The Physics of Time Travel

Is it real, or is it fable?

In H.G. Wells’ novel, The Time Machine, our protagonist jumped into a special chair with blinking lights, spun a few dials, and found himself catapulted several hundred thousand years into the future, where England has long disappeared and is now inhabited by strange creatures called the Morlocks and Eloi. That may have made great fiction, but physicists have always scoffed at the idea of time travel, considering it to be the realm of cranks, mystics, and charlatans, and with good reason.

However, rather remarkable advances in quantum gravity are reviving the theory; it has now become fair game for theoretical physicists writing in the pages of Physical Review magazine. One stubborn problem with time travel is that it is riddled with several types of paradoxes. For example, there is the paradox of the man with no parents, i.e. what happens when you go back in time and kill your parents before you are born? Question: if your parents died before you were born, then how could you have been born to kill them in the first place?

There is also the paradox of the man with no past. For example, let’s say that a young inventor is trying futilely to build a time machine in his garage. Suddenly, an elderly man appears from nowhere and gives the youth the secret of building a time machine. The young man then becomes enormously rich playing the stock market, race tracks, and sporting events because he knows the future. Then, as an old man, he decides to make his final trip back to the past and give the secret of time travel to his youthful self. Question: where did the idea of the time machine come from?

There is also the paradox of the man who is own mother (my apologies to Heinlein.) “Jane” is left at an orphanage as a foundling. When “Jane” is a teenager, she falls in love with a drifter, who abandons her but leaves her pregnant. Then disaster strikes. She almost dies giving birth to a baby girl, who is then mysteriously kidnapped. The doctors find that Jane is bleeding badly, but, oddly enough, has both sex organs. So, to save her life, the doctors convert “Jane” to “Jim.”

“Jim” subsequently becomes a roaring drunk, until he meets a friendly bartender (actually a time traveler in disguise) who wisks “Jim” back way into the past. “Jim” meets a beautiful teenage girl, accidentally gets her pregnant with a baby girl. Out of guilt, he kidnaps the baby girl and drops her off at the orphanage. Later, “Jim” joins the time travelers corps, leads a distinguished life, and has one last dream: to disguise himself as a bartender to meet a certain drunk named “Jim” in the past. Question: who is “Jane’s” mother, father, brother, sister, grand- father, grandmother, and grandchild?

Not surprisingly, time travel has always been considered impossible. After all, Newton believed that time was like an arrow; once fired, it soared in a straight, undeviating line. One second on the earth was one second on Mars. Clocks scattered throughout the universe beat at the same rate. Einstein gave us a much more radical picture. According to Einstein, time was more like a river, which meandered around stars and galaxies, speeding up and slowing down as it passed around massive bodies. One second on the earth was Not one second on Mars. Clocks scattered throughout the universe beat to their own distant drummer.

However, before Einstein died, he was faced with an embarrassing problem. Einstein’s neighbor at Princeton, Kurt Goedel, perhaps the greatest mathematical logician of the past 500 years, found a new solution to Einstein’s own equations which allowed for time travel! The “river of time” now had whirlpools in which time could wrap itself into a circle. Goedel’s solution was quite ingenious: it postulated a universe filled with a rotating fluid. Anyone walking along the direction of rotation would find themselves back at the starting point, but backwards in time!

In his memoirs, Einstein wrote that he was disturbed that his equations contained solutions that allowed for time travel. But he finally concluded: the universe does not rotate, it ex- pands (i.e. as in the Big Bang theory) and hence Goedel’s solution could be thrown out for “physical reasons.” (Apparently, if the Big Bang was rotating, then time travel would be possible throughout the universe!)

Then in 1963, Roy Kerr, a New Zealand mathematician, found a solution of Einstein’s equations for a rotating black hole, which had bizarre properties. The black hole would not collapse to a point (as previously thought) but into a spinning ring (of neutrons). The ring would be circulating so rapidly that centrifugal force would keep the ring from collapsing under gravity. The ring, in turn, acts like the Looking Glass of Alice. Anyone walking through the ring would not die, but could pass through the ring into an alternate universe. Since then, hundreds of other “wormhole” solutions have been found to Einstein’s equations. These wormholes connect not only two regions of space (hence the name) but also two regions of time as well. In principle, they can be used as time machines.

Recently, attempts to add the quantum theory to gravity (and hence create a “theory of everything”) have given us some insight into the paradox problem. In the quantum theory, we can have multiple states of any object. For example, an electron can exist simultaneously in different orbits (a fact which is responsible for giving us the laws of chemistry). Similarly, Schrodinger’s famous cat can exist simultaneously in two possible states: dead and alive. So by going back in time and altering the past, we merely create a parallel universe. So we are changing someone ELSE’s past by saving, say, Abraham Lincoln from being assassinated at the Ford Theater, but our Lincoln is still dead. In this way, the river of time forks into two separate rivers. But does this mean that we will be able to jump into H.G. Wells’ machine, spin a dial, and soar several hundred thousand years into England’s future? No. There are a number of difficult hurdles to overcome.

First, the main problem is one of energy. In the same way that a car needs gasoline, a time machine needs to have fabulous amounts of energy. One either has to harness the power of a star, or to find something called “exotic” matter (which falls up, rather than down) or find a source of negative energy. (Physicists once thought that negative energy was impossible. But tiny amounts of negative energy have been experimentally verified for something called the Casimir effect, i.e. the energy created by two parallel plates). All of these are exceedingly difficult to obtain in large quantities, at least for several more centuries!

Then there is the problem of stability. The Kerr black hole, for example, may be unstable if one falls through it. Similarly, quantum effects may build up and destroy the wormhole before you enter it. Unfortunately, our mathematics is not powerful enough to answer the question of stability because you need a “theory of everything” which combines both quantum forces and gravity. At present, superstring theory is the leading candidate for such a theory (in fact, it is the ONLY candidate; it really has no rivals at all). But superstring theory, which happens to be my specialty, is still to difficult to solve completely. The theory is well-defined, but no one on earth is smart enough to solve it.

Interestingly enough, Stephen Hawking once opposed the idea of time travel. He even claimed he had “empirical” evidence against it. If time travel existed, he said, then we would have been visited by tourists from the future. Since we see no tourists from the future, ergo: time travel is not possible. Because of the enormous amount of work done by theoretical physicists within the last 5 years or so, Hawking has since changed his mind, and now believes that time travel is possible (although not necessarily practical). (Furthermore, perhaps we are simply not very interesting to these tourists from the future. Anyone who can harness the power of a star would consider us to be very primitive. Imagine your friends coming across an ant hill. Would they bend down to the ants and give them trinkets, books, medicine, and power? Or would some of your friends have the strange urge to step on a few of them?)

In conclusion, don’t turn someone away who knocks at your door one day and claims to be your future great-great-great grandchild. They may be right.

How to catch a ripple in spacetime

How do you build a real-world machine to test the most abstract of theories? Janna Levin talks with Rai Weiss, one of the original designers of LIGO, the four-kilometer-long instrument that has now twice detected the distant reverberations of two black holes crashing into one another.

Janna Levin is a theoretical physicist — she works with pen and paper to turn the elegant rules of the universe into theory (and also uses that pen and paper to write books about science; her latest, Black Hole Blues, was recently published by Alfred A. Knopf). Rainer Weiss, or Rai, as he’s known, is an experimental physicist — he thinks about how to find and measure something that may or may not exist outside of theory. Weiss was part of the group that designed and built LIGO, the detector that, in September 2015, for the first time “heard” the sound of two black holes crashing into one another, producing a gravitational wave that stretched spacetime and proving something that Einstein first floated as a theory almost 100 years ago. Earlier this week, the LIGO team announced that on December 26, 2015, they had detected another rippling, spacetime-stretching gravitational wave that erupted 1.4 billion years ago when two other black holes collided at half the speed of light.

In a sparkling conversation held at a Brooklyn art space, Pioneer Works, Levin and Weiss (with help from journalist John Hockenberry) talked about how they work, how LIGO works — and the moment when they realized Einstein had just been proven right. An edited version of the discussion follows.

The 50-year, billion-dollar bet

Janna Levin: The first detection made with LIGO was of a pair of black holes that collided 1.3 billion years ago. The success of the experiment was not only the detection of gravitational waves. That was the first time we detected a pair of black holes, because they’re dark and we can’t see them with telescopes. So it was really an astronomical discovery as well.

The detection happened in September 2015. In August 2015, Rai and I were talking and he said — this after the experiment cost a billion dollars, people had spent 50 years building it — he said, “If we don’t detect black holes, the thing is a failure.” I thought that was a really bold thing to say on the eve of starting the detection, because most people in the scientific community did not think we would detect black holes, at least not for a long time.

Rai Weiss: What was behind that statement — it is slightly facetious, of course — but you have to understand what black holes ultimately mean to people, and what they mean to the theory of gravity that Einstein put together. It’s the one thing that is so Einsteinian, we couldn’t have gotten it any other way. The whole idea of gravity curling up space, that is the epitome of what is going on in a black hole. I would’ve loved to have seen Einstein’s face if he were presented with the data that we actually discovered such a thing, because he himself probably didn’t believe in much of it.

How our sun and Earth warp space and time, or spacetime, is represented here with a green grid. As Albert Einstein demonstrated in his theory of general relativity, the gravity of massive bodies warps the fabric of space and time—and those bodies move along paths determined by this geometry. His theory also predicted the existence of gravitational waves, which are ripples in space and time. These waves, which move at the speed of light, are created when massive bodies accelerate through space and time. Image courtesy T. Pyle/Caltech/MIT/LIGO Lab

How our sun and Earth warp space and time, or spacetime, is represented here with a green grid. As Albert Einstein demonstrated in his theory of general relativity, the gravity of massive bodies warps the fabric of space and time—and those bodies move along paths determined by this geometry. His theory also predicted the existence of gravitational waves, which are ripples in space and time. These waves, which move at the speed of light, are created when massive bodies accelerate through space and time. Image courtesy T. Pyle/Caltech/MIT/LIGO Lab.

When black holes collide

JL: I’m a theoretical physicist. I only work pen-on-paper. I think that’s why I became so enamored of the physicality of the LIGO experiment. It’s one thing to say, “Oh, two black holes are going to collide,” and work it out on paper. It’s quite another thing to build giant L-shaped instruments that span four kilometers on two coasts.

RW: The equations don’t tell you much until you put actual numbers into them and you find out how much energy is involved in making a distortion of space. A gravitational wave is a very slight stretching in one dimension. If there’s a gravitational wave traveling towards you, you get a stretch in the dimension that’s perpendicular to the direction it’s moving. And then perpendicular to that first stretch, you have a compression along the other dimension. It compresses that way, and it stretches that way.

The problem is the energy that’s needed to make that very tiny little distortion in the fabric of spacetime. Space is much stiffer than you imagine; it’s stiffer than a gigantic piece of iron. That’s why it’s taken so damned long to detect gravitational waves: to deform space takes an enormous amount of energy, and there are only so many things that have enough.

JL: Those two black holes were orbiting each other probably for a few billion years, and they were creating gravitational waves the whole time, but the waves were weak because they were still far apart. It wasn’t until the black holes got within a couple of hundred kilometers of each other, in the final 200 milliseconds of their life together, when each one’s going at nearly the speed of light, and they collide to make a new black hole. It’s only that last fraction of this very long process that they’re able to detect at LIGO.

How to measure spacetime, stretching

RW: Here’s the fundamental problem when it comes to measuring the stretching of space: We’re looking for a motion that is 1,000th the size of a proton. It’s 10^-18, which is a decimal point, 17 zeros all in a line, and then a one. You can make the measurement by using a lot of light and an interferometer to look at things that are about 10^-18 meters of motion. You can do that. That’s the easy part. The hard part is how not to detect everything else around you — the people next to you, the leaves blowing on the trees in Louisiana, the winds blowing and howling up in Hanford, Washington. If you’re lucky, if the building is a good building, the amount of motion of the ground you’re dealing with is about a micron, it’s about 10^-6 meters. But you have to somehow get rid of that motion.

John Hockenberry: What were you doing when you thought of the first prototype of LIGO?

RW: I was teaching a course. I was trying to explain what Joseph Weber, the man who first did these gravity wave experiments, was doing, how he had devised a technique I couldn’t understand, and I couldn’t explain it to the class. So I sat and sat and sat, and thought: How can I make the most pristine, simple-minded explanation of how you could detect a gravitational wave? It was very straightforward — at least maybe to me it was: Let me launch a big mass off in space, and put a good clock on it, and over here, I’ll launch another one with another good clock. Then we’ll measure how far they are apart — we’ll send a light beam from one to the other, figure out when it hits, measure it and see what the clock says. Then let the gravitational wave come down, which stretches and compresses space, and see how that changes that time. That was the basis of the idea. I gave that as a problem to the students in the class, and they all were able to do it, and then I put it aside. But in 1971 or so, that’s when I took this idea seriously — I said, maybe this idea is okay. So then it became, how would you do it in a practical way?

JL: When I first went to see the LIGO facility, I was blown away. There’s a sort of corner lab, and it’s a cold room, a clean room, so you go in in bunny suits and laser goggles. And that holds the apex of the machine where a laser comes out and it gets split and sent down two arms. The arms punch right outside the lab in an L-shape, so they cut right out into the — in the case of Hanford, the desert, and in the case of Louisiana, the swamp. Those arms are four kilometers long in each direction, and they just go. There’s not much between the apex and the end stations. So you take a truck, you drive four kilometers down to the end station, and there’s another much, much smaller lab which houses a chamber for the mirrors.

As in Rai’s original thought experiment, one beam of laser light is split into two, and the two beams head off in different directions toward mirrors at the end of each arm. The two beams should take the same amount of time to go out and come back to the apex. But a ripple in spacetime actually changes the distance each laser beam has to travel — so they don’t perfectly meet up back at base. By monitoring the interference pattern where they meet up, we can record the stretching and compression of spacetime.