In 1895, H.G. Wells published his famous novel The Time Machine. Since then, time travel has made its way into all veins of science fiction, from a cyborg sent back in time to kill humanity’s last hope in The Terminator, to two dudes time traveling in a phone booth in Bill and Ted’s Excellent Adventure. But what about time travel in reality? Is it possible to send a human being back and forth through time with just the flip of a switch?
For many years, time travel sat on the periphery of respectable science, dismissed as nothing more than fantasy. However, recently theoretical physicists have taken a closer look at the subject. The incentive was partly for pleasure, as time travel is an interesting subject to ponder, but it also had a more serious draw. The possibility of time travel, even theoretically, would have massive implications for understanding the nature of a unified theory of physics (Davies, 2007).
Before Einstein’s theory of general relativity, we observed the universe through the lens of Newtonian physics. Newton thought that time and space were absolute, rendering the concept of time travel impossible. That all changed in 1905 with Einstein’s release of the special theory of relativity which proposes two important rules: “The laws of physics should look the same to every observer in uniform motion (moving in a straight line at constant speed), and the speed of light in a vacuum should be the same for every observer in uniform motion” (Talcott, 2006). The only way for both of these postulates to be satisfied simultaneously is if time passes at different rates for different observers. In other words, two people who move differently will experience time differently.
The effect described above is known as time dilation. We do not notice these strange time warps in everyday life because they only become significant when movement approaches the speed of light (Davies, 2007). However, experiments do confirm that this phenomenon occurs in everyday life. In 1971, Joe Hafele of Washington University in St. Louis and Richard Keating of the U.S. Naval Observatory in Washington borrowed four atomic clocks and sent them on airplane trips around the world. Even though airplanes travel at a miniscule fraction of the speed of light, the clocks traveled slower than those left at the observatory by a few nanoseconds, the same amount predicted by the theory of special relativity (Talcott, 2006). So time travel to the future is seen in everyday life, even if it occurs in very unremarkable amounts.
For a more dramatic example, we have to look out of the realm of everyday experience. There are subatomic particles called muons that only survive for a few millionths of a second in a laboratory setting. However, when they are moving at nearly the speed of light, they decay much more slowly. When cosmic rays strike our atmosphere, they create muons traveling nearly the speed of light. Under normal circumstances, these particles would not even make it a mile through our atmosphere. While going nearly the speed of light, though, most of the particles make it the full twelve miles to reach the Earth’s surface (Talcott, 2006).
Ten years after inventing his special theory of relativity, Einstein came up with his general theory of relativity. This theory applies to all types of motion and showed that gravity is an expression of space-time curvature (Talcott, 2006). Clocks run a tiny bit faster in space than on the surface of the Earth because we are deeper in a gravitational field. Near the surface of an incredibly massive body such as a neutron star, however, time is slowed as much as thirty percent relative to Earth time. From the view of a neutron star, the universe is going by at fast forward. At the surface of a black hole, space-time is infinitely curved, meaning that time literally stops relative to Earth. If an astronaut could get very close to a black hole, he would return far into the future (Davies, 2007).
This ability to go forward to the future is often described using something called the twin paradox. Say that Tim and Tom are twins. Tim goes on a spaceship to travel the galaxy, and Tom stays home on planet Earth. Tim travels close to the speed of light for a year, but when he returns he finds that ten years have elapsed on Earth. So now Tom is nine years older than Tim even though they were born on the same day. This is clearly a very limited type of time travel. Tim has essentially traveled nine years into Earth’s future (Davies, 2007).
While speed and gravity can propel one into the future, it is much more problematic to travel back to the past. In 1948 Kurt Gödel, an Austrian mathematician, produced the first proposition of how to realistically travel back in time. He came up with a solution to the Einstein field equation that describes a rotating universe. With his solution, an astronaut would be able to travel through space in order to reach his past. This occurs because of the way gravity affects light. The rotation of the universe would drag light around with it, enabling the astronaut to travel in a closed loop in space, which is also a closed loop in time. At no time would he go faster than the speed of light in the close vicinity of the particle. Unfortunately for Gödel, as far as astronomers can tell, the universe is not rotating based on the way galaxies move in relation to one another. His theory was not a complete failure, though. He was able to prove that it is possible to travel back in time without conflicting with the laws of physics (Economist, 1996).
There are a number of other scenarios that would allow for travel to the past. Frank J. Tipler of Tulane University calculated that a massive, infinitely long cylinder spinning on its axis close to the speed of light would create the same sort of time loop as Gödel’s scenario. Richard Gott of Princeton predicted that the cosmic strings produced in the early stages of the big bang could produce similar results. Hovever, in the mid-1980s a more realistic theory was created based on the concept of wormholes (Davies, 2007).
Wormholes are theoretical tubes of space-time which connect two completely different places in space and time in the universe. The idea is that you pilot your spaceship in one mouth of the wormhole and come out in a completely different place and time. Wormholes, if they exist, would solve the speed of light limit issue (Hawking, 2002). Imagine the Earth with a giant tunnel through the middle. It would take you much longer to circumnavigate the Earth than it would to just go through the tunnel and come back again. Wormholes work on the same principle. If you were traveling the galaxy in a spaceship, relativity would demand that you go slower than the speed of light, taking you a very long time. A wormhole would allow you to cross the galaxy and come back in practically no time at all. Another stipulation of wormholes existing, however, is that you could also potentially come back before you even depart (Hawking, 2002).
Wormholes do have their own problems as far a practical time machine might go. Wormholes are tremendously unstable, only forming for just a small amount of time before squeezing off, leaving both openings as black holes. However, physicists theorized a solution to the problem thanks to a science fiction story. Carl Sagan, a Cornell University astronomer, began writing his novel Contact in the early 1980s. In his original manuscript, the heroine of the story goes into a black hole near Earth and comes out of another black hole near a distant star elsewhere in the galaxy. He wanted to make sure that the science in the story made sense, so he sent the manuscript to black hole expert Kip Thorne at Caltech. Thorne immediately saw that the story needed a wormhole, but he realized their limitations. So he and some students set about theorizing a method of keeping the wormhole from collapsing (Talcott, 2006).
Thorne and his students found a solution in something he dubbed “exotic matter” (Talcott, 2006). Exotic matter is, in a sense, less than nothing. Its energy per unit volume, or energy density, is less than zero. While exotic matter may sound like fantasy, some of its effects have been produced in laboratory settings. They come about by Heisenberg’s uncertainty principle, which requires that the energy density of any electric, magnetic, or other field has to fluctuate. Even when the average energy density is zero, as in a vacuum, these fluctuations still occur. Thus, the quantum vacuum constantly has virtual particles popping in and out of existence, which according to quantum theory, still has an energy density of zero. If somehow one could manage to suppress these undulations, the vacuum would have less energy than it usually does, or a negative energy density. The pressure the exotic matter exerts is comparable to the extreme pressure that keeps a neutron star from collapsing. Thus, the exotic matter would force the wormhole to stay open (Lawrence, 2003).
Even with exotic matter, traveling by wormhole is still problematic. Physicists Roman Buniy and Stephen Hsu at the University of Oregon at Eugene calculated the properties of two wormholes coated with exotic matter, one that follows the laws of classical physics and another that follows the laws of quantum mechanics. It turns out that in the quantum wormhole there is no guarantee of where exactly in space in time you pop out. You might come out inside a wall, or at a completely different time than you predicted. Classical wormholes, on the other hand, are much more predictable and would be more effective for space-time travel. The drawback is that they are incredibly unstable. The slightest nudge, say, from a spaceship would cause the wormhole to collapse (New Scientist, 2005).
In order to ensure where one ends up at the end of a wormhole, one would have to make a time machine out of one. How does one go about accomplishing such a task? First, find a wormhole. A good place to look may be in the theoretical quantum foam. Quantum foam is comprised of fleeting time and space that some physicists believe make up empty space at a level a hundred trillion trillion times smaller than an atom. You would then secure one mouth at a sort of “home base” and the other on a spaceship. The spaceship travels close to the speed of light so that time passes more slowly on Earth. The spaceship returns to Earth ten years after its launch, and, say, fifty years have passed on Earth. Because the wormhole is able to provide almost instantaneous connection between the two ends, wormhole end in the spaceship will lead to Earth as it was ten years after the launch, or forty years prior to the moment of return (Economist, 1996).
This same principle applies with incredibly massive bodies as well. If one wormhole opening was left at the surface of a neutron star and the other was sent on a spaceship to float off in space for a while. Time moves more slowly for the neutron star, so you keep the wormhole opening there for as long as the desired time lag is, say ten years. Go one direction in the wormhole, and you go ten years into the past. Go the other direction in the wormhole, and you go ten years into the future. It is effectively the same sort of time loop as was theorized by Gödel (Talcott, 2006).
One major limitation of this form of time travel is that you can only travel within the loop, and therefore you could never travel to earlier than the time machine was invented. This explains why there are no futuristic time travelers amongst us today. However, if the time machine were to be invented in the year 2025, the Earth might become packed with time traveling tourists. It all hinges on the time that the machine comes to exist (Talcott, 2006).
The ability to travel through time opens up a wide range of causal paradoxes. This is classically illustrated in the mother paradox. Consider the time traveler who goes to the past, finds his mother as a young girl, and kills her. If the girl dies, then she can never grow up to be the time traveler’s mother, and if the time traveler was never born, he can never go back in time to murder his mother. This paradox arises when the time traveler tries to change the past, which is clearly impossible. The only thing the time traveler can do is be a part of the past. Suppose the time traveler goes back in time and saves a girl from being murdered, and that girl grows up to be his mother. The causal loop is complete, and there is no paradox (Davies, 2007).
Quantum theory may also make the mother paradox less impossible than it originally seemed. David Deutsch is a proponent of the “multiple worlds” approach to quantum theory. In quantum mechanics objects do not exist in a single state, but rather in a wide array of possibilities with various probabilities. In other words, Deutsch believes that there exists a universe for every possible state of events, which creates a mind-boggling number of universes. According to this theory, wormholes join one possible universe to another rather than a continuous loop in one universe. This resolves the mother paradox because if one were to travel to another universe and kill a parallel version of their mother, they could still return to their own universe and be just fine (Economist, 1996).
Even if time travel is not completely paradoxical, it is certainly strange. Consider the time traveler who jumps ahead a year and reads an article about a new scientific theorem. He travels back in time and teaches the theorem to a student, who then writes an article about it. The article is the same one that the time traveler read. A question comes up: Where did the information come from? It did not come from the time traveler because he read about it. It also did not come from the student because he learned about it from the time traveler. The information apparently came out of nothingness for no reason.
The bizarre repercussions of time travel have led some scientists to deny the possibility entirely. Stephen Hawking has proposed a “chronology protection conjecture” (2002) that would outlaw causal loops. Because relativity actually allows causal loops, chronology protection would rely on some outside force. One idea is that quantum processes might come to the rescue. A time machine would allow particles to loop to there own past, and calculations suggest that the ensuing disturbance would cause a surge of energy that would wreck the wormhole (Hawking, 2002).
Although we know that it is possible to jump forward in time, it remains to be seen if we can travel back in time. A good start would be to find out if wormholes even occur in nature. Also, assuming that they do, it would take an incredible amount of resources and energy to operate a time machine. So in our lifetime, time machines will most likely be confined to the realm of theoretical physics. There is no reason not to make speculation, though. Who knows what we might find.
Davies, Paul. “How to Build a Time Machine.” Scientific American Special Edition. 17.1 (2007): 28-33. Academic Search Premier. EBSCO. Norlin Library, U of Colorado. 14 November 2007.
Ford, Lawrence H. and Thomas A. Roman. “Negative Energy, Wormholes, and Warp Drive.” Scientific American Special Edition. 13.1 (2003): 84-91. Academic Search Premier. EBSCO. Norlin Library, U of Colorado. 14 November 2007.
Hawking, Stephen. “Protecting the Past: Is Time Travel Possible?” Astronomy. 30.4 (2002): 46-50. Academic Search Premier. EBSCO. Norlin Library, U of Colorado. 14 November 2007.
“How to Murder Your Grandfather and Still Get Born.” The Economist. 338.7949 (1996): 81-82. Academic Search Premier. EBSCO. Norlin Library, U of Colorado. 14 November 2007.
Talcott, Richard. “Is Time on Our Side?” Astronomy. 34.2 (2006): 32-39. Academic Search Premier. EBSCO. Norlin Library, U of Colorado. 14 November 2007.
“Wormholes Take You Here, There, or Nowhere at All.” New Scientist. 186.2502 (2005): 19. Academic Search Premier. EBSCO. Norlin Library, U of Colorado. 14 November 2007.