Original Link : https://medium.com/predict/time-travel-ill-be-back-to-the-future-2b939f3a5288

Might time travel be possible someday? And how could it work?

Time travel: It’s the Holy Grail of many a physicist and science fiction writers’ minds.

Is it possible? And if it is, can we find a way to harness it without destroying ourselves, our world, or even our universe?

Time travel holds a special place in many of our dreams. Most of us, at one time or another, wish we might be able to “go back” to some specific instant in our lives and relive it so that we can adjust the trajectory of our life or someone else’s and lead to some better result.

The idea of time travel probably dates back tens — even hundreds — of thousands of years, when one of our early ancestors first regretted some frivolous act that begat some unintended consequence and imagined it all turning out well instead. Surely, that person wasn’t thinking in the exact terms “time travel”, but that was enough to get the gears turning.

Later on, in stories like that of Rip van Winkle and A Christmas Carol, writers would invent ways that their main characters might see or even be transported into a future world.

Before long, H.G.Wells’ The Time Machine began the modern fascination with our ability to explore time as though it were just some other spatial dimension, by building devices that could take us across the years as an ocean liner might take us to another continent.

Time traveling machines would somehow bend space-time enough that it curves back in on itself, making a loop. The traveler (and his machine) enters this loop at one point in space-time and exits at another at some time in the past or future. What the machine uses to achieve this effect varies up the scientist or writer involved, though some imagine a form of exotic matter that possesses what can be called “negative energy density”, resulting in it moving toward a force that pushes it.

Falling under time machines as a category we can place the idea of a faster than light (or FTL) vehicle. Seth lloyd, an MIT researcher, wrote of this:

Finally, we pass the critical speed limit 1 at which the total trip time is a negative number — we’ve gone back in time! In this scenario, a certain time before the spaceship takes off for the planet from the launch pad, a new pair of real spaceships is created on the landing pad, and one takes off toward the planet. Then, the original spaceship takes off at its normal time, and they annihilate as before at the planet.

This kind of possible travel to the “past” always results in two versions of the traveler, both always annihilating one another. Lloyd’s team also concludes that one of the ships is made of that aforementioned exotic negative matter. For anyone interested, Lloyd offers his equation for time travel.

To travel backward in time, the spacecraft’s velocity must exceed:

where u is the velocity of the planet relative to Earth, and c is the speed of light.

A longtime contender for top natural time machine is the inimitable black hole, a singularity of extreme density so high that not even light can escape its pull. The late Dr. Stephen Hawking proposed that if a spaceship crew could fly around the event horizon (the point of no return for light particles) that the would experience time at perhaps only 50% the rate of anyone else. The crew would, in effect, be traveling into the future relative to their friends and relatives on Earth.

Yet another natural phenomenon that might allow for time travel is that of cosmic strings, those theoretical hollow energy bands that might stretch from one end of all creation to the other. Such large objects would contain vast quantities of matter, and therefore mass and gravity, which might warp space-time around them. If cosmic strings exist, they would either stretch on infinitely or loop on themselves.

Borrowing some of the science behind black holes and cosmic strings, the astronomer Frank Tipler proposed something he called the “infinite cylinder”. His construct would be a cylindrical object with the mass of 10 suns, set to spin in the billions of RPMs, around which a spaceship would fly in a tight spiral in order to try and reach a closed, timelike curve. The cylinder would need to be infinitely long, however, which renders any experimentation rather moot.

To add some fuel to the fire, there is actually nothing to be found in Einstein’s General Relativity work that definitively rules out closed timelike curves, tunneling through space-time and the like. In short, time travel could be possible based on physics as we today understand it.

For argument’s sake, let’s accept that one of these time travel options does work, and our potential time traveler — whether that is an intrepid scientific space explorer or any one of us Medium readers who just happens to stumble upon a dusty old time machine in our great-uncle’s basement — partakes in one of the most memorable journeys in human history…and then we find ourselves immediately dreading all the possibilities of various time travel paradoxes.

One of these, familiar to many of us through many interpretations in science fiction literature and cinema, is the “grandfather paradox”. The basis of this paradox is the idea that someone could potentially travel back to the time when their grandfather, or any grandparent for that matter, was an infant and kill them (or by accident cause their death). If you managed to do this, then voila! you have created a paradox. Why? How could you have possibly time traveled in the first place if your ancestor perished before they conceived one of your parents? Thus, a paradox.

Because of this possibility (only if time travel to the past IS possible), theoretical physicists think that either A) time somehow protects itself and would prevent you from ever being able to kill your ancestor, or B) You actually succeed, but when you return to your present — the “future” — you find that you have actually succeeded only in creating a new, alternate timeline in which you were never born. This last is an example of what could be called a parallel universe.

Yet another paradox comes to us from Theodor Polchinski, an American theoretical physicist. He describes an event in which we toss a ball into a wormhole, it goes through (tunneling space-time) and exits in the past, just before the ball was thrown into the wormhole. It then bumps into itself and prevents itself from ever having entered the wormhole in the first place.

Russian physicist Igor Novikov proposed what he called the “Self-Consistency Principle” in the 1980s. This states that while time travel may be possible, time travel paradoxes would NOT be.

Ray Bradbury’s A Sound of Thunder short story provides an example of what’s known as the timeline corruption hypothesis. This is a time travel version of the famous “butterfly effect” notion, that the smallest of actions or events compound over time by influencing ever more actions and events in a rippling snowball effect. In A Sound of Thunder, a time traveler accidentally steps on a single butterfly during a visit to the Jurassic, and when his group returns to their present day they discover a world in which humans never evolved.

Next, we have what we can call a “predestination” paradox, in which a time traveler travels to the past in order to prevent some event, but actually ends up becoming the cause of said event. This is an example of a temporal causality loop: A leads to B leads to A. Some movies where this can be seen in action are Predestination (of course!), The Time Traveler’s WifeLooper, and the modern time travel classic 12 Monkeys.

This leads us to a somewhat related type known as an origin paradox. In this instance, the time traveler — perhaps the inventor of the time machine — goes back in time to give himself the plans for his time machine. So the question becomes: When did those plans ever get devised anyway? The act of time travel, in this case, erased the origin of the plans.

Similarly (and sort of oppositely), we have the paradox that occurs when a time traveler goes back in time to undo something. If you decide to travel back to 1889 and kill Adolf Hitler as a baby in order to prevent his atrocities, and you succeed, you would have then wiped out the very reason you went back in time in the first place. No Hitler, no reason to try and kill him.

A general consensus among many theorists is that time travel to the future — using methods that employ relativity and traveling faster than the speed of light — may well be possible, while time travel into the past is likely not possible due to the many paradoxes, some of which I’ve outlined above.

Maybe one day some astronauts might end up traveling through time via a wormhole or by just hanging out around an event horizon for a while. We’re not at a point to do anything other than entertain the thought of it for now.

We’re all time travelers anyway, moving ever forward into a future. The present doesn’t really exist, because we’re always reacting to or observing events that have already occurred due to the lag between what we sense and when our minds perceive and interpret it. We mentally live in the past while constantly trying to guess what’s coming next.

What’s past is prologue.

— William Shakespeare, The Tempest