A bigger failure comes when we try to put gravity and
quantum field theory together: it's basically a train wreck.
Because QFT must sum over every possible
state, those states must include cases where the energy of one
leg of a "loop" gets infinitely large. In QFTs that work, there
are well-defined ways to make sense of those infinite sums. But
in gravity, the strength of the interaction (corresponding to
*e* in QED) is proportional to the energy of the
particles. Thus, when the energy gets large, the sum over
histories can't converge, and the theory breaks
down.

In short, our understanding of nature is clearly incomplete, because the theory describing gravity simply cannot coexist with the theory describing everything else. That doesn't tell us what form the solution will take, of course! But there is no doubt that something more is needed.

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Copyright © 2004 by Steuard Jensen.