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In the first scene of The Big Bang Theory, Sheldon says:

So if a photon is directed through a plane with two slits in it and either slit is observed it will not go through both slits. If it’s unobserved it will, however, if it’s observed after it’s left the plane but before it hits its target, it will not have gone through both slits.

My question is: Why does he use "either"? Wouldn't it be more appropriate to use "each" ?

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(For context, Sheldon is discussing the famous double-slit experiment in physics.)

Using "each" instead of "either" would change the meaning of the sentence.

Let's work with a simpler version of the sentence:

If either slit is observed, the photon will not go through both slits.

In context, "either" implies a choice between two options. During the experiment, a person may choose to look at the first slit only, or at the second slit only. The outcome will be the same in both cases: "the photon will not go through both slits."

Now, let's look at the alternative:

If each slit is observed, the photon will not go through both slits.

"Each" does not imply a choice between options. This sentence says that if a person looks at the first slit, and a person looks at the second slit, then afterwards "the photon will not go through both slits". (Perhaps one person should look at the first slit and a different person should look at the second slit, or perhaps a single person may look at both of the slits in turn, one after the other. It is not clear from context.)


Let me give another example:

Here is a red marker, and here is a green marker. Take either marker and draw a picture.

This means that I want you take one of the two markers and draw one picture. I do not care whether you use the red marker or the green marker.

Here is a red marker, and here is a green marker. Take each marker and draw a picture.

This means that I want you to draw two pictures. One picture should be drawn with the red marker, and the other picture should be drawn with the green marker. I do not care about the order; you may draw the red picture first and the green picture second, or the green picture first and the red picture second.

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