* Astronomy

Rethinking Einstein: The end of space-time

It was a speech that changed the way we think of space and time. The year was 1908, and the German mathematician Hermann Minkowski had been trying to make sense of Albert Einstein's hot new idea - what we now know as special relativity - describing how things shrink as they move faster and time becomes distorted.
And so space-time - the malleable fabric whose geometry can be changed by the gravity of stars, planets and matter - was born. It is a concept that has served us well, but if physicist Petr Horava is right, it may be no more than a mirage. Horava, who is at the University of California, Berkeley, wants to rip this fabric apart and set time and space free from one another in order to come up with a unified theory that reconciles the disparate worlds of quantum mechanics and gravity - one the most pressing challenges to modern physics.
Read more

AdS/CFT correspondence

In physics, the AdS/CFT correspondence (anti-de-Sitter space/conformal field theory correspondence), sometimes called the Maldacena duality, is the conjectured equivalence between a string theory defined on one space, and a quantum field theory without gravity defined on the conformal boundary of this space, whose dimension  is lower by one or more. The name suggests that the first space is the product of anti de Sitter space (AdS) with some closed manifold like sphere, orbifold, or noncommutative space, and that the quantum field theory is a conformal field theory (CFT).
Read more

Title: A Cosmic Peek at Spacetime Foam
Authors: Wayne A. Christiansen, David J. E. Floyd, Y. Jack Ng, Eric S. Perlman

Plausibly spacetime is "foamy" on small distance scales, due to quantum fluctuations. We elaborate on the proposal to detect spacetime foam by looking for seeing disks in the images of distant quasars and AGNs. This is a null test in the sense that the continued presence of unresolved "point" sources at the milli-arc second level in samples of distant compact sources puts severe constraints on theories of quantised spacetime foam at the Planckian level. We discuss the geometry of foamy spacetime, and the appropriate distance measure for calculating the expected angular broadening. We then deal with recent data and the constraints they put on spacetime foam models. Thus far, images of high-redshift quasars from the Hubble Ultra-Deep Field (UDF) provide the most stringent test of spacetime foam theories. While random walk models (alpha = 1/2) have already been ruled out, the holographic (alpha=2/3) model remains viable. Here alpha~1 parametrises the different spacetime foam models according to which the fluctuation of a distance L is given by ~ L^(1 - alpha) l_P^alpha, with l_P being the Planck length. Indeed, we see a slight wavelength-dependent blurring in the UDF images selected for this study. Using existing data in the HST archive we find it is impossible to rule out the alpha=2/3 model, but exclude all models with alpha<0.65.

Read more (164kb, PDF)

Physicists have struggled to marry quantum mechanics with gravity for decades. In contrast, the other forces of nature have obediently fallen into line. For instance, the electromagnetic force can be described quantum-mechanically by the motion of photons. Try and work out the gravitational force between two objects in terms of a quantum graviton, however, and you quickly run into trouble - the answer to every calculation is infinity. But now Petr HoYava, a physicist at the University of California, Berkeley, thinks he understands the problem. Its all, he says, a matter of time.
The solution, HoYava says, is to snip threads that bind time to space at very high energies, such as those found in the early universe where quantum gravity rules.
At low energies, general relativity emerges from this underlying framework, and the fabric of spacetime restitches.

Read more