Time Travel

Doctor Who's Tardis
Time travels features extensively in fiction, but there is some theoretical basis to the idea

Arguably, we are always travelling though time, as we move from the past into the future. But time travel usually refers to the possibility of changing the rate at which we travel into the future, or completely reversing it so that we travel into the past. Although a plot device in fiction since the 19th Century (see the section on Time in Literature), time travel has never been practically demonstrated or verified, and may still be impossible.

Time travel is not possible in Newtonian absolute time (we move deterministically and linearly forward into the future). Neither is it possible according to special relativity (we are constrained by our light cones). But general relativity does raises the prospect (at least theoretically) of travel through time, i.e. the possibility of movement backwards and/or forwards in time, independently of the normal flow of time we observe on Earth, in much the same way as we can move between different points in space.

Time travel is usually taken to mean that a person’s mind and body remain unchanged, with their memories intact, while their location in time is changed. If the traveller’s body and mind reverted its condition at the destination time, then no time travel would be perceptible.

Time Travel Scenarios

Although, in the main, differing fundamentally from the H.G. Wells concept of a physical machine with levers and dials, many different speculative time travel solutions and scenarios have been put forward over the years. However, the actual physical plausibility of these solutions in the real world remains uncertain.

At its simplest, as we have seen in the section on Relativistic Time, if one were to travel from the Earth at relativistic speeds and then return, then more time would have passed on Earth than for the traveller, so the traveller would, from his perspective, effectively have “travelled into the future”. This is not to say that the traveller suddenly jumped into the Earth’s future, in the way that time travel is often envisioned, but that, as judged by the Earth’s external time, the traveller has experienced less passage of time than his twin who remained on Earth. This is not real time travel, though, but more in the nature of “fast-forwarding” through time: it is a one-way journey forwards with no way back.

There does, however, appear to be some scientific basis within the Theory of Relativity for the possibility of real time travel in certain scenarios. Kurt Gödel showed, back in the early days of relativity, that there are some solutions to the field equations of general relativity that describe space-times so warped that they contain “closed time-like curves”, where an individual time-cone twists and closes in on itself, allowing a path from the present to the distant future or the past. Gödel’s solution was the first challenge in centuries to the dominant idea of linear time on which most of physics rests. Although a special case solution, based on an infinite, rotating universe (not the finite, non-rotating universe we actually find ourselves in), other time travel solution have been identified since then that do not require an infinite, rotating universe, but they remain contentious.

In the 1970s, controversial physicist Frank Tipler published his ideas for a “time machine”, using an infinitely long cylinder which spins along its longitudinal axis, which he claimed would allow time travel both forwards and backwards in time without violating the laws of physics, although Stephen Hawking later disproved Tipler’s ideas.

In 1994, Miguel Alcubierre proposed a hypothetical system whereby a spacecraft would contract space in front of it and expand space behind it, resulting in effective faster-than-light travel and therefore (potentially) time travel, but again the practicalities of constructing this kind of a “warp drive” remain prohibitive.

A worm hole is a feature of space-time that could theoretically provide a short-cut through time and space

Other theoretical physicists like Kip Thorne and Paul Davies have shown how a wormhole in space-time could theoretically provide an instantaneous gateway to different time periods, in much the same way as general relativity allows the theoretical possibility of instantaneous spatial travel through wormholes. Wormholes are tubes or conduits or short-cuts through space-time, where space-time is so warped that it bends back on itself, another science fiction concept made potential reality by the Theory of Relativity. The drawback is that unimaginable amounts of energy would be required to bring about such a wormhole, although experiments looking into the possibility of creating mini-wormholes and mini-black holes are being carried out at the particle accelerator at CERN in Switzerland. It also seems likely that such a wormhole would collapse instantly into a black hole unless some method of holding it open were devised (possibly so-called “negative energy”, which is known to be theoretically possible, but which is not yet practically feasible). Stephen Hawking has suggested that radiation feedback, analogous to feedback in sound, would destroy the wormhole, which would therefore not last long enough to be used as a time machine. Actually controlling where (and when) a wormhole exits is another pitfall.

Another potential time travel possibility, although admittedly something of a long shot, relates to cosmic strings (or quantum strings), long shreds of energy left over after the Big Bang, thinner than an atom but incredibly dense, that weave through the entire universe. Richard Gott has suggested that if two such cosmic strings were to pass close to each other, or even close to a black hole, the resulting warpage of space-time could well be so severe as to create a closed-time-like curve. However, cosmic strings remain speculative and the chances of finding such a phenomenon are vanishingly small (and, even if it were possible, such a loop may well find itself trapped inside a rotating black hole).

Physicist Ron Mallet has been looking into the possibility of using lasers to control extreme levels of gravity, which could then potentially be used to control time. According to Mallet, circulating beams of laser-controlled light could create similar conditions to a rotating black hole, with its frame-dragging and potential time travel properties.

Others are looking to quantum mechanics for a solution to time travel. In quantum physics, proven concepts such as superposition and entanglement effectively mean that a particle can be in two (or more) places at once. One interpretation of this (see the section on Quantum Time) is the “many worlds” view in which all the different quantum states exist simultaneously in multiple parallel universes within an overall multiverse. If we could gain access to these alternative parallel universes, a form of time travel might then be possible.

At the sub-sub-microscopic level – at the level of so-called quantum foam, tiny bubbles of matter a billion-trillion-trillionths of a centimetre in length, perpetually popping into and out of existence – it is speculated that tiny tunnels or short-cuts through space-time are constantly forming, disappearing and reforming. Some scientists believe that it may be possible to capture such a quantum tunnel and enlarge it many trillions of times to the human scale. However, the idea is still at a very speculative stage,

It should be noted that, with all of these schemes and ideas, it does not look to be possible to travel any further back in time that the time at which the travel technology was devised.

Faster-Than-Light Particles

The equations of relativity imply that faster-than-light (superluminal) particles, if they existed, would theoretically travel backwards in time. Therefore, they could, again theoretically, be used to build a kind of “antitelephone” to send signals faster than light, and thus communicate backwards in time. Although the Theory of Relativity disallows particles from accelerating from sub-light speed to the speed of light (among other effects, time would slow right down and effectively stop for such a particle, and its mass would increase to infinity), it does not preclude the possibility of particles that ALWAYS travel faster than light. Therefore, the possibility does still exist in theory for faster-than-light travel in the case of a particle with such properties.

There is a rather strange theoretical particle in physics called the tachyon that routinely travels faster than light, with the corollary that such a particle would naturally travel backwards in time as we know it. So, in theory, one could never see such a particle approaching, only leaving, and the particle could even violate the normal order of cause and effect. For a tachyon, the speed of light is the lower speed limit, while the upper speed limit is infinity, and its speed increases as its energy decreases. Even stranger, the mass of a tachyon would technically be an imaginary number (i.e. the number squared is negative), whatever that might actually mean in practice.

It should be stressed that there is no experimental evidence to suggests that tachyons actually exist, and many physicists deny even the possibility. A tachyon has never been observed or recorded (although the search continues, particularly through analysis of cosmic rays and in particle accelerators), and neither has one ever been created, so they remain hypothetical, although theory strongly supports their existence.

Research using MINOS and OPERA detectors has suggested that tiny particles called neutrinos may travel faster than light. Other more recent research from CERN, however, has put the findings into dispute, and the matter remains inconclusive. Neutrinos are not merely hypothetical particles like tachyons, but a well-known part of modern particle physics. But they are tiny, almost-massless, invisible, electrically neutral, weakly-interacting particles that pass right through normal matter, and consequently are very difficult to measure and deal with (even their mass has never been measured accurately).

Time Travel Paradoxes

The possibility of travel backwards in time is generally considered by scientists to be much more unlikely than travel into the future. The idea of time travel to the past is rife with problems, not least the possibility of temporal paradoxes resulting from the violation of causality (i.e. the possibility that an effect could somehow precede its cause). This is most famously exemplified by the grandfather paradox: if a hypothetical time traveller goes back in time and kills his grandfather, the time traveller himself would never be born when he was meant to be; if he is never born, though, he is unable to travel through time and kill his grandfather, which means that he WOULD be born; etc, etc.

Some have sought to justify the possibility of time travel to the past by the very fact that such paradoxes never actually arise in practice. For example, the simple fact that the time traveller DOES exist at the start of his journey is itself proof that he could not kill his grandfather or change the past in any way, either because free will ceases to exist in the past, or because the outcomes of such
decisions are predetermined. Or, alternatively, it is argued, any changes made by a hypothetical future time traveller must already have happened in the traveller’s past, resulting in the same reality that the traveller moves from.

Theoretical physicist Stephen Hawking has suggested that the fundamental laws of nature themselves – particularly the idea that causes always precede effects – may prevent time travel in some way. The apparent absence of “tourists from the future” here in our present is another argument, albeit not a rigorous one, that has been put forward against the possibility of time travel, even in a technologically advanced future (the assumption here is that future civilizations, millions of years more technologically advanced than us, should be capable of travel).

Some interpretations of time travel, though, have tried to resolve such potential paradoxes by accepting the possibility of travel between “branch points”, parallel realities or parallel universes, so that any new events caused by a time traveller’s visit to the past take place in a different reality and so do not impact on the original time stream. The idea of parallel universes, first put forward by Hugh Everett III in his “many worlds” interpretation of quantum theory in the 1950s, is now quite mainstream and accepted by many (although by no means all) physicists.

>> Quantum Time

Relativistic Time

Albert Einstein
The idea of relativistic time is a direct result of Albert Einstein’s Theory of Relativity

Since Albert Einstein published his Theory of Relativity (the Special Theory in 1905, and the General Theory in 1916), our understanding of time has changed dramatically, and the traditional Newtonian idea of absolute time and space has been superseded by the notion of time as one dimension of space-time in special relativity, and of dynamically curved space-time in general relativity.

It was Einstein’s genius to realize that the speed of light is absolute, invariable and cannot be exceeded (and indeed that the speed of light is actually more fundamental than either time or space). In relativity, time is certainly an integral part of the very fabric of the universe and cannot exist apart from the universe, but, if the speed of light is invariable and absolute, Einstein realized, both space and time must be flexible and relative to accommodate this.

Although much of Einstein’s work is often considered “difficult” or “counter-intuitive”, his theories have proved (both in laboratory experiments and in astronomical observations) to be a remarkably accurate model of reality, indeed much more accurate than Newtonian physics, and applicable in a much wider range of circumstances and conditions.


One aspect of Einstein’s Special Theory of Relativity is that we now understand that space and time are merged inextricably into four-dimensional space-time, rather than the three dimensions of space and a totally separate time dimension envisaged by Descartes in the 17th Century and taken for granted by all classical physicists after him. With this insight, time effectively becomes just part of a coordinate specifying an object’s position in space-time.

It was Hermann Minkowski, Einstein’s one-time teacher and colleague, who gave us the classic interpretation of Einstein’s Special Theory of Relativity. Minkowski introduced the relativity concept of proper time, the actual elapsed time between two events as measured by a clock that passes through both events. Proper time therefore depends not only on the events themselves but also on the motion of the clock between the events. By contrast, what Minkowski called coordinate time is the apparent time between two events as measured by a distant observer using that observer’s own method of assigning a time to an event.

An event is both a place and a time, and can be represented by a particular point in space-time, i.e. a point in space at a particular moment in time. Space-time as a whole can therefore be thought of as a collection of an infinite number of events. The complete history of a particular point in space is represented by a line in space-time (known as a world line), and the past, present and future accessible to a particular object at a particular time can be represented by a three dimensional light cone (or Minkowski space-time diagram), which is defined by the limiting value of the speed of light, which intersects at the here-and-now, and through which the object’s world line runs its course.

Modern physicists therefore do not regard time as “passing” or “flowing” in the old-fashioned sense, nor is time just a sequence of events which happen: both the past and the future are simply “there”, laid out as part of four-dimensional space-time, some of which we have already visited and some not yet. So, just as we are accustomed to thinking of all parts of space as existing even if we are not there to experience them, all of time (past, present and future) are also constantly in existence even if we are not able to witness them. Time does not “flow”, then, it just “is”. This view of time is consistent with the philosophical view of eternalism or the block universe theory of time (see the section on Modern Philosophy).

According to relativity, the perception of a “now”, and particularly of a “now” that moves along in time so that time appears to “flow”, therefore arises purely as a result of human consciousness and the way our brains are wired, perhaps as an evolutionary tool to help us deal with the world around us, even if it does not actually reflect the reality. As Einstein himself remarked, “People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion”.

However, if time is a dimension, it does not appear to be the same kind of dimension as the three dimensions of space. For example, we can choose to move through space or not, but our movement through time is inevitable, and happens whether we like it or not. In fact, we do not really move though time at all, at least not in the same way as we move through space. Also, space does not have any fundamental directionality (i.e. there is no “arrow of space”, other than the downward pull of gravity, which is actually variable in absolute terms, depending on where on Earth we are located, or whether we are out in space with no gravitational effects at all), whereas time clearly does (see the section on The Arrow of Time).

With the General Theory of Relativity, the concept of space-time was further refined, when Einstein realized that perhaps gravity is not a field or force on top of space-time, but a feature of space-time itself. Thus, the space-time continuum is actually warped and curved by mass and energy, a warping that we think of as gravity, resulting in a dynamically curved space-time. In regions of very large masses, such as stars and black holes, space-time is bent or warped substantially by the extreme gravity of the masses, an idea often illustrated by the image of a rubber sheet distorted by the weight of a bowling ball.

Time Dilation
Time dilation is just one consequence of the Theory of Relativity and curved space-time

Also as a result of Einstein’s work and his Special Theory of Relativity, we now know that rates of time actually run differently depending on relative motion, so that time effectively passes at different rates for different observers travelling at different speeds, an effect known as time dilation. Thus, two synchronized clocks will not necessarily stay synchronized if they move relative to each other. There is a related effect in the spatial dimensions, known as length contraction, whereby moving bodies are actually foreshortened in the direction of their travel.

Time dilation (as well as the associated length contraction) is negligible and all but imperceptible at everyday speeds in the world around us, although it can be, and has been, measured with very sensitive instruments. However, it becomes much more pronounced as an object’s speed approaches the speed of light (known as relativistic speeds). If a spaceship could travel at, say, 99% of the speed of light, a hypothetical observer looking in would see the ship’s clock moving about twice as slow as normal (i.e. coordinate time is moving twice as slow as proper time), and the astronauts inside moving around apparently in slow-motion. At 99.5% of the speed of light, the observer would see the clock moving about 10 times slower than normal. At 99.9% of the speed of light, the factor becomes about 22 times, at 99.99% 224 times, and at 99.9999% 707 times, increasing exponentially. In the largest particle accelerators currently in use we can make time slow down by 100,000 times. At the speed of light itself, were it actually possible to achieve that, time would stop completely.

Perhaps the easiest way to think of this difficult concept is that, when an object or person moves in space-time, its movement “shares” some of its spatial movement with movement in time, in the same way as some northward movement is shared with westward movement when we travel northwest. What forces this sharing of dimensions is the invariant nature of the speed of light (slightly less than 300,000km/s), which is a fundamental constant of the universe that can never be exceeded. Thus, the slowing of time at relativistic speeds occurs, in a sense, to “protect” the inviolable cosmic speed limit (the speed of light).

It should be noted that, although a spaceship travelling at close to the speed of light would take 100,000 years to reach a distant star 100,000 light years away as judged by clocks on Earth, the astronaut in the spaceship might hardly age at all as he travels across the galaxy. This characteristic of relativistic time has therefore spawned much discussion of the possibility of time travel (see the separate section on Time Travel).

According to Einstein, then, time is relative to the observer, and more specifically to the motion of that observer. This is not to say that time is in some way capricious or random in nature – it is still governed by the laws of physics and entirely predictable in its manifestations, it is just not absolute and universal as Newton thought (see the section on Absolute Time), and things are not quite as simple and straightforward as he had believed. Some commentators, like the Christian philosopher William Lane Craig, have suggested that there may be a need to distinguish between the reality of time and our measurement of time: according to this line of thinking (which, it should be mentioned, is not a mainstream position in physics), time itself MAY be absolute, but the way we measure it must be relativistic.

One casualty of the Theory of Relativity is the notion of simultaneity, the property of two events happening at the same time in a particular frame of reference. According to relativistic physics, simultaneity is NOT an absolute property between events, as had always been taken for granted up to that point. Thus, what is simultaneous in one frame of reference will not necessarily be simultaneous in another. For objects moving at normal everyday speeds, the effect is small and can generally be ignored (so that simultaneity CAN normally be treated as an absolute property); but when objects approach relativistic speeds (close to the speed of light) with respect to one another, such intuitive relationships can no longer be assumed.

Gravitational Time Dilation

When Einstein extended his Special Theory of Relativity to his General Theory, it became apparent that a similar time dilation effect would also occur in the presence of intense gravity, an effect usually referred to as gravitational time dilation. It is almost as if gravity is somehow pulling or dragging on time, slowing its passage. The closer an object is to another object, the stronger the pull of gravity between them (according to an inverse-square law first identified by Sir Isaac Newton), and thus the more the time drag.

Again, these effects are negligible at the kinds of gravitational differences experienced in everyday life: even though, technically, a person living in a ground floor apartment ages slower than their twin who lives in a top floor apartment of the same building (due to the difference in gravity they experience), the effect might amount to maybe a microsecond over a full lifetime. There is, however, one aspect of modern everyday life where we do experience the effects of gravitational time dilation: it has a noticeable impact on the Global Positioning System (GPS), which many of us now rely on for navigation. The orbiting satellites used by the GPS system experience significantly less gravity than the Earth’s surface, and are also moving very fast, so that the time distortion effects of about 38 microseconds a day have to be specifically factored in or GPS would very quickly begin to accumulate errors.

But, just as with a spaceship travelling at near the speed of light, in the extreme gravity at the edges of a black hole, for example, substantial time differences can become apparent. A black hole spins at close to the speed of light, dragging anything in the vicinity around with it, and the huge gravitational pull of a black hole can bend and warp space-time to a substantial degree. Over the “event horizon” of a black hole – the gravitational point of no return – a hypothetical clock on a spaceship (and indeed the progress of the spaceship itself) would appear from the outside to stop completely due to the infinite time dilation effect. At the gravitational singularity at the centre of a black hole, gravity and density is infinite, and all the normal rules of physics just break down. Time effectively stops, just as there is no time beyond the singularity of the Big Bang (see the section on Time and the Big Bang).

Twins Paradox

The dilation of time also gives rise to the so-called “twins paradox” or “clock paradox”, whereby a hypothetical astronaut returns from a near-light speed voyage in space to find his stay-at-home twin many years older than him (as travelling at relativistic high speeds has allowed the astronaut to experience only, say, one year of time, while ten years have elapsed on Earth). Because of the time dilation effect, a clock in the spaceship literally registers a shorter duration for the journey than the clock in mission control on Earth.

The real paradox, though, as Einstein explained it, arises from the fact that (because there is no “preferred” frame of reference in relativity) we could just as easily consider the traveller in the spaceship as the one remaining at rest, while the Earth shoots off and back at close to the speed of light. In that scenario, Einstein argued, one would expect the astronaut to age much more than the inhabitants of the Earth. In fact the “paradox” is explained by Mach’s Principle: the spaceship is accelerating away at near-light speed from the bulk of the universe, whereas the Earth is not. Hence, it is the spaceship (and its astronaut) that experiences the relativistic time dilation, not the Earth.

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