Ask Ethan: What if gravity is not a fundamental interaction?

The distortion of space-time by gravitational masses

Since Newton first advanced his theory of world gravity, it was believed that the forces that control gravity here on Earth also control the movement and formation of planets, stars, galaxies and large-scale structures of the Universe. With the improvement of scientific knowledge, Newtonian gravity has been replaced by the Einstein General Theory of Relativity, and it is expected that it will in turn be replaced by the full, quantum theory of gravity. So far, no one has been able to quantize gravity. But what if gravity is not a fundamental interaction, which is why its quantization attempts failed? Many readers immediately sent questions about Eric Verlinde's new scientific work: Derivative Gravity and the Dark Universe . Let's get it right.


The four forces (or interactions) in Nature, the carrier particles of these interactions and the phenomena they affect. The three interactions that control the microcosm are much stronger than gravity, and are already combined through the Standard Model.

It is generally accepted that there are four fundamental interactions. Particles and interactions of the Standard Model, which includes quarks, leptons, gauge bosons, and the Higgs particle, describe three of the four forces: electromagnetism, weak and strong nuclear interactions. The last force described by GR is gravity. Its action is taken into account due to the curvature of space-time and the presence of matter and energy. Effects such as gravitational lensing, gravitational radiation and the expansion of the universe are the consequences of this theory, and it is able to include dark matter with dark energy. One of the greatest hopes of many theoretical physicists (and string theory) - although this is not necessary - is the hope of a comprehensive platform that combines all four forces.


A fabric of space-time subjected to deformation and agitation due to mass

Another approach is to consider the possibility that GR itself, space, time, and the force of gravity are not fundamental, but derivative. Perhaps GTR is just a scene on which there is a gravitational representation, and there is a more fundamental basis for what we perceive as gravity. Verlind's approach is to start with the entropy and Hawking temperature of a black hole and then, using ideas from string theory, show the relationship between the theory of quantum information and the appearance of gravity, space and time.


Two possible patterns of entanglement in de Sitter space, representing the entangled pieces of quantum information that can allow space, time, and gravity to appear.

The basic idea is not particularly complicated: imagine that you have two quantum “units” confused with each other. Add a material particle, and it will be able to interact with one of them or with both of them. This particle can change the entanglement of the system, and it is from this change of entanglement that gravity can appear. Since the entropy of a black hole is proportional to its surface area, there is a temptation to consider space in the form of a network of entangled “units” that allow gravity to appear. In addition, one of Verlind's starting elements, the Hawking temperature of a black hole, is proportional to gravitational acceleration on its event horizon.



We do not know what happens in the singularity inside the black hole, but data on the event horizon, including entropy and temperature outside, are well known.

It is hoped that, under the right assumptions, a complete theory of gravity may emerge, which:

• will give you four dimensions of space-time (three spatial and time);
• turn on dark energy as a positive cosmological constant;
• Explains where the gravitational “differences” come from between the predictions of the Standard Model and the observations.

Hope is quite powerful, and Verlinda aspires to it. Others, independently of him, are working in the same direction. This work is a report on how research is progressing. And how are they progressing?


Illustration of one of the stages of the derivation of gravity, according to the idea of ​​entropic gravity

Given the narrow specifics of the assumptions, some progress is visible, but there are many problems. The largest, simply put, is that in order to get something meaningful, you need to make many seemingly random decisions. For example: all the motivation for this approach is based on the space of anti-de Sitter(a space with a negative cosmological constant), but in our Universe there is a positive cosmological constant (i.e. this is de Sitter space), and the mathematics of these two spaces have very different properties. Or, entropy must obey a strict law depending on the location in order to get Einstein’s equations, but in this case you won’t get a cosmological horizon (and the Universe has it). And finally, if you make all the assumptions necessary for gravitational acceleration to appear, you destroy all the successes of GTR on scales larger than galaxies. Verlinde claims that this approach has the right to success, but observations of collisions of galactic clusters refute his reasoning.


The X-ray map (pink) and the general matter map (blue) of various colliding galactic clusters show a clear separation of normal and dark matter.

There are more fundamental disappointments. The Verlinde model allows the gravitational mass to become a derived quantity, but it does not mention the inertial mass, or why they are the same (Einstein's principle of equivalence). Or, many of Verlinde’s confusing assumptions make the calculations true only if they apply the current value of the Hubble expansion to them, despite the fact that during the life of the Universe its expansion rate changed dramatically. Also, to justify his platform, he suggests that dark energy has always been the dominant form of energy in the universe, but in fact, billions of years of dark energy has been negligible. In other words,


Fluctuations of the whole sky are built into the relict radiation, the residual radiation of the Big Bang.

But for the most part this work is an Herculean attempt to develop a radically new idea: starting with the entropy and temperature of quantum “pieces”, we can derive the theory of gravity, including space and time. I would summarize some of the problems of this approach as follows:

• The definitions of entropy and temperature are based on the fact that GTR must first be determined.
• Many assumptions and interpretations are made without clear motivation, except as “mathematics seems to work like that.”
• The composition and structure of the Universe changed over time, but the laws of physics do not, which contradicts Verlinde’s work.
• The work raises many open questions: is it possible to include a standard cosmological picture in it, including the expansion of the Universe, inflation, a complete set of observations of dark matter and dark energy?


The Universe went through an incredible evolution in order to turn into what is now

After all, although this idea is very interesting, it must coincide with the observed Universe. And in order to rise to the level of generally accepted science, it needs to make a prediction on an unresolved issue, which we can verify. She has such potential, but this requires more than just hard work; it is required to be correct, and so, or not, has not yet been determined. New ideas are always interesting, and this can give us amazing discoveries. As Niels Bohr brilliantly said,

The purpose of our description of nature is not to discover the essence of the phenomenon, but to trace the links between the diversity of aspects of our experience as far as possible.

Ethan Siegel - astrophysicist, science popularizer, author of the Starts With A Bang! He wrote the books “Beyond the Galaxy” [ Beyond The Galaxy ], and “Tracknology: the science of Star Trek” [ Treknology ].