The observable Universe around us takes the form of a bounded sphere. Its boundary is defined by the distance travelled by light since the Big Bang. Now, suppose there was a massive object far outside our cosmic horizon that accelerated gravitationally this entire sphere as a whole, including us. Would we notice this uniform acceleration?
The answer is: no. The cosmic sphere is no different than the free-falling elevator in Albert Einstein’s thought experiment. If we happen to be free-falling with the elevator in a uniform gravitational field, the sealed elevator cabin and our bodies would be moving together and we would not sense gravity.
However, if we attach a cable to the elevator, the situation changes. A passenger standing on the surface pulled by the cable would feel being pulled away from that surface, as if there is a negative gravitational force relative to that surface. A passenger on the opposite side of the cabin would feel an attractive gravitational force that is pushing them against the elevator floor. This thought experiment has interesting implications for the dynamics of interstellar objects near the Sun.
The first interstellar object `Oumuamua was discovered in 2017 as it passed near Earth in its trajectory around the Sun. The trajectory exhibited an anomalous non-gravitational acceleration away from the Sun with no sign of cometary evaporation. When `Oumuamua passed near Earth, the magnitude of the anomalous acceleration away was of order 5 micrometers per second squared. A micrometer (micron) is a millionth of a meter or a thousandth of a millimeter.
`Oumuamua was also tumbling with a rotation period of 8 hours. Based on the light curve from its reflection of sunlight, `Oumuamua’s was inferred to possess the shape of a flat disk (pancake) with a radius of order 100 meters, assuming an albedo of 10%. This size was a thousand times too small for our best telescopes to resolve its image. `Oumuamua’s rotation and size imply a centrifugal acceleration of order 5 micrometers per second squared at its outer edge, surprisingly similar in magnitude to its anomalous acceleration away from the Sun.
Finally, assuming a solid density of order a gram per cubic centimeter, the internal gravitational acceleration from a spherical object of size 100 meters is 5 micrometers per second squared, once again surprisingly similar in magnitude to the centrifugal and non-gravitational accelerations. Does this coincidence in magnitude among these three accelerations provide an important clue about the composition or shape of `Oumuamua?
If `Oumuamua was made of independent components that are held together by gravity, then the rotation of `Oumuamua’s disk could have been balanced by its self-gravity, explaining the similar magnitude of the gravitational and centrifugal accelerations. But this would require that a substantial fraction of the mass is in a bulge configuration. A thin disk geometry, as inferred from `Oumuamua’s light curve, carries much less mass than a sphere.
Given the lack of cometary evaporation and the favored disk geometry, the non-gravitational acceleration could have been produced by radiation pressure from sunlight on the disk, as suggested in the paper I wrote in 2018 with my former postdoc, Shmuel Bialy. In that case, the measured non-gravitational acceleration requires a large surface area to mass ratio for `Oumuamua, translating to a thickness of order a millimeter at solid density. This thickness is one part in 100,000 of the estimated-radius of `Oumuamua.
This raises the possibility that `Oumuamua’s disk was actually a thin solid layer that was manufactured technologically, because we are not aware of an astrophysical process that would produce an umbrella-shaped structure of these extreme dimensions. If artificial in origin, `Oumuamua could have been either a light sail or a tough surface layer that was torn apart from a spacecraft or a broken piece of a Dyson sphere, as I noted in a recent paper.
But why would the non-gravitational acceleration of the object be of order its centrifugal acceleration? If the thin disk is held together by gravity from a core that is not affected as much by radiation pressure because of its smaller surface area per unit mass, then the non-gravitational acceleration would be adding positive “gravity” towards the core on one side of the disk and negative “gravity” away from the core on the opposite side, in analogy to a cable acting on the elevator cabin. A thin gaseous disk would have been torn apart if the non-gravitational acceleration was larger than the internal gravitational acceleration which binds it.
For this reasoning to apply, `Oumuamua should have contained a thin debris disk around a central object, resembling a miniature of Saturn’s rings. Remarkably, the ratio of scale-height (~1 kilometer) to radius (~100,000 kilometers) in Saturn’s rings is also one part in 100,000. Did `Oumuamua resemble Saturn?
Probably not. A gaseous disk could not have remained so thin near the Sun. At perihelion, `Oumuamua was four times closer to the Sun than Earth is, and so its surface temperature reached about 600 degrees Kelvin. At this high temperature, the random motion of atoms would have exceeded the gravitational escape velocity from `Oumuamua by a factor of 100,000. To keep atoms from evaporating requires the chemical bonding of a solid. Self-gravity would have not been able to maintain a thin disk.
It is often said that “what goes up must come down,” but this assumed strong gravity whereas a gaseous gas disk around `Oumuamua could not have maintained its integrity by self-gravity and would have evaporated near the Sun. This could have been avoided if `Oumuamua was a solid thin disk manufactured technologically.
Ecclesiastes 1:9 states: “there is nothing new under the Sun.” `Oumuamua may have provided an exception to this rule.
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. His new book, titled “Interstellar”, was published in August 2023.
