By: Adib Bakth
Time, seemingly immutable in its flow; second after second tick by, minute by minute the world ages, year by year we grow, learn, and eventually, we perish. It seems as though time flows forever forwards at a steady unchanging rate; time acts much akin to how a river does, one with a permanent, fixed, and unfightable current dragging all of the universe's matter along it's steady unwavering flow. The reality that resides behind a superficially abstract world of equations tells us a completely different story though. Things aren't always as they seem, and time definitely isn't anything like the notions an everyday Joe would have about it. This unchanging forwards flow of time we perceive is deceiving. Einstein showed us the true nature of time, and his revolutionary conceptualization of time has forever changed our understanding of the universe. A myriad of mind blowing understanding and implications lay just beyond the surface of this mysterious river, so let us dive straight into this world of insight.
Before delving into time's true features and characteristics, let's look at time from an everyday perspective here on Earth. The conceptualisation of time as a river in the introduction generally sums up the everyday notion people have of time. It's unchanging; a second is a second, a minute is a minute, and they'll relentlessly pass for everyone. The seconds have always ticked by, they will continue to tick by, and you'll never get a second of your time back; so spawns the ever dreaded concepts of regret and remorse. The flow of time is unidirectional. Time always moves forward and never backwards; hence our countless movies about time travel showcasing the plethora of mind bending paradoxes and situations that can arise from it. So, in the eyes of the everyday beholder, time appears to be as simple a concept as one plus 2. In reality though, this everyday conception of time is as void of truth as Newton's understanding of what gravity truly was and how it propagates itself through the stage for all things, space; both concepts which Einstein brilliantly, elegantly, and beautifully described.Einstein showed us that time is quite the opposite from unchanging. He unveiled the truth about time; that it isn't the same everywhere or for everyone. Einstein seemed to have a liking to proposing revolutionarily and elegant theories describing the nature of reality, general and special relativity being two of them. A beautiful picture he painted; one depicting space as not a stage just for things to occur upon, but a player effecting the matter and energy immersed within it. Tieing space and time together in a pretty mathematical bowtie, explaining gravity and the true nature of time. We can think of space and time as a sheet. Immersed and existing within space and time resides energy and matter, which, once again was shown to us by Einstein, are interchangable; two sides of the same coin: E=mc2. The very presence of matter bends this sheet of space and time. These bends in the fabric of spacetime manifest into what we perceive to be the force of gravity itself. The three-dimensional paths matter takes along these bends in reality are gravity's effect on the universe. Everything travels in straight paths and bends in the very fabric of space and time cause those straight paths to become bent in trajectory. Orbits, gravitational sling shotting, gravitational lensing, all a result of objects or light following geometrically linear paths along a warped and curved 3D plain. Thus matter, just about everything we see around us, is immersed in and moves through spacetime; just as space bends and twists time does as well. Time twisting and bending?
What exactly does it mean to say: time twists and bends? Einstein displayed that time is relative; variant from thing to thing. It depends on the curvature of spacetime, or in other words, the gravity experienced by the matter, as well as its speed. The faster something is traveling the slower time passes for that something. Consider the following: a clock on a plane travelling around the world vs a clock on the ground. Common sense would tell you that the time for both would remain identical when the plane landed from its journey, as long as they were identical to begin with. What really ends up happening is counter intuitive, but perfectly explained by Einstein. The clock that was on the plane ticks at a time slightly less than the one that was left on Earth when it lands, albeit only by a miniscule fraction of a second; but that's the catch isn't it? The time dilations that occur in a lifetime, let alone in everyday life, are so miniscule that our conception of time is that it's fixed and unchanging. In a lifetime, we may accumulate no more than seconds of slowed time, generally. Some more extravagant and or specific careers can lead to even more time dilation experienced in a lifetime, astronauts or pilots for example. The reason though, that we experience only miniscule effects of times variability in our lives, is because the speeds we work with and travel at pale in the face of the speed of light; the speed limit of the universe. The faster you go the slower time ticks for you, and the closer you get to the speed of light, the truly more counterintuitive the world becomes.To further understand exactly what Einstein illuminated for us, we must delve into his world of relativity. Fundamentally, the elapsing of time is relative to who you're asking. A stationary observer and a moving observer will disagree on how much time has passed; if of course the speeds involved are fast enough to produce a noticeable time dilation. The time experienced by an object, based off speed, is given as so: t'=t1-v2/c2, as shown by Einstein. t' represents the time a moving object (with velocity v) experiences compared to t, which is the elapsed time a stationary observers would say had passed by. Just by playing around with values for v, we can gain incredible insight into how time behaves. Off the bat we can see how things fall apart if we try and have a velocity greater than c (the speed of light). One minus (v/c)2 will result in a negative number; the square root (and any even root at that) of negative numbers have no solutions, in the real numbers plain at least. The speed of light owns its position as a universal speed limit and our equations yell, scream even, this fact into our ears. On the opposite end of extreme speeds, we can see how if v=0, then t' and t will be identical. Stationary observers will agree on the passage of time. Now, a more interesting case. Consider an object moving at very, very near the speed of light. (v/c)2 approaches 1 the closer and closer velocity gets to being the speed of light, thus resulting in the time experienced by the moving object becoming ever smaller and smaller; the faster you go the slower time ticks. Though not physically possible, let us assume that this speeding object is able to accelerate to the speed of light itself! What happens? Well, (c/c)2 is one; One minus one is zero; the square root of 0 is 0; finally, t times 0 is 0. Things travelling at the speed of light experience no time passing at all… With the introduction of special relativity into physics time was revolutionized. No longer looked at as static and constant but dynamic and highly malleable. Lagging in it's relentless progression for some objects more than others, time suddenly sprang to life now full of conceptual insight. Speed and time go hand in hand when trying to describe the universe. The picture of reality that Einstein painted for us is truly beautiful and eye opening. The marriage of space and time has unlocked extremely perplexing realizations about the universe and what makes it tick. Energy and mass are entangled in a web of spacetime and always moving through space, unless stationary, but also constantly through time, expect objects going the speed of light. The act of moving through space results in moving through time slower. But speed isn't the only factor affecting time experienced from observer to observer. The very curvature of spacetime aids to the dilation of time as well; it's only intuitive. Gravity, the manifestation of spacetime twisting and bending, effects not only the paths object take in 3D space, but also the time they experience. Gravity not only bends space, but time as well. Time for an object experiencing gravitational forces will elapse slower than that of an observer immersed in a lesser gravitational field. The stronger the force of gravity is the slower time ticks for that something, or someone. Interstellar (a fantastic movie, give it a watch) displays some of these neat time tricks detailed by Einstein's findings. The crew of an interstellar space expedition travel to alien worlds seeking a new planet to call home, for Earth is slowly but surely dying. Some of these strange candidates for habitable planets are much more massive than earth, and a few orbit a black hole… The more massive an object the larger the curvature it induces onto spacetime thus, the greater it's gravity. A large planet was visited in this movie as a potential candidate to be the cradle of humanity's survival. Some of the crew went down to explore the planet and study it for an hour or so. They left their theoretical physicist behind on the ship orbiting the planet to spend his time, of which he was about to get a ton of, studying and doing calculations for their travels. As an hour and some time elapsed for the crew members exploring the planet, several years passed by for the lonely physicist back on the ship. The movie displays the realities of both extreme speeds as well as gravity and how they draw back time more vigorously for some than others. Hours of aging pass by for a landing crew while years of solitude pass by for a physicist left behind to use his time to the fullest.
The movie goes farther in showing us the extremities that arise and exist lurking out in deep, or maybe even close, space; depicting Matthew Mcconaughey and an advanced robot descending into a black hole. For the readers who are unfamiliar with black holes I digress. There are many structures that have formed in the universe over the unfathomable millennia it has existed. Planets, stars (of which are a variety of forms and types), clouds of gas, galaxies, and even dark matter as well as, possibly, antimatter. Most of these structures are comprised of the remnants of different stars, or even comprised of stars themselves (galaxies). Stars are merely spheres of super hot gas, otherwise known as plasma. The immense pressures and heat within their cores force atoms together (nuclear fusion), thus providing the energy to keep the star from collapsing in on themselves under it's own gravity. Eventually though, stars run out of fuel to burn. Stars inevitably die and the death of a star is nothing you want to be close to. The larger the star the quicker it burns through it's fuel and the more extreme it's death by implosion/explosion is. Their cores collapse in on themselves forming dense strange stars comprised of exotic matter. The biggest of stars have the most spectacular deaths. Gamma ray bursts, a violent explosion firing 2 streams of gamma rays jetting off into space; if one of these gamma ray ray bursts happened close enough to us and was pointed right at us, the Earth and essentially all it's inhabitants would perish from a bombardment of highly radioactive particles. The remnant of the star's core implodes so dramatically that what's left is basically a hole in space and time. A point of immense density, infinite density in fact, forming a well of gravity so powerful that not even light can escape its clutches. The remnants of these dead stars are black holes: singularities immersed in a cloud of eternal darkness from which nothing returns (in other words the event horizon). These gravitational beasts of nature were predicted by the maths brought forth by Einstein, but the very mathematics that spawns them, to this day, fails to describe their true nature and properties at the point of infinite density itself. Our mathematical description of the universe and it's properties under such intense and violent circumstances break down; there's very little we know about the true nature of the heart's of black holes, the singularity is an ever elusive and mysterious subject. With all that out of the way, I un-digress.
These wells of unfathomable gravity have very extreme effects on time. Getting near one of these monsters in the maths will result in countless years of unexperienced time. Decades ( for people away from the black hole and relatively slow moving) will seem like days, minutes, or even seconds depending on exactly how close you stray; talk about a fountain of youth. The heavy warping of space and time around these gravitational perplexities bend, and incases even twist (if the black hole is spinning), the fabric of reality; they illuminate the true nature of time for us to see crystal clear. Einstein so elegantly described to us the reality of gravity, time, and even light, but time's story doesn't end there. We still have some very interesting conceptual hurdles to leap over when it comes to understanding time.
We've just gone over how time's seemingly eternally static flow forwards is far from the truth. Speed and the warping of spacetime can slow the passage of time, and yet, one thing seems to hold true from our naive everyday conception of time: that it always flows forwards. Traveling backwards in time isn't, or at least shouldn't be possible… right? The peculiar thing about all this is that time's perpetually varied ebb and flow exclusively forwards isn't really explicitly defined anywhere in physics. Nowhere in the vast ocean of mathematics that describes our universe has it been written that time must, and only must, tick always ahead and never backwards; yet, here we are, slaves to the constant and relentless march of time dragging us ever closer to our inevitable mortality. The arrow of time is what this tendency of time to do (always passing by rather than the opposite) is referred to as. In physics, only one thing seems to justify time's unidirectional flow: the second law of thermodynamics. Entropy, the law stating that the universe always perpetually increases in disorder. Things in the universe will always go from a state of simplicity to a state of disorder. What this means is simply explained. A plate can be broken more easily than the broken pieces can be reassembled into the original plate. A stack of unstapled papers, when thrown into the air, is unfathomably more likely to land in an unorganised mess then back into a neat pile like it was thrown in to begin with. Over all disorder in the universe increases with time. The reason time seems to only flow forwards is because things constantly go from orderly to disorderly. Stars explode, scattering their guts into space, the universe expands as galaxies drift apart at ever increasing rates, overall, disorder over time prevails. This tendency for order to evolve into disorder, for entropy to increase as time progresses, is the closest thing to a "reason", we have for time only ever flowing forwards in physics.
With, essentially, no mathematical statement saying that time has to flow forewards -besides it being alluded to by the second law of thermodynamics- it seems like all it would take to watch a glass full of water go from being broken to unbroken is reversing the velocities of the shattered pieces and water. Watching this glass going from broken to unbroken would be just like watching the same glass break but rewinding the footage rather than playing it. Much in the same way, we can describe almost everything we describe as time passes forwards, backwards too; just reverse velocities as well as or rotations and voila. We've seen already how easy it is to slow the passage of time; stop it even (just achieve a velocity of the speed of light itself). But who wants years and years of life anyways? Being plunged into an uncertain future, with your beloved, and even not so beloved, acquaintances either long gone and diseased or in their older years of life. What really captures our intrigue is the ability to travel back in time isn't it? To re-experience cherished memories, to change a past decision, to alter a relic of time. Regret and remorse, we all want the ability to extinguish these feelings from our lives, and what better way to do that then to just reverse time and play around with the past with the knowledge of the future in mind? Obviously though, playing around with past occurrences doesn't sound like the best of ideas, and if fact has an array of various mind bending paradoxical implications.
Let us suppose for a second that travelling to the past is possible. If then you were to travel back in time, what would that mean for reality itself progressing on from that point? Would your interactions affect the timeline of reality up until the point you travelled back? If they did affect and change the course of history then things start to get really odd and mind bendingly convoluted. The very act of you travelling back and meddling with the timeline of the universe can lead to plethora of paradoxes. Changing the timeline of what happened up until you travelled back in time could change the course of history so extremely that you travelling back in the first place never happened. What if you go back and accidently prevent your birth? Cause your death? Inadvertently change the flow of happenings in a way that leads to the extinction of the human race. If you went back in time and changed realities timeline in such a way that it means you never travelled back in time to begin with, then you could have never gone back to make the changes that lead to you to not being able travel back in time in the first place. Does your brain hurt yet? Events and time are very delicate concepts. Changing the course of a single air molecule years in the past could lead to a chain reaction of events resulting in a reality vastly different reality from the one we live in today, not to mention what could end up happening if you were to talk to and influence a bunch of people in the past. Everything you affect and alter in the past will be reflected by its own ripple in time; the effects of such ripples you would eventually come to see when you return to your era, being possibly unrecognizably different than the present you came from. A convoluted and unlikely solution to this mind twisting problem with backwards time travel, is the proposition that: sure, you can alter and change reality when going back in time, but reality will find a way to remain more or less the same. Events will happen in such a string that you will still always find your timeline leading to you travelling back, and changing what you changed. The problem with this is that nature is so dependant on events. Small variations, over time, can imply change in many folds of magnitude. It is very farfetched to say that the universe will consciously view us trying to meddle with it's timeline and change the order of events or set up reality so everything falls back into place as if you never meddled with anything from the get go. Another interesting interpretation of time travel deals with the existence of a multiverse, and an infinite (or at least insanely large one) one at that. If an infinite number of universes exist, eventually, a certain number of universes over, their designs and content will start duplicating. Eventually you'll find a universe that is identical to ours, but just slightly different. Perhaps a different history, with individuals that are identical to those who lived through our history but with different lives, personalities, and experiences. Some of these universes will be completely identical to ours, just at different points in time; travelling back in time would just be you travelling to a universe identical to ours but earlier in it's history. Thus your own reality isn't affected by your actions; a parallel one is. You'd merely return to the exact same timeline.
If time slows down the closer and closer one gets to the speed of light and halts all together at the speed of light, then what would happen if one were able to go faster than the speed of light; would time pass faster or even backwards for that object? Off the bat we can dismiss this question as it is very truly nonsensical… The laws of physics have boundaries that tell us about reality. The speed of light is one of those boundaries. To travel faster than the speed of light is impossible, and trying to solve our equations for speeds greater than c results in no answers at all or irrational ones. It is hard to answer the question of what if since the resulting solutions of our equation hardly show us anything conclusive or logical. If v were to be greater than c in the equations for special relativity, then we get the square roots of negatives. The even roots of a negative number aren't real numbers, lying on two planes of numerical sets: the real and imaginary number planes. Once v becomes greater than c, the resulting values we get from Einstein's equation ( t=t'1-(v/c)2) have a component that is real, but also have an imaginary component. What this imaginary component means for real life? It is hard to say. Factoring out the imaginary unit of times results in the equation telling us that this weird time, resulting from faster than light velocities, gets closer and closer the that of what a stationary observer would say elapsed. Eventually, as speeds start exceeding 140% the speed of light, the time that passed for the faster than light traveller would become more than what a stationary observer would say passed. Long durations of time passing for the cosmic speeder would be perceived as a shorter elapsement of time for a stationary observer. As can be seen, the equations themselves don't give us any clear insight into what happens to speeders. This imaginary proponent of time is hard to explain in terms of real life. Once you ignore one of the rules in physics, the maths break down and proceed to spurt out gibberish. So faster than light travel, though clearly impossible, doesn't even open a clear lane for the possibility of backwards time travel.
All these perplexing paradoxes and brain melting possibilities for reality are fun to delve into, but now we need to tackle the real question: is any of this backwards time travel talk do-able? Sadly, for now and a long time to come the answer seems to be no. Any possible ideas we have to achieve time travel are much too technologically advanced and power hungry for us to ever even hope to build. In Fact, the most plausible way of making a time machine stems from the wormholes of Einstein's equations (holes connecting two distance points in spacetime). Creating a wormhole, essentially a tunnel through spacetime, is tough enough a task itself; keeping a wormhole stable and open after it's genesis is an entirely separate hurdle in and of itself. Such an endeavour would require a star's worth of energy, at the least… On top of requiring insane energy requirements, this form of time travel will not allow one to travel back in time any farther then the point in time the wormhole was created. Accelerating to the speed of light is impossible, let alone travelling faster than it (Einstein showed that not only time is affected by speed, mass is too, and the faster something is travelling the more mass it is perceived to have; thus accelerating an object with mass to the speed of light is impossible - it will take infinite energy- since mass approaches infinity as speed approaches c) .The minefield of paradoxes associated with time travel to the past is enough to deter us from believing in it's reality; although very fascinating are the concepts that time travel may spawn. The multiverse idea arising to lay waste to the ever present paradoxical nature of this subject is an interesting one to consider, and even one proposed to help explain away some of the other weird stuff we observe in the universe. Stuff like the collapse of probability waves when observations are being made; why one possibility takes place out of the countless that could have occurred. Though fairly interesting, I'd rather not digress into the realm of quantum mechanics; I'll save that wonderfully wonky topic for another time.Time… The same second as you're brushing your teeth in the morning, someone across the globe could be brushing their teeth at night. Two asteroids somewhere deep in space could be colliding. A star out in a distant galaxy may have just been born and at the same precise time, a stars death may have finally caught up with it. So much going on in the same slices of time, but the passage of time for all things never being quite the same. The aging process laging ever so slowly for objects in motion or trapped in an intense warp in spacetime. Looking out into space is literally travelling backwards in time. The light we observe from all around us is tired and weary for it has journeyed anywhere from years to millions, even billions of years in order to reach our eyes; to brighten our night skies. The light we see of most of the things we see out in the beautifully elegant and diverse universe is, by now, long gone and dead; evolved from what it once was as we see it now, into something completely new and different. The immensity of space and therefore time within the universe paired with it's universal speed limit is what makes telescopes time machines. Even the fastest travelling propagators of information, light (photons), take lifetimes to travel any meaningful distance at all, and even then only meaningful in our perspective, not that of the vast universe itself…We bare witness to the states of stars, gas clouds, and galaxies as they were when that light initially departed from them however many light years away they may reside. As the universe ages, all things within it slowly fall apart in a cycle of ever heightening disorder. As seconds tick by and all things in the universe slowly age, besides light (or particles travelling at precisely the speed of light), time gives meaning to our lives. Time is money; money is time and all our lives are spent devoted to setting up the future, stressing about the present, and trying to cherish or forget the past. Time, the fourth dimension that we all constantly travel through. It bends and twists just as space does. It is the partner in realities tango of two, space and time. It is a part of the fabric we are all immersed within. It was there to bare witness to the universe's birth and will be there to see it all the way through to the bitter end (whatever that end may be) or eternity.
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