84
R. Barrett and P. P. Delsanto
molecules. The precision of his results convinced the doubters, and atoms
became accepted not just as a mathematical artifice to explain chemical reactions, but as real objects. The unobservable had become observed, albeit
indirectly.
But let us return now to the debate, which lasted several centuries, about
the nature of light. As we have seen, Maxwell’s theory seemed to have put
an end to the dispute, but there remained an unexplained but important
phenomenon called black body radiation (see Appendix 5.2). Suppose that
we carry out an experiment by placing a small piece of iron in a blacksmith’s
forge and observe the change as we slowly turn up the heat. At first the iron
starts to glow a dull cherry red, but as it gets hotter, its red colour becomes
lighter and more yellow. If we can make it hot enough, the iron will glow
with a white light, and if it could be made as hot as the inside of some stars,
it would look blue.
This is a commonly observed effect that everybody who has lived in the era
of tungsten filament lamps has witnessed, but for decades physicists strived
in vain to explain it quantitatively. All their calculations predicted that most
of the radiation would be emitted at ultraviolet frequencies, and they dubbed
the phenomenon: the Ultraviolet Catastrophe.
It was not until nearly four decades after Maxwell’s work that Max Planck
in 1900 produced an explanation of black body radiation. He made the
radical and, at the time, arbitrary assumption that electromagnetic radiation
was not emitted continuously, but instead came out in discrete packets, or
quanta. There was no justification for this assumption, which was completely
contrary to the tenets of classical physics (see Appendix 5.3), other than
that it worked! Planck proposed that the energy of these packets is directly
proportional to the frequency of the electromagnetic radiation. The constant
of proportionality was given the symbol h, which is now known as Planck’s
constant.
Another challenge for Maxwell’s theory arose in 1887 when Heinrich
Hertz investigated the photoelectric effect, which consists of the emission
of electrons when light is allowed to fall on a metal surface. Hertz discovered that the energy of the emitted electrons depends on the frequency of the
light, and not on the beam intensity, as expected. In 1905, Einstein explained
the anomaly by proposing that the incident light beam is not a continuous
wave, but rather is composed of discrete wave packets (now called photons). 4
4 It may be worth recalling that Einstein obtained his Nobel Prize in 1921 for the photoelectric effect,
not for his work on Relativity, since the foundation statute required that prizes could be awarded
only for research validated by experimental confirmation, which was not yet the case for General
Relativity.
R. Barrett and P. P. Delsanto
molecules. The precision of his results convinced the doubters, and atoms
became accepted not just as a mathematical artifice to explain chemical reactions, but as real objects. The unobservable had become observed, albeit
indirectly.
But let us return now to the debate, which lasted several centuries, about
the nature of light. As we have seen, Maxwell’s theory seemed to have put
an end to the dispute, but there remained an unexplained but important
phenomenon called black body radiation (see Appendix 5.2). Suppose that
we carry out an experiment by placing a small piece of iron in a blacksmith’s
forge and observe the change as we slowly turn up the heat. At first the iron
starts to glow a dull cherry red, but as it gets hotter, its red colour becomes
lighter and more yellow. If we can make it hot enough, the iron will glow
with a white light, and if it could be made as hot as the inside of some stars,
it would look blue.
This is a commonly observed effect that everybody who has lived in the era
of tungsten filament lamps has witnessed, but for decades physicists strived
in vain to explain it quantitatively. All their calculations predicted that most
of the radiation would be emitted at ultraviolet frequencies, and they dubbed
the phenomenon: the Ultraviolet Catastrophe.
It was not until nearly four decades after Maxwell’s work that Max Planck
in 1900 produced an explanation of black body radiation. He made the
radical and, at the time, arbitrary assumption that electromagnetic radiation
was not emitted continuously, but instead came out in discrete packets, or
quanta. There was no justification for this assumption, which was completely
contrary to the tenets of classical physics (see Appendix 5.3), other than
that it worked! Planck proposed that the energy of these packets is directly
proportional to the frequency of the electromagnetic radiation. The constant
of proportionality was given the symbol h, which is now known as Planck’s
constant.
Another challenge for Maxwell’s theory arose in 1887 when Heinrich
Hertz investigated the photoelectric effect, which consists of the emission
of electrons when light is allowed to fall on a metal surface. Hertz discovered that the energy of the emitted electrons depends on the frequency of the
light, and not on the beam intensity, as expected. In 1905, Einstein explained
the anomaly by proposing that the incident light beam is not a continuous
wave, but rather is composed of discrete wave packets (now called photons). 4
4 It may be worth recalling that Einstein obtained his Nobel Prize in 1921 for the photoelectric effect,
not for his work on Relativity, since the foundation statute required that prizes could be awarded
only for research validated by experimental confirmation, which was not yet the case for General
Relativity.