Light speed memory as a local observable for soft hairs

Creative Commons License

Hajian K.

EPL, vol.136, no.2, 2022 (SCI-Expanded) identifier identifier

  • Publication Type: Article / Article
  • Volume: 136 Issue: 2
  • Publication Date: 2022
  • Doi Number: 10.1209/0295-5075/ac29f0
  • Journal Name: EPL
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus, Aerospace Database, Chemical Abstracts Core, Communication Abstracts, INSPEC, Metadex, zbMATH, Civil Engineering Abstracts
  • Middle East Technical University Affiliated: Yes


Copyright © 2022 EPLA.Einstein observers in flat space-Time are inertial observers which use light to synchronize their clocks. For such observers, speed of light is a constant by construction. However, one can use super-Translations to change coordinates from Einstein to BMS coordinates. From the point of view of BMS observers, speed of light is not a constant all over the space-Time and in all directions. So in general, clocks which are synchronized for Einstein observers are not synchronized for BMS observers, and vice versa. Based on this fact, we propose a local observable for detecting the soft hairs, which is the variations in speed of light for such observers. We also investigate the relation of this observable to gravitational memory, which is a permanent change of position of test particles at infinity, after a gravitational wave passes completely from them. It is shown that the BMS time coordinate is the physical time after a gravitational wave, and it is the legitimate time to be used to calculate the light speed. Based on this argument, the analysis predicts a permanent change in the speed of light rays which propagate in asymptotics after a gravitational wave. Moreover, it is explained how this change is related to the gravitational memory by comparing their significance in observations. ©