have to be to allow life to arise.
[Dawkins] And the fact that the cancellation is so precise
means that the number of universe
in the multiverse you need
to postulate in order to anthropically 
be comfortable with it, is very very large.
[Weinberg] And it must be at least 10 to the 56 or in fact,
if you think you have some idea about
fluctuations at even shorter distances
I think you would say at least 10 to the 120.
In fact, that...that's a little disturbing but...
A completely separate development
not motivated by this at all,
has taken place recently in string theory.
String theory, you know, is our best hope for
a theory unifying all the forces of nature;
gravity and all the other forces, all the particles.
Its the most, its been a little disappointing that it hasn't
led to any specific breakthrough in understanding
what we already know, but its still
the best game in town,
the best hope we have for a really
fundamental understanding.
It was realized a number of years ago
largely through the work of Edward Witten,
that what had seemed to be about half a dozen possible
string theories was really only one string theory,
because there is one string theory which,
it manifests itself in many ways.
