Tryggth’s Blog

As mentioned on  Slashdot, on December 8th at the Dutch Spinoza-instituut,  Erik Velinde gave a talk  describing an approach to the theory of gravity wherein gravity is an emergent property. It was followed by a preprint he posted to the arxiv server on Jan. 6th. The most controversial aspect of his position and paper is the claim that gravity is an epiphenomenon and not a “fundamental” force of nature. In his paper he derives Einstein’s field equations from what he believes are more fundamental assumptions.

It’s been two and half years since Garret Lisi achieved his 70 column-inches of fame with his “Exceptionally Simple Theory of Everything“[PDF]. Also a trained theoretical physicist, Lisi made headlines around the world [here, here and here for example]. The story was made-for-tv material. Bohemian surfer/snowboarder dude unwinds the Rubik’s cube of the universe.  The theory came complete with a pretty picture. But in the end his theory suffered a wipeout:

Distler had demonstrated in his blog that this is a mathematical impossibility.

The Scientific American post-mortem on Lisi’s theory concludes with this graf (my emphasis):

Today the theory is being largely but not entirely ignored. Lisi, naturally, continues to work on it, as does Smolin. Lisi says that even if what Distler claims is true, it would only be true for the variant of E8 (“real E8”) originally used in his paper and that another variant (“complex E8”) would certainly work. Smolin argues that the press coverage gave the false impression that Lisi’s proposal was a finished work. “In reality,” he says, “almost every new theoretical proposal is first presented in a way that is flawed and incomplete, with open issues that need to be filled in…. While Lisi’s proposal has exciting aspects, this is the case with it as well.”

Though covered in the Dutch press, Verlinde’s theory hasn’t achieved the notoriety of Lisi’s. The ‘Exceptionally Simple Theory of Everything’ was written about more often in magazines and newspapers than ever referenced in scientific articles. On the other hand, Verlinde’s January 2010 paper already has more citations than Lisi’s theory ever produced and is serving as the inspiration of roughly one scientific article a day.

In this and subsequent blog posts I will describe, with some context, Verlinde’s paper. And describe what I believe is going on in this small area of physics. My purpose isn’t to advocate for Verlinde’s theory. The work being done – and to come – is more about: 1) understanding the assumptions built into the theory’s underlying components and, 2) the somewhat fascinating – nearly self-referential – creation of the ontology of physics from the entropy reduction in the ’self-organizing’ system of ‘accepted physical theory’.

Some Background

Verlinde readily admits in his blog he isn’t the first person to derive Einstein’s equations from ideas involving thermodynamics, quantum mechanics and boundary assumptions. Jacobson had done this back in 1995. Even then, the interplay between quantum mechanics, thermodynamics and gravity had been “in the air” for awhile going back at least until 1984, if not further. Like many things in modern theoretical physics, it started with the black hole.

In 1972 Jacob Berkenstein argued on largely information-theoretic grounds that:

… black-hole entropy is equal to the ratio of the black-hole area to the square of the Planck length times a dimensionless constant of order unity.

Black holes, already with the reputation that everything “checks in” but nothing “checks out”, were already a bit of an oddity. That they couldn’t exist in thermodynamic equilibrium with other ‘normal’ objects wasn’t much of a stretch for something that could swallow entire galaxies. Thermodynamics, usually considered an emergent property, just didn’t contain in its ‘domain of applicability’ something as exotic as a black hole.

Nevertheless, in 1975 Hawking published a paper describing what has come to be known as “Hawking radiation“. As is well known, stuff can come out of black holes. And importantly the radiation has the frequency distribution of blackbody radiation. Hawking’s calculation was a pretty straight forward application of quantum field theory in curved spacetime where he took into account the boundaries associated with black holes.

The next year, Unruh with the general relativity equivalence principal in mind,  published “Notes on Black Hole Evaporation” which is actually better remember for what is variously called the “Unruh effect”, “Unruh temperature” or “Unruh temperature”. The take-away from that article was:

The behavior of particle detectors under acceleration is investigated where it is shown that an accelerated detector even in flat spacetime will detect particles in the vacuum.

The equation is linked here. He also wrote (remember this for later):

The similarity of this case with the behavior of a detector near the black hole is brought out, and it is shown that a geodesic detector near the horizon will not see the Hawking flux of particles.

So far in this story there are a lot of things going on:

• [1972] Berkenstein thinks black holes should have entropy because in many respects the area of event horizon acts like an entropy measure.
• [1975] Hawking does some quantum field calculations for the curved space around a black hole and finds black holes are thermodynamic objects.
• [1976] Unruh goes further and claims “empty space” will have a temperature relative to an accelerated observer.

There is an important and interesting backdrop to these results. Berkenstein’s original proposal that black holes had entropy faced an objection from Geroch. Here is a description of the problem from Stanford’s philosophy website:

From the time that Bekenstein first proposed that the area of a black hole could be a measure of its entropy, it was know to face difficulties that appeared insurmountable. Geroch (1971) proposed a scenario that seems to allow a violation of the generalized second law. If we have a box full of energetic radiation with a high entropy, that box will have a certain weight as it is attracted by the gravitational force of a black hole. One can use this weight to drive an engine to produce energy (e.g., to produce electricity) while slowly lowering the box towards the event horizon of the black hole. This process extracts energy, but not entropy, from the radiation in the box; once the box reaches the event horizon itself, it can have an arbitrarily small amount of energy remaining. If one then opens the box to let the radiation fall into the black hole, the size of the event horizon will not increase any appreciable amount (because the mass-energy of the black hole has barely been increased), but the thermodynamic entropy outside the black hole has decreased. Thus we seem to have violated the generalized second law.

“… the generalized second law” meaning the second law of thermodynamics.  Not a good thing. Perpetual motion machines and all that. In 1982 Unruh and Wald satisfactorily resolved the apparent ‘paradox’. By applying the ‘Unruh effect’ to the situation of the box they showed the change in the global conservation equations was exactly the amount to eliminate the problem. Exactly the amount.

Something is obviously going on here. Gravity, quantum field theory and the laws of thermodynamics fit together perfectly consistently. That Jacobson was able in 1995 to derive Einstein’s equation from the Unruh temperature, the proportionality of entropy to event horizon surface area and a generalized second law of thermodynamics says something about the components of that relationship.

To be continued…