Multiverse theory posits “a kaleidoscope… of myriad universes”

Read AC Grayling unpacking the philosophy behind experimentation

If a scientific theory is elegant, and is consistent with known facts, does it need to be tested by experiment? Scientific knowledge is supposed to be empirical: to be accepted as scientific, a theory must be falsifiable—that is, it must be possible, at least in principle, to empirically disprove it. This argument was advanced in 1934 by Karl Popper, the philosopher, and is generally accepted by most scientists today as determining what is and is not a scientific theory.

In recent years, however, many physicists have developed theories of great mathematical elegance, but which are beyond the reach of empirical falsification, even in principle. The uncomfortable question that arises is whether they can still be regarded as science. Some scientists are proposing that the definition of what is “scientific” be loosened, while others fear that to do so could open the door for pseudo-scientists or charlatans to mislead the public and claim equal space for their views.

The question of whether highly theoretical scientific ideas can be subjected to experimental testing is an issue for the most advanced and powerful ideas in the world of physics. String theory and the idea of the “multiverse”—the existence of multiple universes—are two leading theories that attempt to explain the most fundamental characteristics of the physical world. Both ideas have immense theoretical appeal. String theory is not intrinsically untestable—but there has been no success yet. In experimental terms, one can imagine some future technology that is—in theory at least—capable of accelerating particles to what is known as the Planck energy scale. This is an energy level a thousand trillion times greater than what can be produced at the Large Hadron Collider (LHC) and the point at which the implications of string theory are predicted to be manifest. Multiverse theory presents apparently insuperable obstacles to experiment as other universes are intrinsically impossible to detect, although even here, physicists are suggesting ways to infer their existence.

Is physics moving towards an era in which elegance will suffice and into the domain of theories that are beyond the reach of experimental proof? Or will empirical evidence remain the arbiter of science?

String theory is an attempt to develop a unified theory of…

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