Robert Bluhm's primary research interests are in the areas of theoretical particle physics, gravity, and cosmology. His dissertation and postdoctoral work were in string theory. However, since the mid 1990s, much of Robert's research has been aimed at looking at ways to test Lorentz and CPT symmetry in particle, atomic, and gravitational systems. Lorentz symmetry is the symmetry behind Einstein's theory of special relativity. It states that the laws of physics are the same for all nonaccelerating observers. In general relativity, Lorentz symmetry becomes a local symmetry, which holds in local inertial frames. CPT is a combined discrete symmetry which essentially says that particles and antiparticles obey the same laws of physics. These symmetries are linked by a famous theorem -- the CPT theorem -- which states that Lorentz-invariant theories describing local particle interactions are also CPT invariant. It has been shown, however, that in certain quantum theories of gravity (including string theory and loop quantum gravity) Lorentz and CPT symmetry might be broken on the ultra-short length scale known as the Planck scale. For a long time, the conventional wisdom was that to probe physics at the Planck scale one would have to perform experiments at extremely high energy -- out of reach of any accelerator -- making these experiments completely unfeasible. However, Robert and his collaborators have shown that another way to search for new physics at the Planck scale is by examining extremely low-energy Lorentz and CPT tests but with very high precision. Robert's work has turned up a number of new candidate signals for testing Lorentz and CPT symmetry in experiments with electrons, muons, hydrogen and antihydrogen, experiments with a spin-polarized torsion pendulum, and clock-comparison experiments in space. A number of on-going and upcoming experiments are continuing to make measurements looking for these signals.
In addition, Robert and collaborators (including some undergraduates at Colby College) have been investigating the effects of spontaneous Lorentz violation in the context of field theory, gravity, and cosmology. When a symmetry is broken spontaneously, the symmetry still holds dynamically; however, the symmetry is hidden when one looks at the solutions to the theory. These investigations have revealed that in theories with spontaneous Lorentz violation, new degrees of freedom emerge that can modify gravity and which could have interesting cosmological effects. These effects and others are the subjects of Robert's on-going investigations.
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