The multiverse pops out of quite a few theories聽 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 Occam’s razor as you could imagine.

But the multiverse might also聽 exist.

Can something that exists be bad science?

In recent episodes we’ve talked about how the  fundamental parameters of our universe are, apparently, finely tuned for the production of聽 entities that can speculate about the fine-tuning of the universe.

One possible explanation聽 that we’ve teased several times now is that there exists a preposterously large ensemble聽 of universes spanning all possible parameters, both habitable and uninhabitable—the  latter being the extreme majority.

We are of course in an inhabitable one,聽 by necessity.

Some would say that this anthropic principle—this argument of an  observer bias—actually predicts that a multiverse exists.

Others say that聽 the multiverse is just bad science.

Before we decide one way or another, let’s  review the types of multiverse and where they come from.

And before we review multiverses,聽 let’s remind ourselves what we mean by uni-verse.

The original meaning of the word was the totality聽 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 spacetime that聽 traces back to a particular big bang.

For that reason we have more concrete terms.

For example,聽 the observable universe refers to everything we can see in that spacetime given the amount of聽 time light has had to travel to us, whose outer boundary we call the particle horizon.

But if you聽 could teleport just beyond the particle horizon, presumably things would be pretty similar—more  stars, more galaxies, etc.

whose light hasn’t reached us yet---and we typically think of those聽 regions beyond as being part of our universe.

So maybe the universe is everything聽 connected to the spacetime that we live in.

But there are conditions that might justify聽 calling some distant but connected region a different universe.

For example, if the laws of聽 physics change over extremely long distances, there could be regions of a vast, connected聽 spacetime that operate very differently to here.

This is often called a quilt聽 multiverse.

A related multiverse comes from the eternal inflation hypothesis, where the “main”  greater spacetime is expanding vastly more quickly than our universe.

Bubbles of more gently聽 expanding space appear and become universes like our own.

In this case, these bubbles may also聽 have different laws of physics to each other.

Other ways to generate universes聽 with different physics include: birthing them inside black holes as in Lee聽 Smolin’s cosmological natural selection; or various cyclic universe options in聽 which the laws shift between cycles.

There’s also the multiverse of  the Many Worlds interpretation of quantum mechanics.

Some of what聽 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聽 of physics might change between universes.

For example, changing the quantum vacuum聽 state changes the physics.

In our universe, the mass of the Higgs boson, and聽 hence of all other particles, depends on the slightly non-zero energy聽 of the Higgs field ground state.

But the ground state of a given field may be a local聽 minimum, and other ground states are possible.

Change the Higgs ground state changes聽 the Higgs mass and our universe looks very different.

For almost all random choices聽 of a Higgs mass the universe is uninhabitable.

Another example where the laws of physics change聽 depending on the configuration of the underlying fields is in string theory.

There, the machinery聽 generating all particles and forces is in the form of compactified extra dimensions, coiled聽 into something called a Calabi-Yau manifold.

The geometry and topology of this manifold聽 and the corresponding string vibrational modes determine the properties of particles聽 and strengths of interactions.

This leads to an enormous number of possible configurations聽 for the ground or vacuum state—at least 10^500 of them in fact—each producing a universe with  different physics.

This is the string landscape—a colossal landscape of possible universes that聽 may or may not actually exist “out there”.

String theory can’t tell us why our  universe’s Calabi-Yau manifold landed on the particular vacuum state that generates聽 the particles and forces of the standard model.

Nor can the standard model of particle physics聽 by itself tell us why the underlying quantum fields have their particular vacuum states.聽 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聽 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聽 the uni-slash-multiverse because we ourselves are unusual.

At some levels that’s not in the least  controversial.

For example, we find ourselves in a very rare environment in our universe: a habitable聽 biosphere.

Even though habitable biospheres occupy an infinitesimal fraction of the volume of the聽 universe, we’re not surprised to find ourselves in one.

We have to have emerged somewhere that we can聽 have emerged.

Extended to the multiverse if there is one, it should be equally unsurprising that we聽 emerged in a universe that can produce habitable biospheres, which in turn requires a very聽 specific configuration of the laws of physics.

That seems to be the case—change the strength  of dark energy or the mass of the Higgs boson or even the number of dimensions of spacetime and聽 our universe likely would not have produced life.

Coupled with the anthropic principle,聽 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 seem finely聽 tuned to allow life.

It’s a selection bias, just like the bias that leads to us observing聽 the universe from an unusually warm, wet, biosphere.

Some would take this much further,聽 and say that this apparent fine-tuning of our universe’s parameters in favor of habitability  is actual evidence for the multiverse.

So now that we have the multiverse and the聽 anthropic principle very loosely defined, 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 unparsimonious—it violates Occam’s razor.

And  two is that the multiverse is scientifically useless in various ways—it’s  unfalsifiable and/or an explanatory dead-end.

Let’s start with one.

The principle of  parsimony, more often called Occam’s razor, can be stated in several ways, but let’s start  with one of the phrasings used by William of Occam himself: “Never posit pluralities  without necessity.” Hm.

A multiverse is about as plural as you can get, so maybe聽 we’ve already annoyed Brother William.

Occam’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 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聽 true and sufficient to explain their appearances.” 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聽 plurality of predictions.

Way before William of Occam we have some enlightening phrasing聽 by Aristotle "We may assume the superiority, other things being equal, of the聽 demonstration which derives from fewer postulates or hypotheses.” A more  modern version is: “All else being equal, simpler explanations should be聽 preferred over more complex ones.” So the thing we’re 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聽 postulates or assumptions.

Basically, the number of new things required to聽 make the hypothesis work.

These could be principles like a law of physics or聽 an initial condition of the universe, or they could be physical stuff, like a new聽 quantum field or a multiverse.

In general, Occam’s razor wants us to be reductionists.

To  either minimise the complexity of the underlying theoretical framework or to minimize the聽 number of fundamental building blocks—to seek theoretical or ontological parsimony.

In聽 fact, these two may ultimately be the same thing.

Occam’s razor basically warns against overfitting.

If you want to find a mathematical description of some arbitrary curve, you can always do it聽 if you have enough degrees of freedom in your function.

For example, a a Fourier series聽 with an arbitrarily large number of terms can fit any continuous curve.

But the resulting聽 mathematical form has no explanatory power—it’s not a general law of nature that can make聽 predictions beyond its overfitted curve.

We could equally describe the universe as a聽 vast map of points indicating the locations of all particles, but that’s complex  and useless compared to the relative parsimony of the standard model plus general聽 relativity and some initial conditions.

So, with that refinement of Occam’s  razor, where does the multiverse stand?

In cases where a theory predicts聽 the existence of a multiverse, it seems that Occam’s razor doesn’t apply.

Especially if the theory was concocted to do a different job and the multiverse just popped聽 out.

In that case, the multiverse is not a prior assumption for the theory—it extends from  it.

So things like the string landscape, the eternal inflation multiverse, and聽 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聽 the situation is muddier.

We could say that the multiverse is predicted by anthropic聽 arguments—-in that fine-tuning plus selection bias demands many untuned universes.

But we聽 could also frame the multiverse as a cause or an assumption in our argument: we observe聽 fine tuning because there’s a multiverse.

In the latter case, we need to pick聽 apart our principle of parsimony a bit more carefully.

If we propose or assume聽 or postulate a multiverse as an explanation, have we multiplied entities聽 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聽 universes doesn’t map to the number of new, prior assumptions that we would use to聽 weigh our hypothesis against Occam’s razor.

And the impulse to argue for the least聽 extravagant universe has failed us in the past.

The ancients rejected the idea that the Earth is聽 moving around the Sun because, if that were so, then the distant stars should appear to change聽 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聽 led them astray.

In the great debate in 1920, Heber Curtis argued that the redshift of the聽 spiral nebulae meant they had to be entire galaxies far beyond the Milky Way, while Harlow聽 Shapely represented the status quo feeling that such distances would mean an uncomfortably large聽 universe.

Now we’re completely comfortable with an observable universe 93-billion light years聽 in diameter and a greater universe hundreds of times larger, or even infinitely large.

So as聽 our measured boundary of the universe expands, so does our acceptance of new,聽 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聽 encompasses regions where the constants of nature shift or stretch to regions聽 where inflation never stopped that’s not necessarily unparsimonious due to the聽 sheer bigness.

Occam’s razor asks that we seek the fewest moving parts, but it doesn’t  say anything about the sizes of those parts.

So, the parsimony of a given multiverse theory聽 is equivalent to the parsimony of the theory that predicts the multiverse, which has聽 little to do with the multiverse that it predicts.

If eternal inflation or string聽 theory or cosmological natural selection or whatever are not too far fetched as solutions to fine tuning themselves then聽 neither is the multiverse.

OK, on to the next point.

Is the聽 multiverse bad science because it’s unfalsifiable or an explanatory dead  end.

I’m only going to touch on this one, because there are details that warrant spending聽 a lot of time on this, which we don’t have.

So the badness of multiverse science depends聽 very much on your definition of science.

If you adhere to a strict Popperian view that science聽 is only that which can be falsified—disproved by doable experiments—then you could argue that the  multiverse is not science.

But by the same logic you would argue that hypothesizing that galaxies聽 exist beyond the particle horizon is bad science because that too can’t be tested.

A more relaxed  view would say that science applies logical reasoning coupled with a whole suite of tools to聽 collectively find a consistent picture of what reality is.

If something is potentially a part聽 of reality, then it’s in principle approachable by scientific methods.

So, if the multiverse might聽 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聽 I won’t get into here although we’ve talked about a couple of them before.

But there are also聽 tests that involve predicting what we should see in this universe if its unusual properties聽 really are a result of anthropic selection.

For example, Steven Weinberg showed that聽 anthropic arguments gave a pretty clear prediction for the value of dark energy under the聽 assumption that our universe should have the most likely value for dark energy that could permit聽 our existence.

The broader topic of anthropic predictions is deep and fascinating,聽 and I’ll also save it for another time.

The fact that various multiverse ideas are聽 testable given sufficient creativity means they also aren’t scientific dead ends.

It’s not true  to say that, by allowing anything to be possible, we no longer have a path to explaining our聽 universe because all universes exist.

In fact, the foundational theory that allows a multiverse聽 to exist will make pretty specific predictions about the distribution of properties of the聽 universes in that multiverse.

And that can place strong constraints on the likely properties聽 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聽 without a concrete mechanism that can be used to make other, testable predictions then聽 your proposition is indeed unparsimonious, unfalsifiable, and an explanatory聽 dead end.

Bad science.

But that’s not how serious proponents of various聽 multiverse-generating theories do it—treated with due care, we can indeed science the聽 prospect of a plurality of spacetime.