Is it Possible to Communicate with Gravitational Waves?
When exploring the frontiers of physics, it is impossible to avoid mistakes. Reality represents one option out of many and not necessarily our favored choice. Risk-taking is therefore an integral part of the scientific learning experience. Experiments that prove our conjectures wrong bring new insights.
At the prime of his career, Albert Einstein made three mistakes in the span of four years –just two years after arriving at the Institute for Advanced Study in Princeton. In a 1935 paper, he argued that quantum mechanics does not have entanglement of particles at large separations, or in his words: `spooky action at a distance’. In a 1936 paper, he argued that gravitational waves do not exist. And in a 1939 paper he argued that black holes do not exist.
Eight decades later, three Nobel prizes were awarded in the span of five years to those who corrected Einstein’s mistakes. In 2017, the Physics Nobel prize was awarded for the detection of gravitational waves. In 2020, the Physics Nobel Prize was awarded for the discovery of the supermassive black hole, Sagittarius A*, at the center of the Milky Way galaxy. And in 2022, the Physics Nobel Prize was awarded for the experimental demonstration of quantum entanglement.
Despite this historic lesson, it has become fashionable these days for theoretical physicists to avoid the guillotine of experiments by promoting untestable conjectures. These ideas cannot be proven wrong. For that reason, they will not merit a Physics Nobel Prize.
The discovery of electromagnetic waves, first theoretically by James Clerk Maxwell and later experimentally by Heinrich Hertz, improved humanity’s communication skills. Could this be the case also with gravitational waves?
So far, nearly a hundred gravitational-wave sources were detected from cosmological distances. This is a remarkable triumph since all of them can be well described by the expected waveforms from mergers of black holes or neutron stars, as predicted by Einstein’s theory of General Relativity. However, this is also a disappointment since we did not detect unexpected sources.
In contrast, electromagnetic waves were first detected in 1888 from human-made sources in Hertz’s laboratory on Earth. Could gravitational wave detectors reach the sensitivity needed to identify terrestrial sources?
For calibration, let us consider the gravitational wave signal expected from the impact of a massive meteor on Earth. The Chicxulub impactor that killed the dinosaurs 66 million years ago had a diameter of about 10 kilometers, comparable to the size of a city. I calculated that its impact on Earth would have resulted in a gravitational wave amplitude at a frequency of about 1 Hertz (inverse of a wave period of a second, a unit named in honor of Heinrich Hertz) that is above the expected sensitivity of a futuristic Lunar Observatory that I proposed in a recent paper with Karan Jani.
Unfortunately for the lunar observatory but fortunately for humanity, such a massive meteor impact only takes place once per several tens of millions of years. It will definitely not occur in the coming century based on existing data from NASA’s survey of the sky for near Earth objects.
Also, unfortunately for the lunar observatory — humanity has no means to propel a Chicxulub-mass projectile of a trillion tons to a Chicxulub-scale speed of tens of kilometers per second. Hence, there is no chance that we will be able to produce a meaningful distortion of spacetime so as to communicate via gravitational waves in the foreseeable future.
But what if there is a more advanced scientific civilization in the Milky Way galaxy? What would it need to do in order to be heard in gravitational waves? Detailed calculations indicate that our observatories would notice them only if they accelerate a Jupiter-mass planet to a fraction of the speed of light, according to two recent papers here and here.
Our best gravitational wave detector is yet to come. Within a decade ESA and NASA plan to send a gravitational-wave observatory to space called the Laser Interferometer Space Antenna (LISA). LISA consists of three spacecraft that are separated by millions of miles behind the Earth as it orbits the Sun. These three spacecraft relay laser beams back and forth in search for transient distortions of spacetime. A Jupiter-mass object on a tight orbit around Sagittarius A* could emit enough power to be observable by LISA, and the energy output of a single star would be sufficient to encode an artificial message in its signal.
The advantage of messaging in gravitational waves is that the message cannot be dissipated or blocked by any intervening astrophysical system, as I showed in a recent paper. A second advantage is that the signal declines only inversely with distance instead of distance squared — as is the case for common detectors which record the energy flux of electromagnetic signals.
Here’s hoping that LISA will detect an unexpected message. And better still, decoding the message will reveal the equation that unifies quantum mechanics and gravity. This will bring closure to Einstein’s quest for a unified theory and will allow us to figure out what lies inside black holes and what preceded the Big Bang. If not from an intelligent civilization, such information could be gathered from a naked singularity that violates the cosmic censorship conjectures and gives us a peek straight in the face of quantum gravity.
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”, is scheduled for publication in August 2023.