Before discussing the physics and science of time travel, we first have to define what we mean by time travel. Here I am presenting several possible types of time travel.

I. What is time? Does time really even exist?

What is time? Where does time come from? In what way is time objective, something actually out there?  In what ways is time not real, but just a way that humans use to describe our perception of the universe?

To some extent, understanding these questions requires knowing something about thermodynamics especially the second law of thermodynamics. Beyond that, a deeper understanding of the nature of time may rely on understanding modern physics, especially quantum mechanics .

The following article discusses entropy and the thermodynamic arrow of time; the question of ‘does time really flow?, the concept of a block universe, and presentism versus eternalism – What is time?

II. Types of time Travel

This text is currently based on the 2008 version of the Wikipedia article.

1. There is a single fixed history, which is self-consistent and unchangeable.

In this view, everything happens on a single timeline which doesn’t contradict itself.

1.1 This can be simply achieved by applying the Novikov self-consistency principle, named after Dr. Igor Dmitrievich Novikov, Professor of Astrophysics at Copenhagen University. The principle states that the timeline is totally fixed, and any actions taken by a time traveler were part of history all along, so it is impossible for the time traveler to “change” history in any way.

The time traveler’s actions may be the cause of events in their own past though, which leads to the potential for circular causation and the predestination paradox; for examples of circular causation, see Robert A. Heinlein’s story “By His Bootstraps”.

The Novikov consistency principle assumes certain conditions about what sort of time travel is possible. Specifically, it assumes either that there is only one timeline, or that any alternative timelines (such as those postulated by the many-worlds interpretation of quantum mechanics) are not accessible.

Given these assumptions, the constraint that time travel must not lead to inconsistent outcomes could be seen merely as a tautology, a self-evident truth that can not possibly be false.

However, the Novikov self-consistency principle is intended to go beyond just the statement that history must be consistent, making the additional nontrivial assumption that the universe obeys the same local laws of physics in situations involving time travel that it does in regions of space-time that lack closed timelike curves. This is clarified in the paper “Cauchy problem in spacetimes with closed timelike curves”, where the authors write:

That the principle of self-consistency is not totally tautological becomes clear when one considers the following alternative: The laws of physics might permit CTCs; and when CTCs occur, they might trigger new kinds of local physics which we have not previously met. … The principle of self-consistency is intended to rule out such behavior. It insists that local physics is governed by the same types of physical laws as we deal with in the absence of CTCs: the laws that entail self-consistent single valuedness for the fields. In essence, the principle of self-consistency is a principle of no new physics. If one is inclined from the outset to ignore or discount the possibility of new physics, then one will regard self-consistency as a trivial principle.

1.2 Alternatively, new physical laws take effect regarding time travel that thwarts attempts to change the past (contradicting the assumption mentioned in 1.1 above that the laws that apply to time travelers are the same ones that apply to everyone else).

These new physical laws can be as unsubtle as to reject time travelers who travel to the past to change it by pulling them back to the point from when they came as Michael Moorcock’s The Dancers at the End of Time or where the traveler is rendered an noncorporeal phantom unable to physically interact with the past such as in some Pre-Crisis Superman stories and Michael Garrett’s “Brief Encounter” in Twilight Zone Magazine May 1981.

 

2. History is flexible and is subject to change (Plastic Time)

2.1 Changes to history are easy and can impact the traveler, the world, or both

Examples include Back to the Future, Back to the Future II, and Doctor Who.

In some cases (such as Doctor Who) any resulting paradoxes can be devastating, threatening the very existence of the universe. In other cases the traveler simply cannot return home. The extreme version of this (Chaotic Time) is that history is very sensitive to changes with even small changes having large impacts such as in Ray Bradbury’s A Sound of Thunder

2.2 History is change resistant in direct relationship to the importance of the event i.e. small trivial events can be readily changed but large ones take great effort.

In the Twilight Zone episode “Back There” a traveler tries to prevent the assassination of President Lincoln and fails but his actions have turned what had originally been the butler of the club that the traveler belonged to into a rich tycoon.
In The Time Machine (2002 film) it is explained via a vision why Hartdegen could not save his sweetheart Emma–doing so would have resulted in him never developing the time machine he used to try and save her.

The Saga of Darren Shan, where major events in the past cannot be changed, but minor events can be affected. Under this model, if a time traveler were to go back in time and kill Hitler, another Nazi would simply take his place and commit his same actions, leaving the broader course of history unchanged.

 

3. Many-worlds interpretation and Parallel universe (fiction)

These terms are often used interchangeably in fiction but mechanically they differ:

The Many Worlds interpretation says time travel creates a coexisting alternate history –

while the second idea says that the traveler actually goes to an already existing parallel world.

In either case the traveler’s original home reality continues to exist unaffected. These versions of time travel are sometimes placed under one of the two above categories.

James P. Hogan’s The Proteus Operation fully explains parallel universe time travel in chapter 20 where it has Einstein explaining that all the outcomes already exist and all time travel does is change which already existing branch you will experience.

Star Trek has a long tradition of using the 2.1 mechanic, as seen in the episodes City on the Edge of Forever, Tomorrow is Yesterday, Time and Again (Star Trek: Voyager), Future’s End, Before and After (Star Trek: Voyager), Endgame (Star Trek: Voyager) and as late as Enterprise’s Temporal Cold War,

The Star Trek episode Parallels had an example of what Data called a quantum realities. His exact words on the matter were “But there is a theory in quantum physics that all possibilities that can happen do happen in alternate quantum realities.” leaving it up the viewer as to the exact nature of these quantum realities.

Michael Crichton’s novel Timeline takes the approach that all time travel really is is travel to an already existing parallel universe where time passes at a slower rate than our own but changes in any of these parallel universe effects the main timeline making it behave as it if was a type 2 universe.

Discussion

While a Type 1 universe will prevent a grandfather paradox it doesn’t prevent paradoxes in other aspects of physics such as the predestination paradox and the ontological paradox (GURPS Infinite Worlds calls this Free Lunch Paradox).

The predestination paradox is where the traveler’s actions create some type of causal loop, in which some event A in the future helps cause event B in the past via time travel, and the event B in turn is one of the causes of A.

For instance, a time traveler might go back to investigate a specific historical event like the Great Fire of London, and their actions in the past could then inadvertently end up being the original cause of that very event.

Examples of this kind of causal loop are found in Timemaster, a novel by Dr. Robert Forward, the Twilight Zone episode “No Time Like the Past”, the 1980 Jeannot Szwarc film Somewhere In Time (based on Richard Matheson’s novel Bid Time Return), the Michael Moorcock novel Behold the Man, and Harry Potter and the Prisoner of Azkaban.

The Novikov self-consistency principle can also result in an ontological paradox (also known as the knowledge or information paradox) where the very existence of some object or information is a time loop.

The philosopher Kelley L. Ross argues in Time Travel Paradoxes that in an ontological paradox scenario involving a physical object, there can be a violation of the second law of thermodynamics. Ross uses Somewhere in Time as an example where Jane Seymour’s character gives Christopher Reeve’s character a watch she has owned for many years, and when he travels back in time he gives the same watch to Jane Seymour’s character 60 years in the past.

 

Time travel to the future in standard physics

There are various ways in which a person could “travel into the future” in a limited sense: the person could set things up so that in a small amount of his own subjective time, a large amount of subjective time has passed for other people on Earth.

For example, an observer might take a trip away from the Earth and back at relativistic velocities, with the trip only lasting a few years according to the observer’s own clocks, and return to find that thousands of years had passed on Earth.

This form of “travel into the future” is theoretically allowed using the following methods:

Using velocity-based time dilation under the theory of special relativity, for instance:

Traveling at almost the speed of light to a distant star, then slowing down, turning around, and traveling at almost the speed of light back to Earth. (see the Twin paradox)

Using gravitational time dilation under the theory of general relativity, for instance:

Residing inside of a hollow, high-mass object;

Residing just outside of the event horizon of a black hole, or sufficiently near an object whose mass or density causes the gravitational time dilation near it to be larger than the time dilation factor on Earth.

Additionally, it might be possible to see the distant future of the Earth using methods which do not involve relativity at all, although it is even more debatable whether these should be deemed a form of “time travel”:

Hibernation

Suspended animation

Time Dilation

Time dilation is permitted by Albert Einstein’s special and general theories of relativity. These theories state that, relative to a given observer, time passes more slowly for bodies moving quickly relative to that observer, or bodies that are deeper within a gravity well. For example, a clock which is moving relative to the observer will be measured to run slow in that observer’s rest frame; as a clock approaches the speed of light it will almost slow to a stop, although it can never quite reach light speed so it will never completely stop.

For two clocks moving inertially (not accelerating) relative to one another, this effect is reciprocal, with each clock measuring the other to be ticking slower. However, the symmetry is broken if one clock accelerates, as in the twin paradox where one twin stays on Earth while the other travels into space, turns around (which involves acceleration), and returns—in this case both agree the traveling twin has aged less.

General relativity states that time dilation effects also occur if one clock is deeper in a gravity well than the other, with the clock deeper in the well ticking more slowly; this effect must be taken into account when calibrating the clocks on the satellites of the Global Positioning System, and it could lead to significant differences in rates of aging for observers at different distances from a black hole.

Time perception

Time perception can be apparently sped up for living organisms through hibernation, where the body temperature and metabolic rate of the creature is reduced. A more extreme version of this is suspended animation, where the rates of chemical processes in the subject would be severely reduced.

Time dilation and suspended animation only allow “travel” to the future, never the past, so they do not violate causality, and it’s debatable whether they should be called time travel.

However time dilation can be viewed as a better fit for our understanding of the term “time travel” than suspended animation, since with time dilation less time actually does pass for the traveler than for those who remain behind, so the traveler can be said to have reached the future faster than others, whereas with suspended animation this is not the case.

 

Mutable timelines

Time travel in a Type 2 universe is much more complex. The biggest problem is how to explain changes in the past. One method of explanation is that once the past changes, so too do the memories of all observers. This would mean that no observer would ever observe the changing of the past (because they will not remember changing the past).

This would make it hard to tell whether you are in a Type 1 universe or a Type 2 universe.

You could, however, infer such information by knowing if a) communication with the past were possible or b) it appeared that the time line had never been changed as a result of an action someone remembers taking, although evidence exists that other people are changing their time lines fairly often.

An example of this kind of universe is presented in Thrice Upon a Time, a novel by James P. Hogan. The Back to the Future trilogy films also seem to feature a single mutable timeline (see the Back to the Future FAQ for details on how the writers imagined time travel worked in the movies’ world). By contrast, the short story “Brooklyn Project” by William Tenn provides a sketch of life in a Type 2 world where no one even notices as the timeline changes repeatedly.

In type 2.1, attempts are being made at changing the timeline, however, all that is accomplished in the first tries is that the method in which decisive events occur is changed; final conclusions in the bigger scheme cannot be brought to a different outcome.

As an example, the movie Deja Vu depicts a paper note sent to the past with vital information to prevent a terrorist attack. However, the vital information results in the killing of an ATF agent, but does not prevent the terrorist attack; the very same agent died in the previous version of the timeline as well, albeit under different circumstances. Finally, the timeline is changed by sending a human into the past, arguably a “stronger” measure than simply sending back a paper note, which results in preventing both a murder and the terrorist attack. As in the Back to the Future movie trilogy, there seems to be a ripple effect too as changes from the past “propagate” into the present, and people in the present have altered memory of events that occurred after the changes made to the timeline.

The science fiction writer Larry Niven suggests in his essay “The Theory and Practice of Time Travel” that in a type 2.1 universe, the most efficient way for the universe to “correct” a change is for time travel to never be discovered, and that in a type 2.2 universe, the very large (or infinite) number of time travelers from the endless future will cause the timeline to change wildly until it reaches a history in which time travel is never discovered.

However, many other “stable” situations might also exist in which time travel occurs but no paradoxes are created; if the changeable-timeline universe finds itself in such a state no further changes will occur, and to the inhabitants of the universe it will appear identical to the type 1.1 scenario.[citation needed] This is sometimes referred to as the “Time Dilution Effect”.

Few if any physicists or philosophers have taken seriously the possibility of “changing” the past except in the case of multiple universes, and in fact many have argued that this idea is logically incoherent, so the mutable timeline idea is rarely considered outside of science fiction.

Also, deciding whether a given universe is of Type 2.1 or 2.2 can not be done objectively, as the categorization of timeline-invasive measures as “strong” or “weak” is arbitrary, and up to interpretation: An observer can disagree about a measure being “weak”, and might, in the lack of context, argue instead that simply a mishap occurred which then led to no effective change.

An example would be the paper note sent back to the past in the film Deja Vu, as described above. Was it a “too weak” change, or was it just a local-time alteration which had no extended effect on the larger timeline? As the universe in Deja Vu seems not entirely immune to paradoxes (some arguably minute paradoxes do occur), both versions seem to be equally possible.

Alternate histories

In Type 3, any event that appears to have caused a paradox has instead created a new time line. The old time line remains unchanged, with the time traveler or information sent simply having vanished, never to return. A difficulty with this explanation, however, is that conservation of mass-energy would be violated for the origin timeline and the destination timeline.

A possible solution to this is to have the mechanics of time travel require that mass-energy be exchanged in precise balance between past and future at the moment of travel, or to simply expand the scope of the conservation law to encompass all timelines. Some examples of this kind of time travel can be found in David Gerrold’s book The Man Who Folded Himself and The Time Ships by Stephen Baxter, plus several episodes of the TV show Star Trek: The Next Generation.