Have you ever dreamed of travelling through time? Not at the standard, boring rate that we normally do??at one second per second??but either: faster, so that you wound up far into the future while remaining the same age, slower, so that you could accomplish much more than anyone else over the same interval, or backwards, so you could return to an era in the past and alter it, perhaps changing the future or even the present? This might seem like out-and-out science fiction, but not all of it belongs to the fiction category: travelling through time is the one thing in science that you cant help yourself from doing no matter what you do! The question is how much you can manipulate it for your own ends, and control your motion through time. An example of a light cone, the three-dimensional surface of all possible light rays arriving at and departing from a point in spacetime. When Einstein put forth special relativity in 1905, the understanding that every massive object in the Universe must travel through time was just one of its astounding implications. For another, we learned that photons??or any massless particle, for that matter??cannot experience time at all in their frame of reference: from the instant one is emitted to the instant its absorbed, only massive observers (like us) can see the passage of time. From the photons reference frame, the entire Universe in its direction-of-travel contracts down to a single point, and absorption and emission happens all at the same time: instantaneously. Photons of vastly different energies are seen to travel at the same speeds. But we have mass. And for anything that has mass, youre limited to always travel at less than the speed of light in a vacuum. Not only that, but no matter how fast youre moving relative to anything else??whether youre accelerating or not??youll always perceive light to be moving at that one constant speed: c, the speed of light in a vacuum. This is a powerful observation and realization, and it comes with a fascinating consequence: if you observe someone in motion relative to you, their clock will appear to run slow. Imagine a light clock, or a clock that works on the principle of light being bounced back-and-forth in the up-and-down direction between two mirrors. The faster the person in motion moves relative to you, the more the lights velocity will move in that transverse (across) direction, rather than in the up-and-down direction, and hence the slower their clock will appear to run. By the same token, your clock will appear to move slow with respect to them; theyll see time passing more slowly for you! Clearly this cant be the case for both of you: when the two of you get together again, one of you will be older and one will be younger. Which one? Thats the nature of the Einstein twin paradox problem. The short answer: assuming you started off in the same frame of reference (at rest on Earth, for example), and you wind up in that same frame of reference at a later time, the person who did the travelling will have aged less, having had time pass at the slow rate, while the person who remained at home will have had time pass at the normal rate. So, if you want to travel ahead in time quickly, just accelerate to a fast (near-light) speed, move at that speed for some time, and then come back to rest at your initial location. (This will involve some turning around!) Do that, and you can??depending on the theoretical quality of your equipment??travel days, months, decades, eons, or billions of years into the future! You can witness the evolution and destruction of humanity; the end of the Earth and Sun; the dissociation of our galaxy; the heat death of the Universe itself. So long as you have enough power in your space ship, you can travel as far into the future as you like. But backwards is another story. Simple Special Relativity, or the relationship between space and time on a basic level, was enough to get us into the future. But if we want to go back??or into the past??well need to go to General Relativity, or the relationship between spacetime and matter and energy. In this case, we treat space and time as an inseparable fabric, and matter and energy is what warps it, or causes changes in that fabric itself. For our Universe as we know it, spacetime is pretty boring: its almost perfectly flat, barely curved at all, and in no way, shape or (discernible) form loops back on itself. The fabric of spacetime, illustrated, with ripples and deformations due to mass. But in some model Universes??in some solutions to Einsteins General Relativity??you can loop back on yourself. If space loops back on itself, you can travel in one direction for a long, long time, and wind up right back where you started: a consequence of a closed Universe. Well, you can not only have solutions with closed space-like curves, but you can have spacetimes with closed time-like curves as well. A closed time-like curve implies that you can literally travel back in time, live through certain conditions, and arrive back at the same point you departed from. But thats a mathematical solution; does that mathematics describe our physical Universe, though? It appears not to be the case. The curvatures and/or discontinuities wed need our Universe to have are wildly incompatible with what we observe, even near neutron stars and black holes: the most extreme examples of curvature in our Universe. Our Universe could be rotating on a global scale, but observed limits on rotation are some 100,000,000 times too stringent to admit the closed time-like curves were craving. If you want to go forward in time, a souped-up DeLorean??assuming souped-up means relativistic??will get you there, as will a souped-up train, which was Einsteins initial idea! But going backwards? Perhaps its better that you cant go back in time, prevent your father from marrying your mother, and creating a time paradox. Futurama aside, the idea of travelling backwards in time will likely continue to fascinate humanity, but that half of time travel??the back half??will almost certainly remain a fiction eternally into the future. Its not mathematically impossible, but the Universe is based in physics, which is a special subset of mathematical solutions. Based on what weve observed, our dreams of correcting our mistakes by going to the past will likely exist in our imaginations alone.
Pionic, 10 July 2017 ;https://pionic.org ;