What is the grandfather paradox?
The grandfather paradox is a contradictory situation that arises in some time travel scenarios exemplified by the impossible scenario in which a person travels back in time only to kill their grandfather (who could no longer continue to produce their parent, and so where does that leave you and your killer event ancestors?). The paradox is sometimes taken as an argument against the logical possibility of going back in time, according to the Stanford Encyclopedia of Philosophy. Within the framework of modern physics, however, there are ways to avoid the paradox without giving up time travel altogether.
Related: 5 possible sci-fi concepts (in theory)
The Grandfather Paradox Explained
Suppose you have a time machine that allows you to travel to the past. While you’re at it, you accidentally kill one of your grandparents – or any other direct ancestors – before they have any offspring. It would alter a whole chain of future events, including your own birth, which would no longer occur. But if you weren’t born in the future, you couldn’t kill your ancestor in the past – hence the paradox. It’s a scenario that became popular in science fiction magazines of the 1920s and 1930s, according to the historical dictionary of science fictionand the name “grandfather’s paradox” was firmly established in 1950.
In fact, you don’t even have to kill anyone; there are many other ways to change history that would lead to your future non-existence. Perhaps the best-known example is the movie “Back to the Future,” in which the time-traveling protagonist inadvertently drives a wedge between his parents before they get married – and then has to work. frantically to bring them together again.
Is the grandfather paradox possible?
Moving from science fiction to scientific fact, one person eminently qualified to talk about the realities of time travel was the late Stephen Hawking, arguably the most brilliant physicist of recent times. In 1999, he gave a lecture on “spatial and temporal distortions”, which showed how Einsteinthe theory of general relativity could make time travel possible, bending space-time back to himself.
One theoretical possibility that would allow for time travel (and therefore the ability to somehow kill a critical ancestor) is a special kind of wormhole. Among the most dramatic consequences of general relativity, wormholes are often described as shortcuts between one point in space and another. But, as Hawking explained in his lecture, a wormhole could possibly revert to an earlier point in time – a situation technically known as a “time-like closed curve” (CTC).
But if physics allows us to go back in time, wouldn’t the grandfather paradox still cause problems? Hawking suggested two possible ways around the paradox in this scenario. First, there is what he called the “coherent histories” model, in which all of time – past, present and future – is rigidly predetermined; this way you can only go back to an earlier point in time if you had already been there in your own story. In this “block universe” model, as it is sometimes called, one could travel to the past, but it wouldn’t change it, depending on the Australian Broadcasting Corporation. From this perspective, the grandfather paradox could never arise. With Hawking’s second option, however, the situation is more subtle.
Grandfather paradox and parallel worlds
This second approach to time travel invokes quantum physicswhere an event can have multiple possible outcomes with different probabilities of occurring.
As described by the Stanford Encyclopedia of Philosophy, the “many-worlds” interpretation of quantum theory sees all of these different outcomes as occurring in different, “parallel” timelines. From this perspective, the grandfather paradox could be resolved if the time traveler begins in a timeline where his grandfather lived long enough to have children, and then – after returning and killing his ancestor – continuing the along a parallel time track in which it will never be born. (Stanford Encyclopedia has a more detailed look at why you can’t move back and forth between parallel timelines at will.) As Hawking pointed out in his 1999 lecture, this seems to be the underlying assumption behind sci-fi treatments such as “Back to the Future”. .”
At the time this film was made in 1985, the “parallel world” explanation of the Grandfather Paradox was just philosophical conjecture. In 1991, however, it was put on firmer ground by physicist David Deutsch, as new scientist reported at the time. Deutsch showed that, while parallel timelines are normally unable to interact with each other, the situation changes in the vicinity of a closed time-like curve (CTC), when a wormhole bends over himself. Here, just as science fiction writers had imagined, the different timelines can intersect – so that when a CTC returns to the past, it’s the past of a different timeline. If proven, then you really could kill an infant grandparent without paradoxically eliminating yourself in the process. In this case, your grandfather would have only ever existed in a parallel world. And you, the grandfather-killer, would have existed only in the other.
The paradox of the grandfather solved?
Surprising as it may seem, there is in fact experimental support for Deutsch’s solution to the grandfather paradox. In 2014, a team from the University of Queensland looked at a simpler time travel scenario that involved a similar logical paradox. The researchers described the work in their paper published that year in the journal Nature Communication. The idea was that a subatomic particle had to travel back in time to flip the switch that resulted in its creation; if the switch wasn’t flipped, the particle would never exist in the first place.
A key feature of Deutsch’s theory is that the various probabilities must be self-consistent. For example, in the Queensland research example, if there is a 50:50 chance that the particle will go back in time, then there must also be a 50:50 chance that the switch will be flipped to create this particle first. In the absence of a time machine, the researchers set up an experiment involving a pair of photons, which they say is logically equivalent to a single photon traveling through time to “create” itself. The experiment was a success, with the results validating Deutsch’s self-consistency theory.
Grandfather paradox and butterfly effect
Killing your grandfather as a child is a surefire way to ensure you never get born. But there are also more subtle possibilities of messing up the timeline. In a sufficiently complex system, even the slightest change can have serious long-term consequences — as in the butterfly Effect, whereby the flapping of a butterfly’s wings can eventually trigger a tornado thousands of miles away. Science fiction writer Ray Bradbury produced a time travel counterpart in his 1952 story “A Sound of Thunder”, which can be read online at the Internet Archive. The protagonist of Bradbury travels to the time of the dinosaurs, where he accidentally steps on a butterfly – then returns to the present to find that society has changed beyond recognition. It’s easy to imagine that, if the societal changes were significant enough, the time traveler could have prevented his own birth as surely as if he had killed a grandparent.
But would this really be the case, using the quantum approach to the Grandfather Paradox? Recent work at Los Alamos National Laboratory indicates that the course of history is more resilient than the butterfly effect suggests. The researchers used a quantum computer to simulate time travel to the past, where information has been deliberately damaged – the computer equivalent of stepping on a Jurassic-era butterfly. But unlike Bradbury’s story, the ripple effect in the computer simulation’s “present” turned out to be relatively small and insignificant. This is, of course, great news for future time travelers. As long as you refrain from blatantly stupid acts like killing a direct ancestor, it may be possible to go back in time without any paradoxical consequences.
Historical Dictionary of Science Fiction. https://sfdictionary.com/view/2178/grandfather-paradox
“Multi-world interpretation of quantum mechanics”, Stanford Encyclopedia of Philosophy, 2021. https://plato.stanford.edu/entries/qm-manyworlds/
“Time travel without paradoxes”, New Scientist, 1992. https://www.newscientist.com/article/mg13318143-000-science-time-travel-without-the-paradoxes/
“The block universe theory, where time travel is possible but passing time is an illusion”, Australian Broadcasting Corporation, 2018. https://www.abc.net.au/news/science/2018-09-02/block-universe-theory-time-past-present-future-travel/10178386
“Experimental simulation of closed temporal curves”, Nature Communications, 2014. https://www.nature.com/articles/ncomms5145
“A sound of thunder”, Ray Bradbury, Internet Archive. https://archive.org/details/Planet_Stories_v06n04_1954-01/page/n5/mode/2up
“Quantum ‘time travel’ simulation disproves the butterfly effect in the quantum realm”, Los Alamos National Laboratory, 2020. https://www.lanl.gov/discover/news-release-archive/2020/July/0728-quantum-time-travel.php