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“I’ve been a skeptic of the landscape for a long time. Some string theorists such as Savdeep Sethi of the University of Chicago welcome the reevaluation that is happening now. “Paradoxically this makes things much more interesting because that means string theory is much more predictive than we thought it was.” “This picture with a big multiverse could be mathematically wrong,” Danielsson says. “Literally anything is possible.” To Steinhardt and others, the newfound problems with dark energy offer string theory a way out. “If it’s really the landscape, in my view it’s death for the theory because it loses all predictive value,” says Princeton University physicist Paul Steinhardt, who collaborated on one of the recent papers. Yet the notion of a string theory landscape that predicts not just one universe but many has put some physicists off.
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Many string theorists contend it is still the most promising direction for pursuing Albert Einstein’s dream of uniting his general theory of relativity with the conflicting microscopic world of quantum mechanics. String theory is an attempt to describe the whole universe under a single “theory of everything” by adding extra dimensions of spacetime and thinking of particles as minuscule vibrating loops. In fact, it may be theoretically impossible to find a valid solution to string theory that includes stable dark energy, says Cumrun Vafa, a Harvard University physicist who led the work on the two papers. Scientists have known many solutions must fall in this swampland for years, but the idea that most, or maybe all, of the landscape solutions might live there would be a major change. But the vast majority of the solutions found so far are mathematically inconsistent, the papers contend, putting them not in the landscape but in the so-called swampland of universes that cannot actually exist. The conversation centers on a pair of papers posted on the preprint server arXiv in June 2018 taking aim at the so-called landscape of string theory-the incomprehensible number of potential universes that result from the many different solutions to string theory’s equations that produce the ingredients of our own cosmos, including dark energy. “This is really something new and it’s led to a controversy within the field,” says Ulf Danielsson, a physicist at Uppsala University in Sweden. The debate was a hot topic in the summer of 2018 in Japan, where string theorists convened for the conference Strings 2018.
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But others say the multiverse is here to stay, and the proposed problem with all those universes is not a problem at all.
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To some, eliminating so many possible universes is not a drawback but a major step forward for string theory, offering new hope of making testable predictions. Now some theorists suggest most-if not all-of those universes are actually forbidden, at least if we want them to have stable dark energy, the supposed force accelerating the expansion of the cosmos. But with so many universes on the table, how can the theory explain why ours has the features it does? It predicts not one but some 10,500 versions of spacetime, each with its own laws of physics. The problem with string theory, according to some physicists, is that it makes too many universes.