About the Nobel Prize in Physics 1921
Albert EinsteinPresentation Speech by Professor S. Arrhenius, Chairman of the Nobel Committee for Physics of the Royal Swedish Academy of Sciences, on December 10, 1922*
Your Majesty, Your Royal Highnesses, Ladies
and Gentlemen.
There is probably no physicist living today whose name has become
so widely known as that of Albert Einstein. Most discussion
centres on his theory of relativity. This pertains essentially to
epistemology and has therefore been the subject of lively debate
in philosophical circles. It will be no secret that the famous
philosopher Bergson in Paris has challenged this theory, while
other philosophers have acclaimed it wholeheartedly. The theory
in question also has astrophysical implications which are being
rigorously examined at the present time.
Throughout the first decade of this century the so-called
Brownian movement stimulated the keenest interest. In 1905
Einstein founded a kinetic theory to account for this movement by
means of which he derived the chief properties of suspensions,
i.e. liquids with solid particles suspended in them. This theory,
based on classical mechanics, helps to explain the behaviour of
what are known as colloidal solutions, a behaviour which has been
studied by Svedberg, Perrin, Zsigmondy and
countless other scientists within the context of what has grown
into a large branch of science, colloid chemistry.
A third group of studies, for which in particular Einstein has
received the Nobel Prize, falls within the domain of the quantum
theory founded by Planck in 1900. This theory asserts that
radiant energy consists of individual particles, termed "quanta",
approximately in the same way as matter is made up of particles,
i.e. atoms. This remarkable theory, for which Planck received the
Nobel Prize for Physics in 1918,
suffered from a variety of drawbacks and about the middle of the
first decade of this century it reached a kind of impasse. Then
Einstein came forward with his work on specific heat and the
photoelectric effect. This latter had been discovered by the
famous physicist Hertz in 1887. He found that an electrical spark
passing between two spheres does so more readily if its path is
illuminated with the light from another electrical discharge. A
more exhaustive study of this interesting phenomenon was carried
out by Hallwachs who showed that under certain conditions a
negatively charged body, e.g. a metal plate, illuminated with
light of a particular colour - ultraviolet has the strongest
effect - loses its negative charge and ultimately assumes a
positive charge. In 1899 Lenard demonstrated the cause to be the
emission of electrons at a certain velocity from the negatively
charged body. The most extraordinary aspect of this effect was
that the electron emission velocity is independent of the
intensity of the illuminating light, which is proportional only
to the number of electrons, whereas the velocity increases with
the frequency of the light. Lenard stressed that this phenomenon
was not in good agreement with the then prevailing
concepts.
An associated phenomenon is photo-luminescence,
i.e.phosphorescence and fluorescence. When light impinges on a
substance the latter will occasionally become luminous as a
result of phosphorescence or fluorescence. Since the energy of
the light quantum increases with the frequency, it will be
obvious that a light quantum with a certain frequency can only
give rise to the formation of a light quantum of lower or, at
most, equal frequency. Otherwise energy would be created. The
phosphorescent or fluorescent light hence has a lower frequency
than the light inducing the photo-luminescence. This is Stokes'
rule which was explained in this way by Einstein by means of the
quantum theory.
Similarly, when a quantum of light falls on a metal plate it can
at most yield the whole of its energy to an electron there. A
part of this energy is consumed in carrying the electron out into
the air, the remainder stays with the electron as kinetic energy.
This applies to an electron in the surface layer of the metal.
From this can be calculated the positive potential to which the
metal can be charged by irradiation. Only if the quantum contains
sufficient energy for the electron to perform the work of
detaching itself from the metal does the electron move out into
the air. Consequently, only light having a frequency greater than
a certain limit is capable of inducing a photo-electric effect,
however high the intensity of the irradiating light. If this
limit is exceeded the effect is proportional to the light
intensity at constant frequency. Similar behaviour occurs in the
ionisation of gas molecules and the so-called ionisation
potential may be calculated, provided that the frequency of the
light capable of ionising the gas is known.
Einstein's law of the photo-electrical effect has been extremely
rigorously tested by the American Millikan and his pupils and
passed the test brilliantly. Owing to these studies by Einstein
the quantum theory has been perfected to a high degree and an
extensive literature grew up in this field whereby the
extraordinary value of this theory was proved. Einstein's law has
become the basis of quantitative photo-chemistry in the same way
as Faraday's law is the basis of electro-chemistry.**
* The Nobel Prize in Physics 1921 was announced on November 9, 1922.
** Being too remote from Sweden, Professor Einstein could not attend the ceremony.
From Nobel Lectures, Physics 1901-1921, Elsevier Publishing Company, Amsterdam, 1967
Copyright © The Nobel Foundation 1922
