Experiments Show Gravity Is Not an Emergent Phenomenon

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AcesHigh
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Experiments Show Gravity Is Not an Emergent Phenomenon

Postby AcesHigh » Sun Aug 28, 2011 12:14 am

http://www.technologyreview.com/blog/ar ... 2/?ref=rss

Experiments Show Gravity Is Not an Emergent Phenomenon

The way gravity effects quantum particles proves that it cannot be an emergent phenomenon, says physicist.

One of the most exciting ideas in modern physics is that gravity is not a traditional force, like electromagnetic or nuclear forces. Instead, it is an emergent phenomenon that merely looks like a traditional force.

This approach has been championed by Erik Verlinde at the University of Amsterdam who put forward the idea in 2010. He suggested that gravity is merely a manifestation of entropy in the Universe, which always increases according to the second law of thermodynamics. This causes matter distribute itself in a way that maximises entropy. And the effect of this redistribution looks like a force which we call gravity.

Much of the excitement over Verlinde's idea is that it provides a way to reconcile the contradictions between gravity, which works on a large scale, and quantum mechanics, which works on a tiny scale.

The key idea is that gravity is essentially a statistical effect. As long as each particle is influenced by a statistically large number of other particles, gravity emerges. That's why it's a large-scale phenomenon.

But today, Archil Kobakhidze at The University of Melbourne in Australia points to a serious problem with this approach. He naturally asks how gravity can influence quantum particles.

Kobakhidze argues that since each quantum particle must be described by a large number of other particles, this leads to a particular equation that describes the effect of gravity.

But here's the thing: the conventional view of gravity leads to a different equation.

In other words, the emergent and traditional views of gravity make different predictions about the gravitational force a quantum particle ought to experience. And that opens the way for an experimental test.

As it happens, physicists have been measuring the force of gravity on neutrons for ten yeas or so. And...wait for the drum roll... the results exactly match the predictions of traditional gravitational theory, says Kobakhidze.

"Experiments on gravitational bound states of neutrons unambiguously disprove the entropic origin of gravitation," he says.

That's an impressive piece of physics. It'll be interesting to see how Verlinde and his supporters respond.

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Postby Giorgio » Sun Aug 28, 2011 1:31 am

The whole idea of gravity as an emergent phenomena is contrary to our observations and Verlinde theory is derived by a twist of logic that does not hold once carefully analyzed.

It is well observed that gravitational interaction between bodies is related to the mass of the bodies. On the other hand an entropic force depends on the system property and not on the mass of the bodies that are part of the system.
It comes immediately clear that the idea of gravity as an entropic force is not supported by our actual observations.

Additionally, we can remember the connection between energy increase and entropy increase.
If we have energy increase than we can have an entropy increase, but a "spontaneous" entropy increase does not produce an energy increase.

So, the entropy increase of the system is caused by its energy increase and not from a tendency of the bodies to redistribute themselves so as to increase entropy. If this tendency does not exist than one cannot theorize the existence of an entropic force (no cause=no effect) and so gravity is not an entropic force.

Sorry for the long rant. Is 3:30 AM and I am suffering from insomnia.

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Postby Giorgio » Sun Aug 28, 2011 11:18 am


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Postby Tom Ligon » Wed Aug 31, 2011 8:16 pm

There is a fond hope (not to be confused with a theory) that gravity is an emergent force, essentially a little leftover effect, of electromagnetism. This leads to ideas of anti-gravity boots and space propulsion systems which don't require rocket thrust. Cue the funny spinning disks.

I'll stand by while the scientific method sorts all this out, but I lean the other way. I personally believe gravity refuses to unify with the other fundamental forces because it is fundamentally different in kind. My own fond hope (not to be confused with a theory) is that gravity is the result of a level of physics we don't even suspect exists.

In fiction, I can weave a pretty good yarn that gravity is part of c-prime physics, which allows faster than light travel. In reality, I'd rather readers not analyze that too deeply. But I do like any theory that keeps gravity apart from the rest of the forces, and from being emergent from something mundane, because it keeps my particular fond hope alive.

I will point out that we're now dealing with two totally mysterious but otherwise well-accepted phenomena: dark matter and dark energy. We have no idea what they are, yet physics has pretty well bought into the idea that they are real. The first one makes gravity, and the second is essentially anti-gravity. Yet, we have not identified any demonstrable physics from which these arise. And I think we can say we really don't know what gravity is, either. Also, cosmologists almost universally accept that the birth of the Universe involved an instant of "inflation", the expansion of space FTL. A bit of handwavium follows saying it is OK for space to expand FTL, but within space nothing can travel FTL. This, for all the kids studying science today, is marvelous news ... there are still discoveries to be made in science, and they cover 95% of everything. At least!

One more little myth of modern physics bugs me. Because "action at a distance is spooky", we have an article of faith that forces must have a mediating particle. Thus, there must be a graviton. Yet, with the galaxy, and and our local group, and our supercluster all held together by gravity, I am expected to believe every particle of matter is exchanging gravitons with every other particle of matter in the Universe. Maybe the poor things run out of steam over 7 billion LY or so, and hence we have apparent Dark Energy. But if gravity is a distortion of space per Einstein's vision, why would we need mediating gravitons? Matter itself would be the mediator that distorts space. Gravitons are a tough sell for me. I'm OK with spooky.

;)
Last edited by Tom Ligon on Wed Aug 31, 2011 8:31 pm, edited 1 time in total.

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Postby djolds1 » Fri Sep 02, 2011 1:33 am

Vae Victis

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Postby Giorgio » Fri Sep 02, 2011 2:36 am



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