PSorry for my excitement, but something big has happened: Physicists have found a new way to solve a long-standing problem—how to merge quantum physics with gravity. The new concept comes from Johnathan Oppenheim, a quantum theory professor at University College London, and he calls it “Post-Quantum Gravity.” I wrote about his new suggestion in Nautilus last month.
Now Oppenheim and his colleagues say that their concept not only reconciles quantum physics and gravity, but also accounts for dark matter and dark energy. “Something significant seems to be happening,” Oppenheim wrote on X (formerly known as Twitter), sharing his research. “We demonstrate that our gravity theory … can explain the universe's expansion and galactic rotation without dark matter or dark energy.”
The issue Oppenheim aimed to address is that quantum physics and gravity don’t gel very well. Quantum physics is non-deterministic, with innate uncertainty and limits on what can be known. We often say that quantum particles can exist in two places at once, but the math suggests it doesn't make sense to say they are in any specific place. They simply don't have defined locations.
On the other hand, gravity is described by Einstein’s theory of general relativity. It assigns a time and place to everything, and is deterministic with the future following from the past, and the only uncertainty comes from our own lack of knowledge. It doesn't mesh well with the non-deterministic behavior of quantum particles.
The alternative explanation to dark matter is that the law of gravity needs to be changed.
Various efforts have been made to unite the two through what is known as a theory of quantum gravity. The most well-known approaches attribute quantum properties to gravity and include string theory, loop quantum gravity, and asymptotically safe gravity. The less popular path is to maintain gravity as a non-quantum theory—a “classical” theory as physicists say—or to adjust something about quantum physics.
One example is Roger Penrose’s concept that gravity explains why we never see large objects in two places. However, these and similar concepts have not been very successful, mainly because the random movements of particles in quantum physics align with our observations, yet gravity cannot accommodate them.
Oppenheim’s new approach overcomes this issue by maintaining gravity as a non-quantum theory, but with a random element. This randomness of post-quantum gravity is fundamental—it's the starting point. This is similar to how the randomness of quantum mechanics is basic, a starting point, and a property of nature.
What Oppenheim has achieved is finding a way to blend the mathematics of both types of randomness into one framework. In this post-quantum theory, gravity remains non-quantum, and particles remain quantum. However, these two aspects interact because particles have gravity and that gravity, in turn, affects the particles. In Oppenheim’s theory, this works without conflicts because these two forms of randomness fit together. But in turn, this subtly alters both quantum physics and gravity.
In a recent paper, which has not been reviewed by other scientists, Oppenheim and his co-author Andrea Russo, a student studying at University College London, claim that this randomness alters the law of gravity in a way that eliminates the need for both dark matter and dark energy.
Dark matter and dark energy are terms that scientists studying stars and galaxies have used to describe two hypothetical parts of the universe. These parts have never been directly seen; scientists have only figured out they exist by noticing their gravitational effects. They introduced dark matter to explain, for instance, why galaxies spin faster than expected and why gravitational lenses are more powerful than anticipated. They added dark energy to explain why the universe not only expands, but the expansion is also speeding up—ordinary matter or energy couldn't cause this to happen.
Another explanation for dark matter and, possibly, dark energy is that the law of gravity needs to be adjusted. It wouldn't have to be changed for large distances because galaxies vary in size and we see noticeable effects of dark matter at different distances from the center. Instead, it seems that the best way to change the law of gravity is when the force acting on a star is very small—exactly what Opppenheim and Russo say they discovered in their post-quantum gravity.
Problem solved? Well, maybe, but I think it won’t work.
This change in Einstein's gravity at low accelerations is called Modified Newtonian Dynamics, MOND for short. The reason this change at low acceleration can look like dark matter is that the acceleration, on average, depends on gravity's strength. If you are close to a planet or a sun, then you feel a noticeable pull of gravity from those heavy objects. If you think of this as a force, then that leads to an acceleration. The farther away you are from heavy objects, the smaller the acceleration.
Yet, the overall gravitational pull coming from an entire galaxy becomes weaker the further a star is from the center because most of the galaxy's mass is in the center. It turns out that assuming gravity gets stronger at low accelerations can explain the observations linked to dark matter in galaxies. This is, basically, the concept of MOND.
Oppenheim now suggests that when he and his colleagues examined the changes to the law of gravity due to the gravitational randomness, they observed a similar pattern: The lower the acceleration of an object, the more it is influenced by the inherent randomness in gravity. They come up with an equation that is similar to Newton’s law of gravity for the Newtonian gravitational potential, which is what physicists use to derive the gravitational force. However, it is not exactly the same equation, and its solutions are unusual. They demonstrate that these extra contributions seem to resemble what MOND provides: They cause galaxies to spin faster. Additionally, one of these contributions appears to be like dark energy. MOND itself does not explain dark energy, so this is a very impressive accomplishment.
Although I want to warn that in this latest work, they only study galaxies, so they have not proven that the quantity they recognize as the simplest form of dark energy (a cosmological constant) actually causes the universe to expand. For this, they refer to another “manuscript in preparation,” so I think we will hear more about this in the future. However, from my own experience, it's not very difficult to get dark energy to come out correctly. (I, too, have a theory which explains both dark matter and dark energy, it’s just that no one cares about it.) Problem solved? Well, maybe, but—call me a grumpy old woman if you must—I think it won’t work. The reason is that the equation they derive is too simple. I suspect that it works for one galaxy at a time, but it will not work for a set of many at once. This is because astrophysicists have noticed that if we combine data from many galaxies, then some observable properties—such as the rotational speed and brightness—are correlated with each other. MOND explains these correlations; dark matter does not. To me this is the great appeal of MOND over dark matter. But the equations which seem to follow from post-quantum gravity do not have the right properties to explain these relations. That is to say, I suspect that as they investigate their equations further, they will find that they don’t work as desired.
Of course I could be wrong. And so for me this is a win-win situation. Either I am right, or I am wrong and we are witnessing the birth of a theory that delivers what many of us have dreamed of.
A new “post-quantum” theory of gravity claims we can say goodbye to dark matter and dark energy. I have my doubts.
Of course I could be wrong. And so for me this is a win-win situation. Either I am right, or I am wrong and we are witnessing the birth of a theory that delivers what many of us have dreamed of.
Lead image: Marcel Drechsler / Shutterstock
