Is Ours the Only Universe? Janna Levin Walks Through the Multiverse Theories

Levin opens by asking whether everything that is possible in the universe is inevitable, if it is allowed by the laws of physics. She then asks whether nature will experiment with every possibility, no matter how improbable.

The question is the framing for the episode, which introduces the multiverse as a way to consider it.

Levin describes a series of blows to humanity's ego. We are not at the center of the solar system, the Sun is not at the center of the galaxy, and the galaxy is not at the center of the universe.

She adds that it is not only that we are not central. Our whole planetary system is one of countless such systems, since there are trillions of planets and trillions of galaxies. That pattern leads to the question of whether we are in the only universe or whether there are others.

Levin says physicists did not have to go looking for the multiverse. The possibility was forced on them by the search for a theory of everything, which would distill all the laws of physics into one law describing one incarnation of the universe. The aspiration was to say that the universe had been predicted and that this is it.

Levin explains that by a universe we mean everything we can observe, including the vast numbers of galaxies, stars and planets, as well as dark energy and dark matter. It also includes the space and time in which all of that resides, and the underlying code that determines how the universe evolves, which are literally the laws of physics.

Levin says the aim of physics is to predict the future from initial conditions, or the past from what we observe. She notes that something is odd, because the initial conditions seem to lie outside the laws of physics themselves.

She uses the example of a ball rolling down a hill. Knowing how fast it was moving and what force was applied lets you predict its path exactly. Similarly, if we knew exactly how fast the universe was expanding and how much energy the big bang contained, we could predict how the universe evolved afterward. Yet we do not know what determines the precise details of that initial state. She asks whether those conditions are predicted by a theory of everything or are simply sampled at random when the universe begins.

She says this question leads toward considering a multiverse.

Levin asks what would happen if the same laws of physics governed a universe with slightly different initial states. It might expand a little more quickly, have a little more dark energy, or have a different number of dimensions. She does not know whether the laws of physics can determine those things, but if these details could be different, she asks why they would not be.

On this view, our universe seems to be just one possibility in a vast landscape of possibilities. The question is whether nature made only this one, or whether all possibilities have been tried and in some sense exist.

Levin says there are many ways nature could produce a multiverse, and she begins with an eternal universe that has always existed and always will, in a state of rapid expansion. The universe is dominated by the energy of empty space, sometimes called dark energy, which is unstable in the primordial universe and drives the expansion exponentially.

Because the energy is unstable, small patches of this spacetime can drop out of the expansion and decay. All of that energy breaks off in a bubble, a kind of bud or plume, which she compares to a shoot growing off a ginger root. That energy is dumped into a primordial explosion that looks like a big bang, and the bubble effectively becomes its own universe. Inhabitants of that bud would look back and see what looks to them like a big bang, and they could not penetrate further than that, so it would feel like an isolated universe.

Each bud could have slightly different conditions, so there could be an infinite number of such bubble universes, which Levin describes as one manifestation of a multiverse.

Levin considers a different idea: the big bang as creation from nothing. The big bang may be the start of space and time themselves, not just an explosion within space. She says this is hard to grasp even theoretically. One idea is that there was literally nothing, with no space and no time, and that a quantum event nucleated the universe, which she calls a random probability from quantum mechanics.

She then asks whether this extraordinary quantum event happened only once, or whether nature experiments with it repeatedly.

Levin compares the process to a slot machine that tries again and again and gets different combinations. Each of these combinations is disconnected from us, not only in space but also in time, so there is no meaning to when they occurred relative to when we occurred.

She asks how we could ever know about them. If we look back at our big bang and cannot see past it, how could we know that other universes exist, and how might we never be able to communicate with or observe another universe?

Asked whether this makes the idea unscientific or not worth our time, Levin says that by understanding the underlying code, which is effectively the laws of physics and which she calls the underlying DNA, we can learn whether that DNA allows for changes and different combinations. If it does, we might know that we have sibling universes out there, which is possible.

She argues it becomes strange to imagine that nature has done this only once, now that we have been removed from the center. She compares it to life producing only one human being, and says these are just existential possibilities.

Levin offers what may be the simplest possibility: our big bang was the only big bang, and our universe was created infinite and is expanding, getting even bigger. She notes that some infinities can be bigger than others. Our observable universe is only a tiny piece of this infinite space.

Beyond what we can see is another region that cannot overlap with ours. In that region, conditions might be a little different, because the conditions created were not completely uniform across all of space. In an infinite universe, a multiverse could arise from that alone. One peculiarity of this view is that somewhere else there might be a planet just like Earth, only slightly different, and people watching a similar show, talking about a multiverse with someone slightly different from Levin.

Levin says that if the universe is truly infinite and has an eternity to unfold, every possibility should be allowed to happen somewhere, and every possibility can happen an infinite number of times.

Levin notes an oddity: the interior of a black hole has properties similar to a big bang, including high energy and extreme curvature. The idea is that the singularity of a black hole could create a big bang, as though a bubble baby universe pinched off and became its own world. That possibility would exist in any universe with black holes, which would then create baby universes in their interiors, so the multiverse becomes a nested possibility within any universe.

She calls this highly conjectural, seemingly inconceivable and utterly improbable. She still asks whether it is possible and, if so, whether nature might try it.

Levin says many people think of the multiverse in terms of quantum mechanics, specifically the many-worlds interpretation, which is perhaps related but also different. The idea is that even within a laboratory, if she performs a quantum measurement, all possible outcomes of that measurement branch off into multiple worlds. These worlds are not the same as universes physically bubbling off. They exist in parallel to each other but are disconnected, and every time a quantum event happens, these multiplicities occur.

She describes this as a purist interpretation of quantum mechanics, because it respects the theory without adding anything new to it. She reminds listeners that quantum mechanics is the most successful paradigm in the history of science. She also acknowledges that the interpretation might challenge our ideas and existential notions, and that quantum mechanics may not be as pure as it looks, so many worlds might not exist.

Levin says she is not a huge proponent of the many-worlds interpretation, but adds that she has been wrong before. If the idea is correct, she jokes that she has been wrong an infinite number of times in an infinite number of worlds.

Levin concludes that the question is whether the initial conditions are simply a random probability, because they determine the entire evolution of the universe afterward. Either this is the only universe and nature made just this one, or nature has tried every possibility, in which case we are not in a universe but in a multiverse.

She suggests that instead of resisting the multiverse and wanting to be the sole, unique, special incarnation in nature, we might have to accept that we are part of it. In that multiverse we would be no more unique, special or central than we are in this universe. She closes with: at least in theory.