Black hole/massive star pair (artist's conception)
When a physicist is on the front page of a newspaper, you know the story is either really bad, or really good. Just before Christmas, the Dutch paper De Volkskrant ran a big story on theoretical physicist Erik Verlinde, who has been making waves with his new theory for the origin of gravity. Since the story ran, Verlinde published a paper explaining his new theory to the Arxiv. In it, he postulates that gravity is an emergent phenomenon resulting from changes in entropy rather than the fundamental force of nature we currently think it is, and demonstrates this using simple thought experiments. Gravity, he says, is an entropic force – a bit like, say, osmosis.
So when Verlinde, who works at the University of Amsterdam, turned up on the colloquium schedule of Leiden’s Lorentz Institute, I thought it was worth checking out.
Gravity has long been the problem child of the fundamental forces. Somehow, it’s everywhere, but how does it arise and how is it mediated? After the electromagnetic, weak and strong forces have been shown to fit nicely within our understanding of quantum mechanics, gravity remains a misfit, poorly understood. Its postulated carrier particle, the graviton, continues to evade detection.
In developing the theory of gravity as an entropic force, Verlinde borrows from the work of Stephen Hawking and Jacob Bekenstein, who pioneered the field of black hole thermodynamics. Black hole thermodynamics tells us that black holes have entropy, and that their entropy, or information content, is directly proportional to the area of the event horizon. This is a little odd, as it relates a property associated with a volume to its bounding area. In an generalised extension of this work, Dutch physicist ‘t Hooft in the early ’90s posited that the entropy of any volume, and thus the information it holds, is in fact proportional to its bounding surface area. This idea is known as the holographic principle, and in this picture the Universe is essentially one giant hologram.
So if a mass is moving towards the event horizon of a black hole, Verlinde explained, at some infinitesimal distance its information content will become indistinguishable from that of the black hole; it will essentially have “thermalised”. This is typically interpreted as acceleration of the mass causing a change in the black hole’s entropy. Verlinde turns this round: what if, he asks, it’s the change in entropy, the change in information on the black hole’s surface, that causes the mass’ acceleration towards the black hole?
He generalises the black hole thought experiment to a generic holographic screen in a non-relativistic space. A mass approaching the screen will change the information content of the screen as it comes infinitesimally close, and Verlinde relates the resulting change in entropy of the space to the displacement of the mass – and assuming that the characteristic temperature associated with the entropy is an Unruh temperature, Newton’s equations of motion naturally emerge. That is, at least, the story.
While I’m not enough of an expert to judge the theory critically, it’s clear that right now the arguments are too heuristic in nature – but Verlinde too acknowledges this. He’s made some assumptions based on our current knowledge to make his idea “fit”, and now has to go back to fill in the details. A certain amount of circularity, he reckons, can’t be avoided at this stage. Around the physics community, the general buzz (here, here, here) seems to be cautiously positive of Verlinde’s work rather mixed, and several papers have been posted to the Arxiv to discuss thoughts, implications and extensions to the theory – I’ve listed some of them below, and collected them on Mendeley. Interestingly, Verlinde himself has taken to responding to comments on the blogs that have offered the most in-depth analysis of the work, such as this one.
The implications of gravity and space as an emergent phenomenon rather than a fundamental force shouldn’t be underestimated, particularly in astrophysics, even if it doesn’t change our actual experience of gravity. All the things we see today as dynamic phenomena – redshift, dark energy – would be reinterpreted as thermodynamic effects.
So perhaps gravity may get to outgrow its misfit status yet – though at a steep cost, by losing its place in the elite club of fundamental forces of nature. There are many fascinating aspects to this theory. I’ll admit that since the colloquium I’ve had a bit of a crush on the idea of space as an emergent phenomenon – even if I’m not too sure what that means. Thoughts? Leave me a comment.
Reading
You can also find these papers in in a public collection on Mendeley, and I’m in the process of gathering some more background reading to add to it.
Erik P. Verlinde (2010). On the Origin of Gravity and the Laws of Newton arxiv arXiv: 1001.0785v1
Jarmo Mäkelä (2010). Notes Concerning “On the Origin of Gravity and the Laws of Newton” by E. Verlinde (arXiv:1001.0785) arxiv arXiv: 1001.3808v3
Lee Smolin (2010). Newtonian gravity in loop quantum gravity arxiv arXiv: 1001.3668v1
Liu Zhao (2010). Hidden symmetries for thermodynamics and emergence of relativity arxiv arXiv: 1002.0488v2
Jae-Weon Lee, Hyeong-Chan Kim, & Jungjai Lee (2010). Gravity from Quantum Information arxiv arXiv: 1001.5445v1
Image: ESO/L. Calçada
And this one too by the theoretical physicist Thanu Padmanabhan. ‘Equipartition of energy in the horizon degrees of freedom and the emergence of gravity.’ http://arxiv.org/pdf/0912.3165v2 coming out almost simultaneously
As for an emergence, absolutely. Wouldn’t it be nice to stop looking at it as a puzzle where the pieces become more and more, and ‘smaller and smaller’, with the dimensions as singular objects that we ‘copy and paste’ as needed to explain some new geometric twist. If SpaceTime is a emergent system it should have a symmetry to it too it seems to me.
And CPT violations would then become?
I’ve felt this way a long time, not that I thought of it as a holographic system though. And using ‘entropy’ as the common nominator seems quite new ? I’ve been considering ‘time’ as that common nominator instead responsible for the ground from where all comes, and times arrow to be what arrange all that we see macroscopically. But maybe ‘entropy’ is it? But if it is, doesn’t that mean that time and it’s arrow is the same thing
I need to know more about his idea, and Thanu Padmanabhan’s too.
Hey, not that I mind, I love learning this kind of things, it’s the best brain gymnastics i know of
Gravity Simplified
Gravity Is The Monotheism Of The Cosmos
A. E=Total[m(1 + D)]
Is the relationship between the cosmic energy(E), mass(m), and spatial expansion distance(D) since the cataclysmic E/m superposition resolution.
At 10^-35 seconds since big bang, D was already a fraction of a second above zero. This is when gravity started. This is what started gravity. At this instance started the energy space texture, the straining of space texture, and started the space-texture-memory, gravity, that most probably will eventually overcome expansion and initiate impansion back to singularity, again.
The clusters of galaxies behave as accelerating classical Newtonian bodies. Their motion is fueled with energy from myriads of mass-to-energy reconversions, in intertwined evolutions within the clusters.
B. The mass-to-energy reconversions continuously diminish m, as D continuously increases
The energy spent on increasing D, the clusters expansion, becomes the potential impansion energy that will eventually re-form singularity. This is gravity. This is the striving of the resolved-from-energy mass to return to its sigularity wholeness.
m are ALL cosmic formats of mass, regardless of size and complexity, including astronomic-to-smallest-particle bodies and all energy-mass organizations such as black holes environs, biospheres-lifes, all sizes and varieties of spin-arrays.
C. Mass is destined to dis-exist. It attempts to postpone-survive this by ingesting of energy
The cosmic expansion will eventually nearly run out of fuel-energy, when at some value of D it will be overcome by gravity and impansion will thus set in. The universe will then revert towards singularity. D will go on a diminishing course and m will enter a growing course of evolution, very different from the present cosmic evolution course.
D. Gravity Is The Monotheism Of The Cosmos
The universe came into being with inflation, with the onset of gravity. Gravity has been setting the course and nature of all the aspects of its evolution. Gravity will eventually terminate cosmic expansion and reverse the course of cosmic evolution.
Gravity Is The Monotheism Of The Cosmos
Dov Henis
(Comments From The 22nd Century)
28Dec09 Implications Of E=Total[m(1 + D)]
http://www.the-scientist.com/community/posts/list/184.page#4587
Cosmic Evolution Simplified
http://www.the-scientist.com/community/posts/list/240/122.page#4427
If nothing else, you have to appreciate that Verlinde’s theory is refreshing, in that Cinderella’s ugly step sisters (string theory’s 11+ dimensional universe, dark matter/energy, quantum loop gravity) now get a rest from trying to cram their crusty feet into the glass slipper (elegant, unified fundamental forces of nature).
The very concept of “Gravity” should really be separated in several parts.
1. The nuclear lumping part.
2. The interaction of celestial objects.
AD 1: When a molecular cloud is hit by electric forces, electromagnetic fields are created and the cloud is set in a swirling movement via the angular momentum, accelerating and heating up gasses and dust to the plasma stage. This plasma are sorted out and lumped together via the electromagnetic circulation, creating larger spheres of gas and dust that becomes stars and planets etc. that are slung out from the swirling centre via the simple centrifugal force when the spheres becomes sufficiently heavier than the momentum in the swirling cloud.
This is the general formation of galaxies and solar systems and here the “gravity contraction” is a build in force of electricity and magnetic field and not a separate force at all.
AD 2: The celestial interaction cannot be understood fully unless it is understood as a pushing force – mostly from the Sun and secondary from the galaxy. The Earth is under pressure form both these forces and the interaction of other celestial objects can only be understood as having a “directly pressure” or any form of “”shaded pressure” when planets and moons are in some kind of alignment or not.
The Pioneer anomaly is just a result of decreasing effect from the solar pressure and increasing effect from the galaxy – the very same interacting forces that creates the excentric orbiting plane of the solar system planets.
That is: The Newtonian laws of attraction should be completely dismissed and ditto with the Einstein “curvation of space” law. Every movement in the space is rotating, orbiting and swirling and as such moves in all kind of curves in the space. It is not the space itself that is curved, but all the movents IN the space.
Kindly Ivar Nielsen
Natural Philosopher