Testing the Weak Equivalence Principle Using Optical and Near-infrared Crab Pulses

The Weak Equivalence Principle states that the geodesics of a test particle in a gravitational field are independent of the particle's constitution. To constrain violations of the Weak Equivalence Principle, we use the one-meter telescope at Table Mountain Observatory near Los Angeles to monitor the relative arrival times of pulses from the Crab Pulsar in the optical (lambda approximate to 585 nm) and near-infrared (lambda approximate to 814 nm) using an instrument that detects single photons with nanosecond-timing resolution in those two bands. The infrared pulse arrives slightly before the visible pulse. Our three analysis methods give delays with statistical errors of.tobs Delta t(obs) = 7.41 +/-.0.58, 0.4. +/- 3.6, and 7.35 +/- 4.48 microseconds (at most 1/4000 of the pulsar period). We attribute this discrepancy to systematic error from the fact that the visible and infrared pulses have slightly different shapes. Whether this delay emerges from the pulsar, is caused by passing through wavelength-dependent media, or is caused by a violation of the equivalence principle, unless there is a fine-tuned cancellation among these, we set the first upper limit on the differential post-Newtonian parameter at these wavelengths of Delta gamma < 1.07 x 10-(10). (3 sigma). This result falls in an unexplored region of parameter space and complements existing limits on equivalence-principle violation from fast radio bursts, gamma-ray bursts, as well as previous limits from the Crab.