We’re only a week and a half into the new decade, but already I’m seeing a trend…
A few days ago Sean Carroll’s book From Eternity to Here came out, promoting the idea that understanding time and cosmology is all about understanding entropy. The same day saw Erik Verlinde’s arXiv preprint On the Origin of Gravity and the Laws of Newton, which argues that
Gravity is explained as an entropic force caused by changes in the information associated with the positions of material bodies.
Verlinde is a well known string theorist, and the paper is somewhat of a repudiation of the motivating idea for string theory unification, that string theory predicts gravity since it has a spin two massless state. But even with the main motivation gone, all is not lost for string theory, since
The presented ideas are consistent with our knowledge of string theory, but if correct they should have important implications for this theory as well. In particular, the description of gravity as being due to the exchange of closed strings can no longer be valid. In fact, it appears that strings have to be emergent too.
Today, it’s yet more entropy, with The Entropic Landscape by Bousso and Harnik, which propounds the Entropic Principle, that:
the number of observers is proportional, on average, to the amount of entropy produced.
and claims that this principle quantitatively predicts six important aspects of cosmology.
While much of physics in the last century was dominated by a highly successful program to identify fundamental degrees of freedom of nature and understand their dynamics using increasingly deep and sophisticated mathematical formalisms, now the trend appears to be very different. Many of the most well-known theorists are pursuing research programs with the remarkable features that:
The last century was a hugely successful one for physics, whether this new order will be equally successful remains to be seen.
Update: Verlinde is adding explanations of points in his paper and conducting a discussion of it on Lubos Motl’s blog here. He now says that, to explain quantum gravity
I am not sure that string theory is the way to go.
Even though under his new framework string theory explains nothing about any fundamental physics, Verlinde refuses to give up on it, arguing that:
It should also be emergent, and it is nothing but a framework like quantum field theory.
In fact, I think of string theory as the way to make QFT in to a UV complete but still effective framework. It is based on universality. Many microscopic systems can lead to the same string theory. The string theory landscape is just the space of all universality classes of this framework. I have more to say about it, but will keep that for a publication, or I will post that some other time.