Is the multiverse actually possible? Physics has a lot to say about multiverses, and these stories only tend to focus on one kind. The multiverse is not a new invention. The concept has existed for countless years. Early versions were just cyclical. These many universes didn’t exist at the same time. They were more like one universe caught in a repeating series of creation and destruction. Then again, maybe you don’t think anything like that should be called a multiverse. So, Let’s see in detail for **What is multiverse theory.**

## What is multiverse theory? The Multiverse Theory Explained

Well, somewhere around 2000 years ago, we started to see a change in thought. According to the ancient Greeks: “It is in the highest degree unlikely that the earth and sky is the only one to have been created, and that all particles of matter outside are accomplishing nothing.” And of course, we all know the ancient Greeks were the pioneers of human philosophy.

The ancient Indians had similar writings: “Who will search through the wide infinities of space to count the universes side by side?” We don’t know exactly when statements like these were written because history is full of fairy tales, and ancient history is sparse. But we do know this idea stuck around. It’s something philosophers now call cosmic pluralism.

Though for a while this concept was intertwined with religion. In 1584, former Dominican friar Giordano Bruno would famously say: “He is glorified not in one, but in countless suns; not in a single earth, a single world, but in a thousand thousand. I say in an infinity of worlds.” The Christian God is referred to as “He” in that sentence.

As you can imagine, this idea wasn’t very popular in the Roman Catholic Church. Things did not end well for Giordano Bruno. Are we back? I think we’re back. Where was I? Right. The multiverse. The actual word “multiverse” wouldn’t enter scientific conversation until about 1896 in a debate between Ludwig Boltzmann and Ernst Zermelo.

They weren’t actually discussing cosmology, though. They were trying to decide if statistical mechanics was sufficient to describe the laws of thermodynamics. Of course, any conversation about entropy quickly grows to cosmic proportions. Pun intended. Anyway, we have two models on the front line of physics: general relativity and quantum mechanics, and both of them have something to say about the multiverse.

When you hear the term multiverse, you probably think of one particular example, and it has to do with people’s choices. Say you’re, I don’t know, trying to decide which flooring to put in your house. Your partner wants easy to clean hardwood while you want that glorious soft carpet. Eventually you decide on the carpet because you’re not a monster. But what if there was another version of you that decided on the hardwood instead? What if that decision split the universe in two?

In one reality, your house has carpet. In the other, there’s hardwood. This exact concept has led to some fantastic stories in our lifetime. Everything Everywhere All At Once is just the latest iteration. Before that, there was the Marvel Cinematic Multiverse. There were the two universes in Fringe. Before that, there was the quantum mirror in Stargate.

Before that, there was Star Trek: The Next Generation, where Worf was married to Troy for a hot minute, but the writers abandoned it almost immediately. Stupid writers. Before Worf and Troy, there was the infamous Mirror Universe from The Original Series in the late 1960s. My point is, this trope has been around probably as long as the hypothesis has been around.

This idea that our choices split the universe in two comes from quantum mechanics. Schrödinger’s equation says wave functions evolve deterministically over time, but our observations show this isn’t always true. Say a photon’s wave function passes through both of these slits. As the photon moves forward, it will be in a superposition of many locations determined by the interference pattern of the waves from each slit.

However, when it reaches the screen, we’ll only see it at a single location. The wave nature of the photon is gone. We call this wave function collapse and it happens suddenly and randomly, in stark contrast to the smooth behavior described by Schrödinger’s equation. Wave function collapse was problematic, to say the least, but it was unavoidable.

That is until 1957, when a physicist named Hugh Everett proposed the many worlds hypothesis. He asked a simple question: What if the wave function never collapses? What if the screen just becomes correlated with the wave function? All possible locations for the photon could still exist just in different universes. At the time Hugh Everett suggested his many worlds interpretation, it seemed extreme.

It was controversial and it had its share of problems. What was keeping observers in each of these worlds from being aware of the others? These problems wouldn’t be solved until 1970, when a type of censorship was discovered we now call “decoherence.”

Here’s what you actually need to know.

- These quantum realities are not some vast distance away. They overlap at the same location.

Your house with carpet occupies the same space as your house with hardwood. They just don’t interact. So you’re completely unaware of each other. It’s not quite as absurd as it seems. Consider these two waves traveling toward each other along a string. They won’t collide.

Instead, they’ll combine into one composite wave for a time. Eventually, they’ll pass through each other entirely and return to their original shape as if the other one wasn’t even there. The same thing happens with waves on the surface of a pond. Schrödinger’s equation is what describes the quantum world, and its solutions are waves. So it’s not that crazy to think the quantum world might behave this way. Each universe being a different ripple in some four dimensional pond.

2. This is not about human choices.

Well, I mean, sure, if you subscribe to the many worlds interpretation of quantum mechanics, your choices would be splitting universes. But that doesn’t make you special. This splitting would be happening for every quantum interaction that releases information to the environment. Your brain just happens to also be made of quantum particles.

3. It’s not the entire universe dividing.

This is a local event. A superposition just splits into measurable states at that location. That separation happens here and only here. The rest of the universe is fine. All the information is still there, too. You just can’t see the whole thing anymore.

4. This isn’t like a tree that branches continuously and gets larger over time.

Other quantum events might bring them back together. So it’s not necessarily permanent. It’s more like a big, messy web of events than a tree full of universes. Fictional stories, like Everything Everywhere All at Once can be fun, but they’re not real. It’s important to look at the science behind these things, too.

We’ve figured out over the last century that the universe has been around for about 13.8 billion years and is currently 93 billion light-years across. But that’s just the observable universe. We know the whole thing is at least 20 times larger than that, but it’s probably infinite, and our finite part is really uniform. Sure, on the smaller scale, you’ve got interesting collections like planets, stars and galaxies.

But if the smallest piece you look at is about 100 million light-years across, the observable universe is basically the same everywhere. There’s no reason to think that’s going to suddenly change beyond the observable edge.

Today we’re going to be estimating how many universes there are. Can you actually calculate that? Yeah! we can do a very crude estimate. Okay, so let’s take a look. Our observable universe has a finite size. Let’s say this is our observable universe. It is ten to the 27 meters across. That’s a lot. That’s a lot of meters. But it is finite.

As far as we can tell, this observable universe is inside of a larger space that is infinite in size. Probably. We don’t know that for sure, but all of our data suggest that it is infinite. Now, if our observable universe has a finite size with a finite number of particles in it, and if those particles can have a finite number of configurations, which stands to reason, if the observable universe is finite, then in an infinite space, those patterns should repeat eventually.

We’d expect them to repeat. And so the next natural question here is: How many particles are in the universe? Now, that’s something we call Eddington’s number, and it’s an estimate we’ve had for a while. It is … 10 to the 80.

There are 10 to the 80 protons in the observable universe. But aren’t there particles other than protons? There are neutrons and electrons, but those don’t actually affect this estimate all that much. See, most of the matter in the universe is inside of stars. It’s mostly hydrogen or ionized hydrogen, which is just protons. Neutrons don’t show up until you make larger elements on the periodic table. And so there aren’t nearly as many of them.

But even if there were, even if there were exactly the same number of neutrons and exactly the same number of electrons, you would just end up with three times 10 to the 80. Which in this case, in the way we’re doing these estimates, that’s still just 10 to the 80. That would round down to 10 to the 80. We’re doing very, very aggressive estimations here, something we call order of magnitude estimation. So we’re only interested in the powers of ten.

Something else we know is we know the diameter of a proton. So this thing has a diameter. It does take up some space because it’s made of smaller particles. And this size is something we call one femtometer, which is 10 to the -15 meters. It’s a very small amount of space, but again, it is a finite space. If we want to know how many possible spaces they could exist in, we need to know how many we could cram in there, like to the brim.

We want to cram it full of protons. How many protons would there be? And that’s actually not that difficult of a calculation. So all we have to do is divide the two sizes. 10 to the 27, the diameter of the universe, and divide it by the diameter of a proton. And we get something like this.

Now, this will tell us how many protons we can fit along the diameter of the observable universe. But what we want is the entire volume. And so all we have to do is cube it. Now, sure, you might want to put like a 4/3 pi out front or something, but again, that’s a fraction or a number outside of the power of ten. And so it doesn’t actually matter. We don’t actually need to know the 4/3 pi for a spherical volume. All we need is the powers of ten.

This fraction cubed comes out to be 10 to the 126 spaces. And so this is how many spaces are available for protons to exist in. In order to figure out how many different configurations these protons can have Again, we’re doing rough estimates. We’re not going to worry too much about how they collected atoms or anything.

Or, to put this another way, we want to choose 10 to the 80 out of 10 to the 126. And that’s very specific language. It’s called combinatorics. To get our final crude estimate, we need three things: the formula for combinations, Sterling’s approximation

(which is a way to find factorials of large numbers more easily), and the fact that 10 to the 126 is much larger than 10 to the 80. When all was said and done, we get a formula that’s very simple. Now all we have to do is plug in the numbers.

So assuming space is infinite, we get approximately 10 to the 10 to the 82 unique universes. Uniqiverses? Anyway, this is a stupid big number. And believe it or not, this is the easiest multiverse to understand. These universes aren’t in some other plane of existence. They’re just a ridiculous distance away. If the expansion of space wasn’t accelerating, we could actually reach them … if we were patient enough.

It’s just a numbers game. But that expansion of space comment has me thinking. There was this era in the early universe called cosmic inflation, where space was expanding at a rate …

What is the fastest thing?

Your first thought might be a cheetah which can reach speeds up to 59 miles per hour. Usain Bolt is the fastest human, and his highest recorded speed is only about 27 miles per hour. Slowpoke. But that’s not the fastest humans have ever gone. According to the Guinness Book of World Records, the fastest speed humans have ever traveled is 24,800 miles per hour aboard the command module for Apollo 10. If you know a little physics, though, you know the universal limit is the speed of light.

Light travels at 670 million miles per hour in a vacuum. So that settles it. Light is the fastest thing. Or is it? The endless chain of cause and effect puts an upper limit on how fast things can move in space. But it puts no such limit on how fast space itself can move. The space contained within the universe has been expanding since the beginning, but not always at the same rate.

Right now, at this very moment, most galaxies are moving away from us faster than light because the space between us is expanding faster than light. However, shortly after the big bang, it temporarily expanded exponentially faster. It had to, otherwise the universe would not have been as uniform as it is today. This begs the question: Why did this period of cosmic inflation stop? But that begs an even better question: Did it stop?

Remember, some infinities are larger than others. There could easily be an infinite space inside of an even larger infinite space. Cosmic inflation began when the observable universe was a trillion times smaller than an atom. It occurred shortly after the big bang and lasted only for a decillionth of a second. That’s a billionth of a trillionth of a trillionth of a second. This period of inflation stopped as quickly as it began, leaving the observable universe about the size of your fist.

Its volume expanded by a factor of 10 to the 78th power. To expand that much again, the universe would take another 13.8 billion years. We know cosmic inflation stopped in our observable universe, otherwise it wouldn’t be here. But cosmologists soon realized something interesting. Inflation doesn’t have to stop everywhere all at once. Our observable universe might just be an infinite bubble inside of an even larger space.

A space experiencing inflation eternally. What implications does this eternal inflation have? That’s coming up right now. Okay. Let’s try this again. Infinite multiverses inside of other infinite multiverses is going to be a bit confusing. So let’s call the larger thing an “omniverse.” It’s a space that contains multiverses. Different multiverses within this omniverse are just regions where inflation has stopped. But in the voids between those bubbles, inflation is still ongoing and no matter can exist there.

That inflation is so fast that even though a multiverse like ours is expanding, it may never collide with another multiverse. Each multiverse bubble would be unreachable by the others. They would be completely separate, which gives us an extra degree of freedom. See, the many universes in our own multiverse may have started with a different set of initial conditions, but all the fundamental constants are still the same.

This is still the gravitational constant and the fine structure constant and this is still Planck’s constant. This is still the mass of the electron. Every one of those universes has the same matter in it, and it all obeys the same rules. That doesn’t necessarily extend to the other multiverses in the omniverse.

Here, every bubble is a separate multiverse, each expanding at a different rate, each with a different number of dimensions, with its own types of particles, with different kinds of structures made from those particles. Our multiverse may be uniform, but there’s no such requirement for the rest of the omniverse. The only constant across the omniverse are the fundamental physical relationships. Whether you think the quantum realities from the many worlds interpretation are real, or you believe that space itself goes on forever and repeats, or even if you think this is all just bogus philosophy which, you know, fair opinion. They might not be real.

But no matter how you feel about this, we should all agree that everything is described by two theories: general relativity and the standard model. Those relationships are what hold the omniverse together. And until next time, remember: It’s okay to be a little crazy. Hugh Arthur was wondering if brown dwarfs could account for dark matter. As nice as that would be, no, they can’t. The number of brown dwarfs you would need is ridiculously larger than could possibly exist.

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