![]() In working out how a particle is allowed to evolve or interact, physicists use the fact that amplitudes never change in a way that disrupts the fixed sum of their squares. As a particle’s state changes (as it flies through a magnetic field, say, or collides with another particle), its amplitudes change too. It’s this twist - the squaring of hidden amplitudes to calculate the outcomes we actually see - that gives unitarity teeth. Unitarity says the sum of these probabilities (really, the squares of all the amplitudes) must equal 1. To calculate the probability of actually observing a particle in a certain state, physicists square the amplitude (or, if the amplitude is an imaginary number, they square its absolute value), which gets rid of the imaginary and negative bits and produces a positive probability. An amplitude is essentially the degree to which a particle is in a certain state it can be a positive, negative or imaginary number. The surprise was that, mathematically, the quantum world operates not by probabilities but by more complicated numbers known as amplitudes. A coin toss, for instance, has a 100% chance of coming up heads or tails.īut a century ago, the pioneers of quantum mechanics made a surprising discovery - one that elevated unitarity from common sense to a hallowed principle. In everyday life, events can’t help but play out in a unitary way. While many physicists are receptive to the isometry proposal - some have even come to similar conclusions independently - opinions vary as to whether the update is too radical or not radical enough. “Unitarity is too strong of a condition.” “You don’t need unitarity,” said Strominger. Andrew Strominger and Jordan Cotler of Harvard University argue that a more relaxed principle called isometry can accommodate an expanding universe while still satisfying the stringent requirements that first made unitary a guiding light. But recently, two quantum gravity theorists may have found a way to loosen unitarity’s buckles to better fit our growing cosmos. “There is a tension there, and it’s something quite puzzling if you think about it,” said Steve Giddings, a quantum gravity theorist at the University of California, Santa Barbara.Ĭoncern over this conflict has been in the air for years. But it means that the future of the cosmos looks totally different from its past, while unitarity demands a tidy symmetry between past and future on the quantum level. This expansion is well described by general relativity. ![]() The main problem is that the universe is expanding. “Unitarity in quantum gravity is a very open question,” said Bianca Dittrich, a theorist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. “It’s a very restrictive condition, even though it might seem a little bit trivial at first glance,” said Yonatan Kahn, an assistant professor at the University of Illinois.īut what once seemed an essential scaffold may have become a stifling straitjacket preventing physicists from reconciling quantum mechanics and gravity. These requirements have long guided physicists as they derive valid quantum formulas. It also ensures that change is a two-way street: Any imaginable event at the quantum scale can be undone, at least on paper. ![]() Unitarity severely limits how atoms and subatomic particles might evolve from moment to moment. ![]() When particles interact, the probability of all possible outcomes must sum to 100%. Unitarity, as the principle is called, says that something always happens. Hints are mounting that at least part of the problem lies with a principle at the center of quantum mechanics, an assumption about how the world works that seems so obvious it’s barely worth stating, much less questioning. But the quest has run up against thorny paradoxes. For 90 years, physicists have sought a reconciliation, a more fundamental description of reality that encompasses both quantum mechanics and gravity. On the other lies general relativity, Einstein’s theory that space and time can bend, causing gravity. On one side lies quantum theory, which portrays subatomic particles as probabilistic waves. ![]()
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