Two decades ago, I gave a colloquium about the then-new-frontier of “Imaging Black Holes.” This was a decade before I served as the founding director of Harvard’s Black Hole Initiative (BHI), inside of which the Event Horizon Telescope (EHT) collaboration was nucleated. The distinguished black hole scientist, Jacob Bekenstein, approached me after my 2005 colloquium. He was not impressed by my enthusiasm and asked bluntly: “Besides the circumstantial patterns of the turbulent accretion flow around black holes, will we actually learn anything fundamentally new from imaging black holes that we do not already know?”
Two decades later, on Monday, January 27, 2025 @ 11AM EST, I will be giving a BHI colloquium (accessible online here) on fundamental aspects of black holes which were not discussed before. One of them, the quantum-mechanical aspects of accretion onto mini black-holes, was posed in 1972 as a research problem to Jacob Bekenstein by his PhD advisor, John Wheeler, but he was distracted by other problems and never published a solution to it. I published the solution a few months ago, followed by six other papers about new aspects of black hole physics. If Bekenstein were alive today, I would have loved to hear his comments on my forthcoming BHI colloquium.
Bekenstein was justified in his criticism. Most of the time, scientists digest past knowledge and add nuances to what is already known. This offers a path of least resistance for academic promotion, since the bricks that pave this path are conventions accepted by the community at large. The incremental improvement through small steps, even if multiplied by the work of thousands of practitioners, gradually leads to a local peak in our knowledge — where the community eventually settles. But to become aware of a higher global peak far away, requires big jumps. Exploring longshots implies taking risks, because a large jump often lands at a meaningless spot in the landscape of possibilities. There is a huge number of possibilities stemming from wishful thinking for any fact we learn about the actual physical reality that we all share. The purpose of tenure in academia is to encourage scholars to take risks in big leaps of knowledge, which they rarely take.
Much of my research is defined by the unknown. When I started my career in cosmology at Harvard 32 years ago, I wrote a paper with my PhD student at that time, Daniel Eisenstein, on a risky model to explain the birth of supermassive black holes that power quasars. We proposed that quasar seeds are likely to form in compact galactic disks which are low-spin outliers of the entire galaxy population at early cosmic times. Now, the Webb telescope discovered evidence for Little Red Dots (LRDs), namely compact galactic disks which are interpreted in the latest analysis as the birth sites of supermassive black holes. I pointed out this connection in a recent research note, but I find it natural that our theoretical proposal was so much ahead of its time that it was forgotten by the discoverers of LRDs today.
If theoretical insights are often forgotten after a few decades, what is the actual reward for publishing them? Based on my personal experience, the genuine reward does not stem from recognition in the form of awards or honors but mainly from the pleasure of learning something new that nobody else figured out before. Taking risks in research projects that were never attempted before is a lot of fun. The pleasure is uninterrupted if one avoids social media as I do. It is fun to witness the imaging of black holes by the EHT and the discovery of LRDs as birth sites of quasars, validating ideas that I personally advocated for long before they became popular.
What is the next thrilling innovation on my agenda? I am currently leading the Galileo Project, in search of technological artifacts near Earth from alien civilizations.
The discovery of alien intelligence (A.I.) would constitute a double win. Not only will it inform us that we are not alone, but it could also educate us about unknown science and technology.
Let me illustrate that with an example. Cosmological data indicates that the cosmic expansion is accelerating because the vacuum has some energy per unit volume. One may wonder whether this “dark energy” can be used by a propulsion system that does not require fuel. Today, the brilliant Professor Yuval Dagan from the department of aerospace engineering at the Technion visited my office at Harvard University to discuss the possible use of vacuum fluctuations for propulsion.
It is well known that two parallel perfectly conducting plates attract each other through the Casimir effect. The electric field vanishes on the surface of a perfect conductor, because it gets cancelled promptly by electric currents. The resulting zero-field boundary conditions on the two plates reduce the available modes of electromagnetic vacuum fluctuations in between them to wavelengths shorter than the plates separation. If the two plates are free to move, the Casimir force produces work by bringing them together until they collide. After the collision, they might bounce elastically or stick together — depending on the level of dissipation in the material that makes them.
The Casimir force is the source of the work done. If the plates bounce, they would subsequently recede away from each other with no change in net momentum. But even if they stick, the final product would conserve the initial momentum, including the momentum carried by the radiation produced during the collision. The center of mass of the entire system will not move, since there is no external force acting on the system. Indeed, a uniformly distributed vacuum has no preferred direction. The accelerated expansion of the Universe is the same in all directions with the net momentum canceling out on average. Nevertheless, in the presence of dissipation, the Casimir force can produce heat that would supply a finite energy reservoir for propulsion.
If we could bottle dark energy, propulsion by the repulsive gravity of the vacuum would be much more effective. Whether that is possible could be discovered theoretically or through the Galileo Project finding evidence for unusual propulsion of extraterrestrial objects in the sky. The sky’s the limit!
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. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
