  Light-by-light scattering ($\gamma\gamma\rightarrow\gamma\gamma$) is a
quantum-mechanical process that is forbidden in the classical theory of
electrodynamics. This reaction is accessible at the Large Hadron Collider
thanks to the large electromagnetic field strengths generated by
ultra-relativistic colliding lead (Pb) ions. Using 480 $\mu$b$^{-1}$ of Pb+Pb
collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV
by the ATLAS detector, the ATLAS Collaboration reports evidence for the
$\gamma\gamma\rightarrow\gamma\gamma$ reaction. A total of 13 candidate events
are observed with an expected background of 2.6$\pm$0.7 events. After
background subtraction and analysis corrections, the fiducial cross section of
the process $\textrm{Pb+Pb}\,(\gamma\gamma)\rightarrow
\textrm{Pb}^{(\ast)}\textrm{+}\textrm{Pb}^{(\ast)}\,\gamma\gamma$, for photon
transverse energy $E_{\mathrm{T}}>$3 GeV, photon absolute pseudorapidity
$|\eta|<$2.4, diphoton invariant mass greater than 6 GeV, diphoton transverse
momentum lower than 2 GeV and diphoton acoplanarity below 0.01, is measured to
be 70 $\pm$ 24 (stat.) $\pm$ 17 (syst.) nb, which is in agreement with Standard
Model predictions.
