Page 32 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 4
measurement would be A in a certain number of cases, B in a different number, and so on. One could predict the
approximate number of times that the result would be A or B, but one could not predict the specific result of an individual
measurement. Quantum mechanics therefore introduces an unavoidable element of unpredictability or randomness into
science. Einstein objected to this very strongly, despite the important role he had played in the development of these
ideas. Einstein was awarded the Nobel Prize for his contribution to quantum theory. Nevertheless, Einstein never
accepted that the universe was governed by chance; his feelings were summed up in his famous statement “God does
not play dice.” Most other scientists, however, were willing to accept quantum mechanics because it agreed perfectly with
experiment. Indeed, it has been an outstandingly successful theory and underlies nearly all of modern science and
technology. It governs the behavior of transistors and integrated circuits, which are the essential components of
electronic devices such as televisions and computers, and is also the basis of modern chemistry and biology. The only
areas of physical science into which quantum mechanics has not yet been properly incorporated are gravity and the
large-scale structure of the universe.

Although light is made up of waves, Planck’s quantum hypothesis tells us that in some ways it behaves as if it were
composed of particles: it can be emitted or absorbed only in packets, or quanta. Equally, Heisenberg’s uncertainty
principle implies that particles behave in some respects like waves: they do not have a definite position but are “smeared
out” with a certain probability distribution. The theory of quantum mechanics is based on an entirely new type of
mathematics that no longer describes the real world in terms of particles and waves; it is only the observations of the
world that may be described in those terms. There is thus a duality between waves and particles in quantum mechanics:
for some purposes it is helpful to think of particles as waves and for other purposes it is better to think of waves as
particles. An important consequence of this is that one can observe what is called interference between two sets of
waves or particles. That is to say, the crests of one set of waves may coincide with the troughs of the other set. The two
sets of waves then cancel each other out rather than adding up to a stronger wave as one might expect Figure 4:1.

Figure 4:1

A familiar example of interference in the case of light is the colors that are often seen in soap bubbles. These are caused
by reflection of light from the two sides of the thin film of water forming the bubble. White light consists of light waves of
all different wavelengths, or colors, For certain wavelengths the crests of the waves reflected from one side of the soap
film coincide with the troughs reflected from the other side. The colors corresponding to these wavelengths are absent
from the reflected light, which therefore appears to be colored. Interference can also occur for particles, because of the
duality introduced by quantum mechanics. A famous example is the so-called two-slit experiment Figure 4:2.

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