Page 33 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 4

Figure 4:2

Consider a partition with two narrow parallel slits in it. On one side of the partition one places a source of fight of a
particular color (that is, of a particular wavelength). Most of the light will hit the partition, but a small amount will go
through the slits. Now suppose one places a screen on the far side of the partition from the light. Any point on the screen
will receive waves from the two slits. However, in general, the distance the light has to travel from the source to the
screen via the two slits will be different. This will mean that the waves from the slits will not be in phase with each other
when they arrive at the screen: in some places the waves will cancel each other out, and in others they will reinforce
each other. The result is a characteristic pattern of light and dark fringes.

The remarkable thing is that one gets exactly the same kind of fringes if one replaces the source of light by a source of
particles such as electrons with a definite speed (this means that the corresponding waves have a definite length). It
seems the more peculiar because if one only has one slit, one does not get any fringes, just a uniform distribution of
electrons across the screen. One might therefore think that opening another slit would just increase the number of
electrons hitting each point of the screen, but, because of interference, it actually decreases it in some places. If
electrons are sent through the slits one at a time, one would expect each to pass through one slit or the other, and so
behave just as if the slit it passed through were the only one there – giving a uniform distribution on the screen. In reality,
however, even when the electrons are sent one at a time, the fringes still appear. Each electron, therefore, must be

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