Experimental basis for The luminiferous ether special relativity • Mechanical waves, water, sound, strings, etc. require a medium • Experiments related to the ether • The speed of propagation of mechanical hypothesis waves depends on the motion of the • Experiments on the speed of light from medium moving sources • It was logical to accept that there must be a • Experiments on time-dilation effects medium for the propagation of light, so that • Experiments to measure the kinetic energy em waves are oscillations in the ether of relativistic electrons • Newton, Huygens, Maxwell, Rayleigh all believed that the ether existed 1 2 The aberration of starlight Consequences of the ether (James Bradley 1727) • If there was a medium for light wave • Change in the apparent position of a star due to changes in the velocity of propagation, then the speed of light must be the earth in its orbit measured relative to that medium • Fresnel attempted to explain this • Thus the ether could provide an absolute from a theory of the velocity of light reference frame for all measurements in a moving medium • According to Fresnel, the ether was • The ether must have some strange properties dragged along with the earth and this – it must be solid-like to support high-frequency gave rise to the aberration effect transverse waves • However, Einstein gave the correct – yet it had to be of very low density so that it did not explanation in terms of relativistic velocity addition. A light ray will have disturb the motion of planets and other astronomical a different angle in different bodies too much relativistic frames of reference 3 4 Fizeau’s measurements of the speed of Michelson-Morley experiment (1887) light in a moving fluid (1851) • Attempt to detect the relative motion of matter • He measured c and got 315,000,000 m/s through the ether • http://galileo.phys.virginia.edu/cla • He used interference effects to attempt to sses/109N/lectures/michelson.ht measure the speed of light in moving water ml • Used interference of light due to path length differences (fringe shift when apparatus was rotated. Optical table was a 1½ ton granite slab floating in a pool • Found no measurable of mercury, to minimize the • He expected to measure c + v, but the effect effects of vibrations, and to magnitude of the result was << expected • NO ETHER allow it to be rotated easily. 5 6 1 Einstein’s postulates Faraday’s law of of special relativity electromagnetic induction I. The laws of physics (mechanics and electrodynamics) are valid in all Einstein was motivated by the fact that the induced inertial frames of reference. There is voltage in the coil did not depend on whether the magnet was moved toward the coil or if the coil was moved no absolute frame of reference. toward the magnet. II. Light is always propagated in empty space with a definite velocity c with respect to any frame of reference, regardless of the state of motion of the emitting body. 7 8 Test of the second postulate of the Experimental setup special theory of relativity in the GeV region (Alvager et al., Phys. Lett. 12, 360, 1984) Used the CERN Proton Synchrotron to accelerate protons to 19.2 GeV/c which target then slammed into a Be target producing 0 beam Bending magnets mesons at 6 GeV = 0.99975. to eliminate The 0’s decay into 2 photons. A time-of- charged particles flight method was used to measure the A A’ collimator d = 31.450 0.0015 m photon speed (A, A’) and (B, B’) are gamma detectors B 9 B’ 0 experiment Muon decay and time dilation • This amounts to measuring c produced on • Muons are produced by decays of ’s in a source (the 0 s) moving at 0.99975 c cosmic ray collisions with nuclei in the • Results c’ = c + kv upper atmosphere. • The half-life of muons at rest is 0 = 1.52 s •k = (3 13) x 105 • The muons move at 0.98c, so in one 0 , they would travel < 500 m, and would not be detected on earth. • Muons are detected on earth 11 12 2 Muon decay and time dilation, Measurements of the speed and kinetic continued energy of relativistic electrons • We observe muons on earth because of • Classically K = ½ m v2, where m = constant the relativistic time dilation effect. • There is no limit on v, so that if a force continually acts on an object, it will eventually • The proper lifetime of the muon is reach a speed in excess of c, in contradiction to Einstein’s second postulate. 0 7.6 s 2 • Two types of experiments: 1 vc – using relativistic electrons emitted by a radioactive source (Am. J. Phys. 77, 757, 2009) • With this lifetime, the muons would travel – Using a Van de Graff device to accelerate electrons roughly 2.25 km, so some would be to high speeds and measuring detected on earth. (Am. J. Phys. 32, 551, 1964). 13 14 Experiment – use radioactive Experiment using a Van De Graff source that emits electrons electron accelerator S1 S2 source collimator L S1 and S2 are very thin scintillation detectors that produce a light Pulse when electrons hit them. The light pulses measure the time interval For the electrons to travel the know distance L, this v is measured. The The kinetic energy of the electrons is measured by the heat produced when Kinetic energy of the electrons emitted by the radioactive nuclei is known. they slam into the aluminum disk at the end (calorimetry). The calorimeter is calibrated by heating it using a resistor embedded in the disk. A thermo- 15 couple is used to measure the increase in temperature. 16 Results: Classical physics fails! Results using a linear accelerator Radioactive sources: 133Ba (25 - 80 keV) and 207Bi (240 -1047 keV) 17 18 3 Results using the van de Graff 19 4.