Matthew Mewes
Visiting Assistant Professor of Physics
Swarthmore College
Ph.D. & M.S.  Indiana University
B.A. Concordia College

Department of Physics and Astronomy
Swarthmore College
Swarthmore, PA 19081

email:   mmewes1@swarthmore.edu
phone: (610) 328-8255  fax: -7895
Matt Mewes


Teaching

Physics 7 - MWF 10:30-11:20
Physics 3 lab - WTh 1:15-4:15
office hours: TF 1:00-4:00




Research

CPT logo
My research involves theoretical studies of fundamental spacetime symmetries.  In particular, most of my attention has been directed towards high-precision tests of Lorentz invariance, the symmetry principle behind special relativity.  Lorentz invariance is the notion that physics does not depend on the orientation or velocity of a system.  So, Lorentz symmetry is the symmetry under rotations (changes in orientation) and boosts (changes in velocity).  A breakdown of these symmetries, Lorentz violation, would point to new and interesting physics.

Below I describe some of the major results of my research.  Papers I have written on Lorentz violation can be downloaded here.  More background information can be found here.   Several animations demonstrating some of the strange effects that arise are posted below.

Photon Clocks:  Some of my work on photons has led to a number of different on-going experiments that test Lorentz invariance using electromagnetic resonate cavities.  These experiments represent the latest versions of the classic Michelson-Morley experiment.  The resonators may be thought of as extremely precise clocks.  A breakdown of special relativity implies that the ticking rate of these clocks could depend on the speed or orientation of the clock.  Searches for these dependences place constraints on possible violations of Lorentz symmetry.  Recently, student Alex Petroff and I explored new cavity geometries that have a net chirality (i.e. a twist in their geometry) to determine if they could provided sensitivity to Lorentz violations that also break parity symmetry (i.e. symmetry under inversions of space). I have also considered what effects Lorentz violation would have on electromagnetic resonances that naturally occur between the Earth's surface and ionosphere.
Cavity papers: Phys. Rev. D 80 015020 (2009), Phys. Rev. D 78, 096008 (2008), Phys. Rev. D 75, 056002 (2007), Phys. Rev. D 66, 056005 (2002)

Rotating Light:  Another way to search for a breakdown of relativity is to look for tiny defects in the way that light propagates.  One possible effect is known as vacuum birefringence.  Birefringence occurs when the speed of light depends on the polarization.  The key result of birefringence is that net polarization of light may change as it propagates through empty space.  This cannot occur if Lorentz symmetry is an exact symmetry of nature.  The best way to search for this effect is to look at light that has traversed extremely large distances.  Using observations of light from astrophysical sources such as radio galaxies, gamma-ray bursts, and the cosmic microwave background, I have extracted constraints on Lorentz violation in photons that are arguably the best existing bounds on deviations from special relativity in any system.
Birefringence papers: Phys. Rev. D 80 015020 (2009), Astrophys. J. Lett. 689, L1 (2008), Phys. Rev. Lett. 99, 011601 (2007), Phys. Rev. Lett. 97, 140401 (2006), Phys. Rev. D 66, 056005 (2002), Phys. Rev. Lett. 87, 251304 (2001)

Rotating Neutrinos:  Neutrinos can rotate in a fashion the rotating light described above, causing what is known as neutrino oscillations.  However, here it is not polarization that changes, but the type of neutrinos.  Neutrinos come in three flavors: electron, muon, and tau.  Oscillations cause a neutrino to change from one flavor to another as it propagates.  The likely explanation for these oscillations is the existence of neutrino mass.  However, a breakdown in relativity can also cause neutrino oscillations.  Moreover, there are some key differences between oscillations caused by mass and those from Lorentz violation.  For example, in the mass case, the amount of oscillation decreases for higher-energy neutrinos.  In contrast, Lorentz-violation-induced oscillations can be independent  of energy or increase with energy.  More unusual is the possibility that the oscillations might depend on the direction the neutrino is traveling.  This strange effect is illustrated in a couple of the animations below.
Neutrino papers: Phys. Rev. D 80, 076007 (2009) Phys. Rev. D 69, 016005 (2004), Phys. Rev. D 70, 031902 (2004), Phys. Rev. D 70, 076002 (2004)





Animations

Click on the image to view the animation, or right click to save to disk.  All animations are .mpg files.  Most are designed to be played in loop mode.

Poincare Sphere
Rotation in Polarization (part 1)

The Poincaré sphere provides an abstract but useful means of visualizing the effects of birefringence.  In general, light is elliptically polarized, meaning that the electric-field vector traces out an ellipse as it propagates.  Each point on the sphere represents a different polarization.  The blue arrow on the left is called the Stokes vector and points to the spot on the sphere that corresponds to the ellipse in the upper right corner.  Simple cases include linear polarization, represented by the equator of the sphere, and circular polarization, given by the poles.  Birefringence causes the polarization ellipse to change as light propagates.  On the Poincaré sphere, these changes give a simple rotation of the Stokes vector about some rotation axis.  The animation shows this effect for a rotation axis (green arrow) that lies in the equatorial plane.
Birefringence
Rotation in Polarization (part 2)

The rotation in the Stokes vector and corresponding changes in polarization are illustrated in this animation.  A very distant galaxy produces linearly polarized light.  The light experiences birefringence on its way towards Earth.  As a result, the polarization is different at points along its journey. When it reaches Earth, it has a different polarization than when it was created.  We can look for this change if we know enough about the source to determine the initial polarization.
Birefringence
Rotation in Polarization (part 3)

For many sources, there is no way of knowing what the original polarization was, so we can't look for a direct change.  However, the effect is frequency dependent for most forms of Lorentz violation.  So, assuming that there is little frequency dependence to begin with, any dependence in the observed polarization would indicate birefringence and Lorentz violation.  This animation illustrates the expected frequency dependence.  It begins at a low frequency and shows how the ellipse and polarization angle change as we move to higher frequencies.
Solar Experiment
Annual Variations

This animation shows neutrinos produced in the Sun as one flavor (yellow) oscillates into another flavor (violet).  In this example, oscillations are caused by a Lorentz-violating background (red arrows).  The result is large oscillations for neutrinos moving parallel to the field and no oscillations in neutrinos moving perpendicular to the field.  A solar-neutrino experiment on Earth then moves through regions of large and small oscillations, resulting in variations in the observed neutrino flux.  A signature of this effect is then an annual variation in the number of neutrinos of each flavor reaching Earth.




Tensor Spherical Harmonics

Below are some graphical representation of the E-parity and B-parity components of some spin-weighted spherical harmonics up to j=2. Spin-weighted spherical harmonics are a form of tensor spherical harmonics. Tensor objects (e.g. scalars, vectors, matrices) can be written as sums of spin-weighted spherical harmonics in the same way a scalar function can be written in terms of the usual spherical harmonics. For more discussion, see Phys. Rev. D 80 015020 (2009), where these harmonics are used to characterize all possible violations of Lorentz invariance in electromagnetism that are consistent with the usual U(1) gauge invariance.


spin weight = 0: These are just the usual spherical harmonics. The size of the circles on the sphere below represent the absolute value of the harmonics at different points on the sphere. Solid circles indicate positive values, while open circles represent negative values. Labels are (spin weight)(parity)(total angular momentum index j)(z-component index m). Click on the image to see a large version.


0E00
0E10
0E11
0E20
0E21
0E22
0E00 0E10 0E11 0E20 0E21 0E22




spin weight = 1: These are vector harmonics. The arrows on the spheres represent the magnitude and direction of the corresponding vector field for each component.


1E10
1E11
1E20 1E21
1E22
1E10 1E11 1E20 1E21 1E22
1B10 1B11 1B20 1B21 1B22
1B10 1B11 1B20 1B21 1B22




spin weight = 2: This case corresponds to symmetric traceless 2-tensors. Any tensor of this type can be represented by two orthogonal vectors associated with the primary axes of the tensor. The lines indicate the magnitude and direction of these axes, but are unoriented because this representation does not depend on the orientation of the two vectors.


2E20 2E21
2E22
2E20 2E21 2E22
2B20 2B21 2B22
2B20 2B21 2B22