The Amazing Story of Quantum Mechanics Read Online Free
Author: James Kakalios
moment to draw his pistol actually impeded the villain, according to Bohr’s theory, while the hero could rely on reflex and simply grab his weapon as soon as he saw the villain move. When some of his students doubted this explanation, they resolved the question as good scientists, via empirical testing using toy pistols on the hallways of the Copenhagen Institute (the experimental data confirmed Bohr’s hypothesis).
In most discussions of quantum mechanics, at both the popular and technical levels, one typically begins with a recitation of the experimental findings that challenged accepted theories and then proceeds to describe how these data motivated physicists to propose new concepts to account for these observations. Let’s not do that. In the spirit of the 1970s television detective show Columbo, 4 I’ll begin with the solution to the mystery of the atom and only then describe its experimental justification.
There are three impossible things that we must accept in order to understand quantum mechanics:
Light is an electromagnetic wave that is actually comprised of discrete packets of energy.
Matter is comprised of discrete particles that exhibit a wavelike nature.
Everything—light and matter—has an “intrinsic angular momentum,” or “spin,” that can have only discrete values.
It is reasonable at this stage to ask: Why wasn’t this brought to our attention sooner? How is it possible to live a careful and wellobserved life and yet never notice the particle nature of light, the wave nature of matter, and the constant spinning of both? It turns out that these are all easy to miss in our day-to-day dealings. While the human eye is physically capable of detecting a single light particle, rarely do we come across them in ones or twos. On a sunny day, the light striking one square centimeter (roughly equivalent to the area of your thumbnail) is comprised of more than a million trillion of these packets of energy every second, so their graininess is not readily apparent.
The second principle discusses the wavelike nature of matter. I show in Chapter 3 that a thrown baseball has a wavelength less than a trillionth the size of an atomic nucleus; it is consequently undetectable. The wavelength of an electron within an atom, in contrast, is about as large as the atom itself, and thus this wavelike property cannot be ignored as we seek to understand how the atomic electrons behave.
Atoms interact with light in minute quantities, and the wavelike nature of the motion of electrons in the atom turns out to be crucial to determining how it can absorb or lose the energy contained in light. Thus any model of the atom and of light that relies solely on our day-to-day experiences fails to accurately account for observation. The influence of the third principle, concerning the “intrinsic angular momentum,” also referred to as “spin,” is fairly subtle and comes into play when two different electrons or two atoms are so close to each other that their matter-waves overlap. This effect turns out to be rather important and is the key to understanding solid-state physics, chemistry, and magnetic resonance imaging.
While it is certainly true that these three basic principles of quantum mechanics seem weird, it is important to note that making counterintuitive proposals about nature is not a unique aspect of quantum mechanics. In fact, putting forth a seemingly weird idea to describe some aspect of the physical world, developing the logical consequences of this weird idea, experimentally testing these consequences, and then accepting the reality of the weird idea if it conforms to observations is pretty much what we call “physics.”
Weird ideas have been the hallmark of physics for at least the past four hundred years. Sir Isaac Newton argued in the mid-1600s, in his first law of motion, that an object in motion remains in motion unless acted upon by an external force. In my personal experience, when I am driving in
In most discussions of quantum mechanics, at both the popular and technical levels, one typically begins with a recitation of the experimental findings that challenged accepted theories and then proceeds to describe how these data motivated physicists to propose new concepts to account for these observations. Let’s not do that. In the spirit of the 1970s television detective show Columbo, 4 I’ll begin with the solution to the mystery of the atom and only then describe its experimental justification.
There are three impossible things that we must accept in order to understand quantum mechanics:
Light is an electromagnetic wave that is actually comprised of discrete packets of energy.
Matter is comprised of discrete particles that exhibit a wavelike nature.
Everything—light and matter—has an “intrinsic angular momentum,” or “spin,” that can have only discrete values.
It is reasonable at this stage to ask: Why wasn’t this brought to our attention sooner? How is it possible to live a careful and wellobserved life and yet never notice the particle nature of light, the wave nature of matter, and the constant spinning of both? It turns out that these are all easy to miss in our day-to-day dealings. While the human eye is physically capable of detecting a single light particle, rarely do we come across them in ones or twos. On a sunny day, the light striking one square centimeter (roughly equivalent to the area of your thumbnail) is comprised of more than a million trillion of these packets of energy every second, so their graininess is not readily apparent.
The second principle discusses the wavelike nature of matter. I show in Chapter 3 that a thrown baseball has a wavelength less than a trillionth the size of an atomic nucleus; it is consequently undetectable. The wavelength of an electron within an atom, in contrast, is about as large as the atom itself, and thus this wavelike property cannot be ignored as we seek to understand how the atomic electrons behave.
Atoms interact with light in minute quantities, and the wavelike nature of the motion of electrons in the atom turns out to be crucial to determining how it can absorb or lose the energy contained in light. Thus any model of the atom and of light that relies solely on our day-to-day experiences fails to accurately account for observation. The influence of the third principle, concerning the “intrinsic angular momentum,” also referred to as “spin,” is fairly subtle and comes into play when two different electrons or two atoms are so close to each other that their matter-waves overlap. This effect turns out to be rather important and is the key to understanding solid-state physics, chemistry, and magnetic resonance imaging.
While it is certainly true that these three basic principles of quantum mechanics seem weird, it is important to note that making counterintuitive proposals about nature is not a unique aspect of quantum mechanics. In fact, putting forth a seemingly weird idea to describe some aspect of the physical world, developing the logical consequences of this weird idea, experimentally testing these consequences, and then accepting the reality of the weird idea if it conforms to observations is pretty much what we call “physics.”
Weird ideas have been the hallmark of physics for at least the past four hundred years. Sir Isaac Newton argued in the mid-1600s, in his first law of motion, that an object in motion remains in motion unless acted upon by an external force. In my personal experience, when I am driving in
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