Escaping Gravity Without Fuel
Electromagnetism involves both positive and negative charges. Einstein described gravity as the curvature of spacetime based on the equivalence principle. This principle identifies the gravitational mass of an object — which defines its coupling to gravity, with its inertial mass — which defines its response to any external force. Einstein’s gravity allows for the existence of a negative mass that satisfies the equivalence principle. This would imply that the negative gravitational mass equals the negative inertial mass.
So far, we had never detected a negative mass in nature. This may mean that the Universe never facilitated the conditions necessary to create it naturally. But it does not mean that a sufficiently advanced technological civilization cannot produce a negative mass artificially. For instance, we know that the expansion of the Universe is accelerating. This is a signature of repulsive gravity, which cosmologists attribute to the existence of dark energy. If advanced extraterrestrial scientists knew the nature of dark energy and found a way to bottle it inside a lightweight container, then they would have produced a negative mass object. How would this object behave?
Because of its negative inertial mass, it would move in a strange way. If a kid ties a string to a negative-mass container and pulls down on the string, the container will move away or upward like a balloon even in the absence of an atmosphere. But when the kid lets go of the string, the container will fall down to the ground because its net gravitational mass is negative. By the equivalence principle, negative and positive masses fall the same way under the influence of gravity. The negative mass would behave like a balloon when you pull on its string and like a rock when you let it go.
We can now imagine the construction of a vehicle without fuel. For that, we place passengers in a carriage and balance the positive mass with an equal negative mass. The passengers merely need to pull a string connecting them to the negative mass. The increased tension in the string will cause the negative mass to move forward together with the rest of the vehicle. As long as the separation between the positive and negative mass remains fixed in the vehicle’s enclosure, the net energy of the system will not change. With tension in the string, the vehicle will accelerate. The gained momentum and kinetic energy of the positive mass will be compensated by the negative momentum and kinetic energy of the negative mass.
Since no energy is being consumed, the propulsion of this vehicle needs no fuel. From the viewpoint of the passengers, the ride would feel like pulling the reins of a horse carriage, except that the horse driving this magical vehicle needs no food supply. In order to bring the vehicle to a stop, the passengers would need to push on the negative mass.
This propulsion scheme can be used to launch a spacecraft without rocket fuel. Increasing the string tension will accelerate the payload upward and deliver it to space. For extraterrestrial space agencies which use negative mass containers, our flagship rocket Starship would appear as a complete waste of mass and energy. Their vehicle with a zero net gravitational mass has no need to overcome gravitational binding to Earth. It floats at zero gravity from the get go, and so lifting it off the ground towards space requires no energy.
As noted by Robert Forward in a 1990 paper, by Richard Price in a 1992 paper and by Geoffrey Landis in a 2019 paper, the rate of acceleration can be adjusted by changing the tension in the string within the vehicle. An object with a zero net mass is weightless. As it floats above the ground without thrust, moving it from one location to another is effortless.
Within alien vehicle factories, transportation of negative-mass containers might be tricky since these objects run away from you if you pull them towards you. But those familiar with herding cats can easily adapt to the psychological challenge of moving negative masses at will. One approach could be to endow the negative mass containers with an electric charge and then use electric or magnetic fields to move them around during the manufacturing process of the vehicles.
As Herman Bondi realized in a 1957 paper, even in the absence of a physical string — the gravitational interaction between a positive-mass payload and a negative-mass carrier would accelerate the pair, since this gravitational interaction effectively serves like a string. However, in low-mass vehicles the impact of this gravitational interaction is negligible.
Could negative-mass vehicles explain the unusual flight characteristics of some Unidentified Anomalous Phenomena (UAPs) near Earth? Clearly, UAPs are a mixed bag, because 97% of the hundreds of military reports on UAPs were already explained as familiar objects in reports issued by the All-Domain Anomaly Resolution Office (AARO) in the Pentagon. A similar fraction of the million objects detected so far by the Galileo Project Observatory were cataloged in a recently published paper as familiar. However, even if one in a million UAPs happens to be propelled by a negative-mass engine, this would constitute the most dramatic discovery in the history of physics. A zero-mass vehicle would float in Earth’s gravity and use no fuel to maneuver. No rocket gas will emerge near it out of an exhaust. The object would accelerate without the familiar signs of an internal engine because its motions would not consume energy.
The 2020 Decadal Survey of the U.S. astronomy community identified as its highest priority the launch of a space-based Habitable Worlds Observatory in search for chemical biomarkers of microbial life on distant exoplanets. The Observatory’s cost will likely exceed 10 billion dollars and its completion will occur no earlier than the 2040s under the most favorable scenario. Wouldn’t it be amazing if we discover negative-mass vehicles near Earth well before the 2040s? Such a low-cost finding in our cosmic backyard will instantly change the priorities of the astronomy community away from a multi-decade plan with a huge price tag. Needless to add, discovering the products of an intelligence superior to ours near Earth is far more exciting than discovering microbes many light years away.
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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.
