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Glossary: M - P

A MACHO, or massive compact halo object, is a localized mass that has a gravitational influence on the matter around it but does not emit any light. Black holes and brown dwarf stars are examples of MACHOs. MACHOs were once thought to make a significant contribution to dark matter; however, gravitational lensing surveys have demonstrated that most of the dark matter must be something else.
A macroscopic object, as opposed to a microscopic one, is large enough to be seen with the unaided eye. Often (but not always), classical physics is adequate to describe macroscopic objects, and a quantum mechanical description is unnecessary.
magnetic moment
The magnetic moment (or magnetic dipole moment) of an object is a measure of the object's tendency to align with a magnetic field. It is a vector quantity, with the positive direction defined by the way the object responds to a magnetic field: The object will tend to align itself so that its magnetic moment vector is parallel to the magnetic field lines. There are two sources for a magnetic moment: the motion of electric charge and spin angular momentum. For example, a loop of wire with a current running through it will have a magnetic moment proportional to the current and area of the loop, pointing in the direction of your right thumb if your fingers are curling in the direction of the current. Alternatively, an electron, which is a spin-1/2 fermion, has an intrinsic magnetic moment proportional to its spin.
magneto-optical trap
A magneto-optical trap, or MOT, uses a combination of laser beams and magnetic fields to confine atoms at temperatures between a few millikelvin and a few microkelvin. Atoms in a MOT are constantly interacting with the laser beams, which cool them to the laser-cooling limit, but no further than that.
Magnons are the quasiparticles associated with spin waves in a crystal lattice.
mass number
The mass number (or atomic mass number) of an atom, denoted by A, is the total number of nucleons (protons+neutrons) in its nucleus. Sometimes, the mass number of an atom is written as a superscript to the left of its chemical symbol (e.g., 6Li) to show which isotope is being discussed. See: atomic number, isotope.
matrix mechanics
Matrix mechanics is the version of quantum mechanics formulated in the 1920s by Werner Heisenberg and his close colleagues Max Born and Pascual Jordan. It makes extensive use of matrices and linear algebra, which was relatively new mathematics at the time. Matrix mechanics, which is mathematically equivalent to Schrödinger's wave mechanics, greatly simplifies certain calculations.
A megaparsec is 1 million parsecs. Galaxies are typically separated by a few megaparsecs, while galaxy clusters are typically separated by around 10 megaparsecs. See: parsec.
The term meson refers to any particle in the Standard Model that is made of one quark and one anti-quark. Murray Gell-Mann arranged the leptons into a periodic-table-like structure according to their electric charge and strangeness (see Unit 1, Fig. 1). Examples of mesons are pions and kaons.
A metastable state has a higher energy than the ground state that a physical system can become trapped in for some length of time. A simple example is a ball sitting on a hilltop. The ball's energy would be lower if it rolled down the hill; but unless something disturbs it, it will remain where it is. Metastable states of atoms are put to use in atomic clocks because they are long lived, and therefore correspond to a clock frequency that can be known very precisely. In biological physics, valleys in the energy landscape correspond to metastable states, as do low-lying peaks in the fitness landscape.
The microkelvin is a unit of temperature equivalent to one-millionth (10-6) of a degree in the Kelvin scale. Laser-cooled atoms are typically at temperatures of a few microkelvin.
The millikelvin is a unit of temperature equivalent to one-thousandth (10-3) of a degree in the Kelvin scale. 3He becomes a superfluid at a temperature of around one millikelvin.
The mini-computer was a precursor to the personal computers that are ubiquitous today. Prior to the development of the mini-computer, scientists doing computer-intensive calculations shared mainframe computers that were expensive multi-user facilities the size of small houses. Mini-computers cost ten times less than mainframe computers, fit into a single room, and had sufficient computing power to solve numerical problems in physics and astronomy when fully dedicated to that purpose. When mini-computers first became available, many areas of scientific research blossomed, including the study of how structure formed in the universe.
MOND, or Modified Newtonian Dynamics, is a theory that attempts to explain the evidence for dark matter as a modification to Newtonian gravity. There are many versions of the theory, all based on the premise that Newton's laws are slightly different at very small accelerations. A ball dropped above the surface of the Earth would not deviate noticeably from the path predicted by Newtonian physics, but the stars at the very edges of our galaxy would clearly demonstrate modified dynamics if MOND were correct.
A monomer is a small molecule that can bind to other like molecules to form a polymer. The amino acids that make up proteins are examples of monomers.
The muon is a fundamental particle in the Standard Model. It is a member of the second generation of leptons. The muon is negatively charged, heavier than the electron, and lighter than the tau.
N-body simulation
An N-body simulation is a computer simulation that involves a large number of particles interacting according to basic physical laws. N-body simulations are used to study how the structures in our universe may have evolved. Typically, many millions of particles are configured in an initial density distribution and allowed to interact according to the laws of gravity. The computer calculates how the particles will move under the influence of gravity in a small time step, and uses the resulting distribution of particles as the starting point for a new calculation. By calculating many time steps, the simulation can track the growth of structures in the model system. Depending on the initial density distribution and cosmological parameters selected, different structures appear at different stages of evolution. N-body simulations have provided strong support to the idea that our universe consists primarily of dark energy and dark matter. These simulations are resource intensive because the number of interactions the computer must calculate at each time step is proportional to the number of particles squared. A sophisticated N-body simulation can require tens of thousands of supercomputer hours.
Nambu-Goldstone theorem
The Nambu-Goldstone theorem states that the spontaneous breaking of a continuous symmetry generates new, massless particles.
natural linewidth
The natural linewidth of an atomic energy level is the intrinsic uncertainty in its energy due to the uncertainty principle.
The neutralino is the superpartner of the neutrino. See: neutrino, superpartner, supersymmetry.
Neutrinos are fundamental particles in the lepton family of the Standard Model. Each generation of the lepton family includes a neutrino (see Unit 1, Fig. 18). Neutrinos are electrically neutral and nearly massless. When neutrinos are classified according to their lepton family generation, the three different types of neutrinos (electron, muon, and tau) are referred to as "neutrino flavors." While neutrinos are created as a well-defined flavor, the three different flavors mix together as the neutrinos travel through space, a phenomenon referred to as "flavor oscillation." Determining the exact neutrino masses and oscillation parameters is still an active area of research.
Newton's law of universal gravitation
Newton's law of universal gravitation states that the gravitational force between two massive particles is proportional to the product of the two masses divided by the square of the distance between them. The law of universal gravitation is sometimes called the "inverse square law." See: universal gravitational constant.
Nodes are the positions in a standing wave that do not move as the wave oscillates. Antinodes are the opposite of nodes—the positions in a standing wave that move up and down the most as the wave oscillates. A plucked guitar string oscillating in its fundamental mode has a node at each end, and an antinode in the middle.
nuclear fission
Nuclear fission is the process by which the nucleus of an atom decays into a lighter nucleus, emitting some form of radiation. Nuclear fission reactions power nuclear reactors, and provide the explosive energy in nuclear weapons.
nuclear fusion
Nuclear fusion is the process by which the nucleus of an atom absorbs other particles to form a heavier nucleus. This process releases energy when the nucleus produced in the fusion reaction is not heavier than iron. Nuclear fusion is what powers stars, and is the source of virtually all the elements lighter than iron in the universe.
The term "nucleosynthesis" refers either to the process of forming atomic nuclei from pre-existing protons and neutrons or to the process of adding nucleons to an existing atomic nucleus to form a heavier element. Nucleosynthesis occurs naturally inside stars and when stars explode as supernovae. In our standard model of cosmology, the first atomic nuclei formed minutes after the Big Bang, in the process termed "Big Bang nucleosynthesis."
open string
In string theory, an open string has two distinct ends. Open strings can have one end attached to another object like a brane, and the two ends of an open string can connect to form a closed string. Open strings and closed strings have different properties, and give rise to different sets of fundamental particles.
optical dipole trap
An optical dipole trap is a type of atom trap that uses only laser light to trap atoms. The laser frequency is tuned to a frequency below an atomic resonance so the atoms do not absorb laser photons as they do in laser cooling or in a MOT. Instead, the electric field from the laser induces an electric dipole in the atoms that attracts them to regions of more intense laser light. Optical dipole traps are only strong enough to hold cold atoms, so atoms are typically cooled first and then transferred into the dipole trap.
optical lattice
An optical lattice is an optical dipole trap made from a standing wave laser beam, so there is a periodic array of regions with a strong and weak laser field. Atoms are attracted to regions of a strong field, so they are trapped in a lattice-like pattern.
optical molasses
Optical molasses is formed when laser beams for Doppler cooling are directed along each spatial axis so that atoms are laser cooled in every direction. Atoms can reach microkelvin temperatures in optical molasses. However, the molasses is not a trap, so the atoms can still, for example, fall under the influence of gravity.
Parity is an operation that turns a particle or system of particles into its mirror image, reversing their direction of travel and physical positions.
A parsec is an astronomer's unit of distance defined as the distance at which the Earth's orbit (1 AU) subtends 1 arc second. This is related to the geometrical method of measuring the distance to stars called parallax. One parsec is equivalent to 19 trillion miles, 3.08 x 1013 km, or 3.26 light years. Stars within a galaxy are typically separated by distances of a few parsecs.
particle accelerator
Particle accelerators are the primary experimental tool used in particle physics experiments. They accelerate beams of charged particles—such as protons, electrons, and ions—to very high speeds. In a particle physics experiment, the fast-moving beams are steered into a collision either with a stationary target or a beam traveling in the opposite direction. These collisions release a tremendous amount of energy that can create new particles. Much of the Standard Model was developed by studying the particles produced in such collisions. See: cyclotron, linac, synchrotron.
Pauli exclusion principle
The Pauli exclusion principle states that no two identical fermions can occupy the same quantum state. It plays an important role in determining the structure of atoms and atomic nuclei, as well as how electrons behave in metals and semiconductors.
In physics, the term phase has two distinct meanings. The first is a property of waves. If we think of a wave as having peaks and valleys with a zero-crossing between them, the phase of the wave is defined as the distance between the first zero-crossing and the point in space defined as the origin. Two waves with the same frequency are "in phase" if they have the same phase and therefore line up everywhere. Waves with the same frequency but different phases are "out of phase." The term phase also refers to states of matter. For example, water can exist in liquid, solid, and gas phases. In each phase, the water molecules interact differently, and the aggregate of many molecules has distinct physical properties. Condensed matter systems can have interesting and exotic phases, such as superfluid, superconducting, and quantum critical phases. Quantum fields such as the Higgs field can also exist in different phases.
phase coherence
If we think of a wave as having peaks and valleys with a zero-crossing between them, the phase of the wave is defined as the distance between the first zero-crossing and the point in space defined as the origin. Two waves are phase coherent (or simply coherent) if the distance between their respective peaks, valleys, and zero-crossings is the same everywhere.
Phonons are the quasiparticles associated with acoustic waves, or vibrations, in a crystal lattice or other material.
Photons can be thought of as particle-like carriers of electromagnetic energy, or as particles of light. In the Standard Model, the photon is the force-carrier of the electromagnetic force. Photons are massless bosons with integer spin, and travel through free space at the speed of light. Like material particles, photons possess energy and momentum.
The term pion refers to any one of three mesons containing up and down quarks and their antiparticles. The positively charged is composed of an up quark and an anti-down quark. Its antiparticle is the negatively charged , which is composed of an anti-up quark and a down quark. The neutral pion, , is made of down, anti-down, up, and anti-up quarks. Pions are the lightest mesons , and play a role in strong interactions in the nuclei of atoms.
Planck length
The Planck length is the fundamental unit of length used in high energy physics, and is a combination of Planck's constant, Newton's constant of universal gravitation, and the speed of light. The Planck length is approximately 1.6 x 10-35 m.
Planck mass
The Planck mass is the fundamental unit of mass used in high energy physics, and is a combination of Planck's constant, Newton's constant of universal gravitation, and the speed of light. The Planck mass is approximately 2.2 x 10-8 kg.
Planck time
The Planck time is the time it takes light to travel one Planck length, and is considered the fundamental unit of time in high energy physics. The Planck time is approximately 5.4 x 10-44 seconds.
Planck's constant
Planck's constant, denoted by the symbol h, has the value 6.626 x 10-34 m2 kg/s. It sets the characteristic scale of quantum mechanics. For example, energy is quantized in units of h multiplied by a particle's characteristic frequency, and spin is quantized in units of h/2. The quantity h/2 appears so frequently in quantum mechanics that it has its own symbol: .
plane wave
A plane wave is a wave of constant frequency and amplitude with wavefronts that are an infinitely long straight line. Plane waves travel in the direction perpendicular to the wavefronts. Although they are a mathematical abstraction, many physical waves approximate plane waves far from their source.
A plasma is a gas of ionized (i.e., electrically charged) particles. It has distinctly different properties than a gas of neutral particles because it is electrically conductive, and responds strongly to electromagnetic fields. Plasmas are typically either very hot or very diffuse because in a cool, relatively dense gas the positively and negatively charged particles will bind into electrically neutral units. The early universe is thought to have passed through a stage in which it was a plasma of quarks and gluons, and then a stage in which it was a plasma of free protons and electrons. The electron gas inside a conductor is another example of a plasma. The intergalactic medium is an example of a cold, diffuse plasma. It is possible to create an ultracold plasma using the techniques of atom cooling and trapping.
Plasmons are the quasiparticle associated with oscillations of charge density in a plasma.
plum pudding model
The Plum Pudding Model is a model of atomic structure proposed by J.J. Thomson in the late 19th century. Thomson had discovered that atoms are composite objects, made of pieces with positive and negative charge, and that the negatively charged electrons within the atom were very small compared to the entire atom. He therefore proposed that atoms have structure similar to a plum pudding, with tiny, negatively charged electrons embedded in a positively charged substrate. This was later shown to be incorrect.
A polar molecule has a nonzero electric dipole moment, so it has a side that is positively charged and a side that is negatively charged.
Some atoms and molecules that have no electric dipole moment in an electrically neutral environment will develop one in an electric field. The polarizability of an atom or molecule is a quantity that describes how susceptible it is to this effect.
The polarization of a wave is the direction in which it is oscillating. The simplest type of polarization is linear, transverse polarization. Linear means that the wave oscillation is confined along a single axis, and transverse means that the wave is oscillating in a direction perpendicular to its direction of travel. Laser light is most commonly a wave with linear, transverse polarization. If the laser beam travels along the x-axis, its electric field will oscillate either in the y-direction or in the z-direction. Gravitational waves also have transverse polarization, but have a more complicated oscillation pattern than laser light.
A polymer is a large molecule that is made up of many repeating structural units, typically simple, light molecules, linked together. Proteins are polymers made up of amino acids. See: monomer.
The positron is the antimatter counterpart to the electron. It has an electric charge of +1 and the same mass as an electron.
potential energy
Potential energy is energy stored within a physical system. A mass held above the surface of the Earth has gravitational potential energy, two atoms bound in a molecule have chemical potential energy, and two electric charges separated by some distance have electric potential energy. Potential energy can be converted into other forms of energy. If you release the mass, its gravitational potential energy will be converted into kinetic energy as the mass accelerates downward. In the process, the gravitational force will do work on the mass. The force is proportional to the rate at which the potential energy changes. It is common practice to write physical theories in terms of potential energy, and derive forces and interactions from the potential.
Precession is a systematic change in the orientation of a rotation axis. For example, the orbits of planets in our solar system precess. Each planet follows an elliptical path around the Sun, with the Sun at one of the focal points of the ellipse. The long axis of the ellipse slowly rotates in the plane of the orbit with the Sun as a pivot point, so the planet never follows exactly the same path through space as it continues to orbit in its elliptical path. The precession measured in Mercury's orbit was found to be different from the prediction of Newtonian gravity but matched the prediction of general relativity, providing some of the first concrete evidence that Einstein's version of gravity is correct.
probability density
The exact location of a quantum mechanical particle is impossible to know because of the Heisenberg uncertainty principle. Rather than specifying the location of a particle such as an electron, quantum mechanics specifies a wavefunction. The probability density, which is a mathematical function that specifies the probability of finding the particle at any location in space, is the square of the wavefunction (technically, its absolute value squared).
probability distribution
In quantum mechanics, the probability distribution is a mathematical function that gives the probability of finding a particle in any small region of space. The probability distribution for a quantum mechanical system is simply the square of the wavefunction.
A pulsar is a spinning neutron star with a strong magnetic field that emits electromagnetic radiation along its magnetic axis. Because the star's rotation axis is not aligned with its magnetic axis, we observe pulses of radiation as the star's magnetic axis passes through our line of sight. The time between pulses ranges from a few milliseconds to a few seconds, and tends to slow down over time.

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