The 'Grandfather Paradox' doesn't rule out time travel after all

The ‘Grandfather Paradox’ doesn’t rule out time travel after all

  • The grandfather paradox is a potential logical problem in which a time traveler can go back in time and erase his existence.
  • Closed time-like curves, or paths through spacetime that lead to the past, allow time travel.
  • The MIT experience suggests that any short trip that leads to a time-travel paradox is preemptively canceled.

It’s a classic science fiction story: a time traveler travels back in time and causes a change in history that has catastrophic effects on the present or even threatens his very existence.

If these changes threaten their ability to travel through time in the first place, then surely the traveler wouldn’t be able to make that change to time, right? But then Can Go back in time again, so you can make those changes again…and so on.

That’s the crux of a trap called the “grandfather paradox,” an idea that’s been used to great effect in books, movies, and TV shows – from Ray Bradbury’s short story Thunder sound to me Futurama to me Back to the future. As fun as this concept is in science fiction, it’s also something physicists and philosophers think extensively about.

“The argument goes like this, if You can “go back in time” and then you can go back to a time before your grandfather had children and killed him,” explains Tim Maudlin, a philosopher of science who researches the metaphysical foundations of physics and logic, popular mechanics. “But if it did, neither of your parents would have been born, so you wouldn’t have been born, so there would be no one for you to go back in time. A contradiction.”

This problem arises from the danger of time travel to one of the most conserved ideas in physics – causation, the idea that causes Should Proceed to influence in all circumstances.

The grandfather paradox is usually presented as a Abbreviation tamper advertisement Or refute the assumption that time travel is possible,” says Maudlin. “So the hypothesis must be impossible because of the grandfather paradox. Time travel – or reverse causation – is not possible.”

Although he does not ultimately believe that time travel is possible, Maudlin believes that the grandfather paradox should not prevent time travel per se. Instead, the paradox only prevents actions that can be performed on a journey through time.

“The grandfather paradox does not prove that you cannot go back in time, only that you cannot go back in time and kill your grandfather,” he says. “There would be no logical error in going back in time and saying ‘hello’ to your grandfather.”

Researchers from the Massachusetts Institute of Technology (MIT) have an idea of ​​how to prevent causality from being violated.

Time travel that protects grandparents

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The Time Turner – Harry Potter and the Prisoner of Azkaban HD

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Seth Lloyd, a professor of mechanical engineering at MIT who describes himself as a “quantum mechanic,” has been conducting research for more than a decade suggesting a way to go back in time and avoid the grandfather paradox altogether.

This includes the physics of time-closed curves (CTCs), paths through time and space back to the starting point, allowed by general relativity – Albert Einstein’s theory of gravity and the effect of mass on space and time, or the individual entity of spacetime.

“If you follow a time-locked curve in your spaceship, you may end up interacting with your former self.”

“A closed time curve is a path through spacetime that leads back into the past,” says Lloyd. popular mechanics. “If you follow a time-closed curve in your spaceship, you may end up interacting with your previous self. This means that curves with a closed time shape allow time travel.”

There are several different types of CTC models, which Lloyd illustrates with examples from popular stories.

There are two different possible models for CFCs. In one – which we, fancifully, call the first type – the time traveler can intervene to change the past as you remember it, at which point you enter into a different quantum branch of the universe – as in Back to the futureAnd the hot tub time machineAnd other stories about time travel. “In such type I theories of time travel, it is entirely possible for a time traveler to kill her grandfather.”

In the other type of CTC model, predictably called the second type, time travel must obey the self-consistency principle. Sometimes called the Novikov self-consistency principle, or Niven’s law of preservation of history, this principle prevents violation of causality by arranging certain events in the same CTC. This self-consistency would prevent our time travelers from lowering their machinery on their grandfather, even if they wanted to. Some effect may always divert her course.

“In type II theories, a time traveler cannot change the past, no matter how hard they try,” says Lloyd. Examples of the second type of time travel novel include Harry Potter and the Prisoner of AzkabanTerry Gilliam, twelve monkeys. “

Terminator Photons: Go back in time with a killing mission

Lloyd and his team set out to explore a version of the second type of CTCs that combines concepts of quantum teleportation with post-selection—the computational factor that allows certain outcomes to be accepted while others are rejected.

“Quantum teleportation is a process in which a quantum system is dematerialized here and then re-embodied elsewhere based on the counterintuitive phenomenon of quantum 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 a closed time curve is closely related to teleportation.”

The quantum mechanic added that the addition of post-selection quantification makes the process deterministic rather than probabilistic, and effectively prohibits events that might prove contradictory.

Lloyd set out to test this idea by developing a time machine “in principle” – a quantum simulation that effectively sends a photon a few billionths of a second back in time to attempt to “kill” its previous self.

The results showed that the closer a photon was to doing something that was not self-consistent, the more frequently the experiment failed. Lloyd’s findings could suggest that time travel might work in the same way—a short trip that might lead to a paradox that is preemptively canceled.

Can quantum physics provide a way out of the grandfather paradox?

Quantum physics may provide another clue to the grandfather paradox. One particular interpretation of quantum mechanics – Hugh Everett’s many interpretations of the world – proposes that for every quantum possibility that exists, a separate and distinct world arises.

Physicist David Deutsch, a pioneer in the field of quantum computing, conceived the idea of ​​multiple worlds in the case of time travel. He envisioned a particle traveling along the CTC cycle through time in quantum superposition—a phenomenon found in quantum physics that allows a system to exist in multiple, potentially contradictory states, simultaneously.

To avoid paradoxes at the end of the journey and to ensure that the particle returns to its starting point as it was when it departed, 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 were possible.

Imagine a hypothetical time traveler, whom we’ll call Susan, taking a CTC-based journey through time to meet her grandfather as a child in 1963. So crafty and precise, this time machine landed exactly where her grandfather was standing. Lynn tension for scrap, London, crushes him dead. Susan is waiting for her to disappear from existence, but the many-worlds interpretation of quantum physics may protect her.

That’s because when Susan arrived in 1963, she created a world different from the one she left. In the world she left, let’s call it Earth 1, her grandfather had not been crushed. He went on to have a granddaughter named Susan who once disappeared into a time machine. So, the child Susan landed on in the past isn’t her grandfather at all, she’s just a copy of him from an alternate world.

Back in the future, Susan will find that it is different from the world she left – not because it was changed by her actions, but because this world, Earth 2, was created by her – it’s not the same world.

His many-worlds interpretation is a consequence of our time traveler; Everett insisted that one of the rules of his theories was that the worlds could not overlap each other. This means that our time traveler cannot return to Earth 1.

If Susan tries to go back in time to 1963 to prevent her grandfather’s death, she creates a third world – Earth 3 – in which two time travelers appeared in a junkyard in Tension Lane in 1963. Let’s go back to Earth 1 or Earth 2.

Somewhere on Earth 1 and in this timeline, Susan’s grieving grandfather is waiting for her return, which will never materialize.

Of course, the grandfather paradox isn’t the only argument against time travel. One very reasonable question is: if time travel was possible, when Do you travel all the time?

“For what it’s worth since we put forward the theory and conducted a proof-of-principle experiment, many people have written to me claiming to be time travelers stuck in time asking if they could use our time machine to go back to our own time,” Lloyd says. “I advise them to wait until the bugs are resolved.”

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