1. What makes up the nucleus? Nucleus is positively charged Different atoms have same electrical properties but different masses Isotopes – same atomic number, different atomic weight.

2. How different are the masses? Masses of isotopes (not “natural” stuff) truly are multiples of basic hydrogen. Hydrogen is positively charged. Could there be neutral particles, same mass as hydrogen atoms? Chadwick discovers the neutron (1930)

3. What holds the nucleus together? All of the positively charged particles in the nucleus should repel each other. Something stronger than electricity is holding the nucleus together. The “Hand of God”??

4. Naming is important... The force is in the nucleus, so we’ll call it... The strong nuclear force! Better than the “Hand of God”?? Why “strong”?

5. An aside on Radioactivity Three types of Radioactivity: Alphas: Positively charged; Helium nuclei; can be stopped by paper Betas: Negatively charged; electrons; stopped by light metals Gammas: Uncharged; electromagnetic radiation; stopped by thick sheets of lead

6. What causes Radioactivity? Radioactivity does not affect the chemical properties of the atom, therefore... Radioactivity is associated with the Nucleus. “Weak” Nuclear Force

7. So what are the forces? Strong Nuclear Force Electromagnetism (1/137) Weak Nuclear Force (10^–5) Gravity (REALLY weak)

8. Describing Forces Action at a Distance Fields An aside on Albert Einstein: The Photoelectric effect shows that light behaves as a particle! So, is EM a wave, a field, a particle, or what?

9. Forces and “their particles”: mediators The photon, a particle of light, is the “particle” associated with the force of electromagnetism. How do mediators work? Does each force have its own mediator?

10. Back to particles... Trying to find the mediator of the strong force... 1937 – a new particle discovered in cosmic rays – the “mu meson”, or muon Couldn’t be mediator of strong Force “Who Ordered that??” I. Rabi

11. Other particles... Strong force mediator – pi meson, or “pion”; other mediators? Neutrino – electron and muon types (at least) Are we done yet? How do we know?

12. Splitting the “particles” Electrons, muons have no structure: these are basic particles (with the tau particle) Neutrons and protons do have structure: they must have “parts” Murray Gell–Mann “quarks”

13. “Three quarks for Muster Mark” Well, really only two at first: up and down Proton = 2 up + 1 down Neutron = 1 up + 2 down Kaons and Sigmas: “Strange” quarks

14. Quarks and Leptons and Bosons, Oh My! What do we have now? 6 quarks, 6 leptons; with antiparticles 4 mediators (one per force) Higgs field?

15. Why do we have all this stuff?? Only need two quarks, an electron, and the electron neutrino to make up all of matter. New theories How far have we come?