# Relativistic Astronomy. II. In-Flight Solution of Motion and Test of Special Relativity Light Aberration

## TL;DR

This paper presents a method to determine the in-flight motion of a trans-relativistic probe using observed point sources and demonstrates how such measurements can test special relativity's light aberration effect and constrain photon mass.

## Contribution

It introduces a novel approach to solve the probe's motion in flight using multiple point sources and applies this to test light aberration and set photon mass limits.

## Key findings

- Probe motion can be accurately determined with minimal error.
- Measurement of point sources can test light aberration in special relativity.
- Photon mass upper limit can be constrained to below 1 eV.

## Abstract

The Breakthrough Starshot project aims to send centimeter-sized, gram-scale "StarChip" probes to Alpha Centauri at a speed of $\sim0.2c$. On the other hand, Zhang \& Li recently proposed that trans-relativistic cameras may be sent to any direction to study astronomical objects and test special relativity. To conduct such "relativistic astronomy", one needs to solve the motion of the probe in flight. We solve the motion of the probe (including the moving direction and the velocity) by comparing the positions of three or more point sources observed in the Earth rest frame and in the probe's comoving frame. When the positions of enough point sources are taken into account, the motion of the probe can be solved with an error which is even smaller than the diffraction limit of the transrelativistic camera. After solving the motion, when the measurement of the position of an additional point source is introduced, one can use the data to test the light aberration effect in special relativity. The upper limit of the photon mass can be placed from the deviation of aberration to slightly lower than the energy of the photon, e.g. $\sim 1 \rm {eV}$.