Thursday, July 15, 2010

Gravity, string theory and emergent phenomena

An interesting report in the New York Times on a paper by Erik Verlinde which outlines the possible start of a new theory of gravity.

Let's start at the beginning. Verlinde is a "string theorist" like about 95% of all physicists currently working in fundamental particle physics. String theory is the current leading (only) candidate for the great desire of a unified theory of physics.

However string theory itself has its critics. In books like Not Even Wrong and The Trouble with Physics the practice of string theory is heavily criticised both for its failure to generate testable results and its failure to provide any kind of explanation.

Without delving into the philosophy of science too far these are the two most common tests of "science" - that its results are testable (or falsifiable) and that the consequence is that something complex is "explained" (typically for example the trajectories of the planets explained by the force of gravity).

As such your standard string theorist would not be the prime candidate for a philosophical discussion. They are better known as incredibly smart individuals creating increasingly esoteric and largely irrelevant mathematical models. As the article states "Dr. Verlinde is not an obvious candidate to go off the deep end. He and his brother Herman, a Princeton professor, are celebrated twins known more for their mastery of the mathematics of hard-core string theory than for philosophic flights."

The article rather inaccurately describes the paper. The paper does not make the "contention that gravity is indeed an illusion". The paper actually claims that gravity - and indeed space itself - can be derived as emergent properties of a simpler universe "using only space independent concepts like energy, entropy and temperature."

Indeed as the newspaper article notes Dr. Verlinde explained “This is not the basis of a theory. I don’t pretend this to be a theory. People should read the words I am saying opposed to the details of equations.”

What is, however, is the first attempt to bring some of the learnings of chaos theory and statistical physics to the issues of fundamental particles. Up till now gravity has been required in the theory to exist between all objects of "mass" - and is a force that has to be reconciled in particle physics. The approach in the paper says that we can construct models of fundamental physics in which the "observable" phenomena of gravity are emergent properties of the underlying system.

Whether Verlinde's paper will be the basis of an actual theory or whether it will merely be a signpost to a theory that treats certain concepts of 20th century physics as emergent phenomena is yet to be revealed.

I have no scientific basis to believe this, nor the mathematics to come to grips with it. But quite frankly it seems "right". As a philosophical concept it requires a reconceptualization of the concept of "reductionism" which has mostly been thought of very mechanistically. In fields as diverse as psychology and neuroscience and economics the failure of the simple concept of reductionism in the face of complexity has been slowly developing. It is not surprising that it will come to physics too.

Note: The NYT article says

Lee Smolin, a quantum gravity theorist at the Perimeter Institute for Theoretical Physics, called Dr. Jacobson’s paper “one of the most important papers of the last 20 years.”

Smolin is the author of "The Trouble with Physics".

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