One hundred and one years ago in 1915, Albert Einstein published his famous General Theory of Relativity (GR). One of the two fundamental postulates of GR is that the so-called "weak equivalence principle", namely, the "gravitational mass" and the "inertial mass" are always equal to each other. As a result, the trajectories of particles in a gravitational field are always the same regardless of their masses. It is said that Galileo once dropped two spheres with different masses from the Leaning Tower of Pisa which landed on ground simultaneously. This was probably the earliest test of this principle.  

   Based on GR, Einstein also predicted the existence of gravitational waves (GWs), weak ripples in the curvature of spacetime that propagate as waves. The breakthrough of detecting these faint waves was finally made recently by the advanced LIGO (Laser Interferometer Gravitational-Wave Observatory) team on Sep. 14, 2015, 100 years after the GR was published. This signal came from an object dubbed GW150914, produced when two astrophysical black holes (BHs), each with mass about 30 times of the mass of Sun, spiral in and merge. Since then the LIGO team reported two more GW events that are also BH-BH mergers.  

   These two amazing legacies of Einstein can be linked together. An international team led by Prof. Xuefeng Wu from Purple Mountain Observatory (PMO), Chinese Academy of Sciences (CAS), reported in a recent paper published in Physical Review D that the GW signals, when combined with possible electromagnetic (EM) signals associated with GW events, would give rise to an unprecedentedly stringent test on the Einstein’s weak equivalence principle. Seven other scientists from Beijing Normal University (Profs. He Gao & Zonghong Zhu), PMO (Dr. Junjie Wei), Penn State University (Prof. Peter Meszaros), University of Nevada, Las Vegas (Prof. Bing Zhang), Nanjing University (Prof. Zigao Dai), and Institute of High Energy Physics, CAS (Prof. Shuangnan Zhang), co-authored the paper. 

  "Using cosmological sources to test the weak equivalence principle is effectively building a huge Leaning Tower of Pisa across the cosmological scale," explained Wu, "As a result, the constraints on the violation of this principle is much more stringent than any other methods carried out so far."

   He Gao, an associate professor at Beijing Normal University, is the co-corresponding author of the paper. "Previous tests of the weak equivalence principle were carried out with photons of different energies," explained Gao, "The significance of having gravitational wave signals detected is that for the first time one can combine such kind of new signals and the traditional photons to perform the test."

In technical terms, by introducing the GW signals, one can set an upper limit on the \Delta\gamma, a quantity to measure violation of the principle, to 10^{-10}. This is better than any previous astrophysical tests of the principle.  

   No EM signal has been firmly detected to be associated with the LIGO GW events. However, one tentative weak gamma-ray signal was claimed to be associated with GW 150914 by the NASA’s Fermi/GBM team. "If this is the case, then the constraint made from the system gives the hitherto most stringent test on the weak equivalent principle." explained Prof. Bing Zhang, "LIGO and other gravitational wave detectors will detect mergers with at least one neutron star (NS) in the binary system in the future. These NS-BH and NS-NS merger systems are predicted to have firm associations with EM signals. This will allow a more definite test on the weak equivalent principle to be performed in the near future.” 

   Testing the weak equivalent principle was listed as one of “the four big cosmological secrets gravitational waves could uncover” in a recent article published in ``New Scientist” 

  (see:https://www.newscientist.com/article/2078129-four-big-cosmology-secrets-gravitational-waves-could-uncover/).

  "In the age of GPS and space travel, where even minute deviations from the assumed theory of gravity would have major consequences, it is of enormous importance," says Wu. "The universe provides a gigantic laboratory to test the correctness of the principle."