Below is the transcript of a wide-ranging interview I had before sunrise this morning.
Where did you grow up as a child and what got you interested in astrophysics and science in general?
I grew up on a farm in a village, Beit Hannan, about 20 kilometers away from Tel Aviv, Israel. As a child, I collected eggs from our 2,000 chickens every day, and drove a tractor to the hills of the village to read philosophy books surrounded by nature. I was interested in the most fundamental questions about human existence. I was drawn to existentialism and read many of the books of Jean Paul Sartre, Albert Camus, and Friedrich Nietzsche.
Given the obligatory military service at age 18, I joined an elite program called “Talpiot” which enabled me to complete a PhD in physics at age 24 from the Hebrew University in Jerusalem in 1986. Between 1983–1988, I led theoretical work on the first international project supported by the Strategic Defense Initiative, dubbed “Star Wars”, of President Reagan. This brought me to visit the U.S. frequently and in one of the visits I was offered a 5-year fellowship at the Institute for Advanced Study, Princeton, under the condition that I will switch my research focus from Plasma Physics to Astrophysics.
In 1993, I was offered a junior faculty position at the Harvard Astronomy department, where I was tenured three years later, after receiving a tenure offer from Cornell. I became chair of the Harvard Astronomy department in 2011 and served three terms until 2020. Since 2005 I am also serving as director of the Institute for Theory and Computation at Harvard. Currently, I am the Baird Professor of Science and head of the Galileo Project. During the past forty years, I wrote more than a thousand scientific papers and eight books during my scientific career. In Washington DC, I served as a member of the President’s Council of Science and Technology in the White House and chaired the Board on Physics and Astronomy of the National Academies (2018–2021).
For additional biographical details, click here.
When thinking about our universe, and everything contained in it, it is very hard to comprehend all of the pieces and how they relate to each other.
Indeed, cosmic history is marked by a transition from simplicity to complexity. The Universe started from a hot soup of elementary particles which was uniform to one part in a hundred thousand. The initial conditions of the Universe at a fraction of a second after the Big Bang, can be summarized statistically on a single sheet of paper. Why did complexity emerge out of these initial conditions? The reason is gravity.
The small primordial fluctuations in the density of matter caused regions that were slightly denser than average to eventually collapse upon themselves. They behaved as over-dense mini-universes which end up in a Big Crunch because their gravitational binding energy exceeds their expansion kinetic energy. This led to the formation of galaxies like the Milky Way, inside of which stars like the Sun were born, around which debris fragmented into planets like the Earth, on the surface of which life-as-we-know it emerged. Today, the universe is so complex that all the books in the world cannot summarize it. This complexity was enabled by gravity.
An often-overlooked point is that we owe our existence to dark matter. This is because ordinary matter was tightly coupled to radiation in the first 400,000 years after the Big Bang. This coupling smoothed out the initial density fluctuations on the scale of galaxies because of radiation diffusion. However, dark-matter maintained memory of the density fluctuations on galaxy scales, because it was not coupled to the radiation. The formation of galaxies was seeded by dark matter. We would not be here without dark matter.
What is dark matter? We still do not know, despite nearly a century of extensive research.
How can you definitely prove that the big bang is 13.8 billion years old given the fact that the universe is expanding?
The initial density fluctuations imprint brightness fluctuations in the cosmic microwave background, left over from the early Universe. Based on the patterns of these brightness fluctuations, we can infer the size of the cosmic horizon at the time that the Universe became transparent, 400,000 years after the Big Bang. It is roughly a degree in the sky, double the angular diameter of the Moon. The amplitude of the brightness fluctuations as a function of angular scale allows us to fit for cosmological parameters and infer the current age of the Universe, 13.8 billion years.
We observe galaxies receding away from us at a speed that increases with distance. The situation is similar to a rising cake with embedded raisins. The raisins recede away from each other as the cake rises. Similarly, galaxies recede from each other as space expands. Einstein’s theory of gravity describes this behavior. The energy densities in the matter, radiation and vacuum that fill up the Universe, dictate its rate of cosmic expansion.
In 1998, two teams of astronomers discovered that the expansion of the Universe had been accelerating in the last third of cosmic history. This is caused by the vacuum energy density, the so-called cosmological constant or dark energy. Here again, we do not know the nature of dark energy.
As we reverse cosmic history and go back in time, the Universe gets dense to a point in time where the density is infinite. This time is called the Big Bang. We do not know what happened before the Big Bang. For that, we need a predictive theory that unifies quantum mechanics and gravity.
Would you be opposed to the idea that “a creator” would want to continue to thwart the discovery of anything outside of our reality?
An advanced quantum-gravity scientist could come up with a recipe for creating a baby Universe. It is possible that our Universe was created by a scientist in a white lab coat. This possibility gives a scientific context to the concept of God.
Do you feel that SETI and SETI-at-Home have been effective in their research?
The public is definitely intrigued by the possibility that some Unidentified Anomalous Phenomena (UAP) near Earth may reflect extraterrestrial technologies. Surely, members of academia are drawn from the public, so deep down many of them must share this sentiment. However, because of stigma they never discuss it openly. Paradoxically, the stigma is promoted by members of the SETI community who banned discussions about UAP in their conferences.
SETI focused on detecting electromagnetic signals. The approach is similar to waiting for a phone call. It is a longshot since nobody may call you while you are waiting. A century is just one part in a hundred million out of the age of the Milky-Way, and radio signals sent a million years ago are a million light years away. A better approach is to search for packages in our mailbox. They can be found even if the sender is dead. NASA launched five interstellar probes at a speed that is ten times slower than needed to escape from the gravitational pull of the Milky-Way. Hence, these probes will remain bound to the Milky-Way. I calculated in a recent paper that Voyager will be on the opposite side of the Milky-Way disk in a billion years. In reciprocity, we can search for space trash of other civilizations in our backyard. Their trash would be our treasure.
Tell us more about the Black Hole Initiative?
In 2016, I served as the founding director of Harvard’s Black Hole Initiative. This is the primary center worldwide for the study of black holes. It brings together astronomers, physicists, mathematicians and philosophers. The first images of black holes were obtained at the conference room of this center. Stephen Hawking visited us for the inauguration festivities.
What do you think about Elon Musk and SpaceX’s plans to build a base on the moon, and for a human to reach Mars safely?
I think it is an excellent plan since we should minimize the existential risk from a single-planet catastrophe. Recently, I came up with a suggestion to Elon Musk of using the moon Phobos as an entry point for SpaceX to Mars.
What are your thoughts about what people believe to be “ghosts” and the idea of a multiverse?
Life is short, so we better focus our time on finding tangible evidence that will guide us through our scientific learning experience. Reliable data, collected by well-calibrated instruments, is key. Reality is under no obligation to make us happy.
I understand that you have a Netflix documentary that will be released in 2025. Congratulations. Tell us more about that.
A Netflix filming crew followed me in my work over the past few years, including the expedition to the Pacific Ocean. They documented how science can be exciting when exploring the unknown.
It is arrogant to think that we are the most intelligent species ever to exist in the ten billion year history of the Milky-Way. Our galaxy contains hundreds of billions of stars and most of them formed billions of years before the Sun. Surely, there was someone out there from whom we can learn how to do better.
On a personal note, and not taking into consideration your occupation, what hobbies do you have? What do you enjoy doing in your spare time?
I jog every morning at sunrise, in the company of birds, wild turkeys and rabbits. It is 5 AM right now and I really need to go out. It was a great pleasure answering your questions but nature is calling me out now.
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.
