Time travel theory avoids grandfather paradox - A new theory proposed by physicists at MIT suggests that this grandfather paradox could be avoided by using quantum teleportation and "post-selecting"
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Time travel theory avoids grandfather paradox
by Lisa Zyga , Phys.org
(PhysOrg.com) -- The possibility of going back in time only to kill your ancestors and prevent your own birth has posed a serious problem for potential time travelers, not even considering the technical details of building a time machine. But a new theory proposed by physicists at MIT suggests that this grandfather paradox could be avoided by using quantum teleportation and "post-selecting" what a time traveler could and could not do. So while murdering one’s relatives is unfortunately possible in the present time, such actions would be strictly forbidden if you were to try them during a trip to the past.
The model of time travel proposed by Seth Lloyd, et al., in a recent paper at arXiv.org arises from their investigation of the quantum mechanics of closed timelike curves (CTCs) and search for a theory of gravity. In simple terms, a CTC is a path of spacetime that returns to its starting point. The existence of CTCs is allowed by Einstein’s general relativity, although it was Gödel who first discovered them. As with other implications of his theories, Einstein was a bit disturbed by CTCs.
In the new paper, the scientists explore a particular version of CTCs based on combining quantum teleportation with post-selection, resulting in a theory of post-selected CTCs (P-CTCs). In quantum teleportation, quantum states are entangled so that one state can be transmitted to the other in a different location. The scientists then applied the concept of post-selection, which is the ability to make a computation automatically accept only certain results and disregard others. In this way, post-selection could ensure that only a certain type of state can be teleported. The states that “qualify” to be teleported are those that have been post-selected to be self-consistent prior to being teleported. Only after it has been identified and approved can the state be teleported, so that, in effect, the state is traveling back in time. Under these conditions, time travel could only occur in a self-consistent, non-paradoxical way.
“The formalism of P-CTCs shows that such quantum time travel can be thought of as a kind of quantum tunneling backwards in time, which can take place even in the absence of a classical path from future to past,” the researchers write in their paper. “Because the theory of P-CTCs relies on post-selection, it provides self-consistent resolutions to such paradoxes: anything that happens in a P-CTC can also happen in conventional quantum mechanics with some probability.”
However, the scientists note that prohibiting paradoxical events would cause unlikely events to happen more often. These “strange and counterintuitive effects” arise due to the nonlinear nature of P-CTCs. Like a movie hero who always manages to escape seemingly imminent death, the grandfather would always somehow manage to survive his grandchild’s murderous plots. “Some little quantum fluctuation would whisk the bullet away at the last moment,” Lloyd explained.
In addition to prohibiting the grandfather paradox, the P-CTC theory also has the advantage that it doesn’t require the distortions of spacetime that traditional time travel theories rely on. These spacetime distortions probably only exist in extreme environments such as inside black holes, making these theories nearly impossible to realize.
Although post-selected computations are nonlinear and have not yet been shown to be possible, some studies have shown that quantum mechanics may be nonlinear and allow post-selected computations, which would potentially make quantum computing a very powerful technique. Such a computer could more efficiently solve a complex problem containing lots of variables by running all possible combinations of values and post-selecting only the combinations that solve the problem. This strategy would work much better than the classical strategy of trying different combinations until you get one that works. On the other hand, other studies suggest that quantum mechanics must be linear, in part due to the seemingly impossible things that post-selection allows.
Still, the scientists hope that future investigations will reveal whether or not their theory is correct. They explain that the effect of P-CTCs can be tested by performing quantum teleportation experiments, and by post-selecting only the results that correspond to the desired entangled-state output.
“P-CTCs might also allow time travel in spacetimes without general-relativistic closed timelike curves,” they conclude. “If nature somehow provides the nonlinear dynamics afforded by final-state projection, then it is possible for particles (and, in principle, people) to tunnel from the future to the past.”
The "grandfather paradox" is a classic thought experiment in time travel that highlights a logical contradiction. It asks what would happen if you traveled back in time and killed your grandfather before he had children, thus preventing your own birth. If you were never born, you couldn't have traveled back in time to kill him, which means he would have lived, and you would have been born, and the cycle repeats endlessly.
The two main theoretical solutions to this paradox are the Novikov self-consistency principle and the multiple-universe theory.
Both the Novikov self-consistency principle and the multiple-universe theory are theoretical solutions to the grandfather paradox, but they propose fundamentally different mechanisms. The Novikov principle suggests a single, deterministic timeline where paradoxes are impossible, while the multiple-universe theory proposes that time travel results in the creation of new, parallel timelines, thus avoiding paradoxes.
Novikov Self-Consistency Principle
The Novikov principle asserts that a time traveler's actions in the past are not changes to the timeline but are, in fact, pre-ordained and already part of the timeline's history. It's a closed-loop view where history is unchangeable. The universe itself would prevent any paradox from occurring. For instance, if you went back to kill your grandfather, you would fail due to an improbable event, a malfunction, or an intervention. This theory maintains that the past is immutable and that your presence in the past is just a part of the causal chain that led to the present you know.
How it Works
The principle operates on the idea that there is only one, fixed timeline. Any attempt to change the past, no matter how small, would either fail or would be the very reason the event occurred in the first place. This concept eliminates the possibility of paradoxes like the "grandfather paradox," where a time traveler goes back in time and kills their own grandfather, thus preventing their own birth. According to Novikov's principle, this would be impossible. The universe would find a way to make the timeline self-consistent. For example, the time traveler's gun might jam, they might change their mind, or they might even accidentally cause their grandfather and grandmother to meet.
Multiple-Universe Theory
The multiple-universe theory offers a solution by suggesting that time travel does not affect the original timeline. Instead, when a time traveler goes to the past, they create or enter a new, alternate universe. In this new universe, you could kill your grandfather, but the original timeline you came from would remain completely unaffected. Therefore, the grandfather paradox is resolved because you are not erasing your own existence from the timeline you originated from. This theory, which is related to the many-worlds interpretation of quantum mechanics, posits that every action or choice creates new branching universes.
Main Types of Multiverse Theories
There are several proposed models for how a multiverse might exist. The most prominent include:
Many-Worlds Interpretation (MWI) of Quantum Mechanics: This is one of the most famous multiverse theories. It suggests that every time a quantum event with multiple possible outcomes occurs, the universe "splits" or "branches" into new, separate universes. In each of these new universes, one of the possible outcomes becomes reality. This interpretation, proposed by Hugh Everett III, addresses the "measurement problem" in quantum mechanics, which deals with how a particle's "superposition" (being in multiple states at once) "collapses" into a single state upon observation. In the MWI, the superposition never collapses; instead, all possible outcomes occur, each in its own universe.
Eternal Inflation: This theory, stemming from cosmic inflation theory, suggests that the rapid expansion of the early universe didn't stop everywhere at the same time. Instead, it might be continuing in different regions of space, creating an "infinite sea" of inflating space, within which "bubble universes" are constantly forming. Each of these bubble universes would have its own Big Bang and its own set of physical laws and constants. Our universe would simply be one such bubble.
Braneworlds (or "Brane" Universes): This theory, derived from string theory, suggests that our universe is a "brane" (a multidimensional membrane) that exists in a higher-dimensional space. Other branes could also exist in this space, and they might be completely separate from our own. Collisions between these branes could be the cause of events like the Big Bang.
Mathematical Universes: Proposed by physicist Max Tegmark, this highly speculative theory suggests that all mathematically possible universes exist. If the structure of the universe is purely mathematical, and we can describe it with equations, then any universe that can be described by a consistent set of mathematical equations must exist.
Evidence and Challenges
It is important to note that the multiverse theory is a speculative hypothesis. There is no direct observational evidence to prove the existence of other universes. However, some theoretical arguments and observations have been used to support the idea:
The Fine-Tuning Problem: The physical constants in our universe (like the strength of gravity, the mass of an electron, etc.) seem to be "just right" for life to exist. The multiverse theory offers a potential explanation for this "fine-tuning." If there are an infinite number of universes with different constants, it's not a surprise that we would find ourselves in one where the conditions are suitable for life.
The Search for "Bruises" in the Cosmic Microwave Background: Some models of eternal inflation predict that if our universe collided with another "bubble universe" in the past, it would leave a "bruise" or a circular pattern in the cosmic microwave background (CMB), the faint radiation left over from the Big Bang. Scientists have looked for these patterns, but so far, none have been conclusively found.
The main challenge for the multiverse theory is its lack of testability. Since other universes are, by definition, outside of our observable universe, it is difficult to find a way to directly test their existence. Critics argue that if a theory cannot be falsified, it is not truly scientific.
While still in the realm of theoretical physics and philosophy, the concept of a multiverse provides a fascinating framework for thinking about the fundamental nature of reality, and it continues to be a subject of intense scientific and philosophical debate.