No Relativistic UAPs More Massive than 50 Megatons Passed Near Earth Over the Past Decade!
The scientific collaboration of the Laser Interferometer Gravitational-Wave Observatory (LIGO), to which the Physics Nobel Prize was awarded in 2017, discovered 90 gravitational wave sources. LIGO detected its first gravitational wave signal from a merger of black holes on September 14, 2015, within days of turning on its improved sensitivity. By now, LIGO collaborates with two other gravitational wave observatories, Virgo in Italy and KAGRA in Japan.
All gravitational wave sources detected sofar involve mergers of stellar-mass astrophysical objects, such as black holes or neutron stars, at cosmological distances.
Today, I realized that the lack of an anomalous gravitational signal since 2015 can be used to rule out Unidentified Anomalous Phenomena (UAPs) of extreme properties near Earth. Let me elaborate.
Imagine a relativistic object moving near the speed of light within a distance from LIGO that is comparable to the radius of the Earth. At closest approach, such an object would generate a gravitational signal over a time period equal to the radius of the Earth divided by the speed of light. The resulting duration of the signal is 0.02 seconds. Its inverse corresponds to a frequency of 50 hertz, to which LIGO is highly sensitive.
The spacetime perturbation that the relativistic UAP would generate is comparable to its gravitational potential, (GM/R), divided by the speed of light squared. Here, G is Newton’s constant, M is the mass of the object, and R is its distance from LIGO. Adopting a distance comparable to the radius of the Earth, what is the minimum mass of a relativistic object that LIGO could have detected?
The design sensitivity of Advanced LIGO for a spacetime perturbation at a frequency of 50 hertz is about ~6e-24. Plugging in the relevant numbers, I had found that LIGO would have been sensitive to the gravitational signal of a relativistic UAP with a mass larger than 50 million tons within a distance comparable to the radius of the Earth. It is not possible to constrain objects at much larger distances or slower speeds because their signal peaks well below the range of frequencies to which LIGO, Virgo or KAGRA are sensitive.
Since no anomalous gravitational signal was reported by the LIGO-Virgo-KAGRA collaboration, we know that over most of the past decade there was no UAP more massive than 50 megatons moving near the speed of light close to Earth.
As a result of the equivalence principle, this constraint must be satisfied by all objects, irrespective of their shape and composition. The military designs materials that minimize electromagnetic signatures, but as I showed in a single-authored paper from 2020 — it is practically impossible to block a gravitational signal.
The null result from LIGO is not surprising because a relativistic spacecraft made of known materials is prone to severe damage from impacts by interstellar gas and dust. In a detailed paper that I wrote with Thiem Hoang and other co-authors in 2017, we calculated that it would be challenging to protect a relativistic craft during an interstellar journey through the entire disk of the Milky-Way galaxy.
At solid density, the upper limit from LIGO on the mass of a relativistic UAP translates to a size limit of order 200 meters. The interstellar object `Oumuamua discovered in 2017 had a size comparable to this limit but was moving 10,000 times slower than the speed of light and reached a distance of closest approach that is 5,000 times farther than considered here. As a result, the frequency of `Oumuamua’s gravitational signal was of order a microhertz, well below the frequency range of past and forthcoming gravitational wave observatories.
Astronomers do not regard the LIGO-Virgo-KAGRA observatories as an alert system to warn humanity from relativistic interstellar crafts. But in retrospect, it is reassuring to know that these gravitational-wave observatories did not find an unexpected signal in their data stream. If they ever detect a massive relativistic object near Earth, the breaking news will change the future of humanity. Until then, we can echo Enrico Fermi’s question: “where is everybody?”
For the related scientific paper, click here.
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.
