The Nobel Prize
in Physics 1932 and 1933The Nobel Prize
in Physics 1932 and 1933Presentation Speech by Professor H. Pleijel, Chairman of the Nobel Committee for Physics of the Royal Swedish Academy of Sciences, on December 10, 1933
Your Majesty, Your
Royal Highnesses, Ladies and Gentlemen.
This year's Nobel Prizes
for Physics are dedicated to the new atomic physics. The prizes, which the Academy
of Sciences has at its disposal, have namely been awarded to those men, Heisenberg,
Schrödinger, and Dirac, who have created and developed the basic ideas of
modern atomic physics.
It was Planck who, in 1900, first expressed
the thought that light had atomic properties, and the theory put forward by Planck
was later more exhaustively developed by Einstein. The conviction, arrived at
by different paths, was that matter could not create or absorb light, other than
in quantities of energy which represented the multiple of a specific unit of energy.
This unit of energy received the name of light quantum or photon. The magnitude
of the photon is different for different colours of light, but if the quantity
of energy of a photon is divided by the frequency of oscillation of the ray of
light, the same number is always obtained, the so-called Planck's constant h.
This constant is thus of a universal nature and forms one of the foundation stones
for modern atomic physics.
Since light too was thus divided into
atoms it appeared that all phenomena could be explained as interactions between
atoms of various kinds. Mass was also attributed to the atom of light, and the
effects which were observed when light rays were incident upon matter could be
explained with the help of the law for the impact of bodies.
Not
many years passed before the found connection between the photon and the light
ray led to an analogous connection between the motion of matter and the propagation
of waves being sought for.
For a long time it had been known that
the customary description of the propagation of light in the form of rays of light,
which are diffracted and reflected on transmission from one medium to another,
was only an approximation to the true circumstances, which only held good so long
as the wavelength of the light was infinitesimally small compared with the dimensions
of the body through which the light passed, and of the instruments with which
it was observed. In reality light is propagated in the form of waves which spread
out in all directions according to the laws for the propagation of waves.
Prince Louis de Broglie concieved the brilliant idea of seeking an analogy
between the path of the light ray and the track of a material point. He wondered
whether the track of a particle of matter, like the path of a ray of light, might
only be an approximate expression for reality, prescribed by the coarseness of
our senses, and whether one here was not also dealing with wave motion. Using
Einstein's theory of relativity, he was equally succesful in representing the
motion of matter as a combination of waves which were propagating themselves with
velocities greater than that of light. Matter is formed or represented by a great
number of this kind of waves which have somewhat different velocities of propagation
and such phase that they combine at the point in question. Such a system of waves
forms a crest wich propagates itself with quite a different velocity from that
of its component waves, this velocity being the so-called group velocity. Such
a wave crest represents a material point which is thus either formed by it or
connected with it, and is called a wave packet. De Broglie now found that the
velocity of the material point was in fact the group velocity of the matter-wave.
De Broglie's theory of matter-waves subsequently received experimental
confirmation. If a relatively slowly travelling electron meets a crystal surface,
diffraction and reflection phenomena appear in the same way as if an incident
beam of waves were concerned.
As a result of this theory on is forced
to the conclusion to conceive of matter as not being durable, or that it can have
definite extension in space. The waves, which form the matter, travel, infact,
with different velocity and must, therefore, sooner or later separate. Matter
changes form and extent in space. The picture which has been created, of matter
being composed of unchangeable particles, must be modified.
One of
the physical phenomena whose correct explanation has proved most difficult, is
the apperance of the spectra of countless lines and bands which are obtained if
light is split up by optical instruments when produced by atoms and molecules
as a result of their vibrations. It has been known for a long time that each such
line corresponds to light of a certain frequency, which varies according to where
the line appears in the various parts of the colour spectrum.
A correct
explanation of the intensities of all these lines and their positions in the spectrum
is of fundamental significance since it gives an insight into the structure of
the atoms and molecules and the relationships within them.
It was
Bohr who, in 1913, expressed the idea that Planck's constant should be taken as
the determining factor for movements within the atom, as well as for the emission
and absorption of light waves.
Bohr assumed, after Rutherford, that
an atom consists of an inner, heavy, positively charged particle, around which
is negative, light electrons circulate in closed paths, held to the nucleus by
the attraction. According to whether the path of the electron is further away,
or closer from the nucleus, the electron possesses different velocitiy and different
energy. Bohr now put forward the hypothesis that only such path exist where the
energy of the electron, as a result of its motion in the path, is a whole multiple
of a quantum of light corresponding to the rotation frequency of the electron.
Light, Bohr now assumed, appears if an electron suddenly transfers from one path
to another, and the frequency of the light ray is emitted, is obtained if the
change of energy experienced during transferis divided by Planck's constant. The
frequencies which Bohr thus obtained held good for a hydrogen atom which has only
one electron, but when his method was applied to more complicated atoms and to
certain optical phenomena, theory and practice did not agree. The fact that Bohr's
hypothesis met the case for the hydrogen atom, however, suggests that Planck's
constant was, in one way or another, a determining factor for the light-vibratons
of the atoms. On the other hand, one had the feeling that it could not be right
to apply the laws of classical mechanics to the rapid movements in the atom. Efforts
made from various sides to develop and improve Bohr's theory proved also in vain.
New ideas were required to solve the problem of oscillations of atoms and molecules.
This solution followed in 1925 upon the works of Heisenberg, Schödinger,
and Dirac in which different starting-points and methods were applied.
I will first of all dwell upon Schrödinger's cotribution since it is more
closely than the others connected to the state of the development which atomic
physics had attained at that period of time, particularly as a result of de Broglie's
above-mentioned theory of matter-waves.
Since the electrons were
the seat of outgoing waves, Schrödinger thought that it should be possible
to find a wave equation for the motions executed by the electrons which would
define these waves in the same way as the wave equation which determined the propagation
of light. From the solution of this wave equation one should be able to select
those oscillations which were feasible for the motons within the atoms. He was
succesful, too, in determining the wave equation for a series of different motions
of the electron, and it turned out that these equations gave finite solutions
only when the energy of the system had specific discrete values, determined by
Planck's constant. In Bohr's theory these discrete energy values of the electron
paths were only hypothetical, but in Schrödinger's, on the contrary, they
appeared as completely determined by the form of the wave equation. Schrödinger
himself, and others after him, have applied his wave theory to various optical
problems including the interpretation of the phenomena accompanying the impact
between light rays and electrons, investigations into the behaviour of atoms in
electric and magnetic fields, the diffraction of light rays, etc. In every direction,
values and formulae have been obtained using Schrödinger's theory, which
have been in closer agreement with experience than the older theories were. Schrödinger's
wave equation has provided a convenient and simple method for handling problems
to do with light spectra, and has become an indispensible tool for the present-day
physicist.
Somewhat before the appearance of Schrödinger's theory
Heisenberg brought out his famous quantum mechanics. Heisenberg started off from
quite different standpoints and viewed his problem, from the very beginning, from
so broad an angle that it took care of systems of electrons, atoms, and molecules.
According to Heisenberg one must start from such physical quantities as permit
of direct observation, and the task consists of finding the laws which link these
quantities together. The quantities first of all to be considered are the frequencies
and intensities of the lines in the spectra of atoms and molecules. Heisenberg
now considered the combination of all the oscillations of such a spectrum as one
system, for the mathematical handling of which, he set out certain symbolical
rules of calculation. It had formerly been determined already that certain kinds
of motions within the atom must be viewed as independent from one another to a
certain degree, in the same way that a specific difference is made in classical
mechanics between parallel motion and rotational motion. It should be mentioned
in this connection that in order to explain the properties of a spectrum it had
been necessary to assume self-rotation of the positive nuclei and the electrons.
These different kinds of motion for atoms and molecules produce different systems
in Heisenberg's quantum mechanics. As the fundamental factor of Heisenberg's theory
can be put forward the rule set out by him with reference to the relationship
between the position coordinate and the velocity of an electron, by which rule
Planck's constant is introduced into the quantum-mechanics calculations as a determining
factor.
Although Heisenberg's and Schrödinger's theories had
differnt starting points and were developed by the use of different processes
of thought, they produced the same results for problems treated by both theories.
Heisenberg's quantum mechanics has been applied by himself and others to
the study of the properties of the spectra of atoms and molecules, and has yielded
results which agree with experimental research. It can be said that Heisenberg's
quantum mechanics has made possible a systemization of spectra of atoms. It should
also be mentioned that Heisenberg, when he applied his theory to molecules consisting
of two similar atoms, found among other things that the hydrogen molecule must
exist in two different forms which should appear in some given ratio to each other.
This prediction of Heisenberg's was later also experimentally confirmed.
Dirac has set up a wave mechanics which starts from the most general conditions.
From the start he put forward the requirement that the postulate of the relativity
theory be furfilled. Viewed from this general formulation of the problems it appeared
that the self-rotation of the electron which had previously come into the theory
as an hypothesis stipulated by experimental facts, now appeared as a result of
the general theory of Dirac.
Dirac divided the initial wave equation
into two simpler ones, each providing solutions independently. It now appeared
that one of the solution systems required the existence of positive electrons
having the same mass and charge as the known negative electrons. This initially
posed considerable difficulty for Dirac's theory, since positively charged particles
were known only in the form of the heavy atom nucleus. This difficulty which at
first opposed the theory has now become a brilliant confirmation of its validity.
For later on, positive electrons, the positrons, whose existence was stipulated
in Dirac's theoretical investigation, have been found by experiment.
The new quantum mechanics has changed to a great extent all our concepts of the
relationships existing within the microscopic world, made up of atoms and molecules.
We have already mentioned that as a result of the new wave mechanics we have had
to modify our conception on the unchangeability of material particles. But more
than this. Heisenberg has shown that according to quantum mechanics it is inconceivable
to determine, at a given instant of time, both the position taken up by a particle
and its velocity. Closer study of quantum mechanics shows in fact that the more
one attempts to fix exactly the position of a particle, the more uncertain the
determination of its velocity becomes, and vice versa. It must be further considered,
that it is impossible to carry out the measurement of the situation in an atom
or molecule without the employed instruments, ilrumination, etc.themselves altering
the situation which is under examination. The light emitted from the electrons
becomes modified in the optical instruments. The relationships go still deeper
however. As a result of the introduction of light quanta, quantum mechanics must
abandon the requirement of causality within the microcosmic world. A ray of light
on being incident upon an optical instrument is resolved. However, the photon
is indivisible. It must be realized then, that some photons will behave in one
way, others in another way at the resolution. The only assertion that can be made
regarding causality is that the physical laws signify a certain probability that
one or another incident will take place. Since we can only perceive average values
because of the imperfection of our senses and instruments, it is probabilities
which are covered in our physical laws, and the question has been raised, whether
in the physical world there is in fact any other accordance with laws than a statistical
one.
Professor Heisenberg.
It has fallen to you whilst young in years, to have given to physics, by means
of the theory of quantum mechanics established by you, a general method for the
solution of the manifold problems which have come to the fore as a result of restless
experimental researches into the theory of radiation. From a study of the properties
of the molecules, you have succeeded, among other things, in predicting that the
hydrogen molecules would appear in two forms, which later has been confirmed.
Your quantum mechanics has created new concepts, and has led physics into fresh
trains of thought, which have now already proved of fundamental importance for
our knowledge of the phenomena of physics.
The Royal Academy of Sciences
has awarded you the Nobel Prize for Physics for 1932 in recognition of these studies,
and I beg you to accept this distinction from the hands of His Majesty the King.
Professor
Schrödinger. Through a study of the wave properties of matter you have succeeded
in establishing a new system of mechanics which also holds good for motion within
the atoms and molecules. With the aid of this so-called wave mechanics you have
found the solution to a number of problems in atomic physics. Your theory provides
a simple and convenient method for the study of the properties of atoms and molecules
under various external conditions and it has become a great aid to the development
of physics.
For your discovery of new fruitful forms of atomic physics
and the application of these, the Royal Academy of Sciences has decided to award
you the Nobel Prize. I request you to receive this from the hands of His Majesty
the King.
Professor Dirac.
The theory of wave mechanics which you have developed is characterized by its
universality, since from the beginning you have imposed the condition that the
postulate of the theory of relativity has to be furfilled. In this way you have
shown that the existence of the spin of electrons and its qualities are a consequence
of this theory and not merely a hypothesis.
Further you have succeeded
in dividing the wave equation into two, which results in two systems of solutions
one of which requires the existence of a positive electron of the same size and
charge as the negative electron. The experimental discovery of the existence of
the positron has in a brilliant way confirmed your theory.
For the
discovery of new fertile forms of the theory of atoms presented by you and for
its applications the Royal Academy of Sciences has awarded you the Nobel Prize,
and I now ask you to receive this prize from the hands of His Majesty the King.
From Nobel Lectures, Physics 1922-1941, Elsevier Publishing Company, Amsterdam, 1965
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