The Nature of Time

Avi Loeb
5 min readJun 19, 2024


Avi Loeb in a Q&A session during the CISO summit in 1440 Multiversity on June 17, 2024 (Image credit: Ilan Fisher)

Today, I was interviewed for a podcast in Los Angeles about the nature of time with the brilliant hosts Jorey Worb and Devon McCutcheon. As questions were flying from both of them, the experience felt like a jazz concert.

In physics, the nature of time is very different from the nature of space because, absent a time machine, there is no way to go back in time whereas we can go back in space. Entropy grows over time according to the second law of thermodynamics. The arrow of time is also apparent through cosmic history, as the Universe started simple and developed complexity over time. The initial conditions that followed the Big Bang can be statistically summarized on a single sheet of paper. They were characterized by a uniform distribution of matter and radiation to within one part in 100,000. As a result, time progressed at the same rate everywhere based on Einstein’s formulation of gravity in terms of the curvature of spacetime. This initial state established a universal cosmic clock, allowing us to understand how the universe looked like in the past by observing radiation that was emitted at great distances from where the light travel time is up to 13.8 billion years.

The cosmic clock was initiated at the Big Bang. How time progressed before that beginning is unknown without a predictive theory of quantum gravity. Yes, our understanding of time is limited to the finite interval that elapsed since the Big Bang. If the dark energy will stay constant, we will be left lonely in an extragalactic vacuum for the foreseeable future until all the stars will burn up, ten trillion years from now. There is plenty of time to adapt technologically to this gloomy future. For now, our main existential threats are limited to Earth.

The arrow of time allows our knowledge about the world to grow but it also has unfortunate circumstances. In particular, it leads to our own death and to the loss of loved ones. The realization that we will die shapes our life. In the near future, artificial intelligence (AI) systems might offer a digital record of people who are not around anymore. This would be far more effective at communicating who we are missing than the tradition of inscribing a few words on a tombstone. It may even rise to the occasion of representing digital immortality.

Sundial. (Credit: Wikimedia)

We measure our lifespan in years, the time unit associated with a full orbit of the Earth around the Sun. This is, of course, a local convention. For example, the orbital time of the nearest exoplanet Proxima b in the habitable zone of the dwarf star Proxima Centauri, is merely 11.2 days. If intelligent beings exist on Proxima b, they celebrate birthdays 32.5 more frequently, making our lifespan about three millennia in their orbital time unit.

The notion of time relies on the fact that identical clocks would tick at the same rate at the same location, like synchronized swimmers in a pool. Synchronized clocks could tick as a result of atomic transitions which emit at a frequency far more precisely defined than the rate of chemical signals in the human brain or the rate of environmental events which define time mentally for us throughout life. The best atomic clocks reach a precision of one part in 10 to the power of 18, corresponding to a seventh of a second over the Earth’s lifetime. The best astrophysical clocks, spinning neutron stars known as pulsars, are much less precise.

Since AI systems process information much faster than the human brain, they can process information much faster. The equivalent of a human lifetime’s worth of data processing by humans might be processed in a matter of hours by a self-improving AI system in the future. What the AI system might do with that knowledge is everyone’s guess.

Our limited lifespan introduces anxiety. Since most of us live less than a century, there are milestones that we are expected to accomplish during our life: find a partner, have children and so on. Jorey and Devon asked me for advice on how to cope with these societal expectations. My advice is to always look forward. Any step along the way is an opportunity for a new beginning and we should not be trapped in the past because it limits our aspirations for what can be accomplished in the future.

Avi Loeb speaking at the CISO summit in 1440 Multiversity on June 17, 2024 (Image credit: Ilan Fisher)

I also gave Jorey and Devon the good news that we could age more slowly by embarking a spaceship that moves near the speed of light or by residing near a black hole. Even on Earth time is ticking more slowly by 56 millionths of a second per day compared to the Moon, as noted by the White House recently. But altogether, it is far more important to promote happiness and fulfill your passion irrespective of how long you live.

Jorey Worb (front left) and Devon McCutcheon (back) with Avi Loeb (June 18, 2024)

That said, my wish is to live forever. There is so much more we can do through a longer lifespan. For now, I try not to think about the end and enjoy the fun while it lasts.


(Image credit: Chris Michel, October 2023)

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.



Avi Loeb

Avi Loeb is the Baird Professor of Science and Institute director at Harvard University and the bestselling author of “Extraterrestrial” and "Interstellar".