A Physicist Claims Gravity Can Exist Without Mass
Some clusters of galaxies may not need dark matter in order for their physics to work.
In new research, a physicist describes a “shell singularity,” where gravity applies without mass.
Today’s theories for dark matter include many sizes and types for different situations.
In the wide landscape of physics, researchers are exploring dozens of ways to either identify or rule out dark matter, all to help humans understand how our visible and invisible worlds work together. In a new paper, which appears in the peer-reviewed journal Monthly Notices of the Royal Astronomical Society, scientists suggest that the fundamental idea of gravity may not need the corresponding fundamental idea of mass in order to make physical sense.
If they’re right—if everything we thought we knew is still not the whole story of gravity—then dark matter may not even exist at all. Has it been experimental physics’ imaginary friend all this time?
That’s a provocative question—but, of course, this is one paper on one theory within a large landscape of dark matter theories. So, the short answer is “no, but what if?” In the paper, physicist Richard Lieu of the University of Alabama describes a specific scenario in which gravity affects a system, but that gravitational field does not contain underlying mass.
It’s an edge case of a “shell singularity” around a bump in empty space. As he computes his way through the physics of what the shell will do, the shell itself moves toward zero mass and seems to disappear. So far, so good.
But, his math supports that the area including the zero mass shell still maintains the characteristics of gravity. “With a sufficient number of such unresolvably closely spaced singular shells,” he concludes, “the potential Φ is effectively a continuous function.” In other words, put enough zero mass shells in a tiny enough space, and you’ll have a sort of ersatz gravitational field with no mass responsible for it.
That seems like a cool math trick—much like the way manufacturers can say diet soda has “0 calories” because the true value of.4 falls under the legal threshold that allows it to be rounded down. But it’s not just a cool trick. The zero mass shells can flesh out a scenario that allows scientists to calculate gravity in a neat way that unites the quantum and classical versions of physics without having to include an X factor of dark matter (DM). “[T]he need for enlisting DM is mitigated, at least in part,” Lieu wrote.
There’s a bit of a twist underpinning this work, though. Lieu’s scenario requires another, somewhat unusual (or, at least, less verifiable) phenomenon of spacetime—a topological defect. “An example is a thin sheet of negative mass wrapping around a wormhole to maintain cylindrical symmetry,” Lieu wrote. That’s kind of a long shot on top of a long shot, and it’s not an everyday scenario that someone will see through a high powered observatory telescope anytime soon.
At the same time, Lieu described the topological defect as “reasonably corroborated,” which is more than we can say right now for dark matter. He concluded that, while his idea does not rule out dark matter or majority of applications that currently rely on the concept, it might apply to specific configurations of galaxies that could cluster and create these topological effects and shell singularities.
And of course, if one edge case exists, there may be others. Lieu discussed some other theories of dark matter within large structures like galaxies, because these behave differently in a huge arena like that than at the micro scale. It could be that the idea “dark matter” contains multitudes—including versions that fill differently sized gaps in our understanding.
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