At the end of a new podcast interview today, the brilliant undergraduate student Oem Trivedi — with whom I had the privilege of co-authoring a scientific paper, asked: “What do you see as the most exciting frontier in cosmology a century from now?” I replied: “Experimental studies of quantum gravity.”
When Albert Einstein attempted to unify Newtonian gravity with Special Relativity, he came up with the theory of General Relativity in which gravity is manifested as the curvature of spacetime. This was a remarkable theoretical achievement. Unfortunately, it gave the illusion that the next grand challenge of unifying General Relativity with quantum mechanics might also be accomplished by pure thought. The reality is different. String theorists are still unable to explain away the singularities of the Big Bang or black holes.
The limitation of human ingenuity is not unprecedented. Quantum mechanics was discovered experimentally a century ago. It was not expected theoretically and is not fully understood at a fundamental level even today. The same may apply to quantum gravity. It is therefore important to pursue an experimental approach that will guide us towards a unique theory of quantum gravity. Are there any suitable environments for this pursuit?
Given today’s standard model of particle physics, we expect quantum gravity effects to appear prominently at the Planck energy, which is 19 orders of magnitude higher than the proton rest-mass energy. Unfortunately, even the highest-energy cosmic-rays are short by a factor of a hundred million relative to the Planck energy. Black hole singularities should be replaced by something else in quantum gravity, but getting close to that thing poses an existential risk — as experimentalists would be torn apart by the huge gravitational tide there.
Luckily, there are more accommodating environments. For example, insights about quantum gravity may be linked to the nature of dark energy which sets the accelerated cosmic expansion. This energy density of the vacuum dominates the current cosmic mass budget, despite the fact that it is smaller by 123 orders of magnitude than the Planck energy density. We can attempt to understand its nature by perturbing the cosmic vacuum in the laboratory or by measuring its evolution over cosmic time — as currently done by DESI.
Another approach is to detect the cosmic graviton background at a temperature below a degree Kelvin. This graviton background constitutes an analog for the cosmic microwave background. Whereas the thermal photon background was released 400,000 years after the Big Bang, the gravitons were thermalized at the Planck time and propagated freely afterwards. As I argued in a 2022 paper with Sunny Vagnozzi, detection of this background would tightly constrain theories of quantum-gravity and rule out cosmic inflation.
In principle, modified gravity at low accelerations, such as MOND, could also have its roots in quantum gravity. In that case, dark matter does not actually exist. Instead, it represents a misinterpretation of the discrepancy between General Relativity and data on dynamics at low accelerations.
Finally, if mini black holes in the mass range of asteroids were produced in the early Universe and one of them is discovered in the Solar system, then the experimental study of its Hawking evaporation or interaction with infalling matter could probe quantum-gravity effects, as I discussed in a new paper,.
I clarified to Oem that these are just a few examples for experimental paths of inquiry into quantum-gravity. Other unexpected routes may open up by new anomalies in the coming decades.
Consider, for example, gravitational wave astrophysics. So far, the detected signals did not reveal new physics. However, future runs of gravitational-wave observatories, such as LIGO-Virgo-KAGRA or LISA, could reveal surprising sources that are shaped by quantum gravity. One such source could be a white hole, the time-reversal of a black hole, in which energy flows out from the vicinity of a General Relativistic singularity. Another could be a wormhole — a spacetime bridge which offers a shortcut between widely separated regions of space. Other quantum-gravity signatures could involve faster-than-light travel or a time machine that delivers information from our future and violates Stephen Hawking’s “chronology protection conjecture.”
All in all, detecting gravitons from the Planck time in cosmic history or observing a white hole are equivalent to staring at a quantum-gravity system straight in the eyes.
A shortcut to uncovering new insights about quantum gravity can be provided by encountering technological products from an advanced alien civilization. In that case, the manufacturers may have benefitted from insights developed over millions of years of their history of science and technology. This could have allowed them, for example, to employ quantum-gravity for propulsion of spacecraft. Reverse-engineering their gadgets might save us time in developing our own insights independently.
With quantum-gravity insights from smarter scientists in our cosmic block, we might be able to figure out what happened before the Big Bang. This might provide us with a recipe for creating a baby universe in the laboratory. With that at hand, not only would we unify quantum mechanics and gravity, but also science and religion.
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.
