I recently wrote a popular science book called "Decoding Reality: The Universe as Quantum Information." The point I was trying to make is that the fundamental building blocks out of which everything around us is constructed are bits of information (rather than energy and matter, or cheese and crackers, or whatever else).
Quantum physics naturally lends itself to this view, as the way we describe the world within the quantum formalism is already very much information-theoretic. (FQXi's Caslav Brukner also talks about this relationship in the article, "The End of the Quantum Road.") First and foremost, we talk about states of a physical system as catalogues of information (a term that Schrödinger himself coined when he invented quantum physics) giving us probabilities with which we should expect experiments to yield different results. The physical evolution of the system is then just a rule telling us how this catalogue of bets on the future states of the system should be updated with time (we call this rule for updating bets the Schrödinger equation).
This is all fine, but, of course, in addition to quantum physics, we have another equally successful physical theory, namely general relativity, that describes gravitational phenomena. And to date quantum physics and general relativity have not be unified. In order to understand the world we seem to need both quantum physics and general relativity. Unifying them is possibly the biggest open problem in physics today. So if I claim that information is fundamental and that quantum physics is naturally a theory of information, what happens with gravity?
The way I dealt with this issue in my book was to invoke a very beautiful paper of FQXi's Ted Jacobson. In 1995 he published a letter in the Physical Review, where he showed how to derive Einstein's gravity from thermodynamics! I adopted his logic in my book to suggest, with a tongue in cheek, that gravity can in fact be derived from information theory (albeit with a little bit of help from quantum entanglement).
I understand that Jacobson's idea was (and still probably is) considered very controversial because it might imply that all those physicists trying to quantize gravity are simply wasting their time (since, according to Jacobson, gravity is not a fundamental force, but merely a kind of thermodynamic quantum noise). This is why I was surprised to see that recently a Dutch physicist Erik Verlinde has been writing to add extra weight to Jacobson's argument (see arxiv.org/abs/1001.0785).
Having skimmed through his most recent paper, my feeling is that Verlinde has in the end not added much to Jacobson. What he did try to do is perhaps bring it closer to everyday physics, rather than engage the full machinery of Riemannian manifolds (which regrettably is necessary to phrase general relativity properly). Verlinde shows (the word "shows" should be interpreted very loosely - he effectively just performs a dimensional analysis of a kind) how to derive Newton's "inverse square" law of gravitational attraction. Given that Jacobson has already shown how to derive the full Einstein field equations from thermodynamics, Newton's law just follows as a special case of Jacobson's derivation. However, it is likely that Jacobson's stunning idea has not had as much impact in the wider physics community simply because of its high level of mathematical sophistication. The merit of Verlinde's publication might then just be to help bring Jacobson to the masses.
Let me now go in medias res. It's very simple to explain how gravity might arise from entropy. First we acknowledge the fundamental thermodynamical relationship that entropy times temperature equals heat. Heat itself is nothing but a form of energy, which according to Einstein equals mass times speed of light squared. The entropy we assume to be proportional to area (the so called Holographic principle, which you can read about in more detail in the article, "The Black Hole and the Babel Fish"), in other words proportional to radius squared. The temperature is, according to FQXi's Paul Davies and Bill Unruh, proportional to acceleration, which in turn is force divided by mass (from Newton's second law). Putting all this together gives us the force equal to the product of masses divided by distance squared, namely Newton's gravity! And that's more or less what Verlinde does.
How watertight is this argument? How much of gravity is really just thermodynamics? The derivation involves two crucial relationships we believe to be true, but which, at present, have no experimental evidence to support them. First of all, the relationship between entropy and area - the aforementioned Holographic principle - has never been tested. Initially argued within black hole theomodynamics by Jacob Bekenstein, the relationship has been elevated to a universally-held principle (i.e. the Holographic principle) by Leonard Susskind. Although this principle is supported by a host of other theoretical ideas (including quantum entanglement, that I used in my book to explain why entropy is proportional to area), it has never been experimentally tested. Secondly, the Unruh-Davies beautiful formula linking acceleration to temperature requires accelerations far beyond what can presently be achieved in order to generate enough detectable heat.
However, lack of experimental evidence aside, there are more fundamental objections that might invalidate the whole logic. One is that the entropy and area connection is not exact and requires additional corrections. This realization comes from a number of directions, including my own, in quantum information theory. Secondly, a physicist Danny Terno, from Macquarie University in Sidney, has argued that the geometric entropy (the one related to area) in quantum field theory is not a Lorentz scalar, while the black hole entropy is. His main point in a 2004 Physical Review Letter is that differently observers would each have to introduce a different temperature, preventing any description in terms of a single Davies-Unruh temperature that features in Jacobson's derivation.
Is this enough evidence to challenge Jacobson (and Verlinde)? Well, there are, as always, two ways of interpreting the more fundamental objections. Fundamental issues might indeed invalidate the Jacobson claim to entropic gravity altogether. This would simply spell an end to gravity being a consequence of thermodynamics and information theory.
But there is a more optimistic possibility. The fundamental objections could be turned on their head. They might mean that, if Jacobson's logic is correct in spirit if not in all its details, we need to make corrections to Einstein's field equations in order to accommodate possible changes to the Holographic principle and the nonexistence of a unique Davies-Unruh temperature. This is obviously exciting. Not only would it confirm that gravity is not a fundamental force, but it would also point to the need to modify Einstein's equations. Much as one would like to speculate about further consequences of all this, it is for now probably best to exercise the scientific method of suspended judgement. Only time indeed can tell what will turn out to be correct.
"Decoding Reality: The Universe as Quantum Information" is published by Oxford University Press, 2010. Check out more about it here.