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Please allow me to introduce you to one of nature's strangest objects: the neutrino.

Neutrinos are everywhere. There are many more of these tiny things than there are atoms in the universe. Trillions of them pass through your body every second but you never notice because they so seldom interact with anything. Moving at nearly light speed, they pass through our planet and out beyond the moon within a second.

We learned in 1998 that 95 percent of the universe is invisible stuff called "dark energy" and "dark matter" that we know little about because we haven't yet detected them in laboratories. Neutrinos are part of the remaining 5 percent that is considered "normal matter," but neutrinos aren't very normal.

Among the neutrino's many mysteries is the magnitude of its mass -- its weight. Indeed, it was long thought neutrinos might be massless, but scientists recently discovered they possess some tiny but poorly known mass. Here's how we know they are not massless.

Neutrinos move fast because basic quantum principles mandate lightweight things must move rapidly. This relates to how we know neutrinos have mass. Photons -- the objects light beams are made of -- have no mass, and quantum physics demands that massless objects must move at light speed. Einstein's relativity tells us time slows down for fast-moving objects, and completely stops for objects moving at light speed. So time "stands still" for a photon, even for one that took 14 billion years to get here from the big bang. So change is impossible for a photon. Three types of neutrinos have been discovered. In 2000, observations convinced physicists that neutrinos "oscillate" spontaneously between the three types, so that a neutrino could spontaneously change from one to another type while traveling here from the sun. But such changes cannot happen to photons and other objects that move at lightspeed. Ergo, neutrinos must move slower than light, so they must have mass, however small.

How small? Electrons, since their discovery in 1897, had long been the lightest known material objects. Electrons are 2000 times lighter than the protons and neutrons comprising the central nucleus of every atom. But the mass of a neutrino is known from experiments to be at least a million times smaller than an electron's mass! That's odd: Why should neutrinos have such tiny masses? Why don't they simply have zero mass? Nobody knows.

Now a laboratory in Germany has undertaken to measure the neutrino's mass with unprecedented accuracy. It's an unbelievably difficult experiment, but its result will be ground-breaking even if this mass turns out to be immeasurably small. The experiment studies the phenomenon that first revealed neutrinos. A certain type of hydrogen atom called tritium happens to be "radioactive," i.e. it can spontaneously create and spit out a high-energy electron from its central nucleus, turning into a helium atom in the process. Back in 1914, when physicists first measured the energy of the emitted electron, they found it to be significantly less than the energy difference between the higher-energy tritium atom and the lower-energy helium atom. A prized physical principle says total energy is "conserved" and hence remains unchanged throughout any spontaneous process, yet energy seemed to be lost during the spontaneous transition of tritium into helium.

Atoms are made of protons, neutrons and electrons, with protons and neutrons in the tiny nucleus and electrons orbiting around the outside. The simplest atom, hydrogen, has a single proton in the nucleus. But there are three types of hydrogen, having respectively zero, one, and two neutrons in the nucleus. The type having two neutrons is tritium.

In 1930, physicists hypothesized that, to conserve energy, tritium must spit out a second, unseen object along with the electron. Dubbed the "neutrino," it was confirmed in 1959.

The German experiment will measure the emitted electron's energy with excruciating accuracy, using a huge tritium source that produces 100 billion electrons per second and a giant electron detector shaped like a small evacuated (airless) zeppelin 23 meters long and 10 meters wide. The detector will measure the energies of the highest-energy electrons emitted from tritium. These electrons would be accompanied by neutrinos having the lowest energies, i.e. by nearly motionless neutrinos, so scientists will be able to determine the energy of a nearly motionless neutrino. But Einstein's E=mc2 tells us this energy is equivalent to a certain amount of mass, so the neutrino's mass can be easily calculated from this energy.

For more, consult the news article in Science magazine, 30 June 2017.

Commentary on 09/19/2017

Print Headline: The lightness of neutrinos

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