Imagine launching a spacecraft at a substantial fraction of the speed of light, equipped with a perfect clock and a laser that transmits pulses towards Earth periodically. The precise arrival times and directions of the pulses would inform us of the location, velocity and acceleration of the spacecraft as it moves away.
According to Albert Einstein’s Theory of General Relativity, gravity is the curvature of spacetime. The spacecraft would surf on a curved spacetime like a surfer riding on the surface of a choppy ocean, and its motion can be used to measure the underlying curvature. What could we infer from the pulse arrival times and directions?
Einstein’s equations relate spacetime curvature to the mass distribution of the matter sourcing it. Initially, the gravitational acceleration of the craft will be dominated by Earth. After escaping Earth, gravity will be dominated by the Sun until the craft will exit the outskirts of the Oort Cloud at a few hundred thousand times the Earth-Sun separation, a distance at which the tidal acceleration from nearby stars or the Milky Way galaxy is as strong as the Sun’s gravity.
If the spacecraft passes along its path near unknown massive objects, like Planet Nine, we could detect them based on the excess gravitational acceleration they induce. In particular, if the dark matter is composed of compact objects, such as primordial black holes, the craft’s trajectory could be deflected by any of them that happen to lie along its path. Conversely, the absence of such deflections would constrain the abundance of dark objects and the overall clumpiness of dark matter. In a paper that I wrote with Thiem Hoang, we showed that electromagnetic forces must be mitigated in order to reach the needed precision.
Of course, dark matter may be a figment of our imagination. Instead, the nature of gravity or inertia might be modified at low accelerations. The precise trajectory of the craft beyond the Oort Cloud could inform us whether dark matter exists or Modified Newtonian Dynamics (MOND) rules Galactic dynamics.
If the precision of the craft’s clock is good enough, it might be possible to detect a passing gravitational wave as a periodic variation in the craft’s location relative to us. The larger is the craft’s distance, the longer is the wavelength of the gravitational radiation that it can probe. An array of many crafts would allow localization of the source. Astronomers are using pulsar timing arrays for the same purpose, but their precision is limited by the uncertainties in the locations of these pulsars.
Within a distance of hundreds of light years from the Sun, the matter density receives comparable contributions from visible and dark matter. But farther away from the Milky-Way center, the dark matter dominates gravity.
The mass budget changes again beyond a few million light years, where the dark energy will cause the craft to accelerate away from Earth at an ever-increasing speed. The recession speed will reach the speed of light as a result of cosmic acceleration within about a hundred billion years of travel time. At that time, the separation in arrival times between pulses will grow and the last pulse will fade and freeze forever. That last pulse represents the time at which the craft reached the speed of light and left our cosmic horizon. The continuing cosmic acceleration would remove the craft from any causal contact with us. Beyond that time, we will stop receiving any additional information about the craft. The experience of indefinite isolation would resemble that encountered by a craft falling into the horizon of a black hole.
Altogether, this futuristic probe could inform us about the composition of the Universe for a hundred billion years. In less than a tenth of that period, the Sun will exhaust its nuclear fuel and its core will condense into a white dwarf, a cold, Earth-size metallic. Before terrestrial life is extinguished by the Sun’s evolution, humans can relocate to the habitable zones around the most common stars with about a tenth of a solar mass. An example from this population is the nearest star to the Sun, Proxima Centauri, which hosts a planet in its habitable zone. The most abundant stars with 7% of the solar mass, will continue to burn their nuclear fuel for 10 trillion years and serve as natural furnaces to maintain longevity of the human species for a thousand times longer than the Sun.
The Breakthrough Starshot Initiative, for which I chair the scientific advisory board, aims to launch a light sail at a fifth of the speed of light out of the Solar system. The original motivation for the project was to carry a camera that would image a habitable-zone planet like Proxima Centauri b and check whether it hosts life.
If Starshot’s vision is realized, we could learn about the composition of the physical Universe in addition the existence of life beyond Earth. The goal of the former search could benefit from the latter search. Most stars formed billions of years before the Sun. Thus, if we encounter intelligent civilizations, we could check whether they have already launched a probe that informed them about the composition of our cosmic environment. This will save us the time required for figuring out the answer ourselves.
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 chairs the advisory board for the Breakthrough Starshot project, and 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.
