Testing the Weak Equivalence Principle using Optical and Near-Infrared Crab Pulses

TL;DR

This study tests the Weak Equivalence Principle by measuring pulse arrival times from the Crab Pulsar in optical and near-infrared wavelengths, setting a new upper limit on potential violations with high precision.

Contribution

It provides the first upper limit on the differential post-Newtonian parameter at optical and near-infrared wavelengths, using pulsar timing to constrain equivalence principle violations.

Findings

Infrared pulses arrive slightly before visible pulses.

Statistical errors in delay measurements are below microsecond level.

The upper limit on the differential post-Newtonian parameter is Δγ < 1.07×10⁻¹⁰ (3σ).

Abstract

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 ($\lambda \approx 585$ nm) and near-infrared ($\lambda \approx 814$ nm) using an instrument which 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 $\Delta t_{obs} = 7.41 \pm 0.58$, $0.4 \pm 3.6$, and $7.35 \pm 4.48$ 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 $\Delta \gamma < 1.07 \times 10^{-10}~(3\sigma)$. 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.