Negative-Mass Binaries Generate Never-Seen-Before Gravitational Radiation
In recent months, two papers (accessible here and here) suggested the existence of an abundant population of negative mass objects in the Universe. Albert Einstein’s theory of gravity, General Relativity, allows negative masses, as discussed in a seminal 1957 paper published here by the physicist Herman Bondi. In 2015, Robert Forward showed in a paper available here how negative masses can be used for propulsion without fuel. But as I explained in a recent essay here, we have no idea whether negative masses can be constructed in reality. In fact, their existence would create fundamental problems in our current understanding of the physical world. For example, negative masses can be used as building blocks in engineering a time machine which would violation causality, as I explained in an earlier essay here.
In principle, there are three types of mass:
1. Inertial mass, namely the mass given by the ratio between the force acting on an object and its resulting acceleration.
2. Active gravitational mass, namely the mass that generates the gravitational field around the object and affects the motion of other objects.
3. Passive gravitational mass, namely the mass that couples the object to the external gravitational field generated by other objects.
Momentum conservation requires that the active and passive gravitational masses be equal. Otherwise, the momentum gained by one object will not necessarily be balanced by the momentum lost by another object when the two interact gravitationally with each other. Momentum conservation is a consequence of spatial translational symmetry, namely the invariance of the laws of physics to shifting a system from one position to another. Therefore, it is a sacred principle that cannot be violated.
The equality of the inertial mass and the passive gravitational mass is a fundamental assumption, called the Equivalence Principle, in Einstein’s theory of General Relativity. It rests on experiments, including by Galileo Galilei and numerous follow-up experiments, which demonstrated that all objects accelerate the same way under the action of gravity irrespective of their mass or composition. In discussing negative mass objects, it is often assumed that conservation of momentum and the Equivalence Principle are both valid, and so all three forms of mass are the same.
However, the Equivalence-Principle rests on observational data for positive masses and could potentially be violated for negative masses. A new paper (available here) that I just co-authored with the brilliant PhD student, Oem Trivedi, shows that a system made of two objects which possess different values of the ratio between their gravitational and inertial masses would emit dipole gravitational radiation instead of the usual quadrupole gravitational radiation. The situation is analogous to a system made of positive and negative electric charges with positive inertial masses. So far, no sign of dipole gravitational radiation has been detected by the LIGO- Virgo-KAGRA (LVK) gravitational wave observatories. Current limits on dipole gravitational radiation are tight, as discussed here.
But even if the Universe contains only negative masses which satisfy the Equivalence Principle, binary systems containing such masses are expected to generate never-seen-before gravitational-wave signals:
1. Systems with a negative total mass are repulsive and short lived, preventing the formation of stable binaries.
2. Systems with a positive total mass, where the negative mass is smaller in magnitude than the positive mass, can form circular orbits. However, their evolution under gravitational radiation leads to expansion rather than inspiral, producing anti-chirp signals with decreasing frequency. Such a waveform is opposite to the chirp observed for all gravitational wave sources so far by LVK. The absence of anti-chirp signals in current gravitational wave catalogs provides a direct observational constraint on negative mass binaries. For an ordinary positive-positive binary, the orbital energy is negative and becomes more negative as the binary separation decreases, so gravitational-wave energy loss drives the system toward inspiral and increasing frequency. For a positive-negative binary with a positive total mass, the orbital energy starts being positive and decreases toward zero as the binary separation grows, so the same loss of gravitational-wave energy causes the system to expand rather than contract.
3. Systems with a zero total mass, where the negative mass cancels the positive mass precisely, correspond to runaway solutions where the pair accelerates up to the speed of light in some direction rather than engage in periodic motion.
So far, the new observational frontier of gravitational wave astrophysics did not lead to the discovery of new unexpected sources in our Universe over the past decade. This is somewhat disappointing and out of line with the history of astronomy, where observatories in a new band of the electromagnetic spectrum always revealed new unexpected sources. This was the case with radio telescopes discovering the cosmic microwave background, complex interstellar molecules or fast radio bursts; X-ray telescopes discovering accreting black holes, the cosmic X-ray background or X-ray clusters, and gamma-ray telescopes discovering gamma-ray bursts, gamma-ray pulsars or the cosmic gamma-ray background.
Our new paper shows that binary systems with negative mass members would generate gravitational-wave signals with never-seen-before properties. They provide novel targets for gravitational wave observatories which could either find them or place the stringest limits on their existence.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
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