The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World by Sean Carroll
Authors: Sean Carroll
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combined in different ways to account for all the variety of the observable world.
At first glance the particle zoo can seem complex and intimidating, but there are only twelve matter particles, which fall neatly into two groups of six: quarks, which feel the strong nuclear force, and leptons, which do not. It’s an amazing story, put together over the course of a century, from the discovery of the electron in 1897 to the detection of the last elementary fermion (the tau neutrino) in 2000. Here we’ll take a whirlwind tour, saving the quantitative details for Appendix Two. When the smoke clears we will have a relatively manageable collection of particles from which everything else is made.
Pictures of atoms
Everyone has seen cartoon images of atoms. They are usually portrayed as tiny solar systems, with a central nucleus surrounded by orbiting electrons. It’s an iconic image, which serves, for example, as the logo of the U.S. Atomic Energy Commission. It’s also misleading in a subtle way.
The cartoon atom represents the Bohr model, named after Danish physicist Niels Bohr, who applied insights from the early days of quantum mechanics to the model of atoms that had been previously developed by New Zealand–born British physicist Ernest Rutherford. In Rutherford’s version of the atom, electrons orbit the nucleus at any distance you might imagine, just like planets in the real solar system (except they are attracted to the center by electromagnetism, not by gravity). Bohr modified that idea by insisting that the electrons can travel only on certain particular orbits, which was a great step forward in fitting the data from radiation emitted by atoms. These days we know that the electrons don’t really “orbit” at all, because they don’t really have a “position” or “velocity.” Quantum mechanics says that the electrons persist in clouds of probability known as “wave functions,” which tell us where we might find the particle if we were to look for it.
Cartoon image of an atom; in this case, helium. A nucleus consisting of two protons and two neutrons sit at the center, while two electrons “orbit” on the outskirts.
All that being granted, the basic cartoon we have in mind of what an atom looks like isn’t that bad, if what we’re looking for is some intuitive grasp of what’s going on. Nucleus in the middle, electrons on the outskirts. The electrons are relatively light; more than 99.9 percent of the mass of an atom is located in the nucleus. That nucleus is made of a combination of protons and neutrons. A neutron is a bit heavier than a proton—a neutron is about 1,842 times as heavy as an electron, while a proton is about 1,836 times as heavy. Protons and neutrons are both called “nucleons,” as they are the particles that make up nuclei (plural of “nucleus”). Aside from the fact that the proton has an electric charge and the neutron is a bit heavier, the two nucleons are remarkably similar particles.
Like many things in life, the nature of an atom is one of exquisite balance. The electrons are attracted to the nucleus by the force of electromagnetism, which is enormously stronger than the force of gravity. The electromagnetic attraction between an electron and a proton is about 10 39 times stronger than the gravitational attraction between them. But while gravity is simple—everything attracts everything else—electromagnetism is more subtle. Neutrons get their name from the fact that they are neutral, having no electric charge at all. So the electromagnetic force between an electron and a neutron is zero.
Particles with the same kind of electric charge repel one another, while opposites live up to the romantic cliché and attract. Electrons are attracted to the protons inside a nucleus because electrons carry a negative charge and protons carry a positive one. So—you may be asking yourself—why don’t the protons packed so closely inside a nucleus push one another apart? The
At first glance the particle zoo can seem complex and intimidating, but there are only twelve matter particles, which fall neatly into two groups of six: quarks, which feel the strong nuclear force, and leptons, which do not. It’s an amazing story, put together over the course of a century, from the discovery of the electron in 1897 to the detection of the last elementary fermion (the tau neutrino) in 2000. Here we’ll take a whirlwind tour, saving the quantitative details for Appendix Two. When the smoke clears we will have a relatively manageable collection of particles from which everything else is made.
Pictures of atoms
Everyone has seen cartoon images of atoms. They are usually portrayed as tiny solar systems, with a central nucleus surrounded by orbiting electrons. It’s an iconic image, which serves, for example, as the logo of the U.S. Atomic Energy Commission. It’s also misleading in a subtle way.
The cartoon atom represents the Bohr model, named after Danish physicist Niels Bohr, who applied insights from the early days of quantum mechanics to the model of atoms that had been previously developed by New Zealand–born British physicist Ernest Rutherford. In Rutherford’s version of the atom, electrons orbit the nucleus at any distance you might imagine, just like planets in the real solar system (except they are attracted to the center by electromagnetism, not by gravity). Bohr modified that idea by insisting that the electrons can travel only on certain particular orbits, which was a great step forward in fitting the data from radiation emitted by atoms. These days we know that the electrons don’t really “orbit” at all, because they don’t really have a “position” or “velocity.” Quantum mechanics says that the electrons persist in clouds of probability known as “wave functions,” which tell us where we might find the particle if we were to look for it.
Cartoon image of an atom; in this case, helium. A nucleus consisting of two protons and two neutrons sit at the center, while two electrons “orbit” on the outskirts.
All that being granted, the basic cartoon we have in mind of what an atom looks like isn’t that bad, if what we’re looking for is some intuitive grasp of what’s going on. Nucleus in the middle, electrons on the outskirts. The electrons are relatively light; more than 99.9 percent of the mass of an atom is located in the nucleus. That nucleus is made of a combination of protons and neutrons. A neutron is a bit heavier than a proton—a neutron is about 1,842 times as heavy as an electron, while a proton is about 1,836 times as heavy. Protons and neutrons are both called “nucleons,” as they are the particles that make up nuclei (plural of “nucleus”). Aside from the fact that the proton has an electric charge and the neutron is a bit heavier, the two nucleons are remarkably similar particles.
Like many things in life, the nature of an atom is one of exquisite balance. The electrons are attracted to the nucleus by the force of electromagnetism, which is enormously stronger than the force of gravity. The electromagnetic attraction between an electron and a proton is about 10 39 times stronger than the gravitational attraction between them. But while gravity is simple—everything attracts everything else—electromagnetism is more subtle. Neutrons get their name from the fact that they are neutral, having no electric charge at all. So the electromagnetic force between an electron and a neutron is zero.
Particles with the same kind of electric charge repel one another, while opposites live up to the romantic cliché and attract. Electrons are attracted to the protons inside a nucleus because electrons carry a negative charge and protons carry a positive one. So—you may be asking yourself—why don’t the protons packed so closely inside a nucleus push one another apart? The
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