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Is time travel possible?

Is time travel possible? Simple answer, no. It violates all known laws of physics, and is logically impossible.

I cannot change the laws of physics, Captain! — Montgomery Scott, Star Trek.

The big problems: Causality, Conservation, Paradoxes.

Causality: Things happen because something happened to make them happen. Nothing ever happens without something to cause it to happen.

Conservation: Nothing is ever created or destroyed, only changed in form. Time travel makes things appear from nothing, or disappear into nothing, and that is not possible in our universe.

Temporal Paradoxes: based on those two key principles, reversing time requires that things happen, which cannot logically happen. Go back in time and change things so you were never born, means that you couldn’t go back in time to change things.

A simple, short paradox story.

You have a time machine, and used it to travel into the past. Things went badly, and you don’t like the horrible present which resulted from your trip. So you go back in time, before you took the first trip, and destroy the time machine before you can use it.

But if you destroyed the time machine, you could never have gone back in time and messed things up, and would therefore not have a time machine to go back in time to destroy it before you could use it.

For more fun, where did you get the time machine from? Turns out, an older person just gave it to you, but you find out later that the older person is in fact your older, time travelling self. But if you got the time machine from yourself, how could you also give it to yourself? The thing was never manufactured, it simply exists with no origin.

Time travel and reality just don’t mix.

These are fundamental properties of what we consider reality. If they aren’t true, then, literally, nothing we know is right!

This is the short answer, but I will go into depth on why this cannot be true in our universe, even in the future. This isn’t a matter of advancing technology, or even our understanding of science.

If the universe is real, then time travel isn’t.

The idea of time travel is old, and is widely used in fiction. Our imagination lets us explore things which aren’t possible. Our ability to tell stories is perhaps an essential part of self aware intelligence. Humans enjoy exercising their minds, and the impossible puzzles surround the concept of time travel are a fascinating challenge.

“Why, sometimes I’ve believed as many as six impossible things before breakfast.” The White Queen to Alice, Through the Looking-Glass.

It is fun to imagine impossible things. Science fiction isn’t restricted to just what is hypothetically possible, no more than any other kind of fiction. Just because it has science in the name doesn’t mean that it has to use real world science or follow the laws of physics, though this is more popularly called science fantasy.

Time travel means a few things. The easy one is travel into the future. Since time passes for everyone, this happens naturally. Going into the future faster doesn’t exactly, truly happen, but there are ways to make the time feel like it is passing faster for the traveler.

Time passes at one second per second for everyone, and only moves forward. This is because time isn’t a location, but a measurement of change. It isn’t a dimension of the real universe, even though we can give a location in three dimensions plus a date in time as a reference point. Neither yesterday nor tomorrow are places you can travel to.

Spacetime has distance and time as dimensions, but both are always positive, never negative. Time in all cases still moves only forward, and cannot be reversed.

But what about relativity? I’ll address this more, but time doesn’t change for the person traveling. They always experience time at the normal — and only — rate it can happen.

Space. The Final Frontier.

Time And Relative Dimensions In Space

A location with different velocity or acceleration from you will appear to experience time slower, an effect called time dilation. It requires a large difference in velocity to get a significant difference in the apparent flow of time. At about 70% of the speed of light, time moves at about half speed. Getting it to slow down enough to make 100 years pass in a few hours would required a great deal of energy, and a ship able to survive that.

Note that for the traveler in the ship, time still moves at the normal, one second per second rate. The traveler perceives time outside the ship to be moving slower (for objects like Earth, which are moving but are much slower). Whose time is actually slowed down? That is the central key of relativity — both are truly slower. One, however, will change speed and direction during the trip, and those changes will cause the traveler to experience less time passing than a relatively stationary (on Earth, for example) person.

Even if you could travel backwards in time, or forward faster than one second per second, you still have a problem. The Earth — and everything else in the universe — is moving. If you simply travel in time without also moving in space, the Earth won’t be there when you arrive.

There are two common solutions in science fiction. First, the time travel device remains in place on Earth, and moves along with it, as in the class H G Wells “The Time Machine”, and its adaptations. You are guaranteed to be at the right location, but you “ride” on the Earth and have to deal with any changes in the landscape over time. This can put you deep underground or underwater.

Second, the time travel device can also travel in space, as in the Tardis from Doctor Who. This is more convenient, as you can go anywhere in space and not worry about your arrival location. But it requires you to have, in addition to a mechanism for time travel, one which includes movement through space.

How fast are you moving? If you are sitting on Earth, you might think that you are motionless. But the Earth is rotating on its axis. If you were to travel 12 hours into the past, the Earth’s rotation would put your location on the opposite side of the planet — with the direction of motion also opposite. You will hit the wall at up to 1500 km/h, not a gentle landing at all.

But wait! The Earth is moving in orbit around the Sun, at around 30 km/s, or about 110,000 km/h. Take a trip to where Earth was six months ago, and the impact on arrival will be far worse than the 12 hour trip.

There’s still more! The Sun, and the whole solar system, is also moving through the Milky Way galaxy at 230 km/s (828,000 km/h). The Galaxy is also moving through the universe, about 585 km/s (2.1 million km/hr).

Any time travel must somehow compensate for these motions. This is a problem with teleportation as well (go to the other side of a rotating earth, and you have the same smash into the wall problem unless your teleportation also changes your velocity).

So, are you standing still, moving at 1500 km/h, 110,000 km/h, 828,000 km/h, or 2,100,000 km/h? Which is the real velocity?

Or are you actually moving at 99.5% of the speed of light?

That’s the trick of relativity. They are all fully real, and valid. Velocity can only be measured relative to some other object.

What about that near light speed thing? A spaceship — or cosmic particle – moving towards the Earth at that velocity will measure the velocity of Earth, relative to it, as 99.5% of the speed of light.

So you, and everyone, is always moving near the speed of light. There is no magical change of velocity needed to do this. Just find a suitably fast object to measure your velocity against.

Time dilation and relativity aren’t all that complicated, but they can be confusing if you don’t grasp a few key elements.

Motion is always relative to some chosen object.

Time and space aren’t separate, but make up a single structure called spacetime.

Spacetime – Wikipedia.

Time dilation – Wikipedia

Time Dilation is the effect of changing the measurement of time between objects having different relative velocities (or gravitational acceleration). There is no minimum velocity difference needed. Drive to the store, fly on an airplane, orbit the Earth, walk across the hall, or use an extremely fast spacecraft to travel to another star, you will experience time passing more slowly compared to another whose velocity is different from yours.

It takes a very large difference in velocity to make the amount noticeable and significant compared to human lifespans, but our computers, satellites, and even air navigation systems are all affected.

But it isn’t time travel. Time still passes for everyone at its normal, and only, rate, of one second per second. It is only compared to others who don’t move as you do that your rate of time passing will be different.

At 99.5% of the speed of light, a very fast speed, time passes roughly one tenth that of a non-moving object. For a spacecraft with an acceleration of 1g (Earth normal gravity), it will take you over five years to reach this speed, and a minimum round trip time over 20 years. I use Earth gravity as a standard for acceleration because we know it is comfortable and survivable for humans. Even so, no spacecraft could sustain that acceleration from internal fuel. It must gather its fuel, somehow, from outside.

If you wish to skip a century, your trip will take this five year time, plus another eight years (one tenth of the remaining 80). Living thirteen years while back on Earth, history and the calendars count 100 years, isn’t too bad, but is it worth the effort just to slow down your time a little?

What about 1000 years? That will cost you another 100. Hope your health care is excellent!

Want to skip one million years? Takes you almost 100,000 years to do that.

This is for a velocity which is difficult and expensive to achieve. Time dilation, as a means to visit the future without having to wait through the normal passage of time, just isn’t an easy method.

The easier way to do it? Just sleep through it. Set your alarm clock, wake up in 100 years, rested and refreshed and ready to go.

The Rip Van Winkle, Buck Rogers, Idiocracy, Futurama method. Find a nice suspended animation sleep chamber and skip all the boring years until things get interesting. Much less expensive, no need to stay awake or worry about what your ship is doing or running into out there.

You still can’t go back, of course, but it is likely to be easier and cheaper than the expensive star traveling way. Travel to other stars may have its own benefits, but time dilation is a side effect, and one which won’t make you stop saying “Are we there yet” all that much quicker.

Either way, this isn’t actual time travel, except in the sense that all of us our moving forward in time, one second every second, awake or asleep.

But what if it was possible?

Imagination can think of things which aren’t possible. Creativity can take the concepts from our imagination, and attempt to find ways to apply them to our real world. Some things turn out to be truly possible to create, while others do not. Our imagination isn’t limited to just what we know is possible.

We happily make up stories about many impossible things, not caring if they could ever be true.

In fiction, we can ignore the impossible premises, and enjoy exploring what could be if they were true. We can suspend disbelief, and choose to, at least temporarily, believe in things we know not to be true.

In reality, believing in things which aren’t true won’t make them real. Reality is what remains real even if you don’t believe in it.

So, time travel is impossible. In science fiction stories, we can get around this by saying, “I know it is impossible. But what if it could be done?”

Let’s do that, and see what could happen.

I’m going to present three cases for time travel. In fiction, travelers never worry about what comes after the end of the story. Perhaps they all live happily ever after, or they go on to the next episode, but no one has to worry about what happens next. This isn’t true in the real world, so if time travel was possible in the real world, it wouldn’t be just a one time, can’t do it ever again, limited tool to resolve the plot of the story.

For a comparison, imagine a story about an Air Travel Machine, written about 200 years ago. The hero invents one, uses it to rescue the princess and fly away from the castle on the island, a wild adventure in the sky, and goes home. Perhaps they think that Air Travel is too dangerous, and destroy the machine, certain that no one else will make one. In any case, the future ramifications of the technology need not be explored within the story.

Icarus – Wikipedia

One disaster ended a whole field of winged flight experimentation. Or it would have, if the story were true. Flight in the real world had many accidents, and they did little to slow down the development of aviation technology.

Time travel doesn’t have that luxury in the real world. If time travel is possible into the past, then the present will be affected by the actions of every future time traveler who reaches the time and location of the original story. The universe is really, really, amazingly large, and the future is filled with more time than the current age of the known universe. Earth and human civilization are tiny, minuscule, almost unnoticeable things in comparison.

Any possible real world time travel therefore will have to consider the effects of its application on the whole of the universe, for the entire history of the universe.

When discussing these cases, I’ll point out some important implications.

First, none of these concepts for traveling in time are remotely close to being possible for our civilization, if there was a way to do them. The action of changing time changes the entire universe, and may require more energy than the entire universe contains. Conservation is a cruel, limiting law of nature. It is trivial to violate it in fiction, but if it was possible to do so in the real world, the results would be far from trivial.

Consider a simple device, which propels a one kilogram object at extremely high speed (say 150000 km/h), and only uses a small flashlight battery for power. This violates conservation, as you get much more energy out than you put in. Now, use this device to push an electrical generator, which then runs thousands of these devices. Just for fun, let each one of the new devices run more thousands of devices. You’ve got a source of infinite energy! Of course, conservation says that the energy must come from somewhere, so you should hope that wherever it is, it doesn’t cause too much trouble for the owners, or your own small area of the universe.

Second, in the real universe the story never ends. There is always more future, except that for time travel, the past is also the future. Stories often have one time traveler with limited resources, whose travel ends when the story ends. There is far too much future for that to be true in a real universe with time travel. Worse, think about how real people you might know would use it, if it existed.

Hitler’s Time Travel Exemption Act – TV Tropes

In fiction, changing anything in the past which would make the present too unrecognizable is rarely done. It is hard to write stories about alternate history, when too much has changed. But in a reality with time travel, you’d have to ask yourself What would Hitler do? And why someone wouldn’t ignore the risks of changing the past, if it might undo some personal loss, regardless of the effects on everyone else?

Third, the implications of time travel in the real world are pure Nightmare Fuel. You can’t do any of these if the universe conforms to the basic laws of physics as we know them. If time travel were possible, then literally, everything you think you know is wrong. In short, there is no such thing as the real world. Discovering real time travel would be an existential challenge for anyone, and a worse challenge for an entire civilization.

If the universe is real, then time travel isn’t.

But…

If time travel is real, the the universe isn’t.

It is a simple corollary, isn’t it? Time travel breaks what we know as reality, so a universe where it is possible simply is nothing like what we think our universe is like. Every last law of physics, everything you think is real, everything that happened, or will happen, or might have happened if there wasn’t time travel, isn’t true.

Of course, we can think about such frightening things, and it shouldn’t hurt us. But you may never be able to enjoy a time travel story again, without the horror of its unmentioned effects after the story ends haunting you forever.

Case 1: Unrestricted, unlimited time travel.

Paradoxes? We don’t need no stinkin’ paradoxes!

Note that this isn’t used much in fiction, because it quickly reveals the unreal nature of the universe when exploited. It is just shy of a full Reality Warper power, and it is difficult to incorporate an infinitely powerful character into a story and have it be relatable or fun. It is much harder if the protagonist, the hero of the story, has this kind of power.

Reality Warper – TV Tropes

It is, however, a much better and quicker way to show the effects of time travel on the universe.

In the real world, when you take action, you are not constrained by any consideration of their interference in future events. You can do whatever you want, only limited by your physical abilities to take those actions. The future is just what happens after you take those actions, and whatever happens after that, and so on.

So our traveler has a time machine, and uses it to go into the past. Let’s say we roll back time 100 years. From the viewpoint of our traveler, time is still moving forward, and that never changed. It is the rest of the universe which has been “reset” back in time. Everyone born after that has been wiped from existence, every event after that point has never happened. The traveler is free to take any action they wish.

Go back and kill your grandfather? Not a nice thing, but one of the classic paradox examples. It doesn’t matter that you won’t be born, again, when time moves forward, since you exist now. Run time forward 100 years, and while no one will know you and your family won’t exist, you are just fine.

With infinite power comes infinite irresponsibility.

With mere great power, you must worry about the consequences of your actions. But with the infinite power of time travel, why worry? You can just go back and do over anything you don’t like. You are, in fact, free to try anything. Nothing can stop you, because you can just travel back and make sure that whoever it is doesn’t have the chance to make that attempt.

But is it really infinite?

Time travel makes for some unstated assumptions, beyond what has already been discussed. Is there free will in the universe? Are all events completely predictable, or is there true random events and chaotic, unpredictable elements like the Uncertainty principle – Wikipedia?

One fine day, you come across a time machine. With no limits, why not try to make a better universe? Let’s kill Hitler!

But that is too cruel, so when you go back in time, you just kidnap baby Hitler, and go back to your future.

Except when you get back, the whole world is different. Not bad, maybe, but not only do you not appear to exist, nor does your family, anyone else you know personally, and your home country is weird and strange.

Well, what is the point of having a time machine with unlimited power to change time if you don’t use it? You go back in time with baby Hitler, a short time after you left, to put him back. But when you get there, you find a baby in the crib, who looks just like the one you took.

You make a quick trip back to your present, and find that history is more or less what you remember. But you, personally, still were apparently never born, even though some of your relatives exist.

No time like the past, you say! So you go back to shortly before you took the baby the first time, to try to solve the mystery. Your previous self hasn’t arrived yet. You don’t know if that will happen, so why not just take the baby again? Now you have two baby Hitlers, and leave none behind, and go back to your present.

This time, things are strange again, and you still don’t exist here. Time for some experimentation!

You go back just a little further than before, and this time, instead of taking the baby, you leave the two you have already. You go forward in time a minute, take the three babies, then go back a minute, and take the three you left behind. Now you have six babies, and you wonder just how many temporal duplicates are possible.

As you continue this conservation-defying and probably pointless duplicating of infants, you eventually let time catch up to when your first trip to the past happened.

To your surprise, you see yourself this time, moving to take the baby. But someone, unseen by you, shoots your past self with a disintegrating beam (or something that just makes you disappear), and destroys your past self’s time machine.

You quickly go back to your machine. Since your past doesn’t affect you in the present, your prior death doesn’t affect you, but what seemed like a safe bit of exploiting the infinite power of time travel has got a lot more complicated. Where can you go, what can do you, to escape or deal with a rival time traveler? Especially one who seems to want to protect baby Hitler!

So this time, you decide to go far into the past. 100 million years should do it, you think. You still have a whole nursery of baby Hitlers to take care of. Who can help you?

Who, in all the universe, can you trust? While pondering this, a time machine identical to yours appears, and you step out of it. “This is what I, er, we, are going to do. Put your time machine into mine, and go back and pick ourselves up.“

How fast can we do this? Time doesn’t matter, we have infinite time to repeat the process. As long as we don’t bump into whoever is interfering with us, we can run loops through time and have as many of us as we want or need. All the time in the world, to try to solve the problem.

Ok, we do have the problem of millions of babies. But since we have lots of time machines and people to do deliveries, it is just a matter of locating orphanages throughout history.

If only it turned out to be that simple. When all of us got back together, this time 150 million years in the past, we compared notes and found that history was different for every one of us. Every single trip resulted in a different timeline, with its own past.

Our efforts to research the future came up with an explanation, simple but terrifying. You thought that going back in time was like reversing a tape, going back to an earlier point in the movie, or something like that. But instead, while it reset the whole universe to the state in your recorded history, in your past, when time moved forward with the changes you made, everything happened in its normal, unpredictable, non-predestined fashion.

Think you didn’t make any changes? Simply being present, popping up from non-existence, has an impact on the universe.

How can you possibly be safe? You do more research, build a whole civilization in the ancient past, very close to the dawn of time, using massive numbers of duplicates of yourself, slightly offset in time.

Until you finally find a solution. One so simple, you wonder why it took you so long. What is the one thing you could change about the universe, which would make it safe from anyone changing your history?

Undo time travel. Make it impossible. Make it so time travel never happened, or will happen, or can happen, etc.

What are the odds of that?

This is the problem of infinities. If you have infinite attempts to solve the problem, then no matter what the odds are, as long as it is possible, it will eventually happen.

Let’s consider a simple goal: win the lottery!

You know the winning lottery numbers, so you go back in time and buy a ticket. But when you get back to the future, the ordinary, natural, unpredictable nature of the universe doesn’t make that future identical to what you remember. You changed it, by going back in time (or for that matter, just sending a message to the past). You would never have bought that ticket with those numbers, if you didn’t have information from the future.

The lottery numbers are entirely different. Why shouldn’t they be? If you roll dice, why would you expect the numbers to any less unpredictable, just because you wrote them down in the future? That future is the past. Your new future is just as unpredictable as if you didn’t travel in time.

How sure are we that random events in the real world are truly random?

Quantum physics has loads of experimental proof that the universe is truly random and unpredictable. We can only predict events by probability, never by certainty.

Fortunately, you can still win! All you have to do is just keep going back and forth in time, until the numbers randomly come up winners!

Do the odds matter? Not if you have infinite trips. Infinite tries makes any probability, no matter how small, a certainty.

It doesn’t even matter how hard you try. Enough time travel trips will eventually cause a universe in which time travel is never invented, by anyone. It is a possibility, and with infinite chances, any probability becomes a certainty.

When does it happen? You can only ask that question because you think that time only can proceed in one direction. Of course, the answer is simple. It already happened, will have already happened in the future, and will always have happened, before anyone even noticed it happening.

If time travel is possible, and the past can be changed, you can never trust what you know of the past.

Result, history is bunk, nothing ever happened the way you think it happened, and…

Time travel will be used to make time travel impossible.

Niven’s laws – Wikipedia

Novikov self-consistency principle – Wikipedia

All the Myriad Ways – Wikipedia – “Theory And Practice Of Time Travel” – Larry Niven

The End of Eternity – Wikipedia – Isaac Asimov

Case 2: History cannot be changed.

The paradoxes resolve themselves.

You already changed history. Therefore, whatever you do (or will do, will have done, or any new combination of strange tenses to reflect changing the past) is what already happened, even if you don’t know it.

This solves the problem of things happening out of order. You can visit the past, but that visit was already a part of the past. No travel to the past will ever happen, if it hadn’t already happened.

In this case for time travel, you don’t actually need to reverse time or change the state of the entire universe. Time can proceed in its orderly, forward only direction, for everyone. Well, except for any time travelers who go to the past, but that is much less of a complication than reversing or resetting time for everyone else.

The whole universe is a giant Groundhog Day loop. Nothing new ever happens. No one has any choice, no one is truly responsible for their actions. Free will is an illusion, and time travel reveals the illusion. Worse, it reveals that some force is causing actions to fulfill a specific history, even when a time traveler specifically acts to change things.

Because all time travel which is ever going to happen has already happened, it is possible to resolve all of it by the normal, forward flow of time. Well, almost, because any travelers to the past will have their personal time flow reversed, but everything else remains perfectly normal. Or at least, normal for this kind of time travel universe.

How does that work?

First, from the point of view of the traveler: On the 26th, we go back in time to the 10th. We spend 4 days doing important things, talk to past selves on the 14th, and return to the future, arriving on the 27th.

Second, in linear time, time proceeds for us normally until the arrival of our future selves on the 10th. At that point, our selves are effectively replicated, resulting in three versions. Why three? First, we have our current selves, who haven’t traveled in time. Second, we have our newly arrived future selves, who now exist simultaneously with us. Third, we have the future selves from before they arrived from the future, traveling backwards in time (from their point of view) to the future. They may exist in whatever space time travel takes place in, rather than the normal universe, but wherever they are, they still exist.

On the 14th, our two sets of duplicates briefly meet. The travelers from the future are still experiencing a reverse time flow, as they are still progressing towards their departure time. Our future selves depart, experiencing a forward flow of time as they also travel to the future, in time travel space, while our “normal” time selves remain to experience time normally.

On the 26th, the time reversed versions of our selves “arrive” at their departure point in time, at which point they disappear and cease to exist. As they are also the normal time versions of ourselves from that point in time, they also cease to exist. There remains our future selves, still traveling forward in time travel space, until they arrive on the 27th. They exit time travel, and proceed to continue living their lives in normal time flow.

Does this sound crazy, or too strange to be true? It is a simple consequence of everything in the universe having already happened, with no possibility of any change.

Gargoyles TV Series, “Vows”

Goliath: If I didn’t fear the damage you would do to the time stream, I’d *gladly* leave you here

David Xanatos: But you won’t, because you didn’t. Time travel’s funny that way.

— Sinclair to Zathras in Babylon 5:”Babylon Squared”

“I tried. I tried to warn them. But it all happened, just the way I remembered.”

Is this really funny? Everyone who time travels becomes a Cassandra – Wikipedia, able to know the future but unable to change anything, ever, no matter how hard they try. Worse than just knowing, they literally can do no action which would cause history to be changed.

Because everything is history. There is no future, only the past. Everything has already happened. We are just watching it, a sort of four dimensional movie, unable to make any edits or other changes.

Predestination (2014) – IMDb

All You Zombies – Wikipedia – Robert Heinlein

This movie is based on the story, and is a mind-blowing example of the effects of time travel in a universe where there ultimately is no way to change the past.

Let’s have an adventure!

After a terrible automobile accident, you find yourself in possession of a device which can let you travel into the past. You know exactly what you want to do: go back and stop the accident.

But when you get there, you are some distance from the location of the accident, so you drive quickly there to make it in time. A pedestrian steps out into the street, and though you swerve, you still hit them. A tragic, fatal accident.

You drive away in a panic, and realize the truth. It was you, going back in time, which caused the accident in the first place. So you go back to the future, and attempt to stop yourself from going back in time to stop the accident.

Except that no matter what you do or who you talk to, the accident still has happened in the past. You eventually find a person who you think supplied you with the time travel device in the first place. They explain what we already knew: time travel cannot change the past.

Unable to believe that there is nothing you can do, no choices you can make, which can change what will happen, you get a very big bomb, take it back to Germany before World War II, and set a timer for it go off when Hitler is giving a public speech.

But it doesn’t go off. Nor can you figure out way to detonate it manually. Frustrated by another example of time travel preventing from doing things freely, you travel into the far future to see if they have any greater knowledge of time travel.

Much of the future Earth is wasteland. You meet someone, who resembles the person who gave you the time travel device, and they tell you the history of the Doomsday War.

“It was a time of great global unrest and tension. A mysterious explosion destroyed a city in Germany. This triggered a devastating global response, bringing about the end of modern civilization. We survivors used what we learned to travel in time, to find a way to avert this catastrophe.”

“So why didn’t you stop it? I was the one who placed the bomb in Germany, but it never went off. Why did you give me the ability to travel back in time, if I was going to bring about the end of the world?”

“We cannot change history. Believe me, we have tried. It was your fate, your history to be given time travel and use it just as you have. It is pointless to be concerned about the consequences. Nothing and no one can change that!”

You scream, “This episode was badly written!”

That doesn’t make sense, and may not make you feel any better, but it is hard living in a universe where you can’t actually make any choices. Everything has already happened.

“Nothing really matters, to me…”

You think that someone is good or bad, based on the actions they take, right? Except that they never really had any choice in what actions to take. No advice, no event, no assistance, nothing anyone could do would ever make them do anything other than what happened. Or what was supposed to happen. Or what will happen.

Nothing can change the shape of things to come.

A universe where no one can actually make choices not only sounds like no fun, but it also doesn’t feel like it can be real. Time travel exposes this lack of reality. There is some mysterious force which prevents us from acting freely, and makes sure that “history” happens exactly the way it is supposed to.

But who is it who wrote this history? Who is the author? If the universe is like a movie where you can rewind and fast forward, but never change any scenes, where did it come from in the first place?

Time travel works fine in fiction. It never seems to run into these kinds of complications. A fictional universe wouldn’t have problems showing time travel.

The world has been called an illusion, and a simulation is a sort of illusion. If the universe were not truly real, would time travel be possible?

Case 3: The Universe Is A Simulation

Simulation hypothesis – Wikipedia

Rules? Where we’re going, there are no rules!

In a fictional universe, time travel is possible. So what if the universe is fictional? A simulation?

Simulations can either be perfect or imperfect. A perfect simulation cannot be distinguished from reality from within it. Imperfect simulations have flaws or limitations which make it possible for inhabitants to detect the artificial nature of their universe.ll

A perfect simulation can be absolutely perfect, with every detail of the universe modeled, down to the smallest subatomic particle.

A virtually perfect simulation, where the details need only be generated when an inhabitant examines them in close observation, while the model must still ha ve no detectable flaws when viewed from greater distances.

A general principle of simulations is that you cannot generate something more complex and detailed than the system it is created with. Thus, simulating our known universe would require a very large, complex — larger than our universe perhaps — system in order to create it. This isn’t impossible for an infinite universe, but would be a problem for finite space. Complex simulations must trade off size (don’t simulate everything), resolution (don’t make every detail, fake things if they won’t be noticed), or speed (who cares how many millions of years it takes to process one year of the simulation, if you are immortal and patient).

Our universe has a known size and known age, and presuming that this information is accurate, it imposes some very daunting requirements for any simulation. Over 14 billion light years in radius, and almost 14 billion years of running time.

It also has inhabitants. That is a key element of any simulation of a universe which is to be perceived as real. If there is no one living in it to notice, there is no requirement that the simulated universe be indistinguishable from “reality.”

Using an imperfect simulation is much, much easier to create, but much harder to make a stable, long term environment for curious inhabitants. Intelligent beings tend to explore and expand, and will find and detect limits.

Computers have a fundamental problem with simulations. They don’t do random numbers!

Extremis (Doctor Who) – Wikipedia

In this episode of Doctor Who, the repetitive pattern of supposedly random numbers tips off the inhabitants of the simulation, revealing its unreal nature. This is a real problem of a simulated universe. The bad guys may have made it a little easier by running multiple scenarios using the same pseudo-random seed, but it applies to any simulation which is entirely contained in a computer program, with no outside access. Computers do pseudorandom numbers, which means they are fake, false, not really random.

Why Computers Can Never Generate Truly Random Numbers

This flaw requires that any perfect simulation must have access to random data drawn from the real universe. This is how true random numbers are created by computers in our world.

Inhabitants can be emergent beings, created by the processes of the simulation itself, or they can be deliberately engineered (programmed) by the authors of the simulation.

It is also possible for inhabitants to be external beings, whether organic lifeforms or self-aware computer/simulated life. A creator of a simulation may wish to observe and interact with the simulation. But unlike native inhabitants, they would have the “problem” of being aware of the nature of the simulation.

In a Matrix sort of simulation, they might not be aware, but that has problems for our discussion here.

How do the inhabitants react when Time Travel is added to the simulation?

An easy method would be to just “load the save game file” for the date in the past you want to reach. Only problem is saving the universe. Literally. It is bad enough that you must have one active copy running of the universe. How much more do you need if you wish to access other points in time?

Do you need every year, every day, every hour, every second, every nanosecond? This pushes the storage requirements into the phenomenally cosmic power range.

Or you could just turn back time, literally, by running the entire simulation in reverse. How fast can the system change time, and how accurate is process? The need to record all of the random events and choices which happened could push the storage requirements up almost as much as storing many copies of the whole thing.

If the inhabitants are a part of the simulation itself, they will be changed by whatever process changes the entire simulation in time. Only any chosen travelers would be unaffected.

But if there are external inhabitants (as in the Matrix), they couldn’t be affected. Unless of course, you can reprogram them, but that complicates things and has its own moral issues.

Alright already!

However it works, we have a simulated universe with inhabitants who believe it is real, and one (and eventually more) inhabitant which has been given access to a Time Travel device (cheat code!).

What rules and limitations apply to time travel? While it is up to the Author of the simulation, it fundamentally falls into the two cases from before. Either the traveler can change past events, or they can’t. The end result is exactly the same as if it wasn’t a simulation. And either case above could be just another aspect of a simulation, not the real universe.

A time travel story with no rules.

You breathlessly step out of your time travel capsule, and run to your friend to tell them of your marvelous adventures and discoveries in time.

“You won’t believe what happened to me in the past.

“First, I went back to find King Arthur. I met Merlin, Lancelot, and a bunch of other knights. We found a unicorn for me to ride, and we went hunting a dragon!”

Your friend says, “Dragons and unicorns aren’t real.”

“They used to be. I saw them, they were real.”

“King Arthur probably wasn’t real either.”

“Well, neither were time machines, and I have one now. I was there, and I’m telling you, the past was totally different from the history books.”

How can this be? Because the past is just as fictional, or simulated, as the present. A few revisions, and history is whatever the books say, no matter what might have really happened. OK, this is “really” in the sense of what the maker of the simulation set up, not really as in the real universe.

But what do we do when we figure out that the universe isn’t real, but a simulation?

Same thing we do every night we find out the universe isn’t real. Try to take over the world!

Time travel is an instant giveaway that the universe isn’t real, if you paid attention to the examples of the two cases for time travel above. There are many others, of course.

Once you know that the universe isn’t real, the natural response is to take advantage of the situation. If you are the only one inside the simulation which knows the truth, you can surely find some way to exploit it. Time travel alone of course would be an amazing exploit.

Or else, as in The Matrix, you could try to escape.

Why would you want to escape a wonderful simulated universe?

Because you never know when some author or administrator might say “Freeze Program” or “Delete Character,” or otherwise use their phenomenal cosmic power to warp reality beyond your control.

Can you escape? It all depends on just how powerful the tools you’ve discovered to hack the system are. If the universe is fixed, unable to be changed, maybe you can just try to do as much damage as possible to get the attention of the administrator. I would think the system might. have some sort of way to detect people who try to beat destiny and change history, even once they know it is impossible.

But if you can change the past, you have an infinite resource of time changes at your disposal. Again, assuming there is some sort of active administrator, doing things which threaten to break the inhabitants’ acceptance of the reality of the simulation might draw a harsh response.

Or, if you are lucky, perhaps the author designed it to draw such a response. Maybe becoming “enlightened” about the nature of the universe could lead to a good reward.

Whatever happens, it is truly beyond your control. You must hope that the author is kind to their simulated inhabitants.

Result: Time travel is impossible in a real universe, therefore its presence in a simulated one reveals the simulation. Can the travelers find a way to the real universe, or a way to hack (Cheat Code!) the simulation?

OK, but even if it is a simulation, doesn’t this make time travel possible?

Nope, negative, that is an incorrect interpretation of the situation.

Since the universe isn’t real, neither is the time travel.

The Flight of the Horse – Wikipedia – Larry Niven

What about the universe the simulation is created in? Well, it gets the above choices too. If time travel is possible in it, then it is a simulation as well.

Somewhere, there must be a real universe, without time travel, in order for a simulated universe to exist.

There are two more scenarios for experiencing something which can look like time travel, but isn’t really. But before I address them, let’s stick with simulations for a short time.

Making Our Own Simulation

We are a long way from being able to simulate a universe like our own, if it is indeed actually possible to ever do so. But we can make some sort of simulations now, and could do better in the future.

If we make our own simulated universe, we can use it to experience a version of time travel. Set the holodeck to 1889 London, go through, that looks and feels like time travel, at least as far as our senses can tell. Same for our own version of The Matrix, or even a complete universe. Lie to any visitors about the nature of the simulation, and they may happily believe they have actually traveled in time.

Virtual and other sensory simulations aren’t the only kinds of simulations possible. Theme parks with actors reenacting the past, especially with the right kind of technology to support the illusions, could be very believable.

With sufficient technology, we could make our own, genuine Jurassic World! An artificial replica of the Earth of millions of years in the past, complete with dinosaurs and other life of the period. Drop someone there, tell them the spaceship was a time machine.

Think that making an artificial planet is too advanced, can’t be possible? Then I think you don’t grasp just how impossible time travel of any sort would be. Making a planet should be child’s play in comparison.

If we do make our own simulated universe, and either develop or allow to evolve free-willed, intelligent inhabitants, how will we treat them?

Video Game Cruelty Potential – TV Tropes

In our computer simulations we call games, we players frequently treat the programmed inhabitants quite badly. We can easily justify it, because we know they are not truly alive or intelligent. Merely a programmed, fake, simulation of intelligence and life.

But what happens if your simulated intelligences come close to acting like they are truly alive? Is there an easy boundary between programmed, fixed, non-intelligent behavior, and free-willed intelligent life?

Based on how synthetic thinking machines like robots and intelligent, talking vehicles get treated in science fiction movies and stories, humans might not care. After all, we have treated other humans in many cruel ways, even knowing that they are, logically, not that different from us.

How intelligent do you need to be to use a simulation of our entire universe? Humans have a hard time keeping track of the contents of a small room. Imagine someone whose awareness of the Earth was about the same as your awareness of your immediate surroundings.

Expand that to someone who can truly be aware, by name and focus, of every star in a galaxy. Go beyond that to someone who can do the same, but with all the planets and all the inhabitants of those planets in the entire universe.

Would such beings think that the degree of intelligence and free will that we humans show is real intelligence? Or would it be too little to count as true intelligent life?

Maybe, if we make a simulated universe, we should make sot me special effort to show kindness to our creations. Just in case our universe is actually a simulation, and its makers don’t take kindly to the cruelty of its “NPCs”.

OK, onward to more ways to not quite travel in time.

Circular Time Theory

This has all happened before, and it will all happen again.

Cyclic model – Wikipedia

The universe begins, time passes, and after a long time, the universe ends.

Then, everything starts over again. This cycle can repeat forever.

We are fairly certain that our universe had a beginning. We don’t know how or if it i will end. But one possibility is a “Big Crunch.” where all of time and space is collapsed by gravity into a single point.

If this were true, it would be possible to travel into a new universe, with a new version of the past, simply by somehow staying outside of, or surviving within, the collapse of the first universe.

Simple, of course, except for the problem of trying to find whatever new world is similar to your own, with similar history, in the vastness of an entire universe.

That assumes that uncertainty and randomness won’t make each new universe almost completely unique. If that happens, it could take a very huge number of repeats to get a universe which happens to have a world close enough to call yours.

In any case, even if you can pull off this, it still isn’t time travel into the past. It is just travel into the future, which just happens to have some location where history actually repeats itself.

Tau Zero – Wikipedia

This story is about surviving into the next cycle in a new universe, and not time travel per se. This method is rarely used in fiction.

I save the best for last. Given that we know that actual time travel into past is not possible within the real universe, we look to alternatives which give us an effect close enough to time travel to accept.

As in, alternative worlds which just happen to just like ours in the past.

Multiple Worlds Theory

Many-worlds interpretation – Wikipedia

Multiverse – Wikipedia

All the Myriad Ways – Wikipedia – Larry Niven, who explored these ideas and many more.

You must combine elements of these two in order to get the effect of time travel. Alone, neither one really offers you that kind of opportunity.

In the many worlds hypothesis, every random event always happens, in every possible way. The coin comes up both heads and tails. They just happen in different versions of the universe. The hypothesis provides no way to get from one of these to the other, but if it did, then you could travel to a world where events happened in a different way than we remember from our history.

The 2nd part is the multiverse element, being able to travel to other, parallel, universes. But multiple universes do not have to include versions which are similar to our past, or where the events and even the landscape (or planets) are the same.

Conservation isn’t a problem, if travel between all these universes means that they are all part of a single, larger, connected multiverse. You do violate it from the viewpoint of each universe, but the whole system remains balanced.

Sliders – Wikipedia

Avengers: Endgame (2019) – IMDb

In both of these stories, the concept of actual travel into the past is rejected, but traveling to another universe which is identical to a past version of ours turns out to be possible.

Sliders as a TV series spent most of the time exploring versions of Earth where it was the same time and date, but past events happened in completely different ways, but there were some episodes where they were able to find worlds where the passage of time was also different. Because they had no way, usually, to control their destination, the possibilities of exploiting it for a kind of time travel was not explored.

Avengers, on the other hand, did just that. But, unlike Sliders, it didn’t also find a way to explore other sorts of parallel worlds other than ones just like our their own past.

But in both cases, the problems of exploring multiple universes and the near infinite possible differences between them are not addressed.

How many different universes do you get, if every quantum randomization produces a new one? The number boggles the mind, as it vastly exceeds the number of all subatomic particles in the universe. Every day, every second, every nanosecond, every however small you want to slice up time, there is a new universe.

Multiple worlds, on the other hand, may be more restricted in number, because they don’t have to be formed from every resolution of random events in a universe. In fact, they may not have any relationship to the history of another universe at all. Of course, these kinds of universes are of no use for time travel. They merely get you to another kind of new world, not that much different from traveling to another planet within the same universe. At least, as far as the interactions of the population are concerned.

How many points in time can you reach? Even if, when you travel to a parallel universe which is identical to your past universe, you don’t plan to make any changes, your mere arrival itself is a change in that universe. Unless, of course, another version of you from another universe did the same thing, earlier in history, in your own, In any case, that parallel universe, offset in time, almost instantly becomes different from your own history.

Which makes you wonder whether you can avoid finding universes with different pasts from your own. Or, for that matter, how do you find your way back home, if the future of that universe is now different from your own present because of the changes you’ve made?

Does it even matter? Once you open the door to a near infinite number of alternate universes to explore, the range of exploits also becomes near infinite.

Let’s consider a simple “time heist”. You want to go back in time, and steal the Mona Lisa. You may even have a good motive, such as if some future disaster destroyed the original. So you go to an alternate universe, similar to the past, where it still exists, and steal it.

OK, that universe lost its masterpiece, but yours gets it back. But why stop there?

If you can steal it once, and there are near infinite times in the past which are accessible via this means, you can repeat this a near infinite number of times. Sure, perhaps those people in those other universes might be upset, but they aren’t your universe.

Obviously, this kind of inter-dimensional theft of resources has some terrifying implications. Even if many would be kind enough not to do such things, in a near infinite number of parallel universes, there would be more than enough to cause serious problems for other universes than their own.

After all, even if they are “real” for their inhabitants, they aren’t your real universe.

If there aren’t a near infinite number of such universes, the problem decreases, but in order to have any kind of controllable, useful, effect which feels like time travel, there has to be an extremely large number of them. Otherwise, many points in time wouldn’t be accessible.

What if you want to change the past? You can’t touch your own past, because you can’t go back in time for real. But if you go to another universe like your past, you can do what you like, then live in that universe and let time unfold in its usual way. You won’t ever get to a universe like our present, of course.

Why? Because uncertainty and randomness in all events will always yield a different future, even if you make no more deliberate changes.

As time travel stories go, this is a kind of cheat. Easy to pull off in a story, because all you need to do is accept that there are parallel universes identical to our past, and a method to travel to them is invented. Like alternative history stories, it is easy to imagine a “what could have been” scenario, and let it play out.

But it doesn’t really let you travel in time, as such, and you can never actually change your own, original, real universe.

Conclusions

The universe is real, the laws of physics are real, free will exists, and time travel does not. Our sense of reality and history are preserved.

That’s OK, because we are imaginative intelligent beings, and usually can tell the difference between reality and fantasy. There is nothing wrong with exploring stories of things which can never exist. Just look at the top box office movies for the last few years, or decades!

Or for that matter, popular fiction for forever. We have always told stories about fantastic things, which we are sure never really happened, simply for entertainment.

But what if we are wrong?

What if time travel actually is possible?

First, any of the many popular science articles about things which might make time travel possible don’t really do that. Even if there is some way for real world physics to support a closed loop in time, a circle which can only be entered from the past and exited from its endpoint in the future still doesn’t give you time travel.

Second, remember the conclusions above. It is possible that our universe is indeed a simulation. If that is the case, we already know what to do! Make plans for ultimate power, or escape. But the simulation hypothesis is not really likely to be true. Our universe is very complex, and if there is a larger, more complex universe out there able to support its simulation, it is also possible that there are many other universes of similar size and complexity which are not simulated.

If a simulated universe is perfect, with no flaws to reveal the simulation, with true random events rather than predictable pseudo-random number generation of algorithmic computers, then unless its makers deliberately reveal its false nature — say, by allowing the impossible to happen, such as time travel — no one within the universe will ever be able to discover that it is not real. And if you can’t tell whether it is real or not, it is both intelligent and wise to assume that it is, actually, real.

Still, if someone out there knows some “cheat codes” that work on the real universe, I’d be happy to take them!

Seriously, though, time travel in the future is only going to happen by faking it. The phony time travel mechanics make look and feel very real, but that doesn’t make them so.

If the starship Enterprise from Star Trek: The Next Generation showed up, and they brought us on board and said, “Go through this doorway. It can take you to any place in the past of your world, or any other.” The holodeck looks and feels real, and if it was set to, say, show Earth of 70 million years ago, you could see and perhaps even hunt dinosaurs. Just make sure it isn’t set to the Klingon safety protocols (where it is only fair that the holo-dinosaurs can eat you!).

If someone shows up in a blue wooden box, and tells you they can take you to anywhere in time and space, consider taking the offer. It could be fun. Maybe send me an invite. But if it looks like they have actually let you travel in time, ask them if they know that time travel is impossible in the real universe. If they know this universe isn’t real, maybe you can find your way to the real one.

Time Travel Story Rules

Time travel stories often work by putting limits on what you can do with time travel. Do too much, and it becomes harder to suspend disbelief and accept the story.

Only travel in time to reach the destination of the adventure. This avoids any concern about the effects of changing the past, or exploiting time travel during the story.

Avoid travel to any place where any of the travelers are present in the past.

Only allow one time travel device per story.

Don’t explain how or why time travel works, or where it came from.

Never cross the streams! Don’t allow future versions of a traveler to interact with their past selves.

Do not, whatever you do, have multiple competing time travelers making changes to each others’ past lives and history.

Avoid having major civilizations which have the potential for thousands, millions, billions, or more time travelers all changing the universe at will.

Doctor Who has been around for over 50 years, telling stories of time travel. Most episodes do stick to limits on what kind of travel can happen in each story. The Doctor never truly explains what the limits are, or how they apply.

Never Tell Me the Rules! – The Doctor, episode “Time Of The Doctor”

But if you are going to break them, make sure that the effect is worthwhile. The Rule Of Funny and Rule Of Cool are good, and doing something Awesome while you break the rules is even better. Most important, don’t leave an opening for such rule breaking to happen often, if at all, in future stories in the series.

The following video is a good example, from the 50th Anniversary Special Episode of Doctor Who.

It was the day when everything went wrong. The Doctor had no good choices, no good options. It was the last day of The War.

What war, you might ask? The last great Time War. The War to end everything. Your choices are everyone dies, mostly everyone dies, everyone good dies, the universe is destroyed but maybe some survive. Pretty much, there is no good ending for the war.

Almost none. The Doctor is a clever hero, and finds a solution. But it is still not a good choice, just one which is slightly better than the others.

All the combatants are in place around the Doctor’s homeworld of Gallifrey. There is a weapon which can destroy them all, guaranteed. But it won’t destroy just Gallifrey’s enemies, and the Doctor’s own people have taken extreme measures in the war, so if they persist in their plan for victory, the universe will be destroyed anyway.

But it can’t spare the innocent, the billions of children, and all the future generations of the Doctor’s people. It is a cruel weapon. But it is the only one the Doctor has which has a chance to work. There isn’t time for another solution.

It has a terrible downside. The user will survive its use, for hundreds, thousands, millions of years. The guilt for their actions will never fade, never leave them.

This event, this day, sets up the entirety of the 2005 Doctor Who series. It is mentioned in many episodes, and the Doctor’s responsibility and history play a major role. It is a fixed point in time, unchangeable even with time travel, with no possibility of undoing it, short of destroying the universe and starting over.

Which isn’t that great a solution, if you ask most people, and certainly the Doctor won’t put up with that.

Hundreds of years living with the guilt takes its toll on a person. Wondering if there was another solution, if only you had more time. Unfortunately, because of the time locked nature of the event, you can’t just go back in time and fix it. No, you’d have to do your fixing before it even happened.

For those not familiar with Doctor Who, the main character can regenerate rather than dying, taking on a new body (and a new actor on the show). In this episode, it starts with three incarnations being drawn together, but escalates beyond that point in this scene.

In a story, even one as epic and long lived as Doctor Who, these kinds of things can work. We just go on to the next episode. Nobody tries to use the amazing new powers of time travel and manipulating the universe for their own gain. The complications which real world users of power would apply just are ignored, for the purposes of telling a fun adventure story.

Some Notes And Comments

The simple answer is easy, and obvious if you know enough physics, but it doesn’t really get into the problems that time travel, if it existed, would cause for a real universe. The concept of “I wish I had never done that” being a real possibility is simply too interesting to ignore in fiction.

If we want a story where we interact with people from the past, especially famous characters from history, we need time travel. If we want to see a vision of our future, we need time travel. And if we want some mind blowing, paradox driven impossible puzzles to unravel, time travel does the job nicely.

It always works in stories, because the plot says so, and the story ends before the full implications of the situation can kick in.

Time travel in one of the big three impossibilities of science fiction mobility,

The other two are teleportation and faster than light travel. Both have no evidence of their existence in the real universe (as does time travel), and both seem impossible to actually achieve despite some far out hypothetical ideas.

Teleportation has a special place in movie and TV, because it is so easy to use. Set up a scene, film, stop the camera, let an actor take their place on the set, restart the camera, add some sound or effect and Presto! You can arrive and leave locations without having to show a vehicle for travel.

Faster than light travel is a Necessary Weasel – TV Tropes. Without it, travel across the stars takes years, and makes it impossible to tell any adventure story where the travelers interact frequently. If you set off on a 20 light year trip, leaving your baby behind, when you come back, you’ll have experienced only a few years, but not only will your baby be a 40+ year old adult, your mate will also be that much older. Longer trips are simply incompatible with human lifespans. This is the reality we live in, but it isn’t as entertaining in fiction.

Time Travel devices tend to have both teleportation and faster than light travel built into them. You can actually get there before you leave. So they add the problems with those concepts to the causality and conservation ones inherent to time travel itself.

But unlike time travel, if there turns out to be some way to teleport or move faster than light, it won’t have to break the laws of physics and render everything we know about the universe unreal.

Science fiction authors have grappled with the problems of time travel for a long time. I mentioned some greats like Robert Heinlein and Isaac Asimov, who certainly knew enough science to realize that time travel and reality weren’t compatible. But Larry Niven wrote an excellent article discussing the problem of time travel (and another on teleportation), and went beyond that to write a series of stories which explored the humorous possibilities in a world where time travel was possible, but only because reality was not.

All the Myriad Ways – Wikipedia – “Theory And Practice Of Time Travel” – Larry Niven