At one point, while reading a paper from Cambridge’s Hitachi Laboratory, the phrase “retroactively change your previous actions” begins to sound less like science fiction and more like something subtly frightening. It doesn’t specifically describe a time machine; rather, it implies—with actual mathematical support—that the distinction between what actually occurred and what might have occurred is more hazy than most of us have ever felt comfortable picturing. It appears that reality might be more brittle than we previously believed. And more and more physicists appear to concur.
In October 2023, David Arvidsson-Shukur and his colleagues published their findings in Physical Review Letters. Since then, the work has garnered a particular kind of slow-burn attention—not the breathless headlines that accompany particle collider announcements, but the more subdued, unsettling realization among physicists that something truly unusual is being formalized here.
| Key Information | Details |
|---|---|
| Institution | University of Cambridge / Hitachi Cambridge Laboratory |
| Lead Researcher | David Arvidsson-Shukur |
| Co-Authors | Nicole Yunger Halpern (NIST & University of Maryland), Aidan McConnell (ETH Zürich) |
| Research Field | Quantum Mechanics, Quantum Metrology, Theoretical Physics |
| Study Published | October 12, 2023 — Physical Review Letters |
| DOI | 10.1103/PhysRevLett.131.150202 |
| Key Concept | Simulating closed timelike curves via quantum entanglement |
| Funding Bodies | Sweden-America Foundation, Lars Hierta Memorial Foundation, Girton College, EPSRC/UKRI |
| Practical Application | Quantum metrology, photon-based measurement experiments |
| Reference Website | Physical Review Letters – APS |
The group showed that they could simulate the physics of backward time travel by controlling quantum entanglement, which allowed them to solve issues that conventional quantum mechanics considers to be unsolvable.
Arvidsson-Shukur’s gift analogy is surprisingly straightforward. On day one, you have to send a gift, but you don’t find out what the recipient wants until day two. You’re out of luck in everyday life and physics. Entanglement, however, provides a workaround in the Cambridge simulation: send the first particle into the experiment, then use the information you learned later to manipulate its entangled twin, thereby nudging the first particle’s previous behavior.
One out of every four times, it works. A 75% failure rate may seem depressing, but keep in mind that the success rate was zero prior to this.
The connection to quantum metrology—the science of employing quantum systems to make incredibly accurate measurements—makes this more than just a cunning physics trick. Practically speaking, before photons reach a sample, they must be prepared in certain ways. Usually, you have to dedicate yourself to that preparation before you are aware of the desired result.
According to this simulation, you might be able to make a retroactive revision to that preparation and then filter for the photons that contain the updated information. The researchers are open about the fact that there is inefficiency. However, the hypothetical door that was previously closed is now slightly ajar.
It’s difficult to ignore how this research is situated at a nexus of concepts that physics has been anxiously pursuing for many years. Physicists such as Audrey Mithani and Alexander Vilenkin have demonstrated through independent theoretical work that the introduction of quantum mechanics can cause universes that appear stable in classical terms to become fatally unstable. Mathematically speaking, entire universes can abruptly collapse into nothingness through a process known as quantum tunneling. The formulas are clear. There are no implications.
Then there is the work of Sung-Sik Lee, who suggests something even more unsettling: that time itself is an emergent characteristic that results from quantum measurement rather than a fundamental aspect of the universe. According to his interpretation, time doesn’t tick because clocks exist; rather, clocks exist because quantum events collapse into specific outcomes, which produce the forward arrow that we perceive as time.
There wouldn’t be any time if the chain of collapses broke. Just potential, suspended in a pre-temporal haze. Perhaps this goes beyond philosophy. It could be the real architecture of life.
All of this is further complicated by the Two-State-Vector Formalism, a framework created over decades by researchers examining the gap between quantum measurements. Typically, we consider a particle to have a clear history from the time it is prepared until it is measured. That’s not quite correct, according to TSVF. Both a particle’s prior preparation and the results of its subsequent measurements influence what it “is” in between measurements.
In other words, it is difficult to distinguish the present from the future. There’s a feeling that reality is constantly negotiating between past and future states at the quantum level, and we only see the final result of that negotiation when we actually look.
The multiverse, a serious theoretical framework arising from inflationary cosmology rather than a science fiction conceit, sits somewhere in the middle of all of this. The universe we live in is one bubble among innumerable others, each expanding in its own pocket of spacetime, if eternal inflation is real. There are stable bubbles among them. Others are not, where a phase transition is triggered by the decay of a false vacuum. They fall apart. Furthermore, there is no assurance that you would see it coming from inside the bubble.
Arvidsson-Shukur takes care to maintain the validity of his assertions. “We are not proposing a time travel machine,” he said, “but rather a deep dive into the fundamentals of quantum mechanics.” That self-control is admirable. Like in journalism, the temptation in physics is to interpret a result as grandly as possible.
However, what the Cambridge team has actually demonstrated is something more subtly profound: that quantum entanglement contains resources that we haven’t yet fully mapped, and that some problems that we previously thought were impossible are actually just very difficult under the correct circumstances.
There is still disagreement over whether time travel, even the probabilistic, simulated version discussed here, reveals anything about the physical world or just mathematical structure. Whether the multiverse is a scientific theory or an incredibly complex metaphor is still up for debate.
Furthermore, whether gravitational wave detectors such as LISA will ever detect the signature of a collapsing vacuum bubble from another universe—a cosmic bruise on the fabric of spacetime—remains an open question. These questions will be tested in subsequent experiments. There will be some responses. In any human timescale, at least, others won’t.
Reading through this corpus of work, you are struck by how consistently the universe defies the notion that it is robust. The picture that emerges is of a reality that is more of a set of very good odds than solid ground, from quantum tunneling erasing stable configurations to time emerging only conditionally from the act of observation to entanglement enabling retroactive corrections. The world holds together most of the time.
The photon lands in its proper location. Time passes. The bubble remains intact. However, it turns out that the mechanism underlying that dependability is more peculiar and contingent than we were ever taught by classical physics. That understanding hasn’t been altered by Cambridge. All they’ve done is make it more difficult to ignore.
