Our Cosmic Cradle Allows Us to Become Interstellar Gardeners

Avi Loeb
5 min readApr 13, 2024
An image of the hydrogen atom. (Credit: A. Stodolna et al., Physical Review Letters)

We should not take our cosmic cradle for granted. A multitude of coincidences should make us happy.

Fundamental constants, such as the charge and mass of the electron, e and m_e, the proton mass, m_p=(1836*m_e), and Planck’s constant, hbar=h/(2*pi), combine to give the mean speed of the electron in the ground state of the hydrogen atom. The ratio between this speed and the speed of light, c, equals the so-called fine-structure constant, alpha=(e²/hbar)=1/137. The binding energy of the electron divided by the total mass of the hydrogen atom, (m_p+m_e)=1837*m_e, implies that the maximum velocity that the atom can get by using this much energy is smaller than alpha times the speed of light by the square root of 1837. This limit translates to 50 kilometers per second.

Chemical fuel releases the binding energy of its atoms or molecules. The released energy per unit mass cannot exceed the above value for the hydrogen atom, which is the lightest atom in nature.

As a result, the exhaust speed of chemical rockets is capped at 50 kilometers per second. The rocket equation allows for a gain in terminal speed equal to the natural logarithm of the ratio between the initial (fuel plus payload) and final (payload) mass. However, under realistic circumstances, this logarithmic gain compensates for the reduction in the exhaust speed as a result of the use of heavier elements than hydrogen in the fuel chemistry. This explains why all chemical rockets from Sputnik to Starship reach a terminal speed below 50 kilometers per second.

Why should this realization make us happy?

First, the speed limit on chemical rockets is close to the value needed to escape from the habitable region around the Sun, about 42 kilometers per second at Earth’s orbit. Taking advantage of the motion of the Earth around the Sun at 30 kilometers per second, the extra boost needed for leaving the solar system can be obtained by chemical propulsion. This is why human-made rockets can reach interstellar space. We should be grateful for that.

So far, we launched 5 spacecraft that will all turn to space trash by the time they exit the solar system. But we can do better with long-lived functional probes navigated by Artificial Intelligence (AI) in the future.

Second, we live at the end of the cosmic era during which chemical propulsion would have allowed spacecraft to arrive at the Milky-Way from neighboring galaxies. In a recent paper, I showed that in the future of our accelerating Universe, chemical rockets will not be able to reach galaxies beyond our nearest neighbor, Andromeda. This nearby galaxy, as I showed in a paper with T.J. Cox, will merge anyway with the Milky-Way in a few billion years.

This makes the present cosmic time special. It is the latest time when our Milky-Way galaxy could have been visited by chemical rockets from other galaxies. We should be grateful for that as well. This timing coincidence involves the value of the cosmological constant (dark energy) which triggers the accelerated expansion of the Universe. Interestingly, Mark Hertzberg and I suggested in a recent Physical Review paper that the cosmological constant may also be related to fundamental constants, such as the electron charge and mass, the speed of light, Newton’s constant and Planck’s constant.

The human species originated on Earth merely a few million years ago. This is the time it takes chemical rockets to traverse the distance from the nearest million stars. The journey of chemical rockets from one side of the Milky-Way disk to the other, takes a billion years and could have been traversed as well since most stars are older than the Sun by a few billion years. Just as the Woolly Mammoth is now being revived using elephant stem cells, it is possible that extraterrestrial intelligence was introduced into chimpanzees as a result of a chemical rocket visiting Earth a few million years ago by intelligent entities from another star.

If interstellar gardeners are watching the fate of their terrestrial seeds of intelligence, are current affairs making them happy or sad? Those gardeners who intended the seeds to blossom, must be disappointed by the focus of humans on conflicts and destruction.

There are galaxies much larger than the Milky-Way where the travel time among most of their stars is ten times longer than within the Milky-Way, making it more difficult for “interstellar gardeners” to spread life by using chemical rockets for transportation. This should make us grateful for our fortunate cosmic cradle for the third time.

Altogether, we are lucky that the fundamental constants ruling chemistry and gravity allowed habitable zones to share life and intelligence between the stars. These circumstances offer us the opportunity to take advantage of our cosmic fortunes and venture out of the solar system towards distant stars.

Humans should not only become a multi-planet species, as advocated in an Astrobiology paper by Elon Musk. Humans should aspire to become interstellar gardeners.


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".