Constraining violations of the Weak Equivalence Principle Using CHIME
  FRBs
  Einstein's General Relativity (GR) is the basis of modern astronomy and
astrophysics. Testing the validity of basic assumptions of GR is important. In
this work, we test a possible violation of the Weak Equivalence Principle
(WEP), i.e., there might be a time-lag between photons of different frequencies
caused by the effect of gravitational fields if the speeds of photons are
slightly different at different frequencies. We use Fast Radio Bursts (FRBs) ,
which are astronomical transients with millisecond timescales detected in the
radio frequency range. Being at cosmological distances, accumulated time delay
of FRBs can be caused by the plasma in between an FRB source and an observer,
and by gravitational fields in the path of the signal. We segregate the delay
due to dispersion and gravitational field using the post-Newtonian formalism
(PPN) parameter $\Delta \gamma$, which defines the space-curvature due to
gravity by a unit test mass. We did not detect any time-delay from FRBs but
obtained tight constraints on the upper limit of $\Delta \gamma$. For
FRB20181117C with $z = 1.83 \pm 0.28$ and $\nu_{obs}$ = $676.5\,{\rm MHz}$, the
best possible constraint is obtained at log($\Delta \gamma$) = $-21.58
^{+0.10}_{-0.12}$ and log($\Delta \gamma$/$r_{\rm E}$) = $-21.75
^{+0.10}_{-0.14}$, respectively, where $r_{\rm E}$ is the energy ratio of two
photons of the same FRB signal. This constraint is about one order of magnitude
better than the previous constraint obtained with FRBs, and five orders tighter
than any constraint obtained using other cosmological sources.
