Tuesday, July 10, 2012

100 years ago: The discovery of cosmic rays

Already in 1785, Charles Coulomb pointed out a puzzle that would take more than a century to solve: An electrically charged conductor will lose charge with time, even if the only way to decharge is through air, which was generally considered a good insulator.

In 1900 the two Germans Julius Elster and Hans Geitel, and independently the Scotsman Charles Wilson, offered the explanation that air becomes partly conductive by the presence of ionizing radiation. It was known at this time that the Earth contains slightly radioactive substances that create a natural background radiation. This was believed to be the origin of the ionizing radiation.

The meterologist Franz Linke, with support from Geitel and Elster, set out to test this hypothesis. If the radiation is emitted by the Earth, its intensity should drop with distance from the ground. In 1902 and 1903 Linke, on board of  a balloon, found tentative evidence that, after an initial decrease between 1000 and 3000 meters, the intensity of ionizing radiation did increase again. He concluded nevertheless that the origin of ionization in the first line should be sought after on Earth. Linke's research was followed up on by Theodor Wulf, who measured the intensity, among other places, high up in the alps, and found no evidence for the increase of intensity, caused by "cosmic radiation." He was the first to coin the term.

But the situation remained inconclusive. Wulf himself went on to measure the discharge of a charge isolated by air on top of the Eiffel tower. He predicted that in that height (about 300m), the radiation should be about 74% less than on the ground. Instead, he found it to be only 13% less. And in 1910, the Italian physicist Pacini argued that, if the ionizing radiation is emitted by the solids in the Earth, then there should be less of it to find on the sea. That however was not the case either.

On August 7th 1912, Franz Hess and his colleague Kolhorster started for the final one of sevel balloon rides, and this final one lead up to 5350 meter. Despite oxygen mask, Hess reported feeling disoriented, and in fact accidentally turned off one of his detectors already below 4000m. Nevertheless, his measurement clearly showed an increase in the ionization. This was the first conclusive evidence for cosmic radiation.

Then the first world war spelled a time-out for academic curiosity. It wasn't until 1921 that the American  physicist Robert Millikan, together with Ira Bowen, got back to this line of research. Their first balloon ride also found an increase in the ionizing radiation, though less pronounced than what Hess found. The New York Times celebrated him as the discoverer of "Millikan radiation." Needless to say, Hess and Kolhorster were not amused.

The measurement of ionizing radiation dramatically improved with the invention of the Geiger counter in 1928 and the spread of bubble chambers. By 1930 there was little controversy left about the existence of cosmic radiation. Franz Hess was awarded the Nobel Prize in physics in 1936.

Today, cosmic radiation is the true high energy frontier, and has lead to a great many discoveries starting with the positron and the muon, and later the Pion, up to the invaluable knowledge that atmospheric neutrinos have brought to the standard model of particle physics. And, who knows, maybe the first evidence for physics beyond the standard model will come from the cosmic ray frontier too.


  1. Hi Bee,

    Cannot help but see influences from the Sun and the interactions as viable processes that are detailed in cosmic particle collisions with the earth's radiation?

    The back drops given in terms of measures helped to greatly elucidate perspective beyond what what we had ever understood so it is an greatly evolving perspective that has helped us to see nature explained in the way of those interactions.

    Of course, we use the LHC as a lab.


  2. "In 1902 and 1903 Linke, on board of a balloon, found tentative evidence that, after an initial decrease between 1000 and 3000 kilometers"! Spacemen! Kill the kilo, I suspect.

  3. You wrote: "[...] found tentative evidence that, after an initial decrease between 1000 and 3000 kilometers, [...]". Did you mean meters rather than kilometers?

  4. Muon Experiment in Relativity

    The calculation will be considered from the Earth frame of reference. The length is then unaffected since it is in the Earth frame. The halflife is in the muon frame, so must be considered to be time dilated in the Earth frame. You may substitute values for the height and the muon speed in the calculation below See:Muon Experiment in Relativity

  5. Hi Peter, Paolo,

    Thanks, I've fixed that. I had originally written 1 and 3 kilometers, and then decided to skip to meters and just added the zeros... Best,


  6. It's a great story. I invited Luke Drury of the Dublin Institute of Advanced Studies to give a talk on this very subject in our college earlier this year.
    Quiz question; which cosmologists died from similar balloon flights?

  7. Black holes are great particle accelerators.

    If the galactic dark matter were composed of trillions of isolated stellar-mass and planetary-mass black holes, then the century-old enigma of the sources of cosmic rays would be solved.

    Stay tuned for NuSTAR results as this new X-ray telescope determines the abundance of black holes in the MW Galaxy. Scientific operations are scheduled to start this week.

    In its high-energy range, it has 10 times the resolution and 100 times the sensitivity of previous X-ray telescopes.

    If you were looking for an occasion to celebrate, the discovery of the true identity of the dark matter would totally eclipse anything else.

    Or you could put your faith in wimps.

    Discrete Scale Relativity
    Fractal Cosmology

  8. Professor R,

    I don't know, who was it? Best,


  9. "Quiz question; which cosmologists died from similar balloon flights? "


  10. http://www.spaceweather.com/
    "Big sunspot AR1520... has a delta-class magnetic field that harbors energy for X-class solar flares."
    The sun can pop 500 MeV protons (11.2 minutes for July arrival) plus 7x10^(-4) W/m^2 x-rays (8.2 minutes for July arrival).
    Nice for a tethered balloon (dielectric fishing line) or a trip through Homeland Severity.

    More fun is fast neutron count from cosmic rays spallating atmospheric atoms. The top of Antarctica and commercial airline pilots are lit.

  11. I noticed that Julius Elster, Hans Geitel, and Franz Linke do not have English wikipedia entries (although they do have German wikipedia entries). It would be good if someone would translate these 3 entries from de.wikipedia.org into en.wikipedia.org .

  12. Wow! Check out Conlon et al posted to arxiv.org [astro-ph] today, and that is 7-11 :). Sections 6+7 especially.

    The ARCADE-2 factor of 6 excess in the radio background may herald a new and previously unrecognized population of > 10^13 radio sources that are not associated with any external galaxies.

    Perhaps an extended halo of trillions of isolated stellar-mass and planetary-mass black holes?

    Now that would be a treat.

  13. Sorry, that's J.J. Condon et al

    "Resolving the radio background..."



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