X-RAY INTERFEROMETRY: Einstein’s theory passes x-ray test

Scientists at the National Institute of Science and Technology (NIST; Gaithersburg, MD) and the Massachusetts Institute of Technology (MIT; Cambridge, MA) combined atomic mass and x-ray wavelength measurements in silicon and sulfur nuclei before and after capture of a solitary neutron to confirm the relationship E = mc² to an accuracy of 0.4 part in 1 million. The measurement was about 55 times more accurate than the previous best direct test of Einstein’s formula (comparing electron and positron masses to the energy released in their annihilation). The mass before and after neutron capture, and the x-ray energy emitted, were measured for each isotope in two different steps.¹

The NIST team, led by the late physicist Richard Deslattes, determined the value for energy in E = mc² by measuring the wavelength of gamma rays emitted by silicon and sulfur isotopes. Deslattes developed methods for using optical and x-ray interferometry to precisely determine the spacing of atoms in a silicon crystal and for using such calibrated crystals to measure and establish accurate standards for the short wavelengths characteristic of x-rays and gamma rays.

“This was Dick’s original vision, that a comparison like this would someday be made,” said Scott Dewey, a member of the NIST research team. “The idea when he started working on silicon was to use it as a yardstick to measure the wavelengths of gamma rays, and to use it in a test of special relativity. It took 30 years to realize his idea.”

The GAMS4, was originally designed and built at NIST and is now located at Institut Laue Langevin (ILL) in Grenoble, France (see figure). It was used in the NIST portion of the test to measure the angle at which gamma rays are diffracted by crystals with known lattice spacings. The ILL facility for colliding nuclei and neutrons and capturing the resulting gamma rays played a key role in obtaining accurate gamma-ray measurements, which were particularly challenging at diffraction angles of less than 0.1°.

The MIT research team, led by David Pritchard, measured the mass numbers by direct comparison of the cyclotron frequencies of two isotopes simultaneously confined in a Penning trap. During measurements, the two isotopes, one with a neutron and one without, were placed on a common circular orbit, on opposite sides of the center of the trap and separated by a distance of about 1 mm. The two-ion technique achieved mass comparisons with fractional accuracies below 1011 by minimizing the effect of many noise sources such as magnetic-field fluctuations.

The MIT mass measurement was about four orders of magnitude more precise than the NIST/ILL wavelength measurement. “This was only possible after a 20-year program that improved the general precision attainable with mass measurements by three orders of magnitude,” Pritchard said.

REFERENCE

1. S. Rainville et al, Nature 438(7071) 1096 (Dec. 22/29 2005).