Albert Einstein famously once said that “The most incomprehensible thing about the universe is that it is comprehensible”. These days, however, it is far from being a matter of consensus that the universe is comprehensible, or even that it is unique.
Fundamental physics is facing a crisis, related to two popular concepts that are frequently invoked, summarized tellingly by the buzzwords “multiverse” and “uglyverse.”
Multiverse proponents advocate the idea that there may exist innumerable other universes, some of them with totally different physics and numbers of spatial dimensions; and that you, I and everything else may exist in countless copies. “The multiverse may be the most dangerous idea in physics” argues the South African cosmologist George Ellis.
Ever since the early days of science, finding an unlikely coincidence prompted an urge to explain, a motivation to search for the hidden reason behind it. One modern example: the laws of physics appear to be finely tuned to permit the existence of intelligent beings who can discover those laws—a coincidence that demands explanation.
With the advent of the multiverse, this has changed: As unlikely as a coincidence may appear, in the zillions of universes that compose the multiverse, it will exist somewhere. And if the coincidence seems to favor the emergence of complex structures, life or consciousness, we shouldn’t even be surprised to find ourselves in a universe that allows us to exist in the first place. But this “anthropic reasoning” in turn implies that we can’t predict anything anymore. There is no obvious guiding principle for the CERN physicists searching for new particles. And there is no fundamental law to be discovered behind the accidental properties of the universe.
Quite different but not less dangerous is the other challenge—the “uglyverse”: According to theoretical physicist Sabine Hossenfelder, modern physics has been led astray by its bias for “beauty,” giving rise to mathematically elegant, speculative fantasies without any contact to experiment. Physics has been “lost in math,” she argues. But then, what physicists call “beauty” are structures and symmetries. If we can’t rely on such concepts anymore, the difference between comprehension and a mere fit to experimental data will be blurred.
Both challenges have some justification. “Why should the laws of nature care what I find beautiful?” Hossenfelder righteously asks, and the answer is: They shouldn’t.
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