Quantum matter in quantum space-time

Quantum matter in quantum space-time is discussed using general properties of energy-conservation laws. As a rather radical conclusion, it is found that standard methods of differential geometry and quantum field theory on curved space-time are inapplicable in canonical quantum gravity, even at the level of effective equations.Comment: 17 pages, v2: further references and more detailed conclusion

Phenomenological implications of an alternative Hamiltonian constraint for quantum cosmology

In this paper we review a model based on loop quantum cosmology that arises from a symmetry reduction of the self dual Plebanski action. In this formulation the symmetry reduction leads to a very simple Hamiltonian constraint that can be quantized explicitly in the framework of loop quantum cosmology. We investigate the phenomenological implications of this model in the semi-classical regime and compare those with the known results of the standard Loop Quantum Cosmology.Comment: 10 pages, 7 figure

On Deflection of Potentially Dangerous Asteroids

ABSTRACT As has been widely discussed recently, our planet may become a target for asteroids. We consider several scenarios proposed to prevent asteroid collisions with Earth. The asteroid 99942 Apophis is considered as a typical representative. Among others, the recent "gravitational tractor" scenario 2 is discussed. For a simplistic toy-model we obtain estimates for both the mass of the tractor and the amount of fuel required to tow a potentially dangerous asteroid off-course so as to avoid a collision with the Earth. In addition, we analyze two more scenarios titled "sling-shot" and "bumping", and comment on their relative efficiency compared to the "towing" scenario. Based on the analysis, the bumping scenario looks most promising. KEYWORDS Near-Earth objects, Gravitational Tractor, Asteroid 99942 Apophis INTRODUCTION After watching a presentation on the possibility of asteroids colliding with the planet Earth, a thought occurred: this may be reality in the near future. In fact, due to one specific asteroid (99942 Apophis), this idea may be even more of a reality than first thought. Situations such as an asteroid impact have been depicted in disaster films since the late twentieth century, all with the same solution -to destroy the asteroid with high yield explosives. There is however an issue with the use of explosives: an asteroid has an original quantity of energy (kinetic, potential, etc.). If the asteroid were to be split into smaller pieces (as an explosion would), then the only change occurring is that the overall energy is then concentrated throughout several pieces instead of just one, and thus after collision the energy is spread over several impact locations as opposed to concentrated at one. Another consideration is that of aerodynamic heating. Most small space born objects collide with the Earth's atmosphere at high velocities, which in turn generates friction between the object and the atmosphere -this friction will tend to disintegrate most of the materials that enter the Earth's atmosphere

Effective constraints of loop quantum gravity

Within a perturbative cosmological regime of loop quantum gravity corrections to effective constraints are computed. This takes into account all inhomogeneous degrees of freedom relevant for scalar metric modes around flat space and results in explicit expressions for modified coefficients and of higher order terms. It also illustrates the role of different scales determining the relative magnitude of corrections. Our results demonstrate that loop quantum gravity has the correct classical limit, at least in its sector of cosmological perturbations around flat space, in the sense of perturbative effective theory.Comment: 44 pages, 6 figure