Heisenberg's uncertainty principle led Neils Bohr to father the concept of complementarity. Bohr said that the reality of particles required *complementary* descriptions— more than one point of view. It doesn't matter that you can't measure both motion and position at the same time; you can't see both sides of a coin at the same time either. As long as we insist on looking at the subatomic world with our everyday perspective, (and what other choice do we have?), we will be stuck with looking at nature one dimension at a time. Take a look at the following figure to see exactly how the principle of complimentarity operates.

When you look at the image, you can either see the outline of the chalice or the outline of the two faces, but you can't see both at the same time. This illustrates the fundamental paradox of the wave/particle duality of matter and shows you the basis for Heisenberg's uncertainty principle and Bohr's concept of complementarity.

**Complementary** ideas are opposing ideas that add up to much more than the sum of their parts. They complement each other like night and day, male and female, yin and yang. One helps to define the other and each is equally important as the other. Waves and particles are complementary ways of describing the nature of light as well as all energy and matter.

For centuries, people have argued over whether light was a wave or a particle (remember Newton's and Huygen's opposing views?). In today's world, we know that light is both and to argue about it is like asking whether the color of the sky is blue or whether it has mathematical properties. Each is true in the proper context. It is rather like the sides of a box or the facets of solving a problem. What you see depends on what you're looking for, which is why light and all energy and matter can show up as quantum chunks in some experiments and as waves in others.

Complementarity makes it easier to accept the innate limits on perception and measurement. Each way of seeing only goes so far. In the same way that we need two eyes to see depth (the combination of two distinct images), we need more than one perspective to see something in all its dimensions.

We now have a number of concepts fundamental to the inner workings of the quantum universe. All of these come together in Bohr's *Copenhagen interpretation*. This is the standard interpretation of what goes on in the quantum world. It held sway from the 1930s to the 1980s and is still taught in most textbooks and many university courses. It is by no means the only way to interpret quantum mechanics, but it is the most popular, owing much of this to the forceful personality of Neils Bohr.

Here's the basic idea of how it works. Let's take an empty box. If we put an electron in the box, even if we don't where it is, it has a definite location. The Copenhagen interpretation says that the electron exists as a wave filling the box and could be anywhere inside. At the moment we look for the electron, the wave function collapses at a certain location. This is similar to an electron in the double-slit experiment. As soon as you observe it, the electron stops behaving like a wave and behaves as a particle.

The **Copenhagen interpretation** got its name from the city where Bohr worked. It is one of the central theories of quantum mechanics. It basically states that there is no meaning to the existence of a quantum particle unless it is observed. In other words, until you see it, it doesn't exist. The Copenhagen interpretation stresses the role of experimentation to understand the quantum world. Bohr insisted that the only thing we can know for sure is what we can measure with our instruments.

Now if we slide a partition in the box without looking, the electron must be in one half of the box or the other. The Copenhagen interpretation says that as long as we don't look, the electron wave still occupies both halves of the box and only collapses on one side of the barrier when we look inside. As long as we don't look, even if we move the two halves of the box far apart, the wave still fills both boxes. Even if the boxes are moved light years apart, it is only when we look into either one that the electron wave collapses, instantaneously, and the electron “decides” which box it is in.

The idea that the electron is in both halves of the box at the same time is based on probability. And according to Heisenberg's uncertainty principle (by observing it we change the outcome) leads to the paradox that somehow the electron knows which box to be in, even if light years apart. How can this information be passed instantaneously when nothing can travel faster than light? It's not called a mystery for nothing.

The idea of the passage of information instantaneously is called action at a distance. It's the same as Newton's idea of how gravity operated—remember? Einstein hated the whole idea of this “spooky” action at a distance. It went against the fact that light was the top speed of anything in the universe. How could two waves light years apart know which box was being opened so that the correct box's wave could collapse into a particle? It's why Bohr theorized that the existence of a quantum particle has no meaning until it is observed.

Some physicists thought this idea was rather absurd. One in particular, our friend Schrödinger, put together his famous thought experiment to show just how nonsensical this whole interpretation was. It's known as “Schrödinger's Cat.” Let's take a box and inside we'll place a radioactive source, a Geiger counter, a hammer, and a glass vial filled with poisonous gas. And, of course, we need a live cat, too (don't worry— no real cat was ever used in this experiment). After a period of time, radioactive decay takes place, the Geiger counter measures this and triggers a device that trips the hammer, which then breaks the vial. Poison is released and the cat dies.

Let's say that in this particular instance, quantum theory predicts that there is a 50 percent probability of one decay particle for each hour from the radioactive source. After one hour, the decay has triggered the entire apparatus and the cat is now either alive or dead. Within the framework of the Copenhagen interpretation, exactly one hour after the experiment began, the box contains a cat that is both alive and dead—a mixture of two states. This is just like the two states of a quantum particle, either it's a wave or a particle. It only becomes a particle after it is observed. Taking this analogy to the cat, it isn't dead until the box is opened and the cat is observed. Until then it's still alive and dead at the same time.

You can see how strange this is. How can the cat be both alive and dead at the same time? Schrödinger and others felt this paradoxical state showed the absurdity of the Copenhagen interpretation. But regardless of how strange it seems, it's still one of the basic ways used to describe the quantum universe. And with Schrödinger's cat either alive or dead, we'll close this introduction to some of the unusual characteristics of quantum interactions.

Excerpted from The Complete Idiot's Guide to Theories of the Universe © 2001 by Gary F. Moring. All rights reserved including the right
of reproduction in whole or in part in any form. Used by
arrangement with **Alpha Books**, a member of Penguin Group
(USA) Inc.

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