NS: Dark matter could meet its nemesis on Earth

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NS: Dark matter could meet its nemesis on Earth

Post by bystander » Mon Mar 01, 2010 6:23 pm

Dark matter could meet its nemesis on Earth
New Scientist - 2010 Mar 01
A SPINNING disc may be all that is needed to overturn Newton's second law of motion - and potentially remove the need for dark matter.

The second law states that a force is proportional to an object's mass and its acceleration. But since the 1980s, some physicists have eyed the law with suspicion, arguing that subtle changes to it at extremely small accelerations could explain the observed motion of stars in galaxies.

Stars move at speeds that suggest that galaxies have far more mass than is visible, which astronomers attribute to dark matter. But if Newton's second law could be modified ever so slightly, it would obviate the need for dark matter. The hypothesis, known as modified Newtonian dynamics (MOND), was proposed in 1981 by Mordehai Milgrom, then at Princeton University.

Ground-based tests of MOND had been thought impossible because of the confounding motions of the Earth. But now, Vitorio De Lorenci of the Federal University of Itajubá, Brazil, and colleagues have devised an experiment to do just that (arxiv.org/abs/1002.2766).

Testing the Newton second law in the regime of small accelerations - VA De Lorenci, M Faundez-Abans, JP Pereira

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TR: How Duality Could Resolve Dark Matter Dilemma

Post by bystander » Mon May 24, 2010 11:03 pm

How Duality Could Resolve Dark Matter Dilemma
Technology Review - 24 May 2010
Astrophysicists need to choose between dark matter or modified gravity to explain the Universe. But a strange new duality may mean they can have both

The debate over the wave or particle-like nature of light consumed physicists for 300 years after Isaac Newton championed particles and Christian Huygens backed the idea of waves. The resolution, that light can be thought of as both a wave and a particle, would have astounded these giants of physics, as indeed, it does us.

What shouldn't surprise us, though, is that other seemingly intractable arguments might be similarly resolved.

But exactly this may be in store for the dark matter conundrum which has puzzled astrophysicists for almost 80s years, according to Chiu Man Ho at Vanderbilt University in Nashville and a couple of buddies,

The problem is that galaxies rotate so fast that the matter they contain ought to fly off into space. Similarly, clusters of galaxies do not seem to contain enough mass to bind them together and so ought to fly apart. Since this manifestly doesn't happen, some force must be holding these masses in place.

Astrophysicists have put forward two explanations. The first is that these galaxies are filled with unseen mass and this so-called dark matter provides the extra gravitational tug. The second is that gravity is stronger at these intergalactic scales and so does the job by itself, an idea called modified Newtonian dynamics or MOND.

There is no love lost between the dark matter proponents and their MONDian counterparts: both say the other is wrong and scour the Universe in search of evidence to damn their opponents. Neither side has convincingly crushed the other's argument so far but all concerned seem to agree that when one triumphs, the other will be ground underfoot.

Perhaps there's another possibility, however: that they're both right.

What makes this possible is a new approach to gravity in which it is an emergent phenomenon related to entropy. We looked at this a few months ago here.

The basic idea is that parts of the Universe have different levels of entropy and this creates a force that redistributes matter in a way that maximises entropy. This force is what we call gravity.

So far, this approach has assumed a simple Universe. But cosmologists know that our Universe is not only expanding but accelerating away from us. What Chui and co have done is derive gravity as an emergent force using the same entropic approach but this time in a Universe that is accelerating.

The result is a form of gravity in which parameters for acceleration and mass share a strange kind of duality: either the acceleration term can be thought of as modified as in MOND; or the mass term can be though of as modified, as in the dark matter theory.

In effect, Chui and co are saying that dark matter and MOND are two sides of the same coin.

Interestingly, the effect of each type of modification seems to be scale dependent. In this theory, the MONDian interpretation works at the galactic scale while the dark matter interpretation works best at the scale of galactic clusters.

That's actually how the observational evidence pans out too. MOND seems to better explain the real behaviour of galaxies while the dark matter approach better explains the structure of galaxy clusters.

Could it be that both are manifestations of the same thing? Only the brave or foolish would rule it out. And stranger things have happened in physics, as Newton and Huygens would surely attest to.

MONDian Dark Matter - Chiu Man Ho, Djordje Minic, Y. Jack Ng
Abstract: We provide a holographic dual description of Milgrom's scaling associated with galactic rotation curves. Our argument is based on the recent entropic reinterpretation of Newton's laws of motion. We propose a duality between cold dark matter and modified Newtonian dynamics (MOND). We introduce the concept of MONDian dark matter, and discuss some of its phenomenological implications. At cluster as well as cosmological scales, the MONDian dark matter would behave as cold dark matter, but at the galactic scale, the MONDian dark matter would act as MOND.

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TR: Gravity Emerges from Quantum Information

Post by bystander » Mon May 24, 2010 11:07 pm

Gravity Emerges from Quantum Information
Technology Review - 26 March 2010
The new role that quantum information plays in gravity sets the scene for a dramatic unification of ideas in physics.

One of the hottest new ideas in physics is that gravity is an emergent phenomena; that it somehow arises from the complex interaction of simpler things.

A few month's ago, Erik Verlinde at the the University of Amsterdam put forward one such idea which has taken the world of physics by storm. Verlinde suggested that gravity is merely a manifestation of entropy in the Universe. His idea is based on the second law of thermodynamics, that entropy always increases over time. It suggests that differences in entropy between parts of the Universe generates a force that redistributes matter in a way that maximises entropy. This is the force we call gravity.

What's exciting about the approach is that it dramatically simplifies the theoretical scaffolding that supports modern physics. And while it has its limitations--for example, it generates Newton's laws of gravity rather than Einstein's--it has some advantages too, such as the ability to account for the magnitude of dark energy which conventional theories of gravity struggle with.

But perhaps the most powerful idea to emerge from Verlinde's approach is that gravity is essentially a phenomenon of information.

Today, this idea gets a useful boost from Jae-Weon Lee at Jungwon University in South Korea and a couple of buddies. They use the idea of quantum information to derive a theory of gravity and they do it taking a slightly different tack to Verlinde.

At the heart of their idea is the tricky question of what happens to information when it enters a black hole. Physicists have puzzled over this for decades with little consensus. But one thing they agree on is Landauer's principle: that erasing a bit of quantum information always increases the entropy of the Universe by a certain small amount and requires a specific amount of energy.

Jae-Weon and co assume that this erasure process must occur at the black hole horizon. And if so, spacetime must organise itself in a way that maximises entropy at these horizons. In other words, it generates a gravity-like force.

That's intriguing for several reasons. First, Jae-Weon and co assume the existence of spacetime and its geometry and simply ask what form it must take if information is being erased at horizons in this way.

It also relates gravity to quantum information for the first time. Over recent years many results in quantum mechanics have pointed to the increasingly important role that information appears to play in the Universe.

Some physicists are convinced that the properties of information do not come from the behaviour of information carriers such as photons and electrons but the other way round. They think that information itself is the ghostly bedrock on which our universe is built.

Gravity has always been a fly in this ointment. But the growing realisation that information plays a fundamental role here too, could open the way to the kind of unification between the quantum mechanics and relativity that physicists have dreamed of.

Gravity from Quantum Information - Jae-Weon Lee, Hyeong-Chan Kim, Jungjai Lee
Abstract: It is suggested that classical Einstein gravity can be derived by using the Landauer's principle applied to an information erasure at causal horizons and Jacobson's idea linking the Einstein equation to thermodynamics. Our result implies that gravity has a quantum informational origin.

On the Origin of Gravity and the Laws of Newton - Erik P. Verlinde
Abstract: Starting from first principles and general assumptions Newton's law of gravitation is shown to arise naturally and unavoidably in a theory in which space is emergent through a holographic scenario. Gravity is explained as an entropic force caused by changes in the information associated with the positions of material bodies. A relativistic generalization of the presented arguments directly leads to the Einstein equations. When space is emergent even Newton's law of inertia needs to be explained. The equivalence principle leads us to conclude that it is actually this law of inertia whose origin is entropic.