After 60 years, here comes the Sun

Abstract

For sixty years, the Sun served neutrino physics more than the reverse: its neutrinos established oscillations rather than probing the star itself. That is now changing. Reactor antineutrinos have pinned down the oscillation parameters governing solar-neutrino flavour conversion so precisely that the propagation-related uncertainty is becoming negligible -- closing the first of two abysses. Solar neutrinos can, at last, be turned on the Sun. Yet a second abyss remains: today's solar-neutrino detection falls an order of magnitude short of the precision the Standard Solar Model already predicts for the dominant pp flux ($>\,$90$\%$) -- a gap no reactor experiment can close. The intrinsic $^{14}\mathrm{C}$ background walls off the pp region; only new detection techniques can breach it. Bridging this second abyss demands a revolution, not an increment. Combined with helioseismology's complementary reading of the Sun's mechanical structure, precision neutrino measurements of the nuclear source terms would enable a multi-messenger heliotomography of the solar interior -- reading our star from both sides at once. A new age of solar discoveries may follow, on one condition: that the field builds the instruments equal to the task.