In 1935, Albert Einstein and His postdoc Nathan Rosen found a hypothical mathematical spacetime structure that bridges between two separate regions of space, potentially allowing a shortcut between these regions. Travel through the throat could be much faster than travel in the familiar external spacetime that connects them. This is an example of a general class of spacetime structures with two mouths connected by a throat, known as `wormholes.’ In 1962, Robert Fuller and John Wheeler demonstrated that the Einstein-Rosen wormhole is unstable and would pinch off before any particle traverses it.
Subsequently, Stephen Hawking, Kip Thorne and collaborators showed that exotic matter with a negative mass (energy) density could stabilize a wormhole and make it hypothetically traversable. Such a substance is known to exist as the `dark energy’ whose repulsive gravity causes the accelerated expansion of the Universe. However, to construct a wormhole one would need to excavate dark energy from the cosmic reservoir and mold it to the shape of a wormhole. We do not know if that is possible since the nature of dark energy is unknown.
A traversable wormhole would allow an advanced civilization to travel back in time. This is because time progresses differently inside and outside the wormhole. As a result, synchronized clocks at either end of the wormhole remain synchronized for an observer passing through the wormhole. Imagine observers at one end of the wormhole that encountered time dilation and aged less by moving around or temporarily visiting a gravitational potential well. Such observers would be able to connect to the older end of the wormhole at the same time, and allow the older version of themselves to meet their younger selves. This constitutes a time machine from the point of view of an outside observer.
In 1992, Stephen Hawking proposed the `chronology protection conjecture,’ arguing that the laws of physics prevent time travel, making the Universe safe for historians. Since we do not have a predictive theory that unifies quantum mechanics and gravity, we do not know whether Hawking’s conjecture is true. If it is, then traversable wormholes cannot be constructed by advanced extraterrestrial civilizations.
This set of arguments suggests that if we ever discover alien visitors near Earth that took advantage of wormholes to travel faster than light, we would know that Hawking’s conjecture is wrong and that time travel is possible. Their scientific feat would have major implications to the theory we develop for quantum gravity. It will also raise ethical questions, such as whether we should request access to their time machine and go back in time to kill Adolf Hitler before the Holocaust. Such an act would allow me to bring to life 65 members of my father’s family who died in Nazi concentration camps.
Having access to a time machine as a tool for correcting human history is one possible benefit of encountering alien quantum-gravity gadgets. Another would involve travel to distant locations in the cosmos within a human lifetime through a wormhole. Which one should we choose to use first? My personal priority would be to fix human history before embarking on interstellar trips. This is because we know what needs to be fixed in our past but we do not know which interstellar destination is worthwhile.
Encountering the technological products of advanced extraterrestrial scientists could be very helpful to our own scientific progress. We discovered quantum mechanics and General Relativity only a century ago, and we have a lot to learn.
Last week, I had a delightful dinner in the company of string theorists from Harvard’s Black Hole Initiative. Three of them sat next to me, and so I could not resist the temptation of asking them: “In case we discover a mini black hole in the solar system and experimentalists will be able to access it and test quantum gravity theories like string theory, would you encourage full funding for the experimentalists to pursue this task even if the results could potentially invalidate your lifelong work on string theory?” They all replied: “Absolutely, yes.” I therefore noted: “Well, that demonstrates that deep inside you are real physicists and not mathematical physicists.”
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.
