The Big Picture
Page 52
303
Big Picture - UK final proofs.indd 303
20/07/2016 10:02:51
T H E B IG PIC T U R E
01
quite zero: about one hundred- millionth of an erg in every cubic centimeter
02
of space. (An erg is not that much energy; a hundred- watt lightbulb uses up
03
a billion ergs per second.) But the vacuum energy could have been enor-
04
mously larger. A back-of-the envelope calculation shows that a reasonable
05
value would have been something like 10112 ergs per cubic centimeter— a
06
full 120 orders of magnitude larger than the actual number.
07
If the vacuum energy had taken on this “natural” value, you wouldn’t be
08
reading these words right now. There would be no such thing as words or
09
books or people. Vacuum energy accelerates the expansion of the universe,
10
pushing things apart from one another. An energy that enormous would
11
rip apart individual atoms, making anything like “life” extremely unlikely.
12
The tiny value of the vacuum energy in the real world seems gentle and life-
13
affirming by contrast.
14
Vacuum energy isn’t the only number that seems to be tuned for life.
15
The way that stars shine (ultimately providing free energy for our biosphere)
16
depends sensitively on the mass of the neutron. Stars run by nuclear fusion.
17
The first step is when two protons come together and one of them converts
18
to a neutron, creating a nucleus of deuterium. If the neutron were a little bit
19
heavier, that reaction would not occur in stars. If it were a little bit lighter,
20
all of the hydrogen in the early universe would have been converted to he-
21
lium, and helium- based stars would have much shorter lifetimes. As with
22
the vacuum energy, the mass of the neutron seems fine- tuned to allow for
23
the existence of life.
24
That might very well be. But there are two subtleties that render this
25
reasoning a bit uncertain.
26
First, we don’t have reliable ways of judging whether the values of vari-
27
ous physical quantities are likely or unlikely. The vacuum energy in our
28
world is much smaller than simple estimates might lead us to guess. But
29
those simple estimates could be wildly misguided, based as they are on our
30
incomplete understanding of the ultimate laws of physics. For example, the
31
maximum entropy that a region of space can contain is higher when the
32
vacuum energy is lower. Perhaps there is a physical principle that prefers
33
space to have a high maximum entropy rather than a low one. If so, that
34
would favor very small values of the vacuum energy, which is exactly what
35S
we observe. We shouldn’t get too excited when physical quantities seem
36N
304
Big Picture - UK final proofs.indd 304
20/07/2016 10:02:51
A R E W E t h E P O I n t ?
unnaturally large or small until we understand the mechanism that sets
01
their values, if there is any. They could be attributable to ordinary physical
02
processes, having nothing to do with the existence of life.
03
Second, we don’t know that much about whether life would be possible
04
if the numbers of our universe were very different. Think of it this way: if
05
we didn’t know anything about the universe other than the basic numbers
06
of the Core Theory and cosmology, would we predict that life would come
07
about? It seems highly unlikely. It’s not easy to go from the Core Theory to
08
something as basic as the periodic table of the elements, much less all the
09
way to organic chemistry and ultimately to life. Sometimes the question is
10
relatively simple— if the vacuum energy were enormously larger, we
11
wouldn’t be here. But when it comes to most of the numbers characterizing
12
physics and astronomy, it’s very hard to say what would happen were they
13
to take on other values. There’s little doubt that the universe would look
14
quite different, but we don’t know whether it would be hospitable to biol-
15
ogy. Indeed, a recent analysis by astronomer Fred Adams has shown that
16
the mass of the neutron could be substantially different from its actual
17
value, and stars would still be able to shine, using alternative mechanisms
18
to the ones employed by our universe.
19
Life is a complex system of interlocking chemical reactions, driven by
20
feedback and free energy. Here on Earth, it has taken a particular form,
21
making use of the wonderful flexibility of carbon- based chemistry. Who is
22
to say what other forms analogous complex systems might take? Fred
23
Hoyle, the astronomical gadfly who liked to cast doubt on the Big Bang and
24
the origin of life, wrote a science- fiction novel called The Black Cloud, in 25
which the Earth is menaced by an immense, living, intelligent cloud of in-
26
terstellar gas. Robert Forward, another scientist with a science- fictional
27
bent, wrote Dragon’s Egg, about microscopic life- forms that live on the
28
surface of a neutron star. Perhaps a trillion trillion years from now, long
29
after the last star has winked out, the dark galaxy will be populated by di-
30
aphanous beings floating in the low- intensity light given off by radiating
31
black holes, with the analogue of heartbeats that last a million years. Any
32
one possibility seems remote, but we know of a number of physical systems
33
that naturally develop complex behavior as entropy increases over time; it’s
34
not at all hard to imagine that life could develop in unexpected places.
S35
N36
305
Big Picture - UK final proofs.indd 305
20/07/2016 10:02:51
T H E B IG PIC T U R E
01
•
02
03
There is another famous complication: we might not have just a universe,
04
but a multiverse. The physical numbers that are purportedly fine- tuned—
05
even supposedly fixed constants, such as the mass of the neutron— could
06
take on very different values from place to place. If that’s the case, the fact
07
that we find ourselves in a part of the mult
iverse that is compatible with life
08
is exactly what we should expect. Where else would we find ourselves?
09
This idea is sometimes labeled the anthropic principle, and the very men-
10
tion of it tends to inflame passionate debate between its supporters and
11
detractors. That’s too bad, because the basic concept is very simple, and
12
practically indisputable. If we live in a world where conditions are very dif-
13
ferent from place to place, then there is a strong selection effect on what we 14
will actually observe about that world: we will only ever find ourselves in a
15
part of the world that allows for us to exist. There are several planets in the
16
solar system, for example, and some of them are much larger than Earth.
17
But nobody thinks it is weird or finely tuned that Earth is where we live; it’s
18
the spot that is most hospitable to life. That’s the anthropic principle in
19
action.
20
The only real question is whether it is reasonable to imagine that we do
21
live in a multiverse in the first place. The terminology can be confusing;
22
naturalism says there is only one world, but that “world” can include an
23
entire multiverse. In this context, what we care about is a cosmological mul-
24
tiverse. That means there are literally different regions of space, very far
25
away and therefore unobservable to us, where conditions are quite different.
26
We call these regions “other universes,” even though they are still part of
27
the natural world.
28
Because there’s been a finite number of years since the Big Bang, and
29
because light moves at a fixed speed (one light- year per year), there are parts
30
of space that are simply too far away for us to see them. It’s completely pos-
31
sible that out beyond our visible horizon, there are regions where the local
32
laws of physics— the equivalent of the Core Theory— are utterly different.
33
Different particles, different forces, different parameters, even different
34
numbers of dimensions of space. And there could be a huge number of such
35S
regions, each with its own version of the local laws of physics. That’s the
36N
cosmological multiverse. (It’s a separate idea from the “many worlds” of
306
Big Picture - UK final proofs.indd 306
20/07/2016 10:02:51
A R E W E t h E P O I n t ?
quantum mechanics, where different branches of the wave function are all
01
subject to the same physical laws.)
02
Some people find this kind of speculation distasteful, as it relies on phe-
03
nomena that are, and will remain, beyond the reach of observation. But
04
even if we can’t see other universes, their existence can affect the way we
05
understand the universe we do see. If there is only one universe, the puzzle
06
of the vacuum energy is “Why does the vacuum energy take on the particu-
07
lar value that it does?” If there are many universes, with different values of
08
the vacuum energy, the question is “Why do we find ourselves in this part
09
of the multiverse, where the vacuum energy takes on this specific value?”
10
These are quite distinct issues, but each is a perfectly legitimate scientific
11
question. Whether or not we live in a multiverse is a perfectly ordinary
12
scientific consideration, to be judged by perfectly ordinary methods: what
13
physical model provides the best account of the data?
14
There would, admittedly, be something disreputable about the multi-
15
verse idea if we were positing all of these different regions of space for no
16
good reason, or only so that we could address fine- tuning problems. That
17
would represent an extremely elaborate and contrived model. Even if it pro-
18
vided a good fit to the data, it would be natural to penalize it severely when
19
it came to assigning prior credences; simple theories are always to be pre-
20
ferred over complicated ones.
21
But in modern cosmology, the multiverse is not a theory at all. Rather,
22
it is a prediction made by other theories— theories that were invented for
23
completely different purposes. The multiverse wasn’t invented because peo-
24
ple thought it was a cool idea; it was forced on us by our best efforts to un-
25
derstand the portion of universe that we do see.
26
Two theories, in particular, move us to contemplate the multiverse:
27
string theory and inflation. String theory is currently our leading candidate
28
for reconciling gravitation with the rules of quantum mechanics. It natu-
29
rally predicts more dimensions of space than the three we observe. You
30
might think that this rules out the idea, and we should move on with our
31
lives. But these extra dimensions of space can be curled up into a tiny geo-
32
metric figure, far too small to be seen in any experiment yet performed.
33
There are many ways to do the curling up— many different shapes the extra
34
dimensions can take. We don’t know the actual number, but physicists like
S35
to throw around estimates like 10500 different ways.
N36
307
Big Picture - UK final proofs.indd 307
20/07/2016 10:02:51
T H E B IG PIC T U R E
01
Every one of those ways to hide the extra dimensions— what string the-
02
orists call a compactification— leads to an effective theory with different
03
observable laws of physics. In string theory, “constants of nature” like the
04
vacuum energy or the masses of the elementary particles are fixed by the
05
exact way in which extra dimensions are curled up in any given region of
06
the universe. Elsewhere, if the extra dimensions are curled up in a different
07
way, anyone who lived there would measure radically different numbers.
08
09
10
11
12
13
14
Different ways that extra dimensions of space could be compactified and hidden from our 15
view. Each possibility leads to different numbers characterizing the physical laws we
16
would measure in that region of space.
17
18
Stri
ng theory, then, allows for the existence of a multiverse. To actually
19
bring it into existence, we turn to inflation. This idea, pioneered by physi-
20
cist Alan Guth in 1980, posits that the very early universe underwent a pe-
21
riod of extremely rapid expansion, powered by a kind of temporary
22
super- dense vacuum energy. This has numerous beneficial aspects, in terms
23
of explaining the universe we see: it predicts a smooth, flat spacetime, but
24
one with small fluctuations in density— exactly the kind that can grow into
25
stars and galaxies through the force of gravity over time. We don’t currently
26
have direct evidence that inflation actually occurred, but it is such a natural
27
and useful idea that many cosmologists have adopted it as a default mecha-
28
nism for shaping our universe into its present state.
29
Taking the idea of inflation, and combining it with the uncertainty of
30
quantum mechanics, can lead to a dramatic and unanticipated conse-
31
quence: in some places the universe stops inflating and starts looking like
32
what we actually observe, while in other places inflation keeps going. This
33
“eternal inflation” creates larger and larger volumes of space. In any par-
34
ticular region, inflation will eventually end— and when it does, we can find
35S
ourselves with a completely different compactification of extra dimensions
36N
than we have elsewhere. Inflation can create a potentially infinite number
308
Big Picture - UK final proofs.indd 308
20/07/2016 10:02:51
A R E W E t h E P O I n t ?
of regions, each with its own version of the local laws of physics— each a
01
separate “universe.”
02
Together, inflation and string theory can plausibly bring the multiverse
03
to life. We don’t need to postulate a multiverse as part of our ultimate phys-
04
ical theory; we postulate string theory and inflation, both of which are
05
simple, robust ideas that were invented for independent reasons, and we get
06
a multiverse for free. Both inflation and string theory are, at present, en-
07
tirely speculative ideas; we have no direct empirical evidence that they are
08
correct. But as far as we can tell, they are reasonable and promising ideas.
09
Future observations and theoretical developments will, we hope, help us
10
decide once and for all.
11
What we can say with confidence is that if we get a multiverse in this