# What Are Popular Interpretations of Quantum Mechanics?

*Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.*

Ask most scientists what discipline leads to many misconceptions and quantum mechanics will frequent the top of any list. It isn’t intuitive. It runs against what we feel reality should be. But experiments have confirmed the accurateness of the theory. However, some things remain outside our realm of testing out, and so different interpretations of the extremes of quantum mechanics exist. What are these alternate views on the implications of quantum mechanics? Astounding, in short. Conflicting, for sure. Easily resolved? Unlikely.

## Hints of Reality Being Not as It Seems, or the Copenhagen Interpretation

Many people like to say quantum mechanics has no macro, or large scale implications. It doesn’t impact us because we are not on the realm of the microscopic, which is the kingdom of quantum. None could be considered a bigger proponent for classical reality than Einstein, who in fact showed how we perceive things depends on our reference frames. His main antagonist (friendly, of course) was Niels Bohr, one of the fathers of quantum mechanics (Folger 29-30).

In the 1920s, several debates and thought experiments went back and forth between these two. For Bohr, his viewpoint was solid: any measurements you take require uncertainty. Nothing is definite, not even properties of a particle, until we take a measurement on it. All we have is a probability distribution for certain events. To Einstein, that was nuts. Lots of things exist without us seeing anything (Folger 30, Wimmel 2).

Such was the main state of quantum mechanics. Measurements remained unfixed. Double slit experiments showed the expected interference pattern that hinted at waves of a single photon. The particle/wave duality was seen. But still, why no macroscopic results? Enter the numerous (understatement) interpretations that challenge us to think even further outside the box (Folger 31).

## Many Worlds

In this interpretation developed by Hugh Everett in 1957, each quantum mechanic wave not only has a probability of happening but *does* in a branching reality. Each outcome happens elsewhere as a new vector (that being the Universe) that branches off orthogonally off each one, forever and ever. But can this really happen? Will Schrodinger’s Cat be dead here but alive elsewhere? Can this even be a possibility? (Folger 31).

The bigger issue is which probability happens *here*. What would cause one event to happen here and not elsewhere? What mechanism determines the moment? How can we math this out? Decoherence usually rules the land, causing a measurement to become solid and no longer a set of superimposed states, but that requires the probability function to work and collapse, which doesn’t happen with Everett’s interpretation. In fact, nothing *ever *collapses with Many Worlds interpretation. And the different branches it predicts are just probabilities of happening, not guarantees. Plus the Born rule, a central tenant of quantum mechanics, would no longer work as it and require sufficient modification, despite all the scientific evidence we have for its veracity. This remains a big issue (Baker, Stapp, Fuchs 3).

## PBR

This interpretation by Jonathan Barrett Matthew Pusey and Terry Rudolph started as an examination of the double slit experiment. They wondered if it showed when the wave function *wasn’t *real (like most people feel it does do – represent a statistic) but through a proof of contradiction showed that the waveform would have to be real and not a hypothetical object. If quantum states are just statistical models, then instantaneous communication of information to *anywhere* could happen. The common viewpoint of a wave being just a statistical probability cannot hold and so PBR shows how a quantum mechanics state has to come from a real wave function that does talk about a physical thing (Folger 32, Pusey).

But is this the case? Is reality just there? Otherwise, PBR holds no ground. Some even say that the result of the contradiction in the form of instant communication should be looked into to see if that is actually true. But most are taking PBR seriously. Stay with this one, everyone. Its going somewhere (Folger 32, Reich).

## De Broglie-Bohm Theory (Pilot Wave Theory) (Bohmian Mechanics)

First developed in 1927 by Louis de Broglie, it presents the particle as not a wave or a particle but both at the exact same time and therefore are real. When scientists perform the double-slit experiment, de Broglie postulated that the particle goes through the slit but the pilot wave, a system of waves, goes through both. The detector itself causes a modification to the pilot wave but not the particle, which acts as it should. We have been removed from the equation, for our observations or measurement isn’t causing the change to the particle. This theory died out because of its lack of testability but in the 1990s an experiment for it was devised. The good old cosmic microwave background, a relic of the early universes, radiates at 2.725 degrees Celsius. On the average. You see, variations exist in it that can be tested against different quantum interpretations. Based on current modeling of the background, the pilot-wave theory predicts the smaller, less random flux seen (Folger 33).

However, pieces of the theory fail with fermion particle predictive power as well as distinguishing between particle and anti-particle trajectories. Another issue is the lack of compatibility with relativity, with many, many assumptions being made before any conclusions can be made. Another issue is how spooky action at a distance can work but the lack of ability to send info along that action can be acted upon. How can this be so, in any practical sense? How can waves move particles and not have a given location? (Nikolic, Dürr, Fuchs 3)

## Relational Quantum Mechanics

In this interpretation of quantum mechanics, a queue from relativity is taken. In that theory, reference frames which relate your experience of events to other frames of reference. Extending this to quantum mechanics, there is no one quantum state but instead are ways to *relate* them via difference frames of reference. Sounds pretty nice, especially because relativity is a well-proven theory. And quantum mechanics already has lots of wiggle room with regards to your frame of observer versus system. The wave function just relates probabilities of one frame to another. But how spooky action at a distance would work with this is tricky. How *would *information on a quantum scale be transmitted? And what does this mean Einstein realism isn’t real? (Laudisa “Stanford”, Laudisa “The EPR”)

## Quantum Bayesianism (Q-Bism)

This one takes the core of science to heart: the ability to remain objective. Science just isn’t true when you want it to be, right? Otherwise, what worth would it hold to explore and define it? That is what quantum bayesianism may imply. Formulated by Christopher Fuchs and Rudiger Schack, it combines quantum mechanics with Bayesian probability, where the odds of success increase as more knowledge of the conditions around it grows. How? The person running the simulation updates it after each success. But is that science? The “experimentalist cannot be separated from the experiment” in this set-up, for all are in the same system. This is in direct contrast to most quantum mechanics, which tried to make it universal by removing the need for an observer to be present in order for it to work (Folger 32-3, Mermin).

So when you measure a particle/wave, you end up getting what you asked from the system and thus avoid any talk of a wave function, according to Q-Bism. And we also get rid of reality as we know it, because those odds of success are governed by you and you alone. In fact, quantum mechanics only arises *because* of the measurements taken. Quantum states are not just out there, freely roaming. But…what would quantum reality *be* then? And how could this be considered legit if it removes objectivity from observations? Is what we consider the present just a misguided view of the world? Maybe it’s all about our interactions with people that govern what reality is. But that itself is a slippery slope… (Folger 32-3, Mermin, Fuchs 3).

## Austere Quantum Mechanics

In our attempts to understand quantum mechanics, can we strip away the Copenhagen Interpretation and instead just start with the wave function and Schrodinger's Equation? Most would say those are on solid, mathematical bedrock. If now a more fundamental quantity, then the eave function really represents reality. Using the Schrodinger Equation to see how the wave function changes, we can try to re-conceptualize the system without measuring anything and just see the behavior of the system itself (Carroll 32).

This is the central underpinning behind austere quantum mechanics, and it presents a conceptual problem for us right away. It doesn’t seem that a collapse is necessary for a fundamental theory of reality and yet the signs point to it being critical. But that is because we are still thinking of the wave function as a probability event whereas now its reality itself. That means everything is some kind of a quantum system, and that means what we conceptualize as discrete objects are really anything but. Particles, energy all end up having a quantum system behind it rather than the traditionally thought of foundations to them (33-4).

Measurements then only offer us a momentary insight into these things, offering an incomplete truth that only appears to be fine in a classical sense. Austere quantum mechanics leads to clouds of probabilities with spread out wave functions applying to all things. Ideas such as nothing and something start to blur now, with reality just being a superposition of many many wave functions (34).

If all of that is true, then how do we reconcile with the collapses of the wave function we seem to have spotted all these years? How do we go from the quantum world to the classical one? This is really just a re-framing of the situation to solve this. Instead of classical being king and quantum being specialized, its really the other way around. If the basics follow quantum mechanics, then so should the rest of reality, with classical being a specialized set of laws for certain circumstances (35).

This implies that the measurement problem is just a perceived problem revolving around the misconception of a classical/quantum divide. In the traditional viewpoint, the measurer and their device would also have to be in some quantum connection to the surroundings, making distinction that much trickier. But under austere quantum physics, all those wave functions are super-positioned across one another, entering an entanglement. It wouldn’t matter then to distinguish the measurer from the device or the surroundings. Instead, what we think of as those divides is just how the superimposed states operate with each other (36-8).

*However*, this doesn’t exactly ring true with the actual observation of a quantum state. We ourselves don’t feel as though we are some superpositioned set of states of several possible measurements. From our vantage point, we are *at* a result, with a probability predicting the outcome. This could be resolved if each of our superimposed states had its own measurement. But that would just be Everett’s Many Worlds Interpretation, where each possible outcome of a system manifests itself in its own branching reality! (38-9)

At this point, you could rightly say we might have gone a bit too far. How could a Universe possibly operate like this? Well, the Universe *itself* would have its own wave function and be a collection of wave functions, all operating in a grand superposition of states! Each one could then manifest itself as its own Universe. Both austere quantum mechanics and Everett’s Many Worlds both predict the same outcome, and it arose when we apply quantum mechanics to everything. But when does the split between realities occur? How many differences are there between worlds? How could we even measure if these worlds exist? Lingering questions will keep us wondering, it seems (39–40).

## Can More Than One Be Right? Any of Them?

Fuchs and Stacey bring several good points to these questions. First and foremost, quantum theory can be tested and edited, just like any theory. Some of these interpretations are actually dismissive of quantum mechanic and offer new theories to develop, or reject. But all should give us predictions to test the validity of, and some of these just flat out cant as of this moment (Fuchs 2). And work is being done on this. Who knows? Maybe the real solution is even *crazier* than anything here. Of course, more interpretations exist than are covered here. Go explore them. Maybe you will find the right one for you.

## Works Cited

Baker, David J. “Measurement Outcomes and Probability in Everettian Quantum Mechanics.” Princeton University, 11 Apr. 2006. Web. 31 Jan. 2018.

Carrol, Sean. Something Deeply Hidden. Dutten, 2019. Print. 17-24, 32-40, 74-9.

Dürr D, Goldstein S, Norsen, T, Struyve W, Zanghì N. 2014 Can Bohmian mechanics be made relativistic? *Proc. R. Soc. A* **470**: 20130699.

Folgar, Tim. “The War over Reality.” Discover May 2017. Print. 29-30, 32-3.

Fuchs, Christopher A. and Blake C. Stacey. “QBism: Quantum Theory as a Hero’s Handbook.” arXiv 1612.07308v2

Laudisa, Federico. “Relational Quantum Mechanics.” *Plato.stanford.edu.* Stanford University, 02 Jan. 2008. Web. 05 Feb. 2018.

---. “The EPR Argument in a Relational Interpretation of Quantum Mechanics.” arXiv 0011016v1.

Mermin, N. David. “QBism Puts the Scientist Back into Science.” *Nature.com*. Macmillian Publishing Co., 26 Mar. 2014. Web. 02 Feb. 2018.

Nikolic, Hrvoje. “Bohmian Particle Trajectories in Relativistic Fermionic Quantum Field Theory.” arXiv quant-ph/0302152v3.

Pusey, Matthew F., Jonathan Barrett, and Terry Rudolph. “The Quantum State Cannot be Interpreted Statistically.” arXiv 1111.3328v1.

Reich, Eugenie Samuel. “Quantum Theorem Shakes Foundations.” *Nature.com*. Macmillian Publishing Co., 17 Nov. 2011. Web. 01 Feb. 2018.

Stapp, Henry P. “the Basis Problem in Many-Worlds Theories.” LBNL-48917-REV.

Wimmel, Hermann. Quantum Physics & Observed Reality. World Scientific, 1992. Print. 2.

**© 2018 Leonard Kelley**