Gino Segre
Gino Segre, a noted theoretical physicist from a distinguished family of physicists, is a professor in the Department of Physics and Astronomy at the University of Pennsylvania. He is also the author of A Matter of Degrees: What Temperature Reveals About the Past and Future of Our Species, Planet, and Universe.
The Big Bang, that primeval explosion more than 13 billion years ago, provides the accepted description of our universe's beginning. We can trace with exquisite precision what happened during the expansion and cooling that followed that cataclysm, but the presence of neutrinos in the earliest phase continues to elude direct experimental confirmation.
Neutrinos, once they were in thermal equilibrium, were supposedly freed from their bonds to other particles about two seconds after the Bang. Since then, they should have been roaming undisturbed through intergalactic space, some 200 of them in every cubic centimeter of our universe, altogether a billion of them for every single atom. Their presence is noted indirectly in the universes expansion; however, though they are presumably by far the most numerous type of material particle in existence, not a single one of those primordial neutrinos has ever been detected. It is not for want of trying, but the necessary experiments are almost unimaginably difficult. And yet those neutrinos must be there. If they are not, our picture of the early universe will have to be totally reconfigured.
Wolfgang Fault's original 1930 proposal of the neutrinos existence was so daring that he didn't publish it. Enrico Fermi's brilliant 1934 theory of how neutrinos are produced in nuclear events was rejected for publication by Nature as too speculative. In the 1950s, neutrinos were detected in nuclear reactors and soon afterward in particle accelerators. Starting in the 1960s, an experimental tour de force revealed their existence in the solar core. Finally, in 1987, a ten-second burst of neutrinos was observed radiating outward from a supernova that occurred almost 200,000 years ago. When they reached Earth and were observed, one prominent physicist quipped that extrasolar neutrino astronomy 'has gone in ten seconds from science fiction to science fact'. These are some of the milestones of twentieth-century neutrino physics.
In the twenty-first century, we eagerly await another milestone - the observation of neutrinos produced in the first seconds after the Big Bang. We have been able to infer their presence, but will we be able to actually detect these minute and elusive particles? They must be everywhere around us, even though we still cannot prove it.