Theories of the Universe: A Little Bit of Quanta Goes a Long Way

A Little Bit of Quanta Goes a Long Way

At this point in the section, I want to cover how light was also discovered to behave as if it were a particle and not a wave. But this falls into the realm of quantum mechanics, which I haven't introduced you to yet. So for the time being I'm going to skip over some material that I'll come back to in the next section. The emphasis of this last section will be on explaining the dual nature of light.

Universal Constants

One of quantum theory's most revolutionary ideas is that all the constituents of matter and light are both wave-like and particle-like at the same time. This is known as wave/particle duality. Neither aspect is more primary than the other. The two complement each other, and both are necessary for any full description of what matter and light really are.

There were a couple of problems in science that classical physics couldn't explain. These problems are what I'll explain in Cracks in a Newtonian World, because out of their solution came the birth of quantum physics. Within the context of these solutions it was discovered that not only could light behave as a wave, but also as a particle. As a matter of fact, all electromagnetic waves can also behave as particles. The name given to this is phenomenon wave/particle duality.

As you know, Newton firmly believed that light was made of particles; however, this view eventually changed, and everyone believed that light was a wave. Now, because of a few problems that classical physics couldn't explain, it was discovered by Max Planck and Albert Einstein that the energy in electromagnetic radiation comes in chunks of energy that Planck named quanta. Later, when referring specifically to light, these chunks of energy were called photons.

Light sometimes behaves like a wave and sometimes like a particle. Which way depends upon the circumstances, or to put it more strongly, how we want it to behave, or how we look at it. This sounds a bit strange, but in the quantum world, weird and unusual are normal. Let's revisit Thomas Young's two-slit experiment and update it to one of the most famous experiments in quantum physics.

Universal Constants

Quantum, or the plural, quanta, comes from the Latin word for “how much.” It describes the discontinuous or intermittent way in which electromagnetic energy radiates under certain conditions.

Many people think that Einstein named a particle of light a photon, but he didn't. Gilbert Lewis, a chemist, named the particle in 1926. Photons, or particles of light, are one form in which light can manifest, the other form, of course, is a wave.

We'll set up the experiment just like before. A light source placed in front of a barrier with two slits in it with a screen behind the barrier. The screen will act as our wave detector, which will enable us to know when light is behaving as a wave. We'll also add two particle detectors, one next to each slit on just the other side of the barrier, so when we turn them on we can measure each photon as it clicks past the detector.

In the original experiment the light source was shown through the two slits and an interference pattern was created on the screen, which meant that the light beam was behaving as a wave. And if we do that now, sure enough we get the same result. However, if we turn on the two detectors to measure the photons individually, we'll notice that they travel through only one slit at a time, not both, and they create a blob of light, rather than an interference pattern. When we shut the detectors off, the photons travel through both slits and create the wave pattern of interference on the screen. Who told them when the detectors were turned on or off? Why do they behave one way when they are counted individually and another way when they act collectively?


The unusual ability of light to behave as both a wave and a particle is a central mystery that lies at the core of quantum physics. The very act of observing light in this context alters the outcome. And this is true of all subatomic particles. You can no longer separate the observer from the experiment. This realization would lead to many other principles and theories that would try to describe the paradoxical nature of the quantum world.

You could say that when a particle is looked for, a particle is found. And when we look for a wave, a wave is found. Somehow there seems to be a direct relationship between how the experiment is set up and outcomes looked for. It's not really possible to say that light is really a wave that sometimes acts like a particle, or vice versa. The nature of light is a deeper and richer phenomena than either of these partial realities. Which side of its dual potential nature it decides to show depends entirely upon the experimental context in which it finds itself. We can never observe light outside of some context.

The potential for light to behave as both a wave and a particle at any given time was a totally new concept in physics and would be the first of many weird characteristics that would be discovered about the structure and behavior of the microcosmic world of quantum physics. And interestingly enough, the opposite paradox arose with solid matter, the objects we interact with in our everyday world. Matter is usually interpreted as consisting of particles, but they began to behave as though they had wavelengths. The small particles that make up solid matter were found to have wave characteristics. Even large objects like apples and ourselves have a wavelength. The wave nature of electrons is the physical basis of the electron microscope, which uses beams of electrons, whose wavelengths are millions of times shorter than those of photons, to view objects too tiny for examination under a light microscope.

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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