Transcript Lecture02
The Ground Rules of Physics (Science) Confirmed or Busted ? New Observation Prediction Reproducible ? New Theory How Does our Knowledge Grow ? • Unexpected observations can lead to a new theory • Mathematical elegance can be a powerful guide (general relativity) • Sometimes, a radically new concept is needed (quantum physics) • New concepts are not accepted easily (Galileo’s trials and tribulations with the church, Einstein’s concerns about quantum theory) • The ultimate test is the experiment (if reproducible) • A new theory does not invalidate a tested theory, it extends it to new territory steady progress (general relativity extends Newton’s gravity) Example 1 : General Relativity Star’s real position Star’s apparent position Starlight deflected by Sun’s gravity Einstein’s logic (Lect. 16): 1) Acceleration and gravity cannot be distinguished. 2) Acceleration deflects light. 3) Therefore, gravity should deflect light. Confirmed by experiment: Stars can be detected close to the Sun during a solar eclipse. Their light is indeed deflected by the Sun’s gravity. Moon Eddington’s Eclipse Expedition 1919 • British astronomer Eddington traveled to Principe Island in the Gulf of Guinea to observe deflection of starlight during a solar eclipse. • After months of drought, it was pouring rain on the day of the eclipse. The clouds parted just in time for the eclipse. • The photographs produced a deflection in agreement with Einstein’s prediction. Einstein instantly became a star. Gravitational lensing • The Sun acts like a lens deflecting the light from stars behind it. • Galaxies can also act as lenses by deflecting light from more distant galaxies. • This gives rise to the faint arcs and streaks in this image. Example 2 : Einstein’s Speed Limit Energy increase Nothing travels faster than light Otherwise the energy of this object would go to infinity. Length decrease And its length would shrink to zero. c = speed of light Do Neutrinos Travel Faster than the Speed of Light? Neutrinos are fundamental particles similar to the electron, but not charged. They interact very little with matter and easily pass through the Earth. On September 23, 2011, a press release from CERN, the European Organization for Nuclear Research, announced a seminar to be held that very afternoon. Members of the OPERA neutrino experiment would describe their observation of what appeared to be a new property of neutrinos. The webcast of a garden-variety CERN seminar attracts an average of a couple of hundred viewers. When the proceedings got under way on the afternoon of September 23, some 120,000 people tuned in. There aren't that many particle physicists in existence, even allowing for parallel universes. Clearly, plenty of regular people were also listening. More than 6000 neutrino news stories appeared on September 23rd and 24th alone, with thousands more in following days. Job applications to CERN climbed by 50 percent on the day of the press conference. Who wouldn't want to work where they make neutrinos that break the speed limit? Judith Jackson, Symmetry Magazine Speedy Neutrinos Einstein: “Do you realize how fast you were going?” Neutrino: “No. Was I speeding?” (Neutrinos kept the strict speed limits of Switzerland but were speeding in Italy.) The Story of the OPERA Experiment Another team of physicists whose apparatus lives right next door to the Opera group — under the Gran Sasso mountain in Italy — reported that they had clocked neutrinos over the same path exactly at the speed of light. The second group, which goes by the acronym Icarus, was led by Carlo Rubbia, a former director of CERN and Nobel-Prize winner. After some searching the OPERA scientists found the flaw. A bad connection of a fiber optic cable caused a 60 nanosecond delay of their clock, such that the neutrinos appeared to arrive 60 nanoseconds earlier. On June 8, 2012, at the 25th International Conference on Neutrino Physics and Astrophysics in Kyoto, CERN's research director Sergio Bertolucci presented data on behalf of four separate instruments at Gran Sasso that had tried to replicate the anomalous result. All of them had failed to do so, finding that the neutrinos respected the universal speed limit. Post-Mortem Analysis of the OPERA Experiment There were warning signs that should have kept OPERA from going to the press prematurely: 1) A previous measurement found that the speed of neutrinos was equal to the speed of light with much higher accuracy: Neutrinos and light from a supernova arrived at the same time. The OPERA team argued that neutrinos from CERN and from the supernova had different energy, therefore different speed. Extraordinary claims require extraordinary evidence. Carl Sagan 2) There was no credible theory explaining the result. Apart from the energy going to infinity, there was a problem with causality. Going faster than light might make it possible to go back in time. That leads to logical problems (and neutrino jokes): You go back in time and kill an ancestor, wiping yourself out in the process. But if you don’t exist, how can you kill someone ? No experiment should be believed until it has been confirmed by theory. Arthur Eddington The Good News: Physics corrects itself.