Some of us lose sleep over looming existential risks, such as devastating pandemics from unintended laboratory leaks in gain-of-function experiments, ignition of a global nuclear war if atomic weapons will reach the hands of terrorist groups, irreversible climate change, catastrophes triggered by artificial intelligence, asteroid impacts, or ultimately — the inevitable brightening of the Sun.
Given the depressing geopolitical news these days, it is my great pleasure to be the bearer of some good news: there is no reason to be concerned that we might be killed by a black hole any time soon. Phrased in practical terms: insurance companies do not need to include this risk in their portfolios or annual policies.
A black hole is the ultimate prison, offering an unavoidable death sentence to any prisoner on a free fall towards its center of gravity. Anyone entering this prison walls, marked by the so-called event horizon, is doomed to be shredded by a diverging gravitational tide near the center. Could a black hole seed encounter the Earth and grow by consuming all of us?
I was asked this question a few days ago by a group of brilliant students during a visit to the elementary school at my childhood village in Israel. In reply, I went over the various types of black holes that one may worry about.
First, consider colliders that smash particles at high energies. Microscopic black holes are expected to be produced in collisions at the Planck energy, but this scale is quadrillion times bigger than the energy attainable by the Large Hadron Collider at CERN. It is evident that we are safe from colliders since higher-energy collisions occur naturally when the most energetic cosmic-rays from the Universe strike the Earth’s atmosphere. But even if microscopic black holes would have been produced by such collisions, they would have evaporated by emitting Hawking radiation before being able to consume background matter, especially given their high speed.
Next, one may worry about primordial black holes, created shortly after the Big Bang in regions where the radiation energy density might have exceeded the critical value for collapse. Primordial black holes with less than the mass of a kilometer-size asteroid would have evaporated by now and those more massive than the Moon are ruled out as the dark matter. The remaining possibilities for primordial black holes as dark matter would have passed through the Earth without causing any damage, since their horizon size is smaller than a human hair. Moreover, given the local mass density of dark matter, collisions of primordial black holes with Earth are highly improbable during our lifespan.
But what about larger black holes which form as a result of the collapse of the cores of massive stars after they consume their nuclear fuel? These black holes are much less abundant and therefore highly unlikely to cross the Earth during the age of the Universe. Bigger black holes that grew by consuming gas from their environments are even rarer.
Finally, one may wonder whether advanced aliens might be able to transport an asteroid-mass black-hole seed and plant it intentionally at the center of Earth. An alien “gardener” can accomplish this task by electrically charging a primordial black hole and carrying it electromagnetically inside a tiny capsule. This would constitute a method for consuming planets or stars as fuel for black holes that represent efficient engines for converting rest-mass to radiation. The likelihood for this speculative risk can be assessed by searching in the sky for low-mass black holes. So far, all astrophysical searches using telescopes or gravitational-wave detectors, did not show any evidence for black holes with less than a few times the mass of the Sun. Until they do, we should not be concerned.
This is all good news, but there is also some bad news. Human society is currently at risk of being shredded by a metaphorical black hole. Its “event horizon” is marked by the ideological bubble of like-minded people who are unable to communicate effectively with the outside world.
That this “event horizon” exists in academia is evident from the latest turmoil at my home institution of Harvard University. The cultural drift to one extreme of the ideological spectrum alienated Harvard’s leadership recently from the values represented by the White House and Congress in Washington DC. The resulting tension threatens to tear apart our campus like the gravitational tide near a black hole. The risk is real: typing the wrong words on two-dimensional computer screens via social media can translate to violence in the three-dimensional reality that we all share.
That this metaphorical black hole is an acute issue became evident for me this week as I was approached by five reporters from the New-York Times, Bloomberg News, the Wall Street Journal, the Harvard Crimson, and the popular television program “Hatsinor” on Channel 13 in Israel. I explained that academia bears the responsibility for moderating societal polarization by fostering dialogues among people on opposite sides of the ideological spectrum.
As an astronomer in pursuit of a cosmic perspective, I focus on the big picture. We can avoid the cultural “event horizon” of this metaphorical black hole by encouraging, rather than suppressing, multiple viewpoints. Professors, students and speakers should not be worried about expressing deviating views, as they feel in the current Orwellian reality where they are at risk of being cancelled if they do not repeat the party’s slogan: “War is Peace … Ignorance is Strength.” Instead, it should be reasonable to argue for war on terrorism and to teach our students history as a remedy for the superficial ignorance promoted by the commercial algorithms on social media.
As responsible citizens, it is our duty to alleviate the existential risk of societal polarization. Free speech should not be given only to those who agree with the party’s slogan, especially when that slogan is stretched too far in one ideological direction and violates common sense within the society at large. Instead of “virtue signaling” to like-minded colleagues, we must listen to other opinions and understand where their advocates are coming from.
Here’s hoping that Harvard’s Corporation will take this advice to heart and avoid our free fall into a cultural “event horizon”. Our campus will not be shredded by this metaphorical black hole if we would be willing to love those who are different from us. In the words of Paula Antonelli, the curator of the Museum of Modern Art in New-York City: “Love the aliens!”
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 chairs the advisory board for the Breakthrough Starshot project, and 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.