Are Time Travel Paradoxes Solved
Can you really go back in time and kill Hitler?
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Huey Lewis sang of the perils of going back in time for the iconic series Back to the Future. Doc Brown frequently spoke of the risks of creating time-based paradoxes resulting from time travel, And, indeed, Marty McFly very narrowly dodged the worst results of them.
Doc’s worries about disrupting the spacetime continuum, however, rest in real issues of concern among physicists and philosophers should humans ever manage to figure out time travel. The simplest explanation is that going back in time could create changes that alter the future to which the time traveler intended to return. Paradoxes arise when those changes abrogate the cause for the time traveler to go to the past in the first place. Or, more abruptly, those alterations could actually forestall the events leading to the creation (birth) of the time traveler altogether. Those are the most obvious of problems, but others exist.
Two groups of researchers claim to have proved that time travel is possible, a concept with which many physicists—including the late, great Stephen Hawking—agree, but have also discovered a solution to some of the time travel paradox problems. Before exploring their rather complex theses, here are some of the seemingly stubborn philosophical implications of time travel.
The Main Temporal Paradox Theories
Predestination Paradox
Imagine traveling back in time with the intent to prevent some incident, only to end up acting as the source of the incident. The website AstronomyTrek.com provides a good example:
A simpl[e] predestination example involves a person traveling back in time to prevent a fire that broke out at a famous museum a century earlier resulting in the destruction of many valuable pieces of art, only to accidentally cause a kerosene lamp to fall, therefore creating the very fire that later motivated them to travel back in the first place.
In essence, a predestination paradox means that the time traveler causes the incident that predicated the desire or gumption to travel backwards in time to begin with.
Bootstrap Paradox
This problem occurs when time travel leads to an infinite cause-and-effect loop. In other words, a critical element to the time travel itself loses (or never has) any definite origin or causation. Seth Lloyd, a professor of mechanical engineering at the Massachusetts Institute of Technology, explains “The bootstrap paradox in time travel occurs when a piece of information that has no right to exist, nonetheless exists.” Lloyd and his team purported to solve the grandfather paradox, yet another time travel problem listed below, but more on their work in a moment. Tim Maudlin, a philosopher of science, described the difference between a bootstrap paradox and the grandfather paradox this way:
The bootstrap paradox occurs in certain logically consistent time travel scenarios. Unlike the grandfather paradox, which is a self-contradictory and hence an impossible scenario, in a bootstrap story, everything fits together in a logically coherent way…
Logical consistency notwithstanding, the paradox arises because certain objects or information become ‘self-caused’ or ‘comes from itself.’ Perhaps the idea is best explained in the 1941 Robert A. Heinlein story, By His Bootstraps:
A time travel machine is built by following the instructions in a book, which itself is sent back from the future. But how did the information get into the book? By being copied into a newly made notebook from the older version of itself.
In other words, the builder of the time machine relies upon guidance written by someone in the future who traveled back to the time of the builder. That historical person builds the time machine, which then informs the future person on how to build one, and the future person writes it down and travels back in time to give it to the historical builder. And the loop goes on and on.
Grandfather Paradox
Perhaps the most well-known paradox involving time travel, the grandfather paradox is quite simple. If one goes back in time and kills his grandfather before his father is born, does the time traveler not immediately erase his own existence? Marty McFly faced this sort of problem in Back to the Future. Similar to this, other named paradoxes (like the “Kill Hitler Paradox”) suggests that if one goes into the past to commit such an act, then the reason for their travel back in time ends with the act and thus they would not travel back to begin with, and time carries on as planned; the killer would not travel back to the past because doing so no longer remains necessary, but then the bad person exists, reigniting the reason to travel back in time to kill him.
If Time Travel were Possible, Where are the Travelers?
The short answer to this is we might never know of them, partly because of the paradoxes noted above. If, for example, a time traveler seemingly causes one, it might dissolve his or her own existence before revealing his or her futuristic knowledge. The moment the paradox arises, the time traveler would disappear, thereby leaving everyone else to carry on uninformed of the traveler’s existence.
Another possibility is that time travelers simply cannot change anything, so their claims of ‘knowing’ futuristic things would ring hollow to people of the requisite time period. They could tell people futuristic things, but those people might perceive them as little more than dreamers or crackpots. In other words, someone who made a change in the timeline and then instantly disappeared would be quickly forgotten following their disappearance, while someone who claimed knowledge of the future could never truly prove it as a result of being unable to change anything.
In both cases, most people would likely dismiss them either out-of-hand or as crazy people not worthy of much attention. Then, there is the issue that traveling from one time to another does not guarantee the possibility of a return trip. If that happens to be the case, then the above would still apply, and the future might look different—but there would be no one who could compare the two results.
It all bends the mind!
Many people have claimed to be time travelers, though they seem to have been mere charlatans using their claims for money, fame, or some other self-interested purpose, and were easily exposed. But what if… It seems that in only a single instance the claims about a time traveler remain unfalsified. The case garnered some attention because one might explain the flaws in a way that left the door open to the possibility of truthfulness.
The story goes like this:
Sergei Ponomarenko came to the attention of police in Kiev, Ukraine, in 2006. Whoever called noted a confused young man wearing rather odd clothing wandering the streets. Police confronted him and asked for ID. Ponomarenko provided one—from the Soviet Union. Since the collapse of the USSR in 1991, former countries of the bloc began creating and distributing their own national IDs, so police in 2006 Ukraine—that had long stopped using Soviet IDs—found this rather odd.
Moreover, the birthdate on the ID was 1932. Even though the picture matched the person they stood staring at, who looked in his 20s, his ID put him around 74 years old. They asked him the day’s date. Ponomarenko responded, April 23, 1958—nearly a half century before. Suspecting that either the man was a liar or lunatic, police transported him to a mental hospital. The treating doctor revealed in his notes that Ponomarenko told him:
It was daytime, and I wanted to go for a walk in the city. I took my camera, but when I left my house, I saw a strange object that had a bell shape, and it was very strange, and it was flying in a strange way. It is difficult to explain what I was seeing. It might be better to look at the photos from my camera…
After hearing this, the doctor examined the camera and film and made a few discoveries. First, the model was old and rare. It contained photo film so old that retail developers in 2006 could not work with it, so the doctor called in a film expert. The expert determined that the film reel was manufactured in 1956, was in excellent condition, and contained pictures of various scenes and people that all comported with features of the 1950s.
To add a bit more intrigue to this situation, one photo contained what appeared to be a bell-shaped UFO, just as Ponomarenko claimed. Moreover, police learned that someone with the same name as this mysterious man had been reported missing by family members around 1960 from Kiev.
On the night of his admission, the hospital staff placed Ponomarenko into a secured room, with barred windows, CCTV watching the door, and a guard stationed in the primary corridor of the wing. Nevertheless, staff the next morning found the room empty—Ponomarenko seemingly vanished. No CCTV footage captured the escape, the bars remained intact, the guard saw nothing. Just gone.
Upon this information, police found and interviewed Ponomarenko’s girlfriend from 1958 Kiev. She told them Ponomarenko disappeared that year, reappeared a few days later, then vanished again sometime in the 1970s never to be seen again. She claimed to have received a photo in the mail some time after the second vanishing, purportedly of Ponomarenko, with a note stating that it was taken in 2050. Its backdrop showed what appeared to be a futuristic city.
Joe Scott, host of Answers with Joe on YouTube and Nebula, conducted a thorough analysis of this story available here. To tell this tale, I relied heavily on his work (except where otherwise noted). Please check out his channel; he makes entertaining and educational videos.
Joe traced Ponomarenko’s story back to an episode of a Ukrainian television show called Aliens that aired in 2012. Of its 10-episode run, it features this story in the third episode, titled “Time Traveler.” Joe noted that a disclaimer admits that any topic presented on the show “cannot be considered correct,” but leaves that decision up to the watcher.
Among the many pieces of “evidence” shown in the episode (some described above), almost all contain enough flaws to discard as illegitimate—something Joe dissected exceptionally well. The research team for Answers with Joe tried hard to locate any scintilla of evidence to give some credence to this narrative. They found none. Nevertheless, Joe said,
Now, I don’t know much about this show, if you are familiar with it, maybe if you’re from Ukraine and know something about it, please educate me in the comments, but it comes across like those History Channel type shows where they take some urban legend and dramatize it. Like, I feel like this was just a good piece of creepypasta that the producers ran across and decided to run with, and they did a great job, it has taken on a life of its own.
He characterized it as a “great story” and probably nothing more. This story contains perhaps the most detail of any, and its airing on a television channel gives great fodder to enthusiasts who want to believe time travel is not only possible, but that some have done it.
Yet, at best the story represents an embellishment of some banal real-world occurrence. Or, more likely, writers simply dreamed it up and created semi-convincing tv props to buttress it. No real evidence backs it up, and Joe’s team could not find contact information to request comment from the show’s producers. Furthermore, no other story comes as close to this one in its persuasiveness, which suggests no viable examples exist (or have been uncovered).
Ponomarenko’s alleged saga highlights another paradox about time travel that remains unsolved—where are the travelers? As noted above, it may be that no one has noticed them, but in a world of billions of people, the lack of a convincing account seems telling. But, assuming it possible that at least a few people have managed the feat, and no one noticed, what of the other problems?
What Is Time?
The concept of time itself confounds even theoretical physicists. Einstein’s Theory of Relativity posits that time travel is indeed possible because time itself represents a symmetric variable. The laws of physics remain the same irrespective of whether the time value is increasing or decreasing (or, moving forward or backward). Moreover, the laws of physics recognize no “now.” In other words, there is nothing special about any given moment in the context of the cosmos. Physicists refer to this concept as a block universe, “a static block of space-time in which any flow of time, or passage through it, must presumably be a mental construct or other illusion.”
Our perception of time moving “forward” may simply reflect our unconscious conceiving of entropy—the measure of disorder within a system. The higher the value of entropy, the more disorder. According to the second law of thermodynamics, entropy does not decrease within a closed system. To give a real-world example, a scrambled egg cannot be unscrambled. In relation to the cosmic system, some call this the “arrow of time.”
Even such a seemingly concrete concept as entropy, however, is not truly settled. As Arieh Ben-Naim explains,
Open any serious textbook on thermodynamics, and you will find that Entropy is a state function. This means that entropy is defined for equilibrium states. Therefore, it is clear that it has nothing to do with time. Yet, surprisingly many authors associate entropy with time, and some even equate entropy to time… Entropy changes are meaningful only for well-defined processes that occur in well-defined thermodynamic systems for which the entropy is defined; once we specify the system, then its entropy is determined and does not change with time.
Put simply, according to Ben-Naim, entropy describes the state of something and is unaffected by the passing of time (or the time variable). Others agree, at least in part, that something other than entropy determines time’s arrow. Three physicists, Tim Koslowski, Julan Barber, and Flavio Mercati, argue that gravity, not entropy, dictate the flow of time. This dictation, however, applies only locally to the observer. Across existence, time can move in any which direction, but to the observer within the local frame, it will appear with a distinct past and future. Still others have posited different interpretations.
So lies the obstinance of the concept of time.
Traveling through time becomes a complicated problem once one realizes that no one knows for sure what time itself even is. To be sure, whatever time is, it is affected by perception. Humans associate time with sight. For example, if we see a thing occur before us, we apply a concept of “now” to the occurrence. “A ball is flying toward me.” This event is taking place right now. Anyone within visual distance of the ball probably agrees. Yet, in reality, for some, the event already happened. The further from the ball the observer stands, the longer it takes for the visual reference—seeing the ball fly—to reach that observer. Thus, though both observers perceive the ball flying right now, for the more distant one, that flight occurred moments before. It has simply taken time for the visual reference to reach them. “Now” for each is slightly different.
Because light moves so fast, we hardly notice the disparity. To better illustrate this, think of a commercial firework. For those directly beneath the show, they conceive the dazzling light and thunderous sound seemingly simultaneously. But for those standing on a bridge a half mile away, the firework explodes in the sky, but the boom follows several seconds later. Both occurred at the same time, but we perceive the difference because the two features—the visual and aural—move at vastly different speeds.
This principle becomes important when attempting to contemplate moving through time because while time remains a mathematical variable, it also exists as a mental construct. When one observer moves near the speed of light and another stays stationary, this differential in the perception—or experience—of time grows to the point that it develops its own paradox. As an entity reaches near to the speed of light, a phenomenon called time dilation occurs. Live Science provides this example to explain it:
Imagine a spaceship traveling at 95% of the speed of light to a planet 9.5 light-years away. A stationary observer on Earth would measure the journey time as distance divided by speed, or 9.5/0.95 = 10 years. The spaceship crewmembers, on the other hand, experience time dilation and thus perceive the trip as taking only 3.12 years… In other words, between leaving Earth and reaching their destination, the crewmembers age a little over three years, while 10 years have passed for people back on Earth.
Scientists have experimented with and proved over and again that Einstein’s equation for calculating time dilation is accurate. The paradox emerges when a pair of twins launches an experiment. One waits on Earth while the other blasts off into space at a velocity near the speed of light. Space twin conducts an around-the-block trip returning to Earth in a short time. Upon arriving home, space twin has aged only some, while Earth twin has aged immensely.
The Twin Paradox, as it is known, results from the mistaken belief that the same time has passed for each, as indicated by their proximity in departing and reuniting, and that both were born in the same Earth year. Yet, one has aged—experienced time—let’s say 5 years, while the other has aged 50. So, is one now older, or are they the same age?
Einstein’s Time Dilation Equation
Is Time Travel Theoretically Possible without Provoking Paradoxes?
Seth Lloyd from MIT, mentioned above, and a group of researchers published a study in 2011 in which they asserted that post-selected closed timelike curves prevent the type of paradox that would forbid a future outcome that itself would prevent time travel. Put way more simply, their research suggests that a person who would go to the past to kill his own grandfather (or create a similar paradox) would be prevented from traveling to the past at all.
Igor Dmitriyevich Novikov established this idea from which Lloyd and others worked, what Novikov called the self-consistency principle. Closed timelike curves (CTCs), proposed under Einstein’s Theory of Relativity, in effect create a circular path upon which an object travels that inevitably leads it to the same point in both space and time. The worldline of the object following the curve does not change, which means anyone traveling to any point of the curve—or any point in time—remains part of that time.
The time traveler cannot change anything anymore than a person standing in his backyard can change yesterday’s weather. This is because by virtue of being part of the time to which the traveler travels, he cannot ‘change’ anything because anything he does is what would have happened (and did happen) anyway. Lloyd’s team showed this in practice using quantum particles.
Germain Tobar, from the University of Queensland in Australia, provided a mathematical representation for a concept of time travel that prevents paradoxes. Tobias noted, “A popular approach holds that the grandfather paradox makes CTCs incompatible with classical physics, while appropriate modifications to quantum physics could restore consistency.” Tobias’s study showed that irrespective of what the traveler does, “space-time can potentially adapt itself to avoid paradoxes.” An example given is that if a time traveler went back to prevent an event, like the fire mentioned above, the event would happen anyway but through some other point of causation. In other words, the traveler could remove the kerosene lamp from the museum example above, but the curator might leave an errant candle in another room that ignites the fire regardless (or some other thing would happen to cause the fire).
But Novikov’s principle and Lloyd’s study cannot resolve all the potential paradoxes of time travel. And Tobias’s idea suggests that while a time traveler can act in any way he or she so chooses, they cannot effect any real change. In exploring this issue, Barak Shoshany and his team propose that multiple histories, or parallel timelines, resolve all paradoxes. The writers of Back to the Future employed this concept in the second movie.
There, old Biff returned to 1955 with an almanac that made his younger self super-wealthy. Growing up an immoral person, Biff engaged in many activities powered by his wealth that turned Hill Valley into a cesspool. Doc Brown postulated that the 1985 he and Marty returned to that was degraded from the previous 1985 they had known, was actually a 1985 on a different timeline. Theirs still existed, but they somehow left it and landed in this one.
Shoshany writes,
After working on time travel paradoxes for the last three years, I have become increasingly convinced that time travel could be possible, but only if our Universe can allow multiple histories to coexist. So, can it? Quantum mechanics certainly seems to imply so, at least if you subscribe to Everett's "many-worlds" interpretation, where one history can "split" into multiple histories, one for each possible measurement outcome – for example, whether Schrödinger's cat is alive or dead, or whether or not I arrived in the past.
But these are just speculations. My students and I are currently working on finding a concrete theory of time travel with multiple histories that is fully compatible with general relativity.
Here is what they say regarding their model for working toward that “concrete theory of time travel” using multiple histories:
In this paper, we propose a new model for resolving time travel paradoxes using parallel timelines. Our model is also based on the MWI [Everett or “many-worlds” interpretation], but it is different from the D-CTC model, and avoids the issues outlined above. In our model, the parallel timelines are created explicitly by entanglement between the time machine and the environment, just as the “worlds” of the MWI are created by entanglement between the observer and the system being observed. For this reason, we named this model “entangled timelines” or the E-CTC model.
The entangled timelines are an emergent concept. Each timeline is not a separate universe, but rather, a separate term in the superposition of the overall quantum state of a single universe. The chain of events within each timeline can be followed continuously via the action of an evolution operator, and each timeline can be related to the next one via the action of a correlation operator.
These researchers examine the universe through a quantum mechanics lens. Entangled timelines mean that the universe is not split asunder, exhibiting multiple realities. Instead,
the timelines are not physically distinct universes, but rather emergent structures which exist due to entanglement between systems, similarly to the “worlds” or “branches” of the Everett or many-worlds interpretation. Timelines are local structures within a single universe, propagating gradually as more and more systems become entangled.
Put differently, the universe accommodates multiple timelines simultaneously, but within the entirety of its structure. In this sense, it seems time travel consists merely of visiting a different part of the universe in which the same—or similar—events have or will occur. Travelers may perceive that they are meddling with history, but the reality is that they are simply creating a novel history elsewhere.
Conclusion
Traveling through time remains beyond the scope of human engineering and mathematical computation. Physicists, however, concede that time travel appears possible within the framework of their understanding of the universe. Unfortunately, it also seems to require speeds near or in excess of the speed of light. As I described previously, traveling at speeds faster than light requires inordinate amounts of energy. Generally, traveling faster than the speed of light requires a great deal of problem solving. On this I previously wrote:
To date, scientists have not been able to produce a scenario in which they can accelerate a single particle to faster-than-light speed. Doing so on the macro level, (i.e., in the form of a manned spacecraft) provokes so many more problems that it may prove intractable enough that scientists will simply deem it impossible.
Whether time travel will ever be possible seems dependent in part on resolving the paradox problems. But, it also requires the solving of computational, theoretical, and energy problems with conducting the needed processes required to conquer the laws of physics. While pondering time travel is a fun intellectual exercise, whatever time actually is, it seems we are stuck within its inexorable path. No one is, has, or will enjoy the opportunity to jump in some magical H.G. Wells Time Machine any time soon.
For my discussion on warp drives and faster-than-light speed, click below:
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I am a Certified Forensic Computer Examiner, Certified Crime Analyst, Certified Fraud Examiner, and Certified Financial Crimes Investigator with a Juris Doctor and a Master’s degree in history. I spent 10 years working in the New York State Division of Criminal Justice as Senior Analyst and Investigator. Today, I teach Cybersecurity, Ethical Hacking, and Digital Forensics at Softwarica College of IT and E-Commerce in Nepal. In addition, I offer training on Financial Crime Prevention and Investigation. I am also Vice President of Digi Technology in Nepal, for which I have also created its sister company in the USA, Digi Technology America, LLC. We provide technology solutions for businesses or individuals, including cybersecurity, all across the globe. I was a firefighter before I joined law enforcement and now I currently run a non-profit that uses mobile applications and other technologies to create Early Alert Systems for natural disasters for people living in remote or poor areas.
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