Robert McNees Brown University (Alum, National Science Bowl `91*)

1. Robert McNeesBrown University (Alum, National Science Bowl `91*) Department of Energy National Science Bowl 2007 * Yes, that was the first one. Some of you were born that year.

2. Outline The System of the World The Early 20th Century: Quantum Mechanics and Special Relativity General Relativity: Curvature and Motion The Standard Model of Particle Physics Faint Supernovae and Glowing Black Holes String Theory

3. The System of the World

4. Isaac Newton 1. 2. with 3.`For every action there is an equal and opposite reaction.’ And, of course, he inventedthe Calculus, did pioneeringwork in Optics, etc…

5. Newtonian Gravity

6. Electricity and Magnetism William Gilbert and the Scientific Method • Observed attraction and repulsion between poles of a magnet • Produced static electricity by rubbing amber • Concluded that Electricity and Magnetism were distinct. It wasn’t what he did. It was how he did it! “In the discovery of secret things, and in the investigation of hidden causes, stronger reasons are obtained from sure experiments and demonstrated arguments than from probable conjectures and the opinions of philosophical speculators.” - De Magnete (1600)

7. Electricity and Magnetism Hans Oersted Electric Current produces a magnetic effect! • Michael Faraday • Electromagnetic Induction: A changing magnetic field induces a current. • Suspected that Electricity and Magnetism were wave phenomenon, related to light, propagate at finite speed • Electrical experiments gave him huge sideburns.

8. Electricity and Magnetism James Clerk Maxwell made theconnection b/t electricity andmagnetism in a beautiful set ofequations UNIFICATION - Phenomena that appear to be unrelated turn out to be aspects of a single, underlying cause.

9. The Early 20th Century: Quantum Mechanics and Special Relativity

10. Blackbody Radiation • A blackbody radiates based on its • Temperature, not its composition. • Radiated energy peaks at a specific frequency. • Drops off at higher frequencies. Classical physics couldn’t come up with the right curve. The physics was saying that the black body should emit more and more at higher frequencies… The Ultraviolet Catastrophe!

11. Max Planck Perhaps energy must be radiated in indivisible units… This assumption leads to the correct result for a blackbody: Fits the data beautifully! It depends on a new constant, called `Planck’s Constant’h = 6.6 x 10-34 Joule-seconds

12. Current Einstein and the Photoelectric Effect Shining light on a metal canproduce a current! Energy ofelectrons depends on frequency of light, not intensity. Einstein suggested that the electrons all havethe same energy because they receive it in whole packets, or quanta, from the light. Quanta are real!

13. The Bohr Hydrogen Atom • An electron can only orbit the nucleus at certain fixed radii. • The orbits are stable. Other orbits are not allowed. • Electrons jumping from one orbit to another release quanta of light. • Each orbit only has room for a certain number of electrons. Bohr’s model correctly predicts the spectral lines for Hydrogen.

14. De Broglie, Schrodinger, and Heisenberg Suggested that electrons can also behave like waves. In fact, any particle can. This was verified in electron diffraction experiments. Erwin Schrodinger developed a quantum mechanical model of the electron that treatsit like a wave. Werner Heisenberg developed a quantummechanical model of the electron that treatsit like a particle.

15. The Uncertainty Principle Heisenberg noticed something important. You can treat the electron like a particle, but there is an inherentuncertainty that goes along with that. The more precisely you try to say where it is, the lessprecisely you can measure its momentum, and vice-versa. As he put it: “we cannot know, as a matter of principle,the present in all of its details.”

16. 1905: A Big Year for Einstein • 1905 was a busy year for • Einstein. • He established the reality of quanta. • He explained Brownian motion. • He laid down the founda-tions of Special Relativity.

17. Special Relativity Consider these three observations, all accepted by physicists at the end of the 19th century. • The laws of physics are the same for a stationary observer and an observer moving at constant speed. • Galileo’s rule for adding velocities is correct. • Light travels at a finite speed, which is a consequence of physical laws described by Maxwell’s equations. Any two of these are mutually consistent. But if you take all Three together, you get contradictions.

18. Galileo: Einstein: Space and time are no longer separate concepts: Lorentz Transformations Einstein said that Galileo’s rule for adding velocities must be wrong.Transformations between frames of reference have to preserve thespeed of light. Consider two observers moving at relative velocity v. The first one uses coordinates (x,t), and the second uses coordinates (x’,t’). Not consistent with Maxwell’s equations. Consistent with Maxwell’s equations.

19. Time Dilation The Lorentz transformations have some pretty weird consequences.For instance, if I see a clock moving with speed v, it looks like it isticking too slow! This has been verified in lots of experiments, with fantastic precision. • Measured in atomic clocks that are sent around the world on a plane. • Measured in the lab directly, as a relativistic Doppler shift.

20. Length Contraction An observer who sees an object moving with a velocity v perceives thatobject’s length as being contracted. It is a small but real effect v = 0.87c v = 0.99c v = 0.999c

21. Special Relativity is not intuitive, but it is true. It has been verified in numerous experiments.Phenomena like length contraction and timedilation are physical effects, as real as anythingelse we experience. But Einstein still felt like something was missing.

22. General Relativity “Matter tells space how to curve, and curved space tells matter how to move.”

23. Some Problems with Newtonian Gravity The orbit of Mercury precesses about 1.5 degrees each century. Influence of other planets account for all but 0.1 degree of this. This excess is not explained by Newtonian gravity.

24. Some Problems with Newtonian Gravity Why are inertial mass and gravitational mass the same thing? And what is gravity, anyway? What causes it? Newton says that it just happens, and it is instantaneous. Action at a distance?

25. General Relativity! Einstein: The structure of spacetimeis influenced by matter and energy • Matter and Energy curve spacetime. • The curvature of spacetime is what causes gravity. • Objects follow geodesics: the `straightest’ lines on a curved surface. Curved Spaces Curved Time?

26. There are a lot of cleverways of representingcurved spaces. The artistM.C. Escher used themin many of his drawings,like this one. This drawing representsa two-dimensional spacewith constant negativecurvature.

27. Consequences and Tests of General Relativity • Curvature of spacetime is larger closer to the sun. • Larger curvature means that GR is more important. • Corrections to Newton from GR are more important for Mercury than for the other planets. Mercury Sun Earth General Relativity accounts for the precession of Mercury’s orbit.

28. Consequences and Tests of General Relativity Changes in the curvature - and the effect of gravity – propagateat the speed of light. Not instantaneous.

29. Gravitational Bending of Light Path that light follows (a geodesic) bends due tothe sun’s gravity. A smallbut measurable effect. Gravitational LensingThis is an image of a distant quasar. Thegravitational effect of a galaxy betweenus and the quasar results in four images.

30. Redshift of Light Due to Gravity Light loses energy as it overcomes gravity, justlike a ball thrown in the air loses kinetic energy. • This effect was measured in 1959 by Pound and Rebka, in a three story tower in Jefferson Lab at Harvard. • This effect is essential in Cosmology. It helps us piece together what the universe looked like along the trajectory of a photon.

31. The Standard Modelof Particle Physics

32. SR + QM = QFT When you combine Quantum Mechanics with Special Relativity,the result is called `Quantum Field Theory’. It is the frameworkthat we use to describe the physics of elementary particles. What is a field? Fields exist everywhere. Sometimes these fields are constant.Excitations – bumps and wiggles in the fields – are what wethink of as particles.

33. Propagation A particles is an excitation of a field. The way it moves – or propagates – follows the rules of Special Relativity. t The excitation can propagate into this region: the `future’ is t > 0. This is where the excitationis right now: t = 0. y The excitation could havewound up where it is nowby starting off somewherein here: the `past’ is t < 0. x

34. Interactions Particles can absorb and emit other particles. There are rules thatgovern the ways this can happen. Forces between two particles are due to one particle emitting an intermediate particle, which is then absorbed by a second particle.

35. Virtual Particles We are interested in Quantum field theory. The Uncertaintyprinciple tells us that a particle and its anti-particle can popinto existence. They can’t stick around for long, but they havereal consequences: In a QFT we have to considerall the ways the particles mightinteract. There are usually aninfinite number of things to keeptrack of!

36. The Building Blocks QFT is a framework – a set of rules we can use to describe particles. There are a lot of possible QFTs. The StandardModel is a specific QFT that describes the real world. It con-tains many different kinds of fields. The Fermions that make up matter are arranged in threegenerations. Everything about particles in a column is thesame except for their mass. FirstGeneration SecondGeneration ThirdGeneration

37. The Fundamental Forces (Well, except gravity) In the standard model forces are due to the exchange of particles called vector bosons. Three forces of this type have been identified:Electromagnetism, the Weak Nuclear force, and the Strong Nuclear Force. The first two are really one force: the Electroweak force. • Electromagnetism: Mediated by the exchange of photons. • Weak Force: Responsible for some forms of nucleardecay. Mediated by three vector bosons: W+, W-, and Z. Only left-handed quarks and left-handed leptons experience this force! • Strong Force: Binds quarks together into baryons (like the proton and neutron) and mesons (like the pion). Mediated by massless vector bosons called gluons.

39. Tests and Predictions of the Standard Model The Standard Model makes numerous predictions. Here are a few of them: • Anomalous magnetic moment of the electron:predicted value:0.0011596521594(230)observed value: 0.0011596521884(43) • Predicts the existence of the Top quark.Discovered in 1995 at Fermilab. • Predicts the W and Z bosons. Discovered in 1983.

40. Some (Big) Open Questions • Why do particles have mass?Most particle physicists assume that a particle known as the Higgs Boson is responsible. Weanticipate that it will be found soon. • Why don’t we see any antimatter outside of the lab?Seems weird, right? We don’t know why natureshould prefer matter over anti-matter. • Why are there three families of particles?We don’t know for sure. Any ideas?

41. The Large Hadron Collider (LHC)

42. Faint Supernovae and Glowing Black Holes

43. The Expanding Universe In 1929 Edwin Hubble reported that the Universe was expanding.Everything seemed to be moving away from everything else. Themore distant galaxies seemed to be receding faster than thecloser ones.

44. Hubble Expansion C A C B A A long time ago, galaxiesB and C were far, far awayfrom galaxy A (that’s us). Now, their distances from A-as measured on the surfaceof the globe- have increased. B

45. The Big Bang What if we follow the expansion back in time? Things must have been very hot and dense. We can only go back so far. Eventually the physics breaks down! The Big Bang refers to the initial event or period from which theuniverse (as we currently understand it) emerged. It is an expansion, but not into anything. It is an expansion of space and time itself.

46. Some say the world will end in fire,Some say in ice.From what I've tasted of desireI hold with those who favor fire.But if it had to perish twice,I think I know enough of hateTo say that for destruction iceIs also greatAnd would suffice. Well what about the future? • Until very recently we assumed that one of two things would • happen to the expansion of the universe: • Gravity stops the expansion. The Universe collapses in a fiery Big Crunch. • Gravity slows down the expansion, but does not stop it. The Universe goes out with a cold and lonely Big Whimper. • The poet Robert Frost had already figured this out in 1923.

47. The Cosmological Constant XX Einstein had added an extra term to his equations, called the cosmo-logical constant. He needed this term to describe a universe that wasstatic. But since the universe is expanding, he could get rid of it!

48. And then, 70 years later…

49. Faint supernova? In the late `90s twogroups of astronomerswere observing distantsupernovae. The type Ia SN arethought to be goodstandard candles. Weknow how bright theyshould be, so we canfigure out how faraway they are. They found somethingtotally unexpected. Thesupernova were too dim.

50. The Accelerating Universe