10 reality-bending facts about the scientific Multiverse

Bring up the concept of the Multiverse, and you’re likely to get a variety of responses. Some will look to it as an idea full of hope: the hope that there’s a version of you out there that made a bolder choice, had a better outcome, or avoided a critical blunder at some point along your life path. Maybe, out there in the Multiverse, is a version of you with a better life, a fatter wallet, a superior job and career, or a version who didn’t suffer the great losses, illnesses, or setbacks you’ve had to reckon with. On the other hand, maybe there are versions of you out there that have suffered far worse than you, including versions where you haven’t made it alive and well to the present day. The Multiverse, at least as most people think of it, is full of our hopes and fears as much as it is of any flavor of physics.

And yet the Multiverse itself comes from physics! Originally there were just two concept for the Multiverse:

• one that arises from the many-worlds interpretation of quantum mechanics,
• and one that arises from the fact that out there, beyond the limits of our observable Universe, is “more universe” that’s forever beyond our reach, perhaps even an infinite amount of it, and perhaps where parts of it are very, very different from the Universe we inhabit and recognize.

I had the pleasure to film a video with Big Think about The secret of the multiverse, and to go along with this video, I’ve compiled a story highlighting 10 facts that challenge the way you think about the reality that arises from the Multiverse. Enjoy the video, and the 10 facts, below.

1.) Our Universe has two parts: an observable part and an unobservable part, and we know very little (except lower limits) about the unobservable part.

When you break down the idea of “Multiverse” into its simplest form, it reduces to the notion that sure, we have our Universe that we know and recognize — our observable Universe — but that there’s certain to be “more Universe” beyond the boundary of what we can even potentially observe.

It turns out this “unobservable part” itself can be broken down into two further components:

• the component that arose from the same hot Big Bang “event” that our observable Universe came from, except that it lies beyond the future visibility limit of what we’ll ever be able to see,
• and the component that arises from other hot Big Bangs — i.e., where inflation ends in causally disconnected regions from our own — which itself can be considered its own “baby universe” distinct from our own Universe.

Although the first of these likely has many other stars, galaxies, planets, and living creatures within it, we have no way to observe and measure it. The second is even tougher to fathom, as there’s no conceivable way to access any of these other baby universes within the currently known rules that nature obeys. And yet, both of these are strongly suspected to exist, as they’re unavoidable consequences of the reality we understand today.

The observable Universe might extend for 46 billion light-years in all directions from our point of view, but that’s not unique to our vantage point; all observers at all locations would experience the same thing. There’s certainly more, unobservable Universe just like ours beyond the limits of what we can see. It’s unfair to associate any particular point with the center, as what we perceive is determined by the amount of time that’s passed since the light observed today was emitted, rather than the geometry of the Universe.

2.) Every “decision” that gets made, including where a “quantum outcome” is realized, reflects only one possible outcome that could’ve occurred. Where do the other possible outcomes reside?

We know this every time we flip a coin: that this was a random process, and that if we flipped the same coin with the same hand over and over again, we wouldn’t be guaranteed to get the same outcome. In fact, if you flip that coin 100 times in a row, and you write down the sequences of “heads vs. tails” that you get, you’d have to perform a hundred flips of that coin roughly a nonillion (~10³⁰) times before you reproduced the sequence you got when you performed your original 100 flips.

With so many possible outcomes, why did our Universe arrive at the one specific sequence you obtained when you flipped that one coin 100 times in a row? What happened to all of the other possible outcomes?

Well, in theoretical physics, all the possible outcomes that could have (but didn’t occur) still exist somewhere in our reality. They’re not necessarily represented by our real, physical space, however: rather, they exist in the mathematical space that all possible quantum outcomes reside in, known mathematically as a Hilbert space. Whether a specific set of outcomes occurs or not can only be determined by a measurement or observation, but until that critical measurement or observation occurs, reality only exists in an indeterminate state: a probability-weighted superposition of all possible outcomes.

We normally conceive of our Universe as having emerged from a preceding period of cosmic inflation, with our Big Bang occurring where one region of inflating space ceased inflating and transitioned to being dominated by matter and radiation. However, in other locations, inflation continues indefinitely, giving rise to other baby (or bubble) universes, potentially with very different properties and conditions from our own.

3.) When you make that critical measurement or observation, however, you determine the one path that describes your lived-in Universe. What happens to the other possible outcomes?

This is where the Multiverse comes into play from the quantum physics perspective. Different interpretations of quantum physics lead to vastly different mechanisms for going from:

• a pre-observation probability distribution of potential outcomes that is not uniquely determined,
• to a post-observation realization of one specific outcome, where all other possible outcomes do not describe our Universe.

In the standard (Copenhagen) interpretation, we simply state that the act of measurement, observation, or a “sufficiently high-energy, low-uncertainty interaction” collapses the wavefunction.

But there are other possible interpretations. There’s the ensemble interpretation, the transactional interpretation, the de Broglie-Bohm (pilot wave) interpretation, and more. The idea of the quantum mechanical Multiverse arises from the many-worlds interpretation, which posits that all of the possible outcomes that could occur actually did occur somewhere within this Hilbert space: an infinite-dimensional vector space. While we only inhabit the one “line” of that vector space that corresponds to the observed/measured outcome for our experiment, the others all still exist: just somewhere beyond our reach in this Hilbert space.

A variety of quantum interpretations and their differing assignments of a variety of properties. Despite their differences, there are no experiments known that can tell these various interpretations apart from one another, although certain interpretations, like those with local, real, deterministic hidden variables, can be ruled out.

4.) Could these “other portions of the Hilbert space” be physically real, but simply somewhere else? That’s the idea behind combining the inflationary multiverse with the many-worlds interpretation of quantum physics.

Imagine you had a single quantum particle, and you wanted to know something about it: say, its spin in the z-direction. If you imagine two identical universes that each have this particle and the measuring device (say, a magnet oriented in the z-direction) to it, having both universes would be enough to hold all possible outcomes. If you instead had a single unstable quantum particle and were waiting to see if it had decayed, and you wanted to know what direction the various decay products would go off in, it wouldn’t be enough to have two identical universes; you’d actually need an infinite number of them.

The reason you’d need an infinitely large number of universes is twofold:

• time is continuous, and so the decay has a non-zero probability of decaying at each and every moment along the way,
• and the direction of a decay is random (and the possibilities are continuous), so all possible directions must be accounted for.

How can you get an infinitely large number of universes? You need something else that grows in an infinite, unbounded fashion. Therefore, the inflationary multiverse is a natural candidate if you’re seeking a place for all of these possible quantum outcomes to exist in.

The quantum fluctuations inherent to space, stretched across the Universe during cosmic inflation, gave rise to the density fluctuations imprinted in the cosmic microwave background, which in turn gave rise to the stars, galaxies, and other large-scale structures in the Universe today. This is the best picture we have of how the entire Universe behaves, where inflation precedes and sets up the Big Bang. Unfortunately, we can only access the information contained inside our cosmic horizon, which is all part of the same fraction of one region where inflation ended some 13.8 billion years ago.

5.) The inflationary Multiverse arises simply by combining two notions: the notion of cosmic inflation with the fact that all physical fields — including, presumably, the inflating one — are inherently quantum in nature.

The theory of cosmic inflation is what precedes and sets up the hot Big Bang. In order for it to work, it has to:

• have the Universe exist in a false vacuum state,
• where a large amount of energy is bound up in “field energy,” or the energy inherent to space itself,
• where it remains there for a sufficiently long time, expanding at an exponential rate (and creating new space) while it inflates,
• and then inflation ends when the field transitions to a lower-energy state,
• converting that field energy into particle energy,
• signaling the start of the hot Big Bang.

The way we typically visualize this is by a ball on top of a flat hill: where inflation (and the exponential expansion of space) continues as long as the ball remains there. When the ball rolls down off of the plateau, inflation ends.

But that “ball on a hill” picture treats inflation as a classical field, when in fact we live in a quantum universe. So now, instead, imagine that field “spreading out” atop the plateau. Sure, some places will see inflation end as the ball rolls off the edge, giving rise to hot Big Bangs and their own “baby universes” where that occurs. But then, other places will see inflation continue for longer, and will continue inflating more and more: creating new space. In this fashion, there will always be new “baby universes” created during this process, but there will also always be new regions of space, going forward, that will continue to inflate forever: eternally, even.

From whatever pre-existing state started it, inflation predicts that a series of independent universes will be spawned as inflation continues, with each one being completely disconnected from every other one, separated by more inflating space. One of these “bubbles,” where inflation ended, gave birth to our Universe some 13.8 billion years ago. Today, dark energy dominates the Universe and causes space to expand exponentially as well. These scenarios may be related, but we have no idea how long inflation persisted for prior to the hot Big Bang: only the ability to say, “at least 10^-32 seconds” or so.

6.) The inflationary multiverse and the many-world interpretation of quantum mechanics can both admit an infinite number of possibilities, but it’s a fundamentally different “type” of infinity for each one.

This is a challenging concept for many: the notion that some infinities are bigger than others. For example, consider the following sequences.

• 1, 2, 3, 4, 5, …
• 2, 4, 6, 8, 10, …
• 1, 4, 9, 16, 25, …
• 2, 4, 8, 16, 32, …
• 1, 10, 100, 1000, 10000, …
• 1, 2, 6, 24, 120, ….

Each of these sequences, if you extend them infinitely forward, to arbitrarily large numbers of terms, clearly always tends towards infinity, as the numbers just get larger and larger. But they don’t all get larger at the same rate.

The first two sequences point to an arithmetic series: they simply add the same number to the prior term to get the next one. These tend towards infinity, but it’s the same “class” of infinity. Although the third sequence is a geometric series, where the difference between successive terms increases (as a power law, in this case), it’s still the same class of infinity as the first two examples. The fourth and fifth sequences, however, are examples of exponential sequences: as some number raised to the n^th power, where n is the number of terms used. This is a larger class of infinity than the first three examples, and describes what happens to the volume of space during inflation. But the sixth and final sequence is a factorial sequence: one based on combinatorics. This is still a larger class of infinity than even exponentials, and it’s a class that describes the possibilities for successive interactions between particles in a physical system, like the number of possible quantum outcomes. The “exponential” and “combinatoric” classes apply to the infinities of the inflationary multiverse and the many-worlds interpretation’s multiverse, respectively.

Inflation ends (top) when a ball rolls into the valley. But the inflationary field is a quantum one (middle), spreading out over time. While many regions of space (purple, red and cyan) will see inflation end, many more (green, blue) will see inflation continue, potentially for an eternity (bottom). The quantum nature of inflation means that it ends in some “pockets” of the Universe and continues in others.

7.) And yet, inflation really does inevitably produce a Multiverse: an exponentially infinite type of Multiverse. Moreover, once inflation begins, it always continues in some pockets of the Multiverse.

If we take the results of fact #5 and fact #6 together, we get something extremely interesting: the notion of an infinite number of continuously-created universes arising from inflation. Think about what happens when inflation — whenever and however it happened — begins. A patch of space, whether small or large, begins expanding exponentially: doubling in a tiny fraction of a second, and then doubling again the next time that fraction-of-a-second elapses, and then again, and again, and again, relentlessly. Because of the quantum nature of inflation, the value of the inflationary field varies randomly all over this space.

• In some regions, the field reaches the edge of the plateau and falls off: giving rise to a hot Big Bang and a baby universe.
• In other regions, the field remains on the plateau and continues inflating: pushing any “baby universes” mutually away from each other.
• Meanwhile, other regions reach the edge of the plateau and fall off, creating new baby universes.

Because there’s always some region in between any two baby universes that keeps on inflating, we say this means inflation is future-eternal: once you start it, it continues forever. You really can create an infinite number of universes within the Multiverse in this fashion; in fact, you cannot avoid doing so. The only issue is that you create these new universes at an exponential rate: a large type of infinity, but not the largest.

The Many Worlds Interpretation of quantum mechanics holds that there are an infinite number of parallel universes that exist, holding all possible outcomes of a quantum mechanical system, and that making an observation simply chooses one path. This interpretation is philosophically interesting, but has no physical meaning if there isn’t enough “universe” out there to physically hold all of these possibilities within it.

8.) However, the number of quantum mechanical possibilities rises more quickly: as a combinatoric infinity. If you start inflation and you start the many-worlds interpretation at the same time, you cannot “fit” the quantum outcomes into the inflationary Multiverse.

Now we run into the big problem of trying to make the Multiverse we’re familiar with — the infinite parallel universes that have an infinite number of copies of you, me, and our shared reality — fit into the inflationary Multiverse that we can actually anticipate physically existing, from a cosmology perspective. At the start of the hot Big Bang, we have an estimated ~10⁹⁰ particles within our observable Universe, including photons, neutrinos, quarks and antiquarks and gluons and charged leptons and antileptons and more: whatever the Universe can create at these high energies, it does.

And those particles (and antiparticles) smash together, create new particle-antiparticle pairs, annihilate, decay, collide, scatter, fuse together, dissociate apart, and so on. This goes on for ~13.8 billion years, even as the Universe expands and cools and gravitates, giving rise to a great cosmic web with stars, planets, and even life within it. Thanks to the mathematics of combinatorics, we can estimate how many possible outcomes there are for such a physical system, and see how that number grows with time. Unsurprisingly, it grows combinatorically — like a factorial — which means it grows too large, too fast, for the number of possibilities to fit within the inflationary Multiverse. It’s too large of a class of infinity to be contained within our other “infinity,” as a combinatoric infinity is greater than an exponential infinity.

A representation of the different parallel “worlds” that might exist in other pockets of the multiverse. As time goes on, more and more possibilities must arise, meaning that the number of Universes that must exist to contain them all must rise as well, at least as quickly, or no two universes will ever be identical.

9.) Yet there remains a plausible way to fit all of the possible quantum outcomes into an inflationary Multiverse: if inflation endured for an infinite amount of time, or if the pre-inflationary universe were born infinite in extent.

So does that mean the Multiverse — or at least, the quantum mechanical, many-worlds interpretation of the Multiverse — is purely science fiction, and entirely divorced from the realm of scientific possibility at all? Not necessarily, as there are a few ways to save it, but only if the cosmos itself is kind to the idea.

• The overall universe, including inflationary and non-inflationary parts, could have been born infinite in extent, and so a truly infinite amount of space began inflating. Given that a combinatoric series only approaches infinity, never reaching it, in a finite amount of time (or from a finite volume of space), this is one scenario where it’s plausible.
• The overall universe, including the part of it that entered an inflationary state, could have been around for an infinite duration of time, even if it were born finite in volume. A region of space that’s been inflating, and expanding exponentially, for an infinite amount of time can always contain even a “larger infinity” if it’s only tending towards infinity, but began a finite time ago from a finite region of space. This is a second plausible scenario.
• Or, even if we had a finite amount of space that began a finite amount of time ago, you can still contain the many-worlds Multiverse, at least for now, if your inflating region of space began long enough ago or was sufficiently large. This will be a temporary situation in this third plausible scenario, as eventually combinatorics will overtake any exponential growth, and the only existing Multiverse will then go back to being inflationary, with just a finite subset of possible outcomes being populated within the Multiverse.

The multiverse idea states that there are very large numbers of Universes like our own out there, and others whose properties might have extreme, fundamental differences. But in order for the many-worlds interpretation of quantum mechanics to be physically real, there must be a place (i.e., a real Universe) for these parallel outcomes to reside in, and unless inflation occurred for an infinite amount of time, the math doesn’t work out right to contain them.

10.) Under all circumstances, there remains no way to communicate, exchange information with, or “switch reality” with any other copies that may exist within the Multiverse.

But this is the most important aspect to understand about the Multiverse: whether it exists or not, it’s not something we can interact with. It’s not something that can affect us in any way at all: the decisions we make, here, in this observable Universe, as well as all of the decisions that get made around us that have nothing to do with our agency, is the only quantum outcome we’ll ever see, know, or experience. What happens in this Universe may not be unique to our Universe, but within the reality that we inhabit, it’ll never be any other way. All of the hopes, fears, the “what ifs” and the “could have beens” that we imagine must remain there: in our imaginations alone.

Even if there is another version of you, me, or our entire known Universe out there, we’ll never know it, and we’ll never be able to find out about it. It’s one of those big ideas that skirts the boundary between science and philosophy, as even though the Multiverse is a science-based idea and a consequence of our best scientific theories, the fact that we can never test, measure, or observe any confirmation or refutation of its existence makes it ultimately speculative and dissatisfying from a scientific perspective. Philosophically, however, I think I said it very well in the Big Think video that just went live:

“What this means is that, in all of the Multiverse, as vast as it is, there’s only you. There’s only this one version of you, with the one life you’ve led up to this point, with the one opportunity to your life to the fullest, with the rest of it that remains. And I encourage you to do exactly that.”