What Is The World Made Of? Ken Krane Academy for Lifelong Learning January 23, 2013 March 12, 2013.

What Is The World Made Of? Ken Krane Academy for Lifelong Learning January 23, 2013 March 12, 2013

Program 1.To the very small Atoms Nuclei, protons and neutrons Probing inside the nucleus the nuclear glue Families of particles: baryons, mesons, leptons Quarks and gluons The Standard Model The Higgs particle BREAK 2.To the very large The large-scale structure of the universe The expansion of the universe Big Bang vs. Steady State theories The Big Bang Theory The formation of the chemical elements Is the expansion accelerating?

Properties of Atoms 1.Fundamental building blocks of matter (but not indivisible) 2.They are small 100,000,000 laid end-to-end would make about one centimeter 3.They are electrically neutral they contain equal amounts of positive and negative charges 4.They are stable they dont spontaneously collapse 5.They emit and absorb various types of electromagnetic radiation light, x rays, etc.

The Periodic Table of the Elements

Constituents of Atoms 1897 J. J. Thomson (England) observed corpuscles of negative charge later called electrons that were emitted from atoms and assumed to be constituents of atoms but only 0.1% of the mass of the atoms. 1911 Ernest Rutherford (England) showed that the positive charge in an atom provided most (99.9%) of its mass and was concentrated in a very small region at the center of the atom called its nucleus. Thomson model Rutherford model

Bohrs Model (1913) 1913 Niels Bohr (Danish, but working in England with Rutherford) developed a planetary model of the atom in which the electrons circulated about the central nucleus like planets about the sun. Instead of gravity, the binding is provided by the electrical attraction of positive and negative charges for one another. Open questions (1913): 1.Why dont electrons fall into nucleus? 2.Why dont all electrons choose lowest orbit (least energy)? 3.What are constituents (if any) of nucleus?

Atomic Number Henry Moseley (British, 1913) from measuring x rays emitted by atoms deduced number of positive charges in nucleus, called the atomic number Z. If atoms are electrically neutral, an atom of atomic number Z with Z positive charges must also contain Z negative charges (electrons).

Rutherfords Transmutation Experiments (1917) N H Alpha particles striking nitrogen release particle with one unit of positive charge. Identical to nucleus of hydrogen Named by Rutherford proton

Constituents of Nucleus The elementary positive charge is identical to that of the simplest nucleus, hydrogen. The mass or weight of an atom is very nearly an integer multiple of that of the lightest atom, hydrogen. This integer is called A, the mass number. This was originally assumed (incorrectly) to be the number of protons in the nucleus. If a nucleus contained A protons, it would have too much positive charge because A is larger than Z. For example, helium has a mass of 4 but a charge of only 2. The protons in the nucleus repel one another. What keeps the nucleus together?

The Neutron In 1920, in order to explain the disparity between atomic mass and atomic number, Rutherford proposed that the nucleus also contained a neutral particle of about the same mass as the proton. Discovered in 1932 by Rutherfords former student, James Chadwick. Being neutral, neutrons experience no electrical force but provide the extra nuclear force to overcome the electrical repulsion of the protons. p p pp p p n n nn X X nn p

Characteristics of Atom Mass of A units (A = total number of protons + neutrons) At its center, a nucleus containing Z positively charged protons and N neutrons (Z + N = A). Nucleus accounts for 99.97% of the mass of the atom. Nucleus is very small if an atom were a large as a football field, the nucleus would be a pea on the 50 yard line. The nucleus is surrounded by a cloud of Z negatively charged electrons.

What Force Holds the Nucleus Together? Yukawa (1934) exchange force A particle (the force carrier) is exchanged between p and n Postulated exchanged particle: meson Observed in 1947

Nuclear Beta Decay (1930s) Joliot-Curie, Fermi Neutron proton Proton neutron Relatively long time scale (hours years) Caused by weak interaction

The Forces of Nature (1800) Magnetism Gilbert (1600) Gravitation Newton (1687) Electrical Coulomb (1785)

The First Great Synthesis Electrical forceMagnetic force

The First Great Synthesis Electromagnetic force Faraday (1800s) Maxwell (1876) Electric generators (rotating magnet produces electricity) Electric motors (electricity produces rotating magnet)

The Four Forces (1930s) Strong nuclear force Electromagnetism Weak nuclear force Gravitation 1 0.01 0.00000001 0.00000000000001 Strong and weak nuclear forces: short range Electromagnetism: neutralized by shielding Gravitation: cumulative 1/1,000,000,000 of the electrons in a penny would provide enough electrical force to launch another penny to the Moon.

The Particle Universe in 1950 Electron (e) Proton (p) 1836 x electron mass Neutron (n) 1839 x electron mass Meson () 274 x electron mass Mu () 207 x electron mass

Digression 1 The Neutrino: Natures Little Joke Beta decay of neutron: neutron proton neutron proton + electron (conservation of electric charge) Conservation of energy: energy of neutron = energy of proton + energy of electron All electrons should emerge from decay with same energy, equal to energy of neutron minus energy of proton Instead, all electrons were observed (1920s) to emerge with less than this energy What happens to the missing energy?

Digression 1 The Neutrino: Natures Little Joke Wolfgang Pauli (1930): suppose there is another particle emitted in the beta decay: neutron proton + electron + x x particle must be electrically neutral (because charge conservation is already taken care of) Electron and x particle share the available energy the missing energy is then simply the energy carried by the unobserved x particle The x particle is now known as the neutrino Neutrinos are elusive and extremely hard to detect not done until 1950s

Digression 2 Antiparticles: Natures Symmetry Every particle has a corresponding antiparticle: electron antielectron or positron (1932) proton antiproton (1955) neutron antineutron (1955) Same mass but opposite electric charge Annihilation: particle + antiparticle energy Pair production: energy particle + antiparticle

The Particle Universe in 1950 Electron (e) Proton (p) 1836 x electron mass Neutron (n) 1839 x electron mass Meson () 274 x electron mass Mu () 207 x electron mass Neutrino () mass zero Antiparticles Now begins era of nuclear accelerators (atom smashers)

The Particle Accelerator Era (1950s-1970s) Beam of high-energy protons directed against a target (usually also protons) Multitude of new, short-lived particles produced Berkeley, Brookhaven, Fermilab, CERN (Geneva), Stanford (electrons), etc.

Experiments in Particle Physics

3 Families of Particles Hundreds of proton-like particles (called baryons, meaning heavy particles) with increasing mass. All have short lifetimes and decay eventually to protons. Hundreds of meson-like particles with increasing mass. All have short lifetimes and decay eventually to electrons and antielectrons (positrons). One electron-like particle (in addition to ) called , unstable and decaying eventually to electrons. Each of the three (e, , ) has a distinct associated type of neutrino. These 6 particles are known as leptons (meaning light particles) and they appear to be point particles with no internal structure. What are the mesons and baryons made of? Are there really hundreds of elementary particles or do they have a simpler set of constituents?

Deep Inelastic Scattering (Stanford, 1969) electron proton High-speed electrons penetrate inside the proton where they encounter something massive that causes them to recoil backward, exactly analogous to Rutherfords discovery of the nucleus. The interior of the proton contains 3 of these massive objects, which are the internal constituents of protons (and neutrons). They are called quarks.

The Quark Model of Baryons and Mesons up quark: charge = +2/3 down quark: charge = -1/3 proton neutron charge = +2/3 + 2/3 1/3 = 1 charge = +2/3 1/3 1/3 = 0

The Quark Model of Baryons and Mesons All of the hundreds of baryons and mesons can be accounted for in terms of 6 elementary and indivisible particles called quarks. Baryons are composed of three quarks, mesons of a quark and an antiquark. Quarks are the fundamental particles of the strong interaction. The force between quarks is based on the exchange of particles called gluons.

The Standard Model Matter is composed of 3 pairs of quarks and 3 pairs of leptons (and their antiparticles). quarks: (u,d) (c,s) (t,b) leptons: (e,) (,) (,) Decay lifetimes of certain particles limit the leptons to 3 types: electron + antielectron mu + antimu tau + antitau Lifetime is determined by number of decay paths.

No Freedom for Quarks! Electrical or gravitational force decreases with increasing separation Strong force between quarks increases with separation (like rubber band) Large energy needed to free a quark appears as the creation of new particles, which form jets of tracks in detector

The Second Great Synthesis Electromagnetic forceWeak force

The Second Great Synthesis Electroweak force Weinberg, Glashow, Salam (1967; Nobel 1979) Force carriers: photon, weak bosons W and Z The unification is not quite perfect: the photon is massless but the W and Z are very massive about 100 times the mass of the proton. Why are the masses so different?

Another Puzzle The 3 charged leptons: electron mu 207 x electron mass tau 3500 x electron mass A similar seeming arbitrariness exists for the masses of the 6 quarks. Why these particular mass values???

Solution: The Higgs Force (Peter Higgs, 1964) A force field (the Higgs field) pervades the universe. Particles get their masses by interacting with the Higgs field. The interaction occurs through the exchange of a particle called the Higgs particle or Higgs boson. (Boson: different way of classifying particles includes mesons and field particles, excludes baryons, leptons, and quarks) Observation of the Higgs boson would not only confirm the theory, but would provide a way to study how particles acquire mass. Higgs particle is expected to be very short-lived and to decay rapidly into 2 other particles, which in turn might either decay or leave visible tracks in the detector. One possible final result: 2 photons or 2 electrons and 2 positrons. Previously unknown boson discovered in 2012 at LHC in CERN; seems consistent with Higgs but so far other explanations cannot be excluded.

Finding the Higgs (2012)

The Standard Model with Higgs

The Next Great Synthesis Electroweak forceStrong force

The Next Great Synthesis Grand Unified Theory (GUT) Many different candidate theories no consensus yet Predictions proton decay, etc. Might explain neutrino oscillations (electron mu or tau)

The Large-Scale Structure of the Universe (1900s) Universe believed to be eternal, static, and infinite Subject to Newtons law of gravity No structure known beyond our galaxy Solar system at center of galaxy Size of galaxy 10,000 light-years Beginning of era of large optical telescopes Einsteins General Theory of Relativity (1916) Requires cosmological constant

The Expanding Universe (1920s) Evidence from observations by Edwin Hubble Nebulae are galaxies with individual stars Galaxies are receding in all directions Speed increases with distance Universe is expanding (therefore not static) No need for Einsteins cosmological constant

Two Competing Theories Steady State Theory Universe is eternal, same from every vantage point and at all times New matter is created to fill voids resulting from expansion Big Bang Theory If galaxies are separating, they must previously have been closer together; therefore earlier universe was denser and hotter Run cosmic clock backward to a single point of creation with infinite density

The Cosmic Microwave Background Radiation (1965) Arno Penzias and Robert Wilson set up microwave receiver at Bell Labs to observe signals from Bells new Telstar communications satellite Annoying background hum in receiver coming from all directions, day and night Deduced to be remnant heat radiation from early hot universe, now cooled to 2.7 degrees above absolute zero Propels Big Bang Theory to forefront

WMAP Satellite (2001-2010) In solar orbit 1 million miles from Earth Temperature map of universe at age 380,000 y (variations by 0.0002 degree)

Dark Matter Light from stars in arms moving toward observer is Doppler shifted toward shorter wavelengths (blue), while light from stars moving away is shifted toward the red Permits measurement of variation of rotation speed with distance from center of galaxy

Dark Matter Galaxies are surrounded by a halo of dark matter not visible in telescopes, not made from ordinary stuff (protons and neutrons)

Dark Matter and Gravitational Lensing

What is the Geometry of the Universe? General Theory of Relativity (Einstein, 1916) Gravity = geometry: matter tells space how to curve, curved space tells matter how to move Curvature depends on relationship between actual density of matter and energy in space and a particular critical density As universe expands, closed universe becomes more closed, open universe becomes more open, but flat universe remains flat closed open flat

The Expanding Universe Closed Flat Open

Two Problems The Horizon Problem: Why do the farthest limits of the observable universe in different directions look the same? How can they achieve an apparent equilibrium? The Flatness Problem: The present universe is flat to within about 1%. Because the drift with time is away from flatness, the early universe must have been within 0.000000..00001% of flatness. Why? Solution: Inflation Hypotheses Just after the Big Bang, the universe underwent a brief period of extreme expansion.

A New Surprise (1998-2000) Observations of supernovas (exploding stars) in the most distant galaxies showed them to be moving away from us much faster than would be predicted by the Hubble expansion The expansion of the universe must be accelerating! What force could be driving this acceleration?

The Expanding Accelerating Universe

Results from Cosmological Experiments Age of universe: 13.77 billion years ( 0.5%) Universe is flat ( 0.4%) Ordinary matter is 5% of universe Dark matter is 23% (unknown form) Remaining 72% is dark energy, a mysterious force that is driving the accelerated expansion (very similar to Einsteins cosmological constant) The number of lepton/quark generations cannot be greater than 3

The Big Bang Cosmology

The Big Bang Chronology Big Bang occurs 13.77 billion years ago Hot dense universe consists of quarks and leptons (+ antiparticles) and radiation Inflation drives possibly non-flat universe to flatness and causally connected regions (in equilibrium) disconnect 1,000,000,001 particles + 1,000,000,000 antiparticles 1 particle + radiation 3 quarks protons and neutrons Protons and neutrons + electrons form hydrogen (75%) and helium (25%) atoms at about 400,000 y (3000 degrees) The universe continues to expand and cool, so that the radiation eventually reaches its present temperature of 2.7 degrees Atoms preferentially condense in low-temperature regions of the radiation field and eventually form stars and galaxies First generation stars explode and die, casting out the heavier elements forged from hydrogen and helium in their interiors From the debris are formed second generation stars and their planets, some of which provide hospitable locations from which physicists can ask fundamental questions about their origins

Open Questions Are there more generations of quarks and leptons? Why dont free quarks exist? How does the Higgs mechanism work? Are there more Higgs particles? What is the correct Grand Unified Theory (strong + weak + electromagnetic)? Is there a Theory of Everything (GUT + gravity)? What is the mass of the neutrinos? What is dark matter? What is dark energy? What causes the accelerating expansion of the universe? What caused inflation? What caused the Big Bang? Was it a quantum fluctuation? Is our universe unique?

What Is The World Made Of? Ken Krane Academy for Lifelong Learning January 23, 2013 March 12, 2013.

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