Modern Physics LECTURE II

Atomic Particles Atoms are made of protons, neutrons and electrons 99.999999999999% of the atom is empty space Electrons have locations described by probability functions Nuclei have protons and neutrons nucleus mp = 1836 me

Leptons An electron is the most common example of a lepton – particles which appear pointlike Neutrinos are also leptons There are 3 generations of leptons, each has a massive particle and an associated neutrino Each lepton also has an anti-lepton (for example the electron and positron) Heavier leptons decay into lighter leptons plus neutrinos (but lepton number must be conserved in these decays)

Types of Leptons Lepton Charge Mass (GeV/c2) Electron neutrino Electron -1 0.000511 Muon neutrino Muon 0.106 Tau neutrino Tau 175

Quarks Experiments have shown that protons and neutrons are made of smaller particles We call them “quarks”, a phrase coined by Murray Gellman after James Joyce’s “three quarks for Muster Mark” Every quark has an anti-quark Modern picture of atom

Types of Quarks Flavor Charge Mass (GeV/c2) Up 2/3 0.003 Down -1/3 0.006 Charm 1.3 Strange 0.1 Top 175 Bottom 4.3 Quarks come in three generations All normal matter is made of the lightest 2 quarks

Combining Quarks proton Particles made of quarks are called hadrons 3 quarks can combine to make a baryon (examples are protons and neutrons) A quark and an anti-quark can combine to make a meson (examples are pions and kaons) meson Fractional quark electromagnetic charges add to integers in all hadrons

Color charges Each quark has a color charge and each anti-quark has an anti-color charge Particles made of quarks are color neutral, either R+B+G or color + anti-color Quarks are continually changing their colors – they are not one color

Gluon exchange Quarks exchange gluons within a nucleon movie

Atomic Forces F = k q1 q2 r2 Electrons are bound to nucleus by Coulomb (electromagnetic) force Protons in nucleus are held together by residual strong nuclear force Neutrons can beta-decay into protons by weak nuclear force, emitting an electron and an anti-neutrino n = p + e + n

Protons and neutrons are made up of quarks bound together by gluons. Like charges repel, so why does the positive charge within a proton not cause the proton to explode? The (Coulomb) repulsion is defeated by a new force: The STRONG force.

Fundamental Forces Gravity and the electromagnetic forces both have infinite range but gravity is 1036 times weaker at a given distance The strong and weak forces are both short range forces (<10-14 m) The weak force is 108 times weaker than the strong force within a nucleus

Force Carriers

The Uncertainty Principle Classical physics Measurement uncertainty is due to limitations of the measurement apparatus There is no limit in principle to how accurate a measurement can be made Quantum Mechanics There is a fundamental limit to the accuracy of a measurement determined by the Heisenburg uncertainty principle If a measurement of position is made with precision Dx and a simultaneous measurement of linear momentum is made with precision Dp, then the product of the two uncertainties can never be less than h/2p

The Uncertainty Principle Virtual particles: created due to the UP

Force Carriers g Each force has a particle which carries the force and is unaffected by it Photons carry the electromagnetic force between charged particles Gluons carry the strong force between color charged quarks

Force Carriers Separating two quarks creates more quarks as energy from the color-force field increases until it is enough to form 2 new quarks Weak force is carried by W and Z particles; heavier quarks and leptons decay into lighter ones by changing flavor

Forces are mediated by particles Photons mediate electric and magnetic forces. (Faraday and Ampère demonstrated that electric and magnetic forces were different manifestations of the same “electromagnetic” force.)

Forces are mediated by particles Gluons mediate the strong force.

There is also the weak force It is responsible for the process by which two protons “fuse” together in the core of the sun. It is “carried” by the W and Z particles. Neutrons transform to protons via beta decay. It is a result of the weak force.

Gravity is the only other force. It so weak as to be negligible in particle physics experiments. Einstein’s “General Theory of Relativity” superseded Newton’s Theory of Gravity in 1915. An “ultimate” theory should explain how gravitons mediate gravity…….?

Unifying Forces Weak and electromagnetic forces have been unified into the “electroweak” force They have equal strength at 10-18 m Weak force is so much weaker at larger distances because the W and Z particles are massive and the photon is massless Attempts to unify the strong force with the electroweak force are called “Grand Unified Theories” There is no accepted GUT at present

Gravity Gravity may be carried by the graviton – it has not yet been detected Gravity is not relevant on the sub-atomic scale because it is so weak Scientists are trying to find a “Theory of Everything” which can connect General Relativity (the current theory of gravity) to the other 3 forces There is no accepted Theory of Everything (TOE) at present

Theory of Everything? Gravity Standard Model Electroweak Strong Weak Glashow, Salam, Weinberg Weak Electromagnetic Ampere, Faraday, Maxwell Electric Magnetic

The Standard Model The weak and electromagnetic forces were unified by Glashow, Weinberg & Salam. Electroweak force GWS also explained how to incorporate QCD, the model of the strong force. Their model defines the laws for all known interactions except gravity.

Force Summary

Spin Spin is a purely quantum mechanical property which can be measured and which must be conserved in particle interactions Particles with half-integer spin are “fermions” Particles with integer spin are “bosons” * Graviton has spin 2

Quantum numbers Electric charge (fractional for quarks, integer for everything else) Spin (half-integer or integer) Color charge (overall neutral in particles) Flavor (type of quark) Lepton family number (electron, muon or tau) Fermions obey the Pauli exclusion principle – no 2 fermions in the same atom can have identical quantum numbers Bosons do not obey the Pauli principle

Standard Model 6 quarks (and 6 anti-quarks) 6 leptons (and 6 anti-leptons) 4 forces Force carriers (g, W+, W-, Zo, 8 gluons, graviton)

Some questions Do free quarks exist? Did they ever? Why do we observe matter and almost no antimatter if we believe there is a symmetry between the two in the universe? Why can't the Standard Model predict a particle's mass? Are quarks and leptons actually fundamental, or made up of even more fundamental particles? Why are there exactly three generations of quarks and leptons? How does gravity fit into all of this? A. The Universe really is not infinite and there is not an infinite number of stars. The size of the Universe is small enough that the number of stars is not enough to light the night sky. B. The farther a star is away, the faster it is receding (Hubble's principle), therefore, the light from the farthest stars is red shifted (a doppler shift for light) below the visible region. Therefore there is not enought visible light to light the night sky. C. The lifetime of a star is about 1010 years. The years for the Universe to reach thermal equilibrium is about 1024 years. Many stars have been born, lived, and died already. Basically at any one time there are not enought stars active to fill the space of the Universe with enough radiation to light the night sky D. Space and the Universe is filled with many clouds of 'space dust'. These clouds absorb visible light. Thus enough light is not available to light the night sky.