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We all have those things that we wish we could undo. We have also dreamed about seeing the distant future or reliving the greatest moments of history. The concept of traveling through time has been a major source of inspiration for countless movies, stories, and even video games. With all of this speculation and imagination running rampant though, the question remains: is time travel possible?
The answer, as with anything this complex, is complicated. Time travel isn’t something that we can accomplish, at least not that we know of, with current technology. Beyond the science portion of it, there are also a number of paradoxes that could result from such a feat. This can all be a little more than confusing so join us as we break it down into normalized terms and see just how much of this is science, and how much is fiction.
Simply put, science does allow for the possibility of time travel, but the means and methods are extremely complicated and in most cases involve technology we don’t have. Going forward is also far easier than going backward it turns out, and involves less theoretical issues. It all begins with the theories of special and general relativity. Let’s take a moment to examine these two concepts, and how into the concept of a space-time continuum.
This theory, posed by Albert Einstein, seeks to describe the way that space and time interact. There are two major assumptions that this theory uses: one is that laws of physics are identical in all aspects of inertial systems, essentially the apparent motion of objects or bodies in a specific frame of reference.
The other assumption that Einstein made was that the speed of light in a vacuum like space is the same for anyone watching it, regardless of the actual light source’s motion. In essence, this version of relativity applies specifically to inertial references frames.
What we have here is the theory that the speed of light is then equal and constant for everyone. This theory was put forth in 1905 when Einstein showed evidence that the speed of light didn’t change when the Earth swung around the sun in orbit.
This realization was a surprise because in most other elements of physics the speed of something depends on what direction the observer themselves in moving. If you see an object approaching, it will look like it is moving faster than if you were following it for example.
We’ll cover this more in detail in a moment, but the takeaway here is that special relativity also shows that time passes by more slowly the closer an object gets to the speed of light. As part of this theory, it is also assumed that no object or ship could travel at the speed of light because its mass would grow to the value of infinity at such a speed.
The famous equation E=mc^2 applies here as the method by which we mathematically show the relationship between mass and energy.
As if one major groundbreaking theory wasn’t enough, Einstein also brought us his general theory of relativity in 1915. This was another major shakeup for the scientific community, and one that took Einstein 10 years to mathematically structure.
To put this into a more bite-sized piece of understanding, Einstein was able to show how gravity affects space-time. If we think of space and time as one concept or continuum as it is known, then we can imagine it like a piece of fabric or cloth.
Large objects or celestial bodies exert a gravitational pull that bends this space-time around it, as if you were to place an apple or rock onto your piece of taut fabric, it would make indentation. There are a number of complex equations surrounding this concept, and many of the results have been proven.
Discoveries have been made that show how these theories of relativity can allow for time to be altered, stretched, or compressed as a result of speed and gravity. Today’s technical discussions of the concept usually tend to avoid the use of “travel” because the person wouldn’t technically be moving in any of the three dimensions that we can see.
Since time itself is referred to as a “fourth dimension” there are theories involving worldlines which track our apparent movement through this separate dimension of time and place. In terms of going backward, there is a theory involving closed time-like curves which would occur if a worldline was to close into a loop that would allow an object to return to a previous time.
These concepts are presented as solutions to certain equations in the field of special relativity. So, science tells us that spacetime could potentially be closed into a loop via an object’s worldlines. Of course, proving physical evidence of these loops is a far more difficult task.
In a more physical sense, relativity tells us that if an object were to leave the Earth at speeds close to the speed of light, and then return they would find that more time had passed on Earth than had passed for them. Say a day went by for them, and a year went by on Earth. Not exact math, but the concept has been proven.
With all of these theories rampaging around in your head, you may be lost, but this where things come together, quite literally in fact. A mathematician by the name of Hermann Minkowski created a type of space where Einstein’s special relativity allowed for time to be interwoven into the fabric of our three dimensions.
Normally, in Euclidean space, time is a fourth dimension and separate from our world of three dimensions, but here it’s all uniform. This allowed for a number of physical theories to put into practice. With this in mind, it’s easier to picture how gravity warps time in some of our upcoming concepts.
This in and of itself proves that one could travel to the future by placing themselves at relativistic speeds and returning to where they came from. What we know for certain then, as a result of relativity, is that time itself can be stretched and compressed, so time travel is possible in a sense.
Keep in mind that these considerations, while awesome, are very theoretical in some cases. In others, they have been proven to an extent. Going forward seems to be the only real possibility, because you cannot go back to a time before you created your time machine, so unless you use another method that doesn’t include a machine or phenomenon, going backwards wouldn’t work.
We’ve never seen one of these in person before, but they are possible according to the theory of general relativity. As we know, space-time can be stretched and compressed with enough gravity or energy involved. If we somehow had an incredible amount to use, we could create a shortcut between two points by warping space to close the gap in distance.
The most common visual example is taking a sheet of paper and drawing two dots. Then, fold the paper in half, thus eliminating the distance between the two dots. Push your pen or pencil through the paper and you have a real world example of what a wormhole would do.
The issue with these, however, is that they would collapse almost instantly. There is something called traversable wormholes that are theorized to be stable enough, but they also require something called exotic matter which encompasses theoretical substances like dark matter, negative mass, or imaginary mass.
Besides being a quick shortcut, these could also be used for time travel by accelerating one of the two ends of the wormhole. Another option is to place one of the ends near a large gravitational field. In both of these scenarios, time dilation occurs which causes time to pass more slowly for the object entering the modified end of the wormhole.
You could go into one side and come out in a future time, but you would not have aged. Going backwards would be possible as well, but you not are able to travel back to a time before the wormhole was created. So, this would work, but we would need to first understand the nature and abilities of the previously mentioned “exotic matter.”
Black holes are famous for being enigmatic and mysterious. Their gravitational pull is so strong that not even light can escape from it. There’s a point called the event horizon where nothing escapes and beyond that, we can only guess what is within. Theories suggest that there is a central point called a singularity where gravity is so strong that physics themselves break down.
These are known as Schwarzschild black holes, but there is another type that has more possibility of being usable by us. These are called Kerr holes which are a rotating type. In 1963, Roy Kerr put this theory forward and said that dying stars could potentially collapse into a rotating ring of neutrons that result in a black hole without a singularity.
This type of black hole could be used to pass through into other universes or other times, without being crushed by a singularity, but it’s a subject that is argued to be impossible by famous physicists such as Kip Thorne.
These are theoretical strings that were created during the formation of the universe. They are thought to potentially still exist and run across the length of our entire universe. They would be even thinner than an atom though and cause massive gravitational pulls on anything that gets near them.
If we could find and attach a ship to one of these, we would be able to travel fast enough to cause time dilation and essentially move through time without aging ourselves. With two strings, we could even create the aforementioned closed time-like curves that would allow us to jump into the past, but not before the ship was made.
We know that if these strings do exist, that we could not create any more of them, so we would have to find them first, then understand how to harness them.
There are many different methods that we could potentially move through time. These are three potential examples, but more options are always appearing as we progress in both knowledge and technology. As it stands though, we have not discovered any true way of traveling through time.
What if we could though? Let’s assume that moving backward and forward is possible. What problems would we run into? What issues would we face? Let’s find out.
This is where things get interesting, or at least less complicated. If we put aside our theories and our impossible notions for a moment and assume that, yes, we can travel backwards and forwards in time, what issues or problems could that create? Well, as it turns out, there’s a lot of things that could (theoretically, the word of the day) go wrong.
The most common problem is something called a paradox. In common terms, these are situations where you create an event that contradicts itself. Several physicists have argued that time travel is not possible based on the sole fact that there is potential to cause these types of situations. Let’s explore some possible impossibilities that our trip through time could cause:
This is one of the most famous “time travel paradoxes” and was first put forth by Nathaniel Schachner in his science fiction story Ancestral Voices. The idea here is that you go back and time and kill your grandfather before he meets your grandmother. By doing this, you alter the timeline so that neither you, nor your parents, were ever born.
This paradox refers to any situation where physical items or information is passed from the future to the past. These items eventually become part of a loop that makes it impossible to know where they originated. The paradox here is the impossibility of knowing where the items or information truly came from.
The name comes from the phrase “pulling yourself up by your bootstraps.” The situation was first posed in the Robert A. Heinlein story, By His Bootstraps
This is a very popular concept in science fiction. The situation that causes it involves a set of events that are part of a loop. This is also known as a temporal causality loop. It begins with a past event that eventually results in a secondary event occurring, however, that second event is also what caused the first one to occur.
An example helps to better illustrate this. Let’s say you lost a person dear to you in an accident and you created a time machine to back and save them. In your attempt to save them though, you caused the accident to occur yourself. A loop is created where you were the cause of your original motivation to go back in time, so the process repeats infinitely.
In quantum mechanics, there is a theory that our world is one of many different parallel universes. This theory, known as the “many-worlds interpretation” would account for possible paradoxes in the event that someone did travel through time and create one. Every possible outcome of a situation creates a separate path or universe that then continues to move parallel to all of the others.
With all possibilities currently happening, a paradox would simply result in a change in the timeline or the universe. An alternate history will be presented to resolve the paradox. It sounds simple, but both science fiction and real scientists like Stephen Hawking argue how plausible it is, given what we know about the nature of quantum mechanics.
Daniel Greenberger and Karl Svozil presented a new theory that allowed for time travel without paradoxes. The idea of their theory is based on the fact that quantum mechanics states that there are a number of possible outcomes, but once you’ve measured the state of something, those outcomes shrink into one, removing any element of uncertainty.
In their example they state that if you know for certain that your father is alive in the present, that you would be physically unable to kill him in the past. What has happened is the only possible outcome. Something would get in your way or stop you from going through with it.
Now, if you weren’t certain that your father was alive, there would be a chance that you could kill him, but the fact remains that the outcome has already been decided, and there’s nothing you can do to change it. This doesn’t resolve all paradoxes, but it does remove the possibility of several never occurring in the first place.
Beyond our desire to do something over again in the hopes that things will go differently, we typically don’t think of time travel in our daily lives. This idea that someone could move forwards into the future or backwards into the past is something that doesn’t have solidified roots in any one piece of research or fiction, but there are signs of the concept dating back to very early times.
One such example is the story of King Raivata Kakudmi in Hindu Mythology. In this story, the king leaves Earth to visit the creator of all things: Brahma. When he returns, he discovers that 108 Yugas have passed which we believe to be the equivalent of 432 million years. Kakudmi is told that time flows differently in different planes of existence, which is why so much had passed while he was gone.
Interesting, no? Another instance of time travel, albeit it one that occurs in a different manner, is in the Quran concerning the cave of Al-Kahf. It tells a story of persecuted people who sought shelter in a cave where God put them into a deep sleep like hibernation. When they woke up, 309 years had passed.
Again, we see a similar story in the form of a Japanese tale regarding a fisherman named Urashima Taro. In the story, Urashima stumbles upon an underwater palace and stays for only three days. However, when he returns home, 300 years have passed. Even this story dates back to the 15th century.
These are all examples of time travel that takes the protagonist into the future, but the past is also just as popular in the subject matter. An early example of this concept comes in the form of a piece entitled Memoirs of the Twentieth Century which was written in 1733 by Samuel Madden.
In this story, the main character receives documents from 1997 and 1998 despite the fact that he is living in the year 1728. The explanation is that his guardian angel came to him with the futuristic documents. This is a vague idea of the concept, another Russian novel whose title translates to The Forebears of Kalimeros: Alexander, son of Philip of Macedon is widely regarded as one of the earliest novels to use time travel.
The first time a machine was used to move through time as in the 1881 story The Clock that Went Backward by Edward Page Mitchell. It was published in the New York Sun and featured a clock that when wound, would run backward and transport people near it through time.
The first time an actual vehicle was created for the purpose of traveling through time was in the 1887 book El Anacronopete. The term “time machine” was not used here though, that phrase came later.
The novel that truly brought this concept to the forefront of the general public’s imagination was The Time Machine by H.G Wells. Published in 1895, this was the first time the term “time machine” was coined, and it was also the first instance where the protagonist could travel back and forward at will.
So, is time travel possible? The answer is yes, but only in theory. It would also most likely only be possible to go forward by bending space-time and causing time dilation. Even then, that isn’t the science fiction answer most of us would like to hear. The closest possible solution would be the use of a wormhole.
It’s fascinating to see how fiction and fact can sometimes come together in various ways. As it stands, we don’t have any mainstream methods of time travel to utilize, but we’ve got some ideas in place and plenty of theories. For now, you can continue traveling forward through time at a rate of one hour per hour.
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