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In a mathematical model, multiple histories can prevent certain time-travel paradoxes—but that is not evidence that time machines or parallel universes exist. Physicists Jacob Hauser and Barak Shoshany proposed that a traveler who goes into the past could arrive in a different history from the one they left. The proposal explores what would follow if backward time travel were possible; it does not demonstrate a way to do it.
Why time travel creates paradoxes
The classic grandfather paradox goes like this: a traveler goes back in time and prevents their grandparent from having children. The traveler would then never be born—and could not have made the trip that caused the change. The contradiction arises if the traveler can alter the very past that produced them.
A bootstrap paradox is different. Imagine a traveler carrying a musical score into the past. A composer copies and publishes it; decades later, the traveler finds that published score and takes it back. The score circulates in a causal loop, but it appears to have no original composer. Hauser and Shoshany’s research considers both kinds of paradox.
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The proposal’s key move is to let the traveler’s actions affect a different history from the one they departed. In a simplified picture:
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History A: original past → traveler is born → traveler enters a time machine
└── arrives in History B's past
History B: altered past → grandfather may be killed → traveler remains presentIf the traveler changes events in History B, those events do not erase their birth in History A. They still have an origin, so the grandfather paradox does not arise in this setup. The traveler has changed another history’s past, not rewritten their own.
That distinction is also the proposal’s trade-off: the traveler cannot use the trip to alter the history they came from. And reaching a different history does not automatically mean they can return to their original one; that depends on the model’s rules and structure.
What “parallel universes” means here
“Parallel universe” is a convenient popular shorthand, but the technical discussion is more carefully framed in terms such as multiple histories, parallel timelines, branching spacetimes and covering spaces. These are mathematical ways to represent alternative causal histories. The model does not establish that separate, accessible universes exist in the science-fiction sense.
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- Time Travel Rule 1: Your destination must be within your lifetime
- Rule 2: Each trip lasts just 90 seconds
- Rule 3: You can only watch, not interact
- These rules are absolute and unbreakable
Nor does it mean that a traveler can select any branch, move between branches at will or communicate with people in them. Those are additional physical claims that the model does not prove.
What the research established—and what it did not
Hauser and Shoshany posted the preprint “Time Travel Paradoxes and Multiple Histories” on November 25, 2019. A peer-reviewed version appeared in Physical Review D on September 24, 2020, as volume 102, article 064062 (paper record; full text).
The work develops mathematical models in which multiple histories can address paradoxes that are difficult to resolve with a single mutable timeline. It also examines whether the histories in certain models must be infinite: finite, cyclic arrangements can work under particular assumptions. That does not tell us how many histories exist in reality—or establish that any do.
The paper’s logic is conditional: if time travel is possible, multiple histories offer a way to represent it without certain contradictions. A mathematically consistent scenario is not an experiment, a working machine or proof that nature permits the scenario.
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How this differs from Novikov self-consistency
The Novikov self-consistency principle takes another route. It allows a single history containing a closed timelike curve, but says events on it must remain globally consistent. A traveler may try to cause a contradiction, but circumstances would prevent any outcome that makes the history inconsistent.
- Novikov approach: one history; only self-consistent events can occur.
- Multiple-history approach: an intervention affects another history, leaving the traveler’s origin history intact.
Hauser and Shoshany argue that certain paradoxes are not resolved by Novikov-style self-consistency alone within their analysis. This is a result about the models and assumptions involved, not an experimental disproof of Novikov’s principle.
Why wormholes enter the discussion
A traversable wormhole is a hypothetical shortcut through spacetime often used in theoretical discussions of time machines. In some general-relativistic constructions, differences in how much time passes at the wormhole’s two mouths could create a route associated with a closed timelike curve—a path through spacetime that returns to an earlier event.
A later paper, “Wormhole Time Machines and Multiple Histories”, analyzes a multiple-history scenario using a Morris–Thorne wormhole model in 3+1 dimensions. The wormhole is part of a theoretical construction, not an observed or engineered object. The mathematical existence of a spacetime solution does not show that a traversable wormhole can form, remain stable or be built with physically available matter and energy.
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Not automatically. The many-worlds interpretation is an interpretation of quantum mechanics in which the universal quantum state evolves without collapse, with apparently separate “worlds” emerging through decoherence. A multiple-history time-travel model is a proposed way to represent causal histories when time travel is assumed. Related vocabulary does not make them the same theory, and many-worlds does not by itself provide a way to travel between branches.
A 2023 preprint by Barak Shoshany and Zipora Stober, “Time Travel Paradoxes and Entangled Timelines”, proposes an entangled closed timelike curve (E-CTC) model within the Everett, or many-worlds, framework. It treats timelines as emergent from entanglement between a hypothetical time machine and its environment. This is a theoretical proposal, not an experimental realization or confirmation that people can move between branches.
Does “paradox-free” mean every problem is solved?
No. In this context, “paradox-free” means a modeled sequence of events avoids a logical contradiction under its rules. It does not guarantee that a traveler reaches their intended destination, can get home, or has a conventional origin for every object or piece of information. A bootstrap loop, for example, may be represented consistently while leaving the origin of the score unexplained.
Other open questions include how histories would be generated, which history a traveler would enter, how matter, energy and information would behave across histories, and what theory of quantum gravity would govern a real closed timelike curve. The papers investigate pieces of these problems, but they do not provide a complete physical theory of time travel.
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The headline’s “possible” refers to a mathematical possibility under stated assumptions, not an available technology or confirmed feature of the universe. No time machine has been demonstrated, and the cited work does not report an observed parallel timeline or a method for transporting people between histories.
The accurate takeaway is narrower: multiple histories can provide a mathematically coherent way to avoid some time-travel paradoxes if backward time travel is possible. Whether nature permits the required time machines, wormholes or accessible histories remains unproved.
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