AFFIRMING THE CONSEQUENT

More and more frequently I see science journalists, and sometimes even professional physicists, committing various elementary fallacies in logic. The most common is “affirming the consequent.” You see statements like, “If covariance is found to be violated, at least in rare circumstances, that indicates that Loop Quantum Gravity is correct!” Or, “If Supersymmetry is finally found to be a valid symmetry, then String Theory is shown to be correct!”

String Theory depends on Supersymmetry, but Supersymmetry in no way depends on String Theory. Even if a future discovery unexpectedly validated Supersymmetry (very unlikely!), this would provide no support whatsoever for String Theory. But lack of experimental evidence for Supersymmetry invalidates String Theory, because that theory incorporates Supersymmetry from the beginning. Similarly, as far as we know, all exact physical theories must be covariant. A discovery of a process that violates covariance to some tiny degree would not support Loop Quantum Gravity, because there are already a huge number of physical theories (Newtonian physics, for example!) that violate covariance in some limit, and are therefore experimentally only approximate.  [I hasten to add that there are also fully covariant versions of Loop Quantum Gravity.]

The other problem with quantum theories of gravity is that they tend to make only predictions that are in principle unverifiable. The White Knight in the Alice books “was thinking of a plan, to dye one's whiskers green, and always use so large a fan, that they could not be seen.” Various different quantum theories of gravity tend to make predictions only in places where it is physically impossible to look, such as the center of a black hole!








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