What if gravity is an illusion, a cosmic side effect of something else going on at deeper levels of reality? The classical theory of gravity is in dire need of new approaches, since it doesn’t combine well with quantum physics. Both theories, crown jewels of 20th century physics, cannot be true at the same time. A Dutch theoretical physicist and string theorist, Erik Verlinde, proposes a starkly different theory – the theory of emergent gravity. “For me gravity doesn’t exist,” he says. “We might be standing on the brink of a new scientific revolution that will radically change our views on the very nature of space, time and gravity.”
“Emergent Gravity” –Verlinde’s Starkly Different Theory
“We have evidence that this new view of gravity actually agrees with the observations, ” says Verlinde. “At large scales, it seems, gravity just doesn’t behave the way Einstein’s theory predicts.” Einstein’s theory of general relativity is elegant and accurate. It makes many correct predictions, including the formation of gravitational waves, the observations of which won the Nobel Prize in physics in 2017. Yet if we use general relativity to predict the motion of galaxies, we get the wrong answer.
The favored response to this is to invent a new form of so-called ‘dark’ matter. Adding the right amount of this to the visible matter in the galaxies brings theory and observation into agreement. But this is not just a minor correction – there needs to be much more of the dark matter around than normal matter.
Dark Matter –”Doesn’t seem to be made up of quarks and electrons”
Moreover, dark matter doesn’t seem to be made up of quarks and electrons like all other matter. In fact, it doesn’t seem to be made up of any of the particles in the Standard Model of particle physics. Theorists postulate new extensions to the Standard Model, containing new particles, from which dark matter might be made. Physicists are actively searching for such new particles, smashing sub-atomic particles together at extreme energies at the Large Hadron Collider at CERN or using sensitive underground detectors to search for weakly interacting particles incoming from space.
But there may be another way. Perhaps general relativity can be modified instead, to give a new theory, in which the motions of galaxies, and the structure of the universe, are correctly predicted, without the need for dark matter. Maybe even the accelerating expansion of the universe, another current conundrum of physics, can be explained in such a theory.
Dark Matter –”A mythical beast, a figment of our ignorance”
Perhaps dark matter is a mythical beast, a figment of our ignorance which will evaporate as we explore nature more thoroughly. The problems arise in extreme conditions: near black holes or during the Big Bang.
A new theory of gravity might explain the curious motions of stars in galaxies. Emergent gravity, as the new theory is called, predicts the exact same deviation of motions that is usually explained by invoking dark matter.
“Emergent Phenomenon” –Gravity is not a fundamental force of nature
In 2010, Verlinde surprised the world with a completely new theory of gravity. According to Verlinde, gravity is not a fundamental force of nature, but an emergent phenomenon. In the same way that temperature arises from the movement of microscopic particles, gravity emerges from the changes of fundamental bits of information, stored in the very structure of spacetime.
In his 2010 article (On the origin of gravity and the laws of Newton), Verlinde showed how Newton’s famous second law, which describes how apples fall from trees and satellites stay in orbit, can be derived from these underlying microscopic building blocks.
Deciphering Curious Motions of Stars in Galaxies
Extending his previous work and work done by others, Verlinde now shows how to understand the curious motions of stars in galaxies.
The outer regions of galaxies, like our own Milky Way, rotate much faster around the center than can be accounted for by the quantity of ordinary matter like stars, planets and interstellar gases. Something else has to produce the required amount of gravitational force, so physicists proposed the existence of dark matter. Dark matter seems to dominate our universe, comprising more than 80 percent of all matter. Hitherto, the alleged dark matter particles have never been observed, despite many efforts to detect them.
According to Erik Verlinde, there is no need to add a mysterious dark matter particle to the theory. In a new paper, Verlinde shows how his theory of gravity accurately predicts the velocities by which the stars rotate around the center of the Milky Way, as well as the motion of stars inside other galaxies.
Verlinde’s Theory –“A totally different starting point”
At first glance, Verlinde’s theory presents features similar to modified theories of gravity like MOND (Modified Newtonian Dynamics). However, where MOND tunes the theory to match the observations, Verlinde’s theory starts from first principles. “A totally different starting point,” according to Verlinde.
The Holographic Principle
One of the ingredients in Verlinde’s theory is an adaptation of the holographic principle, introduced by his tutor Gerard ‘t Hooft (Nobel Prize 1999, Utrecht University) and Leonard Susskind (Stanford University). According to the holographic principle, all the information in the entire universe can be described on a giant imaginary sphere around it. Verlinde now shows that this idea is not quite correct—part of the information in our universe is contained in space itself.
This extra information is required to describe that other dark component of the universe: dark energy, which is believed to be responsible for the accelerated expansion of the universe. Investigating the effects of this additional information on ordinary matter, Verlinde comes to a stunning conclusion.
Whereas ordinary gravity can be encoded using the information on the imaginary sphere around the universe, as he showed in his 2010 work, the result of the additional information in the bulk of space is a force that nicely matches that attributed to dark matter.
Source: Emergent Gravity and the Dark Universe, E. P. Verlinde, 7 Nov 2016. arxiv.org/abs/1611.02269
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