Traveling through the vast distances of interstellar space currently seems impossible, and time traveling is purely fictional. Undreamt of technologies in the future could change all this—but if so, why haven’t we met our future selves?
Someone was stealing trashcans. New Yorkers did not see them go, but during the spring and summer of 1950, metal bins were being spirited away from Manhattan’s streets at an alarming rate. At the same time, a rash of flying saucer sightings was gripping the country, and a cartoon appeared in The New Yorker portraying a fleet of aliens returning to their home planet with their cache of dustbins.
“It’s a neat solution to both phenomena,” said physicist Enrico Fermi, when he heard a description of the cartoon one summer lunchtime at Los Alamos National Laboratory, New Mexico, and it led him into a discussion with colleagues about the possibility of faster-than-light interstellar travel.
Einstein’s Special Theory of Relativity states that the speed of light is an absolute speed limit throughout the Universe. Nothing can travel through space faster than light and this stands in the way of interstellar spaceflight, because the stars are so far away from one another. The record holder for the fastest manmade object is the spacecraft Helios 2; launched in 1976, it reached speeds of about 250,000 kilometers per hour (155,000 miles per hour) during a series of close fly-bys of the Sun. However, compare this with the speed of light, approximately 1.1 billion kilometers per hour (0.7 billion miles per hour), and it looks like a snail’s pace. Traveling at the maximum speed of Helios 2, it would take 18,500 years to reach even our nearest star.
Even if we could travel at the speed of light, there are only eleven stars (excluding the Sun) within a distance of ten light years. It would take 4.3 years traveling at the speed of light to reach the nearest star, a triple star system known as Alpha Centauri, 4.3 light years away. Confined to speeds below that of light, the rest of the Galaxy will forever remain beyond our reach.
The universal speed limit came about as a consequence of the principle that the speed of light is a constant. By this Einstein meant that regardless of the speed of the measurer, the speed of light always appeared to be the same. This is contrary to what was expected, because in our normal experience velocities “add” together; two cars traveling at 50 kilometers per hour toward each other pass with a combined, or relative, velocity of 100 kilometers per hour. This is not true for light. If you measure the speed of light the value will always be the same, no matter whether you approach the light beam head-on, from the side, or are running away from it. This had been experimentally proven by the Michelson-Morley experiment in 1887.
“Space isn’t remote at all. It’s only an hour’s drive away if your car could go straight upward.”
FRED HOYLE 20TH CENTURY ASTRONOMER
American physicists Albert Michelson and Edward Morley originally designed their experiment to provide incontrovertible evidence for the ether. At the time it was widely believed that light needed a medium to travel through, rather like sound traveling through air, and the postulated medium in which the Earth was immersed was the ether. Michelson and Morley reasoned that as Earth traveled around its orbit at 30 kilometers per second (19 miles per second) it should experience an ethereal wind. The wind would sweep across Earth in different directions and at different speeds, depending on the time of year and the direction in which the Earth was moving. To measure the effects of this wind, Michelson and Morley split a beam of light into two using a semisilvered mirror so that half of the light passed straight through while the other half was reflected. They then sent these identical beams down two paths at right angles to one another, and eventually combined the beams on their return journey. Traveling in different directions relative to the Earth’s motion, the two light beams should have been affected differently by the ethereal wind: one beam would meet it head-on, the other would feel it broadside. However, the light beams were not affected in any discernible way. No matter when the experiment was conducted, the same null result was obtained: it was as if the ether did not exist at all.
THE MICHELSON-MORLEY EXPERIMENT
Eventually this is what the physics community decided: there is no ether. Light does not need a physical medium, and its perceived speed does not depend on the speed of the observer. Accepting this result, Albert Einstein set about investigating what consequences this would have, and it led him to the Special Theory of Relativity. “Relativity” here refers to the fact that speeds can only ever be measured relative to something else. There is no universal standard, no absolute framework of space that speeds are measured against; one object must always be compared to another. “Special” refers to the fact that this is not a result that can be easily applied to all forms of motion; at first, Einstein investigated only nonaccelerated motion, in other words the simple case where objects were not changing their speed or direction in any way. He subsequently extended his investigation to accelerated forms of motion in his General Theory of Relativity (see Was Einstein Right?).
While working on special relativity, Einstein found that once an object’s velocity exceeds ten percent the speed of light, previously unimaginable effects manifest themselves. As bizarre as it may seem, the object becomes more massive the faster it travels. This increase in mass means an increase in inertia: more energy is required to make the object go faster. So, as the object accelerates further, becoming more massive all the time, greater and greater amounts of energy would be needed; in fact to accelerate an object to the speed of light would require an infinite amount of energy. It follows that it is impossible to attain the speed of light: it is a fundamental speed limit. On the face of it, we seem restricted forever to meander around the confines of the Solar System like ants on Earth’s surface, restricted to a minuscule area because we simply do not live long enough to cross the vast distances between the stars. It is a frustrating thought—but there is a get-out clause.
Special relativity tells us that nothing can travel through space at more than the speed of light, but the theory does not prevent space itself from expanding or contracting faster than the speed of light. This is exploited in the theory of inflation (see How Did the Universe Form?), which postulates that the early Universe underwent a sudden period of extreme expansion. The inflating Universe flung itself outward, distributing matter and energy at speeds far faster than light. Even today distant galaxies are moving away from us faster than the speed of light because of the expanding space between us and them. But as they are not actually moving through space at such speeds, Einstein’s laws are not broken. This is the weird world of relativity, and astronomers have to get used to it or the observed motions in the Universe make no sense.
Einstein’s predictions are now verified on a daily basis. Without relativity we would not have global positioning systems (GPS). Only by taking into account special and general relativity can GPS devices provide accurate locations. In other words, every time you check your position on your cell phone or your GPS, you are relying on Einstein’s theories. But applications more much exciting than GPS can be envisaged.
Relativity allows us to imagine the basic concept of a Star Trek–like “warp drive.” Think of it as an engine that can stretch space like a piece of elastic, with your spacecraft being carried along in the stretching. Once you have reached your destination, you move off to one side and let space return to normal behind you. Making this work seemed pure science fiction until theoretical physicist Miguel Alcubierre Moya mathematically modeled a way of forcing space into a configuration that would create a bubble of warped space–time, rather than an elastic strip, which could travel through the Universe. The trick is to arrange an energy field around a spacecraft so that space behind it is forced to expand and space in front of it is forced to contract. Between these two warped regions sits a bubble of flat space–time in which a spacecraft could safely rest and travel at “warp speed.”
“Any sufficiently advanced technology is indistinguishable from magic.”
ARTHUR C. CLARKE 20TH CENTURY SCIENCE FICTION WRITER
The difficulty with the warp drive model is that to create a contracting region of space–time requires a hypothetical form of matter called exotic matter. Exotic matter, unlike antimatter, has negative mass; this means that it would feel repulsion in a gravitational field. Although there are hints of “exotic” behavior in the Universe, for example, dark energy acting as a kind of “repulsive” gravitational force (see What Is Dark Energy?), there is nothing that proves exotic matter actually exists. Nevertheless, Alcubierre’s work set people thinking.
Between 1996 and 2002, NASA funded a small research team called the Breakthrough Propulsion Physics group. It was their job to survey physics, looking for chinks in our understanding that might bring about a revolution in space propulsion. One of the things they looked at was the possibility of gravity control. For example, could we “turn down” the gravitational pull of a launch pad so that a rocket could lift off more easily? Could we reduce the inertia of a spacecraft so that it can move more easily in space? These investigations led nowhere, but the team also looked at the concept of short cuts through space–time, intriguingly called “wormholes.”
A WORMHOLE: A SHORT CUT THROUGH SPACE AND TIME
If you remember Alice’s Adventures in Wonderland, you can think of a wormhole in space–time as the rabbit hole. To visualize how it works, imagine that a piece of paper is a two-dimensional Universe. To travel from one corner to the furthest corner, a two-dimensional inhabitant would have to walk the diagonal length of the paper. Now imagine that you fold the Universe around, so that the far corner, which the flatlander wants to reach, is sitting just at the corner where he starts. Now, all he has to do is hop upward through the third dimension, which is a totally alien dimension to him, and he will miraculously find himself on the other side of the Universe. He has traveled through a wormhole, which is literally a short cut through a dimension that we cannot directly perceive.
Investigating the physics of wormholes, the physicists discovered that unfortunately, like Alcubierre’s warp drive, they require exotic matter to make them traversable. The group concluded that no fundamental breakthrough in space travel was imminent, but they did identify anomalies in current knowledge that inspired new thoughts. Two of these anomalies, now well known to space engineers, hint at new forces and energy sources that have so far defied explanation.
The “Pioneer anomaly” affects the Pioneer 10 and 11 space probes, which have been coasting away from us through space since their encounters with the giant planets Jupiter and Saturn in the 1970s. They are being mysteriously decelerated by a tiny amount, so every second their speed decreases by about one billionth of a meter per second. An international consortium of scientists and engineers are investigating the spacecraft’s data in the hope of understanding exactly when the strange deceleration began, whether it built up gradually or just switched on, and whether it can be explained by some onboard malfunction. To date, they have explained some of the anomaly as heat from the radioactive generators onboard, but the major part of the deceleration remains unaccountable and suggests that something fundamental in our understanding of gravity is not quite right.
The second unexplained effect is the “fly-by anomaly.” More significant than the Pioneer anomaly, it has affected a number of different spacecraft, including the Jupiter-bound Galileo, the Near Earth Asteroid Rendezvous (NEAR), the Saturn probe Cassini and the comet chaser Rosetta. The anomaly appears in the form of an unexpected acceleration that imparts an extra few millimeters per second onto the velocity of a spacecraft when it flies past a planet. There have been attempts to explain this by errors in the mathematics, but the more spacecraft that experience the bizarre acceleration, the less likely it seems that so many different teams can all be getting their sums wrong. Other possible solutions include some kind of natural decrease in the spacecraft’s inertia, making it more responsive to the gravitational field it is moving through.
It is not yet possible to say whether either of these effects will lead to a revolution in propulsion technologies, but understanding them is a prime focus for physicists. History has shown that profound scientific breakthroughs often begin with the recognition of small anomalies.
If all the talk of warp drives and suchlike sounds fanciful, putting those ideas into practice will be a walk in the park compared to building a time machine. First, we need to understand what we mean by time. Time is an immensely difficult concept to define; unlike electrical charge or mass, it is not something that can be measured. This may not be obvious, as we are so used to monitoring the passage of time in our lives. But clocks do exactly this—mark the passage of time—by using some phenomenon in which time is an integral factor, such as the oscillation of a quartz crystal or the decay of a radioactive isotope; nothing actually measures time itself. Nor is time like shape, taste or color, because it cannot be perceived by our traditional senses; yet we are constantly aware of its passage by the changing nature of events around us, or just by our own changing thought patterns. We are traveling through time, one-way, into the future.
The passage of time can be slowed down, however: special relativity tells us how to do this. As well as the increase in mass experienced when something travels close to the speed of light, another bizarre special relativistic correction involves time. Known as “time dilation,” this states that the faster an object moves, the slower time passes for it. This seems to offer something of a solution for interstellar travel because, if it were possible to accelerate a spacecraft to relativistic velocities, time would slow down inside it, allowing humans to reach the stars within their lifetimes. The downside is that outside the spacecraft time would continue to run at its normal rate and many years would pass, perhaps centuries. Imagine that one of a pair of identical twins becomes an astronaut and leaves Earth on a spaceship capable of traveling at a significant fraction of the speed of light. Upon his return, he has barely aged but his twin will now be an old man because he has experienced the passage of time differently.
General relativity, too, offers some ideas here. It describes how time is slowed down in the presence of a gravitational field. Where the field is weaker, for example at the altitude of the International Space Station, time would pass quicker: a clock on the space station would gain about 1 second every 10,000 years compared to an identical clock on the Earth’s surface. This may sound small but with atomic clocks accurate to better than one part in a trillion, the time-dilating effects of general relativity can be easily be verified.
“When a man sits with a pretty girl for an hour, it seems like a minute. But let him sit on a hot stove for a minute and it’s longer than any hour. That’s relativity.”
ALBERT EINSTEIN 20TH CENTURY PHYSICIST
In science fiction, time travel has often involved traveling to the past in some sort of time machine. A far cry from the contraptions suggested by fiction writers, the time machine proposed by physicist Frank Tipler was a sound mathematical possibility. In 1974, he showed that a rotating cylinder would drag the space–time continuum around with it, rather like the way a spatula stirs up honey. If the cylinder were rotating fast enough, Tipler’s calculations showed that routes into the past might be opened up. The trouble was that Tipler had to assume the cylinder was infinitely long. He suggested that a shorter cylinder might be capable of the same behavior if it rotated faster. Interestingly, some of the recent theoretical analysis of rotating black holes mimics Tipler’s mathematics, suggesting that time travel might be possible in the twisted space–time region close to a black hole known as the “ergosphere” (see What is a Black Hole?). Other theoreticians, notably Stephen Hawking, have suggested that only exotic matter, with its negative mass, would be capable of opening routes into the past.
If time travel is possible then there are a number of paradoxes that immediately spring to mind, such as the scenario of going back in time and murdering a grandparent. A number of scholars have suggested that something would always happen to prevent logical paradoxes. But to most physicists this has the tinge of a supernatural hand of fate and they prefer to believe that either the Universe might split into parallel realities (see Are There Alternative Universes?) or that time travel is simply impossible.
Let us now return to that lunch with Enrico Fermi. After the brief discussion about the possibility of faster-than-light travel, the conversation turned to other topics and lunch continued. Fermi’s mind continued to work though, and he suddenly exclaimed, “Where is everybody?” He elaborated by explaining that, if you assume that there are many extraterrestrial civilizations in the Galaxy, then the extreme age of the Universe implies that some are older than human civilization and so should have developed space travel. As a result, the Galaxy should be teeming with technologically advanced civilizations and we should have been visited many times in the past and present. So where are they?
Fermi discounted the weird and wacky UFO sightings as evidence of anything, and his simple question became known as the “Fermi Paradox.” He used it to argue that practical interstellar space travel was impossible; otherwise the evidence of extraterrestrial visitation should be all around us. The same argument can be advanced about time travel. If it is possible, someone somewhere at sometime in the future will invent a time machine; at which point people will begin traveling into the past, and should be among us today. So, if the laws of physics really permit time travel, where are the visitors from the future?
The balance of evidence suggests that interstellar travel and time travel may indeed be impossible. But, before we become too pessimistic, think about technical knowledge and modes of travel just a few centuries ago. The natural philosophers of the 17th century no doubt believed that interplanetary travel was impossible, too.