change of speed with respect to time

alpha particle

A kind of radioactivity product which is actually the nucleus of the Helium atom (2 protons and 2 neutrons) which is so tightly bound that is a standard decay product of many unstable nuclei.


To every particle in Nature, there corresponds an antiparticle with opposite electrical charge. Some particles, like the photon, are their own antiparticles. Some other neutral particles like the neutron, have a distinct antiparticle, the antineutron in this case.

atomic number

Most easily thought of as the position of an element in the periodic table, but more specifically it is both the number of electrons in an element or the number of protons in an element.

atomic mass

The mass of an atom expressed in terms in which Carbon’s atomic mass is 12. When an element has the same number of neutrons and protons, then the atomic mass is essentially the same as the number of nucleons.


Any Fermionic hadron which is made up of 3 quarks.

beta decay

A kind of radioactivity product which is actually an electron, the “beta particle.” This is old-time language.

Big Bang

The theory in cosmology in which the Universe, space, and time, have an origin.

black hole

According to the General Theory of Relativity, spacetime is bent around a mass. There is an extreme density in which the bending is so severe that light cannot escape. This happens when all of the mass is inside of a threshold radius, called the Schwarzschild Radius.


Quantum mechanical particles with integer quantum spin. Boson states prefer to be in the same quantum state.

Conservation law

Various quantities in physics appear to be unchanged through events…Something happens to a system and then later various quantities are unchanged among the participants. The two important mechanical conservation laws are energy conservation and momentum conservation.


the study of the birth, evolution, and future of the universe.


Charles-Augustin de Coulomb first investigated the force of attraction and repulsion between electrically charged objects. He published his results in 1785. From him now name the force between two charges ![Q_1][84] and ![Q_2][85] “Coulomb’s Law” which is:     ![[F = k \frac{Q_1 Q_2}{R^2_{1,2}}]][86] where

![k = 8.99 \times 10^8 \mbox{ Nm}^2\mbox{C}^{-2}][87] in free space. Notice that the sign of the charges matter so that if they are the same ![++][88] or ![–][89] the force on say, ![1][90] due to ![2][91] will point away from ![1][90] (and visa versa) and if they are different, the force will point the other way, toward ![1][90] or visa versa.


Electrical current is the transport of electric charge during some time. This is usually in a wire, but it needn’t be. It’s defined as if the charge carriers in a wire are positively charged (but we now know that the carriers are actually electrons and so the actual motion of charges is the opposite from how current is defined. So we live with the convention that current flows from the positive to the negative terminal in a battery. Current is defined as:     ![[I = \frac{\Delta Q}{\Delta t}]][92] The units are Ampere’s (aka “Amps,” or A). 1 A is equal to 1 Coulomb per second.

dark matter

This is the hypothesized substance assumed to account for the non-Newtonian motions observed in almost all galaxies and among large galactic clusters. It is also presumed to be a quantum object of unknown type. Whatever the Dark Matter particle is it has no electric charge (because it does not radiate in any known wavelength). It just gravitates, so it has a mass.

deBroglie wavelength

deBroglie showed that “matter” particles have wave properties in the same way in which light waves have particle properties. The deBroglie wavelength relates the momentum and wavelength: ![\lambda = h/p][93].

Dirac Equation

Dirac found an equation that correctly combined quantum mechanics with special relativity called the Dirac Equation. The solutions to it were four Quantum Fields (rather than the one of Schroedinger’s Equation) which naturally incorporate both quantum spin and antimatter.

Eightfold Way

A classification of hadrons introduced by Murray Gell-Mann in 1961. In this scheme, baryons (and eventually mesons) were found to cluster in groups of 1, 8, or 10 by mass and arrange themselves in patterns according to certain of their quantum numbers. Eventually this became associated with the way in which quarks are bound together to make up those baryons and mesons.

Electric field

The “disturbance” in space caused by any electrically charged object. The Electric Field is a vector, E, or ![\overrightarrow{E}][94] and the field lines go away from a positive charge and towards a negative charge.  ![\overrightarrow{E}][94] will cause a force on any charged object along the direction of the electric field direction (opposite, if the charge is negative).

electric potential energy

The potential energy due to an electric field. Since a charge is accelerated by   ![\overrightarrow{E}][94], then it will accelerate, and if it accelerates it will increase its velocity, and that means that its kinetic energy will increase, which means that it can do work. Whew. For a uniform charge, the Electric Potential Energy is ![U=QV][95] where ![V][96] is the Electric Potential, or voltage difference. Electric potential is the voltage difference between two points in a field and is expressed in units of Volts.

electromagnetic radiation

When a charged particle is accelerated, it radiates electromagnetic energy. This is in the form of coupled ![\overrightarrow{E}][94] and ![\overrightarrow{B}][97] fields which travel as waves when the acceleration is oscillatory, such as in radio transmission.

electron volt, or eV

1 electron volt, or 1eV is the energy gained by a charged particle of the fundamental electric charge of ![e = 1.6 \times 10^{-19} \mbox{ C}][98] accelerating through a potential difference of 1 Volt. One can convert any energy expressed in Joules to eV by multiplying by the conversion factor:     ![[\frac{1 \mbox{ eV}}{1.6 \times 10^{-19} \mbox{J}}]][99]


An attribute of all physical objects and entities which can do Work (in the physicist’s sense!). For Work, look at that glossary entry.

energy level

Every quantum system has levels of energy that it can possess which are called excited states. This includes atoms, but also nuclei, molecules, and quark-combinations. These levels are distinct and can be counted, and so they are labeled with integers.


It was presumed that light and all electromagnetic waves required a substance which “waved” much like air oscillates back and forth as sound. This substance was dubbed “ether” in sympathy with a slightly similar notion of the Greeks called the “aether,” presumed by Aristotle to be the substance outside of the orbit of the Moon. The ether was subsequently found to be a concept that could not be measured and hence could therefore not exist.

Faraday’s Law

That a changing Magnetic Field creates an Electric Field. This was shown by the demonstration of moving the magnet towards and away from the coil of wire.


A quantum mechanical particle with a half-integer, quantum spin. Fermions cannot be in the same quantum mechanical state.

** Feynman Diagrams **

These are pictorial representations of elementary particle interactions and decays. In their “professional” use, they also serve as a pneumonic for the assembly of a complicated calculation which leads to the evaluation of the probability that this reaction might occur.

** gamma, ![\gamma][100]**

The relativistic gamma function is the quantity ![\gamma = \frac{1}{\sqrt{1-u^2/c^2}}][101] where ![u][102] is the speed of a rest frame moving co-linearly and at at constant velocity relative to the observer’s frame. I’ve been calling the frame in which an observer makes measurement the “home frame” and any inertial frame moving relatively to it the “away frame.”

gamma decay

A kind of radioactivity in which a gamma particle (a high energy photon) is emitted when a nucleus goes from an excited state to a lower energy state, much in principle like in atomic transitions.

gamma particle

A very high energy photon.


Massless, spin 1 Bosons that are exchanged by quarks and propagate the strong force.

gravitational lens

Unseen, but very massive objects can bend light from more distant objects into multiple images of that distant object. These gravitational lenses were predicted by Einstein.


Elementary particles which are made of quarks and participate in the Strong Interaction. Distinct from Leptons, which are themselves elementary and do not participate in the Strong Interaction.

half life

Any unstable quantum object has a lifetime that is characterized by a single number. That is the shape of the number of undecayed objects is the same for all, but the function that describes that shape is stretched out (long-lifetimes) or shrunken (short lifetimes) by the half-life. Quantitatively, when a set of unstable particles have lived for one half-life, on average half of them will have decayed. After another half-life in time, half of those will remain (1/4 of the original). And so on. Half lives can range from fractions of a second to millions of years.

ionization detector

When a charged particle passes through matter, it ionizes the atoms of the material. That is the electric field of the intruding charged particle can be so strong that it affects the electrons of the atoms and forces them away from their home-atom. If this ionization happens in a material immersed in an electric field, the liberated electrons and remaining positive ions are attracted to the opposite poles of the container or configuration that created the field. Typically one of these acts as a wire and the arrival of the liberated charge becomes a little current pulse which can be detected on that wire. Such currents can be related to the location of the wire (and sometimes the time of the signal’s arrival) to pinpoint where the charged particle went through the detector. With many ionization detectors close to one another, the reconstructed current pulse locations reveal the trajectory of the charged particle.


An atom in which electrons have been removed (or added) making them electrically positive (or negative). The properties are still described by the number of protons, which is unchanged.


Particles appear to arrange themselves in sets of particles of different electric charge, but nearly the same mass. They are individually labeled with a quantum number of -1, 0, 1, or -1/2 and %2B1/2 or other multiples of these. Hadrons have Isospin. Leptons have a similar arrangement and are labeled with “weak isospin” according to their roles in the Standard Model. This is not the same as generic “isospin.”


The chemical properties of an element is governed by the atomic number. But the element is also characterized by the number of protons. The number of neutrons for a given element need not be the same as the number of protons and there exist different isotopes which are the same chemical element but with different atomic masses because there are a different number of neutrons. Typically isotopes are unstable.

kinetic energy

Kinetic energy is the manifestation of energy due to motion. In non-relativistic objects it is ![K=1/2 mv^2][103].


Elementary particles with spin 1/2 which do not participate in the Strong Interaction. As distinct to hadrons which are composite, made of quarks, and which do participate in the Strong Interaction. Examples of leptons are electrons, muons, neutrinos.

magnetic field

The “disturbance” in space caused by currents. The symbol is B and it’s a vector,   ![\overrightarrow{B}][97] It can exert a force on any charged object which is perpendicular to both the direction of the field and the velocity of the charge and the signed charge value. The direction is determined by using the Right Hand Rule, where your fingers of your right hand sweep through, first the velocity vector, then through the field vector and then your thumb will point in the direction of the force that a positive charged object will feel. If the moving charge is negative, then reverse the direction of that force.


Any Bosonic hadron made up of a quark and an antiquark.


The product of mass and velocity. A vector. Absolutely conserved.

muon****, ![\mu^{\pm}][104]

The next to lightest, charged lepton, cousin of the electron. It is much heavier: ![m_\mu \sim 200 \times m_e][105].

neutrino****, ![\nu][106]

A neutral lepton which is paired with the charge lepton. Hence, there are electron-neutrinos ![\nu_e][107], muon-neutrinos ![\nu_\mu][108], and tau-neutrinos ![\nu_\tau][109]. They appear to actually mix among themselves, which indicates to physicists that they must have a mass. Originally they were presumed to be massless.

neutron****, ![n][110]

The electrically neutral baryon which has a mass just a bit above that of a proton. The neutron is unstable and is the cause of beta decay in a nucleus.

nuclear force

The force that holds the nucleus together. Originally thought to be the exchange of pions, as suggested by Yukawa. Pions are now known to not be elementary themselves, but quarks which are held together by gluons.


The generic name for neutrons and protons, reflecting the fact that their Strong Interactions are identical.

particle physics

the study of the most elementary (fundamental) bits of matter.

Pauli Exclusion Principle

No two Fermions can occupy the same state. This means that in atomic shells, that the electrons can fill shells only insofar as to keep spins opposite. The same thing holds true for nucleons in a nucleus and even neutrons in a neutron star.

photon****, ![\gamma][100]

The spin-1 Boson, propagator of the electromagnetic force is the quantum of the electric and magnetic field. In my terminology, the “messenger particle” of electromagnetism.

pion****, ![\pi^{\pm,0}][111]

The lightest meson, a Boson of spin zero made up of up and anti-down quarks, or visa versa. Pions then have charges of %2B1, -1, or 0 ![e][112].


The original name for the anti-electron.

potential energy

Potential energy is the energy that seems to be “stored” in an object because of its position. If you hold something above a table, it has acquired potential energy because if you drop it, it could do work. If you compress or expand a spring, the same thing can happen. Work could be done by a mass attached to the end of a spring or the spring itself could do work. Finally there is potential energy between two massive objects like planets. This one is defined in a funny way, but its definition also works in atomic physics and chemistry. That is when one object is bound to another (like a planet in the solar system or an electron in a hydrogen atom) its potential energy is negative. This negative energy definition is a convention that is useful. Since both the gravitational and electric force rules have no limit in their spatial extent – they become zero only when ![R=\infty][113].

proton****, ![p][114]

The electrically charged (![+e][115]) baryon which has a mass just a bit below that of a neutron.

Quantum Electrodynamics

The mathematical theory of electrons and photons as first figured out by Feynman. It was the origin of the use of Feynman Diagrams and the difficult treatment of combining them to create reactions within Quantum Field Theory.

Quantum Number

Quantum mechanics is full of integers which describe certain quantum states. In atoms, these quantum numbers come from the solutions to the Schroedinger equation, in which the principle quantum number, n, is identical to the original idea of Bohr’s that electron orbits are fixed in radius and energy. There are quantum numbers for angular momentum, spin, and other characteristics. Transitions among states follow “selection rules” that relate the before and after values of the quantum numbers of the atomic states. In elementary particle physics Quantum Numbers appear to be inherent to particular particles and are “additive” meaning that in many cases they are conserved…that the sum before an interaction or decay must equal the same ones after the interaction or decay. This is especially true in the Strong Interactions, but also in the Weak Interactions. Examples of inherent quantum numbers are: Baryon Number, Lepton Number, Electric Charge, Isospin, and Strangeness.


These spin-1/2 Fermions are thought to be the most basic building blocks of all of hadronic matter. They are thought to be on par with the leptons as elementary.  We know of 6: up, down, charm, strange, top, and bottom which differ only in their masses.

Relativistic Quantum Field Theory

The mathematical and physical framework that is used to calculate and understand the interactions among elementary particles. It relies on the notion that particles are created and annihilated from the vacuum.

rest frame

Any environment in which everything is stationary with respect to everything else is a rest frame. Rest frames can move relative to one another and observers in each can see and make measurements of events in both.

Schroedinger Equation

The equation of the Wavefunction, or Quantum Field describing the way in which non-relativistic particles behave bound in atoms and freely.


change of distance with respect to time


A quantum number that describes the way in which hadrons interact. This quantum number is conserved in Strong Interactions, but violated in Weak Interactions.


A synchrotron is a particle accelerator in which the beam rotates in a closed beam pipe at a fixed radius. The acceleration is done by passing the beam repeatedly through the same Radio Frequency cavity region where an electric field is present. The beam is bent by magnets which must repeatedly increase their field strength in order to contend with the increasing speeds of the beams.

Uncertainty Relations

Due to Heisenberg, the Uncertainty Relations require that no object possesses absolutely precise values of momentum and position, or energy and time.


a quantity that carries both magnitude and direction. For velocity, speed is the magnitude. For displacement, distance is the magnitude.

W and Z Bosons,**** ![W^{\pm}, Z^0][116]

Spin 1 Bosons, propagators of the Weak Interaction (![W][117]) and a combination of Weak and Electromagnetic Interaction ![Z][118]. They were predicted by Weinberg in his formulation of the Standard Model that unified the Weak and Electromagnetic Interactions.


Work is a very specific term in physics which is related to a force which pushes on something over some distance. It’s defined as ![W=Fx][119], where the force is along the distance through which the object moves.