This Day in Tech History

On This Day . . .

Rays Zipping Through Their Eyeballs


November 11, 1925: Robert Millikan Announces Cosmic Rays

On this day in 1925, Robert Millikan announces discovery of cosmic rays. “There is a very powerful, very penetrating ray which reaches earth from space.” Dr. Millikan recognized this ray and called it the cosmic ray. He found that it could penetrate 50 ft. of lead.


The discovery of his law of motion of a particle falling towards the earth after entering the earth’s atmosphere, together with his other investigations on electrical phenomena, ultimately led him to his significant studies of cosmic radiation (particularly with ionization chambers). Millikan proved that this radiation is indeed of extraterrestrial origin, and he named it “cosmic rays.”

Artist concept of cosmic rays.

Artist concept of cosmic rays.

What are cosmic rays?

Particles that bombard the Earth from anywhere beyond its atmosphere.

Cosmic rays are pieces of atoms, flying through space at incredible speeds.
Sometimes they hit Earth, and some even hit us. About 30 cosmic rays fly through our bodies every second.


Let’s clarify what is meant by incredible speeds.
Most energetic cosmic rays detected have been found to travel at about:  0.999999999999999999999995 the speed of light (that’s 23 9’s, plus half of that)

This is faster than any known process could possibly launch a particle. In fact it would have taken 40 million times more energy than our most powerful particle accelerator can pack into an atom for those particles to reach those speeds.

Hmmm, not nuclear fusion in the sun… not exploding stars if they come from black holes or colliding galaxies? How do they reach those speeds!?


So where do they come from?

Most galactic cosmic rays are probably accelerated in the blast waves of supernova remnants. The remnants of the explosions can last thousands of years, and this is where cosmic rays are believed to be accelerated. Bouncing back and forth in the magnetic field of the remnant randomly lets some of the particles gain energy, and become cosmic rays. Eventually they build up enough speed that the remnant can no longer contain them, and they escape into the Galaxy.


However, cosmic rays have been observed at much higher energies than supernova remnants can generate, and where these ultra-high-energies come from is a big question.

Astronauts and Cosmic Rays

The situation is quite different if you happen to be in space.

During the Apollo 11 mission in 1969 (the first mission to the moon), crew members reported seeing something unusual while in orbit: flashes of light in the dark – even when they closed their eyes! Subsequent Apollo missions confirmed this phenomenon, and later performed tests to study it. As you may have guessed, the astronauts were witnessing cosmic rays zipping through their eyeballs!

Build a Cosmic Ray Detector

Of Note:

In 1912, Victor Hess carried three enhanced-accuracy Wulf electrometers to an altitude of 5300 meters in a free balloon flight. He found the ionization rate increased approximately fourfold over the rate at ground level. He concluded: “The results of my observation are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above.” Hess received the Nobel Prize in Physics in 1936 for his discovery.


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