In a public event about black holes, an audience member expressed deep concerns and recurring nightmares about the possibility of death by a black hole. I clarified that indeed an interception of her body with a black hole would mean bad news for her longevity, but the chance of that happening is much smaller than the chance that she will meet a serial killer on a dating app.
Does this imply that we may ignore existential risks to life on Earth from black holes during the entirety of Earth’s history? Not at all. Additional concerns involve action at a distance.
When a massive star collapses to a black hole, some of the energy gets channeled into a pair of tightly-collimated beams of intense gamma-rays that could annihilate life on a nearby habitable planet like Earth. The radiation originates from two opposing jets launched by the newly formed black hole at the center of its parent star. The jets drill through the stellar envelope at nearly the speed of light. Once they break out from the edge of the star into free space, they generate a burst of gamma-rays towards any observer that happens to be aligned with their beam. The resulting gamma-ray bursts are so bright that we observe them all the way from the edge of the Universe. These bursts were discovered in 1967 by the Vela military satellites which were looking for covert nuclear explosions and monitoring Soviet Union compliance with the 1963 Partial Test Ban Treaty.
What is the chance of a mass extinction event on Earth triggered by these “cosmic death rays”?
In a paper that I wrote with David Sloan and Rafael Batista in 2017, we calculated that the chance of a complete sterilization of life on Earth by a gamma-ray burst is negligible. However, this and earlier estimates were based on the known statistics of gamma-ray bursts in the cosmos prior to October 2022.
On October 9, 2022, the Brightest Of All Time (BOAT) gamma-ray burst was observed from a distance of 2.4 billion light years, and labeled GRB 221009A. The individual energies of photons from BOAT reached values above those produced by CERN’s Large Hadron Collider. A new preprint suggests that BOAT belongs to a potentially new population of gamma-ray bursts with narrow jets.
Before my morning jog, I did a simple calculation. There are about ten million galaxies like the Milky-Way within the volume associated with the distance to the BOAT. This extremely bright burst was detected during 55 years of gamma-ray monitoring of the sky. This suggests that such an event could occur in our own Milky Way galaxy and illuminate Earth once per 550 million years. How bright would a Milky-Way BOAT be?
To find out the answer, we multiply the fluence of GRB 221009A by the square of its distance divided by the characteristic distance of a Milky-Way source. This calculation gives about a mega-joule per meter squared in gamma-rays. Such a fluence would have a devastating impact on life on Earth, as it is equivalent to a nuclear fireball carrying 30,000 megaton of TNT, much more than the worldwide nuclear arsenal. By comparison, the total chemical energy of the ozone layer in the atmosphere is only 300 megatons of TNT. The illumination of Earth by a Milky-Way BOAT would also have a large impact on the chemistry of atmospheric nitrogen.
As much as this energy release sounds impressive, it is dwarfed by asteroid impacts on Earth. For example, the Chicxulub impactor that struck Earth 66 million years ago released 72 teraton of TNT, a few thousand times more than a Milky-Way BOAT, and 5 billion times the energy of the atomic bomb dropped on Hiroshima. During 550 million years that separate Milky-Way BOATs, there should be 8 such asteroid impacts.
To those who lose sleep thinking about death by a Milky-Way BOAT, I would say: “indeed gamma-ray illumination by a Milky-Way BOAT would mean bad news for your longevity, but the chance of that happening is much smaller than the chance that you will be killed by an asteroid.”
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.
