In anticipation for my next public lecture, the organizer requested the title of my lecture. I suggested: “Hunting for Aliens.” The organizer expressed concern that some members of the audience might confuse me for a U.S. government employee in search of illegal aliens near the southern border wall. I explained that no two-dimensional wall erected on Earth would protect us from extraterrestrials because they will arrive from above. It is just a matter of time until we notice interstellar travelers arriving without a proper visa. A policy of deporting them back to their home exoplanet will be expensive — over a billion dollars per flight. The trip will also take a long time — over a billion years with conventional chemical propulsion. We will have to learn how to live with these aliens, and promote diversity and inclusion in a Galactic context.
The Sun formed in the last third of cosmic history, so we are relatively late to the party of interstellar travelers. Experienced travelers might have been engaged in their interstellar journeys for billions of years. To properly interpret their recorded diaries and photo albums in terms of the specific stars they visited, we would need to accurately interpret their time measurements.
Imagine an interstellar tourist wearing a mechanical analog watch. Such a timepiece is at best accurate to within 3 seconds per day, or equivalently 30,000 years per billion years. This timing error is comparable to the amount of time it takes to hop from one star to another with chemical propulsion. Interstellar travelers must wear better clocks in order to have a reliable record of time.
Our best atomic clocks are precise to a fractional uncertainty of about a billionth of a billionth. They employ the natural transition frequency of atoms, such as ytterbium. Higher-frequency transitions enable greater precision. This was recognized in a recent paper, which suggested the use nuclear transitions in future clocks.
What is the ultimate precision attainable by clocks? In the context of our current understanding of quantum-mechanics and gravity, there could be no transition between discrete quantum states in which the emitted particle carries more than the Planck energy. Dividing this maximum energy by Planck’s constant yields the maximum frequency of any clock. This suggests that no clock could employ a timing cycle shorter than the inverse of the Planck frequency, 5.4×10^{-44} seconds. The age of the Universe is 8×10^{60} Planck units. Since each clock cycle provides an independent measurement, the Central Limit Theorem in statistics implies that the temporal precision of a clock could improve inversely with the square root of the number of cycles. A Planck clock operating over the entire cosmic history would achieve at best a precision of 4×10^{-31} Planck times or 2×10^{-74} seconds. This is good enough for all practical purposes of interstellar travel.
So far, we assumed that time is continuous as postulated in modern physics. But what if time is realized as a series of discrete points? We would not notice the difference between these cases if we use a clock that cannot resolve the discreteness of time because its inherent timing errors introduce fuzziness that makes the discrete time steps appear as a continuous sequence. When probing a collection of atoms with a poor spatial resolution, much larger than the separation between the atoms, the system appears as a continuous fluid. When watching a movie with a frame rate that exceeds the sampling rate of our visual sensory system, the movie appears continuous. In reality, the movie is a collection of snapshots.
Is reality a collection of snapshots?
Since the speed of light is a universal constant, discreteness in time translates to discreteness of space. The spatial pixel length is the temporal pixel duration times the speed of light.
Computer simulations adopt discrete pixels in space and time. If our world is a computer simulation, as suggested in 2003 by the philosopher Nick Bostrom, then improved clocks could eventually resolve the discrete time intervals of the simulation. Precise clocks would detect a physical reality composed of snapshots, in which nothing happens between discrete time steps.
A movie watcher with an exceptionally high sampling rate will see a sequence of snapshots. For the same reason, future clocks could reveal experimentally that we live in a simulation. But winning the Physics Nobel Prize for such a discovery will not be particularly satisfying, since the Prize ceremony is part of the simulation and not real in any meaningful way. The discovery must have been choreographed by the simulation creator.
Here’s hoping that our interstellar visitors will carry a Planck clock which would demonstrate to the best possible precision that time is continuous and that we do not live in a simulation. In that case, the reality of their visit would feel much better than a Hollywood script for an artificial sequence of snapshots.
But there is also a cultural nuance to our future encounters with aliens. The way we express the age of the Universe as 13.8 billion years reflects our anthropocentric worldview, in which we document cosmic history based on the orbital period of our home planet around the Sun. Aliens might use the orbital period of their home planet. Just keep that in mind when scrolling through their photo albums in chronological order.
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.
