  Tidal disruption events (TDEs) are transient, multi-wavelength events in
which a star is ripped apart by a supermassive black hole. Observations show
that in a small fraction of TDEs, a short-lived, synchrotron emitting jet is
produced. We observed the newly discovered TDE AT2022cmc with a slew of radio
facilities over the first 100 days after its discovery. The light curve from
the AMI-LA radio interferometer shows day-timescale variability which we
attribute to a high brightness temperature emitting region as opposed to
scintillation. We measure a brightness temperature of 2x10^15 K, which is
unphysical for synchrotron radiation. We suggest that the measured high
brightness temperature is a result of relativistic beaming caused by a jet
being launched at velocities close to the speed of light along our line of
sight. We infer from day-timescale variability that the jet associated with
AT2022cmc has a relativistic Doppler factor of at least 16, which corresponds
to a bulk Lorentz factor of at least 8 if we are observing the jet directly on
axis. Such an inference is the first conclusive evidence that the radio
emission observed from some TDEs is from relativistic jets because it does not
rely on an outflow model. We also compare the first 100 days of radio evolution
of AT2022cmc with that of the previous bright relativistic TDE, Swift J1644,
and find a remarkable similarity in their evolution.
