Nov 18, 2025 08:15 PM
https://www.quantamagazine.org/old-ghost...k-20251117
INTRO: The force we experience most intimately remains the most mysterious. Physicists understand how vast migrations of particles called photons light up our homes, and how swarms of “gluon” particles hold together the cores of our atoms. But they can’t say what gravity particles, if any, delight us as babies by enabling our spoons to plummet to the floor.
The force of gravity has proved so difficult to account for in terms of particles that many physicists have abandoned that approach altogether. They consider the possibility that gravity — and with it, reality as a whole — might instead be made of tiny strings or other strange things.
But in one corner of the theoretical-physics world, the particle approach is staging a comeback. A growing band of physicists has been using the typical approach to particle physics, known as quantum field theory, for gravity. Although this use of the theory was long considered fatally flawed, these physicists are now finding that it works far better than their predecessors expected.
“So far there is no hint telling us that we should throw quantum field theory away; actually, it’s the opposite,” said Luca Buoninfante, a theoretical physicist at Radboud University in the Netherlands whose calculations have helped shore up the old theory. When you apply the standard quantum field theory to gravity, you don’t just get a unique theory called quadratic gravity, he said. “You also get new predictions.”
Those predictions have not yet been tested. And on purely theoretical grounds, quadratic gravity has eerie features that still spook many physicists.
But quadratic gravity enthusiasts aren’t put off by its abnormalities. On the contrary, they view these features as previously unappreciated possibilities that may be permitted by quantum field theory. Perhaps effects occasionally sneak ahead of their causes at the microscopic level, for instance. And perhaps negative-energy “ghost” particles that arise in quadratic gravity can exist safely in the equations without creating paradoxes in experiments.
Ghosts, Bonfanti said, may be “new objects that appear when we try to understand gravity and quantum field theory at a deeper level.” (MORE - details)
INTRO: The force we experience most intimately remains the most mysterious. Physicists understand how vast migrations of particles called photons light up our homes, and how swarms of “gluon” particles hold together the cores of our atoms. But they can’t say what gravity particles, if any, delight us as babies by enabling our spoons to plummet to the floor.
The force of gravity has proved so difficult to account for in terms of particles that many physicists have abandoned that approach altogether. They consider the possibility that gravity — and with it, reality as a whole — might instead be made of tiny strings or other strange things.
But in one corner of the theoretical-physics world, the particle approach is staging a comeback. A growing band of physicists has been using the typical approach to particle physics, known as quantum field theory, for gravity. Although this use of the theory was long considered fatally flawed, these physicists are now finding that it works far better than their predecessors expected.
“So far there is no hint telling us that we should throw quantum field theory away; actually, it’s the opposite,” said Luca Buoninfante, a theoretical physicist at Radboud University in the Netherlands whose calculations have helped shore up the old theory. When you apply the standard quantum field theory to gravity, you don’t just get a unique theory called quadratic gravity, he said. “You also get new predictions.”
Those predictions have not yet been tested. And on purely theoretical grounds, quadratic gravity has eerie features that still spook many physicists.
But quadratic gravity enthusiasts aren’t put off by its abnormalities. On the contrary, they view these features as previously unappreciated possibilities that may be permitted by quantum field theory. Perhaps effects occasionally sneak ahead of their causes at the microscopic level, for instance. And perhaps negative-energy “ghost” particles that arise in quadratic gravity can exist safely in the equations without creating paradoxes in experiments.
Ghosts, Bonfanti said, may be “new objects that appear when we try to understand gravity and quantum field theory at a deeper level.” (MORE - details)