How to Teach Physics to Your Dog (30 page)

The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics
by Robert Oerter (Plume, 2006) gives an excellent overview of the state of modern physics,
including basic quantum theory, QED, and issues in particle physics that are not discussed in this book.

Richard Feynman’s
QED: The Strange Theory of Light and Matter
(Princeton University Press, 2006) is a wonderfully accessible explanation of the details of quantum electrodynamics. His autobiographical books (
Surely You’re Joking, Mr. Feynman
and
What Do You Care What Other People Think?
) have less physics, but are great fun.

For more mathematically inclined readers,
The Quantum Challenge, Second Edition: Modern Research on the Foundations of Quantum Mechanics
by George Greenstein and Arthur G. Zajonc (Jones and Bartlett, 2005) gives an excellent overview of many of the experiments that have demonstrated the strange features of quantum mechanics.

James Gleick’s
Genius: The Life and Science of Richard Feynman
includes a description of the development of QED, and the rivalry between Feynman and Julian Schwinger.

On an artistic note, Michael Frayn’s play
Copenhagen
makes powerful use of quantum ideas in exploring the famous falling-out between Niels Bohr and Werner Heisenberg.

And finally, George Gamow’s “Mr. Tompkins” stories (collected in
Mr. Tompkins in Paperback
[Cambridge University Press, Canto Imprint, 1993]) are the original whimsical exploration of modern physics, through the daydreams of an unassuming bank clerk. No physics book involving a talking dog could possibly fail to mention them.

allowed state
: One of a limited number of states in which an object may be measured in quantum mechanics. For example, a dog at rest can either be found on the floor, or on the couch, but never halfway between the floor and the couch.

antimatter
: Every particle in the universe has an antimatter equivalent, with the same mass and the opposite charge. When a particle of ordinary matter encounters its antiparticle, the two annihilate, converting their mass into energy.

Bell’s theorem
: A mathematical theorem proved by John Bell, showing that entangled quantum particles have their states correlated in ways that no local hidden variable (LHV) theory can match.

classical physics
: Physics theories developed before about 1900, describing the behavior of everyday objects. Core components are Newton’s laws of motion, Maxwell’s equations for electricity and magnetism, and the laws of thermodynamics.

coherence
: A property of waves or wavefunctions, roughly defined as behaving as if the waves came from a single source. Adding together two coherent waves gives a clear interference pattern; adding together two incoherent waves gives a rapidly
shifting pattern that smears out and becomes indistinct. The process of “decoherence” destroys the coherence between two waves from a single source through random and fluctuating interactions with a larger environment.

conservation of energy
: The law of conservation of energy states that energy can be changed from one form to another, but the total energy of a given system is always the same. For example, a dog can convert the potential energy stored in food into kinetic energy as she chases a squirrel, but she cannot gain more kinetic energy than the total energy available in her food.

Copenhagen interpretation
: The philosophical framework for quantum mechanics developed by Neils Bohr and colleagues at his institute in Denmark. The Copenhagen interpretation insists on an absolute split between microscopic systems, which are described by quantum mechanics, and macroscopic systems, which are described by classical physics. The interaction between a microscopic quantum system and a macroscopic measuring apparatus causes the wavefunction to “collapse” into one of the allowed states for that system.

decoherence
: A process by which random, fluctuating interactions with the environment destroy our ability to see an interference pattern for a quantum particle. Decoherence is particularly important for the many-worlds interpretation, where it ensures that different branches of the wavefunction of the universe will not affect one another.

diffraction
: A characteristic behavior of waves, in which waves passing through a narrow opening or around an obstacle spread out on the far side. A dog can hear a potato chip hitting the kitchen floor from the living room because sound waves diffract through the kitchen door and around corners.

Einstein, Podolsky, and Rosen (EPR) paradox
: A famous paper by Albert Einstein, Boris Podolsky, and Nathan Rosen that used entangled particles to argue that quantum mechanics was incomplete. Their argument has been proven wrong by experiments testing Bell’s theorem, but has led to the development of quantum teleportation and other quantum information technologies.

energy
: A measure of an object’s ability to change its own motion or the motion of another object. Energy comes in many forms, such as kinetic energy, potential energy, and mass energy (Einstein’s
E = mc
²
). Energy may be converted from one form to another, but cannot be created or destroyed.

energy-time uncertainty
: A variation of the Heisenberg uncertainty principle stating that it is impossible to know both the exact energy of some object and the exact time at which it was measured. This limits the lifetime of virtual particles in quantum electrodynamics.

entanglement
: A quantum “connection” between two objects whose states are correlated in such a way that measuring one also determines the state of the other. A classical analogy is two dogs in the same room: either both will be awake, or both will be asleep. If you measure one dog to be awake, you immediately know that the other is also awake. Similar correlations exist for quantum particles, but their states are indeterminate until one of the two is measured, at which time the state of both is instantaneously determined, no matter how far apart they are.

Feynman diagram
: A picture representing a possible sequence of events for a charged particle interacting with light. Each diagram stands for a calculation in QED, and the energy of the interacting particle is found by adding together
all the possible diagrams for that particle. The diagrams are named after Richard Feynman, who invented them as a calculational shortcut.

gyromagnetic ratio/ “g-factor”
: A number, given the symbol
g,
that determines how an electron interacts with a magnetic field. The simplest theory of quantum mechanics predicts that
g
= 2, but QED predicts a value that is very slightly larger. The experimentally measured value of
g
agrees with the QED prediction to fourteen decimal places.

Hawking radiation
: A process by which “virtual particles” cause black holes to evaporate. When a particle-antiparticle pair appears near a black hole, one of the two can fall into the black hole, while the other escapes. In order to conserve energy, the black hole must lose a tiny bit of mass. Over time, the black hole is whittled down to nothing, one particle mass at a time.

interference
: A phenomenon that occurs when two or more waves are added together. If the peaks of one wave line up with the peaks of the other (“in phase”), the result is a much larger wave. If the peaks of one wave line up with the valleys of the other (“out of phase”), the result is no wave at all. Interference patterns involving single particles are the clearest demonstration of quantum behavior.

kinetic energy
: Energy associated with a moving object. For everyday objects, the kinetic energy is equal to half the mass times the speed squared (
½ mv
²
). A Great Dane has more kinetic energy than a Chihuahua moving at the same speed, while a hyperactive Siberian husky has more kinetic energy than a sleepy bloodhound of the same mass.

local hidden variable (LHV) theory
: A theory of the sort preferred by Einstein, Podolsky, and Rosen. In an LHV theory, measurements made in one position are independent of measurements made at other positions (“local”), and particles are always in definite states, though the exact values are unknown (“hidden variables”). LHV theories cannot duplicate all the predictions of quantum mechanics (according to Bell’s theorem), and have been disproven in experiments by Alain Aspect, among others.

many-worlds interpretation
: The philosophical framework for quantum mechanics developed by Hugh Everett III at Princeton in the 1950s. The many-worlds interpretation avoids the “wave-function collapse” problem of the Copenhagen interpretation by saying that all possible measurement outcomes take place in different branches of the wavefunction—in some part of the wave-function, every dog eats steak. Sadly, we only perceive a single branch. The other branches of the wavefunction are effectively separate universes, due to decoherence, which prevents the different branches from having a measurable effect on one another.

measurement
: In quantum mechanics, an active process that changes the state of the system being measured. Before a measurement is made, a quantum object will be in a superposition of all the allowed states; after the measurement, the object will be in one and only one state. The Copenhagen interpretation and the many-worlds interpretation offer two different ways of describing what happens during a measurement.

modern physics
: Physics theories developed after about 1900, consisting principally of relativity and quantum mechanics.

momentum
: A quantity associated with motion that determines what will happen to an object during a collision. In classical
physics, momentum is mass times velocity (
p = mv
); a small Chihuahua must be moving much faster than a Great Dane to have the same momentum. In quantum mechanics, the momentum of a particle determines its wavelength, through the de Broglie relation
λ = h/p
.

no-cloning theorem
: A mathematical theorem showing that it is impossible to make a perfect copy of a quantum object without knowing its state in advance.

particle-wave duality
: A feature of quantum mechanics, in which objects have both particle and wave properties. Classical physics says that light is a wave, but quantum physics says that a beam of light is also a stream of photons. Classical physics says that an electron is a particle, but quantum physics says that an electron also has a wavelength that depends on its momentum. Electrons and photons are each “quantum particles,” a third class of object that is neither particle nor wave, but has properties of each.

photoelectric effect
: An effect discovered in the late 1800s, where light falling on metal knocks out electrons. Einstein explained the photoelectric effect in 1905 by applying Planck’s quantum hypothesis to light directly, describing a beam of light as a stream of photons.

photon
: A “particle” of light. A beam of light may be thought of as a stream of particles, like kibble being poured into a dog’s bowl. Each photon has an energy given by Planck’s constant multiplied by the frequency associated with that color of light (
E = hf
).

Planck’s Constant (
h
)
: The constant that relates energy to frequency or momentum to wavelength in quantum physics. The
measured value is
h
=6.6261×10
^-34
J-s, or 0.00000000000000 00000000000000000006261 J-s, which is a very small number indeed.

polarization
: A property of light, corresponding to the direction of oscillation of the light wave in classical physics. Any polarization can be described as a combination of vertical and horizontal polarizations, and will determine the probability of that light passing through a vertical or horizontal polarizer.

polarizer/ polarizing filter
: A material that allows through light polarized at some angle, and blocks light polarized 90° away from that angle. Light polarized at some intermediate angle has a probability of passing through that depends on the angle.

potential energy
: Energy associated with an object that is not currently moving, but has the potential to start moving. A dog always has potential energy, even when sleeping: at the slightest sound, she can leap up and start barking at nothing.

probability
: The wavefunction for a quantum object describes the probability of finding the object in any of its allowed states when a measurement is made. For example, there is a high probability of finding the dog in the kitchen, a high probability of finding the dog in the living room, and a very low probability of finding the dog on the couch, if she knows what’s good for her.

quantum computer
: A computer made up of “qubits” that not only can take values of “0” and “1” like the bits in a classical computer, but also superpositions of “0” and “1.” Such a computer could solve certain types of problems, such as the factoring of large numbers, much faster than a classical computer. The difficulty of factoring large numbers is the basis for modern cryptography, so a quantum computer would let an evil squirrel
decipher your credit card transactions, and clean out your bank account to buy birdseed.

quantum electrodynamics
: “QED” for short. The theory describing the interactions between charged particles and light, developed by Richard Feynman, Julian Schwinger, and Shin-Ichiro Tomonaga around 1950. Feynman’s formulation is the best known version, which describes interactions in terms of the exchange of “virtual particles.”

quantum eraser
: A demonstration of quantum measurement in which an interference pattern is destroyed by making it possible to measure the exact path taken by a particle, but recovered by doing something to confuse that measurement.