You’re probably quite familiar with the ‘many-worlds theory’, about how there is an infinite number of universes however different or similar to yours, as depicted in many sci-fi novels and movies. But what does it actually mean, and is there any scientific basis that actually backs it?

This theory, along with countless others, is only one interpretation of what **quantum mechanics** – the study of how the universe functions at the small scale of particles and atoms – actually means about our reality. You see, the current issue related to quantum mechanics is that, although we have a fairly thorough understanding of how to use it in scientific experiments and technological applications, we still don’t truly understand it on a conceptual level.

My aim in this post is to, by use of __Sean M. Carroll’s piece on quantum mechanics__, give you a foundation in this field so that we can then discuss two popular interpretations of it. Just as a small warning: quantum mechanics gets extremely weird and takes quite a few conceptual leaps to comprehend. It is therefore by no means intuitive! Just hang in there, and remember that even if it seems unreal, it is indeed how our world actually functions. Even if that’s definitely not what it looks like in our everyday lives.

**The Wave Function**

Let’s begin our discussion with something called the **wave function**. All matter has a wave associated with it. This wave has peaks and troughs, just like, say, a sound wave has peaks and troughs. However, the weird thing about it is that it's a so-called ‘probability wave’ – it describes the *probabilities* of different outcomes (e.g., where the particle is, or what its momentum is) that we would see if we were to observe the particle. For instance, a wave function describing a particle’s position may have a greater amplitude at one spot than another (i.e., its peak or trough is further away from its equilibrium, center position), which would imply that the probability of us observing it there is greater there than at some other spot.

(To be exact, since a wave function can have negative amplitudes yet probabilities cannot be less than zero, the square of the wave function is used to predict these probabilities. For instance, even if the amplitude of the wave function at one point is, say, -0.5, the probability associated with that point is negative one half squared (-0.5)^2 = 0.25).

Another thing that is very important to note here is that these waves, although they sound utterly abstract since they describe probabilities, still behave like real waves. Consequently, like the aforementioned sound wave, different wave functions can interact with each other in different ways, bringing rise to lots of weird quantum phenomena.

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One such phenomenon intrinsically related to the wave function is that of **superposition**. Here, you must carefully notice how the description of wave functions is phrased: it describes the different probabilities assigned to the different states a particle can be in, and not the state it actually is in right now. That’s because it is in a sort of mix of all the different states that have a probability assigned to them – this is called superposition. As an example of this, electrons have a property called spin, which can either be ‘up’ or ‘down’. When in a certain state of superposition, they are neither up nor down, but they are both at once! To take this further, that also means that this electron could be in multiple places at once.

**The Measurement Problem**

This now leads us to one of the greatest mysteries in quantum mechanics, known as the **measurement problem**: when we observe an object that is in superposition, such as the electron that is in multiple places simultaneously, we always end up observing it in only one state. Before we looked, it was in several places; after we looked, it was in only one.

This is best exemplified by the famous __double-slit experiment__ wherein a beam of electrons is fired at two narrow side-by-side slits. When we don’t look at the slits, the electrons behave as waves and therefore interfere, producing an interference pattern typical for waves. Even if you only shoot a single electron, its wave function passes through both slits at once (per superposition) and it will still interfere with itself! But when we do look, we get quite a different result: the electrons behave as particles and so we only see two areas where electrons hit the screen. Here’s a visual for this:** **

By now, you’re probably quite confused. How can this be? Why would the mere act of our observing cause the electrons to alter their state, and behave as particles rather than waves? (Note that an observer is not necessarily a human, and that it has nothing to do with ‘consciousness’ – a camera filming the double-slit experiment would yield the exact same result! So, a clearer way of describing an observer is any macroscopic object placed near the quantum system). Why do we never see them in a state of superposition but always as particles? This is exactly why I called this such a mystery – we just don’t know! However, there are lots of interpretations of what this could mean about the world we live in. In an attempt to understand this, we will inspect two of the more popular interpretations: the Copenhagen Interpretation and the Many-worlds Interpretation.

**The Copenhagen Interpretation**

In the Copenhagen Interpretation, first thought up by Niels Bohr, the wave function is said to ‘collapse’ upon our inspection of the quantum system (the quantum system being the electron in our previous example).

To understand what this means, let’s take the following example: a particular electron’s wave function provides us with the information that there is a 50% chance of us observing it at position A, and a 50% chance of us observing it at position B (recall that these probabilities simply mean that, at positions A and B, there are two equally-sized peaks or troughs in the electron’s wave function). It is therefore in a superposition between A and B. Imagine that we now observe it and see that it is at position B. The wave function is said to have ‘collapsed’ such that, now, there is a 100% probability of us finding it at B and a 0% probability of us finding it at A; the wave function literally changed to have a single peak/trough only at point B.

This interpretation is the most popular among modern physicists; however, it has one major limitation: it assumes that there is some sort of division, a major difference, between the quantum system and the observer. But that doesn’t make much sense to me! Since quantum mechanics describes the behavior of tiny particles, and we ourselves are made of those tiny particles, it necessarily also applies to ourselves! We are not exempt from the laws of quantum mechanics, and so there is no division between us and the quantum system we’re inspecting.

**The Many-worlds Interpretation**

This limitation of the Copenhagen Interpretation leads us to a different approach to understanding the measurement problem, one that takes into account our own quantum nature as observers: __the Many-worlds Interpretation__. First presented by Hugh Everett III, the idea goes as follows: when we observe a quantum system in superposition, the wave function never actually collapses. Instead, all of the possible observable outcomes take place, and ‘you’ observe all of them – just in different ‘worlds!’

Going back to the electron appearing at position B, *you* ‘you’ sees it at B, whereas *another* ‘you’ sees it at A. All of the outcomes take place. Before that observation, those two ‘yous’ were one and the same person; after the observation, ‘you’ split into two different branches of the wave function. To better understand why we do end up seeing the different alternatives rather than a superposition of those alternatives, has to do with two related quantum phenomena: entanglement and decoherence.

**Entanglement** occurs when the wave functions of two objects, upon interacting (say, they bump into each other), become inextricably linked. Following their entanglement, both objects are now described only by one wave function. Thus, if you observe one object’s state, you ‘collapse’ (to use the Copenhagen interpretation) not only its wave function but also that of the object it’s entangled with! This means that you can know the state of the other object without ever looking at it, no matter how far away it is.

To understand this better, consider the following example: two electrons, both in a superposition of spin-up and spin-down, interact and become entangled in such a way that, if measured, their spins are always opposite from one another (as is the case for two electrons in the same atomic orbital). So, you now take a look at electron X, ‘collapsing’ its wave function to see that it is now spin-up. Instantaneously, you now know that electron Y, the one X is entangled with, must be spin-down!

This phenomenon of entanglement plays an important role when it comes to quantum **decoherence**. Decoherence takes place when an object gets entangled with so many different things in its environment that its wave function cannot interfere with other wave functions anymore.

Let’s take yourself as an example: you’re huge compared to, say, a single electron, and so you interact way more with your environment. As you do so, you get entangled with more and more other objects and your wave function gets entangled with those of the other objects. Your wave function is consequently so complex that it ceases to be able to interfere with others. Recall from the double-slit experiment that wave functions interfere only when the objects are in a superposition, and so are still wave-like. However, once such interference stops taking place (as with the electrons ‘materializing’ into actual small balls of matter, rather than waves), the electrons behave as particles. They behave just like you or any other macroscopic object behaves – as a single object free from the weird phenomena of quantum mechanics such as superposition.

Here’s a small visual to show how the messiness of a wave function that has become entangled with many other things prevents it from cleanly interfering with another wave function:

It is important to note, though, that decoherence is a fairly new field in quantum physics. If accurate, it provides a good explanation for why an observer (like ourselves, or a camera) doesn’t enter a state of superposition upon looking at a quantum system that is in superposition, but instead enters one of the ‘branches’ of the wave function. That is to say, it enters two different worlds.

**So… What Does This All Mean About Reality?**

Well, the issue of which interpretation is correct (and there are many more than just the two outlined above!) still prevents us from drawing any hard conclusions on what this all means about the world we live in. However, I do personally find the Many-worlds Interpretation more convincing than the Copenhagen Interpretation. The idea of how there is some fundamental difference between a minuscule quantum system and a macroscopic observer, which is the very basis of the Copenhagen Interpretation, makes me uncomfortable. Since quantum mechanics describes how particles function, and we are made of those very particles, I don’t find it logical to believe that different rules should apply to observers. Plus, the argument of how decoherence allows us to exist as stable macroscopic objects does seem quite convincing.

But even if this Many-worlds Interpretation turns out to be true, I’m still unsure about what it actually means about reality. The idea of there being a quasi-infinite number of ‘me’s all living however similar or different lives to mine, and on a bigger scale the even larger number of worlds where every possible outcome ever is taking - or has taken - place, discomforts me.

__T____his raises an important question about who we are as human beings__. Am ‘I’ solely the version of myself living in this very world, or am ‘I’ all of the ‘me’s that have ever existed? If the former is the case, then I’m only a tiny fraction of my whole self!

This interpretation also gives rise to an odd dilemma regarding free will: does the infinite number of worlds prove the existence of free will (since all possible outcomes do take place) or does it mean that free will is just an illusion (since all of these outcomes are simply determined by the probabilities ascribed to them by the wave function of the universe, rather than by my ‘decisions’)?

Another implication of this interpretation, and of quantum mechanics as a whole, is that nature is fundamentally probabilistic. Does this somehow imply that, if there is a God, this God basically has no control over the universe?

But even if this interpretation is correct, I don’t feel like it matters too much on a personal level. Sure, it can cause existential crises and lots of regret and jealousy if you keep thinking of how those other versions of you are just so lucky to have had the better outcome in any life event. But then again, it’s still not sure if that’s actually the case.

One thing remains certain: the quantum world is just freaking weird!

**References and Further Resources**

*Videos*

__Domain of Science's__excellent video that describes lots of weird quantum phenomena in an extremely clear way. It really helped me conceptualize it all!__A video that helps visualize quantum entanglement and decoherence__.

*Articles*

__Sean M. Carroll’s chapter on quantum mechanics__. I primarily used this to write my blog post, and it definitely describes what I’ve covered in a lot more detail!__Sean M. Carroll’s case for the Multiple-worlds Interpretation__.__Sean M. Carroll’s explanation of the____double-slit____experiment__.

*Encyclopedias*

__Britannica Encyclopedia’s__entry on quantum mechanics provides more technical as well as historical information to deepen your understanding.__Stanford Encyclopedia of Philosophy’s__entry on the Many-Worlds Interpretation goes more into the philosophical implications of this interpretation, including the idea of the self and what it means to be ‘you’.

**Works Cited**

Carroll, Sean. From Eternity to Here. Oneworld Publications, 2011.

Carroll, Sean M. "The Notorious Delayed-Choice Quantum Eraser." Preposterous Universe, 21 Sept. 2019, __www.preposterousuniverse.com/blog/2019/09/21/the-notorious-delayed-choice-quantum-eraser/__.

"Why the Many-worlds Formulation of Quantum Mechanics Is Probably Correct." Preposterous Universe, 30 June 2014, __www.preposterousuniverse.com/blog/2014/06/30/why-the-many-worlds-formulation-of-quantum-mechanics-is-probably-correct/__.

Domain of Science. "If You Don't Understand Quantum Physics, Try This!" YouTube, 25 Feb. 2019, __www.youtube.com/watch?v=Usu9xZfabPM__.

Squires, Gordon L. "Quantum Mechanics." Encyclopedia Britannica, 27 Nov. 2019, __www.britannica.com/science/quantum-mechanics-physics__.

Vaidman, Lev. "Many-Worlds Interpretation of Quantum Mechanics." Stanford Encyclopedia of Philosophy, 17 Jan. 2014, __plato.stanford.edu/entries/qm-manyworlds/__.

Vulgarisation. "quantum superposition of states and decoherence." YouTube, 19 June 2019, __www.youtube.com/watch?v=7B1llCxVdkE__.

Great jobb!!

This amazing work Yann, I really enjoyed the read!