Volume 25 · Number 3 · Spring 2008
A glossary
GPS and relativity: GPS navigation systems work by picking up the “tick-tock” of atomic clocks on satellites high above the Earth. As a result of relativity, the clocks on the satellites appear to run a few micro-seconds a day faster than a clock on the ground. That’s enough to build up errors of miles per day if engineers had not accounted for these effects.
Hadron: A hadron is a subatomic particle made up of smaller fundamental particles called quarks. Protons and neutrons, the particles that make up the nucleus of an atom, are both hadrons.
Energy tied to mass: A consequence of special relativity is that energy and mass are essentially the same thing. Einstein’s famous equation E=mc2 shows that a small amount of mass is equivalent to a large amount of energy.
Space-time: The concept of uniting the three dimensions of space with the fourth dimension of time.
Higgs boson/Higgs field: The Higgs field was proposed by Peter Higgs in 1964 to explain why particles have mass. According to the theory, particles get mass by pushing through this field. In quantum theory, all fields are associated with a fundamental particle, and the Higgs boson is the particle associated with the Higgs field.
Symmetry: In science, something has symmetry when it remains essentially unchanged regardless of what you do to it. If I pick up a pencil and put it behind my ear, it is still the same pencil that lay on my desk—it has just moved in space.
Science is based on a profound symmetry: the laws of nature work in the same way everywhere in the universe and for all time.
The Standard Model includes symmetries, but these must be broken under some circumstances in order to account for observed facts—such as the existence of mass.
Zero-point energy: In quantum physics, zero-point energy is the lowest amount of energy a system can have. It is not exactly zero, because of the uncertainty principle in quantum theory.
Zero-point energy is also the amount of energy in empty space due to the random appearance and
disappearance of particles.
Gravitational lensing: If a beam of light passes close to a massive object, it will be slightly bent by gravity. This prediction was one of the first tests used to verify Einstein’s theory of general relativity, which says that gravity is actually due to a curvature of space-time.
Dark matter does not interact with light at all, so it is completely invisible. But it does exert gravity, so light passing through or near a massive clump of dark matter will get slightly bent, much as a lens bends the light passing through it.
Quantum foam: Some theories try to unite quantum theory with gravity and relativity by proposing that space itself is made up of discrete units, or “quanta,” at an extremely small scale. “Quantum foam” describes the structure of space in some of these scenarios.