‘Grandfather Paradox’ Doesn’t Rule Out Time Travel After All

It’s a classic science fiction trope: a time traveler journeys back in time and causes a change in history that has disastrous effects on the present or even threatens their very existence.

If these changes jeopardize their ability to travel back through time in the first place, then surely the traveler can’t make that change to time, right? But then they can go back in time again, so, can make those changes again … and so forth.

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That’s the essence of a trap called the “grandfather paradox,” an idea that has been used to great effect in books, films, and TV shows—from Ray Bradbury’s short story A Sound of Thunder to Futurama to Back to the Future. And as much fun as this concept is in science fiction, it’s also something that actual physicists and philosophers are intensely thinking about.

“The argument runs like this, if you could ‘go back in time’ then you could go back to a time before your grandfather had had any children and murder him,” Tim Maudlin, a philosopher of science who investigates the metaphysical foundations of physics and logic, explains to Popular Mechanics. “But if that happened, then one of your parents would not have been born, so you would not have been born, so there would be no you to go back in time. Contradiction.”

This problem arises from the risk time travel would present to one of the most preserved ideas in physics — causality , the idea that cause must proceed effect in all circumstances.

“The grandfather paradox is usually presented as a reductio ad absurdum, or a refutation of the proposition that time travel is possible,” Maudlin says. “So the hypothesis must be impossible because of the grandfather paradox; time travel — or reverse causation — is not possible.”

Though he doesn’t ultimately think travel backward through time is possible, Maudlin thinks that the grandfather paradox shouldn’t prevent time travel in and of itself. Instead, the paradox just prevents what actions can be conducted on a trip through time.

“The grandfather paradox does not prove that you can’t go back in time, just that you can’t go back in time and kill your grandfather,” he says. “There would be nothing logically wrong with going back in time and, say, saying ‘Hello’ to your grandfather.”

Researchers from the Massachusetts Institute of Technology (MIT) have an idea of just how causality violation could be prevented.

Time Travel That Protects Granddad

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Seth Lloyd, a professor of mechanical engineering at the Massachusetts Institute of Technology and a self-described “quantum mechanic,” has been conducting research for over a decade that suggests a way of going back in time and avoiding the grandfather paradox altogether.

This involves the physics of closed timelike curves (CTCs), paths through time and space that return to their starting point, which are allowed by general relativity — Albert Einstein’s theory of gravity and the effect mass has on space and time, or the single entity of spacetime.

“A closed timelike curve is a path through spacetime that leads to the past,” Loyd tells Popular Mechanics. “If you follow a closed timelike curve in your spaceship, you can end up interacting with your former self. That is, closed timelike curves allow time travel.”

There are a few different types of CTC models, which Lloyd illustrates with examples from popular fiction.

“There are basically two different possible types of models for CTCs. In one — which we call, imaginatively, Type I — the time traveler can intervene to change the past as she remembers it, at which point she enters into a different quantum branch of the universe — as in Back to the Future, Hot Tub Time Machineand other time-travel narratives,” he explains. “In such Type I theories of time travel, it’s perfectly possible for the time traveler to kill her grandfather.”

In the other type of CTC model , which is predictably called Type II, time travel has to obey a principle of self-consistency. Sometimes called the Novikov self-consistency principle, or Niven’s Law of the conservation of history, this principle prevents causality violation by placing some events in order on the same CTC. This self-consistency would prevent our time-traveler from landing her machine on granddad, even if she wanted to. Some effect would always divert her course.

“In Type II theories, the time traveler cannot change the past, no matter how hard she tries,” Lloyd says. “Examples of Type II time travel narratives include Harry Potter and the Prisoner of Azkabanand the Terry Gilliam film, Twelve Monkeys.”

Terminator Photons: Back in Time With a Mission to Kill

Lloyd and his team set about exploring a version of Type II CTCs that combine the concepts of quantum teleportation with post-selection — the factor in a computation that allows certain results to be accepted while others are rejected.

“Quantum teleportation is a process in which a quantum system dematerializes here and then rematerializes somewhere else based on the counter-intuitive quantum phenomenon of entanglement [the idea that two or more particles can be linked in such a way that a change in one instantaneously causes a change in the other no matter how distant they are],” Lloyd says. “In the quantum theory of CTCs that we developed, travel through the closed timelike curve is closely related to teleportation.”

The quantum mechanic added that adding post-selection to quantum measurement makes the process deterministic rather than probabilistic and it effectively bans events that would prove to be paradoxical.

Lloyd set about testing this idea by developing an “in principle” time machine — a quantum simulation that effectively sends a photon a few billionths of a second backward in time to have it attempt to “kill” its previous self.

The results showed that the closer a photon got to doing something self-inconsistent, the more frequently the experiment failed. Lloyd’s results could hint that time travel might work in the same way — any jaunt that would lead to a paradox is canceled preemptively.

Could Quantum Physics Provide an Exit to the Grandfather Paradox?

Quantum physics might provide another out to the Grandfather Paradox. One particular interpretation of quantum mechanics — Hugh Everett’s Many World Interpretations — suggests that for every quantum possibility that exists, a separate and distinct world emerges.

Physicist David Deutsch , a pioneer in quantum computing , imagined the Many Worlds idea in the case of time travel. He envisioned a particle traveling along a CTC loop through time in a quantum superposition — a phenomenon that exists in quantum physics that allows a system to exist in multiple, potentially contradictory, states at once.

To avoid paradoxes at the end of the journey and ensure a particle arrives back at its starting point the same as it was when it left, a world is created for each possible state. Let’s see how that would work for a human traveler in time if such a thing was possible.

Imagine a hypothetical time traveler ,  who we’ll call Susan , takes a CTC-based journey back through time to meet her grandfather as a child in 1963. Being hyper-literal and overprecise, she lands this time machine exactly where granddad was standing in Totter’s Lane scrapyard, London, squishing him dead. Susan waits to disappear from existence, but the Many Worlds interpretation of quantum physics may protect her.

This is because when Susan arrived in 1963, she created a world that is distinct from the world she left. In the world she left , let’s call it Earth 1 , her grandfather wasn’t squashed. He went on to have a granddaughter called Susan who once disappeared in a time machine. So, the child Susan landed on in the past isn’t her grandfather at all, just a version of him from an alternative world.

Traveling back to the future, Susan would find it different from the world she left—not because it’s been altered by her actions, but rather because this world, Earth 2, was created by her — it’s not the same world.

The Many Worlds Interpretation has a consequence for our time traveler; Everett insisted that one of the rules of his theorems was that worlds couldn’t interfere with each other. This means that our time traveler can’t get back to Earth 1.

If Susan attempts to travel back in time to 1963 to prevent the death of her grandfather, she creates a third world — Earth 3 — in which two time-travelers appeared in Totter’s Lane scrapyard in 1963. She travels forward again realizing she now can’t get back to Earth 1 or Earth 2.

Somewhere on Earth 1 and in that timeline, Susan’s wistful grandfather awaits her return, which will never come about.

Of course, the Grandfather Paradox isn’t the only argument against time travel. One very sensible question is: if time travel is possible, when are all the time travelers?

“For what it’s worth since we put forward the theory and performed the proof of principle experiment, many people have written to me claiming to be time travelers who are stuck in time and asking me if they can use our time machine to get back to their own time,” Lloyd says. “I advise them to wait until the bugs have been worked out.”