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Making sense of many universes

This story was originally published by Knowable Magazine.

Almost anybody who has ever thought deeply about the universe
sooner or later wonders if there is more than one of them. Whether
a multiplicity of universes — known as a multiverse — actually
exists has been a contentious issue since ancient times. Greek
philosophers who believed in atoms, such as Democritus, proposed
the existence of an infinite number of universes. But Aristotle
disagreed, insisting that there could be only one.

Today a similar debate rages over whether multiple universes
exist. In recent decades, advances in cosmology have implied (but
not proved) the existence of a multiverse. In particular, a theory
called inflation suggests that in the instant after the Big Bang,
space inflated rapidly for a brief time and then expanded more
slowly, creating the vast bubble of space in which the Earth, sun,
Milky Way galaxy and billions of other galaxies reside today. If
this inflationary cosmology theory is correct, similar big bangs
occurred many times, creating numerous other bubbles of space like
our universe.

Properties such as the mass of basic particles and the strength
of fundamental forces may differ from bubble to bubble. In that
case, the popular goal pursued by many physicists of finding a
single theory that prescribes all of nature’s properties may be in
vain. Instead, a multiverse may offer various locales, some more
hospitable to life than others. Our universe must be a bubble with
the right combination of features to create an environment suitable
for life, a requirement known as the anthropic principle.

More Knowable Q&As

But many scientists object to the idea of the multiverse and the
anthropic reasoning it enables. Some even contend that studying the
multiverse doesn’t count as science. One physicist who affirms that
the multiverse is a proper subject for scientific
investigation is John Donoghue of the University of Massachusetts,
Amherst.

As Donoghue points out in the 2016 Annual Review of Nuclear
and Particle Science
, the Standard Model of Particle Physics — the theory
describing the behavior of all of nature’s basic particles and
forces — does not specify all of the universe’s properties. Many
important features of nature, such as the masses of the particles
and strengths of the forces, cannot be calculated from the theory’s
equations. Instead they must be measured. It’s possible that in
other bubbles, or even in distant realms within our bubble but
beyond the reach of our telescopes, those properties might be
different.

Maybe some future theory will show why nature is the way it is,
Donoghue says, but maybe reality does encompass multiple
possibilities. The true theory describing nature might permit many
stable “ground states,” corresponding to the different cosmic
bubbles or distant realms of space with different physical
features. A multiverse of realms with different ground states would
support the view that the universe’s habitability can be explained
by the anthropic principle — we live in the realm where conditions
are suitable — and not by a single theory that specifies the same
properties everywhere.

Knowable Magazine quizzed Donoghue about the meaning of
the multiverse, the issues surrounding anthropic reasoning and the
argument that the idea of a multiverse is not scientific. His
answers have been edited for brevity and clarity.

Can you explain just what you mean by
multiverse?

For me, at least, the multiverse is the idea that physically out
there, beyond where we can see, there are portions of the universe
that have different properties than we see locally. We know the
universe is bigger than we can see. We don’t know how much bigger.
So the question is, is it the same everywhere as you go out or is
it different?

If there is a multiverse, is the key point not just the
existence of different realms, but that they differ in their
properties in important ways?

If it’s just the same all the way out, then the multiverse is
not relevant. The standard expectation is that aside from random
details — like here’s a galaxy, there’s a galaxy, here’s empty
space — that it’s more or less uniform everywhere in the greater
universe. And that would happen if you have a theory like the
Standard Model where there’s basically just one possible way that
the model looks. It looks the same everywhere. It couldn’t be
different.

The Standard Model of Particle Physics

Isn’t that what most physicists would hope
for?

Probably literally everyone’s hope is that we would someday find
a theory and all of a sudden everything would become clear — there
would be one unique possibility, it would be tied up, there would
be no choice but this was the theory. Everyone would love that.

But the Standard Model does not actually specify all the
numbers describing the properties of nature, right?

The structure of the Standard Model is fixed by a symmetry
principle. That’s the beautiful part. But within that structure
there’s freedom to choose various quantities like the masses of the
particles and the charges, and these are the parameters of the
theory. These are numbers that are not predicted by the theory.
We’ve gone out and we’ve measured them. We would like eventually
that those are predicted by some other theory. But that’s the
question, whether they are predicted or whether they are in some
sense random choices in a multiverse.

The example I use in the paper is the distance from the Earth to
the sun. If you were studying the solar system, you’d see various
regularities and a symmetry, a spherically symmetrical force. The
fact that the force goes like 1 over the radius squared is a
consequence of the underlying theory. So you might say, well, I
want to predict the radius of the Earth. And Kepler tried to
do this
and came up with a very nice geometric construction,
which almost worked. But now we know that this is not something
fundamental — it’s an accident of the history. The same laws that
give our solar system with one Earth-to-sun distance will somewhere
else give a different solar system with a different distance for
the planets. They’re not predictable. So the physics question for
us then is, are the parameters like the mass of the electron
something that’s fundamentally predictable from some more
fundamental theory, or is it the accident of history in our patch
of the universe?

How does the possibility of a multiverse affect how we
interpret the numbers in the Standard Model?

We’ve come to understand how the Standard Model produces the
world. So then you could actually ask the scientific question: What
if the numbers in the Standard Model were slightly different? Like
the mass of the electron or the charge on the electron. One of the
surprises is, if you make very modest changes in these parameters,
then the world changes dramatically. Why does the electron have the
mass it does? We don’t know. If you make it three times bigger,
then all the atoms disappear, so the world is a very, very
different place. The electrons get captured onto protons and the
protons turn into neutrons, and so you end up with a very strange
universe that’s very different from ours. You would not have any
chance of having life in such a universe.

Are there other changes in the Standard Model numbers
that would have such dramatic effects?

My own contribution here is about the Higgs field [the field
that is responsible for the Higgs
boson
]. It has a much smaller value than its expected range
within the Standard Model. But if you change it by a bit, then
atoms don’t form and nuclei don’t form — again, the world changes
dramatically. My collaborators and I were the ones that pointed
that out.

There’s some maybe six or seven of these constraints —
parameters of the Standard Model that have to be just so in order
to satisfy the need for atoms, the need for stars, planets, et
cetera. So about six combinations of the parameters are constrained
anthropically.

By “anthropically,” you mean that these parameters are
constrained to narrow values in order to have a universe where life
can exist. That is an old idea known as the anthropic principle,
which has historically been unpopular with many
physicists.

Yes, I think almost anybody would prefer to have a
well-developed theory that doesn’t have to invoke any anthropic
reasoning. But nevertheless it’s possible that these types of
theories occur. To not consider them would also be unscientific. So
you’re forced into looking at them because we have examples where
it would occur.

Historically there’s a lot of resistance to anthropic reasoning,
because at least the popular explanations of it seem to get
causality backwards. It was sort of saying that we [our existence]
determine the parameters of the universe, and that didn’t feel
right. The modern version of it, with the multiverse, is more
physical in the sense that if you do have these differing domains
with different parameters, we would only find ourselves in one that
allows atoms and nuclei. So the causality is right. The parameters
are such that we can be here. The modern view is more physical.

If there is a multiverse, then doesn’t that change some
of the goals of physics, such as the search for a unified theory of
everything, and require some sort of anthropic
reasoning?

What we can know may depend on things that may end up being out
of our reach to explore. The idea that we should be searching for a
unified theory that explains all of nature may in fact be the wrong
motivation. It’s certainly true that multiverse theories raise the
possibility that we will never be able to answer these questions.
And that’s disturbing.

Does that mean the multiverse changes some of the
questions that physicists should be asking?

We certainly still should be trying to answer “how” questions
about how does the W boson decay or the Higgs boson, how does it
decay, to try to get our best description of nature. And we have to
realize we may not be able to get the ultimate theory because we
may not be able to probe enough of the universe to answer certain
questions. That’s a discouraging feature. I have to admit when I
first heard of anthropic reasoning in physics my stomach sank. It
kills some of the things that you’d like to do.

Don’t some people even argue that though a multiverse
would seem to justify anthropic reasoning, that approach should
still be regarded as not scientific?

It’s one of the things that bothers me about the discussion.
Just because you feel bad about the multiverse, and just because
some aspects of it are beyond reach for testing, doesn’t mean that
it’s wrong. So if it’s worth considering, and looking within the
class of multiverse theories to see what it is that we could know,
how does it change our motivations? How does it change the
questions that we ask? And to say that the multiverse is not
science is itself not science. You’re not allowing a particular
physical type of theory, a possible physical theory, that you’re
throwing out on nonscientific grounds. But it does raise long-term
issues about how much we could understand about the ultimate theory
when we can just look locally. It’s science, it’s sometimes a
frustrating bit of science, but we have to see what ideas become
fruitful and what happens.

An important part of investigating the multiverse is
finding a theory that includes multiple “ground states.” What does
that mean?

The ground state is the state that you get when you take all the
energy out of a system. Normally if you take away all the
particles, that’s your ground state — all the background fields,
the things that permeate space. The ground state is described by
the Standard Model. Its ground state tells you exactly what
particles will look like when you put them back in; they will have
certain masses and certain charges.

You could imagine that there are theories which have more than
one ground state, and if you put particles in this state they look
one way and if you put particles in another state they look another
way — they might have different masses. The multiverse corresponds
to the hypothesis that there are very many ground states, lots and
lots of them, and in the bigger universe they are realized in
different parts of the universe.

Kepler's mathematical explanation for the layout of the known solar system involved a series of geometrical solids, including a sphere, cube, pyramid and other shapes.

In the early 1600s, German astronomer Johannes Kepler sought a
mathematical theory for explaining the distances of the six known
planets from the sun. He found his answer in the ratios of
geometrical solids, or polyhedra. By embedding one within another,
Kepler showed that their dimensions roughly corresponded to the
planetary distances. But he didn’t know about other planets and
other solar systems; planetary distances differ from star to star
and are determined by local conditions, not a mathematical formula.
Some scientists today think that the properties of the universe are
similarly not determined by a formula, but they differ in different
realms of space composing a multiverse.

CREDIT: NORTH WIND PICTURE ARCHIVES / ALAMY STOCK
PHOTO

Even if a theory of particles and forces can accommodate
multiple ground states, don’t you need a method of creating those
ground states?

Two features have to happen. You have to have the possibility of
multiple ground states, and then you have to have a mechanism to
produce them. In our present theories, producing them is easier,
because inflationary cosmology has the ability to do this. Finding
theories that have enough ground states is a more difficult
requirement. But that’s a science question. Is there one, is there
two, is there a lot?

Superstring theory encompasses multiple ground states,
described as the “string landscape.” Is that an example of the kind
of theory that might imply a multiverse?

The string landscape is one of the ways we know that this
[multiple ground states] is a physical possibility. You can start
counting the number of states in string theory, and you get a very
enormous number, 10 to the 500. So we have at least one theory that
has this property of having a very large number of ground states.
And there could be more. People have tried cooking up other
theories that have that possibility also. So it is a physical
possibility.

Don’t critics say that neither string theory nor
inflationary cosmology has been definitely
established?

That’s true of all theories beyond the Standard Model. None of
them are established yet. So we can’t really say with any
confidence that there is a multiverse. It’s a physical possibility.
It may be wrong. But it still may be right.

This story was originally published by Knowable Magazine. Knowable Magazine is an independent journalistic endeavor from Annual Reviews.

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