Last month, long-awaited proceedings of a new theory of gravity were published by one of the physics institutes at the University of Amsterdam. The author is Erik Verlinde, a professor of physics who specializes in String theory. In 2010, he caused quite a stir when he first announced his theory of Emergent Gravity to the world. Some expressed enthusiasm for his theory, while others critiqued it fiercely. Six years of radio silence later, the promises put forward in his newest paper still manage to spark the hopes of many. That said, couldn’t this all be a big fuss about “fried air”? There are several intriguing motivations for this framework of Emergent Gravity that seem to tell us otherwise. For one, his theory intends to unify gravity and quantum mechanics. Also, recent developments in cosmology have led to the introduction of two dark components of the energy budget of the universe: dark matter and dark energy (these two are very different things, see below). Potentially, Verlinde’s theory could tell us what dark energy is and absolve the need for dark matter altogether. These are some of the greatest problems facing physics these days. Let’s review them shortly and see what Verlinde’s work might do to ameliorate them.
Problem 1: Gravity refuses to talk with quantum mechanics
Currently, the best theory we have of gravity is Einstein’s theory of General Relativity. Its extremely precise predictions have been confirmed time and time again. It does a good job at describing our universe on the large scales. The main problem is that it refuses to be united with the other big player in town, the one which governs the small scales: quantum mechanics. Quantum mechanics provides a very successful description of the microscopic world of elementary particles. Some theorists have made attempts at unifying gravity and quantum mechanics using String theory. The main problem with such theories is that it’s difficult to subject them to observational tests.
Verlinde’s framework of Emergent Gravity circumvents the unification problem by stating that gravity does not exist on the quantum scales. Instead, he posits that gravity emerges from the building blocks of spacetime called quantum bits or “qubits”. In his view, spacetime depends fundamentally on the quantum information stored in those bits. The information in those quantum bits tends to become less ordered over time (i.e., its degree of disorder or entropy increases). This tendency would then give rise to the predictable effects we observe as gravity. With several simplifying assumptions, Verlinde managed to rederive Newtonian gravity. This way of looking at gravity could be compared to our understanding of temperature:
- Temperature is a property of a gas that emerges from the behavior of countless molecules on the smaller scales.
- Gravity is a property of spacetime that emerges from the behavior of countless qubits on the smaller scales.
Problem 2: Where is all the extra mass?
Observations of galaxies and galaxy clusters have been telling us that there is much more mass in the universe than meets the eye. This has been determined using Einstein’s theory of gravity. For lack of a more precise term, astronomers call this missing mass “dark matter”. Presumably, it consists of particles that are only noticeable due to their gravity. They don’t emit light, making them difficult to observe. Large amounts of research time (and money!) have been allocated to identify the particles that make up dark matter, so far without success.
Verlinde believes a better solution than positing dark matter would be, instead, to improve our theory of gravity. His theory of Emergent Gravity produces the result that the laws of gravity behave differently at different distances. For several static cases, Verlinde has shown that it departs from Einstein’s theory in such a way that dark matter is not needed.
Problem 3: What on earth is dark energy?
The universe is expanding. Not only that, evidence indicates that the expansion itself is accelerating. This has led to the inclusion of “dark energy” in the cosmological standard model. This energy fills the vacuum throughout the universe and tends to push apart the galaxies. We know hardly anything about this energy beyond that Einstein’s equations allow for it! Taken together, dark matter and dark energy make up 95% of the energy in the universe. This leaves us with only 5% of all the energy (!) for everything that modern physics studies (namely, matter and light).
If true, Verlinde’s theory could actually tell us what dark energy is. To picture that, consider a mass-containing sphere. The quantum information we talked about earlier could then be stored on its surface, in its volume, or both. By considering only the quantum information on the surface, Verlinde derived “conventional” gravity (plus at least some of the effects attributed to dark matter). By including the volume elements too, he derived effects that could be interpreted as dark energy. There’s no need to wrap your head around this completely—this is just to give you a taste of the theory.
Can Emergent Gravity shed light on our dark universe?
The title of Verlinde’s paper is “Emergent Gravity and the Dark Universe.” Part of what makes his theory interesting is that it promises to place our dark universe in a brighter interpretative light. That is why it has generated so much media attention. However, a cautionary note would be appropriate here. It is exciting to hear news about such novel developments in science, but it’s not always clear whether true progress has been made. For all that it purports to solve, Verlinde’s theory completely undermines the predominant view of spacetime. (Besides, his most recent paper has not been through the review process yet.) It’s great that there is room within the sciences for conceptual revolutionaries such as Verlinde. It goes to show that the scientific community is open to new ideas, even if these could overturn deeply-cherished theories. But new frameworks, such as Verlinde’s proposed theory, need time to be fleshed out in light of theory and observations to see whether they withstand (or succumb to) critical review.
Is Emergent Gravity the way to go? At this point, not quite. This framework is still too underdeveloped to know whether it can compete with the cosmological standard model. For example, Verlinde only managed to describe gravity when spacetime has reached “equilibrium” (i.e., stable situations). However, the most important evidence for dark matter comes from rapidly changing circumstances (e.g., clusters of galaxies in collision). These will be the toughest problems to tackle in the future. That will really tell us whether Verlinde’s Emergent Gravity has merit or not.
Whatever direction this will be heading, it is a beautiful illustration of how science works. The vast majority of the scientists working in this field are exploring the possibilities within the confines of the cosmological standard model. That’s perfectly fine, it is what makes collaboration efficient. At the same time, there’s considerable room for dissenters such as Verlinde. When someone really has a sound basis for an alternative position, his or her ideas should be able to withstand the pressure of critical peer review in the scientific community. If they really hold water, they can eventually contribute to a shift in the paradigm as a whole. It should be noted again that Verlinde’s latest publication has not been through the peer-review process yet. We eagerly await other qualified string theorists to review his recent work to help us understand how promising these ideas are.
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