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The artist imagines an electron surrounding the nucleus of an atom.Nicolle R. Fuller, National Foundation for Science.
Electrons are interesting things. They help shape the structure of atoms and molecules, they can flow through conductive materials to create electric currents, and they have a strange quantum behavior that is sometimes similar to a particle or a wave. And as far as we can judge, they are perfectly spherical. This has interesting implications for advanced physics.
When we imagine what electrons look like, we usually have a bad idea. Electrons are often described as small particles gravitating around a nucleus, like planets gravitating around a star. But as quantum objects, electrons are not particles. They do not gravitate around the nucleus of an atom, but rather surround it in a fuzzy quantum cloud. Electrons can exhibit particle-like behavior in some experiments, but they are not hard and solid objects in the way we perceive particles. In addition, electrons are elementary particles. While the nucleus of an atom is composed of neutrons and protons, themselves made up of quarks, the electrons are just electrons. They do not consist of even smaller particles.
So, what does it mean to say that these fuzzy quantum objects are spherical? It all boils down to what is called an electric dipole moment. A dipole moment occurs when the charge of an object is not distributed evenly. For example, water molecules have a dipole moment because the molecular bonds between the oxygen atom and the hydrogen atoms mean that the negatively charged electrons are slightly shifted positively charged nuclei . Many molecules have electric dipole moments because the molecules are not elementary particles. Since electrons do not consist of smaller particles, it would seem obvious that electrons can not have a dipole moment.
But there is a problem. Although they are elementary particles, electrons also have a property called spin. This is similar to the kinetic moment of a rotating object, except that it is simply an inherent property of the electrons. In the standard model of quantum physics, the electron spin does not distort the charge distribution in an electron, so the electrons should not have a dipole moment. In other words, they should be spherical. But there are clues that the standard model might be wrong, and some alternative models predict that super massive (undiscovered) particles could interact with the electron spin to give the electrons a tiny dipole moment. If an experiment showed that the electrons were not perfectly spherical, it would show that the standard model is false.
The ACME collaboration therefore focused on measuring the dipole moment of the electrons. ACME stands for Advanced Cold Molecule Electron EDM. In their experiment, they projected lasers on thorium monoxide (ThO) molecules. This brought the electrons into the molecules to emit light. By measuring the light emitted by the electrons, they could determine how much the electrons are spherical. Their experience was so accurate that if an electron had the size of the Earth, they could say it was non-spherical for a tiny fraction of a human hair. But at the limits of their experience, the electrons seem to be perfectly spherical.
As far as we can judge, the standard model of electrons is correct. This means that some of the alternatives to the standard model must be false.
Paper: ACME collaboration. Improved limit of the electric dipole moment of the electron. Nature, volume 562, pages 355-360 (2018).
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The artist imagines an electron surrounding the nucleus of an atom.Nicolle R. Fuller, National Foundation for Science.
Electrons are interesting things. They help shape the structure of atoms and molecules, they can flow through conductive materials to create electric currents, and they have a strange quantum behavior that is sometimes similar to a particle or a wave. And as far as we can judge, they are perfectly spherical. This has interesting implications for advanced physics.
When we imagine what electrons look like, we usually have a bad idea. Electrons are often described as small particles gravitating around a nucleus, like planets gravitating around a star. But as quantum objects, electrons are not particles. They do not gravitate around the nucleus of an atom, but rather surround it in a fuzzy quantum cloud. Electrons can exhibit particle-like behavior in some experiments, but they are not hard and solid objects in the way we perceive particles. In addition, electrons are elementary particles. While the nucleus of an atom is composed of neutrons and protons, themselves made up of quarks, the electrons are just electrons. They do not consist of even smaller particles.
So, what does it mean to say that these fuzzy quantum objects are spherical? It all boils down to what is called an electric dipole moment. A dipole moment occurs when the charge of an object is not distributed evenly. For example, water molecules have a dipole moment because the molecular bonds between the oxygen atom and the hydrogen atoms mean that the negatively charged electrons are slightly shifted positively charged nuclei . Many molecules have electric dipole moments because the molecules are not elementary particles. Since electrons do not consist of smaller particles, it would seem obvious that electrons can not have a dipole moment.
But there is a problem. Although they are elementary particles, electrons also have a property called spin. This is similar to the kinetic moment of a rotating object, except that it is simply an inherent property of the electrons. In the standard model of quantum physics, the electron spin does not distort the charge distribution in an electron, so the electrons should not have a dipole moment. In other words, they should be spherical. But there are clues that the standard model might be wrong, and some alternative models predict that super massive (undiscovered) particles could interact with the electron spin to give the electrons a tiny dipole moment. If an experiment showed that the electrons were not perfectly spherical, it would show that the standard model is false.
The ACME collaboration therefore focused on measuring the dipole moment of the electrons. ACME stands for Advanced Cold Molecule Electron EDM. In their experiment, they projected lasers on thorium monoxide (ThO) molecules. This brought the electrons into the molecules to emit light. By measuring the light emitted by the electrons, they could determine how much the electrons are spherical. Their experience was so accurate that if an electron had the size of the Earth, they could say it was non-spherical for a tiny fraction of a human hair. But at the limits of their experience, the electrons seem to be perfectly spherical.
As far as we can judge, the standard model of electrons is correct. This means that some of the alternatives to the standard model must be false.
Paper: ACME collaboration. Improved limit of the electric dipole moment of the electron. Nature, volume 562, pages 355-360 (2018).
