What are the main time travel theories?
Most time travel stories, and most of the serious thinking about time travel, have to answer one question: what happens if you go back and change something? There are three common answers, and Harrison Densmore’s infographic puts them side by side with an example and the films that use each one.
- Fixed timeline. There is one history and it cannot be changed. Whatever a time traveller does in the past was always part of it.
- Dynamic timeline. There is one history, but it can be rewritten. Change the past and the present changes with it.
- Multiverse. Going into the past starts a separate branch of history. You can change that branch as much as you like, but the timeline you came from carries on as before.
These are not competing scientific findings. Nobody has travelled into the past, so none of them has been tested. They are ways of dealing with the logical problems that backwards time travel throws up, and physicists and philosophers have argued about each of them.
Fixed timeline and the Novikov self-consistency principle
In a fixed timeline the past has already happened and nothing can undo it. A time traveller can go back and act, but their actions were part of history all along. The infographic’s example shows how this plays out: someone goes back to kill Adolf Hitler as a baby to prevent the Second World War, swaps him for an orphaned baby so the family won’t notice, and returns home to find that the replacement baby grew up to become Adolf Hitler.
The physics version of this idea is the Novikov self-consistency principle, developed by the Russian physicist Igor Novikov in the mid 1980s [2]. Some solutions of Einstein’s general relativity allow closed timelike curves, paths through spacetime that loop back to their own past, so in theory they allow time travel. Novikov’s principle says that if an event would cause a paradox, or any change to the past at all, the probability of that event is zero [2]. You can visit the past, but you can only do things that are consistent with what already happened.
Stories built this way often turn into a causal loop, where an event ends up causing itself. The Terminator is the usual example: Kyle Reese is sent back to protect Sarah Connor and becomes the father of John Connor, who sent him. Harry Potter and the Prisoner of Azkaban and 12 Monkeys follow the same rule.
Dynamic timeline and the grandfather paradox
In a dynamic timeline the past can be changed, and the present changes to match. This is what most people imagine when they think about time travel, and it is the model used in Back to the Future, where Marty McFly’s family starts to fade from a photograph after he interferes with how his parents met.
The problem with a changeable past is the grandfather paradox. If you go back and kill your grandfather before your parent is born, you are never born, so you never travel back, so your grandfather is never killed, so you are born and go back after all. Each answer leads to the opposite one. The Stanford Encyclopedia of Philosophy describes the grandfather paradox as the most important objection to the logical possibility of backwards time travel, and notes that it has convinced many people that such travel is impossible [1].
There are replies. The philosopher David Lewis argued in 1976 that a time traveller could have the ability to kill their grandfather in one sense, yet would always fail “for some commonplace reason”, such as the gun jamming or a slip on a banana peel [1]. That reply effectively turns a dynamic timeline back into a fixed one.
Multiverse and alternate timelines
The third theory avoids the paradox by giving the time traveller somewhere else to go. In a multiverse, travelling into the past creates a new timeline that splits off from the original one. As the infographic puts it, you could go back and kill all your grandparents and nothing would happen to you: you have simply made a timeline in which you will never be born, while your own timeline carries on. The catch is that you can’t get back to where you started.
This idea borrows from the many worlds interpretation of quantum physics, which holds that all possible quantum events can happen in separate histories [3]. Some philosophers argue that it isn’t really time travel, because the traveller arrives in someone else’s past rather than their own [3]. Stephen Hawking also argued that even if many worlds is correct, each time traveller should still experience a single self-consistent history [3].
Fiction uses the multiverse to let characters change history without breaking the plot. The infographic lists Terminator 2 and 3, the British series Misfits and the 2009 Star Trek film, where the arrival of a ship from the future creates the separate “Kelvin” timeline. Which films belong in which group is open to debate, and fans of the Terminator series in particular disagree about how its timelines work.
Is time travel possible according to physics?
Travel into the future is possible, and it happens all the time on a very small scale. Einstein’s relativity says that time passes more slowly for something moving fast or sitting deeper in a gravitational field. In 1971 the physicist Joseph Hafele and the astronomer Richard Keating flew four atomic clocks around the world on commercial airliners and compared them with clocks at the US Naval Observatory. The clocks flown westward gained 273 nanoseconds and those flown eastward lost 59, in line with what relativity predicted [4]. GPS satellites have to correct for the same effect. Without it, sat nav positions would drift by about 10 km a day [5].
The same effect means an astronaut who spends a long time travelling fast ages slightly less than a twin who stays at home. The BBC points to the twins Scott and Mark Kelly, who did this for real when Scott spent months in space, although at speeds nowhere near that of light [5]. The difference is fractions of a second, not years.
Travel into the past is a different matter. General relativity has solutions with closed timelike curves, the first described by the logician Kurt Gödel in 1949, and wormholes are allowed in theory [5]. Kip Thorne, Michael Morris and Ulvi Yurtsever revived interest in wormhole time machines with a paper in 1988 [2]. But there is no evidence that wormholes exist, and any that did would likely be extremely short lived [5]. In a 1992 paper Hawking proposed the chronology protection conjecture, the idea that the laws of physics stop time machines forming at all [6]. Physicists can’t settle the question without a theory of quantum gravity [7].
Which time travel theory is most likely?
Nobody knows, because nobody knows whether travel into the past is possible at all. Of the three, self-consistency has had the most attention from physicists. The physicist Joseph Polchinski challenged it with a thought experiment in which a billiard ball goes back through a wormhole and knocks its earlier self off course, so it never enters the wormhole. Two Caltech students, Fernando Echeverria and Gunnar Klinkhammer, found that the scenario also had consistent solutions, such as a glancing blow that sends the ball into the wormhole after all [2]. That doesn’t prove the fixed timeline is right, but it showed the idea could hold up in the maths.
The dynamic timeline is the hardest to defend, because it leads straight into the grandfather paradox. The multiverse avoids the paradox but depends on an interpretation of quantum mechanics that is itself still argued over.
For anyone writing or watching time travel fiction, the useful point is that each model has its own rules. A story that switches between them part way through is usually where the plot holes come from.
Questions about time travel theories
What are the three theories of time travel?
A fixed timeline, where the past cannot be changed; a dynamic timeline, where changing the past changes the present; and a multiverse, where changing the past creates a separate timeline and leaves the original one alone.
What is the grandfather paradox?
It is the contradiction that comes from going back in time and killing your own grandfather before your parent is born. If you succeed you are never born, so you can’t travel back to do it [1].
What is the Novikov self-consistency principle?
A principle developed by Igor Novikov in the mid 1980s which says that any event that would change the past or cause a paradox has a probability of zero. Time travel would be possible, but only in ways consistent with history [2].
Is time travel to the future possible?
Yes, in a limited sense. Relativity means fast-moving or high-altitude clocks run at a different rate, which has been measured with atomic clocks on aircraft and has to be corrected for on GPS satellites [4][5]. The effect is tiny at the speeds people can reach.
Has anyone proved time travel to the past is possible?
No. Some solutions of general relativity allow it in theory, but there is no evidence that the conditions they need exist, and Hawking’s chronology protection conjecture suggests physics may rule it out [5][6].
Sources
- Stanford Encyclopedia of Philosophy, Time Travel.
- Wikipedia, Novikov self-consistency principle.
- Wikipedia, Temporal paradox.
- Wikipedia, Hafele-Keating experiment.
- BBC Future, Is time travel really possible? Here’s what physics says, November 2023.
- Wikipedia, Chronology protection conjecture.
- Wikipedia, Time travel.
- Tumblr (inkydavid), 3 Theories of Time Travel by Harrison Densmore, 30 August 2014 (earliest copy of the infographic we found).
Last reviewed 4 November 2026
