How often have you found yourself sighing, wondering about the good old days of yore, and how they would never come back again? How often have you languished in the sunny warmth of nostalgia, mulling over the past, adding color to touch up old memories? And each time you looked back at the past, have you not wished that there was some way by which you could relive those moments in time? Time―an eternal paradox, and in more ways than one can imagine. Forget nostalgia, forget futuristic fiction with its flying cars and intergalactic missiles and what have you―the paradox has now evolved to real problems of physical science. Such as the space-time continuum. Or the famous, famous grandfather's paradox. Or the most important question since H G Wells―would it ever be possible to travel back and forth through time? Before we discuss that, let us first grab some basics of modern science related to the concepts of space and time. The Newtonian UniverseSir Isaac Newton, revered by many as the father of modern theoretical physics, propounded that time is a constant factor in physics, regardless of the reference frame. In his theories and equations, time was variable which could move only in one direction and at a constant rate, independent of whether the reference frame was static or in motion. The Einstein Approach Albert Einstein, a Nobel Laureate physicist of German origin, first put forth a four-dimensional concept of the universe. In 1905, Einstein produced his Special Theory of Relativity, which sought to explain the behavior of light waves in different reference frames. Later, in 1915, he extended this theory to explain gravitational forces and the geometry of the universe with his general theory of relativity. In both theories, time was treated as a relative variable―the observations being different for different frames of reference. The Current StatusAfter Einstein and the Lorentz Transformation (an equation developed by mathematician Hendrik Lorentz), the concept of time as an immutable variable slowly faded away, giving rise instead to an entirely new concept, that of the space-time continuum. In this approach, space and time are not treated as individually distinct entities; rather the four dimensions of length, breadth, height, and space are intricately interrelated to form the space-time continuum; the shape being determined by the matter contained in the continuum. A little too complicated to understand? To simplify, imagine a two-dimensional sheet of cloth held taut at all four ends. Now, if one were to drop an object with some mass anywhere on the sheet, we would observe the sheet to 'dip' at that particular place. Further, if an object of smaller mass were to be placed in the vicinity of the 'dip' (within the outer perimeters of the 'dip', i.e.; where the 'dip' begins), one would observe this body of lighter mass to roll inwards towards the body of greater mass. Now imagine the space-time continuum as this two-dimensional sheet of cloth, and suppose that the object of greater mass is the Sun, and the one with the smaller mass is the Earth, and voila, we have explained gravity! It is the 'dip' in the 'universe-sheet' which pulls the planets towards the center of the solar system, with their own axial rotations counter balancing the tremendous force of attraction. The survival of the planets is also dependent on their size/mass and distance from sun; the three variables of axial speed of rotation, mass, and distance determining which planets would stay in perpetual orbits and which would be 'sucked in'. It is perhaps such precarious balance which leads one to ask whether God could have created the universe differently. On Traveling Through TimeSo, we have understood gravity, and got a fair idea of what the General theory of relativity is all about. But where does time travel come into the picture? The answer lies in Time Dilation. Einstein's theory states that the faster a body moves, the slower is the passage of time for that body. This dilation is caused by the warping effects in the space-time continuum by the twin effects of mass and speed, and it could perhaps hold the key to time travel. Imagine a person A flying off in a spaceship whose speed is very near the speed of light (the speed of light being the critical speed). Let us also assume there is a person B on Earth, who is observing the spacecraft. Now, according to Einstein's theory, the passage of time for B would be much faster than that of A, since B is static and A is in motion. Thus, if A decides to travel back to Earth in five years' time, he would come back to meet B who has aged by decades. Still sounds like fiction? The effects of speed on time have been measured right here on Earth (on a much smaller scale) using atomic clocks, and the results have been astoundingly spectacular―time does indeed slow down if speed speeds up! Thus, if we were to enclose a person in a spherical device and rotate it in a loop at very high speeds (approaching the speed of light), we could enable that person to move through years in what would seem a few seconds to him. Of course, the tremendous gravitational forces at work at that speed would tear this person apart, but once we have tackled the problem of establishing the theory of time travel we can always sort out such minor details! WormholesSo, now we know that we can move forward in time, if only theoretically as yet. But that is not the time travel that we are looking for, right? I mean, how about a round trip, the one where a person goes to place and 'comes back'? Well, that is not an impossibility, but there are limitations. To understand, let us first study black holes. Yes, every child who knows his television these days has certainly come across the term 'black hole' more than once. But what exactly is it? Simply put, a black hole is a body of tremendous mass squeezed into an infinitesimally small area, giving rise to such a tremendous warp in the space-time continuum as to defeat the physical laws known to modern science. Some theorize that black holes empty out into another sphere of space-time curvature, presumably one where the warping effects on time are greater than the warping effects of space, as opposed to what happens in our sphere of space-time. This could undoubtedly lead to a movement to the past. However, there is no empirical evidence that black holes do empty out in another region, and if they do, the physics of that region would undoubtedly be too complex for our scientific understanding as of now. The way to override this technical difficulty is to use the concept of black holes to develop 'worm holes'. Devised by Professor Kip Thorne of Caltech, the theory propounds the creation of two space-time warps which are interconnected. For example, let us go back to our sheet of cloth and create two 'dips' at two ends of the cloth. Now, if we fold the cloth such that the two 'dips' touch each other, what we have are two interconnected 'dips' (please ignore thoughts like the 'ball at the lower end would fall due to gravity'―we are discussing space-time here using the cloth sheet as an example!). These 'dips' would be the two mouths of what is known as a Worm Hole. Thus in the wormhole we have created, we would take a fraction of time to travel from one mouth to another, even though we would have taken a much longer period to travel between the two points had we tried to move in a straight line on the sheet. Now, assume that you have access to a portable wormhole. You keep one 'mouth' in your backyard, and let your friend take off in a very fast spaceship with the other 'mouth'. Time, for your friend, obviously slows down. So, ten years later if you step into that wormhole and persuade your friend to turn back towards Earth, you would have traversed a period of twenty years (including the return trip) of Earth time, but if your friend were to step into that wormhole, he would be coming back to an Earth of say eight years before the day he actually stepped into the wormhole (by his time). Exciting, isn't it? Are We Ready Yet?!There is one limitation to time travel through worm holes. Such a 'time machine' could carry you to the past only so far back as the date on which the wormhole was created (or where multiple wormholes are used, to the date when the first wormhole was created). So, it would perhaps be right to bid adieu to the good old days―they are not coming back. All we can do right now is to hope that we don't age too much before the first wormhole appears! We could, of course, spend the time on a spaceship to slow down memories! And in the meantime, we could save up for the round trip. Would it not be exciting to have a fund where we could put some money and watch it grow till the day science has developed the first time machine? Ridiculous as it may sound, there actually is a Time Travel Fund―which proposes to bring you back to life using time machines once the technology is perfected. Personally, I do not believe in the proposition, nor do I advocate it―life force is an object of metaphysics and though natural sciences may prolong or even defeat death, resurrection is something that may best be left to the creator alone. Bon Voyage!