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The concept has literally given story writers and movie producers plenty of material to work with; such science fiction motion pictures like Star Trek, Super Man, Back to the Future, The Twelve Monkeys, and the like come to mind. Why not? There is almost no limit to what can be imagined, as a theme around time travel, whether it has to do with visiting new worlds in our universe, someone accidentally waking up in very different time from his/her own, or someone yearning to change a situation to his/her advantage in their present time, with the aid of changing the course of history by altering some event or another, and the list can just go on.
In all seriousness though, just how practical is the concept of managing the course of history with the manipulation of time?
The idea of traveling faster than the speed of light, just as fast or else close to it, has been paraded around as one possible step towards achieving that end (time travel). Needless to say, humanity has not yet attained technology that can make traveling at such a rate possible, and there are several reasons for that.
For one, even if humans did get their hands on such extraordinary technology, humans themselves would not be able to be physical participants of such travel through time and space. The human body is simply not designed or conditioned to travel at such atrociously high speed; the acceleration that would be needed to even come anywhere near the speed of light comes with some health hazards. This thus only leaves one alternative under an unlikely scenario that humans have said technology in possession: sending inanimate self-operating intelligent systems like say, specially designed robots. It would be unfortunate that such should be the case, since there is nothing quite like experiencing events in live time, with our own physical human-presence. However, it would be the best thing next to nothing.
For another, the human brain is simply too primitive and limited in its capacity to come up with a device that would allow such a travel. While the human mind can come up with estimates of what speed is needed to outpace the speed of light, putting together the technology needed to achieve this feat is entirely another matter. There is very few directions of path, if any, that light cannot travel along at extraordinary speed; something of that nature would be very challenging for humans to overcome.
Human culture and social interaction is also another matter that presents a daunting challenge to achieving technology of this feat. Humans do not simply get along well enough to unconditionally pool their resources and work together in developing technology at this level.
States are more likely than not, to hide away from fellow states—and even from the greater society, be it their own or of others', any step forward in implementing traveling either at the speed of light or faster. Such information will likely find itself in "classified" material rather than open cooperation between states. To even come close to any technology at this level, humans would have to cultivate a totally different lifestyle and mindset than they presently do.
Other ideas floating around in the chat avenues of the research community speak of the prospect of managing such features of the universe as worm holes or black holes, as a means to cheat on time and space travel. It needs to be kept in mind, however, that humans have no idea of what really goes on inside these entities other than pure speculation, even if it is considered educated guesswork.
Traveling in a black hole is not guaranteed to direct humans to a safe destination back into the universe, or if at all, to any destination in some other corner of our own universe. If there's any indicator, it is more likely that humans would be crushed by tidal forces even before any headway is achieved, given the seeming ultra density—and associated tidal force—at the center of black holes. Yet upon listening to some commentators within the research community, one may get the impression that humans are intellectually and socially developed enough to come up with technology that would overcome such a barrier.
Supposing humans were able to travel at or near the speed of light to very distant parts of the universe, then an observer (which could be represented by any of the arrived human crew-members) located in a destination point implicated at such distance, may well facilitate a peek at our Earth at an earlier time, given the distance light had to travel to reach said destination, just as the light cone—as the trajectory of light from said destination—may give an observer on Earth an insight into the state of that location at an earlier time frame. In that sense, we can get an idea of what the past looked like.
An unlikely scenario of traveling at faster than the speed of light would mean that the above-mentioned crew will reach their destination at the distant location before light gets there for the said event. The implication of this, is that the traveling crew, as the principal observers at the destination location, will not be able to see the state of the Earth during the event in question. Instead, if any, the traveling crew may see the state of Earth during some other past event, for which the past light cone will have already been available for the crew.
However, if the traveling crew were to travel back to Earth from that hypothetical location in the universe, there is no inevitability that said humans would get to Earth at an earlier time frame or even considerably later than the time frame in which the observers originally belong, as suggested by some commentators and in science fiction movies. The visualizations an observer gets at one end of the light cone is simply a reflection of the speed of light, and the time it took light to reach the destination point from the point where the event originally took place. It doesn't really alter the course of events that would have taken place as light travelled from point A to point B.
The only way a person can go back to a past event, is to beat any flash of light that would have accommodated said event. This would mean that if the person was at point B, and the event took place at point A, then the person would not only have had to leave point B, but would also have had to arrive at point A even before a light flash occurred when said event was taking place.
In the above scenario, it would essentially appear to the traveling person as though time stood still, as the person went from point A to point B. This calls to mind such science fiction flicks as Back to the Future, wherein time freezes as the principal time-traveling characters went to the past, and then returned to the time frame they belonged.
From the standpoint of the time frame in which the traveler belonged, that is to say, the "present", the time-travel will have been so fast that it would appear as though the traveler had never moved from one time frame to another, and likewise, from point A to point B. The time-traveling event would have been extraordinarily fast that the people who would have surrounded the time-traveler in the "present", would not have noticed the absence of said traveler.
In Back to the Future this point is seemingly communicated in the form of the time-traveling characters just disappearing into thin air, and then appearing in a past time frame (or in the future, in other situations) from nowhere, again in thin air. Here is the glitch in the practicality of such time-travel: the events that the time-traveling characters would have gone back to, would have already taken place, and in accommodation with said events, light would have already traveled in every case and would have engendered past & future light cones accordingly.
What about a future travel, as also portrayed in the Hollywood flick mentioned above? As implied in the movie itself, the prospect of affecting the course of events in a future time-travel would require foreknowledge of certain events before they supposedly occurred.
The last scenario would hence require foreknowledge of the potential causal processes/events of the future event, as well as the specific date, time and location of where the future event is set to take place. If said event is far out into the future, like the example suggested in the aforementioned movie, then this would require skipping a considerable interval of time.
It's one thing to have foreknowledge of the specifics of a future event, but it is quite another, overcoming the logistics of skipping time at such considerable depths. These matters can be again correlated to the concept of light cones of any event in a space-time framework; knowing the causal event of a future one will amount to connecting the dots between an earlier event (coinciding with the "present" in this case) and a future one in some space within the future light cone of the earlier event.
The success of connecting said dots would imply being able to locate respective events, i.e. in the present and in the future, well within the light cone(s) under consideration. Anything outside of the light cones will not be subject to alteration.
Events or locations outside the cone in say, the past light cone of an event under consideration, is very likely an event that has had no effect on the event now under consideration. Either that, or the causal event and the future event would have been too far apart, such that it would be very difficult to tie the two. This can perhaps be liken to an event being unobservable to an observer, because the distance light has to travel is so great, that light has not yet reached the observer.
Light travel is used as the standard against which to consider the rate at which one event can affect another in a space-time framework, given that light is generally considered the fastest known element in the universe. This means an event, and its possible effect on other events, can only occur at either less than the speed of light or equal to it.
In short, it's essentially impossible to skip time. The only near-practical way to alter a future event, is if it unquestionably is tied to one that had occurred in a distant or recent past, and hence forth, if that past event were altered. The logistics of altering a past event has already been described in preceding passages.
For the same reason it is impractical to skip time, any time travel into the past, as noted above, will have to have been accommodated with a virtual standstill of time, from the standpoint of the time traveler, and essentially no "unaccounted for" time, from standpoint of everyone else living in the present or live time. There cannot be any skip in time.
Checks and balances have already been put in place, as governed by the laws of our universe, so as to render any alteration of the past, and any corresponding time-travel, impossible. So, even in the very unlikely scenario that humans were able to overcome the obstacles of time-travel, they would still not be able to alter the course of certain events. For instance, causing someone living in the present, not to be born at time and place they would have been born, or someone altering situations, such that he/she would in effect take the procreative place of one of his/her parents.
The need to alter the course of events in the greater universe will necessarily be required in any hypothetical scenario of traveling back in time, in say, a planet like Earth. Spinning the Earth in the reverse direction does not come across as a practical means of either turning back the clock or traveling back in time, as entertainingly implied in a Super Man movie of the 70s.
|Still from Superman, 1978. The still features Superman flying fast, supposedly faster than the speed of light, around the Earth, to make it spin in the reverse direction, so that time can be reversed. Click on the image to expand.|
|Click on the image to expand. Images: In the left image—Our Milky Way galaxy from a lateral view, and in the right image—top view of the Milky Way.|
The light we see today, as the cosmic microwave background, has traveled over 13 billion years to reach us. Within this light are infinitesimal patterns that mark the seeds of what later grew into clusters of galaxies and the vast structure we see all around us today.
Patterns in the big bang afterglow were frozen in place only 380,000 years after the big bang, a number nailed down by this latest observation. 
"WMAP's "baby picture of the universe" maps the afterglow of the hot, young universe at a time when it was only 375,000 years old, when it was a tiny fraction of its current age of 13.77 billion years. The patterns in this baby picture were used to limit what could have possibly happened earlier, and what happened in the billions of year since that early time. The (mis-named) "big bang" framework of cosmology, which posits that the young universe was hot and dense, and has been expanding and cooling ever since, is now solidly supported, according to WMAP." 
This puts into perspective, the amount of time that elapsed as light made it to our space in the universe, so that we can get a glimpse of an earlier state of our universe, and this is light we are talking about—the fastest known entity in universe! That state of the universe only exists as a "frozen" snapshot taken when emitted light during the event first dispersed in all directions.
Any travel, at or faster than the speed of light, is not going to overcome the time elapsed in the scenario presented above. By the time a hypothetical traveling crew gets to the distance covered by the light, which brought us said images of the earlier universe, said crew will have found a situation completely different. Point A, from which the light initially dispersed, will now be in its present state.
Hypothetically spinning the Earth in reverse may for example, only effect unnatural, and possibly a peculiarly-drastic, change in how the Sun light is distributed on Earth at a given moment, and potentially, climate in locations of the Earth. Again, said scenario does not rationally register as a practical way of reversing the course of the past, and henceforth, the aging of living creatures on Earth or even that of the Earth itself for that matter.
Think of the universe metaphorically as a lake, whereby a stone is thrown into it to create one ripple after another. Altering the universe, to effect the turning back of the clock with respect to the universe itself, might be the feat that could feasibly create ripples that have the potential of reversing or pushing forward time.
It is hard to imagine a time-traveler going back in time on Earth, without concurrently going back in time with respect to the universe. Such a scenario would create a paradox, whereby the time frame (in the past) on Earth is out of sync with that featuring the universe in which it would have resided during the elapsed time frame in question. It would be like say, traveling back in time and expecting to be in say, 10000 BC Earth years, whereby the 10000 BC Earthly world is placed in the Universe of the present, as opposed to what the condition would have been like in the Universe, back when it was 10,000 BC on Earth.
Perhaps much would not have been considerably different, as far as the universe is concerned, in say, back 10,000 BC on Earth, since after all, our moment on Earth will seem like just a very tiny point on the temporal trajectory of the universe. Still, traveling back in time on Earth should logically coincide with traveling back in time in the universe, from the time-traveler's standpoint.
|Image captions: Possibly an early period of our solar system, when many of its planets were at their earliest states of development? Second image on the right features what appears to be an early state of our very own Earth|
That's just it, however; there has to be constraints, since going as far back as the "big bang" event would engender the absence of our solar system, galaxy, and needless to say, the very existence of our own planet, Earth. So, an Earthling time-traveler would be seriously violating the law of the universe by his/her very existence at the moment of the so-called "big bang". It goes back to the aforementioned checks and balances put in place by the laws that govern our universe, precisely to ensure such a situation is impossible under any circumstance.
Time, it must be pointed out, is not an actual tangible entity. It is an abstract of human creation, to enable humans to deal with our universe in a meaningful way. It allows humans to measure intervals between a series of events, and henceforth, reconstruct a chronology of events and put said events in order of occurrence.
Without a starting point, an abstract concept like time will make no sense. Needless to say, the prospect of time-travel also disappears. The "big bang", as noted on this site before, sets the clock ticking for our universe. Before the "big bang", time as an abstract with meaning, ceases to have meaning. One way to extend the concept of time into an existence preceding and/or beyond the "big bang" moment can be recapped as follows, quoting Stephen Hawking:
It seems that Quantum theory, on the other hand, can predict how the universe will begin. Quantum theory introduces a new idea, that of imaginary time. Imaginary time may sound like science fiction, and it has been brought into Doctor Who. But nevertheless, it is a genuine scientific concept. One can picture it in the following way. One can think of ordinary, real, time as a horizontal line. On the left, one has the past, and on the right, the future. But there's another kind of time in the vertical direction. This is called imaginary time, because it is not the kind of time we normally experience. But in a sense, it is just as real, as what we call real time...
If space and imaginary time are indeed like the surface of the Earth, there wouldn't be any singularities in the imaginary time direction, at which the laws of physics would break down. And there wouldn't be any boundaries, to the imaginary time space-time, just as there aren't any boundaries to the surface of the Earth. This absence of boundaries means that the laws of physics would determine the state of the universe uniquely, in imaginary time. But if one knows the state of the universe in imaginary time, one can calculate the state of the universe in real time. One would still expect some sort of Big Bang singularity in real time. So real time would still have a beginning. But one wouldn't have to appeal to something outside the universe, to determine how the universe began. Instead, the way the universe started out at the Big Bang would be determined by the state of the universe in imaginary time. Thus, the universe would be a completely self-contained system. It wouldnot be determined by anything outside the physical universe, that we observe. 
Speaking of Mr. Hawking, it appears that he is conceding the impracticality of traveling back in time, in this peace, as he examined the feasibility of utilizing wormholes to travel back in time:
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Any kind of time travel to the past through wormholes or any other method is probably impossible, otherwise paradoxes would occur. So sadly, it looks like time travel to the past is never going to happen. A disappointment for dinosaur hunters and a relief for historians. - Stephen Hawking 
Mr. Hawking then goes onto essentially urge listeners to not be discouraged, given the feasibility of traveling into the future; he identifies several options of making that happen:
Orbiting the black hole at extraordinarily high speeds...
I do believe in time travel. Time travel to the future. Time flows like a river and it seems as if each of us is carried relentlessly along by time's current. But time is like a river in another way. It flows at different speeds in different places and that is the key to travelling into the future...
They [clocks] run fast because time itself runs faster in space than it does down below. And the reason for this extraordinary effect is the mass of the Earth. Einstein realised that matter drags on time and slows it down like the slow part of a river. The heavier the object, the more it drags on time. And this startling reality is what opens the door to the possibility of time travel to the future.
Right in the centre of the Milky Way, 26,000 light years from us, lies the heaviest object in the galaxy. It is a super massive black hole containing the mass of four million suns crushed down into a single point by its own gravity. The closer you get to the black hole, the stronger the gravity. Get really close and not even light can escape. A black hole like this one has a dramatic effect on time, slowing it down far more than anything else in the galaxy. That makes it a natural time machine.
I like to imagine how a spaceship might be able to take advantage of this phenomenon, by orbiting it. If a space agency were controlling the mission from Earth they'd observe that each full orbit took 16 minutes. But for the brave people on board, close to this massive object, time would be slowed down. And here the effect would be far more extreme than the gravitational pull of Earth. The crew's time would be slowed down by half. For every 16-minute orbit, they'd only experience eight minutes of time. 
In the scenario above, the spaceship crew would have to be at a substantial enough distance, which would be great indeed, to avoid being sucked into the black hole. If they get close enough, time will slow down alright, and they will speed up alright, i.e. right into the black hole, and not live to tell Earthlings the story.
Orbiting Earth at very high speeds, approaching the speed of light...
Imagine that the train left the station on January 1, 2050. It circles Earth over and over again for 100 years [at high speeds approaching the speed of light; Hawking couldn't have put it simply: "no matter how much power the train has, it can never quite reach the speed of light, since the laws of physics forbid it"] before finally coming to a halt on New Year's Day, 2150. The passengers will have only lived one week because time is slowed down that much inside the train. When they got out they'd find a very different world from the one they'd left. In one week they'd have travelled 100 years into the future. Of course, building a train that could reach such a speed is quite impossible. But we have built something very like the train at the world's largest particle accelerator at CERN in Geneva, Switzerland. 
|Image Caption: Hadron Collider, i.e. the "particle accelerator", at CERN, Geneva.|
Imagine a child running forwards up the train. Her forward speed is added to the speed of the train, so couldn't she break the speed limit simply by accident? The answer is no. The laws of nature prevent the possibility by slowing down time onboard.
Now she can't run fast enough to break the limit. Time will always slow down just enough to protect the speed limit. And from that fact comes the possibility of travelling many years into the future. 
The occupants of this hypothetical train will more than likely feel the health side-effects of attaining the speed needed to get the results noted in Hawking's narrative. Just as the slowing down of time would have the effect of keeping them physically younger than they would otherwise be, with the passage of time, the acceleration events—needed to get the speeding train to arrive at the speed necessary to slow down time enough to make a considerable leap into the future—would have their own physiological effects on the occupants. The gravitational pull of the Earth can only be handled at certain levels for only so long, not leaving out the effect of inertia. The following provides a colorful example of this:
We experience higher or lower g forces when we are rapidly changing speeds or directions. Normal humans can withstand no more than 9 g's, and even that for only a few seconds. When undergoing an acceleration of 9 g's, your body feels nine times heavier than usual, blood rushes to the feet, and the heart can't pump hard enough to bring this heavier blood to the brain. Your vision narrows to a tunnel, then goes black. If the acceleration doesn't decrease, you will pass out and finally die. The Air Force's F-16 can produce more g's than the human body can survive. We're forced to limit the acceleration of planes and spacecraft to a level humans can survive.
If we need to accelerate for extended periods, the level we can withstand is even lower. We can withstand 5 g's for only two minutes, 3 g's for only an hour. For the sake of argument, though, let's try to tough it out at 3 g's for a little longer. For Han to take off from Mos Eisley and accelerate at 3 g's to half the speed of light would take him two and a half months—hardly the makings of an exciting movie. Even at 9 g's, it would take him nineteen days to reach half the speed of light, though he'd be dead long before the ship reached that speed. 
However, the effects of gravity can be overcome if one were to try one's hand at approaching the speed of light in space, as opposed to orbiting bodies of great mass like planets, stars or black holes. In space-traveling science fiction movies like Star Wars, whose Han Solo character was used in the extract above, or Star Trek, viewers see that the spaceship crew are able to stay stuck to the floor of the spaceship. For that to happen in a real situation, some other phenomenon besides gravity would have to have been applied to keep the crew member on the floor of the spaceship, as opposed to floating in near-empty space.
It's taken for granted that the space crew had managed to overcome lack of gravity in space far away from any body or object with great mass; viewers are not treated with details of how this was attained. Generally the only time crew members experience "weightlessness", is when they intend it, by working the necessary instrument(s) of the spaceship to put it into effect.
While gravity may not be much of an issue for high-speed travel in space, i.e. in a vessel/vehicle, as opposed to orbiting a massive objective, the traveler may still not be in the clear, in terms of physiological risks to the human body. Acceleration events would still be necessary to approach the speed of light, aside from the prerequisite power and energy. The effect of inertia at 1 g may hardly be noticeable in normal situations, but at any speed approaching speed of light, the inertial effect can be potentially damaging; consider the following example:
Han Solo talks about making the "jump to light speed." If the Millennium Falcon is somehow jumping to light speed, it implies a nearly instantaneous acceleration. The Falcon might be traveling along at 50 miles per hour, and then suddenly it's traveling at 186,000 miles per second. Let BMW try to beat that acceleration! It's no problem for Han to accelerate the Falcon from zero to 60 miles per hour in five seconds. Inertia will push him slightly back in his seat. But accelerating from zero to 186,000 miles per second in five seconds will push Han back so forcefully that he'll become a splat on that fine vinyl upholstery.  - Jeanne Cavelos, The Science of Star Wars.
Indeed, if one were traveling at the speed of light or close to it, everything else in motion, but at considerably lower speeds, will appear as though they've come to a halt. In that hypothetical scenario of the speeding train orbiting Earth , it is said that the occupants would be traveling into the future, given the slowing down of time on the train at extraordinary speeds approaching the speed of light: however, unlike the situation portrayed in many science fiction flicks, the occupants would not be "skipping" time on Earth to visit the future or have a "dual presence" at any moment in time. That would create a paradox, and heighten the potential for disaster.
For instance, if one recalls Back to the Future, the time travelers bypassed a good amount of time, before appearing into the future, at a certain date. In this scenario though, while the time travelers were making that journey, along the time intervals—supposedly in "real time" into the future—between the day they left Earth in their "present" time and the day they would have arrived in the "future", said travelers' persons were also experiencing worldly events on Earth.
Henceforth, said time travelers were visiting the future to "change" the course of events that would have been triggered by what had already transpired. So in one sense, said time travelers did not "skip" any moment on Earth, yet in another sense, they would have skipped time on Earth and instead would have spent it in the time traveling machine, wherein time would have considerably slowed when compared to that on Earth. These two phenomena would have occurred parallel to one another, and so creates a paradox, because the time travelers appear to have been at two places simultaneously!
Given the above, the time traveling characters were shown to have seen themselves in the future, but the writers of the story sought to overcome the paradox, by suggesting that the time travelers sought to visually conceal themselves from their 'future" selves. Simply put, the time travelers made attempts to hide themselves from their "future" selves; naturally, their "future" selves looked visibly "older" than their "time traveling" selves.
|Image Caption: Back to the Future time traveler featuring clothing worn in the "present", "future" and the "past".|
This time however, the story writers sought to circumvent the paradox, by insinuating that the time traveler could meet and speak with his "past" self but not physically touch the other, and vice versa. Still, the time traveler has a dual presence simultaneously, although in the very same space in this case. Henceforth, the paradox has not been resolved or circumvented by a lack of physical contact between the dual personalities of the time traveler!
Conclusion:There is nothing wrong with finding comfort in dreaming about all the possibilities out there, in our universe, waiting for the human mind to unlock them and possibly use it to the advantage of humankind. This certainly applies to human desire to fulfill the long-held wonderment about the universe we reside in, and possibilities that come with manipulating time to effect change to one's advantage. This is what movie flicks seek to simulate in more vivid visualizations.
Time travel however, is more than likely one of those endeavors that will never be within the human grip. Humanity is not only too intellectually primitive to overcome the obstacles of time-travel, but also too socially primitive.
Time travel would require technological access to extraordinarily high speed, like that approaching speed of light. Beating the speed of light is out of question, since nothing known in our universe is faster than light. Still, even attempting to approach speed of light at any capacity, requires tremendous power and energy; that is one great challenge to overcome, both technologically and economically. The appropriate machinery needed to make such high speed possible for a manned-transportation, is yet another challenge to worry about, and even if humans managed to acquire such technology, the physiological hazards that come with traveling at extraordinary speeds for any substantial amount of time, may render such travel unfeasible.
Movie flicks and fictional story books have told stories of "future worlds" with "flying cars", "teleportation" devices, super high-rise buildings—considerably taller than anything available today, suburbs littered with homes which are nearly entirely controlled by super computers that are "verbally" commanded to manage a wide range of domestic services from climate control to hygiene-oriented maintenance, and so forth, whereupon said "futures" have now become our past, and yet, still no working infrastructure of such "conveniences".
A future with say, "flying cars", can potentially be right around the corner, even if it may practically prove to be a logistical nightmare, provided the will is there. Time travel, on the other hand, looks to be a much more distant into the future kind of thing; perhaps too distant for humans to ever attain!
*Subject to revision or modification upon reception of new or additional information.
—: NASA/WMAP Science Team
—: Stephen Hawking, The Beginning of Time.
—: Stephen Hawking, 2010, How to build a time machine.
—: Jeanne Cavelos, The Science of Star Wars.
—Movies: Back to the Future, Superman I (1978), Time Cop, Star Trek, etc.