Recently, the Gaia collaboration discovered the most massive black hole that originated from the collapse of a star in the Milky Way galaxy. It was identified through the orbital motion of its visible companion, a yellow giant star with an orbital period of 11.6 years.
This black hole, called Gaia BH3, has 33 times the mass of the Sun, in striking resemblance to the masses of the first black holes detected through gravitational waves by LIGO in 2015 involving a merger of 29 and 36 solar masses. Gaia BH3’s motion through the galaxy implies that it originated from a disrupted star cluster in the halo of the Milky Way where the oldest stars reside. Indeed, the iron-to-hydrogen ratio of its companion is 320 times smaller than the solar value, suggesting that Gaia BH3 is a relic of the early Universe where heavy elements were scarce.
Indeed, my research from three decades ago suggested that the first stars were much more massive than present-day stars because the gas clouds that birthed them contained primarily hydrogen and helium, with limited ability to cool and fragment into stars with less than ten solar masses. As a result, star forming regions in the early universe were efficient factories of massive black holes, such as Gaia BH3. After formation, these black holes followed the collision-free dynamics of the dark matter and assembled into the extended halos of galaxies like the Milky Way.
On the other hand, the disk of the Milky Way contains younger stars because it formed later in cosmic history. The disk is estimated to contain about one black hole per thousand stars, yielding a population of about 100 million black holes. Given the local density of stars and their characteristic random motions, I calculated that over the age of the Solar system, one of the black holes entered the outer envelope of the Oort cloud at 20,000 times the Earth-Sun separation. This was likely the closest encounter ever of a black hole near Earth, lasting tens of thousands of years.
Such an encounter had a negligible impact on terrestrial life if the black hole was dormant. However, if the black hole was fed with mass from a tightly bound companion star, then it could have produced an X-ray flux larger than the solar X-ray flux by up to a factor of a hundred.
This would have been the closest encounter of a black hole, unless there is a population of primordial black holes which were produced by exotic physics within a second after the Big Bang. If primordial black holes make a significant contribution to dark matter and they have masses of mile-scale asteroids, then some of them may have passed through Earth during its lifespan. However, these passages had a negligible impact since the horizon of these black holes spans a scale comparable to an atomic nucleus. Even with their Hawking radiation, these black holes released less power than meteors of the same mass.
There are potentially better opportunities for black hole enthusiasts as interstellar tourists. For adventurers who wish to witness a black hole up close, the nearest destination at any given time should be about 30 light-years away. Spacecraft moving faster than half the speed of light could traverse this distance during a human’s lifespan. With conventional chemical rockets, the trip would take about a million years. It would make sense for interstellar travel agencies to advertise such trips only if AI-assisted medicine will bring humans to the longevity escape velocity.
The engineers of the trip could design the spacecraft to be made of sufficiently strong materials that survive horizon crossing. A ten-meter scale spacecraft made of steel would have the material strength to withstand the gravitational stress exerted on it at the horizon of a 33 solar mass black hole, like Gaia BH3.
Under such circumstances, the trip to the black hole can end with a magnificent spectacle of gravitational lensing and gravitational time dilation, as the courageous passengers engage in a one-way visceral plunge into the horizon, from where there is no return.
Contemplating a journey to a black hole as a thought experiment brings along a practical benefit. As the Dalai Lama noted: “Analysis of death is not for the sake of becoming fearful but to appreciate this precious lifetime.”
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.
