Despite intense efforts during the last 25 years, the physics of
unconventional superconductors, including the cuprates with a very high
transition temperature, is still a controversial subject. It is believed that
superconductivity in many of these strongly correlated metallic systems
originates in the physics of quantum phase transitions, but quite diverse
perspectives have emerged on the fundamentals of the electron-pairing physics,
ranging from Hertz style critical spin fluctuation glue to the holographic
superconductivity of string theory. Here we demonstrate that the gross energy
scaling differences that are behind these various pairing mechanisms are
directly encoded in the frequency and temperature dependence of the dynamical
pair susceptibility. This quantity can be measured directly via the second
order Josephson effect and it should be possible employing modern experimental
techniques to build a `pairing telescope' that gives a direct view on the
origin of quantum critical superconductivity.Comment: 19 pages, 9 figures; minor changes in the experimental part; added a
new appendix section calculating the pair susceptibility of marginal Fermi
liqui
