A new peer reviewed paper from Lloyd, Ji, and Wilde sets a quantitative ceiling on information flow through a closed timelike curve, with the caveat that the underlying spacetime structures may not exist in our universe.
Around a rotating black hole, general relativity permits a kind of geometric contortion in which a forward-moving traveler would eventually meet an earlier version of themselves. That possibility, a closed timelike curve, has lived in the margins of physics for decades, more useful as a thought experiment than as a route to a working time machine. A new paper in Physical Review Letters gives the question something it has rarely had: a number. Seth Lloyd of MIT, together with Kaiyuan Ji and Mark Wilde of Cornell, calculate the maximum amount of information that could pass through such a curve without breaking the rules of quantum mechanics, according to Scientific American.
The result is a bound, not a feat of engineering. It says nothing about whether any rotating black hole actually hosts a closed timelike curve, and nothing about whether such structures exist in our universe at all. What it does is constrain what the geometry of spacetime would permit if one did. "You're innocently going forward in time and then you meet yourself in the past," Lloyd told Scientific American. The line captures the geometry. It is not a claim of discovery.
The mechanism comes from the way spacetime can warp near a spinning mass. In 1963, the New Zealand mathematician Roy Kerr solved Einstein's equations for a rotating black hole and found a solution that, in principle, contains a region where the natural direction of "forward in time" loops back on itself. Whether nature actually builds such regions is a separate question, one that no observation has answered and that many physicists suspect may be settled by quantum-gravity corrections the classical equations cannot see. The Lloyd-Ji-Wilde bound is best read as a study of what Kerr-style geometry says is allowed, with the conditional attached.
The information bound sits inside the broader problem of consistency. If a message could travel to the past and change something, the universe would inherit paradoxes of the "kill your own grandfather" variety. The standard way around them, the so-called self-consistency condition, is to require that any signal arriving from the future must match what was already there. Lloyd and his coauthors turn this into a calculable constraint: a closed timelike curve can carry information, but only up to a limit set by the geometry itself, Scientific American reports. That limit is what makes the paper quantitative where most CTC discussions are not. The bound tells you, for a given spacetime, how many bits could in principle make the round trip without producing logical contradictions. It is the kind of answer that physicists can disagree about the interpretation of, while agreeing on the math.
What it is not is a blueprint. No device, no transmitter, no clever arrangement of matter and energy can turn the bound into a message. The constraint is a property of the geometry, not of anything an experimenter can build. And even if a future telescope were to find a black hole with the right spin and the right surroundings, the closed timelike curve region would still be hidden behind an event horizon, invisible and unreachable.
The work also leaves open the question that haunts all CTC physics: whether the structures general relativity permits are the structures the universe actually contains. Quantum-gravity approaches have long suggested that the singularities and exotic regions of classical general relativity may be smoothed out at the Planck scale, removing the curves altogether. A bound on information transfer is a bound on a hypothetical object. Until physicists find one, the calculation will keep its conditional.
For now, the cleanest way to read the result is as a sharpening. Spacetime's geometry, under specific assumptions, can be asked exactly how much information a closed timelike curve could carry. The answer is finite, calculable, and small. The scenario in which it applies is the one that has not been built, and may not be available.