The Quantum Gravity Wars

A quantum gravity theorist discloses that the acrimonious partisanship between proponents of the two main theories of quantum gravity is stifling advance in the field. Reluctantly, because of this state of affairs, he reports that he has decided to leave the field, for the moment.

“[O]ur two current best theories” in physics are “the Standard Model, which tries to explain all the forces except gravity” with help of quantum mechanics, and “general relativity, which tries to explain gravity” but excludes quantum mechanics. The disciplinary field of quantum gravity has as its object the formulation of “a theory of gravity that takes quantum mechanics into account, but reduces to General Relativity” when quantum effects are ignored.

There are two main approaches to quantum gravity. The more popular and certainly far more ambitious one is called string theory. String theory aims, not only to be a theory of quantum gravity, but  a theory of everything. What is  its record of success?

According to mathematician John Baez, of the University of California, not much. It’s made no predictions that have been confirmed experiment. In fact, its made few predictions that we have any hope of testing anytime soon!” Further, because of “the vast ‘landscape’ of string theory models that researchers are uncovering, it keeps getting harder to squeeze specific predictions out of this theory.”

The other main approach to quantum gravity is called loop quantum gravity. It is far less ambitious than string theory in that it does not claim to be a theory of everything, only a theory of quantum gravity. Baez, himself, worked on this approach for “about a decade.” It painted a picture of reality “in which spacetime resembles a random ‘foam’ at very short distance scales, following the laws of quantum mechanics.”

Despite being far less ambitious than string theory, loop quantum gravity, too, has failed to deliver. As Baez reports: “We can write down lots of theories of this general sort, However, we have never yet found one for which we can show that General Relativity emerges as a good approximation at large distance scales – the quantum soap suds approximating a smooth surface when viewed from afar, as it were.”

What has made matter worse, Baez report, is the acrimonious partisanship of the respective proponents of the two rival theories. Meanwhile, the observational cosmologists “were making wonderful discoveries left and right, getting precise data about dark energy, dark matter, and inflation” – data which should have helped “resolve the string-loop war” but which didn’t because neither of the two theories were sufficiently rigorous to “make predictions about the numbers the cosmologists were measuring!”

This missed opportunity made Baez realize that he “didn’t have enough confidence in either theory to engage in these heated debates.” Reluctantly, Baez decided to quit quantum gravity, for the moment, and devote his time, instead, to “questions where I could actually tell when I was on the right track, questions where researchers cooperate more and fight less.”

This was a most painful decision: Baez had expected to devote his entire life to helping develop a theory of quantum gravity. Still, he reports, his decision, once executed, has been “tremendously liberating.” Dr. Baez reports that he is “making more progress understanding the universe than I ever did before.”

(Reference: Baez, John. 2008. “Should I be thinking about quantum gravity?”

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