NOTE: This is a mathematical, statistical preprint (arXiv) — not an experimental or peer‑reviewed physical proof.
On a summer evening in 2009, Stephen Hawking threw a party. He filled a room at the University of Cambridge with champagne, balloons, and finger food. Then he sat and waited.
Nobody came.
This was not a scheduling mishap. Hawking had deliberately invited no one. The invitations — complete with date, time, and coordinates — were published only after the party ended. The idea was elegant: if time travel ever becomes possible in the future, some curious tourist from the year 3000 (or 30,000) might read about the party, step into a machine, and show up for the free champagne. Hawking's empty room, he argued, was proof that backward time travel cannot happen.
It was a charming experiment. It was also, by the standards of evidence, a single data point with no control group.
Now a researcher at the University of Edinburgh has proposed something more interesting than a party: a mathematical model showing that even if time travel is physically possible — even if the machines work, the tourists pack their bags, and the champagne is poured — we still should not expect visitors. Not because time travel is forbidden by physics, but because time travel, once invented, has a habit of un-inventing itself.
In other words: time travel might be real, and that might be exactly why we never see it.
The Physics Does Not Say No
The Grandfather Paradox and Its Discontents
Jackson's model sidesteps this paradox entirely by making the paradox self-resolving: any action that would prevent time travel from being invented simply ensures that the action itself never happened.
In 1949, the logician Kurt Gödel (better known for his incompleteness theorems) published a paper in Reviews of Modern Physics that changed how physicists think about time. Gödel found a solution to Einstein's field equations — the equations that describe gravity and the shape of the universe — in which the universe rotates. In this rotating universe, called the Gödel metric, certain paths through space and time loop back on themselves. A traveler following one of these paths would return to their own past. These loops are called closed timelike curves, and their existence in a valid solution to Einstein's equations means that general relativity, by itself, does not forbid backward time travel.
This is not a fringe idea. Gödel's paper was dedicated to Albert Einstein on his seventieth birthday. Einstein himself reportedly found the result philosophically unsettling.
Of course, the Gödel universe is not our universe. It rotates, which ours apparently does not. It requires a negative cosmological constant, which does not match observations. But the point stands: the equations of our best theory of gravity admit time travel. They just do not seem to describe a universe where it actually happens.
This is where Hawking's chronology protection conjecture comes in. In 1992, Hawking proposed that the laws of physics might contain a kind of "cosmic censor" — a mechanism that prevents closed timelike curves from forming in the real world. Quantum effects near time machines might destroy them before they could be used. It was a suggestion, not a proof, and it remains unproven.
Other physicists have taken different approaches. In 1990, Novikov and colleagues (including Thorne and Morris) proposed that quantum mechanics might enforce self-consistency: time travel is allowed, but only if the events it causes do not create logical contradictions. More recently, researchers have explored the many-worlds interpretation of quantum mechanics, arguing that time travelers simply branch into alternate timelines, avoiding paradoxes entirely.
None of these frameworks, however, explains the complete absence of time travelers. If time travel is technically possible, and if the future contains billions or trillions of humans, it seems probable that at least one of them would have visited us by now — accidentally or on purpose.
Andrew Jackson, a researcher at the University of Edinburgh's School of Informatics, thinks the answer lies not in physics but in statistics.
The Self-Suppressing Timeline
Jackson's paper, titled Where Are All The Tourists From 3025? and published on arXiv in August 2025, does not assume time travel is impossible. In fact, it assumes the opposite: time travel is theoretically possible, the machines can be built, and nothing prevents them from visiting our era. And yet, Jackson argues, we should still expect an empty room.
The argument works like this.
Imagine the entire history of the universe as a single timeline. At some point in the future — say, the year 3025 — humans invent a time machine. The first tourist packs a bag, steps inside, and travels back to, say, the year 2000. That tourist breathes different air, steps on different blades of grass, maybe shifts a pebble. Each of those tiny actions changes the world in some small way.
Now here is the critical step: because the past has changed, the future that leads to the invention of the time machine in 3025 might also change. Maybe the butterfly effect of that single shifted pebble means a different person becomes a physicist, or a different company gets funding, or a different war happens — and the time machine is never built.
What‘s a Markov Chain?
The Rule of "Memorylessness" In probability, this is called the Markov Property. It means the past doesn't matter—only the present state determines the future.
How Your Phone Uses It Think about your smartphone's predictive text. If you type the word "peanut," the software doesn't analyze the last five paragraphs you wrote. It simply looks at your current state ("peanut") and calculates the odds for the next state: an 80% chance for "butter" and a 5% chance for "allergy."
Where Else Are They Used?
- Video Game AI: Deciding an enemy's next move based solely on its current stance.
- Google Search: Predicting how a random user might click from one web link to another.
- Spam Filters: Guessing if an email is junk based on the probability of certain words following each other.
Jackson formalizes this intuition using a mathematical tool called a Markov chain — a way of modeling how a system moves between different states based only on its current condition. In his model, each "state" is not a moment in time but an entire possible world, defined by one number: how many time machines are ever constructed in that world. He calls this the construction number.
A world with construction number zero has no time machines at all. A world with construction number one has exactly one. A world with construction number five hundred has five hundred. Each state is a broad category — what physicists call a macrostate — that bundles together all possible worlds sharing the same construction number, regardless of their other differences.
The model's rules are straightforward. The more time machines exist in a world, the more likely that world is to change its own past — because more travelers means more disruptions. Each disruption can increase or decrease the construction number by one. The rate of these changes is proportional to the construction number itself.
Jackson then proves something striking: as the model runs forward, the probability of being in a world with zero time machines approaches one. Every other state — every state with one, ten, or ten thousand time machines — eventually becomes statistically impossible.
In mathematical terms: lim P₀(t) = 1 as t → ∞. In plain English: given enough time, a world with no time machines becomes a near-certainty. The only stable timeline is the one where no time machine was ever built.
Orthogonal Time: The Tourist's Clock
There is a subtlety here that Jackson emphasizes and that makes the model especially clever. The "time" in his Markov chain is not ordinary time. It is what he calls orthogonal time — the time experienced by the time travelers themselves.
Think of it this way. From the perspective of someone standing still in 2025, a time traveler might appear and disappear instantly. The traveler spends a week in ancient Rome, but to the stationary observer, no time passes at all. The traveler experiences time "orthogonally" to the normal flow — at a right angle to it, in a sense.
This means that from our perspective, all the rewriting of history happens in a single instant. We do not see the intermediate timelines. We do not remember the version where time machines existed and then did not. We simply wake up in the final, stable timeline — the one where no time machines were ever invented.
As Jackson puts it: "We jump to the final conclusion after an infinite amount of orthogonal time."
It is a bit like watching a video on fast-forward. You see the opening frame and the closing frame, but all the middle frames blur together. For us, the only observable timeline is the end result: the state where the probability of zero time machines has reached 100 percent.
What the Model Actually Proves and What It Doesn't
It is important to be clear about what Jackson's speculative paper is and is not.
It is not a proof that time travel is possible. It assumes time travel is possible and then asks what the consequences would be.
It is not a physical theory in the traditional sense. The model does not describe specific mechanisms, particles, or fields. It is a statistical framework — a way of reasoning about probabilities across possible worlds.
It is not a prediction that can be tested tomorrow. The model's conclusion is that we should not see time travelers, which is the same conclusion we already had from looking around. The value of the model is that it offers a reason for the absence that does not require assuming physics forbids time travel.
What the paper is, then, is a philosophical argument with mathematical backing: a demonstration that even under generous assumptions, time travel contains the seeds of its own non-existence. The timeline is unstable. The more time travel happens, the more the past gets rewritten, and the more likely it becomes that the conditions for inventing time travel never arise.
Jackson is aware of the model's limitations. He notes that the model assumes "orthogonal time" can run to infinity — that time machines can keep operating long enough for the statistical convergence to complete. He also notes that a future society might deliberately suppress timeline changes by keeping the rate of changes extremely low, though he argues this would not change the end result unless that rate were driven to exactly zero.
There is also the question of whether time machines could maintain themselves. If a working time machine can travel back to its own construction for repairs, or if future societies can cannibalize old machines to build new ones, the resource limits that bound the construction number might not apply. Jackson acknowledges this but argues that the model still captures the essential instability.
Sci-fi Got There First
Jackson was not the first to suggest this idea. After publishing the first version of his paper, he learned that science fiction author Larry Niven had proposed essentially the same conclusion fifty years earlier.
In his 1971 collection All the Myriad Ways, Niven stated what he called "Niven's Law":
"If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe."
Niven arrived at the conclusion through narrative intuition, not differential equations. But the logic is the same: a universe with changeable time travel is a universe where time travel undermines its own possibility. Jackson's contribution is to put Niven's intuition on formal mathematical footing.
It's worth noting that Niven's Law and Jackson's model share a critical assumption: that time travelers can change the past. If time travel exists but the past is fixed — if travelers can only observe, not alter — then the self-suppression mechanism does not apply. The model also assumes that changes to the past propagate forward in ways that affect the invention of time machines. A tourist who visits the Jurassic Period and steps on a fern might not change human history at all.
Conclusions
Jackson's paper is clever, mathematically careful, and refreshingly modest. He does not claim to have proven time travel is real. He claims only that the absence of time travelers does not prove time travel is impossible — and he supports that claim with a formal model.
Whether the model describes reality is another question entirely. The assumption that time travelers experience "orthogonal time" is mathematically convenient but physically speculative. The treatment of timelines as states in a Markov chain abstracts away most of the physics that would actually govern time travel. And the model's central result — convergence to zero time machines — depends on assumptions about how past changes propagate into the future that are essentially untestable.
In other words, this isn't evidence, it's a possibility. A well-constructed, mathematically interesting possibility, but a possibility nonetheless.
Hawking's party remains the most direct evidence we have. One empty room, one bottle of undrunk champagne, and one astrophysicist with a sense of humor. Jackson's model suggests the empty room is not proof of impossibility — but it is also not proof of self-suppression. It is just an empty room.
The tourists from 3025, if they exist, may have rewritten themselves off the invitation list.
Works Cited
Deutsch, David. "Quantum Mechanics Near Closed Timelike Lines." Physical Review D, vol. 44, no. 10, 1991, pp. 3197–210.
Friedman, John, et al. "Cauchy Problem in Spacetimes with Closed Timelike Curves." Physical Review D, vol. 42, no. 6, 1990, pp. 1915–30.
Gödel, Kurt. "An Example of a New Type of Cosmological Solutions of Einstein's Field Equations of Gravitation." Reviews of Modern Physics, vol. 21, no. 3, 1949, pp. 447–50.
Greenberger, Daniel M., and Karl Svozil. "Time Travel, Path Integrals, and the Many Worlds Interpretation of Quantum Mechanics." arXiv:2003.05051 [quant-ph], 2020.
Hawking, Stephen W. "Chronology Protection Conjecture." Physical Review D, vol. 46, no. 2, 1992, pp. 603–11.
Jackson, Andrew. Where Are All The Tourists From 3025? arXiv:2508.09157 [physics.gen-ph], 5 Aug. 2025, rev. 4 Oct. 2025.
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Niven, Larry. All the Myriad Ways. Ballantine Books, 1971.
Vaidman, Lev. "Many-Worlds Interpretation of Quantum Mechanics." The Stanford Encyclopedia of Philosophy, edited by Edward N. Zalta, Fall 2021 ed., Metaphysics Research Lab, Stanford University, 2021.
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Edited on July 18, 2026
Fact Checked on July 18, 2026