Original Link : https://medium.com/predict/we-might-live-in-a-multiverse-but-which-kind-97a14895ec40

On the many theories on multiverses. Pick your favorite.

You might have heard theories about living in a Multiverse. You might think that one Universe should be more than enough, but a multiple of them? But wait, there is ore: There are also multiple Multiverse-theories.

Of course there are. As long as we don’t actually know and are not able to prove the existence of what lies beyond our measurable space, there are as many theories as there are smart people thinking about it (or maybe less, at least some are thinking about the same ideas). And it’s a good thing too since chances of getting something right are higher there more ideas we get and more hypothesis we suggest. One of them might happen to be correct or we get closer to a solution by trying to fit reality to different ideas.

Some physical reasoning behind the idea would be reasonable to expect though. Is there a way to test the theory or falsify it? How else can it be a scientific theory?

In this article, I will present:

**Many World Interpretation**: An alternative interpretation of the quantum theory.**String Theory and multiple dimensions**: There exist more than 3 spatial dimensions.**The infinite patchwork universe**: We live in an infinite universe, which consists of rather uniform “patches” while space in between and beyond our horizon is rarely uniform.

When someone mentions that we might be living in a multiverse, what kind of multiverse is (s)he talking about? Let’s explore some of these possibilities and the theoretical physics behind them.

# Many Worlds Interpretation

The Many Worlds Interpretation (MWI) was first proposed by the physicist Hugh Everett in 1957, supervised by John Wheeler, as a consequence of quantum mechanics. Quantum mechanics states, that particles exist in a superposition (e.g. in multiple places or states at once) until it is observed or measured. When observed it collapses to one state only. This is what we see, our Universe. This is the one and only outcome.

This outcome is non-deterministic, meaning that you cannot know the precise outcome of a quantum measurement. The Copenhagen Interpretation operates with this non-deterministic model, where you can only predict the probability distribution of a given measurement’s possible results, but not the exact result. The probabilities are given in a wave function and observing the quantum system will make the wave function collapse to just one single outcome — the one we see at the time of observation.

Example: A particle can be in either in position A or B and we don’t know until we measure. When we measure, we might find it in A. This is what our reality has collapsed into and we can not see it at position B. But there exists another reality, where the particle is still in position B as well, where it just as well could have been measured, because, in reality, it exists both places. This is the reality of multiverses that MWI is about.

MWI states that all of the possible outcomes are real, but we as an observer will only see one of them. The cat is both dead and alive. If we see it as alive, it’s dead in another universe.

A human example: Since we all consist of particles and our mind operates by electromagnetic impulses and signals, this also applies to whatever decisions we make. Do you decide to eat that cake even though you are on a diet? Or do you decide not to? It might be just a small difference in electromagnetic impulses on a quantum level that finalizes your decision.

You might say yes to that cake (it’s a brownie — really hard to resist!). The diet goes off track since one failure can lead to a “f*ck it” the rest of the day. And the week. But in a different universe, the electromagnetic impulses form so that you say “No thanks”, and it leads to you going for a run the same evening and small steps lead to a healthier you. Both of the yous exist in parallel universes.

Since there are infinite quantum wave constructions, there are infinite multiverses. All of those infinite numbers of universes exist at once, but they cannot interact. Because of that, this theory cannot be falsified, which doesn’t make it a good scientific theory, exciting as it might be.

Brian Cox supports the Many World Interpretation and states that it is possible that we live in such a Multiverse, where all possible outcomes exist at once. The question is: Why do we perceive only one and not all outcomes?

That’s a good question for a philosopher.

# String theory and 11 dimensions

The seeds to the string theory were planted with The Kaluza-Klein theory (KK) published in 1921. It would explain fundamental forces with additional spatial dimensions.

Just like gravity is as a distortion of the 4D spacetime in Einstein’s relativity theory, so was electromagnetism explained by ripples in a 5th dimension. The math using 5-dimensional tensors (matrices) and just adding electromagnetism to Einstein’s 4-dimensional tensors seemed to add up!

This specific theory didn’t get to develop further because of issues when trying to incorporate the electron and its mass and charge. It didn’t add up and gave mismatches between theoretical and experimental results.

But the idea of using extra dimensions to explain the fundamental forces and unify them was the precursor to the current string theories.

Hang on, we’ll get to the multiverse part soon enough.

In string theory, the point-like particles are replaced by 1-dimensional vibrating strings, that are curled into higher dimensions along our known extended three dimensions. The vibrations of these strings will define the properties we see in particles (electrons, neutrinos, quarks), such as mass and charge.

In superstring theory, we operate with 10 dimensions, where 6 of the dimensions could be a 6-dimensional space curled up along our 3-dimensional space. The dimensions are really small — particle size. And therefore, they seem to be hidden in our perception.

M-Theory tries to unify all the existing string theories — here we operate in 11 dimensions.

Why these numbers? Because the math adds up. And not only that, scientists who have worked with these theories, experienced that Einstein’s equations came out as a result of the calculations, even though they were not given, to begin with.

What has all of this to do with multiverses, you ask?

String theory deals with the singularity problem and the fact that physical laws break down when we go back to the big bang. What was before the big bang? This is where the superstring theories operate with multiverse theory.

According to string theory, there are multiple universes and those universes can either collide to create a new universe or split into two. This is what might have been the case with our big bang, where two universes split and it caused what we call the big bang.

Michio Kaku, a known supporter of the superstring theory, says:

“Inflation is continuous and eternal, with big bangs happening all the time, with universes sprouting from other universes. In this picture, universes can ‘bud’ off into other universes, creating a ‘multiverse.’”

All of the universes exist with different values of the cosmological constant. But, of course, we live within the one, which has a value that made it possible for life to exist. That is the anthropic principle.

“The intelligent beings in these regions should therefore not be surprised if they observe that their locality in the universe satisfies the conditions that are necessary for their existence. It is a bit like a rich person living in a wealthy neighborhood not seeing any poverty.” — Stephen Hawking on the Anthropic Principle

Is there a way to falsify the string theory? It seems like the answer is no, for now. The theory might make sense. It might add up. But it cannot be falsified. This makes it a questionable and controversial scientific theory.

Nevertheless, it is still interesting to think about, and it mathematically, it cannot be ruled out as a possibility.

# The infinite and the patchwork or quilted multiverse

There is yet another kind of multiverse — and it is not an actual multiverse but just one single infinite universe.

What’s so “multiversy” about that, you might ask? It’s still the single well-known Universe, that stretches forever in infinity. Old news, maybe?

Here’s the thing: If the universe is truly infinite, like in truly mindboggling forever in space (and time?), then every single configuration of particles will be bound to happen someplace and sooner or later. It means that if we continue forever, the configuration of Earth and everything on it (You! And me) are bound to be repeated. That’s the idea of a multiverse inside one single infinite Universe.

Not only do we get repeated — we get repeated an infinite number of times. Our one Universe becomes a Multiverse of everything that can happen that will happen.

But wait, there is more. Another theory in the same mindset os the patchwork Universe. According to this theory, the inflation that was expanding our Universe in the beginning at extreme rates, are still going on continuously. The Universe keeps inflating but in patches of areas, the universe no longer inflating but expands more slowly.

Inside these patches, the Universe is uniform, as we have measured our space to be (it is also one of the mysteries — why is it uniform?). The theory is that space between the uniform patches such as our own, is not uniform but continues to expand at the inflation rate, which is at an *enormous* speed. We will never be able to catch up with the patch “borders”. What we see as our whole Universe, is actually just a patch in a greater Universe.

We are limited by the speed of light to only seeing the observable Universe. Since our Universe (patch) is expanding faster than the speed of light, it is not possible to see beyond 46.5 billion light-years.

Yes, the Universe can expand faster than light, and no, it doesn’t break any laws of physics by doing it. While it is true, that nothing inside the Universe can travel faster than light, the Universe itself can expand faster, since it is not traveling inside the fabric of space itself. it IS the fabric of space.

The quilted multiverse works only in an infinite universe. With an infinite amount of space, every possible event will occur an infinite number of times. However, the speed of light prevents us from being aware of these other identical areas. — wikipedia

These were just a few of the multiverse theories that exist. I tried not to go into too many details, but it’s hard not to get excited about this peculiar topic.

Other theories include the cyclic universe that keeps repeating itself with periodical big bangs and big crunches, Universes inside black holes, wormholes and hidden paths between multiverses and much more.

The problem with most of these theories is that they are hard to falsify, which makes them more controversial as scientific theories. It seems though like the theories of the possible existence of multiverses are much more accepted in this century. The giggle factor has decreased.

“A theory of science which appears too ridiculous to be seriously considered is said to have a “**high giggle factor**.” — urbandictionary.com

What will the future bring of new scientific breakthroughs of that matter? Only time will tell!