That is the question author Stuart Clark addresses in the cover story “Differently equal” of the 19 January 2013 NewScientist edition, in which he reports sciences quest to find an exception to the equivalence principle .
Clark quotes Ben Gripaios of the University of Cambridge to explain the reason for finding an exception, “Any theory of quantum gravity must violate the equivalence principle at some level.” He also points out that finding any difference in inertial and gravitational mass is a thorny issue as there are no theories or explanation to inertia; again quoting Gripaios, “We do not yet know how to define it. We know it must be related closely to mass, but until we can define it precisely and know how to measure it, there can be no theory for it.”
The article, after discussing various related experiments and theories, concludes “… the deadlock remains until someone delivers, either an experiment that expresses the equivalence principle as a sham, or a theoretical idea that shows why it must be just so. But if in the end gravitational mass is indeed inertial just mass in another guise – whatever inertial mass is – then it will be the quantum theories of gravity, including string theories, that will find themselves laid upon the sacrificial altar. Paths to a theory of everything will become even more winding if gravity is not a force, but truly an illusion that springs from the warping of space, as described by general relativity …”
There are easier ways to falsify Einstein’s relativity theory than finding a difference in the inertial and gravitational masses. A simple thought experiment, described in the blog “Looking beyond Einstein” disproves Einstein’s relativity postulates.
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