Quantum mechanics as-we-know-it predicts that the vacuum energy density, the so-called `vacuum zero-point’ or `dark energy’, should have been much larger than the value inferred from the expansion rate of the Universe. If the value had been indeed much higher, as expected, the Universe would have expanded too rapidly for galaxies like the Milky-Way to form and give birth to stars like the Sun, and for us to have fun under the Sun.
The measured mass density of the vacuum based on the accelerated cosmic expansion, 6.5×10^{-30} grams per cubic centimeter, is lower than air density by 26 orders of magnitude. The vacuum energy is so tiny that even if we were to harvest it in full across a huge cube which is 20 kilometers on a side — as long as Manhattan Island and as thick as twice the maximum altitude of commercial airplanes, and convert all the dark energy in this volume into electricity, it would have been able to power merely a single 100-Watt light bulb for less than a minute. Despite claims to the contrary, the vacuum energy density is so dilute that it cannot power alien craft across our sky. There is a limit to how much we can contemplate technologies that go beyond the standard model of physics. Energy conservation must be satisfied.
Some speculators argue that the Casimir effect provides evidence for the potential of advanced technologists to tap the energy of the vacuum. The effect results from placing two conducting plates parallel to each other and excluding electromagnetic vacuum fluctuations with wavelengths larger than their separation. As a result of the boundary conditions imposed by the plates, the vacuum energy density is lowered between them, giving the illusion that one can manipulate the vacuum to artificially create a negative mass. However, the energy associated with the conducting plates far exceeds the energy density of the vacuum between them, implying that the total energy of the system is positive. No physics can give rise to a negative mass object, as far as we know.
If it was possible to manufacture a negative mass, we could have had access to novel propulsion systems that travel faster than light and as a bonus — we could have also built a time machine. Therefore, we can confidently say that no Jewish scientists will be able to travel through a time machine in our future, or else they would have traveled back to Germany before World War II and killed Hitler to save the lives of six million Jews that he authorized to kill.
This conclusion was reinforced in a conversation I had last night with the brilliant Princeton physicist, Juan Maldacena, at the annual conference of Harvard’s Black Hole Initiative, for which I served as the founding director a decade ago. Juan provided multiple reasons for why negative masses do not exist in nature. They would have violated the null energy condition, allowed faster than light transmission of information and created untenable pathologies in quantum field theory. In the absence of negative masses, there is no known path to creating stable traversable wormholes, time machines, or faster-than-light propulsion systems.
But even without these exotic extensions of the standard model of physics, we must seek evidence for extraterrestrial technologies. They might extend well beyond our capabilities and provide us with new insights about what is possible. For example, our rockets never exceed a speed beyond 0.01% of the speed of light, so there is plenty of room for improvement in our propulsion technologies by orders of magnitude. At dinner, I debated the gifted MIT physicist, Daniel Harlow, about the low prior probability that he assigns for extraterrestrial gadgets arriving near Earth. I pointed out that without data collection and analysis, we will never discover something new. The search for near-Earth artifacts of interstellar origin requires a shift in the funding priorities of mainstream astronomers who are focused on the search for microbes. The only way to make this shift is by revising our priors for the possibility of detecting alien technologies. This will be money well spent. The artificial intelligence (AI) software and advanced sensors that will be developed through this search for alien artifacts could also be used for national security purposes by the U.S. Department of Defense. As I argued in the U.S. Congress on May 1, 2025, the investment of resources in the search for advanced technological gadgets near Earth is a win-win proposition as it will serve both national security and frontier science. The history of science reveals that our most useful discoveries were made possible by allowing scientists to explore the unknown.
This morning, I received an email from Emanuel Rozik, founder of the Dyson Swarm Corporation, who wrote: “I’m working on a long-term initiative called Dyson Swarm Corp — a speculative but structured roadmap to build the first Dyson Swarm infrastructure around the Sun within the next 150–200 years.” In reply, I wrote: “Thank you for the inspiring vision. The main engineering challenges for constructing a Dyson sphere involve the required mechanical strength of its materials, the likely damage by the impact of micrometeorites which are thirty times faster than a bullet in the vicinity of Earth (as the Webb telescope which is only ten meters in size was impacted multiple times in recent years), the dynamical stability of the sphere to perturbations by solar flares, and the assembly of the required materials on such a large scale. There is discussion of some of these issues in various papers, including here and my paper here.”
New physics beyond the standard model must exist, as we do not know the nature of dark energy and dark matter. But whether this knowledge is helpful in designing vehicles for interstellar travel remains to be seen. We need to look up in order to find out. After all, the sky’s the limit.
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.
