Tuesday, December 4, 2012

Two Provocative Papers On Gravity

Gravity can be neither classical nor quantized
Sabine Hossenfelder
(Submitted on 3 Dec 2012)
I argue that it is possible for a theory to be neither quantized nor classical. We should therefore give up the assumption that the fundamental theory which describes gravity at shortest distances must either be quantized, or quantization must emerge from a fundamentally classical theory. To illustrate my point I will discuss an example for a theory that is neither classical nor quantized, and argue that it has the potential to resolve the tensions between the quantum field theories of the standard model and general relativity.
7 pages, third prize in the 2012 FQXi essay contest "Which of our basic physical assumptions are wrong?"

Hossenfelder's essay considers the possibility that Planck's constant runs with the energy level of the environment, in an amount only discernable at very high energies, and reaches zero at the temperatures seen in the early days of the Big Bang and inside black holes.  This causes the gravitational constant to run to zero at these energy levels, so rather than reaching infnity and creating singularities, gravity turns itself off when temperatures get great enough.

Not on but of
Olaf Dreyer
(Submitted on 3 Dec 2012)
In physics we encounter particles in one of two ways. Either as fundamental constituents of the theory or as emergent excitations. These two ways differ by how the particle relates to the background. It either sits on the background, or it is an excitation of the background. We argue that by choosing the former to construct our fundamental theories we have made a costly mistake. Instead we should think of particles as excitations of a background. We show that this point of view sheds new light on the cosmological constant problem and even leads to observable consequences by giving a natural explanation for the appearance of MOND-like behavior. In this context it also becomes clear why there are numerical coincidences between the MOND acceleration parameter, the cosmological constant and the Hubble parameter.
9 pages. This article received a forth prize in the 2012 FQXi essay contest "Questioning the Foundations".

Dreyer constructs a formalism in which particles are excitations of space-time.  In this construction, ground state energy in the vacuum is zero and the Casimir effect can be explained by means other than vacuum energy. 

The energy of the vacuum in this formulation is described by changes in the gravitational ground state energy, in a vacuum at zero temperature without considering entropy, produces the familiar Newtonian gravitational constant. 

But, when entropy is considered, gravity weakens at non-zero temperatures (in a derivation based on the definition of free energy in a system with entropy).  The entropic effect is composed of contributions from wavelengths of all lengths, but the wavelengths longer than the size of the universe (the maximal wavelengths) cannot contribute.  Taking this limitation into effect, it is remarkable that the Hubble scale for the size of the universe, and the square root of the inverse of the cosmological constant both produce cutoff scales that change the strength of the gravitational constant at a cutoff value on the same order of magnitude of Milgrom's MOND regime's empirically determined cutoff value. 

Hence, entropy effects on a gravitational force in the context of particles seen as excitations of space-time naturally produces MOND-like gravitational effects that mirror an empirically derived modification of gravity that captures essentially all effects attributed to dark matter at the galactic scale and some of the effects attributed to dark matter at the galactric cluster scale (where ordinary but invisible matter could in theory account for the additional dark matter effects observed in galactic clusters whose composition and structure are still not fully understood).

Dreyers cites many recent papers in the literature at the close of his essay that have combined the ideas of Verlinde and Milgrom in a similar way, arguing that his approach is notable because it does not use a holographic approach and is three dimensional. 

Other essays in the contest can be found here.

As a bonus, a recent paper on quark-lepton complementarity that integrates the latest empirical data can be found here and here and here and here by an overlapping group of Chinese investigators.  They examine different parameterization regimes to determine which ones might be consistent with both QLC and the empirical data and suggests that measurements of CP violation in neutrino oscillations is critical to discriminating between the possible options.  Another investigator, independent of that group, looks at the issues here.  A third team looks at related problems here.  A related investigation in the CKM matrix itself is here and focuses on a number of interesting relationships between the mass matrix and mixing matrixes there.


Maju said...

How does Hossenfelder explain black holes then?

andrew said...

He argues that gravity is almost identical until the infinity in the equation of general relativity almost appears and then gravity becomes weaker and weaker as one gets closer and closer to a true singularity. So, the outer vicinity of a black hole is very similar.

But, while the event horizon is almost a complete boundary between inside and outside in his analysis, it isn't a perfect platonic ideal of a boundary and can be overcome at the quantum level, giving rise to Hawking radiation.

The interior of a black hole is in his analysis a gravity-free super high temperature zone in which information is preserved rather than destroyed, with a boundary that is ever so slightly permiable.

Maju said...

Hmmm... according to her (Sabine is a woman's name, right?) there can't be no such things as singularities, so you cannot "get close to a true singularity".

Regardless, where is mass concentrated then inside a black hole, always according to her theory, In some sort of hollow ball?

If so, what happens with this hollow ball as more and more mass enters the black hole: does it expand its diameter? Why? How?

How do attraction forces neutralize each other inside the black hole, as they approach the turning point of her equation and (1) more and more mass falls from the outside (outwards gravity) and (2) the mass of the hollow ball grows.

Without a repellent anti-gravity force (something like the strong force, which behaves differently depending on distance) I don't see how the singularity can be avoided, because the hollow ball would tend to collapse onto itself, as there's nothing repelling the components and keeping them at a distance.

Well, just brainstorming on the matter. Many questions. Also I'm "intuitively certain" that there must be an antigravity force of some sort and that it will be once shown to be linked to the strong force and hence to the electronuclear force. But I can well be wrong, of course.

andrew said...

The singularity is avoided by turning off gravity in places of very high matter-energy density. If gravity is zero rather than infinite when matter is sufficiently dense, then there is no singularity.