Tuesday, April 1, 2014

What Quantum Theory says about Physical Reality

Is there a physical world, and if there is, what is it like? In other words, what is the nature of physical reality? Although this may seem like an obsessively philosophical question, our best current scientific theory about the basic stuff that makes up the physical world, quantum theory, seems to force us to try to answer this question. Even the scientific founders of this theory were driven to address this issue.

There is no quantum world ... only an abstract quantum description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.  (Niels Bohr)

This may look like an attempt to avoid engaging in “metaphysical speculation”. But it does states a view about the nature of physical reality. It seems that no one who takes physics seriously as an attempt to account for what the world is like can avoid such issues.

In this paper I will discuss two views about what quantum theory has to say about the nature of physical reality. Let’s call one of these views “the Many Worlds view”, and let’s call the other “the many minds view”. In describing and comparing these views I will explain what each view says about the nature of the physical world and how it is described by or related to quantum theory. In particular I will describe what each view says about the quantum measurement problem.

Before proceeding, let me first give a brief overview of the quantum measurement problem.  Quantum mechanics can not predict a particular position or particular velocity for a particle, it may only predict the probability that said particle will be in a particular position or have a particular velocity.  The wave function is what quantum mechanics uses to make this prediction.  The problem arises when a measurement is taken.  Because the measuring device must enter the system or interact with it, the measuring device alters the system.  For example, should one attempt to measure the position of an electron through a microscope the position is determined by a photon of light from the microscope which reflects off of the electron and returns back to the eye of the observer.  But that photon likely altered the state of the electron.  In addition the observer will only know where the electron was not where it currently is. Therefore measuring itself inherently causes measurements to be flawed.

The first interpretation of quantum theory I will discuss is the Many Worlds view. This view says that every possible action or outcome happens either in our universe or in another.  That is to say that if something could happen in our universe, but doesn’t, it does happen in another of an infinite number of universes.  The Many Worlds view says that reality consists of an infinite or largely finite amount of parallel universes all of which are increasing in dissimilarity over time; as time moves forward more and more parallel universes are created.  Similar to the limbs of a tree: all the separate small branches all emerged from one initial trunk.  According to the Many Worlds view all these other universes are real because it holds the universal wavefunction which is the quantum state of all that exists, the “basic physical entity.”  The reality that Many Worlds describes is a quantum reality which makes up the macroscopic reality and thereafter makes up the universal reality.  Many Worlds does not require the inclusion of the mind; it allows for the mind to make decisions but the mind itself does not determine the universe.

As a result of the double-slit experiment in which light particles pass through the double slit and appear as particles and not as waves, the system, the light particles(“the observed” and the measuring device(“observer”), does not need a well defined state because all systems are relative to each other. 

The Many Worlds view settles the EPR paradox, Schrodinger’s cat, boundary problem, and double-slit experiment.  Many Worlds Interpretation allows the mechanics of quantum systems to be deterministic and localized while losing counterfactual definiteness, which is to say that the measurement of each physical system does not result in a specific outcome.  Because Many Worlds allows for an infinite number of universes to exist it easily explains for the existence of life; as long as the potential for life exist than life will exist in at least one universe according to Many Worlds.

One of the main concerns for Many Worlds is that when the branching of universes occurs is not well defined.  Situations where a quark could become an up or a down(limited choice/possibility) should be simple whereas choosing the color of shirt(unlimited choice/possibility) make for a much tougher definition.
The second interpretation of quantum theory I will discuss is the Many Minds view.  Many Minds is an extension of Many Worlds with one key distinction:  Many Minds says that the mind plays a role in accounting for quantum phenomena whereas Many Worlds says that nothing accounts for quantum phenomena other than quantum mechanics.  They both say that systems do not take on the specific properties they are measured to have.  In other words that the quantum systems do exist in the way that we measure them to. 

Many Minds is an interpretation which says the distinction between worlds should be made in the mind of each individual observer.  The different worlds that are created in the Many Worlds view actually exist only in the mind of the observer.  Minds evolve at random and independent to the mental states that correspond to the differing states of perception.  When a measurement is performed, the mind of the observer forms mental states that adhere to perceptions of the different possible outcomes.  The mind then chooses only a single one of these identities to be its non-random reality.  Since there is a continuum of minds an infinite number of minds will always exist within any being.

The biggest issue with Many Minds is that it adds on to the theory by introducing an additional step, and it includes what seems to be a generally random process.  Secondly, the mind must be separate from the body; mind and brain must not be the same.  This of course is still up for debate in philosophy. 
I believe that the Many Worlds Interpretation is the better of the two for multiple reasons: 1. MWI adheres to Occam’s razor much closer because Many Minds adds in the fact that the mind must exist separate from the body            2. It more clearly addresses the measurement problem (where are the minds when there is no observer)    3.  It also allows for the study of quantum cosmology since there would be an infinite number of possible Big Bang scenarios

4.  Because it is realist and deterministic it is more alike classical physics than most 5. Few physicists and philosophers have dedicated themselves to the Many Minds view

This article originally written May 6th, 2008 as the final paper for OU PHIL 3623 - Physics and Cosmology.

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