Theories of the Universe: That Old Quantum Theory

That Old Quantum Theory

Einstein's two theories of relativity have shown us that when things move very fast or when objects get massive, the universe exhibits very strange properties. The same is also true of the microscopic world of quantum interactions. The deeper we delve into the macrocosm and the microcosm, the further we get away from the things that make sense to us in our everyday world.

At the heart of relativity theory is the idea that what you experience is real for you but not necessarily for someone else—it depends on your frame of reference. This can also be said of the quantum world, but with less certainty. Because what we find within the framework of quantum mechanics is that events are based on probability and not certainty. And the probable outcome of any event often depends on what you're looking for.

In “The Dual Nature of Light,” I discussed the wave/particle duality of light and touched on some aspects of the quantum. This section will extend that discussion by inquiring into just why energy has to come in chunks, or quanta, and how this would lead in the next section into the bizarre nature of quantum interactions where everything is so uncertain. We'll also look at the key theories of some of the great physicists who helped to define this weird quantum landscape. Next stop, the quantum zone.

Founding Fathers

There are many physicists who have contributed to the development of quantum mechanics. For our purposes we're going to look at the handful whose theories laid the foundation that others would build upon. Here's a list of those that we'll be taking a look at. If I omit any, it's not because their contributions weren't significant, it's only because I can only cover so much as an introduction to this huge body of material.

• Max Planck is often called the father of quantum mechanics. He coined the term quanta and developed the solution to the ultraviolet catastrophe (discussed in “Cracks in a Newtonian World”). His main contributions were the theory that electromagnetic radiation happens in discrete quanta and the discovery that the size of each quantum is associated with a universal constant, a physical ration or proportion that stays the same in all circumstances and all frames of reference.
• Albert Einstein described the photoelectric effect that explains how electrons are released from a substance under the influence of light or other electromagnetic radiation. This was a key step toward the concept of wave/particle duality.
• Niels Bohr developed the theoretical model of the atom that most of us are familiar with. This model has electrons orbiting around the nucleus like planets orbiting around the Sun. He also promoted the Copenhagen interpretation of quantum mechanics.
• Louis de Broglie proposed that all material particles such as electrons could also be described in terms of waves. This led to the replacement of Bohr's model with one that explained the orbits of electrons as waves.
• Werner Heisenberg discovered his now famous uncertainty principle that simply places the role of uncertainty as a core characteristic of the quantum world.
• Erwin Schrdinger described quantum mechanics in terms of wave mechanics and had a famous cat named after him.

These six physicists along with three others—Wolfgang Pauli, Max Born, and Paul Dirac—were all together in 1927 at the Solvay Conference. It was during this conference that the essential concepts of quantum theory were formulated. What I will do for the rest of this section is to discuss each of the founding fathers listed above and explain some of the basic concepts of their contributions. Let's begin with a little overview of just what quantum mechanics is all about.

Quantum Leaps

What is quantum mechanics? Well, essentially it's the mechanics of quantized things. Okay, so what does that mean? In physics, mechanics is not about fixing your car, but is the explanation of the way things work in terms of energy, forces, and motion. And anything that is quantized is simply expressed in multiples of some small measurable unit. As you already know, before the turn of the century, the way things worked was explained by Newtonian mechanics or classical physics. And the central feature of Newtonian mechanics was that everything was continuous; things flowed smoothly through space; energy could come in an infinite range of amounts; light undulated in a continuous wave; there was no minimum amount of anything.

Quantum mechanics changed all that. Now energy, light, force, and motion all came to be quantized. For something to be quantized, you can't have just any old amount; you can only have multiples of certain minimum quantities. Quantum mechanics meant that all the qualities of subatomic things, and by extension, all things, were precisely quantifiable. Nature revealed herself to be somewhat grainy or jerky, jumping from one quantum amount to the other, never traversing the area in between. This leads to an uneasy uncertainty about what is going on between those quantum states, or to put it into a popular phrase, quantum leaps. In fact, as it turns out, there is no way of knowing the exact state of things between quantum states. In our perception (and that is a key ingredient), there is no transition between quantum states. You can have one or two or three units of energy or momentum or light or force or matter or whatever, but there is no such thing as one and one-half or two and three-quarters units.

An example of this might help you understand. Imagine that a child is jumping up the stairs. A quantum leap is the transition from one state to the next state, the same as a child jumping from the first stair to the second stair. Depending on the amount of energy she has, she can also jump up to the third stair, the fourth, or the sixth. But she can't jump in between the stairs and land safely.

Everything in the quantum mechanical universe, which is the universe we live in, happens in quantum leaps. The uncertainty associated with these leaps will be explained when we get to Werner Heisenberg in “Chunks of Uncertainty.” The important point I want to mention is that all of this comes into acute focus when we actually set out to measure things as small as atoms and try to describe them in language of typical Newtonian systems. Lost? Don't worry, it will become quantumly clear shortly.