How to Teach Physics to Your Dog (31 page)

quantum field theory
: A theory that combines quantum mechanics with Einstein’s relativity, to cover particles moving at speeds close to the speed of light, and the interactions between such particles. The simplest quantum field theory is quantum electrodynamics.

quantum interrogation
: A technique using the quantum Zeno effect to detect the presence of an object without letting it absorb even a single photon. Its applications to bunny stalking should be obvious to any dog.

quantum mechanics/ quantum physics/ quantum theory
: The subject of this book, quantum mechanics was developed in the first half of the twentieth century, and describes the behavior of and interactions among atoms, molecules, subatomic particles, and light.

quantum teleportation
: A procedure for transferring the exact state of a quantum particle from one place to another without
measuring it or moving it, using entangled particles as a resource. Sadly, it does not allow dogs to beam themselves into places where they can easily catch squirrels.

quantum Zeno effect
: A demonstration of quantum measurement in which an object can be prevented from changing states by repeatedly measuring its state. The classical equivalent is a dog who prevents her owner from napping by constantly asking “Are you asleep?”

relativity
: The theory developed by Albert Einstein to describe gravity and the behavior of objects moving at speeds close to the speed of light.

Schrödinger equation
: The mathematical formula that physicists use to find the wavefunction for a particular quantum system, and predict how it changes in time.

Schrödinger’s cat
: A thought experiment proposed by Erwin Schrödinger, intended to show the absurdity of quantum superpositions. He imagined a cat enclosed in a box with a device that had a 50% chance of killing the cat within one hour; quantum physics says that at the end of the hour the cat is equal parts alive and dead, until its state is measured. This experiment has made him a hero to canine physicists.

semiclassical argument
: A description of a physical system that is mostly based on classical physics, with a few modern ideas added in an ad hoc manner. Examples of semiclassical models include the “Heisenberg microscope” (page 38) and the Bohr model of hydrogen (page 49).

state/ quantum state
: A particular collection of properties (position, momentum, energy, etc.) describing an object. For example,
“sleeping in the living room,” “sleeping in the kitchen,” and “running around the house” are three different possible states for a dog.

superposition state
: In quantum mechanics, an object can exist in a superposition of two or more allowed states at the same time, until a measurement is made. Such superposition states give rise to interference patterns, which can be detected experimentally, even though the system can only be measured in one allowed state.

thermal radiation
: Also called “black-body radiation,” the light that is emitted by a hot object, such as the characteristic red glow of a hot burner on a stove. The spectrum of this light depends only on the temperature of the object. Explaining this spectrum led Max Planck to introduce quantum mechanics.

tunneling
: A quantum phenomenon in which a particle that does not have enough energy to pass over a barrier passes through the barrier anyway, appearing on the other side, like a bad dog digging a hole under a fence.

uncertainty principle/ Heisenberg uncertainty principle
: One of a set of mathematical relationships limiting the precision with which complementary properties can be measured. The best known uncertainty principle is between momentum and position, and says that it is impossible to know both exactly where a bunny is, and exactly how fast it is moving. Any attempt to specify the position more precisely will lead to increased momentum uncertainty, and vice versa. The energy-time uncertainty relationship is also important, as it determines the length of time that virtual particles can exist in QED.

virtual particle
: A particle in a Feynman diagram that appears and disappears too quickly to be measured directly. These usually
appear as pairs of one normal particle and one antimatter particle, most often one electron and one positron. In principle, anything can show up as a virtual particle, even a bunny made of cheese.

wavefunction
: A mathematical function whose square gives the probability of finding an object in any of its allowed states. In quantum mechanics, all objects are described by wavefunctions.

zero-point energy
: The tiny amount of energy that is always present in a quantum object, thanks to the wave nature of matter. Confined quantum particles are never perfectly at rest—they’re like puppies in a basket, always squirming and wiggling and shifting around, even when they’re asleep.