Natural Science Forum / Physics / General Physics / June 2007

Two relativity tests are better than one
  http://physicsweb.org/articles/news/11/6/17/1

  28 June 2007

  Simultaneous tests of Einstein's special theory of relativity
  performed in Europe and Australia have allowed physicists to conclude
  that the speed of light is constant in all directions, without having
  to make an important assumption that had limited the validity of
  previous tests. The researchers performed two different types of
  Michelson-Morley experiment -- one on each continent -- which allowed
  them to rule out, for the first time, violations of Einstein's theory
  that could affect the physical properties of the experimental
  equipment and change the outcome of the measurement
  (arXiv:0706.2031v1).

  Einstein's 1905 special theory of relativity is based on the idea
  that the speed of light is constant in all directions regardless of
  the relative motion of the observer. A consequence of Lorentz
  invariance, this property was first demonstrated by Albert Michelson
  and Edward Morley in their famous experiment of 1887.

  Michelson and Morley split a beam of light in two and sent the beams
  off at right angles to two different mirrors. The beams were
  reflected back and recombined to form an interferometer. If the beams
  travelled at different speeds in the two directions -- as they would
  if they were passing through a stationary aether through which the
  Earth was moving -- then the two beams would be out of phase when
  they were recombined, leading to an interference pattern. No such
  pattern was revealed -- ruling out the existence of an aether -- and
  over the past 120 years the Michelson-Morley experiment has been
  refined and repeated to confirm that the speed of light is constant
  to one part in 10^16.

  However, there is one lingering doubt surrounding the
  Michelson-Morley experiment, according to Holger Mueller, a physicist
  at California's Stanford University. Any Michelson-Morley experiment
  is also sensitive to possible changes to the length travelled by the
  light, and to changes in the refractive index of the medium that the
  light travels through.

  Such physical changes could be caused by violations to Einstein's
  theory and could prevent the Michelson-Morley experiment from
  detecting changes in the speed of light. For example, if the both the
  speed of light and the length travelled by a light beam changed by
  the same factor, the changes would cancel each other out. Indeed,
  such violations are a consequence of some theories that attempt to
  reconcile gravity with quantum mechanics -- such as the standard
  model extension (SME).

  In the past, most physicists simply assumed that the physical
  properties of Michelson-Morley experiments do not change, and have
  interpreted the results as proof of the Lorentz invariance of light.
  Now, however, Mueller and colleagues in Australia, Germany and France
  have worked out a way to separate possible changes in the speed of
  light from the variations in the physical properties of the
  apparatus.

  The team performed two different Michelson-Morley experiments \u2013
  one in Berlin involving infrared light in optical cavities and the
  other in Perth, employing microwave radiation in a pair of resonating
  cavities. The research will be described in an upcoming issue of
  Physical Review Letters.

  The researchers used the SME to calculate possible changes to
  physical properties of both experiments as well as to the speed of
  light. While the SME predicts that the speed of light in both
  experiments should change by the same factor, the theory says that
  changes in the physical properties of the two experiments will change
  by different factors. By doing two experiments, Mueller obtained sets
  of equations that can be solved to separate the possible changes to
  the speed of light from physical changes.

  Mueller told Physics Web that the experiments were both run for over
  a year, which meant his team could measure Lorentz violations that
  become evident only by a modulation of the experiment's rotation
  relative to an inertial frame, such as the modulation provided by the
  Earth's orbit. Making the measurements in different geographical
  locations meant that the experiments were sensitive to different
  combinations of Lorentz violations, which would boost their ability
  to separate the changes.

  In all, the team was able to say that Lorentz invariance is not
  violated in 14 parameters associated with SME to an accuracy of
  around one part in 10^16. While the team were able to boost the
  accuracy of some parameters by a factor of 50 over previous
  experiments, Mueller believes the real significance of the team's
  work is the ability to simultaneously confirm the Lorentz invariance
  of light and matter without assuming the Lorentz invariance of the
  physical properties of the other.