Wednesday, 29 May 2013

Time- A Traveller's Guide

This moment is priceless. Or is it? You feel as if it began out in the future, suddenly became the present and will soon be dismissed into the past. But what if our common sense was less sensible than we ever envisaged? What if our deep down, fundamental intuitions about the fabric of reality were somehow flawed? Time is no exception. Throughout human inquiry, it seems to have lost its flair and panache as a measure of all things causal; Newton liken time to an undeviating arrow, a master clock, but it was Boltzmann who proved that the laws of motion work just as well in reverse and the discrimination between past and future is nothing more than the result of a thermodynamic asymmetry (we remember the past because of its state of low entropy). Einstein comes along and crushes the notion of absolute simultaneity, showing that two observers traveling at different velocities have disagreement regarding where and when events take place despite being unanimous to the space-time coordinates. He then smashes the Newtonian idea of synchronised time, noting that gravity can distort it and the universe cannot governed under a single chronological parameter. The fall of time as an illustrious entity is furthered with a final blow to our common sense of its 'flow'; the present moment or 'now' seems to move in the future direction and our consciousness seems to leap from one moment to the sequential one. But this is silly! (Let's try a thought experiment). Imagine a dart fired into the air, we like to think of time as a line with the motion of the dart captured in a set of successive snippets or photographs stretching out from past to future. Now we circle an arbitrary point as the 'present moment'. But hang on. This is deceptive as it shows the present as stationary and popping into being at a particular instant and disappearing soon after. Our common sense tells is us the present moment is moving into the 'open' future, so lets we circle all the photos of the dart's motion to satisfy this condition. Now the motion of the dart is better envisioned but the flow of time becomes an illusion, there is no objective present moment except a subjective one. Circling all the photos of the dart's motion reveals that we don't really experience time passing; we just conjure variations between present realisations and present memories of past realisations and to give the illusion that time flows or that the present moment moves via time. So there is no moment that is privileged to be more 'now' than any other moment just like no position is privileged to be more 'here' than others as David Deutsch put it. But the possibility of time travel emerges, one that was once been reserved for the likes of H.G. Wells and 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.

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