Can the conflict between dark matter and gravity be resolved?
by Jessica Hall
Dutch string theorist Dr. Erik Verlinde insists on saying gravity doesn’t exist. The good professor clearly does not believe that if he stops holding onto his briefcase, it will do anything other than fall. But he definitely believes we’re thinking of it all wrong. And if it turns out he’s right, it could change the way we think about the large-scale structure of the universe, and make a scientific shot over the bow at the idea of dark matter. Physicists have taken up the challenge, and the first results are in — and they support Verlinde’s ideas.
Gravity, Verlinde contends, is just an emergent phenomenon stemming from entropy, and we don’t need dark matter to explain the way we see galaxies behave. In his theory, when particles get closer under the influence of gravity, they’re actually just relieving entropic strain: falling into a lower-energy state. Because gravity operates at great distances, Verlinde believes that this change in our expectations of gravity could account for the difference between the gravitational behavior we expect galaxies to exhibit, and what we see. It could remove the “place” that the idea of dark matter is “holding.”
This new explanation builds on Verlinde’s 2010 work, wherein he made a lot of grand statements in a long, proof-style paper that reads with the breathtaking, perfect confidence only a career mathematician can muster.
It is well known that Newton was criticized by his contemporaries, especially by Hooke, that his law of gravity acts at a distance and has no direct mechanical cause like the elastic force. Ironically, this is precisely the reason why Hooke’s elastic force is nowadays not seen as fundamental, while Newton’s gravitational force has maintained that status for more than three centuries. What Newton did not know, and certainly Hooke didn’t, is that the universe is holographic. Holography is also an hypothesis, of course, and may appear just as absurd as an action at a distance.
One of the main points of this paper is that the holographic hypothesis provides a natural mechanism for gravity to emerge. It allows direct “contact” interactions between degrees of freedom associated with one material body and another, since all bodies inside a volume can be mapped on the same holographic screen. Once this is done, the mechanisms for Newton’s gravity and Hooke’s elasticity are surprisingly similar. We suspect that neither of these rivals would have been happy with this conclusion.
To support his theory, he went on to experimentally vary the value of ħ: the Planck constant, pronounced h-bar, which Wikipedia defines as “a physical constant that is the quantum of action, central in quantum mechanics.” Changing this parameter apparently helps to resolve the difference in how fast galaxies should be moving through space relative to the mass they have. At the end, Verlinde put forth a master equation that could be used to test his ideas. This year, Dr. Margo Brouwer put his predictions to the test by looking at 33,000 galaxies, and found that using Verlinde’s equations, our mathematical predictions suddenly line up with the data, and the galaxies move at the same speed we expect them to.
Verlinde’s ideas turn over more than just the labels we use to talk about gravity. One of the biggest questions in physics is why the force of gravity that we see around a galaxy is so much stronger than what Einstein’s general theory of relativity would predict, even at great distances. To date, we’ve accounted for this by invoking dark matter and dark energy, which together apparently make up much of the universe. But the infamous “placeholder” still offends many scientists’ sense of reason. If it’s so important that it alters the fate of galaxies, so ubiquitous that there could be seasons of dark matter as the Earth moves with or against the galactic current, why can’t we see it or test for it or otherwise interact with it in any way? Epicycles accounted for everything we could see — until suddenly we could see more, and that explanation was no longer sufficient. At least antimatter has the good manners to light up when it interacts with matter. How many exotic regimes of physics must there be?
Verlinde agrees in his recent work: “[The] fact that 95% of our Universe consists of mysterious forms of energy or matter gives sufficient motivation to reconsider this basic starting point.” Instead, Verlinde believes it possible to describe the observed distribution of gravity without resorting to substances like dark matter and dark energy if gravity is considered an emergent phenomenon arising from entropy — a consequence of thermodynamics, as outlined in his 2010 theory.
This is where I start to have problems. Nobody seems to have asked this yet: If gravity is a byproduct of the universe’s tendency to maximize disorder, then why does it seem to pull things together into a more ordered state? Is this related to the way polar and non-polar liquids exclude one another where they make contact? How does entropic gravity work in light of the long-term tendencies of clumps of matter in our universe? What else depends on the Planck constant, and does changing it break anything else? Relabeling gravity as a different kind of omnipresent force is one thing. If gravity arises from entropy and dark matter isn’t necessary to resolve the conflict between Einstein’s theories and the distribution we see, then we have a lot of thinking to do.
This isn’t a one-shot disproof of dark matter. Earlier in 2016, the JPL made observations of galaxy clusters that tied their physical structure in 3-space to the density of dark matter around them. Cosmologists related the age and size of the dark matter formations within galaxy clusters to how tightly their resident galaxies were packed. It was the first time a property other than mass had ever been ascribed to dark matter, which currently we think must be made of light, weakly interacting particles something like neutrinos. It could also provide a window into studying dark energy; if dark energy and dark matter interact, we could get a sense of how dark energy influences dark matter by studying how it moves. Is this a pivot in the field of physics? Either way, we’re starting to make moves along the frustrating path to fully characterizing dark matter — or discarding the theory.
Now read: What is dark matter?
Originally published at www.extremetech.com on January 3, 2017.
