Nobel Lecture, December 12, 1936

Unsolved Problems in Physics: Tasks for the Immediate Future in Cosmic Ray Studies

From a consideration of the immense volume of newly discovered facts in the field of physics, especially atomic physics, in recent years it might well appear to the layman that the main problems were already solved and that only more detailed work was necessary.

This is far from the truth, as will be shown by one of the biggest and most important newly opened fields of research, with which I am closely associated, that of cosmic rays.

When, in 1912, I was able to demonstrate by means of a series of balloon ascents, that the ionization in a hermetically sealed vessel was reduced with increasing height from the earth (reduction in the effect of radioactive substances in the earth), but that it noticeably increased from 1,000 m onwards, and at 5 km height reached several times the observed value at earth level, I concluded that this ionization might be attributed to the penetration of the earth’s atmosphere from outer space by hitherto unknown radiation of exceptionally high penetrating capacity, which was still able to ionize the air at the earth’s surface noticeably. Already at that time I sought to clarify the origin of this radiation, for which purpose I undertook a balloon ascent at the time of a nearly complete solar eclipse on the 12th April 1912, and took measurements at heights of two to three kilometres. As I was able to observe no reduction in ionization during the eclipse I decided that, essentially, the sun could not be the source of cosmic rays, at least as far as undeflected rays were concerned.

Many esteemed physicists in Europe and America have tried since then to solve the problems of the origin of cosmic rays. The fluctuations of intensity of the radiation already incidentally observed by me in 1912 have been thoroughly studied using apparatuses which have been constantly improved and perfected. An influence from specific sky zones which individual research workers (1923-1927) believed they had found, could not be confirmed later.

In the autumn of 1931 a small observatory for the continuous recording of the fluctuations in intensity of the cosmic rays was set up by me on a 2,300 m high mountain, the Hafelekar at Innsbruck in Austria. A great number of results are already available from there which will only be mentioned here briefly. The determination of a small, regular, daily fluctuation of radiation according to solar time (maximum at midday), which were attributed to atmospheric influences, particularly electrical and magnetic effects in the highest layers of the atmosphere. Further indications of a still smaller fluctuation according to stellar time, which would speak in favour of Prof. A.H. Compton’s hypothesis published a year ago, according to which the cosmic rays come from milky-way systems external to, and far-distant from, our own. Further, evidence of simultaneous radiation fluctuations from day to day at two measuring devices spaced at 6 km from each other at heights of 600 and 2,300 m (fitted with ionization chambers, as well as with counting tubes).

On what can we now place our hopes of solving the many riddles which still exist as to the origin and composition of cosmic rays? It must be emphasized here above all that to attain really decisive progress greater funds must be made available. The further improvement of the method of sending up automatically recording instruments to heights above 25 km using pilot balloons, so successfully employed by Prof. Regener (Stuttgart), must be still further expanded and perfected. In conjunction, the many trial methods of automatic radiotelegraphic transmission of observation data as used in America for stratospheric flights will serve a useful purpose. It may well be said that the answer to the question: Of what do the cosmic rays in fact consist before they produce their familiar secondary radiation phenomena in the earth’s atmosphere? can only be obtained from numerous measurements in the stratosphere. In conjunction with this, the study of the occurrence of the so-called showers and Hoffmann’s bursts (release of enormous quantities of ions resulting from atomic disintegration processes) of cosmic rays at various heights will provide new knowledge about the effects of these rays.

In addition, the tracing of the occurrence of these “showers” in the depths of the earth, in mines and through the immersion of recording apparatus in water to some hundreds of metres depth will yield very important results.

In order to make further progress, particularly in the field of cosmic rays, it will be necessary to apply all our resources and apparatus simultaneously and side-by-side; an effort which has not yet been made, or at least, only to a limited extent. Simultaneous recording with superimposed ionization chambers and Wilson chambers, ionization chambers and sets of counting tubes, has not yet been carried out. The photographic method of observing the tracks of the particles of cosmic radiation, first successfully tried out by Prof. Wilkins (Rochester, USA) merits great attention. The application of a strong magnetic field enables the measurement of the energy of the most penetrating particles to be carried out, and the method may be capable of still further extension and improvement.

The investigation into the possible effects of cosmic rays on living organisms will also offer great interest.

The investigation of the tracks of cosmic rays in strong magnetic fields by means of the Wilson cloud chamber method has led to the discovery of the positron (positively charged electrons), that is, one of the hitherto unknown fundamental components of matter; this was carried out by Prof. Carl Anderson (Pasadena) who was in 1936 awarded the Nobel Prize for this work, at the same time as I myself received the award.

It is likely that further research into “showers” and “bursts” of the cosmic rays may possibly lead to the discovery of still more elementary particles, neutrinos and negative protons, of which the existence has been postulated by some theoretical physicists in recent years.