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R. Barrett and P. P. Delsanto
very core of this masterwork is a caveat, an essential, rather dodgy, sleight-ofhand, to remind us that we are but human, and our knowledge is therefore
imperfect.
First, however, it is necessary to refresh and extend our understanding of
two concepts that we have encountered already in previous chapters: particles
and fields.
8.2 Particles
In Chap. 5, we have seen how the idea that matter in the universe is composed
of elementary particles, called atoms, has been around at least since the time
of the early Greeks. By the start of the 20th Century, it was recognised
that electromagnetic radiation also exhibits corpuscular properties in some
circumstances. Any doubt about the atomic structure of matter—and there
had been many doubters because of the difficulty of observing atoms—had
been dispelled by Einstein’s paper in 1905 explaining Brownian motion (see
Chap. 5).
The 19th Century had also seen the discovery of the electron, the fundamental particle of electricity. In 1897, the British physicist J. J. Thompson
reported in The Philosophical Magazine experiments on the strange cathode
rays emitted from a heated metal in a vacuum. His ingeniously designed
experiments used deflections of the cathode rays by electric and magnetic
fields to verify that the rays were indeed comprised of electrified particles, now
known as electrons, and went on to determine the ratio of the electric charge
of the particles to their mass.
A number of other rays were also discovered towards the end of the 19th
Century. These were X-rays, produced by Wilhelm Röntgen in 1895, and the
alpha, beta and gamma rays emitted during the radioactive decay of uranium,
and discovered in 1899 and 1900 by Henri Becquerel, Paul Villard and Ernest
Rutherford, working independently. Further experiments to determine the
electric charge-to-mass ratio of the alpha and beta particles found that the
positively charged alpha particle was most probably a helium atom with its
two electrons stripped away, and the negatively charged beta particles were
electrons. Gamma rays were a very energetic form of electromagnetic radiation. Both X-rays and gamma rays, and also visible light, heat rays (infra-red
radiation) and chemical rays (ultra-violet radiation), are now known to be
made of photons of different frequency (see Fig. 5.1).
So at the end of the 19th Century there was a widespread belief that
apart from a few wayward examples (electrons, and the particles emitted in
R. Barrett and P. P. Delsanto
very core of this masterwork is a caveat, an essential, rather dodgy, sleight-ofhand, to remind us that we are but human, and our knowledge is therefore
imperfect.
First, however, it is necessary to refresh and extend our understanding of
two concepts that we have encountered already in previous chapters: particles
and fields.
8.2 Particles
In Chap. 5, we have seen how the idea that matter in the universe is composed
of elementary particles, called atoms, has been around at least since the time
of the early Greeks. By the start of the 20th Century, it was recognised
that electromagnetic radiation also exhibits corpuscular properties in some
circumstances. Any doubt about the atomic structure of matter—and there
had been many doubters because of the difficulty of observing atoms—had
been dispelled by Einstein’s paper in 1905 explaining Brownian motion (see
Chap. 5).
The 19th Century had also seen the discovery of the electron, the fundamental particle of electricity. In 1897, the British physicist J. J. Thompson
reported in The Philosophical Magazine experiments on the strange cathode
rays emitted from a heated metal in a vacuum. His ingeniously designed
experiments used deflections of the cathode rays by electric and magnetic
fields to verify that the rays were indeed comprised of electrified particles, now
known as electrons, and went on to determine the ratio of the electric charge
of the particles to their mass.
A number of other rays were also discovered towards the end of the 19th
Century. These were X-rays, produced by Wilhelm Röntgen in 1895, and the
alpha, beta and gamma rays emitted during the radioactive decay of uranium,
and discovered in 1899 and 1900 by Henri Becquerel, Paul Villard and Ernest
Rutherford, working independently. Further experiments to determine the
electric charge-to-mass ratio of the alpha and beta particles found that the
positively charged alpha particle was most probably a helium atom with its
two electrons stripped away, and the negatively charged beta particles were
electrons. Gamma rays were a very energetic form of electromagnetic radiation. Both X-rays and gamma rays, and also visible light, heat rays (infra-red
radiation) and chemical rays (ultra-violet radiation), are now known to be
made of photons of different frequency (see Fig. 5.1).
So at the end of the 19th Century there was a widespread belief that
apart from a few wayward examples (electrons, and the particles emitted in