means to say that you “see” something. In order for you to see these words, for example, light from some source (the sun or a lamp) must strike the book and then travel to your eye. A complex chemical process in your retina converts the energy of the light into a signal that travels to your brain.
Think about the interaction of light with the book. When you look at the book, you do not see it recoil when the light hits it, despite the fact that floods of photons must be bouncing off every second in order for you to see the page. This is the classic Newtonian way of thinking about measurement. It is assumed that the act of measurement (in this case, the act of bouncing light off the book) does not affect the object being measured in any way. Given the infinitesimal energy of the light compared to the energy required to move the book, this is certainly a reasonable way to look at things. After all, baseballs do not jitteraround in the air because photographers are using flashbulbs, nor does the furniture in your living room jump every time you turn on the light.
But does this comfortable, reasonable, Newtonian viewpoint apply in the ultrasmall world of the atom? Can you “see” an electron in the same way that you see this book?
If you think about this question for a moment, you will realize that there is a fundamental difference between “seeing” these two objects. You see a book by bouncing light off it, and the light has a negligible effect on the book. You “see” an electron, on the other hand, by bouncing another electron (or some other comparable bundle) off it. In this case, the thing being probed and the thing doing the probing are comparable in every way, and the interaction cannot leave the original electron unchanged. It’s as if the only way you could see a billiard ball was to hit it with another billiard ball.
There is a useful analogy that will help you think about measurement in the quantum domain. Suppose there was a long, dark tunnel in a mountain and you wanted to know whether there was a car in the tunnel. Suppose further that you couldn’t go into the tunnel yourself or shine a light down it—that the only way you could answer your question was to send another car down that tunnel and then listen for a crash. If you heard a crash, you could certainly say that there was another car in the tunnel. You couldn’t say, however, that the car was the same after your “measurement” as it was before. The very act of measuring—in this case the collision of one car with the other—changes the original car. If you then sent another car down the tunnel to make a second measurement, you would no longer be measuring the original car, but the original car as it has been altered by the first measurement.
In the same way, the fact that to make a measurement on anelectron requires the same sort of disruptive interaction means that the electron (or any other quantum particle) must be changed whenever it is measured. This simple fact is the basis for the uncertainty principle and, in the end, for many of the differences that exist between the familiar world and the world of the quantum.
The uncertainty principle is a statement that says, in effect, that the changes caused by the act of measurement make it impossible to know everything about a quantum particle with infinite precision. It says, for example, that you cannot know both the position (where something is) and velocity (how fast it’s moving) exactly—the two pieces of data that are significant in describing any physical object.
The important thing about the uncertainty principle is that if you measure the position of a tiny particle with more and more precision, so that the error becomes smaller and smaller, the uncertainty in velocity must become greater to compensate. The more care you take to know one thing, the more poorly you know the other. The very act of measuring changes the thing you are measuring, so you must always be uncertain
Think about the interaction of light with the book. When you look at the book, you do not see it recoil when the light hits it, despite the fact that floods of photons must be bouncing off every second in order for you to see the page. This is the classic Newtonian way of thinking about measurement. It is assumed that the act of measurement (in this case, the act of bouncing light off the book) does not affect the object being measured in any way. Given the infinitesimal energy of the light compared to the energy required to move the book, this is certainly a reasonable way to look at things. After all, baseballs do not jitteraround in the air because photographers are using flashbulbs, nor does the furniture in your living room jump every time you turn on the light.
But does this comfortable, reasonable, Newtonian viewpoint apply in the ultrasmall world of the atom? Can you “see” an electron in the same way that you see this book?
If you think about this question for a moment, you will realize that there is a fundamental difference between “seeing” these two objects. You see a book by bouncing light off it, and the light has a negligible effect on the book. You “see” an electron, on the other hand, by bouncing another electron (or some other comparable bundle) off it. In this case, the thing being probed and the thing doing the probing are comparable in every way, and the interaction cannot leave the original electron unchanged. It’s as if the only way you could see a billiard ball was to hit it with another billiard ball.
There is a useful analogy that will help you think about measurement in the quantum domain. Suppose there was a long, dark tunnel in a mountain and you wanted to know whether there was a car in the tunnel. Suppose further that you couldn’t go into the tunnel yourself or shine a light down it—that the only way you could answer your question was to send another car down that tunnel and then listen for a crash. If you heard a crash, you could certainly say that there was another car in the tunnel. You couldn’t say, however, that the car was the same after your “measurement” as it was before. The very act of measuring—in this case the collision of one car with the other—changes the original car. If you then sent another car down the tunnel to make a second measurement, you would no longer be measuring the original car, but the original car as it has been altered by the first measurement.
In the same way, the fact that to make a measurement on anelectron requires the same sort of disruptive interaction means that the electron (or any other quantum particle) must be changed whenever it is measured. This simple fact is the basis for the uncertainty principle and, in the end, for many of the differences that exist between the familiar world and the world of the quantum.
The uncertainty principle is a statement that says, in effect, that the changes caused by the act of measurement make it impossible to know everything about a quantum particle with infinite precision. It says, for example, that you cannot know both the position (where something is) and velocity (how fast it’s moving) exactly—the two pieces of data that are significant in describing any physical object.
The important thing about the uncertainty principle is that if you measure the position of a tiny particle with more and more precision, so that the error becomes smaller and smaller, the uncertainty in velocity must become greater to compensate. The more care you take to know one thing, the more poorly you know the other. The very act of measuring changes the thing you are measuring, so you must always be uncertain
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