Spring 2004 - Fall 2005

In 1886, Michelson and Morley conducted an experiment that proved that there was no “luminous ether.” The idea that light waves could propagate without a propagation media contradicted everything that was known about wave physics at the time, but this crucial realization was the foundation of Einstein's theory of special relativity. In essence, if there is no ether, then there is no preferred rest frame in the universe, so all non-accelerating observers should observe the same physical phenomena.

But what if there were a preferred rest frame? Today, we know that there is an interesting rest frame, namely the Cosmic Microwave Background, but this does not disprove Einstein's claim. This would be like holding up a stapler to prove that the universe is not rotationally invariant: the stapler itself is not rotationally invariant, but the law of physics that govern the stapler are indeed rotationally invariant. Similarly, we have shown to a very high accuracy that not only is physics invariant under spacial rotations, but it is also invariant under “space-time” rotations, and we call the combination of these two symmetries Lorentz invariance.

So the more interesting question is: What if the vacuum state of the universe had a preferred rest frame? This is known as spontaneous Lorentz-violation, and I study the theoretical basis and experimental consequences of this unlikely but theoretically consistent possibility.

Fall 2004 - Fall 2005

The theory of Ghost Condensation is a consistent way to break Lorentz symmetry, Einstein's famed symmetry between space and time. (See my work on Ghost Condensation under A New Spin-Dependent Force.) However, Lorentz symmetry is part of the foundation of General Relativity, Einstein's wildly successful theory of gravity, and in general, Ghost Condensation is well constrained by precision tests of gravity. In the fall of 2004, we invented/discovered Gauged Ghost Condensation, a slight deformation of Ghost Condensation that allows Lorentz symmetry to be violated without making violent modifications to gravity.

With Hsin-Chia Cheng, Markus Luty, and Shinji Mukohyama, we study the implications of the theory of Gauged Ghost Condensation and show how it is connected to many other proposals for slightly modified gravity. Because this theory is a simple extension of the consistent framework of Ghost Condensation, Gauged Ghost Condensation is in our oponion the easiest way to understand the dynamics of these modified gravity theories. Because Gauged Ghost Condensation allows violations of space-time symmetry to be almost entirely independent from modifications of gravity, we have the freedom to ponder exotic new scenarios for cosmic dynamics.

• Spontaneous Lorentz Breaking at High Energies.
  Hsin-Chia Cheng, Markus A. Luty, Shinji Mukohyama and Jesse Thaler.
  JHEP 0605:076 (2006), hep-ph/0603010.

Spring 2005

Because neutrinos have such weak interactions, they are sensitive to possible Lorentz-violating effects over cosmological scales. We found that measurements of the neutrino flux from supernova SN1987A and the energy content of the universe from cosmic microwave background measurements severely restrict any Lorentz-violating couplings for neutrinos.

• Neutrino Constraints on Spontaneous Lorentz Violation.
  Yuval Grossman, Can Kilic, Jesse Thaler, and Devin G. E. Walker.
  Phys. Rev. D72:125001 (2005), hep-ph/0506216.

Spring 2004

In the fall of 2003, Nima Arkani-Hamed and colleagues presented a novel way of breaking Lorentz invariance. They call the theory Ghost Condensation, and it provides a way to study Lorentz violations within a completely consistent theoretical framework. Starting in the spring of 2004, I worked with Nima to study the bounds on Lorentz violations from existing experiments to see whether or not Ghost Condensation is an experimentally viable theory. We placed various bound on various parameters of the theory and proposed a new experiment to search for the novel physical phenomena that the theory predicts.

Claire Cramer performed a test of a possible Lorentz-violating spin-dependent force while she was a graduate student in the Eöt-Wash group.

• Universal Dynamics of Spontaneous Lorentz Violation and a New Spin-Dependent Inverse-Square Law Force.
  Nima Arkani-Hamed, Hsin-Chia Cheng, Markus A. Luty, and Jesse Thaler.
  JHEP 0507:029 (2005), hep-ph/0407034.