Occam's Machete: Cosmology

Since Einstein we have known that gravity was somehow special. Physicists have a unified standard model of forces and elementary particles but everyone knows that the big problem is gravity. String theory contains gravitons and explains gravity like any of the other forces. But String theory has a lot of things, in fact it is more a tool set or framework than a single theory.

But what if gravity is just a shadow on the cave wall? Or, to put it in terms a physicist would use, what if gravity is emergent—the holographic affect of entanglement entropy? I have written about it before because the idea seems to be gaining steam, at least among wannabe crackpots (like me perhaps).

Ted Jacobson (no crackpot) wrote the seminal paper in 1995 describing how to derive Eistein’s General Relativity equations from, “the form of black hole entropy together with the fundamental relation Q = TdS.” It is a short elegant paper that has been sited often of late. Back in 1995 it was pretty hard for an amateur to follow controversy in the physics world so I didn’t notice a big reaction to Jacobson.

Then came Verlinde who shook things up in January of this year. I’ve already written about the reaction.

Verlinde seems to have started a lot of people thinking because there have been several new papers jumping on the bandwagon (1,2). In a recent FQXi post Vlatko Vedral shills his own coverage of the territory in his book, Decoding Reality.

I adopted his (Jacobson’s) 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). Vlatko Vedral.

For whatever intuition is worth, the ideas of entropy (which is tied to the arrow of time) and entanglement (spooky action at a distance) do seem to be special too.  And wasn’t it that honored cosmologist David Brin who said our universe is just a gigantic simulation?

Update: FQXi covers several of the papers and posts you’ve seen here but you might like the writing better. Check it out.

Woit and Motl agree on something and that is that Verlinde is on the wrong track, maybe even a crackpot.  But neither seem to have torn apart the math with a critical eye and, well let’s just say they have strong pre-existing opinions about the proper way to think.  They are both smart but extreme enough that I sometimes question their opinions only because they hold them.  What’s an amateur to do to get a balanced analysis?

Enter Sabine Hossenfelder or Bee as she is known to her blog readers.  She has recently spent time examining Verlinde’s paper producing a blog post and arXiv paper.  She offers some thoughtful criticism and asks the question, “so what?”  Some sort of equivalence between entropy and gravity is only useful if it can be used as a bridge to apply gravity to the quantum world.  And she finds that part still lacking.

The year of Entropy continues with a paper referencing Verlinde but describing gravity and dark energy as effects of entanglement entropy. Read it on arXiv.

If Roger Zelazny had written the Amber books today I’m sure he would have had a Courts of Entropy instead of Chaos.  But I digress…

The Holographic Principal states that a 5-D universe can be encoded holographically on a 4-D universe.  Just as a 3-D object can be captured in a flat hologram.   The implications are that the supertring universe of 5 dimensions can be seen by thinking of our own universe as a hologram.  Using this principal some really smart guys (including George Smoot of Nobel fame) proposed on arXiv that dark energy in our universe is only a holographic affect of an entropic force in higher dimensions.

They develop their theory and use it to predict the amount of dark energy we should see in our universe.  Unless there are mistakes in their reasoning, which I cannot judge, the most interesting thing is how close the prediction is to the dark energy seen in the supernova data—you know the data that showed our universe is expanding at an accelerating rate.  The other interesting thing is this how people are beginning to find observable predictions in String/M-theory.

I’m going to have to pay more attention to this Entropy stuff.

BTW Motl has some thoughts on this paper too.

Lately I’ve seen a several papers describing how one fundamental way of measuring the universe or another is actually emergent from a different way of looking at things.  There are those that claim time does not exist in any fundamental way, we are just looking at things wrong. So what is fundamental?

Entropy.

Understand entropy and gravity emerges.  Understand it in a moving frame and General Relativity emerges.  The idea is published on arxiv (On the Origin of Gravity and the Laws of Newton, Erik Verlinde) and even made it to Slashdot.  Woit surveys other entropy related ideas.

As a computer scientist there is something appealing in the notion that information is fundamental and that all the laws of nature are derived from the way information works.

Update: There is a long post from Verlinde on Motl’s blog here.  It illuminates some of his ideas and explains their history.  If you can stomach Motl’s bullying read the comments too, there are many.