There’s something unnerving about unifying physics. The two theories that need to be unified, quantum field theory and Einstein’s general theory of relativity, are both highly successful. Both make predictions good to as many decimal places as experimentalists can manage. Both are grounded in compelling principles. Both do have flaws — including an unfortunate tendency to produce the number ∞ — but those flaws remain safely behind the scenes, never undermining the theories’ empirical successes.
And yet, if the theories are incompatible, something has to give. That is what makes unification so hard. In conferences, I see physicists go down the list of assumptions that underpin their theories. Each, it seems, is rock solid. But they can’t all be right. Maybe one will, on closer inspection, prove to be not like the others. Or maybe physicists have left the culprit off their list because it is so deeply embedded in their way of thinking that they don’t even recognize as an assumption. As economist John Maynard Keynes wrote, “The difficulty lies, not in the new ideas, but in escaping from the old ones, which ramify… into every corner of our minds.”
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