Fringe has always been full of riddles and mysteries, but now our favorite weird-science show is one giant paradox. How did Peter change history, and what are the repercussions? We may find out more in tonight's episode — but for now, here's a brilliant explanation of what we know about time travel in the real world, and how Fringe plays with the space-time continuum. Stephen Cass, senior editor with the Technology Review, takes us inside Fringe's time paradoxes, in an excerpt from the new book Fringe Science: Parallel Universes, White Tulips, and Mad Scientists, edited by Kevin R. Grazier.


Out of thin air, a man appears on a crowded commuter train-and is shaken, but not surprised, to see that everyone else in the car is now dead. He is Alistair Peck, MIT astrophysicist and time traveler. Peck soon finds himself being tracked down by Fringe Division, and the pursuit hinges on the rules and consequences of time travel in the fringe Universe.


Thinking about what the rules and consequences of time travel might be in the real universe has drawn the attention of some of the greatest scientists for over a century. The laws of physics, as we currently understand them, don't seem to forbid time travel, but they're not very clear on how real time travel might work, either. If we could really understand time travel it would mean understanding some of the deepest questions we have about our cosmos and its destiny. What is time? Is everything in the Universe predestined? Why do we feel ourselves existing in the instant of the present, ever moving from the past into the future?

To begin to get a handle on these questions, we're going to have to follow the example of Walter Bishop staring at the equations scribbled all over Peck's apartment . . .

WALTER: If I comprehend this correctly, then this Alistair Peck has taken Einstein's Theory of Relativity and turned it on its ear. I grasp portions of it.

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. . . and try to grasp a few portions of Albert Einstein's theories of Special and General Relativity ourselves.

Thinking very deeply about what time is and how we measure it was critical to the creation of Special Relativity. Special Relativity is probably best known for stating the relationship between energy and mass that is the principle behind nuclear reactors and atomic bombs, captured in the famous equation E=mc2. But more importantly for us and Peck, it also established the existence of a four-dimensional space-time continuum, where the three dimensions of space are inexorably intertwined with the fourth dimension of time.


Introducing the space-time continuum meant that for the first time, physicists had to consider that time travel wasn't just nonsense dreamt up by science-fiction authors, which must have made H.G. Wells feel a little smug. Wells published his book The Time Machine ten years before Einstein published his theory of Special Relativity in 1905. In previous time travel books, such as Mark Twain's A Connecticut Yankee in King Arthur's Court, characters were transported by some inscrutable bit of magic, but The Time Machine popularized the idea of the deliberate development of time travel technology, grounded in the scientific principles of a four-dimensional Universe.

Still, time travel in Special Relativity is very different from Wells' imagining. In The Time Machine, the traveler sits in a contraption that moves into the past and future while remaining in the same spot in space. In Special Relativity, time travel is directionally limited; you can go from the present into the future, but not back into the past. And you can't do it sitting still.


To understand why, let's look at what Special Relativity did to the concept of time. Before Einstein, the notion of absolute time was wildly popular in scientific circles, ever since Isaac Newton described it in 1687, alongside a few other clever ideas such as the Three Laws of Motion. Absolute time means that every point in the Universe is ticking along to the same clock. If ten seconds pass in Boston, then exactly ten seconds pass on the surface of Mars, too; time everywhere is in perfect sync, and how objects move through space has no effect on it.

This corresponds to our everyday experience. If you phone a friend, she will report experiencing time at the same rate as you, even if she is on the other side of the country, or traveling in a car (if she is in a different time-zone, her clock will read a different time than yours, but the rate at which the seconds tick by will be the same).


But according to Einstein, the time experienced by an object depends on how it moves through the space-time continuum-in other words, how you move through the three dimensions of space can affect how you move through time. In Special Relativity, objects moving at different velocities relative to each other (hence the name of the theory) have clocks that tick at different rates. Picture one object that's standing still and one object that's zooming through space. From the point of view of the still object (in physics speak, this object is said to be at rest), the moving object's clock will appear to be running slow, a phenomenon known as time dilation. (It's important to note that from the point of view of the moving object, its clock will appear to be running normally. A second of time will still feel like one second. It's only by comparing the moving and still clocks that the time dilation can be detected.)


The faster the object is moving, the bigger the effect, which is why we don't notice it when talking to friends on the phone-people simply don't normally move fast enough. But imagine an astronaut zooming past the Earth on a spaceship moving at 99.499 percent of the speed of light. For every ten seconds that ticked by for an observer (or Observer!) on the surface of the Earth, the astronaut would only experience one second of time. If the astronaut moved at 99.995 per cent of the speed of light, 100 seconds on Earth would pass for every second experienced by the astronaut. A century on Earth would be just a year of astronaut time. The astronaut would be traveling on a one-way trip into the far future (this is the fate that befell Charlton Heston and his crew in Planet of the Apes).

As odd as it sounds, time dilation is not just a theory-it's been measured in particle accelerators. Not for nothing did Peck's old MIT colleague Carol Bryce explain to Peter and Olivia that Peck had been obsessed with particle acceleration. In a particle accelerator, unstable subatomic particles can be boosted to speeds very close to that of light. Normally, at rest, one of these particles might decay in a thousandth of a second, but close to light speed, the measured lifetimes of these particles are increased many times over-time is running more slowly for them, relative to the time experienced by the physicists running the accelerator. Similarly for our astronaut, as she comes closer and closer to light speed, the slower and slower her personal clock ticks from an external perspective, until at light speed, time for her would stop completely. (Remember, time inside the spaceship appears to be moving normally; for her, it's the universe outside that's speeding up as she gets closer to light speed. She would never realize time had stopped, because you need time to perceive and think.)


So, if Special Relativity says we can slow and then stop the clock by going to light speed, could we start it running backward by going just a little bit faster than light? The logic is a little hairy, but if we could, then the result would be full-fledged forward-and-backward time travel-we could visit the past, not just the future. So, can we make an object's clock go backward? Well, yes, followed by a big no. First the yes, and back to Walter looking at Peck's equation-covered apartment. Noted Walter: "Tachyons are depicted here . . . "

Physicists call the hypothetical particles that travel faster than light tachyons (meaning "swift ones"), and it's true that any such particle would be able to travel back in time. But, for now at least, they are entirely theoretical. If they actually exist, these particles must be very different from the kinds of particles that make up you and me, which brings us to the big no for using Special Relativity for time travel into the past.


All the protons, electrons, and neutrons that make up the atoms of our bodies have one thing in common-they all have mass. It takes energy to increase the velocity of a particle with mass, and the bigger the mass of the particle, the more energy is required. Right here is where Special Relativity's E=mc2 (which states that energy is equivalent to mass, and mass is equivalent to energy) comes in to bite us in the rear. Making an object go faster-accelerating it so it's traveling, say, one extra mile per hour-increases its kinetic energy. And, because E=mc2, that increase in energy means that now the particle's mass has been increased, too. So, in order to accelerate the object by the same amount a second time, you need more energy than you did the first time, because now the particle is just that little bit more massive.


The mass/energy effect is minute and unnoticeable at everyday speeds, but it becomes painfully apparent as you get close to the speed of light-in fact, reaching light speed would require an infinite amount of energy, because the particle's mass would effectively become infinite, too. (Photons, which are particles of light, can travel as fast as they do because they don't have any mass of their own.)

Like time dilation, this increased-mass effect has been verified experimentally in particle accelerators and is the reason why particle accelerators grew from bench-top devices in the early twentieth century, through room- and then building-sized machines, until finally reaching the scale of the current Big Daddy in the accelerator game: the Large Hadron Collider, or LHC. The LHC straddles the border of France and Switzerland and takes the form of an underground ring seventeen miles around filled with superconducting electromagnets. The LHC is designed to pump huge amounts of electricity into the electromagnets in order to accelerate two streams of protons to 99.999999 percent of light speed and smash them together; at the moment of collision each proton will be 7,460 times heavier than the protons that are currently making up the nuclei of the atoms in your body.


Scientists are interested in accelerating particles to such speeds because collisions at higher and higher energies lead to more and more interesting physics that further reveal the fundamental forces and particles that make up our Universe. Pursuing interesting physics without the need for large amounts of pricey European real estate and a staggering power bill is presumably what first motivated Peck to-as Carol Bryce described it-find ways to accelerate particles without a particle accelerator.


So if Special Relativity forbids going backward in time by making crossing the light speed barrier impossible, are there any other possibilities? Well, proving that scientists like sequels as much as movie studio executives, it turns out that there are other possibilities, buried in Einstein's Relativity: Part Two. Properly known as the Theory of General Relativity, the follow-up to Special Relativity was published in 1915. (Today, scientists usually combine both theories under the single banner of the Theory of Relativity.) Where Special Relativity just dealt with the comparatively simple case of objects moving with uniform velocities, General Relativity includes acceleration, allowing the theory to handle more types of motion and incorporate gravity.

It wasn't immediately obvious that General Relativity had opened the door to the possibility of traveling back in time, so it was decades before physicists seriously began to consider the idea. Things really got moving in 1974, when Frank Tipler was studying solutions to Relativistic equations as they pertained to motion around massive, very long, rotating cylinders and realized something weird.


To understand what, let's first take a paragraph and look at how gravity works according to General Relativity. Every object that has mass produces a gravitational field. The more mass an object has, the bigger the gravitational field, which is why gravity on the Earth is six times stronger than the gravity on the surface of the Moon-the Moon is a lot smaller than the Earth. But why does this field produce an attractive force? Einstein showed that the attractive force arises because gravitational fields warp space. Think of an ant marching across a piece of paper. This ant is not the brightest member of the colony and only knows how to march in a straight line. Now, imagine bending or twisting the piece of paper (be careful not to squash the ant!). If you looked at the ant from above and ignored the paper, it would look like the ant was walking along curves or turning corners. But the ant is really still just walking in a straight line-it's the terrain that's curved. Similarly, it's the warping of space that causes things like planets to orbit a star or people walking around on Earth to feel a force pulling them toward the center of the planet. Everyone, and everything, is just trying to move in a straight line through the space-time continuum.

Now back to Tipler's cylinder. Around a rotating object, the warping due to gravity has the effect of constantly twisting space along with the object, like a piece of cloth getting wrapped around the axle of a car, an effect known as frame dragging. But remember that space and time are part of a continuum, so warping space affects motion through time as well. Tipler realized that the frame dragging effect would mean that if a spacecraft (I did say massive and very long cylinder!) flew a certain trajectory around the cylinder, it could go backward in time.


The spacecraft would follow what's known as a closed timelike curve, returning to the same point in time and space over and over-just like Peck, who can only return to specific times and places in his own past, looping back to an earlier point in the space-time continuum, whether it be a rush-hour commuter train or a field with a balloon he visited one afternoon.


So now we have the blueprint for a real time machine, right? All we need to do is convince NASA to build us a giant spinning cylinder in space and off we go. Unfortunately, Stephen Hawking shot the Tipler Cylinder down in 1992, proving that the cylinder had to be not just long, but infinitely long for the math to work out. But by then other physicists had been inspired to see if General Relativity allowed other methods of time travel.

In fact there have been a slew of proposed Relativistic approaches, which include finding flaws in the universe left over from the Big Bang known as cosmic strings (these are different to the subatomic strings that feature in String Theory). These very long cosmic strings would be incredibly dense and consequently would greatly warp space. In theory, if two such cosmic strings passed close to each other, a spacecraft could fly along a closed timelike curve around both strings to travel back in time. But perhaps the most potentially practical approach is that of Kip Thorne, who, along with some colleagues in 1988, published a paper showing how a wormhole could be turned into a time machine.


Thorne's work is why, when Carol Bryce said that the goal of Peck's particle acceleration research was to create wormholes, Peter immediately guessed that Peck's work was related to time travel. Later, Walter illustrated the basic wormhole idea using a piece of paper with a line drawn on it, marked with two times, 10 a.m. and 11 a.m.: he bent the paper over so that the timeline looped back on itself and the 10 and 11 a.m. points coincided. But notice that there was still a problem-in the two-dimensional world of the surface of the paper, the two points were not yet connected by the folding-once Walter released his hand the two points would fly apart. If you want to move from the 11 a.m. point to the 10 a.m. point, you first have to find a way to fuse, or build a stable bridge between, these two spots of the space-time continuum, creating a closed timelike curve.

That's just what a wormhole is-it's a shortcut, a bridge between two points in the space-time continuum that would otherwise be far apart. Wormholes are typically thought about in terms of joining two points that are physically very distant (for example, as seen in the Stargate movie), but Thorne showed how they could be used to connect points that are far apart in time rather than space (a dodge occasionally employed in the Stargate TV spin-offs).


This is how it would work: assuming you somehow managed to create a wormhole in the first place (a short one, for convenience), you then take one of the ends of the wormhole-you can move the end of a wormhole around by pulling it along with an electrical or gravitational field-and accelerate it to near light speed. Time dilation will slow down the rate of time at the moving end. If you can't travel fast enough, General Relativity says you can also use gravity to produce the required dilation-gravitational fields cause time to slow down. So by dangling the end of the wormhole in a high gravitational field, such as around a black hole, you would cause time to run more slowly at that end. Pull the end out of the gravitational field (or return it from its high-speed trip through outer space), and any time dilation effect will stop. Time will resume ticking at the same rate at both ends of the wormhole.

But because one end spent a chunk of time with its clock running slow relative to the other end, you've introduced a permanent time difference between one end and the other. Say, that using time dilation, you establish a one-hour time difference between the two ends of the wormhole, which you put in opposite ends of the same room. You turn up at 11 a.m., go through the end that wasn't subjected to time dilation, and pop out on the other side of the room at 10 a.m.


So all we need to do is create a wormhole, right? Usually, such a construction project is imagined as being the province of super civilizations, like the aliens that send the instructions for building a wormhole machine in the book and movie Contact. But in a few years, depending on whether or not certain theories turn out to be right, human beings could be creating wormholes at the LHC-microscopic ones, but wormholes nonetheless. In these theories, the energy of the collisions could be enough to warp tiny pieces of the time-space continuum, so much so that they are twisted into the shape of a wormhole. But the big problem with wormholes is keeping them open.

Any normal wormhole, of the sort that might be created by the LHC, would be so unstable that it would collapse the moment a single particle entered it-there wouldn't be enough time for the particle to make even a one-way journey. But Thorne and colleagues have proposed that the wormhole could be stabilized and become a traversable wormhole.


For this trick, Thorne turned to quantum mechanics, invoking something known as the Casimir effect. Quantum mechanics tells us that there's no such thing as perfectly empty space. Instead, even the emptiest void in deep space is a foaming mass of subatomic particles and waves, popping in and out of existence. We don't normally notice these virtual particles all around us because they only exist for a very short period of time, but there is a way to feel their presence. Two metal plates placed a millimeter or so apart will exclude long wavelength virtual electromagnetic fields. So, on either side of the plates you have long and short wavelength electromagnetic virtual fields, while inside the plate you just have short wavelength virtual fields. The result of this imbalance creates a small but measurable pressure that pushes the plates together. Thorne and his colleagues believe that this same technique might be used to stabilize a wormhole: the quantum low pressure produced by the Casimir effect could act to pull open the mouth of the wormhole, preventing it from collapsing.


In the laboratory, flat metal plates are used to demonstrate the Casimir effect. But to stabilize a wormhole, you'd need to enclose the entrance in three dimensions. Many times in fiction, the entrance of a wormhole is depicted as a two-dimensional portal that people step through, but an actual wormhole entrance would be a three-dimensional volume of space: a sphere instead of a circle. This, perhaps, is why Peck implanted a conductive mesh underneath his skin. Walter identifies that as a "Faraday Mesh," but what he was talking about is more often called a faraday cage. They take their name from their inventor, nineteenth century British physicist Michael Faraday, who made key early discoveries about electricity and magnetism. Faraday cages are containers made of a conducting solid or mesh, usually metal: an electromagnetic wave can't penetrate the mesh if the holes are a good bit smaller than its wavelength. They're often used to shield electronics from either producing or receiving electromagnetic interference, as when the alternate Brandon demands a Faraday cage be built before turning on the Doomsday machine in the episode "6:02 a.m. E.S.T." (3-20).

Nearly all of us have a Faraday cage in our homes, in the form of the metal mesh in the door window of a microwave oven. Microwaves and visible light are both electromagnetic waves, but the microwaves used in an oven have wavelengths close to five inches long, while visible light has wavelengths measured in a few hundreds of thousandths of an inch. So visible light passes through the sixteenth-of-an-inch holes in the oven door mesh, but the microwaves are blocked.


A Faraday cage would block virtual electromagnetic radiation, too. Perhaps Peck was using the mesh to stabilize the entrance to a wormhole he was opening within himself, expanding the volume of the opening until his body could fit inside the entrance and he could then move through the wormhole to the past. (That Peck did not have to cover his head and hands in mesh we can attribute to either artistic license or some details of wormhole physics that he alone was privy to.)

But what would happen if you really could go back in time? It could lead to all kinds of problems, under the broad heading of causality violation. Let's go back to our room, with its one-hour wormhole time machine . You go through at 11a.m.and emerge at 10 a.m. What if you chose to hang around for an hour after going through the wormhole? Just before 11 a.m. you'd see the earlier you come into the room and get ready to step through the wormhole. What if you stopped yourself from going through? If you stopped yourself going though at 11 a.m., then how did you get back to 10 a.m. to wait around for yourself? In other words, how can you have an effect (arriving at 10 a.m.) if you eliminate the cause (going through the wormhole)? This specific problem is known as the grandfather paradox, typically told as the story of a time traveler who goes back in time and shoots her grandfather before he has any children, thus preventing the birth of the time traveler.


Stephen Hawking figured that causality violation would cause such difficulties with the basic operation of the Universe that in 1992 he proposed the Chronology Protection Conjecture. This conjecture states that the laws of the Universe, known and unknown, must fit together in a way that makes time travel impossible. Just like faster-than-light travel, or Tipler's cylinder, every time travel strategy that gets dreamt up will prove to have some fatal flaw upon closer examination.

Other scientists believe that causality violation can be avoided if time travel instead follows a rule known as the Novikov self-consistency principle, developed by Igor Novikov in the 1970s. This states that the laws of the Universe don't forbid time travel, they just forbid paradoxes. If you arrive at the one-hour time machine to find yourself waiting for yourself, then it is guaranteed that you will go through the wormhole on time. Any effort to stop you made by the later version of yourself will inevitably be doomed to failure: If you try to shoot yourself with a gun, you'll miss, or the bullet will be a dud. If you try to physically block yourself from going into the wormhole you might trip and fall, or have a heart attack; you can no more stop yourself going through than you can walk to the Moon.


At first glance, the Novikov self-consistency principle may seem to be saying that even if time travel is possible, it's impossible to alter the past in any way. Wouldn't any change, however small, cause ripples that would eventually produce a paradox?

But there may be many ways the past could be altered and still produce the same present, at least from the point of view of the time traveler. Imagine in the one-hour time machine room you keep a box. At 9 a.m., a robot puts a set of blocks into the box in some pattern-a cross, say. The box has the feature that the pattern of the blocks can be changed just once, before the pattern is locked. If you come in at 11 a.m. and see the cross pattern before you go through the time machine, then it's guaranteed that whatever you do at 10 a.m., you won't change the pattern. That part of the past is fixed. But now let's imagine the box has a lid, so when you come in at 11 a.m., you can't see the pattern. Now when you go back to 10 a.m., you're free to change the pattern and alter that part of the past or not-either way, what you saw before you entered the time machine (the closed box lid) is the same, and so both possibilities are consistent with the Novikov principle.


This is what the Walter of 2026 is trying to explain to Peter in the third-season finale, "The Day We Died" (3-22):

WALTER: I know the pieces [of the Doomsday machine] were buried millions of years ago, but how did they get there? So deep in the past. But now I understand. I sent them there. The wormhole in Central Park. I sent them back through time. Peter, you can stop the destruction before it occurs. PETER: If that's the case just don't send the machine back. Then we'll never discover it and I'll never destroy the other universe. WALTER: No. It doesn't work that way. I have already done it. Therefore, I have no choice but to do it again. PETER: Walter, that doesn't make any sense. WALTER: It does. It's a paradox. I can't change what happens because it's already happened, but you can make a different choice within what happened.


What Walter means is that by 2011, he believes that the Fringe Division has done the equivalent of seeing a part of the pattern of blocks from the one-hour time machine example above. That part of the pattern-discovering pieces of a machine that had been sent back through time-is now fixed, not least because it's implied that Walter needs the machine to send Peter's consciousness from 2026 back to 2011. If someone did stop the machine from being sent back, it wouldn't be available for Walter and Peter to use to warn 2011. Stopping the machine from going back could also create a new timeline starting 250 million years ago, the era that the wormhole in Central Park opens to. This would probably make 2011 unrecognizable, depending on how much impact the machine has had in the Fringe backstory (remember, the Sam Weisses have been around for generations!). For 2011 as we know it to exist, the machine must go back. Once Peter goes back in time, though, everything else since 2011 is up for grabs. It's true that this would mean that the people of 2026 would have their timeline replaced by a new history, but even if they knew about it, I doubt the 2026ers would object-their world is about to end, after all. Just so long as someone in some future sends the machine pieces back in time, history will remain consistent with the existence of the 2011 Fringe Division.

The Novikov principle is at the heart of another type of real-world time machine, one that doesn't rely on using Relativity to twist the space-time continuum into knots. It relies on quantum mechanics, and, appropriately enough, was proposed by a real-life MIT professor, Seth Lloyd, late in 2010.


MIT was a good choice for Peck's home institution (and I'm not just saying that because as an editor for Technology Review, I'm also an MIT employee!). Scientists at MIT have worked on the issues of time travel for decades, and even the students here are pretty time travel savvy: in 2005, they organized a Time Traveler's Convention, publishing the time and location in as many places as possible in the hopes that time travelers from the future would come across them and journey back in time to the convention. Sadly, no travelers arrived-or if they did, they stayed incognito.

Lloyd's time machine relies on the way quantum mechanics describes things like particles. In quantum mechanics, each particle is associated with a wave function. The wave function gives the probability of a particle being located at a particular point in space when you measure its location. Imagine calling someone on their cell phone to find out where they are-they might have an 80 percent probability of being at their desk, a 10 percent chance of being in a meeting, a 5 percent chance of being in the elevator, and so on, but you won't know exactly where they are until you call. Now here's where it gets weird-for something like a person, regardless of whether or not you call them to find out where they are, they are always still in some one specific location.


But according to quantum mechanics, before you look to see where a particle is, it is, in a sense, in all the places it could be, simultaneously. It's only when you make a measurement that a single location for the particle is selected, with the probability of the location given by the wave function (taking a measurement is known as collapsing the wave function). Once you've stopped observing the particle, a new wave function emerges. Just exactly how wave functions collapse is still the topic of a great deal of research and speculation. But it is established, through experiments, that it's possible to use wave functions of multiple particles to reinforce or cancel each other out, enhancing or eliminating the possibility of finding particles at a particular spot. It's even possible to make a single particle's wave function interfere with itself in this manner.

More weirdness: before it is collapsed, the tail ends of a wave function that describes a single particle never quite fade away to nothing, infinitesimally extending off toward infinity, at least until you try to observe it. This means that there's always a tiny chance that a particle could suddenly appear a long way away from where it's supposed to be when you do try to observe it, a process known as quantum tunneling.


Lloyd and his colleagues reasoned that if the wave function effectively smears the existence of a particle across space, maybe it smears it in time, too. If particles can tunnel through space, why not time? We don't normally detect, say, a particle in an accelerator appearing before the collision that created it, because most of time-tunneling events would violate the Novikov principle in some way and the wave function would get cancelled out. But Lloyd realized how the Novikov principle might be used to create the ultimate computer, capable of performing computations instantly-in essence, he set up a (theoretical) system where all the wrong answers to a problem become the equivalent of grandfather-murdering time travelers and their wave functions get cancelled out. The right answer is the equivalent of the time traveler who spares his grandfather and therefore has a wave function that can go back in time without being cancelled out. By seeing which wave function survives its quantum tunneling to the past, the answer is revealed.

But Peck doesn't seem to follow either Hawking's or Novikov's rules. He's built a machine that allows him to traverse closed timelike curves, breaking the Chronology Protection Conjecture, and then he creates a grandfather paradox, when he chooses to die with his fiancée rather than trying to change time to save her life.


This implies a view of time that's similar to what's described by Brandon in the episode "August" (2-8):

BRANDON: We think of time as linear, right? Life is a journey. You're born, and then you die. And to get from one end to the other, there's only one way through. [Brandon demonstrates by holding up a tube that's open at both ends and pouring water through it.] Unless you look at it like this. [Brandon refills the tube with water and then traps the liquid inside the tube with his fingers.] Then, you can see any point. It's all happening at once.


In this view, every instant of our Universe's existence, from the Big Bang to the big crunch, or heat death, or whatever other fate may be in store for the cosmos, exists simultaneously with every other moment, like frames coexisting along a strip of movie film (or, for younger readers, like a video file sitting on a hard disk). Our consciousness is like playing that movie film or video file: we see time passing from moment to moment, but the past doesn't cease to exist, and the future already exists-we just haven't gotten to it yet.

This may sound like saying that, like a movie, the past and the present must therefore be fixed. But not necessarily. To see why, a few years ago I created a little computer simulation to explore the grandfather paradox. This simulation creates a very simple and short-lived artificial universe. In this universe, a particle is set up to go through a time machine, return to the past, annihilate the earlier version of itself, and continue on its way. The simulation displays the entire history of the artificial universe as a series of panels, like a comic book, calculating what the entire state of the universe should be at each moment before moving on to the next. So how does the simulation handle the grandfather paradox?


When the simulation is started, the particle goes toward the time machine unimpeded, and then vanishes once it reaches the time machine. The universe continues on into the future without the particle. Once the simulation has finished calculating the entire history of the universe, it goes back to the beginning and starts again. Again, the particle begins to move toward the time machine, but this time, its future self emerges from the machine and "kills" the earlier version of the particle. The simulation runs through the rest of the history of the universe with the future version of the particle going its own way. But the next time the simulation begins running through the history of the universe, everything returns to the way things were during the simulation's first run, where the particle doesn't meet its future self and goes through the machine. The grandfather paradox forces the simulated universe to oscillate back and forth between two possible histories. Note that from within the universe, no oscillation appears to be happening-each timeline has a complete history: but an observer from outside the universe can easily see that two histories are swapping back and forth.

My simulation is very basic and has a number of drawbacks-for one, it uses absolute time, dividing the history of the universe into slices according to uniform ticks of a cosmic clock. And Stephen Hawking would consider it utterly ridiculous, because violating the Chronology Protection Conjecture-allowing a time machine-is built right into the simulation's rules. But the point is that this computer simulation hints at a way that the past, present, and future could all exist simultaneously, and yet still be changeable without paradoxes or the Novikov principle.


One could even imagine that in the real Universe and its lack of absolute time, rather than the entire history of the universe being changed instantly at the moment an alteration is made, changes would propagate down the timeline, like a ripple on a river altering the surface of the water, moment by moment. One could even imagine a series of changes, each working their way along the history of the Universe at the speed of time, creating a number of alternate histories that would all coexist along the timeline, like a chain of boats bumping down a stream. Perhaps this is what the Observer September meant when he said to Walter in the episode "The Firefly" (3-10), "Various possible futures are happening simultaneously."

In this picture, each particle (or time traveler) carries with it its own history that's independent of the rest of the Universe. This would be how Peck was able to send Walter a drawing of a white tulip-despite it coming from a future that would no longer happen from the point of view of Fringe Division. The new history Peck created-the one where he died with his wife, and the team never investigated the mysterious deaths of train passengers-overwrote the original timeline, second by second. But all that's important to the laws of physics-and to Walter's search for a sign of forgiveness-is that, once upon a time, that future did happen.


Originally from Ireland and now based in Boston by way of Brooklyn, Stephen Cass is a senior editor with Technology Review, working just across the river from where Fringe Division's headquarters is supposed to be. One of his latest projects is Technology Review's The Best New Science Fiction. He has written about space, physics, and technology topics for over ten years for various publications and was the founding editor of Discover magazine's Sci-Fi blog Science Not Fiction.