When the doors of perception are cleansed, man will see things as they truly are, infinite.

-- artist/poet William Blake

Modern physics has some interesting things to say about how reality behaves: the equivalence of mass and energy, the duality of wave and particle behavior, the impossibility of measuring anything precisely. It also raises a more fundamental question: How many realities are there? One or many?

The question arises from the discovery of the wave equation of particles, and not having a verifiable interpretation of it. What does the wave equation mean? And what does it say about us?

The wave equation describes the state of a particle (for example, an electron or photon). Everything we see is made up of these particles. (Remember that scientists can demonstrate both wave behavior and particle behavior of things, so calling them "particles" doesn't mean that they are really particles. We don't have a word for what they really are because we don't know!)

The funny thing about the wave equation is that it describes the state of a particle as a sort of collection of all possible states that the particle can be in. In other words, the wave equation describes the particle as being both over here, and over there; as moving fast, and moving slow; as spinning up, and spinning down.

The problem is that whenever scientists actually measure a particle, they find only one state: it's over here or over there, but not both. So the question is, what does the wave equation say about reality?

There are many interpretations of the wave equation, around half a dozen to a dozen popular ones. The most widely accepted one is called the "Copenhagen interpretation" after the location of the schools that first developed it.

The Copenhagen view says that the wave equation gives probabilities of finding a particle in a certain state when it is measured. When the particle is not being measured however, it exists in all possible states, in what is called "superposition." When a measurement is made, the superposition of the wave function "collapses" into one state.

What is so special about measurement? In the Copenhagen view, the special part is consciousness. Consciousness is assumed to be the mechanism of collapse. When a human being isn't looking, the world exists in many states; when a human being looks, the world exists in only one state.

One of the most popular alternative interpretations of the wave function is called the "Many Worlds" theory.

This one is really simple: there is no collapse. The superposition is real; everything that is possible, really exists. One of the possible states that exists is consciousness which is limited to seeing only its state, and none of the others. That's how we fit in. We can only directly observe our world, but it is only one of many. There may be billions or trillions of worlds "split off" every second.

This may seem like a purely academic exercise, and one may be forgiven for taking Einstein's view: The bizarreness of quantum physics merely shows that we still don't have it right. Einstein never accepted the truth of quantum physics, but even by the time of his death he was in the minority, and today it would be hard to find one who disagreed. Experiments conducted after Einstein's death show that the wave function is as real as anything.

In one experiment, a photon (a particle of light) is split into two weaker photons, and then bounced off mirrors so that they both land on a "beam splitter" at the same time. What the beam splitter does is create a 50%/50% chance that any one photon will pass through the splitter, or reflect back off it. Two photon detectors past the splitter then record where the photons went.

By simple logic, the two photons should be independent of each other, and should randomly hit the two photon detectors. We should sometimes see both photons hit the top detector, sometimes see both photons hit the bottom detector, and sometimes see one hit the top and one hit the bottom.

However, reality doesn't operate according to simple logic, it operates according to the wave function. What actually happens is that both photons always hit the same detector! We may observe both hitting the top one, or both hitting the bottom one, but we never observe one hitting the top and one hitting the bottom.

Why is this? There is a superposition of four worlds involved. In one world, both photons hit the top; in another, both hit the bottom; in another, photon "A" hits the top and photon "B" hits the bottom; in another, photon "B" hits the top and photon "A" hits the bottom. These last two worlds cause interference with each other, just like any other kind of colliding waves, and cancel each other out. When we make an observation, we can only see one of the first two worlds.

With this and other experiments, scientists have demonstrated that our world is the result of a superposition of waves, some of which interfere and cancel each other out, so that we only see one world.

Is there something special about our one world? It is "really real" and the others "kinda sorta real?" Does our consciousness "collapse" the many worlds into one, or is the appearance of one world an illusion due only to the limitations of our senses? Scientists debate these questions, but do not have an answer.

Modern physics represents a meeting of science and philosophy. Traditional scientific inquiry has led to fundamental questions about the nature of reality, being, and consciousness. Nowhere is the nature of this meeting better illustrated than in the interpretation of the wave function.

The wave function, a purely mathematical description of how reality behaves in experiments, has many interpretations, each of which delves into the realm of philosophy. Today's theoretical physicist cannot avoid also being a philosopher.

The existence of the wave function dramatically raises the question of the limitations of science. If a conscious observer "collapses" the wave function into one state from many, is the observer's mental and emotional state relevant to the experiment? If many worlds are equally as real as the one this conscious observer perceives, are truth and falsehood absolute, or only relative to a particular observer? Are any of the interpretations of the wave function "right," or are they all deceptive?

Next: What a Tangled Web We Weave

For Further Exploration

The technically minded and curious are encouraged to see chapter 7 of Paul Davies' About Time, and these sources elsewhere on the web: