Every human being alive shares the same understanding of time. The past causes the present. The present causes the future. You cannot be burned by a fire before the match is struck. You cannot hear a gunshot before the trigger is pulled. This is not a scientific theory open to revision. It is the operating system of reality itself, the one rule that has never bent, never failed, and never needed defending. Until now.
Physicists at the University of Vienna have proven, in a physical laboratory with measurable results, that cause and effect do not have to run in a fixed direction. Two events can cause each other simultaneously. An action can both precede and follow its own consequence at the same time. The result was not a close call. The experiment produced a score of 1.8328 against a hard mathematical boundary of 1.75, landing 18 standard deviations above the limit that classical causality cannot cross. In science, five standard deviations ends debates. Eighteen does not leave room for one.
The experiment was published in March 2026 in the journal PRX Quantum by a team at the University of Vienna’s Faculty of Physics. Using pairs of entangled photons, they built a system in which the order of two physical operations was not fixed by any external instruction but held in genuine superposition until measurement forced a resolution. When they tested whether any classical causal order could account for their results, the score came back at 1.8328. The ceiling for classical causality is 1.75. That gap, 18 standard deviations wide, is the sound of a foundation cracking.
To be clear about what superposition of causal order actually means in plain language: imagine you ask two people to walk through two doors. In normal reality, one person goes first and one goes second. The order is fixed. In the Vienna experiment, the order was not fixed. Both operations existed in a state where each one was both first and second until the moment of measurement collapsed it into one. This is not a metaphor or a model. It is what the photons in that laboratory were physically doing, and the numbers confirm it beyond any reasonable scientific threshold.
Previous experiments had hinted at this possibility for years. Every one of them carried a vulnerability. They relied on trusting the equipment used to measure the result. If the detectors behaved unexpectedly, if the photon source had hidden flaws, the conclusions could be questioned. Scientists have a word for these vulnerabilities. They call them loopholes. The Vienna experiment was built specifically around a mathematical test, the VBC inequality, designed so that no assumption about the equipment could change the outcome. Whether the devices were perfect or imperfect, whether the photon source was flawless or noisy, the only thing that could produce a score above 1.75 was a universe that genuinely permits indefinite causal order. The score was 1.8328. The loophole of blaming the equipment is closed.
This matters because science has been here before with a different but related question. Decades ago, physicists debated whether quantum entanglement was real or whether it was an artifact of hidden variables, unseen factors in the equipment or the particles that only made it look like two particles were instantly connected across distance. The debate was settled by Bell inequality tests, experiments designed so that no hidden variable theory could explain the results. The Vienna experiment does the same thing for causality. It is a Bell test for the arrow of time, and the arrow failed.
Time as human beings experience it moves in one direction. Memories run backward into the past. Consequences follow actions. Coffee cools, it does not spontaneously heat. This forward direction is so fundamental to experience that most people never question whether it is a property of reality or a property of the scale at which human beings happen to exist. The Vienna result points firmly toward the second option. At the quantum scale, the forward march of time is not enforced. Events can influence each other without respecting sequence. Cause and effect become a loop rather than a line.
Quantum technologies have been exploiting the advantages of indefinite causal order for several years in experimental settings. Systems using quantum switches, the devices at the center of the Vienna experiment, have already demonstrated superior performance at quantum communication tasks, noise reduction, precision measurement beyond standard limits, and quantum key distribution for secure information transfer. Each of those advantages depends on indefinite causal order being a genuine physical resource. The Vienna result is the hardest confirmation yet that it is, and it will accelerate work on every one of those applications.
Beyond technology, a question sits at the edge of this result that physicists have not yet answered. A serious and growing body of opinion holds that a genuine superposition of causal orders cannot arise from photons alone. Under this view, true indefinite causal order requires a superposition of gravitational fields, which means a superposition of spacetime itself. Not just particles in superposition. Not just order in superposition. The container that holds all of reality, space and time as a unified structure, existing in multiple configurations simultaneously. The Vienna experiment does not prove this. It produces a result that makes the question impossible to set aside. If closing the remaining experimental gaps confirms that the indefinite causal order observed in photonic experiments is the real thing and not a simulation of it, then the gravitational interpretation demands that spacetime itself is not fixed at the quantum level. It can be in two states at once, just like the photons that revealed the crack in causality in the first place.
The remaining gaps are specific and the Vienna team names them plainly. The measurement parties in the experiment sat less than one meter apart on a single optical table. A fully airtight version requires them to be physically separated by distances large enough to rule out any classical communication between them during the measurement. Detection efficiency through the full apparatus ran at approximately one percent, far below what a loophole-free result demands. These are engineering problems, not conceptual ones. The path forward is laid out. It is a matter of building it.
In that Vienna laboratory in March 2026, a number came back from an experiment that no classically ordered universe could produce. The boundary was 1.75. The result was 1.8328. Eighteen standard deviations separated measured reality from the world in which cause always precedes effect. The photons did not obey the arrow of time. They ran both ways at once, and the mathematics confirmed it with a margin that left nothing to argue about.
Source:
Richter, C.M.D., Antesberger, M., Cao, H., Walther, P., and Rozema, L.A. (2026). Toward an Experimental Device-Independent Verification of Indefinite Causal Order. PRX Quantum 7, 010354. https://doi.org/10.1103/5t2y-ddmt
