While the idea of time travel gives rise to discussions of topics ranging from science fiction to ethics, understanding the possible effects of time travel gives us a better grasp of the foundations of general relativity and quantum theory. Most work in the area has focused on the theoretical aspects of time travel, but there are also attempts to simulate the effects of time travel experimentally.

In physics, a time machine is known as a closed timelike curve (CTC). Basically, an object makes a loop through spacetime to interact with its past self. In a recent work published in Nature, a team simulated the possible effect of a time machine using polarized light. Since they couldn’t actually make a beam of light travel back in time, they used two separate beams of light, with one beam mirroring an earlier state of the other. Their focus was to study how quantum computers might be affected by a CTC.

The DWave chip is promoted as a quantum computer. Credit: DWave

Quantum computers use the fuzzy aspects of quantum mechanics to perform calculations. Rather than discrete bits of 0s and 1s, a quantum computer uses quantum states or q-bits. The challenges of quantum computing are huge, but they have the potential to perform some incredibly difficult computations with relative ease. In the early 1990s, David Deutsch demonstrated that if a CTC is self-consistent on a quantum level, then quantum computers could solve computational problems known as PSPACE-complete. In other words, it would be the supercomputer of all supercomputers.

Deutsch’s model is controversial because it relies on an interpretation of quantum mechanics that invokes “parallel universes.” And without a real time machine, his ideas are impossible to prove. For this simulated time machine, the team tweaked the states of their light beams to see what results they could get. They found that the results were self-consistent as Deutsch proposed, and they also completely agreed with relativity. This doesn’t mean that Deutsch is right, but rather if Deutsch is right the effects would work as he claims. There are other quantum models that would also prevent time-traveling paradoxes, but wouldn’t allow for the construction of a super-duper supercomputer.

The results of this work aren’t particularly surprising, but it’s an excellent demonstration of just how subtle and sophisticated optical experiments can be. And until someone is able to make a real time machine, simulated time machines like this one are the only way we can study time travel experimentally.

Paper: Martin Ringbauer, et al. Experimental simulation of closed timelike curves. Nature Communications 5, Article number: 4145 (2014)

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In the discussion of hypothetical time travel, someone almost always brings up Hitler. After all, if you’ve got access to a nuclear DeLorean or blue police box, who better to eliminate from the annals of time? In internet conversations, Godwin’s law is the observation that in any online argument someone will eventually bring up Hitler to make a point. “Yeah? You know who else liked vanilla ice cream? Hitler!” So when the New York Times decided to poll readers on whether they would kill baby Hitler, they enacted the Godwin’s law of time travel. It was only a matter of time.

The idea of killing Hitler is such a common trope in science fiction that the phrase “Everyone kills Hitler on their first trip.” has become a meme. From a physics standpoint, the challenges of killing Hitler are almost as big as the challenge of time travel itself. As I wrote about in an earlier post, there are metaphysical problems with the physics of time travel, and those would affect whether you could or should kill Hitler to prevent WWII.

One of the problems is the grandfather paradox, where if you go back in time to kill your grandfather before he has children, you wouldn’t have been born and couldn’t have killed your grandfather. The same paradox occurs with killing Hitler. If you eliminate baby Führer and prevent the rise of the Nazis, then you create a world where WWII didn’t occur, and thus you have no reason to travel back in time. Thus, Hitler survives to adulthood and we’re back where we started. Acting as a time-traveling executioner simply creates a paradox.

If time travel is self-consistent, then you can’t change history. You could, however, travel back in time to cause Hitler’s rise to power. So that’s not a good idea.

What about the many-worlds idea, where traveling back in time creates a parallel universe? In that case eliminating Hitler would create a new timeline without Hitler, but the old timeline would also still exist. Your time tripping does nothing to eliminate the pain and suffering of the original timeline. It might also create a new timeline with even more pain and suffering. After all, if Hitler didn’t rise to power, who’s to say that someone worse wouldn’t replace him?

So overall it seems traveling through time to kill Hitler is inconsequential at best, and could be downright harmful. That whole period of history is probably best for novice time-travelers to avoid.

I hear 1985 is nice.

This post originally appeared on Forbes.

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According to Back To The Future Part II, October 21, 2015 was the day Doc and Marty arrive from the wonderful world of 1985. They’re greeted by a world of flying cars, hoverboards and self-lacing shoes.

As far as time travel is concerned, making the journey from 1985 to 2015 is easy. Many of us took the long road, traveling at the usual 1 second per second through time. But we know it’s possible to make the journey in less time. A central property of special relativity is time dilation, where an object moving at some speed relative to you will appear to experience time at a slower rate. We see this effect in particle accelerators, where unstable particles moving at high speed decay at a slower rate than ones at rest relative to us.

Using time dilation, you could in principle travel away from Earth at high speed and return 30 years later. If you traveled at speeds approaching light, then your journey might only take a month for you while decades pass on Earth. The power required for such a journey is far beyond the total energy production of Earth, but it’s possible within known laws of physics. Traveling forward in time is easy.

When folks talk of a time machine, however, they mean one that can travel backward in time. We want to be able to visit key moments in history, watch dinosaurs walk the Earth, or accidentally prevent our mother from dating our geek of a father. While it’s mostly just a fun plot device, time travel has been studied by physicists in great detail. Not necessarily with the goal of building such a machine, but rather to explore the theoretical possibilities of general relativity. What they’ve found is that backwards time travel is extremely difficult at the very least, but not necessarily impossible.

In physics a time machine is known as a closed timelike curve. Objects move through time and space, but can never travel through space faster than light. Any path through spacetime that obeys this rule is known as a timelike curve. If a timelike curve could somehow loop back on itself (if we could meet our younger self for example) it would be a closed timelike curve (CTC), hence a time machine.

It turns out that CTCs are possible within general relativity. In 1949 Kurt Gödel found a solution to Einstein’s equations with CTCs. It described a universe that rotated, and the rotation caused some timelines to loop back upon themselves. While Gödel’s universe is a mathematical solution to general relativity, it isn’t a physical one. The real universe doesn’t rotate the way Gödel’s model does. But Gödel showed that CTCs were at least theoretically possible within relativity, and so other solutions have been explored.

Wormholes and warp drive could be used to make a time machine. Credit: Wikipedia user Raude. CC BY-SA 3.0.

We now know that anything that allows travel faster than light could be used to create a time machine. If we had warp drive, then we could use time dilation and warp drive to create a CTC. The same is true for wormholes that allow us to create shortcuts across cosmic distances. As far as we know neither of these exist, and it seems the speed limit of light is also a rule against time travel.

But even if we assume time travel is somehow physically possible, there are also metaphysical problems with time travel. The most famous one is known as the grandfather paradox. It was first proposed by science fiction author Nathaniel Schachner in 1933, and is alluded to in the Back To The Future series. Shachner’s paradox had a time traveler journey to the past and kill his grandfather before he married and had children. Since this prevented the time traveler from being born, he couldn’t have killed his grandfather, hence the paradox. In Back To The Future, Marty accidentally prevents his parents from going to the school dance, thus putting his very existence in danger.

Marty introduces Rock and Roll to Chuck Berry, but Marty first heard it from Berry. Credit: Universal Pictures

Interestingly, the grandfather paradox is easily resolved in general relativity through what is known as the Novikov self-consistency principle. If we assume that (somehow) CTCs are possible, then the Novikov principle requires that such time loops be self consistent. So you could travel back in time in an attempt to assassinate your grandfather, but only end up wounding him. He’s rushed to the hospital, where the attending doctor is none other than your grandmother. Your time-trip caused them to meet, and hence you were born, which is perfectly consistent. According to the Novikov principle, it would be physically impossible to create a paradox. This self consistent approach was used in that other 80’s time travel movie, Bill & Ted’s Excellent Adventure.

In the many worlds model any quantum outcome leads to both results. Christian Schirm.

Back To The Future uses a different take on time travel, specifically that traveling back in time would spawn a new timeline. Even though Marty reunites his parents by the end of the first film, there are small differences when he returns to 1985. As the trilogy progresses a wide range of timelines are formed (up to 21 depending on how you count them). This draws upon what is known as the many worlds model.

In quantum theory, the measurement of a quantum object leads to an outcome that is probabilistic. Rather than a deterministic cause and effect, as seen in Newtonian physics, quantum theory can only give us the likelihood of a certain result. In an effort to bring determinism to quantum measurements, Hugh Everett proposed a “relative state” interpretation of quantum theory in 1957. In this interpretation, all outcomes of a quantum measurement occur, but the act of measurement causes the outcomes to separate from each other. Basically, a quantum measurement causes the universe to split into separate “universes,” each with a different outcome. While Everett’s model isn’t that simple, the idea that events or choices can split the universe has become a popular trope in science fiction. The idea of time traveling between parallel worlds isn’t plagued with the paradoxes of single-universe time travel, but it relies upon a loose interpretation of an unproven model.

Because of the challenges and paradoxes of backwards time travel, most physicists don’t think time travel is possible. All the metaphysical problems go away if a time machine is simply unphysical, and in physics the simplest answer is usually the right one. So perhaps we should focus on things that, though highly unlikely, are in principle physically possible. Such as the Cubs winning the World Series.

Paper: Gödel, K. (1949). “An example of a new type of cosmological solution of Einstein’s field equations of gravitation”. Rev. Mod. Phys. 21 (3): 447–450.

Paper: Friedman, John; Michael Morris; Igor Novikov; Fernando Echeverria; Gunnar Klinkhammer; Kip Thorne; Ulvi Yurtsever (1990). “Cauchy problem in spacetimes with closed timelike curves”. Physical Review D 42 (6): 1915

Paper: Everett, Hugh (1957). “Relative State Formulation of Quantum Mechanics”. Reviews of Modern Physics 29: 454–462.

This post originally appeared on Forbes.

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In fiction there are lots of ways to travel through time, from reaching 88 miles per hour in a DeLorean to walking through a circle of ancient Scottish stones. While the idea of time travel is centuries old, it had no basis in reality. But when Einstein developed his gravitational theory of general relativity, some physicists began to wonder if time travel might actually be possible.

One of the central ideas of general relativity is that there is no cosmic clock. While we always experience time at a normal rate, our rate can seem faster or slower relative to another observer. The relations between space and time are distorted by the presence of mass, and for objects such as black holes that distortion can be extreme. So could time be warped in such a way that it loops back on itself? In the 1970s, some physicists began to study this question in a serious way. Not as a serious effort to build a time machine, but as a way to explore the broader aspects of relativity.

A light cone representing the “sphere of influence” around a point in spacetime.

One of the ways we can study the behavior of space and time is through what are known as light cones. Imagine a flash of light at a single point. The light would travel away from that point in all directions, so that the flash of light is an expanding sphere. Since nothing can travel faster than the speed of light, then that expanding sphere marks the limit of your influence. In general, you can’t move outside that sphere, because you can’t travel faster than light. Since it’s difficult to visualize 4-dimensional spacetime, we often imagine space as a two-dimensional surface, like a sheet of paper, and time as being the third dimension. You can imagine it as a stack of paper, with a cartoon drawing on each page, then flipping through the drawings would make them appear to move. Each sheet of paper would be a moment in time, and different pages in the stack of paper would be different moments in time. The only difference is that time is a continuum rather than discrete instants. In this visualization, an expanding sphere of light would start at a point, and with each higher page would be a larger and larger circle. If you flipped through the pages you would see an expanding circle of light. If instead you could see through the paper and just look at the circles, then you would see they form a cone starting at a particular point. This is known as a light cone, and it defines the limit of influence of an object at the point. An object at the starting point cannot influence anything outside of that light cone, nor can it ever move beyond the limit of that light cone.

Lightcone path of a CTC vs normal time. Credit: Ben Tippett and David Tsang

The path of an object that stays within the light cone is called “timelike,” since it follows the motion of the object through time. Relativity requires that the path of all objects be timelike. So in general relativity the question becomes whether light cones can be warped in such a way that the timelike path of an object connects with its own past. If you found yourself in such a wibbly-wobbly spacetime it would be possible to meet your younger self. Such a loop is known as a closed timelike curve (CTC), and would be an actual time machine.

A CTC in a Gödel universe.

Interestingly, CTCs are absolutely possible within the theory of general relativity. One place where CTCs appear is in a solution to Einstein’s gravity equations known as the Gödel universe. This is a general relativistic description of a universe with an inherent rotation to it. Of course we know observationally that our universe doesn’t have such an inherent rotation, so the Gödel model doesn’t match reality. CTCs also appear inside a rotating black hole. A rotating mass causes space and time to swirl around it a bit (an effect known as frame dragging). Once you are within the event horizon of the black hole this swirling becomes strong enough there are CTCs. But this is only true if there is a singularity within the black hole, and its most likely that quantum physics prevents such a singularity from forming.

Using lasers to create a time machine is a long shot at best.

It doesn’t look like time machines occur naturally, so what about creating an artificial one? In the 1980s, Kip Thorne proposed using a wormhole as a time machine. Wormholes are purely hypothetical, and if they did exist they would collapse before you could travel through one. But Thorne found that if you could magically prevent a wormhole from collapsing, it could be used to create a time machine. Thorne’s model was simply a “what-if” scenario intended to test the limits of general relativity, but others such as Ron Mallett think it might be possible. Mallet has found a solution to general relativity that allows for CTCs without an event horizon. What Mallett has shown is that light can curve space and time in the same way as mass. By creating a rotating ring of laser light it is possible to distort space and time in a way similar to the way it is distorted by the rotating mass of a black hole, but without the black hole. This, he argues, opens the door to the possibility of creating a time loop. Critics have pointed out that Mallet’s solution still contains a singularity, so it isn’t a valid physical solution, but Mallett argues the singularity in his solution is an artifact that doesn’t affect the physics.

As for who’s right, time will tell. It’s easy to create a theoretical time machine within general relativity, the physics of matter and energy seem to prevent the creation of a real time machine. So could we ever build a time machine? Almost certainly not.

But that’s not a no.

Paper: Ronald L. Mallett. The Gravitational Field of a Circulating Light Beam. Foundations of Physics, Vol. 33, No. 9 (2003).

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When someone mentions time machines, you might think of fantastical machines such as Dr. Who’s TARDIS or the DeLorean in Back to the Future, but several physicists have made a serious study of time machines. Most of this work focuses on “what if” scenarios, which are really about testing the limits of a particular theoretical model, rather than actually engineering a device that can travel to the past.

The physics of time travel is based upon general relativity. If you’ve ever taken a physics course you might remember that the motion of objects is due to forces acting on them. That is, by pushing or pulling on them—either directly or by gravitational or electric fields—you can cause them to move. This is Newton’s physics, where objects fall because a gravitational force acts upon them.

But Einstein had a different way of looking at things. Through his theory of general relativity, Einstein demonstrated that gravity occurs because matter and energy distort space and time. For example, the mass of the Earth curves space around it. The motion of anything near the Earth, such as a satellite, is changed because of this spatial curvature, as if there were a force of gravity acting on it. Since space and time are connected, the mass of the earth also distorts time, which means a clock on the satellite ticks at a slightly different rate than a clock on the Earth. This effect on a satellite’s time is small (on the order of microseconds) but it is a measurable effect. In fact the satellites of the global positioning system have to take this time distortion into effect in order to work properly. If you’ve ever used a GPS receiver to find your way, you’ve counted on Einstein being right.

Although the mass of the Earth really does distort time, it doesn’t allow you to create a time machine. The clocks in satellites tick at different rates because of their motion around the Earth, but they always still tick forward. It is only the rate of their ticking that changes relative to other clocks on Earth. According to general relativity you can change the rate at which time flows but you can never quite stop time completely, and you can never cause your clocks to tick backwards. If that’s the case, it would seem that a true time machine—one that would let you travel into the past—is impossible.

But general relativity leaves the time-travel door open just a little. In Einstein’s theory time is connected to space, which means time can be bent in ways similar to the way space is bent by the Earth’s mass. So in principle time can be bent into a loop in such a way that it connects with its own past. If you found yourself in such a wibbly-wobbly space-time it would be possible to meet your younger self. Such a loop of time would be an actual time machine. As strange as this seems, there are examples of these time loops—what physicists call closed timelike curves (CTCs)—in general relativity.

One place where CTCs appear is in a solution to Einstein’s gravity equations known as the Godel Universe. This is a general relativistic description of a universe with an inherent rotation to it. If this were an accurate description of our universe then we would observe a rotational effect where distant galaxies are not only moving away from us, but also appearing to rotate about us. We don’t see any cosmic rotation among distant galaxies, so the Godel model doesn’t apply to our universe. While it is an interesting model, it is non-physical.

However CTCs also appear inside a rotating black hole. In general relativity, a rotating mass causes space and time to swirl around it a bit. This effect is known as frame dragging, and it has been observed experimentally by a satellite known as gravity probe b. Near a rotating black hole this effect is larger, but still not large enough to make a time machine. However, once you are within the event horizon of the black hole there are CTCs. This would imply that a time machine might be possible inside a black hole. The problem is that though they might exist inside a black hole, you would have to go into a black hole to travel in time, and once inside the black hole you would be trapped there forever. You couldn’t travel to this cosmic time machine, go into the past, and arrive back on Earth in the 1700s. The other problem is that just because general relativity works outside a black hole doesn’t mean it applies inside a black hole. The matter inside a black hole is so small and dense that quantum mechanics and particle physics comes into play, and we don’t have a solid understanding of quantum gravity. There might be something that prevents CTCs from forming inside a black hole.

Most physicists figure this must be the case, because CTCs create all sorts of problems with traditional physics. For example, CTCs can violate the principle of causality (basically cause and effect). This is popularized by the so-called grandfather paradox. Suppose you have a time machine, travel to the past, and accidentally kill your grandfather before he has a chance to woo your grandmother. By preventing their offspring you have prevented your own existence. But that means you couldn’t have travelled back in time, so you couldn’t have killed your grandfather. But that means you didn’t kill your grandfather, which means you were born, which means you did kill your grandfather, which means…

So what would really happen in this case? The answer is unclear, because such a time loop violates causality. The cause and effect contradict each other. In many science fiction stories this is solved by simply declaring that history rewrites itself, or that there are parallel timelines and such. We’ll look at parallel universes later, but this doesn’t solve the problem. The CTCs that general relativity allows exist in a single universe. Following the physics, we can’t simply invoke parallel universes to solve a tricky problem.

One possible solution is to impose what is called the “self-consistency” principle. This requires that any “time machine” example must be self consistent. So the grandfather paradox mentioned above is forbidden because it is not self-consistent. What would be allowed is for you to go back in time and wound your grandfather. While in the hospital he meets a kindly doctor who turns out to be your future grandmother. So your trip back in time caused your grandparents to meet, which allowed you to be born. Perfectly self consistent.

But this solution doesn’t prevent every problem. Suppose when you were 16 a stranger gives you a book. As you read through the book you find it is a set of instructions for building a time machine. It even includes all the background physics necessary to make it work. You go to college, study physics, and your doctoral dissertation is on the physics of time travel (which you got from the book). This groundbreaking work wins you the Nobel prize, and with the prize money you build a time machine, travel back in time and present your younger self with the book on time travel you once received… from yourself.

This is self consistent, but we seem led to ask where the book came from. Yes, you got it from yourself, but that doesn’t seem to be a satisfying answer. Where did the knowledge of time travel originate? The only answer is that the book is itself a closed timelike curve. It doesn’t have an origin. It just is.

Various theories have been proposed to provide a more satisfying answer to examples such as this. They invoke aspects of quantum mechanics, thermodynamics, entropy, information theory, and on and on down the rabbit hole. None of these models provide a completely satisfying description of time travel that makes sense. This is why most physicists figure time travel is impossible. There isn’t a clear way for it to make physical sense. Stephen Hawking went so far as to propose a chronology protection conjecture, which proposes that all macroscopic CTCs are physically impossible.

Still, there are a few physicists who think time machines are possible. For example, Ron Mallett at the University of Connecticut has found a solution to general relativity that allows for CTCs without an event horizon. What Mallett has shown is that light can curve space and time in the same way as mass. By creating a rotating ring of laser light it is possible to distort space and time in a way similar to the way it is distorted by the rotating mass of a black hole, but without the black hole. This, he argues, opens the door to the possibility of creating a time loop you could step into. Critics have pointed out that Mallet’s solution still contains a singularity, so it isn’t a valid physical solution, but Mallett argues the singularity in his solution is an artifact that doesn’t affect the physics.

Even if Mallett is right, his time machine would not allow you to travel anywhere in time. The CTCs could only form in the span of time in which the time machine existed. So you can only go back as far as the moment the time machine was turned on, and you could only travel from the future in which it was still on. In other words, if you wanted to travel 10 years back in time, you’d have to turn on your time machine, keep it running continuously for 10 years, so that you could climb into the machine and arrive when you started. To go back to the future you’d simply have to hang around for another 10 years.

Of course the real question is whether it is possible to distort space and time strongly using only laser light, and whether that distortion could be made into a time machine. Mallet has proposed an experiment to test his model, but so far it hasn’t been performed. Until that happens (and succeeds) time travel is still very hypothetical.

In the end, time will tell.

Tomorrow: Warp Drive. NASA is rumored to be working on it, does that make it so? Find out tomorrow.

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While fantastical stories have been with us as long as we’ve been human, in the early 1800s a new type of story appeared. Often Mary Shelly’s Frankenstein is named as the first example of this genre. Also known as The Modern Prometheus, it gives us the tale of a mad scientist who creates a creature from alchemy and science. By the late 1800s H. G. Wells wrote tales of The Time Machine, and an alien invasion with The War of the Worlds, and Jules Verne gave us adventure stories of an atomic powered submarine in 20,000 Leagues Under the Sea, and the first astronauts in From the Earth to the Moon.

It’s not surprising that the earliest works of science fiction were about time travel, space aliens and starships. We love dreaming of new horizons, and science fiction can create entire tales from the the mere whiff of scientific possibility. This can be both a blessing and a curse. On the one hand science fiction lets us explore the possibilities, and can inspire an interest in science. On the other hand, the trappings of science fiction can make awesome feats of human engineering seem trivial. Who cares if we can land a rover on Mars when science fiction lets us travel to the stars. On the gripping hand, many of the concepts of science fiction are now deeply rooted in our culture. The once futuristic idea of touch screens and verbally asking your shipboard computer for information on a particular star system are now very real. Even some of science fiction’s wildest ideas like teleportation and quantum computers are at the cutting edge of real science.

But some common ideas from science fiction are based only on the most tenuous science. While we all know what time machines, warp drive and wormholes are in the context of science fiction, scientifically these ideas are speculative at best. They also happen to show up all the time in popular science articles. You’ve likely come across the “NASA working on warp drive” articles, or “scientist wants to build time machine”. Popular media often portrays them as merely engineering challenges rather than speculative science. They are also ideas I get asked about all the time.

It’s because I’m so often asked about these topics that I decided to do this particular series. I’ll focus on the ones with a connection to astronomy and astrophysics. So the topics for this week are

• Time Travel
• Warp Drive
• Wormholes
• Ansibles
• Parallel Universes
• Aliens

I’ll focus on what we know both experimentally and theoretically. We’ll go to the very edge of current science, but separate scientific possibility from wild fantasy.

Starting tomorrow: Time travel. Is it really possible to go back in time? If your Mom falls in love with Calvin Klein instead of your Dad, does that mean you’ll cease to exist? Allons-y!

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