 The second reason for the absence of consensus is that there are no experiments in quantum gravity, and little in the way of observations that might qualify as direct or indirect data or empirical evidence. This stems in part from the lack of theoretical predictions, since it is difficult to design an observational test of a theory if one does not know where to look or what to look at. But it also stems from the fact that most theories of quantum gravity appear to predict departures from classical relativity only at energy scales on the order of 1019 GeV. (By way of comparison, the proton-proton collisions at Fermilab have an energy on the order of 103 GeV.) Whereas research in particle physics proceeds in large part by examining the data collected in large particle accelerators, which are able to smash particles together at sufficiently high energies to probe the properties of atomic nuclei in the fallout, gravity is so weak that there is no physically realistic way to do a comparable experiment that would reveal properties at the energy scales at which quantum gravitational effects are expected to be important—it would take a particle accelerator of galactic size to even approach the required energies. (In a little more detail, the weakness of gravity can be compared to the strength of the electromagnetic interaction — cf. Callender and Huggett (eds.) 2001, p. 4. An electron couples to the electromagnetic field with a strength of 10−2, while the coupling of a mass to the gravitational field is 10−22. Feynman (1963, p. 697) gives an example that highlights this difference in magnitudes more dramatically by showing how the gravitational coupling between a proton and an electron in a hydrogen atom would shift the wave-function of an electron by just 43 arcseconds over a time period of 100 times the age of the Universe! Hence, quantum gravity is more of a theorist’s problem.)