The 'Most Mysterious' Elementary Particle Turns 100

Next time you turn on the TV, give a thought to 19th-century British physicist Joseph John Thompson. He announced his discovery of the electron 100 years ago this month. Beams of these electrically charged particles paint the pictures on your TV screen. In fact, the picture tube itself is just a sophisticated version of the equipment "J.J." Thompson used.

A century later, physicists still don't know exactly what Thompson discovered. The electron was the first elementary particle known. But we don't know if it is truly elementary or if it is made up of other entities. We don't even know why it has a small amount of mass.

Commenting on this in the journal Nature, Nobel laureate Steven Weinberg at the University of Texas in Austin observes that, given this basic ignorance, "the electron is the most mysterious of all the elementary particles."

What we do understand is its importance. As Thompson himself observed, electrons are a major constituent of atoms, "the substance from which the chemical elements are built up." Roughly speaking, electrons clothe an atomic nucleus in such a way that the entire ensemble - nucleus plus surrounding electrons - make up a complete atom. When atoms of various elements combine in chemical reactions, it is the electrons that mediate.

All of chemistry - including the biochemistry of plant and animal bodies - depends on this interplay of atomic electrons. As chemists have come to understand that interplay in detail, they have produced a vast array of useful products.

Then there is the world of electronics with its computers, communications, and TV tubes. This, too, has emerged from an ever more sophisticated understanding of how electrons behave in vacuum, in gases, and in solid materials. In a basic sense, then, modern technological civilization depends on the electron.

Meanwhile, physicists are trying to understand Thompson's discovery better. According to the current "standard model" of elementary particles, an electron has mass and charge but no intrinsic spatial extend. It's just a point. The theory also predicts, however, that this point-like particle will be surrounded by so-called virtual particles that pop in and out of existence. They buzz around the electron like bees around a honey pot.

At Purdue University in West Lafayette, Indiana, David Koltick and colleagues are probing that virtual cloud. They are part of a group that ran experiments on Japan's Tristan accelerator at the National Laboratory for High Energy Physics in Tsukuba. The virtual-particle cloud cloaks the electron's charge somewhat as measured from the outside. Dr. Koltick's group has been able to measure this effect, showing that the electron's electromagnetic force becomes stronger as they penetrate the cloud and come closer to the electron core. This research will help them better understand the electron's properties.

Thompson did his work at the Cavendish Laboratory at Britain's Cambridge University. There, Michael Pepper, his modern successor, heads a team that is doing something Thompson couldn't dream of trying. They have found a way to isolate a single electron using sound waves. They use it to measure the electric current carried by that single electron. Professor Pepper expects this will give physicists a more reliable standard method for electric current measurement than they have had before.

At the DESY laboratory for particle physics near Hamburg, Germany, experiments now hint that the electron may not be so elementary after all. In these experiments, positrons scatter off quarks. Positrons are the positively charged antimatter counterparts of electrons. Quarks are basic constituents of atomic nuclei. At certain energies, the distinction between the positrons and the quarks becomes blurred. If these results are confirmed, they could open a new era in understanding matter's basic nature.

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