Sep 26, 2022 09:34 PM
Physicists rewrite a quantum rule that clashes with our universe
https://www.quantamagazine.org/physicist...-20220926/
INTRO: A jarring divide cleaves modern physics. On one side lies quantum theory, which portrays subatomic particles as probabilistic waves. On the other lies general relativity, Einstein’s theory that space and time can bend, causing gravity. For 90 years, physicists have sought a reconciliation, a more fundamental description of reality that encompasses both quantum mechanics and gravity. But the quest has run up against thorny paradoxes.
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.
Unitarity, as the principle is called, says that something always happens. When particles interact, the probability of all possible outcomes must sum to 100%. Unitarity severely limits how atoms and subatomic particles might evolve from moment to moment. It also ensures that change is a two-way street: Any imaginable event at the quantum scale can be undone, at least on paper. These requirements have long guided physicists as they derive valid quantum formulas. “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.
But what once seemed an essential scaffold may have become a stifling straitjacket preventing physicists from reconciling quantum mechanics and gravity. “Unitarity in quantum gravity is a very open question,” said Bianca Dittrich, a theorist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada.
The main problem is that the universe is expanding. This expansion is well described by general relativity. 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. “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.
Concern over this conflict has been in the air for years. But recently, two quantum gravity theorists may have found a way to loosen unitarity’s buckles to better fit our growing cosmos. 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.
“You don’t need unitarity,” said Strominger. “Unitarity is too strong of a condition.”
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... (MORE - details)
US particle physicists envision future of the field
https://physicstoday.scitation.org/doi/1.../PT.3.5097
INTRO: Gordon Watts was nervous when US particle physicists began the most recent grassroots exercise in formulating how to further their science. Watts is a high-energy physicist at the University of Washington and one of the organizers of the 2022 Snowmass process, which began more than two years ago.
“I thought people would view the next steps for the field as already decided—for the High-Luminosity Large Hadron Collider at CERN and neutrino experiments in the US—and conclude, ‘Why should I spend my time thinking about what’s next?’ ”
Instead, “a glorious range of approaches were presented” over 10 days in July, when about 1200 people met online and in person at the University of Washington, says Watts. “There were 511 white papers! The amount of work that represents is insane. I am very happy to be wrong.”
At Snowmass, which takes place roughly once a decade under the aegis of the division of particles and fields of the American Physical Society (APS), particle physicists share ideas for research directions, projects, and facilities. They hone their science goals and find synergies across their subfields.
Some projects and approaches drew clear support and enthusiasm, but ranking them was left for later: Reports from Snowmass will serve as guides for the Particle Physics Project Prioritization Panel (P5), which will rank projects for the US Department of Energy and NSF.
Hitoshi Murayama, a particle physicist at the University of California, Berkeley, is chairing P5. “It’s a scary assignment,” he says. “We need to be realistic and maximize the science worldwide; the field is international. Community buy-in is key.” The P5 recommendations will be due in the second half of 2023... (MORE - details)
COVERED: Neutrino flagship ..... Complications and complementarity ..... A Higgs factory, now ..... “The dark sector” ..... Health of the field
https://www.quantamagazine.org/physicist...-20220926/
INTRO: A jarring divide cleaves modern physics. On one side lies quantum theory, which portrays subatomic particles as probabilistic waves. On the other lies general relativity, Einstein’s theory that space and time can bend, causing gravity. For 90 years, physicists have sought a reconciliation, a more fundamental description of reality that encompasses both quantum mechanics and gravity. But the quest has run up against thorny paradoxes.
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.
Unitarity, as the principle is called, says that something always happens. When particles interact, the probability of all possible outcomes must sum to 100%. Unitarity severely limits how atoms and subatomic particles might evolve from moment to moment. It also ensures that change is a two-way street: Any imaginable event at the quantum scale can be undone, at least on paper. These requirements have long guided physicists as they derive valid quantum formulas. “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.
But what once seemed an essential scaffold may have become a stifling straitjacket preventing physicists from reconciling quantum mechanics and gravity. “Unitarity in quantum gravity is a very open question,” said Bianca Dittrich, a theorist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada.
The main problem is that the universe is expanding. This expansion is well described by general relativity. 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. “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.
Concern over this conflict has been in the air for years. But recently, two quantum gravity theorists may have found a way to loosen unitarity’s buckles to better fit our growing cosmos. 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.
“You don’t need unitarity,” said Strominger. “Unitarity is too strong of a condition.”
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... (MORE - details)
US particle physicists envision future of the field
https://physicstoday.scitation.org/doi/1.../PT.3.5097
INTRO: Gordon Watts was nervous when US particle physicists began the most recent grassroots exercise in formulating how to further their science. Watts is a high-energy physicist at the University of Washington and one of the organizers of the 2022 Snowmass process, which began more than two years ago.
“I thought people would view the next steps for the field as already decided—for the High-Luminosity Large Hadron Collider at CERN and neutrino experiments in the US—and conclude, ‘Why should I spend my time thinking about what’s next?’ ”
Instead, “a glorious range of approaches were presented” over 10 days in July, when about 1200 people met online and in person at the University of Washington, says Watts. “There were 511 white papers! The amount of work that represents is insane. I am very happy to be wrong.”
At Snowmass, which takes place roughly once a decade under the aegis of the division of particles and fields of the American Physical Society (APS), particle physicists share ideas for research directions, projects, and facilities. They hone their science goals and find synergies across their subfields.
Some projects and approaches drew clear support and enthusiasm, but ranking them was left for later: Reports from Snowmass will serve as guides for the Particle Physics Project Prioritization Panel (P5), which will rank projects for the US Department of Energy and NSF.
Hitoshi Murayama, a particle physicist at the University of California, Berkeley, is chairing P5. “It’s a scary assignment,” he says. “We need to be realistic and maximize the science worldwide; the field is international. Community buy-in is key.” The P5 recommendations will be due in the second half of 2023... (MORE - details)
COVERED: Neutrino flagship ..... Complications and complementarity ..... A Higgs factory, now ..... “The dark sector” ..... Health of the field