Thank you to Displate for supporting PBS. The multiverse pops out of quite a
few theories in physics and has been
proposed as a solution to certain vexing
problems. But it's also been argued that
the very idea of a multiverse is just
bad science. That it's unfalsifiable and
a dead end to inquiry and as bad a
violation of Okam's razor as you could
imagine. But the multiverse might also
be real. Can something that exists be
bad science?
[Music] Hey everyone, a couple of quick
and one with an all black hole. Links in the description. In recent episodes,
we've talked about how the fundamental
parameters of our universe are
apparently finely tuned for the
production of entities that can
speculate about the finetuning of the
universe. One possible explanation that
we've teased several times now is that
there exists a preposterously large
ensemble of universes spanning all
possible parameters, both habitable and
uninhabitable, with the latter being the
extreme majority. We are of course in an
inhabitable one by necessity. Some would
say that this anthropic principle, this
argument of an observer bias actually
predicts that a multiverse exists.
Others say that the multiverse is just
bad science.
Before we decide one way or the other,
let's review the types of multiverse and
where they come from. And before we
review multiverses, let's remind
ourselves what we mean by universe.
The original meaning of the word was
just the totality of all existence which
makes the idea of a multiverse
nonsensical. If it exists, it's in the
universe. But in modern usage, universe
tends to mean this particular continuous
spaceime that traces back to a
particular big bang. For that reason, we
have more concrete terms. For example,
the observable universe refers to
everything we can see in that spaceime
given the amount of time light has had
to reach us whose outer boundary we call
the particle horizon. But if you could
teleport just beyond the particle
horizon, presumably things would be
pretty similar, more stars, more
galaxies, etc. whose light just hasn't
reached us yet. And we typically think
of those regions beyond as being part of
our universe. So maybe the universe is
everything connected to the spaceime
that we live in. But there are
conditions that might justify calling
some distant but connected region a
different universe. For example, if the
laws of physics change over extremely
long distances, there could be regions
of a vast connected spaceime that
operate very differently to here. This
is often called a quilt multiverse. A
related multiverse comes from the
eternal inflation hypothesis where the
main greater spaceime is expanding
vastly more quickly than our universe.
Bubbles of more gently expanding space
appear and become universes like our
own. In this case, these bubbles may
also have very different laws of physics
to each other. Other ways to generate
universes with different physics include
birthing them inside black holes, as in
Lee Mullen's cosmological natural
selection, or various cyclic universe
options in which the laws shift between
cycles. There's also the multiverse of
the many world interpretation of quantum
mechanics. Some of what I get to today
is relevant to that, but I'm going to
focus on the multiverses that allow
varying laws of physics. There are
concrete reasons that the laws of
physics might change between universes.
For example, changing the quantum vacuum
state changes the physics. In our
universe, the mass of the Higs Bzon and
hence the masses of all other particles
depend on the slightly nonzero energy of
the Higs field ground state. But the
ground state of a given field may be a
local minimum and other ground states
are possible. Changing the Higs ground
state changes the Higs mass and our
universe looks very different. For
almost all random choices of a Higs
mass, the universe is uninhabitable.
Another example where the laws of
physics change depending on the
configuration of the underlying fields
is in string theory. There the machinery
generating all particles and forces is
in the form of compactified extra
dimensions coiled into something called
a kabi yao manifold. The geometry and
topology of this manifold and the
corresponding string vibrational modes
determine the properties of particles
and the strengths of interactions. This
leads to an enormous number of possible
configurations for the ground or vacuum
state. at least 10 to the^ of 500 of
them. In fact, each producing a universe
with different physics. This is the
string landscape. A colossal landscape
of possible universes that may or may
not actually exist out there. String
theory can't tell us why our universe's
calabi ya manifold landed on the
particular vacuum state that generates
the particles and forces of the standard
model. Nor can the standard model of
particle physics by itself tell us why
the underlying quantum fields have their
particular vacuum states. It would be
nice to find such a mechanism. But what
if there just isn't one? What if all
possible vacuum states can happen and we
just happen to be in one of the versions
capable of producing life? That's the
anthropic principle. In its most
reasonable form, it argues that we
shouldn't be surprised to find ourselves
in an unusual part of the uni/multiverse
because we ourselves are unusual. At
some levels, it's not in the least
controversial. For example, we find
ourselves in a very rare environment in
our universe, a habitable biosphere.
Even though habitable biospheres occupy
an infinite decimal fraction of the
volume of the universe, we're not
surprised to find ourselves in one. I
mean we have to have emerged somewhere
that we can have emerged
extended to the multiverse if there is
one. It should be equally unsurprising
that we emerged in a universe that can
produce habitable biospheres which in
turn require a very specific
configuration of the laws of physics.
And our universe's laws of physics do
seem specific. Change the strength of
dark energy or the mass of the Higs
boson or even the number of dimensions
of spacetime and our universe likely
would not have produced life. Coupled
with the anthropic principle, the
multiverse gets us out of having to
explain why these specific laws and in a
way also gives an explanation for why
the physics of our universe seems
finally tuned to allow life. It's a
selection bias just like the bias that
leads us to observing the universe from
an unusually warm wet biosphere. Some
would take this much further. Some say
that this apparent finetuning of our
universe's parameters in favor of
habitability is actual evidence for the
multiverse. So now we have the
multiverse and the anthropic principle
very loosely defined. So let's decide if
this is bad science. We're going to
break this into two broad categories of
criticism. One is that the multiverse is
unparimious.
It violates Okam's razor. and two is
that the multiverse is scientifically
useless in various ways. It's
unfalsifiable andor an explanatory dead
end. Let's start with one. The principle
of parimony, more often called Okam's
razor, can be stated in several ways,
but let's start with one of the
phrasings used by William of Okam
himself. Never posit pluralities without
necessity. Hm. Well, a multiverse is
about as plural as you can get. So maybe
we've already annoyed brother William.
Okam's edict has been refined over the
years. Another common phrasing is
entities must not be multiplied beyond
necessity. Again, sounds damning, but we
can at least start to pick this apart.
What's an entity in this context? Isaac
Newton offers some guidance. We are to
admit no more causes of natural things
than such as are both true and
sufficient to explain their appearances.
Well, this is a critical refinement.
We're told to limit the plurality of
causes in an explanation which is very
distinct from limiting say the plurality
of predictions. Now, way before William
of Okam, we have some enlightening
phrasing by Aristotle.
We may assume the superiority other
things being equal of the demonstration
which derives from fewer postulates or
hypotheses.
A more modern and parimonious version of
that is all of us being equal, simpler
explanations should be preferred over
more complex ones. So the thing we are
advised against multiplying is not the
outcome or predictions of a hypothesis
but rather the number of entities in the
explanation. The causes not the
predictions according to Newton. But
what are these entities? Well, they are
the number of postulates or assumptions.
Basically, the number of new things
required to make a hypothesis work.
These could be principles like law of
physics or an initial condition of the
universe. Or they could be physical
stuff like a new quantum field or a
multiverse. In general, Okam's razer
wants us to be reductionists to minimize
either the complexity of the underlying
theoretical framework or to minimize the
number of fundamental building blocks to
seek theoretical or ontological
parimony. In fact, these two things may
ultimately be the same thing. Okam's
razor basically warns against
overfitting. If you want to find a
mathematical description of some
arbitrary curve, you can always do it if
you have enough degrees of freedom in
your function. For example, a for series
with an arbitrarily large number of
terms can fit any continuous curve. But
the resulting mathematical form has no
explanatory power. It's not a general
law of nature that can make predictions
beyond its overfitted curve. We could
equally describe the universe as a vast
m of points indicating the location of
all particles, but that's complex and
useless compared to the relative
parimony of the standard model plus
general relativity and some initial
conditions. So with that refinement of
Okam's razor, where does the multiverse
land? In cases where a theory predicts
the existence of a multiverse, it seems
that Okam's razor doesn't apply.
Especially if the theory was concocted
to do a different job and the multiverse
just popped out. In that case, the
multiverse is not a prior assumption of
the theory. It extends from it. So
things like the string landscape, the
eternal inflation multiverse, and
honestly even the splitting timelines of
many worlds shouldn't be considered a
problematic plurality when it comes to
assessing these ideas. When we bring in
anthropic arguments, the situation is
muddier. We could say that the
multiverse is predicted by anthropic
arguments in that fine-tuning plus
selection bias demands many untuned
universes.
But we could also frame the multiverse
as a cause or an assumption in our
argument. We observe fine-tuning because
there's a multiverse. In the latter
case, we need to pick apart our
principle of parimony a bit more
carefully. If we propose or assume or
postulate a multiverse as an
explanation, have we multiplied entities
beyond necessity? Have we overfit? Some
would say yes. I don't agree. It's
tempting to want to assume the least
extravagant universe possible, but the
size or number of universes doesn't
necessarily map to the number of new
prior assumptions that we would use to
weigh our hypothesis against Okam's
razor. And the impulse to argue for the
least extravagant universe has failed us
in the past.
The ancients rejected the idea that the
earth is moving around the sun because
if that were so, then the distant stars
should appear to change position
relative to each other due to parallax.
They didn't consider that it was even
reasonable for the stars to be so far
away that parallax was too small to
observe. Their bias towards smallness
led them astray. In the great debate in
1920, Heb Curtis argued that the red
shift of the spiral nebula meant they
had to be entire galaxies far beyond the
Milky Way, while Harlow Shapely
represented the status quo feeling that
such distances would mean an
uncomfortably large universe. But now
we're completely comfortable with an
observable universe 93 billion
lightyears in diameter and a greater
universes hundreds of times bigger or
even infinitely large. So, as our
measured boundary of the universe
expands, so does our acceptance of new
increasingly preposterous sizes. For
this reason, it's no longer compelling
to argue against the multiverse based
only on a sense of excessive bigness. If
that bigness encompasses regions where
continents of nature shift or stretch to
regions where inflation never stopped,
that's not necessarily unparimious due
to the sheer bigness alone. Okam's razor
asks that we seek the fewest moving
parts, but it doesn't say anything about
the sizes of those parts. So the
parimony of a given multiverse theory is
really equivalent to the parimony of the
theory that predicts the multiverse
which has little to do with the
multiverse that it predicts. If internal
inflation or string theory or
cosmological natural selection or
whatever are not too far-fetched as
solutions to fine-tuning themselves,
then neither is the multiverse. Okay, on
to the next point. Is the multiverse bad
science because it's unfalsifiable or an
explanatory dead end? I'm going to only
touch on this one because there are
details that warrant spending a lot more
time on this which we don't have. So,
the badness of multiverse science
depends very much on your definition of
science. If you adhere to a strict
poparian view that science is only that
which can be falsified, disproved by
doable experiments, then you could argue
that the multiverse is not even science.
But by the same logic, you could argue
that hypothesizing that galaxies exist
beyond the particle horizon is also bad
science because we can't test those
either. A more relaxed view would say
that science applies logical reasoning
coupled with a whole suite of tools to
collectively find a consistent picture
of what reality is. If something is
potentially part of reality, then it's
in principle approachable by scientific
methods. So if the multiverse might
exist, then it's very much in the realm
of science. And in fact, it's not true
that we can't test multiverse theories.
There are a few proposals for direct
tests for other universes that I won't
get into here, although we've talked
about a couple of them before. But there
are also tests that involve predicting
what we should see in this universe if
its unusual properties really are a
result of anthropic selection. For
example, Steven Weineberg showed that
anthropic arguments gave a pretty clear
prediction for the value of dark energy
under the assumption that our universe
should have the most likely value for
dark energy that could permit our
existence. The broader topic of
anthropic predictions is deep and
fascinating and I'll also save it for
another time. The fact that various
multiverse ideas are testable given
sufficient creativity means they also
are not scientific deadends. It's not
true to say that by allowing anything to
be possible, we no longer have a path to
explaining our universe because all
universes exist. In fact, the
foundational theory that allows a
multiverse to exist will make pretty
specific predictions about the
distribution of properties of the
universes in that multiverse. And that
can place strong constraints on the
likely properties of our universe under
anthropic selection. So, the multiverse
doesn't have to be bad science, but it
also can be. If the idea is proposed as
a blanket answer to fine-tuning without
a concrete mechanism that can be used to
make other testable predictions, then
your proposition is indeed unparimious,
unfalsifiable, and an explanatory dead
end. Bad science. But that's not how
serious proponents of various multiverse
generating theories do it. treated with
due care, we can indeed science the
prospect of the plurality of spacetime.