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3 Popular Time Travel Theory Concepts Explained

Time travel theory. It's one of the most popular themes in fiction. But every plotline falls into one of these three Time Travel Theories.

Time travel is one of the most popular themes in cinema . Although most time travel movies are in the sci-fi genre, every genre, even comedy, horror, and drama, have tackled complicated storylines involving time travel theory. Chances are, you’ve seen at least a few of the movies listed below:

• But what about...

The Possibility Of Time Travel

Time travel theory.

• Bill & Ted’s Excellent Adventure (1989)
• The Time Machine  (2002)
• Timeline (2003)
• Time Cop (2004)
• Back to the Future  (1985)
• 12 Monkeys  (1995)
• Terminator Series (1984)
• Star Trek (2009)
• Harry Potter and the Prisoner of Azkaban (2004)
• Freejack (1992)
• Looper (2012)

But one thing you might not have realized, even if you’ve seen hundreds of time travel-related films, is that there are only  3 different theories of time travel. That’s it. Every time travel movie or book that you’ve ever enjoyed falls into one of these time travel theories.

Fixed Timeline: Time Travel Theory

Want to change the future on Earth by modifying the past or present? Don’t even bother according to this time travel theory. In a fixed timeline, there’s a single history that is unchangeable. Whatever you are attempting to change by time-traveling is what created the problems in the present that you’re trying to fix ( 12 Monkeys ). Or you’re just wasting your time because the events you are trying to prevent will happen anyway ( Donnie Darko ).

Dynamic Timeline: Time Travel Theory

History is fragile and even the smallest changes can have a huge impact. After traveling back in time, your actions may impact your own timeline. The result is a paradox. Your changes to the past might result in you never being born, like in Back to the Future (1985), or never traveling in time in the first place. In The Time Machine (2002), Hartdegen goes back in time to save his sweetheart Emma but can’t. Doing so would have resulted in his never developing the time machine that he used to try and save her.

One common way to explore this paradox theory is by killing your own grandfather. The grandfather paradox is when a time traveler attempts to kill their grandfather before the grandfather meets their grandmother. This prevents the time travel’s parents from being born and thus the time traveler himself from being born. But if the time traveler was never born, then the traveler would never have traveled back in time, therefore erasing his or her actions involving the death of their grandfather.

Multiverse: Time Travel Theory

Travel all over time and do whatever you want. It doesn’t matter because there are multiple universes and your actions only create new timelines. This is a common theory used by the science fiction TV series, Doctor Who . Using the multiverse theory of time travel, it’s assumed that there are multiple coexisting alternate timelines.

Therefore, when the traveler goes back in time, they end up in a new timeline where historical events can differ from the timeline they came from, but their original timeline does not cease to exist. This means the grandfather paradox can be avoided. Even if the time traveler’s grandparent is killed at a young age in the new timeline, he/she still survived to have children in the original timeline, so there is still a causal explanation for the traveler’s existence.

Time travel may actually create a new timeline that diverges from the original timeline at the moment the time traveler appears in the past, or the traveler may arrive in an already existing parallel universe. There’s just one problem… you can’t go back ( The One , 2002).

But what about…

Some may argue that people who are “trapped” in time are time travelers as well. This happens in countless time travel movies including Robin Williams ‘ character in the 1995 film Jumanji who gets trapped inside a board game. The list of “people who are cryogenically frozen and then successfully thawed out in the future” is even longer and includes Austin Powers: The Spy Who Shagged Me  (1999), Planet of the Apes (1968) and so on.

Although these characters are “moving” through time, they are doing so by pausing and then rejoining the current timeline. The lack of a time machine device disqualifies them from technically being “time travelers” and included in this list of theories on time travel.

So will time travel ever be possible? All we know for sure is that the experts don’t agree. According to the Albert Einstein theory of relativity, time is relative, not constant and the bending of spacetime could be possible. But according to  Stephen Hawking , time travel is not possible. The Stephen Hawking time travel theory suggests that the absence of present-day time travelers from the future is an argument against the existence of time travel — a variant of the Fermi paradox (aka where the hell is everybody?). But it’s fun to think about.

NERD NOTE:  What happens to time in a black hole? We don’t know for sure, but according to both Stephen Hawking and Albert Einstein’s theory, time near a black hole slows down. This is because a black hole’s gravitational pull is so strong that even light can’t escape. Since gravity also affects light, time would also slow down.

If you could successfully travel into the future, or back in time, what would you do? Warn people about natural disasters? Buy a winning lottery ticket ? Try to prevent your own death? What do you think about these time travel theory ideas or the time travel movies that we included in this article? Please tell us in the comments below.

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Frank Wilson is a retired teacher with over 30 years of combined experience in the education, small business technology, and real estate business. He now blogs as a hobby and spends most days tinkering with old computers. Wilson is passionate about tech, enjoys fishing, and loves drinking beer.

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Mar 24, 2015 at 11:24 PM

are there really only 3 theories? i feel like there are more but i cant think of any besides the movies listed here. hummmmmmmmm

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The Different Types Of Time Travel And How They Work

Have you ever watched a movie or read a book where the characters travel through time? Maybe they go back to prevent a tragedy or forward to see what the world will look like in the future. Time travel is a fascinating concept that has captured our imaginations for centuries. But have you ever wondered about the different types of time travel and how they work?

Let's take the example of Marty McFly from Back to the Future. In this classic film, Marty travels back in time and accidentally changes his parents' meeting, leading to him potentially being erased from existence. This type of time travel is known as a fixed timeline or predestination paradox, where events that occur in the past are predetermined and cannot be changed. However, there are other types of time travel that allow for altering events in history or exploring alternate timelines altogether. Join us as we dive into the different types of time travel and how they work!

Fixed Timeline or Predestination Paradox

Dynamic timeline or multiverse theory, definition and explanation, examples in popular culture, theoretical implications, wormholes and black holes, the philosophy of time travel, the nature of time, the ethics and consequences of time travel, the role of free will and determinism, frequently asked questions, is it possible to travel through time without creating a paradox, what are the ethical implications of time travel, can time travel be used to alter history, how would time travel affect the concept of free will, are there any real-life experiments or technologies that could potentially enable time travel.

The Fixed Timeline, also known as the Predestination Paradox, asserts that events in the past cannot be changed no matter what actions are taken in the present or future. This theory is based on the idea of temporal mechanics, which suggests that time is a fixed and unchangeable entity. In other words, everything that has happened in the past has already been determined and cannot be altered.

However, this does not mean that there are no consequences to our actions. Even though we cannot change the past, our present and future actions can still have an impact on alternate timelines. These alternate timelines may exist alongside our own reality and could potentially lead to different outcomes depending on the choices we make. With this understanding of temporal mechanics and alternate timelines, let's explore another type of time travel: dynamic timeline or multiverse theory.

So, let's talk about the dynamic timeline or multiverse theory. This concept suggests that when someone travels back in time and changes something, they don't actually alter their own timeline but instead create a new universe where those changes have already occurred. In simpler terms, every decision creates a new branching reality where every possibility exists simultaneously. Some examples of this can be seen in popular culture such as Marvel's "What If" series or the movie "The Butterfly Effect." The theoretical implications of this theory are mind-boggling as it suggests that there could be an infinite number of parallel universes with different versions of ourselves living out different realities.

You'll quickly understand the ins and outs of time travel once you grasp how each method takes you on a unique journey through the labyrinth of time. One such method is the dynamic timeline or multiverse theory, which posits that every action taken in the past creates an alternate universe in which those actions had different outcomes. This means that if one were to go back in time and change something, they would not be altering their own history but creating an entirely new reality altogether.

To better understand this concept, consider these emotional responses:

• Fear: The idea that any action taken in the past could potentially lead to disastrous consequences can be overwhelming.
• Fascination: The thought of multiple realities existing simultaneously can spark curiosity and wonder about what other versions of ourselves may exist out there.
• Discomfort: The realization that our actions may not have as much impact on our own lives as we once believed can be unsettling.

Examples in popular culture further illustrate this concept, from Marvel's "What If?" series to Christopher Nolan's "Interstellar." These stories showcase how even small changes made in the past can create vastly different futures, each with their own set of consequences.

Now let's explore some popular culture references that have made time travel a fascinating concept. One of the most iconic examples is the Back to the Future trilogy, where Marty McFly uses a DeLorean time machine to travel between different eras and alter his family's history. This classic film series not only introduced us to the concept of time travel but also explored the idea of changing one's future by altering events in the past.

Another example is Doctor Who, a science fiction television series that has been on air since 1963. The show follows an alien known as The Doctor who travels through time and space in a spaceship called TARDIS. Through this character, we see how different actions can have significant consequences throughout time and how even small changes can lead to drastic outcomes. These pop culture references not only entertain us but also make us question our own understanding of time travel and its implications on our lives.

As we delve further into these examples, it becomes clear that they raise important theoretical implications about the nature of time itself.

As you explore the theoretical implications of time travel, your mind begins to unravel the mysteries of the universe and you feel as though you are floating through a vortex of endless possibilities. One of the most fascinating aspects is the concept of the butterfly effect, where even small actions in the past can have major consequences on future events. This means that if someone were to go back in time and change even one minor detail, it could drastically alter the course of history as we know it.

Another significant theory is The Grandfather Paradox, which poses an interesting dilemma: if someone were to travel back in time and kill their own grandfather before they had children, would they still exist? This paradox highlights one potential consequence of time travel – that any changes made in the past have potentially irreversible effects on future events. These theories are just some examples of how complex and thought-provoking time travel can be. With such profound implications at stake, it's no wonder this topic has captivated audiences for generations.

With so many theories surrounding time travel and its potential impacts on our world and existence, it's clear that there is much more to explore. In fact, some scientists believe that certain types of time loops may actually be possible based on current research into quantum mechanics. As we delve deeper into these topics, we can only hope to uncover more about ourselves and our place within this vast universe.

So, let's talk about time loops. A time loop is when a specific event or sequence of events repeats itself over and over again in a cyclical manner. This concept has been explored in various forms of popular culture such as the movie "Groundhog Day" and the TV show "Supernatural." Not only is it fascinating to think about the possibilities and consequences of being stuck in a time loop, but it also has some profound theoretical implications for our understanding of the nature of time itself.

You probably know by now how time travel actually operates and its various forms. One of these forms is the Time Loop, which occurs when a certain event or series of events repeats itself indefinitely. This means that every action taken by an individual in the loop has already happened before and will continue to happen again and again, creating an endless cycle.

Time Loops have theoretical implications in the sense that they challenge our understanding of causality and free will. If every action we take is predetermined and destined to repeat itself, then do we truly have control over our own lives? Additionally, Time Loops have scientific applications such as studying the effects of repeated actions on physical objects or even exploring alternate timelines.

• A Time Loop can be triggered by a specific event or decision.
• The loop can be broken by making a different choice or taking a different action.
• Time Loops often involve character development as individuals must learn from their past mistakes in order to break the cycle.

Examples in popular culture range from classic films like Groundhog Day to contemporary television shows like Russian Doll. In each instance, characters are forced to confront their own limitations and weaknesses while facing seemingly insurmountable odds. However, through perseverance and self-reflection, they are able to break out of their respective loops and find redemption.

Take a look at how popular culture has tackled the concept of Time Loops, from characters repeating the same day over and over again in Groundhog Day to a woman reliving her death in Russian Doll. Time travel is not just limited to cinema and television shows, but can also be found in video games and literature. In video games, time travel often takes the form of rewinding time or jumping between different points in history. The popular game series Assassin's Creed incorporates this mechanic by allowing players to explore historical events and even alter them through their actions.

In literature, time travel has been explored for centuries with classics like H.G. Wells' The Time Machine and more recently with Audrey Niffenegger's The Time Traveler's Wife. These stories often examine the consequences of changing past events or exploring different timelines. The concept of time travel allows authors to explore philosophical questions about fate, free will, and causality. It raises questions about whether our actions have predetermined outcomes or if we can truly change our future. These theoretical implications make time travel an endlessly fascinating concept to explore across all forms of media.

Exploring the theoretical implications of time travel can lead us to question our understanding of fate and free will, as we grapple with the possibility of altering past events and shaping our own future. The concept of time travel challenges our perception of cause and effect, as we consider the potential consequences of changing even a single event in history. This raises philosophical considerations about whether or not we have control over our destiny, or if our path is predetermined.

Furthermore, time travel forces us to confront ethical dilemmas that arise from manipulating historical events for personal gain. If we are able to change the past, what responsibility do we have to ensure that those changes do not harm others? As we continue to explore the different types of time travel and their possible consequences, it becomes clear that this topic raises complex questions about human nature and morality. With these considerations in mind, let us delve into the fascinating world of wormholes and black holes.

As you approach a black hole or wormhole, you'll feel the intense gravitational pull that could potentially allow you to travel across space and time. This is due to the effects of time dilation caused by extreme gravitational forces near these objects. Time dilation is a phenomenon in which time appears to move slower for an observer who is closer to a stronger gravitational field. This means that as you get closer to a black hole or wormhole, time will appear to slow down for you compared to someone who is far away from these objects.

This effect can be harnessed for interstellar travel and time travel technology, but it comes with significant risks and challenges. The immense gravity of these objects can easily destroy any spacecraft attempting to enter them, making it difficult for us to explore their potential benefits. Additionally, there are still many unknowns about how exactly we could use wormholes and black holes for time travel, leaving this possibility largely in the realm of science fiction at this point. With all of these uncertainties surrounding the practical applications of wormholes and black holes for time travel, it's important to consider the philosophical implications behind this concept as well.

Hey, let's talk about the philosophy of time travel! It's a fascinating subject that raises some big questions about the nature of time itself. We'll explore the ethics and consequences of time travel, as well as the role of free will and determinism in shaping our understanding of this complex topic. So buckle up and get ready for a mind-bending ride through the twists and turns of temporal theory!

Time is a mysterious force that we can never truly control, but as the saying goes, 'time heals all wounds.' The subjective experience of time is something that varies greatly based on our individual perceptions. Some days seem to drag on forever while others fly by in the blink of an eye. Theories of time perception suggest that our brains may alter our sense of time based on external stimuli or internal emotions.

One sub-list suggests that external stimuli like music or movies can make us feel as though time is passing faster or slower than it actually is. Another sub-list proposes that internal emotions such as fear or excitement can also distort our sense of time, making moments seem longer or shorter than they really are. Lastly, some theories suggest that our brain's internal clock may be responsible for how we perceive the passage of time. Understanding these various theories about the nature of time helps us appreciate just how complex and mysterious this concept truly is.

As we delve into the ethics and consequences of time travel, we must consider how actions in one moment can ripple throughout history and change everything that comes after.

You're about to explore the dark and unpredictable consequences of messing with the fabric of reality, and it's going to make your heart race with both fear and excitement. Time travel is an exciting concept, but it comes with a serious set of ethical implications that cannot be ignored. Imagine traveling back in time to prevent a tragedy from happening, only to realize that by doing so, you've inadvertently caused another one. This is known as the butterfly effect - the idea that even the smallest change in the past can have significant repercussions in the present.

The ethical implications of time travel are not limited to accidental outcomes like this either. What if someone were to go back in time and kill Hitler before he rose to power? Would they be justified in doing so? Or would they be altering history in such a way that it ultimately leads to a worse outcome? These are difficult questions without easy answers, and they highlight just how complex time travel can be. With so much at stake, it's no wonder that people are both fascinated by and afraid of this concept.

As we delve deeper into this topic, we will explore another crucial aspect of time travel: its relationship with free will and determinism.

Now, imagine you could go back in time and change a decision you regret; would the outcome still be predetermined or does your free will play a role in altering it? This question brings up the long-standing philosophical debate of determinism vs free will. Determinism is the belief that all events are predetermined and inevitable, while free will asserts that humans have the ability to make choices independent of external factors. Time travel adds another layer to this already complex issue as altering past events can create paradoxes and affect causality.

To understand the role of free will and determinism in time travel, we must first consider the paradoxes that arise when attempting to change past events. The grandfather paradox is one such example where traveling back in time and killing your own grandfather before he has children would mean you were never born, making it impossible for you to travel back in time to commit the act. This paradox highlights how changing past events can lead to contradictions and inconsistencies. Additionally, if we assume that all events are predetermined, then any attempt at altering them through time travel would ultimately fail because those events were always meant to occur. However, if we believe in free will, then it's possible that our actions could alter future outcomes despite their predetermined nature. Ultimately, whether determinism or free will reigns supreme depends on your personal beliefs about fate and choice.

As much as we would love to travel through time without causing any paradoxes, it seems like a tricky business. The idea of alternate timelines comes into play when considering the possibility of avoiding the Grandfather paradox, where traveling back in time and altering something could prevent your own existence. However, even with alternate timelines, there is still the risk of creating new paradoxes and complications that could have unforeseen consequences. While it's fun to imagine the possibilities of time travel, it's important to consider the potential ramifications and embrace the present moment.

When it comes to time travel, there are a lot of ethical considerations to take into account. For starters, what impact will our actions have on the course of history? Will we be altering the past in ways that could negatively affect the future? Additionally, how will our presence in different cultures and time periods impact those around us? It's important to approach time travel with sensitivity and respect for the people and places we encounter. Furthermore, we must consider the cultural impact of introducing modern ideas and technologies into ancient societies. While time travel may seem like an exciting adventure, it's crucial to think about the potential consequences of our actions before jumping headfirst into such an endeavor.

Alternate realities and butterfly effects are two concepts that come to mind when considering the possibility of altering history through time travel. The very idea of changing something in the past can lead to a sense of excitement and curiosity, but it also raises many ethical questions. What if altering one event leads to unintended consequences in the future? Would we be willing to take responsibility for those outcomes? It's easy to get lost in the allure of changing history, but we must remember that every action has a reaction, and even the smallest alteration could have drastic effects on our present-day reality. As intriguing as the idea may be, we must approach it with caution and consider all possible outcomes before making any decisions about altering history.

The philosophical debate surrounding time travel centers on the question of whether or not it would affect the concept of free will. Some argue that if time travel were possible, our past actions would be predetermined and therefore we wouldn't truly have agency over our choices. However, others believe that even with knowledge of the future, individuals would still have the ability to make their own decisions. While there is no scientific evidence to support either side of this argument, it remains a fascinating topic for discussion and speculation.

When it comes to time travel, there are several theories and experiments that have been explored by physicists. Two of the most popular ones are Quantum Entanglement and Wormhole Theories. Quantum Entanglement suggests that two particles can be connected in a way that their states remain correlated, regardless of distance or time. This means that manipulating one particle could potentially affect its entangled counterpart, even if it's light-years away in space or years into the future or past. Wormhole Theories propose the existence of shortcuts through space-time via hypothetical tunnels called wormholes. If these tunnels could be utilized for travel, they could potentially allow us to move through time as well. While these theories have yet to be proven experimentally, the potential for them to enable time travel is certainly exciting and worth continued exploration.

So there you have it - the different types of time travel and how they work. From fixed timelines to dynamic ones, from time loops to wormholes and black holes, each theory offers a unique perspective on the possibility of traveling through time.

But no matter which theory you subscribe to, one thing is certain: time travel remains a fascinating topic that captures our imaginations and challenges our understanding of the universe. As the philosopher Heraclitus once said, "no man ever steps in the same river twice," reminding us that time is constantly moving forward and changing. Whether we will ever be able to manipulate it for our own purposes remains to be seen, but one thing is for sure - we'll keep dreaming about it as long as we live.

The Three Major Theories of Time Travel

Above we have a rather interesting (and thankfully rather short) infographic from Harrison Desnmore about pretty much all the kinds of time travels you see in movies. Did you time traveling actions make history the way it’s always been? Did they screw up time and reality itself? Or did you create an entirely new universe based on your f-ing around in the past?

The middle one is what’s known as a “plot hole” in most instances, but the other two could fall under that category too if you expand on them enough. I’m of the firm belief that all time travel movies ultimately never make sense once you start trying to extrapolate them to their logical conclusion. Still, this is all enough to make your brain hurt for a while.

Not pictured: Primer, which is it’s own category that can’t even be explained by human words.

I think I'm a part of the first generation of journalists to skip print media entirely, and I've learned a lot these last few years at Forbes. My work has appeared on TVOvermind, IGN, and most importantly, a segment on The Colbert Report at one point.

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10 Comments

I have always been a fan of the multiverse concept. It has always made the most sense to me.

All time travel talk is incomplete unless Dr. Who is taken into account.

Dynamic Lazy Timeline Time travelers from the future affected by present events will be altered in the easiest way possible for the universe. If you go back in time and kill your grandfather, you will cease to exist from that moment forward. The new timeline will see that you never existed to travel back in time, but it will see that you arrived before the murder and the actions leading up to that point.

As seen in Looper. This style greatly reminds me of lightning. It takes the path of least resistance.

My understanding is that Primer follows Multiverse. The infograph Paul pointed to for Primer explained that as you left a timeline, that timeline no longer had you in it. When you witness yourself going into a box, they were leaving for the next timeline while you gained the opportunity to take their place in this timeline.

Primer infograph: https://unrealitymag.com/index.php/2011/09/30/at-last-a-definitive-timeline-for-primer/

Misfits also have fixed timeline, different timetraveling abilities work differently so it really depends on the timetravele.

One of the things that I found weird about this year’s MIB3 is(spoilers) That Will Smith travels to the past from an altered time to correct it. He prevents another character’s death at the hands of a time traveler, restoring the timeline.

But he also travels back a few minutes within the past to more effectively defeat his enemy after being wounded. But when he travels back, he doesn’t find himself standing there, planning to travel in time. If he went back a few minute to a timeline which exists because of his own actions, shouldn’t he bump into his past, time traveled self?

But I’m thinking way too much about this, aren’t I?

Something time travel movies don’t take into account – you don’t travel just back in time, but also move distance.

The earth is moving around the sun, and the earth is also rotating (Not exactly 24 hours for one rotation).

Doctor Who can do it because the TARDIS travels in time and space.

Physic nerd alert!

I’ve always considered the fixed timeline as the only possible structure for time in the universe. Consider that you travel to the future. From that perspective our present is in the past and a matter of record. Our actions in the present do not result in constantly changing newspaper headlines to the future based time traveler any more than our newspaper heading are not in flux from events happening (in a simultaneous) yesterday.

Other multiverse versions of our universe may or may not exist, but they must surely exist separately from our own even if the split point hasn’t occurred yet, because if a universe is created for every difference that occurs then what’s the cut off point? Human decisions? The change in the charge of an electron? Do these new time lines then split? and split again? It would be less a split and more of an exponential explosion. Where would the energy come from for that?

Nicholas: The Time Machine (2002) shows the machine physically staying where it was as time rapidly moved past. Looper suggest that it’s machine is able to place a person in different locations, though they don’t show that the future is actively calculating where the destination is. I do agree that your point is highly valid.

Monstrinho: I’ve tried to figure out potential plots for a time movie where the time line is a matter of record in that people, even those from alternate times or those holding newspapers or the like from alternate times, but without some being able to at some point recognize changes a story has characters that have bouncing motivations and goals that likely leave the audience confused.

Timelines that lend themselves well to theory don’t always lend themselves to Hollywood.

Going with the fixed timeline direction with the multiverse splitting, perhaps all of the decisions that are ever made (where ever their split point is at) already exist in the first place, with all lines being identical at the beginning, then half of them become the left side of the first ‘split’ and the other half the right. No need for energy creation among these already existing parallels.

The problem with the Dynamic Timeline model is that it assumes time travel is possible and history can be changed, but it still insists that there’s only one timelike dimension.

While a single timelike dimension works fine for the Fixed Timeline model, the Dynamic Timeline model requires a minimum of *two* timelike dimensions. One of the timelike dimensions corresponds to how events play out within the timeline, and the other timelike dimension corresponds to the timeline being changed by time travelers. There’s also an implication that if time travel is possible in the first place, then it probably occurs naturally at the quantum level, not just when a living creature invents a time machine. Random quantum events could be changing history underneath us all the time.

“Not pictured: Primer, which is it’s own category that can’t even be explained by human words.” – Loved this line; because it is so true.

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Inconceivable Paradoxes: 3 Theories of Time Travel

Time travel is one of those things that seems like it should be possible. After all, it’s been a plot device in countless works of fiction and pop culture . But serious physicists have never been able to come up with a way to do it. These theories explain why we’re not zipping around through time as much as we’d like to—and why you shouldn’t get your hopes up about ever going on a Star Trek-style adventure through history.

The Fixed Timeline

The Fixed Timeline is the theory of time travel where you can only travel to a point in your own future. It’s like time is like a train track and everything that happens has always happened and will always happen. The train cannot go backwards, it cannot go off the track and if you’re on the train you can only move forward with it. This means that if we were to travel back in time then our actions would already have been done before they could even happen in front of us again! This theory also suggests that there are parallel universes out there where every single event that ever happened or will ever happen has already happened somewhere else (and probably more than once).

The Dynamic Timeline

The Dynamic Timeline is a theory that says that the timeline is dynamic and can be changed. The theory is that the timeline is not fixed and can be changed by time travelers or other events in one’s life. This means that it’s possible for you to change your past, present and future if you travel back in time like in Back to the Future or 12 Monkeys.

The Multiverse

The multiverse theory , also known as the many-worlds interpretation (MWI), is a theory that states that our universe is just one of many universes. In this model, all possible alternative histories and futures actually exist; it’s just that we can’t see most of them. The MWI suggests that time travel is possible because we are constantly traveling through time—just in different directions than we normally experience in our everyday lives. The MWI has its roots in quantum mechanics, where it was first developed by Hugh Everett III during his PhD work at Princeton University. Basically, according to this theory you could go back in time if there were infinite universes: You’d simply hop across one of these other universes—and therefore a different timeline—when you tried to change something about your life or world line. It sounds confusing but it makes sense when explained more clearly:

Time travel is possible . You’ve probably heard of three different theories about time travel: the fixed timeline, dynamic timeline and multiverse. The first theory is that time travel isn’t possible. The second theory states that it is possible, but only in one direction (from future to past). The third says you can go back and forth between past and future in both directions. In the end, I think we’re going to have to accept that time travel is possible in some form. Whether it’s in the future or right now, the question isn’t whether there are people who can get around through time. It’s how they do it and why we haven’t seen them yet. They may have been there all along without us noticing—or maybe they’re still working on figuring out their own theories about what time really means in order for us humans (and our brains) to understand!

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In This Article Expand or collapse the "in this article" section Time Travel

Introduction, general overviews.

• David Lewis’s Analysis, Its Forerunners and Critics
• Gödel and the Ideality of Time
• Models and Issues from Relativity
• Models and Issues from Quantum Theory
• Causal Loops and Probability
• Time Travel in Many Worlds and the Autonomy Principle
• Travel in Dynamic Time and Multi-Dimensional Time
• General Metaphysical Issues

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Time Travel by Alasdair Richmond LAST REVIEWED: 28 May 2019 LAST MODIFIED: 26 October 2015 DOI: 10.1093/obo/9780195396577-0295

Time travel is a philosophical growth industry, with many issues in metaphysics and elsewhere recently transformed by consideration of time travel possibilities. The debate has gradually shifted from focusing on time travel’s logical possibility (which possibility is now generally although not universally granted) to sundry topics including persistence, causation, personal identity, freedom, composition, and natural laws, to name but a few. Besides metaphysical discussions, some time travel works draw on the philosophies of science, spacetime, and computation. Some interesting forerunners notwithstanding, serious physical interest in time travel begins with Gödel’s 1949a demonstration that general relativity permits space-times that are riddled with closed timelike curves (“CTCs” henceforth). A key philosophical text on time travel is Lewis 1976 and its argument for the logical possibility of certain backward time travel journeys and even for the possibility of casual loops. Lewis concludes that time travel could occur in a possible world, albeit perhaps a strange world that would feature (or seem to feature) strange restrictions on actions. In Lewis’s analysis, a traveler can arrive in the past of the same history they come from provided that the traveler’s actions on arrival are consistent with the history that they come from. So other worlds or multiple temporal dimensions are not necessary to make time travel consistent. Granted, the physics, persistence conditions, agency, and epistemology of agents in such worlds might look weird indeed. Since Lewis, philosophical time travel questions include the following: given that a traveler into the past cannot create any paradoxical outcomes on arrival, what then would stay their hand? Are the constraints on a traveler’s actions admissible within our ordinary understanding of physical law or human agency? Is time travel compatible with dynamic time or even with the existence of time itself? Can backward time travel be physically possible within a single history? If a time traveler meets another stage of him- or herself, is the traveler in two places at once, and what theory of persistence can cope with this puzzling multiplication? Can time-travel spacetimes resolve otherwise intractable computational problems?

Despite several hundred philosophical and scientific articles, book chapters, and Internet resources devoted to philosophical problems posed by time travel, there is currently no full-length monograph or anthology on the subject. The best introduction to the topic in general so far is chapter 8 of Dainton (second edition 2010), Dainton 2010 being the best general philosophical resource available on time and space. The key work is Lewis 1976 , a defense of the logical possibility of backward time travel, from which a large number of subsequent treatments take their cue. A useful overview, albeit largely from a physical science perspective, is Nahin 1999 . Also largely physical in emphasis but comprehensive and thorough is Earman 1995 . Richmond 2003 surveys philosophical work on time travel to date. Arntzenius 2006 details the problems of free action and nomological constraint posed by backward time travel. Arntzenius and Maudlin 2005 is helpful on (especially) problems of physical law. Carroll 2008 is perhaps the best single online resource available on any aspect of time travel. Le Poidevin 2003 is a highly commendable introduction to the philosophy of time in general but especially good on problems of time travel. Bourne 2006 offers some useful arguments and clarifications centered on Gödel’s arguments about time travel and the relations between time travel and the status of times themselves. Earman and Wüthrich 2006 offers scientifically well informed but approachable and philosophically cogent discussions of what physics might, and might not, allow by way of time travel.

Arntzenius, Frank. “Time Travel: Double Your Fun.” Philosophy Compass 6 (2006): 599–616.

DOI: 10.1111/j.1747-9991.2006.00045.x

Entertaining survey of the philosophical terrain around time travel that concentrates particularly on the constraints on action likely to be suffered by travelers in the past. An excellent introduction to the nomological contrivance problem and more. Available online for purchase or by subscription.

Arntzenius, Frank, and Tim Maudlin. “ Time Travel and Modern Physics .” In Stanford Encyclopedia of Philosophy . Edited by Edward N. Zalta. 2005.

Notably acute survey of physical possibilities for time travel, including detailed arguments that backward time travel threatens to create correlations that conflict with standard quantum predictions.

Bourne, C. A Future for Presentism . Oxford: Oxford University Press, 2006.

DOI: 10.1093/acprof:oso/9780199212804.001.0001

Although primarily devoted to defending presentism, chapter 8 offers one of the best treatments of Gödel’s ideality argument around and pp. 132–134 offer some interesting sidelights on the possible compatability of time travel and presentism.

Carroll, John W. A Time Travel Website . 2008–.

Extremely thorough, engagingly-written, well-designed, and continually evolving online resource that offers helpful discussions, well-chosen readings, and helpful animations to boot.

Dainton, Barry. Time and Space . 2d ed. Durham, NC: Acumen, 2010.

Revised and expanded edition of Dainton’s classic 2001 introduction to the philosophy of space and time. Can be highly recommended but notable here for its extensive, essential treatments of time travel, relativity, and Gödel’s “ideality” argument.

Earman, John. “Recent Work on Time Travel.” In Time’s Arrows Today . Edited by Steven F. Savitt, 268–310. Cambridge, UK: Cambridge University Press, 1995.

DOI: 10.1017/CBO9780511622861

Thorough discussion of the then-current state of play in the philosophical and physical literature on time travel. This is still a valuable resource.

Earman, John, and Christian Wüthrich. “ Time Machines .” In Stanford Encyclopedia of Philosophy . Edited by Edward N. Zalta. 2006.

Comprehensive discussion of physical resources for time travel, among other intriguing suggestions, develops the view that physically realistic time machines might be uncontrollable even if they become a possiblility.

Le Poidevin, Robin. Travels in Four Dimensions: The Enigmas of Space and Time . Oxford: Oxford University Press, 2003.

Engaging and clearly written introduction to the philosophy of space and time. Often offers problems and discussions that lend themselves to time travel interpretation. An excellent introductory and pedagogical resource.

Lewis, David. “ The Paradoxes of Time Travel .” American Philosophical Quarterly 13 (1976): 145–152.

The philosophical time travel work. Includes Lewis’s discrepancy definition of time travel: the most useful by far. Invokes the notion of compossibility to disambiguate “Grandfather paradox” arguments and argues that backward time travel and causal loops can occur in (nonbranching) possible worlds. Usefully distinguishes between replacement change and counterfactual change. (This is often cited and sometimes rebutted but never refuted.)

Nahin, Paul. Time Machines: Time Travel in Physics, Metaphysics and Science Fiction . 1st ed. New York: American Institute of Physics, 1999.

DOI: 10.1007/978-1-4757-3088-3

Engaging and comprehensive attempt at surveying all the scientific, philosophical, and fictional literature on time travel. Perhaps slightly more at ease with physics and fiction than with philosophy, but this is a detailed and thorough treatment.

Richmond, Alasdair. “Recent Work: Time Travel.” Philosophical Books 44 (2003): 297–309.

DOI: 10.1111/1468-0149.00308

Survey of the time travel debate from Lewis 1976 onward, sketching links with debates in persistence, philosophy of spacetime and temporal topology. Available online by subscription.

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Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered. Timothy A. Clary/AFP via Getty Images hide caption

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered.

"The past is obdurate," Stephen King wrote in his book about a man who goes back in time to prevent the Kennedy assassination. "It doesn't want to be changed."

Turns out, King might have been on to something.

Countless science fiction tales have explored the paradox of what would happen if you went back in time and did something in the past that endangered the future. Perhaps one of the most famous pop culture examples is in Back to the Future , when Marty McFly goes back in time and accidentally stops his parents from meeting, putting his own existence in jeopardy.

But maybe McFly wasn't in much danger after all. According a new paper from researchers at the University of Queensland, even if time travel were possible, the paradox couldn't actually exist.

Researchers ran the numbers and determined that even if you made a change in the past, the timeline would essentially self-correct, ensuring that whatever happened to send you back in time would still happen.

"Say you traveled in time in an attempt to stop COVID-19's patient zero from being exposed to the virus," University of Queensland scientist Fabio Costa told the university's news service .

"However, if you stopped that individual from becoming infected, that would eliminate the motivation for you to go back and stop the pandemic in the first place," said Costa, who co-authored the paper with honors undergraduate student Germain Tobar.

"This is a paradox — an inconsistency that often leads people to think that time travel cannot occur in our universe."

A variation is known as the "grandfather paradox" — in which a time traveler kills their own grandfather, in the process preventing the time traveler's birth.

The logical paradox has given researchers a headache, in part because according to Einstein's theory of general relativity, "closed timelike curves" are possible, theoretically allowing an observer to travel back in time and interact with their past self — potentially endangering their own existence.

But these researchers say that such a paradox wouldn't necessarily exist, because events would adjust themselves.

Take the coronavirus patient zero example. "You might try and stop patient zero from becoming infected, but in doing so, you would catch the virus and become patient zero, or someone else would," Tobar told the university's news service.

In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time.

"No matter what you did, the salient events would just recalibrate around you," Tobar said.

The paper, "Reversible dynamics with closed time-like curves and freedom of choice," was published last week in the peer-reviewed journal Classical and Quantum Gravity . The findings seem consistent with another time travel study published this summer in the peer-reviewed journal Physical Review Letters. That study found that changes made in the past won't drastically alter the future.

Bestselling science fiction author Blake Crouch, who has written extensively about time travel, said the new study seems to support what certain time travel tropes have posited all along.

"The universe is deterministic and attempts to alter Past Event X are destined to be the forces which bring Past Event X into being," Crouch told NPR via email. "So the future can affect the past. Or maybe time is just an illusion. But I guess it's cool that the math checks out."

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Can we time travel? A theoretical physicist provides some answers

Emeritus professor, Physics, Carleton University

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Peter Watson received funding from NSERC. He is affiliated with Carleton University and a member of the Canadian Association of Physicists.

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Time travel makes regular appearances in popular culture, with innumerable time travel storylines in movies, television and literature. But it is a surprisingly old idea: one can argue that the Greek tragedy Oedipus Rex , written by Sophocles over 2,500 years ago, is the first time travel story .

But is time travel in fact possible? Given the popularity of the concept, this is a legitimate question. As a theoretical physicist, I find that there are several possible answers to this question, not all of which are contradictory.

The simplest answer is that time travel cannot be possible because if it was, we would already be doing it. One can argue that it is forbidden by the laws of physics, like the second law of thermodynamics or relativity . There are also technical challenges: it might be possible but would involve vast amounts of energy.

There is also the matter of time-travel paradoxes; we can — hypothetically — resolve these if free will is an illusion, if many worlds exist or if the past can only be witnessed but not experienced. Perhaps time travel is impossible simply because time must flow in a linear manner and we have no control over it, or perhaps time is an illusion and time travel is irrelevant.

Laws of physics

Since Albert Einstein’s theory of relativity — which describes the nature of time, space and gravity — is our most profound theory of time, we would like to think that time travel is forbidden by relativity. Unfortunately, one of his colleagues from the Institute for Advanced Study, Kurt Gödel, invented a universe in which time travel was not just possible, but the past and future were inextricably tangled.

We can actually design time machines , but most of these (in principle) successful proposals require negative energy , or negative mass, which does not seem to exist in our universe. If you drop a tennis ball of negative mass, it will fall upwards. This argument is rather unsatisfactory, since it explains why we cannot time travel in practice only by involving another idea — that of negative energy or mass — that we do not really understand.

Mathematical physicist Frank Tipler conceptualized a time machine that does not involve negative mass, but requires more energy than exists in the universe .

Time travel also violates the second law of thermodynamics , which states that entropy or randomness must always increase. Time can only move in one direction — in other words, you cannot unscramble an egg. More specifically, by travelling into the past we are going from now (a high entropy state) into the past, which must have lower entropy.

This argument originated with the English cosmologist Arthur Eddington , and is at best incomplete. Perhaps it stops you travelling into the past, but it says nothing about time travel into the future. In practice, it is just as hard for me to travel to next Thursday as it is to travel to last Thursday.

Resolving paradoxes

There is no doubt that if we could time travel freely, we run into the paradoxes. The best known is the “ grandfather paradox ”: one could hypothetically use a time machine to travel to the past and murder their grandfather before their father’s conception, thereby eliminating the possibility of their own birth. Logically, you cannot both exist and not exist.

Read more: Time travel could be possible, but only with parallel timelines

Kurt Vonnegut’s anti-war novel Slaughterhouse-Five , published in 1969, describes how to evade the grandfather paradox. If free will simply does not exist, it is not possible to kill one’s grandfather in the past, since he was not killed in the past. The novel’s protagonist, Billy Pilgrim, can only travel to other points on his world line (the timeline he exists in), but not to any other point in space-time, so he could not even contemplate killing his grandfather.

The universe in Slaughterhouse-Five is consistent with everything we know. The second law of thermodynamics works perfectly well within it and there is no conflict with relativity. But it is inconsistent with some things we believe in, like free will — you can observe the past, like watching a movie, but you cannot interfere with the actions of people in it.

Could we allow for actual modifications of the past, so that we could go back and murder our grandfather — or Hitler ? There are several multiverse theories that suppose that there are many timelines for different universes. This is also an old idea: in Charles Dickens’ A Christmas Carol , Ebeneezer Scrooge experiences two alternative timelines, one of which leads to a shameful death and the other to happiness.

Time is a river

Roman emperor Marcus Aurelius wrote that:

“ Time is like a river made up of the events which happen , and a violent stream; for as soon as a thing has been seen, it is carried away, and another comes in its place, and this will be carried away too.”

We can imagine that time does flow past every point in the universe, like a river around a rock. But it is difficult to make the idea precise. A flow is a rate of change — the flow of a river is the amount of water that passes a specific length in a given time. Hence if time is a flow, it is at the rate of one second per second, which is not a very useful insight.

Theoretical physicist Stephen Hawking suggested that a “ chronology protection conjecture ” must exist, an as-yet-unknown physical principle that forbids time travel. Hawking’s concept originates from the idea that we cannot know what goes on inside a black hole, because we cannot get information out of it. But this argument is redundant: we cannot time travel because we cannot time travel!

Researchers are investigating a more fundamental theory, where time and space “emerge” from something else. This is referred to as quantum gravity , but unfortunately it does not exist yet.

So is time travel possible? Probably not, but we don’t know for sure!

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6 Is Time Travel Possible?

• Published: July 2013
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The static theory of time treats temporal location a lot like spatial location. In accordance with contemporary physics, spacetime is treated in this theory as an unchanging four-dimensional block. Each temporal slice of the continuum is just as real as any other, just as any part of space is just as real as any other. Some might suggest that this means that travel to other parts of time should be possible, at least in theory, just as travel to other parts of space is possible. Thus an investigation of the possibility of time travel is at the same time an investigation of the implications of the static theory of time .

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Time Travel

There is an extensive literature on time travel in both philosophy and physics. Part of the great interest of the topic stems from the fact that reasons have been given both for thinking that time travel is physically possible—and for thinking that it is logically impossible! This entry deals primarily with philosophical issues; issues related to the physics of time travel are covered in the separate entries on time travel and modern physics and time machines . We begin with the definitional question: what is time travel? We then turn to the major objection to the possibility of backwards time travel: the Grandfather paradox. Next, issues concerning causation are discussed—and then, issues in the metaphysics of time and change. We end with a discussion of the question why, if backwards time travel will ever occur, we have not been visited by time travellers from the future.

1.1 Time Discrepancy

1.2 changing the past, 2.1 can and cannot, 2.2 improbable coincidences, 2.3 inexplicable occurrences, 3.1 backwards causation, 3.2 causal loops, 4.1 time travel and time, 4.2 time travel and change, 5. where are the time travellers, other internet resources, related entries, 1. what is time travel.

There is a number of rather different scenarios which would seem, intuitively, to count as ‘time travel’—and a number of scenarios which, while sharing certain features with some of the time travel cases, seem nevertheless not to count as genuine time travel: [ 1 ]

Time travel Doctor . Doctor Who steps into a machine in 2024. Observers outside the machine see it disappear. Inside the machine, time seems to Doctor Who to pass for ten minutes. Observers in 1984 (or 3072) see the machine appear out of nowhere. Doctor Who steps out. [ 2 ] Leap . The time traveller takes hold of a special device (or steps into a machine) and suddenly disappears; she appears at an earlier (or later) time. Unlike in Doctor , the time traveller experiences no lapse of time between her departure and arrival: from her point of view, she instantaneously appears at the destination time. [ 3 ] Putnam . Oscar Smith steps into a machine in 2024. From his point of view, things proceed much as in Doctor : time seems to Oscar Smith to pass for a while; then he steps out in 1984. For observers outside the machine, things proceed differently. Observers of Oscar’s arrival in the past see a time machine suddenly appear out of nowhere and immediately divide into two copies of itself: Oscar Smith steps out of one; and (through the window) they see inside the other something that looks just like what they would see if a film of Oscar Smith were played backwards (his hair gets shorter; food comes out of his mouth and goes back into his lunch box in a pristine, uneaten state; etc.). Observers of Oscar’s departure from the future do not simply see his time machine disappear after he gets into it: they see it collide with the apparently backwards-running machine just described, in such a way that both are simultaneously annihilated. [ 4 ] Gödel . The time traveller steps into an ordinary rocket ship (not a special time machine) and flies off on a certain course. At no point does she disappear (as in Leap ) or ‘turn back in time’ (as in Putnam )—yet thanks to the overall structure of spacetime (as conceived in the General Theory of Relativity), the traveller arrives at a point in the past (or future) of her departure. (Compare the way in which someone can travel continuously westwards, and arrive to the east of her departure point, thanks to the overall curved structure of the surface of the earth.) [ 5 ] Einstein . The time traveller steps into an ordinary rocket ship and flies off at high speed on a round trip. When he returns to Earth, thanks to certain effects predicted by the Special Theory of Relativity, only a very small amount of time has elapsed for him—he has aged only a few months—while a great deal of time has passed on Earth: it is now hundreds of years in the future of his time of departure. [ 6 ] Not time travel Sleep . One is very tired, and falls into a deep sleep. When one awakes twelve hours later, it seems from one’s own point of view that hardly any time has passed. Coma . One is in a coma for a number of years and then awakes, at which point it seems from one’s own point of view that hardly any time has passed. Cryogenics . One is cryogenically frozen for hundreds of years. Upon being woken, it seems from one’s own point of view that hardly any time has passed. Virtual . One enters a highly realistic, interactive virtual reality simulator in which some past era has been recreated down to the finest detail. Crystal . One looks into a crystal ball and sees what happened at some past time, or will happen at some future time. (Imagine that the crystal ball really works—like a closed-circuit security monitor, except that the vision genuinely comes from some past or future time. Even so, the person looking at the crystal ball is not thereby a time traveller.) Waiting . One enters one’s closet and stays there for seven hours. When one emerges, one has ‘arrived’ seven hours in the future of one’s ‘departure’. Dateline . One departs at 8pm on Monday, flies for fourteen hours, and arrives at 10pm on Monday.

A satisfactory definition of time travel would, at least, need to classify the cases in the right way. There might be some surprises—perhaps, on the best definition of ‘time travel’, Cryogenics turns out to be time travel after all—but it should certainly be the case, for example, that Gödel counts as time travel and that Sleep and Waiting do not. [ 7 ]

In fact there is no entirely satisfactory definition of ‘time travel’ in the literature. The most popular definition is the one given by Lewis (1976, 145–6):

What is time travel? Inevitably, it involves a discrepancy between time and time. Any traveller departs and then arrives at his destination; the time elapsed from departure to arrival…is the duration of the journey. But if he is a time traveller, the separation in time between departure and arrival does not equal the duration of his journey.…How can it be that the same two events, his departure and his arrival, are separated by two unequal amounts of time?…I reply by distinguishing time itself, external time as I shall also call it, from the personal time of a particular time traveller: roughly, that which is measured by his wristwatch. His journey takes an hour of his personal time, let us say…But the arrival is more than an hour after the departure in external time, if he travels toward the future; or the arrival is before the departure in external time…if he travels toward the past.

This correctly excludes Waiting —where the length of the ‘journey’ precisely matches the separation between ‘arrival’ and ‘departure’—and Crystal , where there is no journey at all—and it includes Doctor . It has trouble with Gödel , however—because when the overall structure of spacetime is as twisted as it is in the sort of case Gödel imagined, the notion of external time (“time itself”) loses its grip.

Another definition of time travel that one sometimes encounters in the literature (Arntzenius, 2006, 602) (Smeenk and Wüthrich, 2011, 5, 26) equates time travel with the existence of CTC’s: closed timelike curves. A curve in this context is a line in spacetime; it is timelike if it could represent the career of a material object; and it is closed if it returns to its starting point (i.e. in spacetime—not merely in space). This now includes Gödel —but it excludes Einstein .

The lack of an adequate definition of ‘time travel’ does not matter for our purposes here. [ 8 ] It suffices that we have clear cases of (what would count as) time travel—and that these cases give rise to all the problems that we shall wish to discuss.

Some authors (in philosophy, physics and science fiction) consider ‘time travel’ scenarios in which there are two temporal dimensions (e.g. Meiland (1974)), and others consider scenarios in which there are multiple ‘parallel’ universes—each one with its own four-dimensional spacetime (e.g. Deutsch and Lockwood (1994)). There is a question whether travelling to another version of 2001 (i.e. not the very same version one experienced in the past)—a version at a different point on the second time dimension, or in a different parallel universe—is really time travel, or whether it is more akin to Virtual . In any case, this kind of scenario does not give rise to many of the problems thrown up by the idea of travelling to the very same past one experienced in one’s younger days. It is these problems that form the primary focus of the present entry, and so we shall not have much to say about other kinds of ‘time travel’ scenario in what follows.

One objection to the possibility of time travel flows directly from attempts to define it in anything like Lewis’s way. The worry is that because time travel involves “a discrepancy between time and time”, time travel scenarios are simply incoherent. The time traveller traverses thirty years in one year; she is 51 years old 21 years after her birth; she dies at the age of 100, 200 years before her birth; and so on. The objection is that these are straightforward contradictions: the basic description of what time travel involves is inconsistent; therefore time travel is logically impossible. [ 9 ]

There must be something wrong with this objection, because it would show Einstein to be logically impossible—whereas this sort of future-directed time travel has actually been observed (albeit on a much smaller scale—but that does not affect the present point) (Hafele and Keating, 1972b,a). The most common response to the objection is that there is no contradiction because the interval of time traversed by the time traveller and the duration of her journey are measured with respect to different frames of reference: there is thus no reason why they should coincide. A similar point applies to the discrepancy between the time elapsed since the time traveller’s birth and her age upon arrival. There is no more of a contradiction here than in the fact that Melbourne is both 800 kilometres away from Sydney—along the main highway—and 1200 kilometres away—along the coast road. [ 10 ]

Before leaving the question ‘What is time travel?’ we should note the crucial distinction between changing the past and participating in (aka affecting or influencing) the past. [ 11 ] In the popular imagination, backwards time travel would allow one to change the past: to right the wrongs of history, to prevent one’s younger self doing things one later regretted, and so on. In a model with a single past, however, this idea is incoherent: the very description of the case involves a contradiction (e.g. the time traveller burns all her diaries at midnight on her fortieth birthday in 1976, and does not burn all her diaries at midnight on her fortieth birthday in 1976). It is not as if there are two versions of the past: the original one, without the time traveller present, and then a second version, with the time traveller playing a role. There is just one past—and two perspectives on it: the perspective of the younger self, and the perspective of the older time travelling self. If these perspectives are inconsistent (e.g. an event occurs in one but not the other) then the time travel scenario is incoherent.

This means that time travellers can do less than we might have hoped: they cannot right the wrongs of history; they cannot even stir a speck of dust on a certain day in the past if, on that day, the speck was in fact unmoved. But this does not mean that time travellers must be entirely powerless in the past: while they cannot do anything that did not actually happen, they can (in principle) do anything that did happen. Time travellers cannot change the past: they cannot make it different from the way it was—but they can participate in it: they can be amongst the people who did make the past the way it was. [ 12 ]

What about models involving two temporal dimensions, or parallel universes—do they allow for coherent scenarios in which the past is changed? [ 13 ] There is certainly no contradiction in saying that the time traveller burns all her diaries at midnight on her fortieth birthday in 1976 in universe 1 (or at hypertime A ), and does not burn all her diaries at midnight on her fortieth birthday in 1976 in universe 2 (or at hypertime B ). The question is whether this kind of story involves changing the past in the sense originally envisaged: righting the wrongs of history, preventing subsequently regretted actions, and so on. Goddu (2003) and van Inwagen (2010) argue that it does (in the context of particular hypertime models), while Smith (1997, 365–6; 2015) argues that it does not: that it involves avoiding the past—leaving it untouched while travelling to a different version of the past in which things proceed differently.

2. The Grandfather Paradox

The most important objection to the logical possibility of backwards time travel is the so-called Grandfather paradox. This paradox has actually convinced many people that backwards time travel is impossible:

The dead giveaway that true time-travel is flatly impossible arises from the well-known “paradoxes” it entails. The classic example is “What if you go back into the past and kill your grandfather when he was still a little boy?”…So complex and hopeless are the paradoxes…that the easiest way out of the irrational chaos that results is to suppose that true time-travel is, and forever will be, impossible. (Asimov 1995 [2003, 276–7]) travel into one’s past…would seem to give rise to all sorts of logical problems, if you were able to change history. For example, what would happen if you killed your parents before you were born. It might be that one could avoid such paradoxes by some modification of the concept of free will. But this will not be necessary if what I call the chronology protection conjecture is correct: The laws of physics prevent closed timelike curves from appearing . (Hawking, 1992, 604) [ 14 ]

The paradox comes in different forms. Here’s one version:

If time travel was logically possible then the time traveller could return to the past and in a suicidal rage destroy his time machine before it was completed and murder his younger self. But if this was so a necessary condition for the time trip to have occurred at all is removed, and we should then conclude that the time trip did not occur. Hence if the time trip did occur, then it did not occur. Hence it did not occur, and it is necessary that it did not occur. To reply, as it is standardly done, that our time traveller cannot change the past in this way, is a petitio principii . Why is it that the time traveller is constrained in this way? What mysterious force stills his sudden suicidal rage? (Smith, 1985, 58)

The idea is that backwards time travel is impossible because if it occurred, time travellers would attempt to do things such as kill their younger selves (or their grandfathers etc.). We know that doing these things—indeed, changing the past in any way—is impossible. But were there time travel, there would then be nothing left to stop these things happening. If we let things get to the stage where the time traveller is facing Grandfather with a loaded weapon, then there is nothing left to prevent the impossible from occurring. So we must draw the line earlier: it must be impossible for someone to get into this situation at all; that is, backwards time travel must be impossible.

In order to defend the possibility of time travel in the face of this argument we need to show that time travel is not a sure route to doing the impossible. So, given that a time traveller has gone to the past and is facing Grandfather, what could stop her killing Grandfather? Some science fiction authors resort to the idea of chaperones or time guardians who prevent time travellers from changing the past—or to mysterious forces of logic. But it is hard to take these ideas seriously—and more importantly, it is hard to make them work in detail when we remember that changing the past is impossible. (The chaperone is acting to ensure that the past remains as it was—but the only reason it ever was that way is because of his very actions.) [ 15 ] Fortunately there is a better response—also to be found in the science fiction literature, and brought to the attention of philosophers by Lewis (1976). What would stop the time traveller doing the impossible? She would fail “for some commonplace reason”, as Lewis (1976, 150) puts it. Her gun might jam, a noise might distract her, she might slip on a banana peel, etc. Nothing more than such ordinary occurrences is required to stop the time traveller killing Grandfather. Hence backwards time travel does not entail the occurrence of impossible events—and so the above objection is defused.

A problem remains. Suppose Tim, a time-traveller, is facing his grandfather with a loaded gun. Can Tim kill Grandfather? On the one hand, yes he can. He is an excellent shot; there is no chaperone to stop him; the laws of logic will not magically stay his hand; he hates Grandfather and will not hesitate to pull the trigger; etc. On the other hand, no he can’t. To kill Grandfather would be to change the past, and no-one can do that (not to mention the fact that if Grandfather died, then Tim would not have been born). So we have a contradiction: Tim can kill Grandfather and Tim cannot kill Grandfather. Time travel thus leads to a contradiction: so it is impossible.

Note the difference between this version of the Grandfather paradox and the version considered above. In the earlier version, the contradiction happens if Tim kills Grandfather. The solution was to say that Tim can go into the past without killing Grandfather—hence time travel does not entail a contradiction. In the new version, the contradiction happens as soon as Tim gets to the past. Of course Tim does not kill Grandfather—but we still have a contradiction anyway: for he both can do it, and cannot do it. As Lewis puts it:

Could a time traveler change the past? It seems not: the events of a past moment could no more change than numbers could. Yet it seems that he would be as able as anyone to do things that would change the past if he did them. If a time traveler visiting the past both could and couldn’t do something that would change it, then there cannot possibly be such a time traveler. (Lewis, 1976, 149)

Lewis’s own solution to this problem has been widely accepted. [ 16 ] It turns on the idea that to say that something can happen is to say that its occurrence is compossible with certain facts, where context determines (more or less) which facts are the relevant ones. Tim’s killing Grandfather in 1921 is compossible with the facts about his weapon, training, state of mind, and so on. It is not compossible with further facts, such as the fact that Grandfather did not die in 1921. Thus ‘Tim can kill Grandfather’ is true in one sense (relative to one set of facts) and false in another sense (relative to another set of facts)—but there is no single sense in which it is both true and false. So there is no contradiction here—merely an equivocation.

Another response is that of Vihvelin (1996), who argues that there is no contradiction here because ‘Tim can kill Grandfather’ is simply false (i.e. contra Lewis, there is no legitimate sense in which it is true). According to Vihvelin, for ‘Tim can kill Grandfather’ to be true, there must be at least some occasions on which ‘If Tim had tried to kill Grandfather, he would or at least might have succeeded’ is true—but, Vihvelin argues, at any world remotely like ours, the latter counterfactual is always false. [ 17 ]

Return to the original version of the Grandfather paradox and Lewis’s ‘commonplace reasons’ response to it. This response engenders a new objection—due to Horwich (1987)—not to the possibility but to the probability of backwards time travel.

Think about correlated events in general. Whenever we see two things frequently occurring together, this is because one of them causes the other, or some third thing causes both. Horwich calls this the Principle of V-Correlation:

if events of type A and B are associated with one another, then either there is always a chain of events between them…or else we find an earlier event of type C that links up with A and B by two such chains of events. What we do not see is…an inverse fork—in which A and B are connected only with a characteristic subsequent event, but no preceding one. (Horwich, 1987, 97–8)

For example, suppose that two students turn up to class wearing the same outfits. That could just be a coincidence (i.e. there is no common cause, and no direct causal link between the two events). If it happens every week for the whole semester, it is possible that it is a coincidence, but this is extremely unlikely . Normally, we see this sort of extensive correlation only if either there is a common cause (e.g. both students have product endorsement deals with the same clothing company, or both slavishly copy the same influencer) or a direct causal link (e.g. one student is copying the other).

Now consider the time traveller setting off to kill her younger self. As discussed, no contradiction need ensue—this is prevented not by chaperones or mysterious forces, but by a run of ordinary occurrences in which the trigger falls off the time traveller’s gun, a gust of wind pushes her bullet off course, she slips on a banana peel, and so on. But now consider this run of ordinary occurrences. Whenever the time traveller contemplates auto-infanticide, someone nearby will drop a banana peel ready for her to slip on, or a bird will begin to fly so that it will be in the path of the time traveller’s bullet by the time she fires, and so on. In general, there will be a correlation between auto-infanticide attempts and foiling occurrences such as the presence of banana peels—and this correlation will be of the type that does not involve a direct causal connection between the correlated events or a common cause of both. But extensive correlations of this sort are, as we saw, extremely rare—so backwards time travel will happen about as often as you will see two people wear the same outfits to class every day of semester, without there being any causal connection between what one wears and what the other wears.

We can set out Horwich’s argument this way:

• If time travel were ever to occur, we should see extensive uncaused correlations.
• It is extremely unlikely that we should ever see extensive uncaused correlations.
• Therefore time travel is extremely unlikely to occur.

The conclusion is not that time travel is impossible, but that we should treat it the way we treat the possibility of, say, tossing a fair coin and getting heads one thousand times in a row. As Price (1996, 278 n.7) puts it—in the context of endorsing Horwich’s conclusion: “the hypothesis of time travel can be made to imply propositions of arbitrarily low probability. This is not a classical reductio, but it is as close as science ever gets.”

Smith (1997) attacks both premisses of Horwich’s argument. Against the first premise, he argues that backwards time travel, in itself, does not entail extensive uncaused correlations. Rather, when we look more closely, we see that time travel scenarios involving extensive uncaused correlations always build in prior coincidences which are themselves highly unlikely. Against the second premise, he argues that, from the fact that we have never seen extensive uncaused correlations, it does not follow that we never shall. This is not inductive scepticism: let us assume (contra the inductive sceptic) that in the absence of any specific reason for thinking things should be different in the future, we are entitled to assume they will continue being the same; still we cannot dismiss a specific reason for thinking the future will be a certain way simply on the basis that things have never been that way in the past. You might reassure an anxious friend that the sun will certainly rise tomorrow because it always has in the past—but you cannot similarly refute an astronomer who claims to have discovered a specific reason for thinking that the earth will stop rotating overnight.

Sider (2002, 119–20) endorses Smith’s second objection. Dowe (2003) criticises Smith’s first objection, but agrees with the second, concluding overall that time travel has not been shown to be improbable. Ismael (2003) reaches a similar conclusion. Goddu (2007) criticises Smith’s first objection to Horwich. Further contributions to the debate include Arntzenius (2006), Smeenk and Wüthrich (2011, §2.2) and Elliott (2018). For other arguments to the same conclusion as Horwich’s—that time travel is improbable—see Ney (2000) and Effingham (2020).

Return again to the original version of the Grandfather paradox and Lewis’s ‘commonplace reasons’ response to it. This response engenders a further objection. The autoinfanticidal time traveller is attempting to do something impossible (render herself permanently dead from an age younger than her age at the time of the attempts). Suppose we accept that she will not succeed and that what will stop her is a succession of commonplace occurrences. The previous objection was that such a succession is improbable . The new objection is that the exclusion of the time traveler from successfully committing auto-infanticide is mysteriously inexplicable . The worry is as follows. Each particular event that foils the time traveller is explicable in a perfectly ordinary way; but the inevitable combination of these events amounts to a ring-fencing of the forbidden zone of autoinfanticide—and this ring-fencing is mystifying. It’s like a grand conspiracy to stop the time traveler from doing what she wants to do—and yet there are no conspirators: no time lords, no magical forces of logic. This is profoundly perplexing. Riggs (1997, 52) writes: “Lewis’s account may do for a once only attempt, but is untenable as a general explanation of Tim’s continual lack of success if he keeps on trying.” Ismael (2003, 308) writes: “Considered individually, there will be nothing anomalous in the explanations…It is almost irresistible to suppose, however, that there is something anomalous in the cases considered collectively, i.e., in our unfailing lack of success.” See also Gorovitz (1964, 366–7), Horwich (1987, 119–21) and Carroll (2010, 86).

There have been two different kinds of defense of time travel against the objection that it involves mysteriously inexplicable occurrences. Baron and Colyvan (2016, 70) agree with the objectors that a purely causal explanation of failure—e.g. Tim fails to kill Grandfather because first he slips on a banana peel, then his gun jams, and so on—is insufficient. However they argue that, in addition, Lewis offers a non-causal—a logical —explanation of failure: “What explains Tim’s failure to kill his grandfather, then, is something about logic; specifically: Tim fails to kill his grandfather because the law of non-contradiction holds.” Smith (2017) argues that the appearance of inexplicability is illusory. There are no scenarios satisfying the description ‘a time traveller commits autoinfanticide’ (or changes the past in any other way) because the description is self-contradictory (e.g. it involves the time traveller permanently dying at 20 and also being alive at 40). So whatever happens it will not be ‘that’. There is literally no way for the time traveller not to fail. Hence there is no need for—or even possibility of—a substantive explanation of why failure invariably occurs, and such failure is not perplexing.

3. Causation

Backwards time travel scenarios give rise to interesting issues concerning causation. In this section we examine two such issues.

Earlier we distinguished changing the past and affecting the past, and argued that while the former is impossible, backwards time travel need involve only the latter. Affecting the past would be an example of backwards causation (i.e. causation where the effect precedes its cause)—and it has been argued that this too is impossible, or at least problematic. [ 18 ] The classic argument against backwards causation is the bilking argument . [ 19 ] Faced with the claim that some event A causes an earlier event B , the proponent of the bilking objection recommends an attempt to decorrelate A and B —that is, to bring about A in cases in which B has not occurred, and to prevent A in cases in which B has occurred. If the attempt is successful, then B often occurs despite the subsequent nonoccurrence of A , and A often occurs without B occurring, and so A cannot be the cause of B . If, on the other hand, the attempt is unsuccessful—if, that is, A cannot be prevented when B has occurred, nor brought about when B has not occurred—then, it is argued, it must be B that is the cause of A , rather than vice versa.

The bilking procedure requires repeated manipulation of event A . Thus, it cannot get under way in cases in which A is either unrepeatable or unmanipulable. Furthermore, the procedure requires us to know whether or not B has occurred, prior to manipulating A —and thus, it cannot get under way in cases in which it cannot be known whether or not B has occurred until after the occurrence or nonoccurrence of A (Dummett, 1964). These three loopholes allow room for many claims of backwards causation that cannot be touched by the bilking argument, because the bilking procedure cannot be performed at all. But what about those cases in which it can be performed? If the procedure succeeds—that is, A and B are decorrelated—then the claim that A causes B is refuted, or at least weakened (depending upon the details of the case). But if the bilking attempt fails, it does not follow that it must be B that is the cause of A , rather than vice versa. Depending upon the situation, that B causes A might become a viable alternative to the hypothesis that A causes B —but there is no reason to think that this alternative must always be the superior one. For example, suppose that I see a photo of you in a paper dated well before your birth, accompanied by a report of your arrival from the future. I now try to bilk your upcoming time trip—but I slip on a banana peel while rushing to push you away from your time machine, my time travel horror stories only inspire you further, and so on. Or again, suppose that I know that you were not in Sydney yesterday. I now try to get you to go there in your time machine—but first I am struck by lightning, then I fall down a manhole, and so on. What does all this prove? Surely not that your arrival in the past causes your departure from the future. Depending upon the details of the case, it seems that we might well be entitled to describe it as involving backwards time travel and backwards causation. At least, if we are not so entitled, this must be because of other facts about the case: it would not follow simply from the repeated coincidental failures of my bilking attempts.

Backwards time travel would apparently allow for the possibility of causal loops, in which things come from nowhere. The things in question might be objects—imagine a time traveller who steals a time machine from the local museum in order to make his time trip and then donates the time machine to the same museum at the end of the trip (i.e. in the past). In this case the machine itself is never built by anyone—it simply exists. The things in question might be information—imagine a time traveller who explains the theory behind time travel to her younger self: theory that she herself knows only because it was explained to her in her youth by her time travelling older self. The things in question might be actions. Imagine a time traveller who visits his younger self. When he encounters his younger self, he suddenly has a vivid memory of being punched on the nose by a strange visitor. He realises that this is that very encounter—and resignedly proceeds to punch his younger self. Why did he do it? Because he knew that it would happen and so felt that he had to do it—but he only knew it would happen because he in fact did it. [ 20 ]

One might think that causal loops are impossible—and hence that insofar as backwards time travel entails such loops, it too is impossible. [ 21 ] There are two issues to consider here. First, does backwards time travel entail causal loops? Lewis (1976, 148) raises the question whether there must be causal loops whenever there is backwards causation; in response to the question, he says simply “I am not sure.” Mellor (1998, 131) appears to claim a positive answer to the question. [ 22 ] Hanley (2004, 130) defends a negative answer by telling a time travel story in which there is backwards time travel and backwards causation, but no causal loops. [ 23 ] Monton (2009) criticises Hanley’s counterexample, but also defends a negative answer via different counterexamples. Effingham (2020) too argues for a negative answer.

Second, are causal loops impossible, or in some other way objectionable? One objection is that causal loops are inexplicable . There have been two main kinds of response to this objection. One is to agree but deny that this is a problem. Lewis (1976, 149) accepts that a loop (as a whole) would be inexplicable—but thinks that this inexplicability (like that of the Big Bang or the decay of a tritium atom) is merely strange, not impossible. In a similar vein, Meyer (2012, 263) argues that if someone asked for an explanation of a loop (as a whole), “the blame would fall on the person asking the question, not on our inability to answer it.” The second kind of response (Hanley, 2004, §5) is to deny that (all) causal loops are inexplicable. A second objection to causal loops, due to Mellor (1998, ch.12), is that in such loops the chances of events would fail to be related to their frequencies in accordance with the law of large numbers. Berkovitz (2001) and Dowe (2001) both argue that Mellor’s objection fails to establish the impossibility of causal loops. [ 24 ] Effingham (2020) considers—and rebuts—some additional objections to the possibility of causal loops.

4. Time and Change

Gödel (1949a [1990a])—in which Gödel presents models of Einstein’s General Theory of Relativity in which there exist CTC’s—can well be regarded as initiating the modern academic literature on time travel, in both philosophy and physics. In a companion paper, Gödel discusses the significance of his results for more general issues in the philosophy of time (Gödel 1949b [1990b]). For the succeeding half century, the time travel literature focussed predominantly on objections to the possibility (or probability) of time travel. More recently, however, there has been renewed interest in the connections between time travel and more general issues in the metaphysics of time and change. We examine some of these in the present section. [ 25 ]

The first thing that we need to do is set up the various metaphysical positions whose relationships with time travel will then be discussed. Consider two metaphysical questions:

• Are the past, present and future equally real?
• Is there an objective flow or passage of time, and an objective now?

We can label some views on the first question as follows. Eternalism is the view that past and future times, objects and events are just as real as the present time and present events and objects. Nowism is the view that only the present time and present events and objects exist. Now-and-then-ism is the view that the past and present exist but the future does not. We can also label some views on the second question. The A-theory answers in the affirmative: the flow of time and division of events into past (before now), present (now) and future (after now) are objective features of reality (as opposed to mere features of our experience). Furthermore, they are linked: the objective flow of time arises from the movement, through time, of the objective now (from the past towards the future). The B-theory answers in the negative: while we certainly experience now as special, and time as flowing, the B-theory denies that what is going on here is that we are detecting objective features of reality in a way that corresponds transparently to how those features are in themselves. The flow of time and the now are not objective features of reality; they are merely features of our experience. By combining answers to our first and second questions we arrive at positions on the metaphysics of time such as: [ 26 ]

• the block universe view: eternalism + B-theory
• the moving spotlight view: eternalism + A-theory
• the presentist view: nowism + A-theory
• the growing block view: now-and-then-ism + A-theory.

So much for positions on time itself. Now for some views on temporal objects: objects that exist in (and, in general, change over) time. Three-dimensionalism is the view that persons, tables and other temporal objects are three-dimensional entities. On this view, what you see in the mirror is a whole person. [ 27 ] Tomorrow, when you look again, you will see the whole person again. On this view, persons and other temporal objects are wholly present at every time at which they exist. Four-dimensionalism is the view that persons, tables and other temporal objects are four-dimensional entities, extending through three dimensions of space and one dimension of time. On this view, what you see in the mirror is not a whole person: it is just a three-dimensional temporal part of a person. Tomorrow, when you look again, you will see a different such temporal part. Say that an object persists through time if it is around at some time and still around at a later time. Three- and four-dimensionalists agree that (some) objects persist, but they differ over how objects persist. According to three-dimensionalists, objects persist by enduring : an object persists from t 1 to t 2 by being wholly present at t 1 and t 2 and every instant in between. According to four-dimensionalists, objects persist by perduring : an object persists from t 1 to t 2 by having temporal parts at t 1 and t 2 and every instant in between. Perduring can be usefully compared with being extended in space: a road extends from Melbourne to Sydney not by being wholly located at every point in between, but by having a spatial part at every point in between.

It is natural to combine three-dimensionalism with presentism and four-dimensionalism with the block universe view—but other combinations of views are certainly possible.

Gödel (1949b [1990b]) argues from the possibility of time travel (more precisely, from the existence of solutions to the field equations of General Relativity in which there exist CTC’s) to the B-theory: that is, to the conclusion that there is no objective flow or passage of time and no objective now. Gödel begins by reviewing an argument from Special Relativity to the B-theory: because the notion of simultaneity becomes a relative one in Special Relativity, there is no room for the idea of an objective succession of “nows”. He then notes that this argument is disrupted in the context of General Relativity, because in models of the latter theory to date, the presence of matter does allow recovery of an objectively distinguished series of “nows”. Gödel then proposes a new model (Gödel 1949a [1990a]) in which no such recovery is possible. (This is the model that contains CTC’s.) Finally, he addresses the issue of how one can infer anything about the nonexistence of an objective flow of time in our universe from the existence of a merely possible universe in which there is no objectively distinguished series of “nows”. His main response is that while it would not be straightforwardly contradictory to suppose that the existence of an objective flow of time depends on the particular, contingent arrangement and motion of matter in the world, this would nevertheless be unsatisfactory. Responses to Gödel have been of two main kinds. Some have objected to the claim that there is no objective flow of time in his model universe (e.g. Savitt (2005); see also Savitt (1994)). Others have objected to the attempt to transfer conclusions about that model universe to our own universe (e.g. Earman (1995, 197–200); for a partial response to Earman see Belot (2005, §3.4)). [ 28 ]

Earlier we posed two questions:

Gödel’s argument is related to the second question. Let’s turn now to the first question. Godfrey-Smith (1980, 72) writes “The metaphysical picture which underlies time travel talk is that of the block universe [i.e. eternalism, in the terminology of the present entry], in which the world is conceived as extended in time as it is in space.” In his report on the Analysis problem to which Godfrey-Smith’s paper is a response, Harrison (1980, 67) replies that he would like an argument in support of this assertion. Here is an argument: [ 29 ]

A fundamental requirement for the possibility of time travel is the existence of the destination of the journey. That is, a journey into the past or the future would have to presuppose that the past or future were somehow real. (Grey, 1999, 56)

Dowe (2000, 442–5) responds that the destination does not have to exist at the time of departure: it only has to exist at the time of arrival—and this is quite compatible with non-eternalist views. And Keller and Nelson (2001, 338) argue that time travel is compatible with presentism:

There is four-dimensional [i.e. eternalist, in the terminology of the present entry] time-travel if the appropriate sorts of events occur at the appropriate sorts of times; events like people hopping into time-machines and disappearing, people reappearing with the right sorts of memories, and so on. But the presentist can have just the same patterns of events happening at just the same times. Or at least, it can be the case on the presentist model that the right sorts of events will happen, or did happen, or are happening, at the rights sorts of times. If it suffices for four-dimensionalist time-travel that Jennifer disappears in 2054 and appears in 1985 with the right sorts of memories, then why shouldn’t it suffice for presentist time-travel that Jennifer will disappear in 2054, and that she did appear in 1985 with the right sorts of memories?

Sider (2005) responds that there is still a problem reconciling presentism with time travel conceived in Lewis’s way: that conception of time travel requires that personal time is similar to external time—but presentists have trouble allowing this. Further contributions to the debate whether presentism—and other versions of the A-theory—are compatible with time travel include Monton (2003), Daniels (2012), Hall (2014) and Wasserman (2018) on the side of compatibility, and Miller (2005), Slater (2005), Miller (2008), Hales (2010) and Markosian (2020) on the side of incompatibility.

Leibniz’s Law says that if x = y (i.e. x and y are identical—one and the same entity) then x and y have exactly the same properties. There is a superficial conflict between this principle of logic and the fact that things change. If Bill is at one time thin and at another time not so—and yet it is the very same person both times—it looks as though the very same entity (Bill) both possesses and fails to possess the property of being thin. Three-dimensionalists and four-dimensionalists respond to this problem in different ways. According to the four-dimensionalist, what is thin is not Bill (who is a four-dimensional entity) but certain temporal parts of Bill; and what is not thin are other temporal parts of Bill. So there is no single entity that both possesses and fails to possess the property of being thin. Three-dimensionalists have several options. One is to deny that there are such properties as ‘thin’ (simpliciter): there are only temporally relativised properties such as ‘thin at time t ’. In that case, while Bill at t 1 and Bill at t 2 are the very same entity—Bill is wholly present at each time—there is no single property that this one entity both possesses and fails to possess: Bill possesses the property ‘thin at t 1 ’ and lacks the property ‘thin at t 2 ’. [ 30 ]

Now consider the case of a time traveller Ben who encounters his younger self at time t . Suppose that the younger self is thin and the older self not so. The four-dimensionalist can accommodate this scenario easily. Just as before, what we have are two different three-dimensional parts of the same four-dimensional entity, one of which possesses the property ‘thin’ and the other of which does not. The three-dimensionalist, however, faces a problem. Even if we relativise properties to times, we still get the contradiction that Ben possesses the property ‘thin at t ’ and also lacks that very same property. [ 31 ] There are several possible options for the three-dimensionalist here. One is to relativise properties not to external times but to personal times (Horwich, 1975, 434–5); another is to relativise properties to spatial locations as well as to times (or simply to spacetime points). Sider (2001, 101–6) criticises both options (and others besides), concluding that time travel is incompatible with three-dimensionalism. Markosian (2004) responds to Sider’s argument; [ 32 ] Miller (2006) also responds to Sider and argues for the compatibility of time travel and endurantism; Gilmore (2007) seeks to weaken the case against endurantism by constructing analogous arguments against perdurantism. Simon (2005) finds problems with Sider’s arguments, but presents different arguments for the same conclusion; Effingham and Robson (2007) and Benovsky (2011) also offer new arguments for this conclusion. For further discussion see Wasserman (2018) and Effingham (2020). [ 33 ]

We have seen arguments to the conclusions that time travel is impossible, improbable and inexplicable. Here’s an argument to the conclusion that backwards time travel simply will not occur. If backwards time travel is ever going to occur, we would already have seen the time travellers—but we have seen none such. [ 34 ] The argument is a weak one. [ 35 ] For a start, it is perhaps conceivable that time travellers have already visited the Earth [ 36 ] —but even granting that they have not, this is still compatible with the future actuality of backwards time travel. First, it may be that time travel is very expensive, difficult or dangerous—or for some other reason quite rare—and that by the time it is available, our present period of history is insufficiently high on the list of interesting destinations. Second, it may be—and indeed existing proposals in the physics literature have this feature—that backwards time travel works by creating a CTC that lies entirely in the future: in this case, backwards time travel becomes possible after the creation of the CTC, but travel to a time earlier than the time at which the CTC is created is not possible. [ 37 ]

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Taking a Deeper Dive Into Fandoms and Conventions

4 Theories of Time Travel

With Tenet having come out the past few weeks, I’ve been thinking of my favorite time travel movies. I’m not entirely sure if the new Nolan flick is time travel in the traditional sense, as it seems that people travel in reverse, real-time, but I still wanted to talk about it, so let’s see how it goes.

All time travel stories seem to subscribe to one of three theories on how time travel, if it could work, would work. Below is a cool graphic, that I’ll be expanding on.

Fixed Timeline

This is my favorite form of time travel, because it’s the only one that holds up to scrutiny according the laws of physics that we currently understand. Everything that has happened, will happen because it has. Also, things that will happen, will happen regardless of your actions. This is the least dramatic of all time travel, because there is no way you can change the past or the future via your actions. My first foray into the idea of time travel, if I remember properly, was in Harry Potter and the Prizoner of Azkaban . In it, they try to save Buckbeaks life, by going back in time, and when they do, they realize that they were warned of the executioners arrival via themselves, in that moment. Not only that, but when they initially saw Buckbeak’s death, they didn’t actually see him, just heard the sound of the axe chopping, which they later realize was through a pumpkin out of frustration, meaning they had always succeeded. They went to the past, and didn’t change any outcomes, but instead ensured what happened will happen, as would always happen. They saved the day, but didn’t change anything.

In Tenet , the time travel is fixed, and the characters travel in that fixed timeline, backwards. We see this in the trailer, when one character gets into a fight, and it’s clear that the person he’s fighting’s actions are influenced by that fight, but backwards.

Dynamic Timeline

Next, we have a the Dynamic Timeline. This is the basis of most time travel movies, because to put it simply, this is the one that you have the power to control, and meddling with it could cause problems. If you go back in time, you can prevent something that has happened, but in doing so, you could cause the future to change, so much so that you might even prevent your own birth.

The most popular version of this is in Back to the Future , where Marty McFly goes back in time, and almost erases his own existence. This causes a paradox, as he starts to vanish from reality itself.

This is the theory of time travel that is rising in popularity, especially in comic book movies. Whether it’s the Worlds of DC universe, with the Flashpoint movie (both Keaton and Affleck Batmans! Batmen?) or the Marvel universe, in Endgame when they took the Infinity Stones out of their time, and the upcoming Doctor Strange in the Multiverse of Madness . In both of these stories, they rely on the theory that every decision that is made in the world, splits reality into two parallel universes, wherein each of those decisions are made, and so there are infinite parallel universes, depending on every single combination of possibilities over all of time. So whenever you go back in time, you are actually going to that alternate reality where your actions were the truth. This is, in effect, similar to the Fixed Timeline theory, it’s just that there are multiple Fixed timelines. You aren’t only travelling in time, but also in space, between the multiverses.

Lastly, we have the logic of Doctor Who , which is a mix of all of these. There are times when The Doctor goes back in time and changes history, and there are other times when history happens the same way, regardless of his involvement. The show calls these “fixed points in time”, or moments that are too important to be changed, and will happen no matter what you do. The show used these fixed points primarily as a way to teach about history, as the shows original intent was educational.

In The Umbrella Academy , we see an almost Whovian style of time travel with the Commission, who ensures that specific moments in time play out the way they are supposed to. They find out that different events might be changed by specific people doing things a certain way, and they must travel through time to kill the right person, to ensure that the event should happen. Of course, the addition of problems in time, that they find out over time, doesn’t make sense by traditional time theories. At one point, they find out that the Hindenburg might not blow up, which is, in essence, a “fixed” point in time. Therefore Five decides that a butcher needs to die, so that his unhygienic son would take over the business, making the problem person unable from stopping the destruction of the airship. But why do they find out at that point that the problem person is a problem in the first place?

The answer is that time is just a “big ball of wibbly-wobbley, timey-wimey, stuff.”

What is your favorite theory of time travel? Let us know in the comments below!

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Hello. I’m fascinated with your theories on time travel & fixed points in time. Is there basis for this in science? I’ve read that time travel is probable if not yet possible.

Hi Kathryn. I’d say there is no basis in science, this is purely regarding movies and storytelling. I don’t believe that we will see time travel in our lifetimes.

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Time travel 101 – a time travel blog with basics for the beginner

3 theories of time travel

I found this excellent infographic over at techeblog.com which explains 3 theories of time travel; fixed timeline, dynamic timeline and multiverse.

As I commented on the original site, I have a problem with multiverses! They’re too much of an easy ‘get-out-of-jail-free’ card and I can’t find any sound scientific basis for the creation of all the additional energy and mass required to go round creating new universes willy nilly…let alone specify a defined moment for divergence.

So many problems, so little time…I think I have an idea for another blog post!

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Published by Paul Wandason

Paul is a time travel enthusiast and writes for time2timetravel.com as Paul Wandason. As a father of two growing daughters, Paul’s eager to avoid the grandfather paradox and has no intention of annoying any of his future grandchildren. If he had any free time he’d probably fill it by looking at the stars and other wonders of the universe. And if he had a time machine he’d probably hide it from his grandchildren. View all posts by Paul Wandason

Oh, cool. Actually, there’s a 4th theory that’s a bit of a hybrid. I can’t tell you about it, though, because a future version of my great grandson told me that telling you would destroy the space-time continuum.

Hi Justin, I’m very happy the (great) grandfather paradox didn’t get a hold! Naturally I’d be very interested in the 4th theory…but not at the expense of the space-time continuum…there are rumours it’s quite important!

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Watch CBS News

Solar eclipse maps show 2024 totality path, peak times and how much of the eclipse people could see across the U.S.

By Aliza Chasan

Updated on: April 9, 2024 / 5:00 AM EDT / CBS News

A total solar eclipse  crossed North America Monday with parts of 15 U.S. states within the path of totality. Maps show  where and when astronomy fans could see the big event  as skies darkened in the middle of the day Monday, April 8.

The total eclipse first appeared along Mexico's Pacific Coast at around 11:07 a.m. PDT, then traveled across a swath of the U.S., from Texas to Maine, and into Canada.

About 31.6 million people live in the path of totality , the area where the moon fully blocked out the sun , according to NASA. The path ranged between 108 and 122 miles wide. An additional 150 million people live within 200 miles of the path of totality.

Solar eclipse path of totality map for 2024

The total solar eclipse started over the Pacific Ocean, and the first location in continental North America that experienced totality was Mexico's Pacific Coast, around 11:07 a.m. PDT, according to NASA. From there, the path continued into Texas, crossing more than a dozen states before the eclipse enters Canada in southern Ontario. The eclipse exited continental North America at around 5:16 p.m. NDT from Newfoundland, Canada.

The path of totality included portions of the following states:

• Pennsylvania
• New Hampshire

Small parts of Tennessee and Michigan also experienced the total solar eclipse.

Several major cities across the U.S. were included in the eclipse's path of totality, while many others saw a partial eclipse. These were some of the best major cities for eclipse viewing — though the weather was a factor :

• San Antonio, Texas (partially under the path)
• Austin, Texas
• Waco, Texas
• Dallas, Texas
• Little Rock, Arkansas
• Indianapolis, Indiana
• Dayton, Ohio
• Cleveland, Ohio
• Buffalo, New York
• Rochester, New York
• Syracuse, New York
• Burlington, Vermont

Map of when the solar eclipse reached totality across its path

The eclipse began in the U.S. as a partial eclipse beginning at 12:06 p.m. CDT near Eagle Pass, Texas, before progressing to totality by about 1:27 p.m. CDT and then moving along its path to the northeast over the following few hours.

NASA shared times for several cities in the path of totality across the U.S. People could have also  checked their ZIP code on NASA's map  to see when the eclipse was to reach them if they were on, or near, the path of totality — or if they saw a partial eclipse instead.

How much of the eclipse did people see if they live outside the totality path?

While the April 8 eclipse covered a wide swath of the U.S., outside the path of totality observers may have spotted a partial eclipse, where the moon covers some, but not all, of the sun, according to NASA. The closer they were to the path of totality, the larger the portion of the sun that was hidden.

NASA allowed viewers to input a ZIP code and see how much of the sun was to be covered in their locations.

Could there be cloud cover be during the solar eclipse?

Some areas along the path of totality had a higher likelihood of cloud cover that could interfere with viewing the eclipse. Here is a map showing the historical trends in cloud cover this time of year. 

You could have checked the latest forecast for your location with our partners at The Weather Channel .

Where did the solar eclipse reach totality for the longest?

Eclipse viewers near Torreón, Mexico, got to experience totality for the longest. Totality there lasted 4 minutes, 28 seconds, according to NASA. 

Most places along the centerline of the path of totality saw a totality duration of between 3.5 and 4 minutes, according to NASA. Some places in the U.S. came close to the maximum; Kerrville, Texas, had a totality duration of 4 minutes, 24 seconds.

What is the path of totality for the 2044 solar eclipse?

The next total solar eclipse that will be visible from the contiguous U.S. will be on Aug. 23, 2044.

Astronomy fans in the U.S. will have far fewer opportunities to see the 2044 eclipse they had on April 8. NASA has not yet made maps available for the 2044 eclipse but, according to The Planetary Society , the path of totality will only touch three states.

The 2024 eclipse will start in Greenland, pass over Canada and end as the sun sets in Montana, North Dakota and South Dakota, according to the Planetary Society.

Aliza Chasan is a digital producer at 60 Minutes and CBSNews.com. She has previously written for outlets including PIX11 News, The New York Daily News, Inside Edition and DNAinfo. Aliza covers trending news, often focusing on crime and politics.

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