*Back to the Future*, with plutonium fueled Deloreans and Star-Trek warp drives dominating the silver screen. However, we are all time travelers at a lowly rate of one second per second (still not satisfied?). You could travel into the future by boarding a space-craft, traveling near the speed of light to a distant galaxy, slowing, and then turning back and traveling near the speed of light to earth. But what about going to the past? General relativity allows paths in space-time where proper time reverses or loops back upon itself into the past, such closed time-like curves (CTCs) may be the ultimate candidate. But other solutions like Godel's rotating fluid universe (if you walk along the direction of rotation you would end up back where you started but backwards in time), Gott's cosmic strings (topological defects caused by phase transitions in early universe) and Kerr's worm holes stand out as equally probable. In fact, certain anomalies such as the grandfather and information paradoxes have forced some to propose a protected chronology postulate. However when we combine quantum teleportation with post-selection, we get a CTC with the ability to choose what types of states may be teleported and by extension, preventing a particle or person from preventing their existence from the word go in principle just as Novikov's self-consistency condition avoids paradoxes by making their probability zero.

## Wednesday, 29 May 2013

### Time- A Traveller's Guide

## Thursday, 16 May 2013

### Gravity- Taking a Quantum Leap

Gravity. It's what makes the tides come in and out, what makes the sun go up and down. Yet it's nothing more than matter telling space how to curve and space telling matter how to move; a universal force arising from a universal dialogue. But among the holy grails of modern physics, the challenge of marrying quantum mechanics (QM) with general relativity (GR) is proving just as quirky as placing square pegs in round holes. One where loops, strings and twistors go head to head to unearth gravity at the quantum regime; where our current picture of space-time may potentially transfigure and where theory of everything may eventually, finally catch up. Let's start at the start. Once upon a time Newton looked at space as a sort of stage or fixed background on which matter moves, then Einstein showed in GR that gravity necessitated explanation in terms of a field that then emerged as the 'fixed background' that Newton proposed; the acceleration in

*F=ma*became a part of a field entity and not an absolute view of space. Einstein had checkmated the Newtonian view, there is no background space-time. From then on, it becomes easy to see how QM views the gravitational field in terms of a discrete, grainy and granular framework where the 4D continuum is broken (a sort of 'fields over fields' and not on background space). The unfavorable encounter with ultraviolet divergences in quantum field theory has seduced many to the idea that strings are the thing to better unite QM with GR, assuming the extravagance of extra compactified dimensions, super-symmetry and even proton decay; but string theory ain't the only game in town. When we consider Faraday's own field concept as a variable in Yang-Mills theory, they become excited fluctuations of the quantum field and form closed loops when there are no charges; they are probably also excitations of the gravitational field. Here loop quantum gravity (LQG) comes to the rescue as a background independent format with no need of unification and super-partners for particles; the loops cross over at nodes forming spin networks, with each node constituting a monomer of space, hence Newton's picture is substituted with spinfoam (the histories of the spin networks). Space becomes the spin network and the space-time becomes the spinfoam, where the histories of points, nodes and lines are surfaces or faces become connected; it's an approach very similar to Feynman's formalism of summing over histories, here we sum over the possible geometries and get rid of the divergences into infinities caused by perturbative quantum field theory. The loops are space per se and a state of space is therefore defined by a network of intersecting loops. Another road to quantum gravity is one championed by Penrose, the so called twistor: initially we thought of spinors at the lego bricks of discrete space-time as they distinguish between different spins, indirectly implying they can generate their own spaces. But an entity was needed to combine the notion of spin with linear momentum, in other words it must be an entity moving and rotating along with quantum and relativistic properties, hence the twistor. Penrose combined the concepts of rays of light and complex numbers to create a 'twistor space' as an analogue to space-time, like the spinfoam in LQG, a Riemann sphere represents all the possible histories of light rays. Here the sequences of events don't vary and fluctuate as one would expect from the quantum scale but instead the timing and location of events change in twistor space. As for real-world applications and tests of such theoretical frameworks, we can expect breaches of Lorentz invariance (different wavelengths of light travelling at varied velocities), a possible elimination of the initial singularity (as attempted by the Hartle-Hawking proposal), explanation of the inflationary era of the early universe, insight into black hole temperature and entropy in addition to greater insight as to what what the planck scale really looks like.
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