Theories of the Universe: Bohr's Atomic Theory

Bohr's Atomic Theory

Neils Bohr is considered the grandfather of quantum mechanics. If Planck is the father of quantum mechanics, then Bohr must be older than him, which in fact he's not, they're not even related—another strange quantum anomaly.

We're now going to cover three of Bohr's most significant contributions (not all in this section, but the next section as well).

  • Bohr's theory of the atom and its quantum structure
  • The theory of complementarity, which helps explain wave/particle duality
  • The Copenhagen interpretation, which is the standard explanation of what goes on in the quantum world

Did you know that about 1,000 billion (1012) photons of sunlight fall on a pinhead each second? Even when you look at a faint star, your eye receives a few hundred photons from that star each second. And some of them have traveled thousands of light years to get to you.

Having studied the ideas presented by Planck and Einstein, Bohr wanted to theorize about the quantum property of all forms of energy. In order to do that, he would have to explain how energy was released at the atomic level. He went about this by developing a better picture of the atom's structure. The current one that had been developed by Ernst Rutherford needed some tweaking to explain how atoms could emit light and yet not collapse in on themselves. As I hope you remember, light is created when energy is released from matter in the form of electromagnetic radiation. No one really knew how energy was released, they just knew that it was.


We've used our imagination and have done some thought experiments to understand some of the theories we've discussed so far. And when it comes to quantum mechanics, we'll also be explaining these theories and concepts through familiar analogies, metaphors, and other images. In most cases, these will suffice to get the ideas involved across even though the theories are much more complex. However, this is the place where many of the physicists began to formulate their understanding of quantum interactions as well. When you're not really sure how something new operates, its often the best way to begin. Remember what Einstein said, “Imagination is more important than knowledge.”

In the early 1920s, Bohr came up with a way to understand the stability and exactness of atoms using the analogy of standing waves. You can create your own standing waves by using a jump rope secured at both ends. If you pump energy into it and get it swinging, it can vibrate only in a certain number of predetermined ways. A violin string is another example. It can vibrate in its fundamental frequency, or twice, three times, or four times that frequency—in other words, its characteristic harmonics. It can't vibrate at two and one-half times that frequency. And if you can imagine an electron acting as a wave (remember that electrons can act as either a particle or a wave) within an atom in much the same way, you can see how it would be forced to assume only a certain number of predetermined vibrational states.


The quantum nature of the universe is not limited to the subatomic world. It seems that some things have to come in whole chunks: children, snowflakes, memories, experiences, paintings, and a whole host of other things. This also seems to embody an irreducible yes/no, on/off quality that lies at the core of computer technology. Cultures, perceptions, beliefs, and even phases of life can often seem discretely separated as individual quantum states, which is why we can feel transformed when we move from one to another.

Let's look at another analogy. Remember our stair analogy discussed previously? Our child can't land safely or remain stable at step two and one-half, or three and one-third. She needs a minimum amount of energy before she can attain the next step or state. If she doesn't have quite enough energy to make it to step four, she'll remain at step three. Atoms, too, act in the same way and will not absorb radiation unless the energy they receive contains the minimum to make the next quantum leap.

A child jumping down stairs also behaves somewhat like an electron changing states within an atom. In this case, she gives off energy to the floor as she jumps to a lower stair or state. But Bohr realized, like Planck and Einstein, that this energy can only come in chunks or quanta. A jump from step four to step two gives off two steps' worth of energy or a jump from step five to step two gives off three steps of energy.

An electron jumping to a lower state gives off its energy in the form of light. A jump from orbit or step five to orbit three might radiate quanta of red light; a jump from six to two might radiate more energy (higher frequency), so it could be a blue light. In terms of other types of electromagnetic radiation, a jump from orbit one to the ground might give off low energy radio waves, while a jump from orbit eight to the ground might give off high energy x-rays. This then was the way that Bohr described the quantum leap and how light was emitted.

But while this theory explained the quantum nature of atoms, some other unusual questions were raised, questions that have now become an accepted characteristic of quantum reality. For example, an electron can't exist between quantum states, not even for an instant. There is no such thing as in between. It's like jumping from one hour to the next without passing through the minutes in between, or disappearing from one end of a room to miraculously reappear at the other end. This quantum leaping in and out of existence can be very unnerving. How does a quantum state mysteriously materialize out of nowhere? How do you get from one place to another without crossing the territory in between?

This unanswerable characteristic is true of the entire microcosmic quantum world. Virtually everything in the subatomic world is quantized. Not only energy and light, but also matter, momentum, electric charge, and many other exotic qualities of subatomic things, such as “strangeness” and “charm” (we'll get to these last two terms when we discuss properties of subatomic particles). An atom has to absorb energy by swallowing it whole and spits it back out in quantum chunks. This means the very stuff of the universe can't be smoothed out past a certain point, it has a grainy, lumpy texture.

So with this first of many unexplainable qualities under your quantum belt, I'll close this section with a quote from Richard Feynman, one of the most brilliant and gifted physicists/teachers of quantum mechanics. He said:

  • There was a time when the newspapers said that only twelve men understood the theory of relativity. I do not believe there ever was such a time. There might have been a time when only one man did because he was the only guy that caught on, before he wrote his paper. But after people read the paper a lot of people understood the theory of relativity in one way or another, certainly more than twelve. On the other hand I think I can safely say that nobody understands quantum mechanics.
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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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