(well maybe.)New ideas on gravity would vanquish dark matter

["David Farber" <farber () gmail com>]

Begin forwarded message:

From: Dewayne Hendricks <dewayne () warpspeed com>
Subject: [Dewayne-Net] New ideas on gravity would vanquish dark matter
Date: January 18, 2017 at 12:48:42 PM EST
To: Multiple recipients of Dewayne-Net <dewayne-net () warpspeed com>
Reply-To: dewayne-net () warpspeed com

[Note:  This item comes from friend David Rosenthal.  DLH]

New ideas on gravity would vanquish dark matter
New ideas on gravity predict rotation curves of galaxies without dark matter.
By CHRIS LEE
Jan 18 2017
<http://arstechnica.com/science/2017/01/new-ideas-on-gravity-would-vanquish-dark-matter/>

Throughout the Universe, there are lots of signs that there's more gravity out there than there is visible matter to 
produce it. Over the last few decades, physicists have slowly come to the conclusion that it is not the laws of gravity 
that need to be changed, but rather that a massive particle is responsible for the extra gravity.

Now, it should be pointed out that this is not a whim. The distribution of dark matter describes all manner of 
gravitational phenomena at all scales, including some really weird things, like the Bullet Cluster. So, understandably, 
particle physicists and cosmologists get a bit touchy when people say that we should just modify gravity instead.

Erik Verlinde, a Dutch theoretical physicist, doesn't seem to care. The alternative title to Verlinde's talk, presented 
at a conference I'm attending, could be "How to piss off a room full of physicists in under 20 minutes."

Verlinde is not a crank, and his work is at the heart of a nationally recognized effort to understand the nature of 
gravity. He recognizes that any modification to gravity has to explain a whole range of phenomena, while also agreeing 
with general relativity at the right scales. That's a tall order.

Nevertheless, Verlinde thinks he is on to something.

Gravity as a consequence of quantum information

Now, before I go further, let me hasten to admit my ignorance. If I removed my brain, stomped on it, and put it back 
in, I would be no more ignorant of Verlinde's work than I am now. With that disclaimer in place, let's continue with 
caution.

His central idea is that gravity is a consequence of information. In particular, gravity is driven by entropy, and 
entropy is related to available configurations of quantum states and entanglement between particles. All of these ideas 
were derived in the context of black hole physics. The big struggle was (and still is) to understand how a black hole 
dealt with quantum states. Could they cross the event horizon without destroying information?

Verlinde's insight was to see that any particular point in the Universe also has a horizon, given by the distance at 
which the expansion of the Universe occurs at speeds faster than light's. That means that some of the same issues that 
apply to black holes apply anywhere in the Universe. Which means we can use some of the same tools used to examine 
quantum information and black holes on the Universe as a whole.

When that's done, gravity naturally emerges.

> From the perspective of someone outside of this horizon, the same physics applies. The heavy lifting is to figure out 
> how the view from outside corresponds to the internal gravity. This is exactly what Verlinde has done. The 
> consequence, Verlinde claims, is that there is extra gravity compared to the mass. What's more, these gravitational 
> contributions naturally occur at just the right scales to explain many phenomena that we explain with dark matter at 
> the moment.

[snip]

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