What is the Grandfather Paradox?

If you’ve ever seen Back to the Future, you probably know that you shouldn’t be messing with things your parents did before you were born. At least, not unless you want to erase yourself from existence. The plot of Robert Zemeckis’ 1985 blockbuster is an excellent example of the Grandfather Paradox. However, the concept is much richer and more profound than that.

The Grandfather Paradox is a self-canceling contradiction that arises from changing the past. A person who travels through time to and kills their grandfather before the birth of their own parents is the classic example.

The change caused by the time traveler would logically negate their own existence. However, since the traveler never existed, then they never created that difference. Contradictorily, the absence of the traveler means the change never took place. Is the traveler born after all? 

Name notwithstanding, the Grandfather Paradox does not solely depend on killing one’s own grandfather to prevent one’s birth. Any action that changes the past in a contradictory way fulfills the definition.

The Origins of the Grandfather Paradox

The earliest known appearance of the Grandfather Paradox is a novelette called Paradox, written by Charles Cloukey in 1929. Although Cloukey was only 16 at the time of publishing, the story outlines many intricate paradoxes of time travel. In the end, his book concludes that backward time travel is, in fact, impossible. Cloukey writes, “the fact that I was pre-sent to kill my unfortunate grandfather would show that I had been born. Therefore, I could not have killed my grandfather. It was hopeless.” 

Many other science fiction stories of the early 20th century dealt with this paradox, with examples arising in works such as Ancestral Voices by Nathaniel Schachner and Future Times Three, by René Barjavel in 1943.

Variants of the Grandfather Paradox

The grandfather paradox includes any contradictory change to the past, and the concept has many variations. A notable example is Physicist John Garrison’s 1998 publication entitled Superluminal Signals: Causal Loop Paradoxes Revisited. In it, Garrison describes a machine which receives a signal from itself to turn itself off before that signal is sent.

Other variations include going back in time to kill yourself as a child, or going back in time to kill Adolf Hitler before his rise to power. 

Hitler’s murder is an exceptional version of this paradox. Instead of preventing the action of time travel, killing Hitler removes any reason for time travel at all. However, as Hitler’s existence was so pivotal, anyone born after the war would have their life influenced in some way. It’s an interesting thought experiment.

Some ideas support a parallel universe answer to the grandfather paradox. This variant suggests that when a time traveler kills their grandfather, they are creating (or arrived in) a separate timeline, individual, and apart from the one the traveler came from. Some may argue that this isn’t real time travel, but it does solve the paradox.

The Philosophies Behind the Grandfather Paradox

Regardless of whether time travel to the past is possible, we can use logic to show that changes to the past can result in a logical contradiction. If events in history happened in a certain way, and we know our records to be accurate, then events can’t have occurred in any other way. A time traveler should not be able to change the past from the way it is, meaning it is only possible for them to act in a manner consistent with those events.

The study of the Grandfather Paradox has led some to conclude that time travel is paradoxical by its very nature and, therefore, impossible. The philosopher Bradley Dowden made this argument in the textbook Logical Reasoning, maintaining that the likelihood of creating a contradiction entirely rules out time travel to the past. 

Still, some philosophers and scientists believe that time travel into the past is not logically impossible, provided care is taken not to change said past. Bradley Dowden himself was persuaded to change the views outlined in Logical Reasoning after a discussion with famed philosopher Norman Swartz:

I found your argument about the visitor from 2045 very convincing. I guess I’ve never thought very seriously about the issue of time travel. Yes, I now believe it is logically possible to build a time machine which allows travel into the past where one does not change the past. This would be ‘real’ time travel, not just a disembodied existence passively viewing the scene. The person could contribute to making the past just the way it was.

Previously when I’ve thought about time travel into the past I’ve thought of that kind of travel that meets the demand “Oh, I wish I could go back and make the Toronto Blue Jays lose the World Series, or go back and make Adolf Hitler slip on a banana peel and die at the age of seven.” That sort of time travel, in which the past does get changed, is logically impossible, and that is the only kind of time travel I was considering when I wrote page 202 of my textbook. So I’ll change my ways in the future. I’ll change the first sentence at the top of page 202 to say “Nobody has ever built a time machine that could take a person back to an earlier time to change what has happened.” No, I believe the whole paragraph should be rewritten.

Full conversation can be found here:

Causal Loops and the Grandfather Paradox

Time travel to the past, which does not create a Grandfather Paradox, creates a causal loop. The Novikov self-consistency principle declares one view on how backward time travel would be achievable without the creation of paradoxes. According to the theory, physics in or near closed timelike curves (or a time machine) can only be compatible with those of the universal laws of physics. This means that only self-consistent events can occur, so actions taken by a time traveler must have been part of history all along. 

Does this imply that a time traveler can do anything they want without consequence? Novikov used an example by physicist Joseph Polchinski for the Grandfather Paradox. A billiard ball is heading towards a time machine, and the ball’s older self emerges from it and strikes its younger self. Therefore, the younger version of itself never enters the time machine. Novikov explains:

There is a mistake in the previous discussion (the reason for the ‘paradox’): when at the beginning of our discussion we continued the trajectory a after point z, we did not take into account the influence of the impact and considered the motion of this ball along the trajectory a2 without taking into account this impact. This means that we did not take into account the influence of the future on the past.


MIT’s Seth Lloyd and other researchers have proposed an augmented version of the Novikov principle, in which probability molds itself to stop paradoxes from occurring at all. These outcomes would become stranger and more incomprehensible as one approaches a forbidden act, as the universe must favor improbable events to avoid impossible ones. This theory is explored at length in Stephen King’s novel, 11/22/63, in which the protagonist faces increasingly unexplainable obstacles while trying to prevent the assassination of John F. Kennedy.

The Grandfather Paradox and Quantum Physics

Physicists, David Deutsch, Daniel Greenberger, and Juergen Tolks have asserted that quantum theory allows for time travel where the past must be self-consistent. Deutsch reasons that backward time travel only produces only self-consistent solutions, and that violating chronology imposes constraints that are not apparent through classical reasoning. 

In 2014, researchers published a simulation validating Deutsch’s model using photons in an experiment:

In their new simulation Ralph, Ringbauer and their colleagues studied Deutsch’s model using interactions between pairs of polarized photons within a quantum system that they argue is mathematically equivalent to a single photon traversing a CTC. “We encode their polarization so that the second one acts as kind of a past incarnation of the first,” Ringbauer says. So instead of sending a person through a time loop, they created a stunt double of the person and ran him through a time-loop simulator to see if the doppelganger emerging from a CTC exactly resembled the original person as he was in that moment in the past.

Deutsch uses the language of “multiple universes” in his article to represent the quantum phenomena, but notes that this terminology is inadequate. Others have taken this to mean that this model of time travel involves emerging in a different universe, which avoids the grandfather paradox.