Title: Experimental basis for special relativity
1Experimental basis forspecial relativity
- Experiments related to the ether hypothesis
- Experiments on the speed of light from moving
sources - Experiments on time-dilation effects
- Experiments to measure the kinetic energy of
relativistic electrons
2The luminiferous ether
- Mechanical waves, water, sound, strings, etc.
require a medium - The speed of propagation of mechanical waves
depends on the motion of the medium - It was logical to accept that there must be a
medium for the propagation of light, so that em
waves are oscillations in the ether - Newton, Huygens, Maxwell, Rayleigh all believed
that the ether existed
3Consequences of the ether
- If there was a medium for light wave propagation,
then the speed of light must be measured relative
to that medium - Thus the ether could provide an absolute
reference frame for all measurements - The ether must have some strange properties
- it must be solid-like to support high-frequency
transverse waves - yet it had to be of very low density so that it
did not disturb the motion of planets and other
astronomical bodies too much
4The aberration of starlight(James Bradley 1727)
- Change in the apparent position of a star due to
changes in the velocity of the earth in its orbit - Fresnel attempted to explain this from a theory
of the velocity of light in a moving medium - According to Fresnel, the ether was dragged along
with the earth and this gave rise to the
aberration effect - However, Einstein gave the correct explanation in
terms of relativistic velocity addition. A light
ray will have a different angle in different
relativistic frames of reference
5Fizeaus measurements of the speed of light in a
moving fluid (1851)
- He measured c and got 315,000,000 m/s
- He used interference effects to attempt to
measure the speed of light in moving water - He expected to measure c v, but the magnitude
of the result was ltlt expected
6Michelson-Morley experiment (1887)
- Attempt to detect the relative motion of matter
through the ether - http//galileo.phys.virginia.edu/classes/109N/lect
ures/michelson.html - Used interference of light due to path length
differences (fringe shift when apparatus was
rotated. - Found no measurable effect
- ? NO ETHER
Optical table was a 1½ ton granite slab floating
in a pool of mercury, to minimize the effects of
vibrations, and to allow it to be rotated easily.
7Einsteins postulates of special relativity
- I. The laws of physics (mechanics and
electrodynamics) are valid in all inertial frames
of reference. There is no absolute frame of
reference. - 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.
8Faradays law of electromagnetic induction
Einstein was motivated by the fact that the
induced voltage in the coil did not depend on
whether the magnet was moved toward the coil or
if the coil was moved toward the magnet.
9Test of the second postulate of the 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 then slammed into a Be target
producing ?0 mesons at 6 GeV ? ?? 0.99975. ?
The ?0s decay into 2 photons. A time-of-flight
method was used to measure the photon speed
10Experimental setup
11?0 ? ? experiment
- This amounts to measuring c produced on a source
(the ?0 s) moving at 0.99975 c - Results? c c kv
- k (?3 ? 13) x 10?5
12Muon decay and time dilation
- Muons are produced by decays of ?s in cosmic ray
collisions with nuclei in the upper atmosphere. - The half-life of muons at rest is ?0 1.52 ?s
- The muons move at 0.98c, so in one ?0 , they
would travel lt 500 m, and would not be detected
on earth. - Muons are detected on earth
13Muon decay and time dilation, continued
- We observe muons on earth because of the
relativistic time dilation effect. - The proper lifetime of the muon is
- With this lifetime, the muons would travel
roughly 2.25 km, so some would be detected on
earth.
14Measurements of the speed and kinetic energy of
relativistic electrons
- Classically K ½ m v2, where m constant
- There is no limit on v, so that if a force
continually acts on an object, it will eventually
reach a speed in excess of c, in contradiction to
Einsteins second postulate. - Two types of experiments
- using relativistic electrons emitted by a
radioactive source (Am. J. Phys. 77, 757, 2009) - Using a Van de Graff device to accelerate
electrons to high speeds and measuring(Am. J.
Phys. 32, 551, 1964).
15Experiment use radioactive source that emits
electrons
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 Kinetic energy of the
electrons emitted by the radioactive nuclei is
known.
16Experiment using a Van De Graff electron
accelerator
The kinetic energy of the electrons is measured
by the heat produced when 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- couple
is used to measure the increase in temperature.
17Results Classical physics fails!
Radioactive sources 133Ba (25 - 80 keV) and
207Bi (240 -1047 keV)
18Results using a linear accelerator
19Results using the van de Graff