The philosopher René Descartes published in 1637 his first philosophical principle which in Latin reads “cogito, ergo sum,” meaning “I think, therefore I am,” followed in 1641 by “ego sum, ego existo” namely “I am, I exist.”
Remarkably, it is possible to exist without much thinking. Those who spend their life on social media often believe a different variant: “I read about things, therefore they exist.” But the truth is that the world realizes only a small fraction of what is possible on the internet, and what is realized does not necessarily follow what we imagine it to be. The difference between our thinking and reality makes science a learning experience. Let me use two examples to illustrate the humbling scientific experience of learning from evidence rather than from our imagination.
When quantum mechanics was discovered a century ago, the response of prominent theoretical physicists, including Albert Einstein, was: “who ordered this?” We still do not have an intuitive understanding of the quantum measurement process through which the probability function of a system is reduced to a specific outcome, nor do we understand quantum entanglement — which Einstein referred to as “spooky action at a distance.” The human body contains numerous elementary particles whose quantum uncertainties average out. This allows classical physics to describe our daily experiences and makes it difficult to understand the quantum world. In the language of Richard Feynman’s path integral, the quantum world involves a sum over all possible paths that a system can take between its initial and final state. The classical physics limit involves the path of least action.
In two weeks, I will host a reception for the beginning of the academic year at my home, in my role as director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics. My colleague, Mark Reid, emailed me his regrets: “I had planned to come to the reception, but I will be in Madrid that day, on my way to a meeting on the Hubble tension in Barcelona.” In my reply to Mark, I admitted: “I truly regret that you are a classical object, because in the quantum world your wave function could have overlapped with my home at the same time that you are visiting Spain. On the other hand, communicating with you would have been very frustrating if you were a quantum object, because the quantum uncertainty principle would have meant that I can never get a straight answer from you. The experience would have been as bad as speaking to a politician.”
The second example for the learned difference between reality and our imagination involves the habitability of stars. Naively, we would have assumed that life is possible around any star. In that case, the most common stars are red dwarfs, having about a tenth of the mass of the Sun but an abundance that is an order of magnitude larger.
However, observational data indicates that life is not easy near these abundant stars. Dwarf stars are much fainter than the Sun and so their habitable zone is much closer-in than the Earth-Sun separation. A rocky Earth-mass planet with an atmosphere needs to be closer to the nuclear furnace in order to maintain liquid water and the chemistry of life-as-we-know-it on its surface. For example, our nearest neighboring star, Proxima Centauri, has 12% of the mass of the Sun and 0.16% of the luminosity of the Sun. The habitable zone distance scales approximately as the square-root of the star’s luminosity, and is therefore 25 times closer in comparison to that for the Sun. As it turns out, this dwarf star hosts an Earth-size planet, Proxima b, at about that distance.
The proximity of a habitable planet to a dwarf star has two implications. First, the planet becomes tidally locked, having a permanent dayside and nightside — similarly to the Moon showing the same side toward Earth throughout its orbit. The dayside is hot and the nightside is cold. When I mentioned this to my daughter, she said that if we ever move to Proxima b, she wants us to have a house on the strip that separates these two sides where the climate is most comfortable and we can see Proxima’s sunset forever. I pointed out to her that there may be strong winds between the two sides because of their different temperatures.
Another challenge for life is the strength of the stellar wind at a close-in distance, resulting in stripping the atmosphere of the planet. If that happens, liquid water will evaporate and the planet will become a desert like Mars after the loss of the Martian atmosphere 2 billion years ago. On top of that, dwarf stars have strong flares in UV and X-rays that can further strip any remnants of the planet’s atmosphere.
Finally, the surface temperature of dwarf stars like Proxima Centauri, is half that of the Sun, peaking in the infrared. This might suppress photosynthesis except during UV flares.
Ocean worlds are not helpful because life-as-we-know-it requires the interface between land and water. Puddles that dry up tend to concentrate chemical nutrients. In addition, a recent study suggested that rain droplets might have given rise to the earliest biological cells on Earth, and this is a phenomenon that requires landmass.
Dwarf stars live much longer than the Sun. Their lifespan is up to ten trillion years — a thousand times longer than that of the Sun. If intelligent life had been common around dwarf stars, we would have been most likely to live in the future, as I argued in a paper with Rafael Batista and David Sloan.
The above-mentioned challenges to habitability may explain why we reside at the current cosmic epoch on a planet near a rare star rather than being near the most abundant stars in the future.
All in all, scientific reasoning in astronomy and physics can be summarized by the phrase: “I think, therefore I understand why I exist.”
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.
