Historically, astronomy started by observing light from sources in the sky. The reason is simple.
Survival of the fittest selected our eyes to be sensitive to optical light, made of an abundant stream of photons from a nearby star, the Sun. As these photons bounce off surfaces of physical objects on Earth, their detection helps in seeking food and triggering alerts from predators. On a clear night, our optical sensors revealed numerous other stars at great distances with no immediate practical benefits. With no distraction from electronic screens or city lights, early humans wondered about the nature of these stars and whether they affected their life. By now, we know that we are made of heavy elements fused in the core of other stars, and that many of the distant stars host planets like the Earth. If any of these exoplanets gave rise to intelligent beings, the darkness of the sky would feel more welcoming. We could learn from our neighbors and visit their homes. At such a future time, astronomical observations will resemble looking through the windows of our home at our cosmic neighborhood, and interstellar space exploration would resemble walking down the street to greet our neighbors.
But even through the limitations of remote observing, the universe offers a wealth of additional information beyond the photons visible to our eyes. In fact, not all animals have the same spectral sensitivity. For example, the eyes of the Mantis shrimp can sense ultraviolet light. If we were born on a planet near a neutron star, we might have developed X-ray eyes. This would have allowed us to see the hot X-ray gas in clusters of galaxies.
Modern astronomers compensate for the limited sensitivity of our eyes by using sensors across the electromagnetic spectrum from radio waves to gamma-rays. The lowest energy photons from outside the Solar system were detected in 1970 at a frequency of 1 megahertz by long wire-antenna during the Apollo program. Lower frequency photons are absorbed in scatterings of interstellar electrons and protons, through a process named free-free absorption. The record for the highest energy photons is still being pursued. In February 2024, the Large High Altitude Air Shower Observatory (LHAASO) discovered individual photons with an energy up to 2.5 peta-electron-Volt (a quadrillion times the energy of an optical photon), emitted by the Cygnus star forming region. Absorption of a single photon with this much energy, would kick a ping-pong ball to a speed of 2 kilometers per hour.
The highest-energy particles from the sky appear in the form of cosmic-rays made of atomic nuclei. The energy record is held by the “Oh-My-God particle” with 320,000 peta-electron-Volt, detected in 1991 by the Fly’s Eyes camera in Utah. Absorbing this energy would kick a ping-pong ball to a speed of 700 kilometers per hour, 3.6 times above the fastest table-tennis ball shot recorded in human history.
Over the past sixty years, observations of the cosmos extended beyond electromagnetically interacting particles. This started with the detection of neutrinos from the Sun and from supernova 1987A. In June 2023, the IceCube Collaboration reported the detection of high-energy neutrinos from the plane of the Milky-Way galaxy.
The LIGO experiment pioneered yet another cosmic messenger, gravitational waves. On September 14, 2015, about a century after Albert Einstein first imagined gravitational waves as ripples in spacetime, LIGO’s two interferometers detected a strong signal. The measured waveform matched the predictions of Einstein’s equations of General Relativity for a merger between two black holes. This source involves pure structures of spacetime with no electromagnetic counterpart, ushering a new era of gravitational wave astrophysics.
Expanding astronomical detectors across the electromagnetic spectrum from the radio to gamma-rays revealed new astronomical sources. However, so far gravitational waves revealed only previously known source types, such as black holes or neutron stars.
Whereas the Universe was opaque to light up to 400,000 years after the Big Bang, it was always transparent to gravitational waves. Hence, gravitational waves allow us to probe the cosmic conditions immediately after the Big Bang. If we ever detect gravitational waves from the cosmic beginning, they will likely unravel new physics.
Are there any additional types of cosmic particles that we might hope to observe in the future? One class of such particles are hypothesized to be streaming through Earth from the Milky-Way halo. They constitute dark matter, a substance that makes 85% of the cosmic matter budget. It is inferred to exist through its gravitational effect on visible matter.
Another class of new cosmic messengers are interstellar objects, detected for the first time over the past decade. These objects could be a mix of natural rocks and technological products manufactured by our cosmic neighbors over billions of years. Finding space probes from another civilization will be far more intriguing than any of the messengers we detected from the cosmos so far.
Are there any clues for interstellar gadgets in the sky? Next month, a new paper will address this question by analyzing data from the Galileo Project observatory at Harvard University. Other clues might be discussed in the next congressional hearing on Unidentified Anomalous Phenomena.
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.
