What is gravity without mass? From Newton’s revolutionary laws describing its universal effect to Einstein’s proposal of a dimpled space-time, we have thought that gravity is exclusively in the realm of matter.
Now, an unusual new study suggests that gravity can exist without mass, conveniently eliminating the need for one of the most elusive substances in our universe: dark matter.
Dark matter is a hypothetical, invisible mass believed to make up 85 percent of the total mass of the universe. Originally created to account for galaxies holding together at high spin rates, it has yet to be directly observed, prompting physicists to come up with all sorts of ideas to avoid using this elusive material as a way to plug the holes in current theories.
The latest proposal in this vein comes from astrophysicist Richard Liu of the University of Alabama in Huntsville, who suggests that instead of dark matter binding galaxies and other bodies together, the universe may contain thin, shell-like layers of “topological defects” which give rise to gravity without any underlying mass.
Lieu began trying to find another solution to Einstein’s field equations, which relate the curvature of spacetime to the presence of matter in it.
As Einstein described in his 1915 theory of general relativity, spacetime warps around bundles of matter and streams of radiation in the universe, depending on their energy and momentum. This energy is, of course, related to mass in Einstein’s famous equation: E=mc2.
So the mass of an object is related to its energy, which bends space-time – and this curvature of space-time is what Einstein described as gravity, a little more refined than Newton’s 17th-century approximation of gravity as a force between two objects with mass . In other words, gravity seems inextricably linked to mass.
Not so, says Liu.
In his work, Lieu sets out to solve a simplified version of Einstein’s field equations that allows for a finite gravitational force in the absence of any detectable mass. He says his efforts were “driven by my frustration with the status quo, namely the idea that dark matter exists despite the lack of any direct evidence for a century.”
The Lieu solution consists of shell-like topological defects that can occur in very compact regions of space with very high matter density.
These sets of concentric shells contain a thin layer of positive ground sandwiched inside an outer layer of negative ground. The two masses cancel each other out, so the total mass of the two layers is exactly zero. But when a star lies on this shell, it experiences a large gravitational force that pulls it toward the center of the shell.
“The claim of my paper is that at least the shells it suggests are massless,” says Liu. If these controversial proposals have any weight, “there is no need to continue this seemingly endless search for dark matter,” Liu adds.
The next question then is how to possibly confirm or reject the shells proposed by Lieu through observations.
“The increasing frequency of observations of ring and shell-like galaxy formations in the Universe provides evidence for the type of source at play here,” Lieu wrote in his paper. Although he admits that his proposed solution is “highly suggestive” and cannot by itself discredit the dark matter hypothesis.
“At best, it can be an interesting mathematical exercise,” Liu concludes. “But it’s the first [mathematical] proof that gravity can exist without mass.”
The study was published in Monthly Notices of the Royal Astronomical Society.
