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By Root 2056 0

it’s still the most widely used method for introducing quantum theory’s essential ideas. Here’s how to think about it. When Davisson and Germer heated the tarnished sample, they caused numerous small nickel crystals to meld into fewer larger ones. In turn, their electron beam no longer reflected off a highly uniform surface of nickel but instead bounced back from a few concentrated locations where the larger nickel crystals were centered. A simplified version of their experiment, the setup of Figure 8.1, in which electrons are fired at a barrier containing two narrow slits, highlights the essential physics. Electrons emanating from one slit or the other are like electrons bouncing back from one nickel crystal or its neighbor. Modeled in this way, Davisson and Germer were carrying out the first version of what’s now called the double-slit experiment.

To grasp Davisson and Germer’s startling result, imagine closing off either the left or the right slit and capturing the electrons that pass through, one by one, on a detector screen. After many such electrons are fired, the detector screens will look like those in Figure 8.2a and Figure 8.2b. A rational, nonquantum trained mind would therefore expect that when both slits are open, the data would be an amalgam of these two results. But the astounding fact is that this is not what happens. Instead, Davisson and Germer found data, much like those illustrated in Figure 8.2c, consisting of light and dark bands indicating a series of positions where electrons do and do not land.

Figure 8.1 The essence of the Davisson and Germer experiment is captured by the “double-slit” setup in which electrons are fired at a barrier that has two narrow slits. In the Davisson and Germer experiment, two streams of electrons are produced when incident electrons bounce off neighboring nickel crystals; in the double-slit experiment, two similar streams are produced by electrons that pass through the neighboring slits.

These results deviate from expectations in a way that’s particularly peculiar. The dark bands are locations where electrons are copiously detected if only the left slit or only the right slit is open (the corresponding regions in Figures 8.2a and 8.2b are bright), but which are apparently unreachable when both slits are available. The presence of the left slit thus changes the possible landing locations of electrons passing through the right slit, and vice versa. Which is thoroughly perplexing. On the scale of a tiny particle like an electron, the distance between the slits is huge. So when the electron passes through one slit, how could the presence or absence of the other have any effect, let alone the dramatic influence evident in the data? It’s as if for many years you happily enter an office building using one door, but when the management finally adds a second door on the building’s other side, you can no longer reach your office.

What are we to make of this? The double-slit experiment leads us inescapably to a conclusion hard to fathom. Regardless of which slit it passes through, each individual electron somehow “knows” about both. There’s something associated with, or connected to, or part of each individual electron that is affected by both slits.

But what could that something be?

Figure 8.2 (a) The data obtained when electrons are fired and only the left slit is open. (b) The data obtained when electrons are fired and only the right slit is open. (c) The data obtained when electrons are fired and both slits are open.

Quantum Waves

For a clue as to how an electron traveling through one slit “knows” about the other, look more closely at the data in Figure 8.2c. The light-dark-light-dark pattern is as recognizable to a physicist as a mother’s face is to her baby. The pattern says—no, it screams—waves. If you’ve ever dropped two pebbles into a pond and watched as the resulting ripples spread and overlap, you know what I mean. Where the peak of one wave crosses the peak of another, the combined wave height is big; where the trough of one crosses the trough of another,