Page 37 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 5
is much larger than the size of an atom, we cannot hope to “look” at the parts of an atom in the ordinary way. We need to
use something with a much smaller wave-length. As we saw in the last chapter, quantum mechanics tells us that all
particles are in fact waves, and that the higher the energy of a particle, the smaller the wavelength of the corresponding
wave. So the best answer we can give to our question depends on how high a particle energy we have at our disposal,
because this determines on how small a length scale we can look. These particle energies are usually measured in units
called electron volts. (In Thomson’s experiments with electrons, we saw that he used an electric field to accelerate the
electrons. The energy that an electron gains from an electric field of one volt is what is known as an electron volt.) In the
nineteenth century, when the only particle energies that people knew how to use were the low energies of a few electron
volts generated by chemical reactions such as burning, it was thought that atoms were the smallest unit. In Rutherford’s
experiment, the alpha-particles had energies of millions of electron volts. More recently, we have learned how to use
electromagnetic fields to give particles energies of at first millions and then thousands of millions of electron volts. And so
we know that particles that were thought to be “elementary” thirty years ago are, in fact, made up of smaller particles.
May these, as we go to still higher energies, in turn be found to be made from still smaller particles? This is certainly
possible, but we do have some theoretical reasons for believing that we have, or are very near to, a knowledge of the
ultimate building blocks of nature.

Using the wave/particle duality discussed in the last chapter, every-thing in the universe, including light and gravity, can
be described in terms of particles. These particles have a property called spin. One way of thinking of spin is to imagine
the particles as little tops spinning about an axis. However, this can be misleading, because quantum mechanics tells us
that the particles do not have any well-defined axis. What the spin of a particle really tells us is what the particle looks like
from different directions. A particle of spin 0 is like a dot: it looks the same from every direction Figure 5:1-i. On the other
hand, a particle of spin 1 is like an arrow: it looks different from different directions Figure 5:1-ii. Only if one turns it round
a complete revolution (360 degrees) does the particle look the same. A particle of spin 2 is like a double-headed arrow
Figure 5:1-iii: it looks the same if one turns it round half a revolution (180 degrees). Similarly, higher spin particles look
the same if one turns them through smaller fractions of a complete revolution. All this seems fairly straightforward, but the
remark-able fact is that there are particles that do not look the same if one turns them through just one revolution: you
have to turn them through two complete revolutions! Such particles are said to have spin ½.

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