A year ago, in my essay on quantum entanglement, I looked at how quantum physics dismantles our everyday intuition of space. Now a series of experiments on “negative time” invites us to question an even more intimate habit of mind: how we understand the flow of time and our place in history.
When physicists at the University of Toronto recently fired pulses of light through a chilled cloud of rubidium atoms and measured how long the photons spent exciting the atoms inside, their instruments returned a duration below zero. In their 2024 experiment, the atomic excitation appeared to end before the photons had entered the cloud.
Why do results like these provoke such a peculiar mixture of fascination and vertigo? When I wrote about quantum entanglement last year, the philosophical shock was based on our perception of space: measure one entangled particle, and you instantly fix the state of its partner miles away without any signal crossing the gap between them. We can live with a strangeness in space because space stays outside us. Time is different. Immanuel Kant saw why: while neither space nor time belongs to “things in themselves”—both are a priori forms through which the human mind organizes experience—space is merely the form of our outer sense. Time is the form of inner sense: the intimate architecture of self-awareness, memory, and history. When a report in BBC Future explores the measurement of “negative time,” it sounds as though the foundation of human experience has been cracked. Yet these experiments also suggest something Kant never anticipated: if sequential time is our own construct, and our instruments can now peer past it, then that construct is not a permanent prison. We can change how we think and inhabit time.
Still, no signals are arriving from tomorrow, and Einstein’s speed limit remains unbroken. What happens if we look past the headlines and ask what these experiments actually reveal about light, causality, and our experience of time?
Two Ways Time Runs Below Zero
To understand what happened in Toronto, we have to distinguish between two different temporal anomalies in quantum optics. The first dates back to the 1990s, when Aephraim Steinberg, Paul Kwiat, and Raymond Chiao conducted their landmark 1993 tunneling experiments, confirming predictions of negative group delay: the phenomenon in which a wave packet—a localized bundle of light waves—passes through a barrier and sees its outgoing peak emerge before the peak of the incoming bundle arrives. That earlier effect turned out to be a result of wave reshaping: the barrier absorbs and reflects the trailing portion of the pulse while allowing the leading edge to pass through and reorganize into a new peak. Nothing outruns the speed of light.
The recent University of Toronto work—spanning a foundational 2022 rubidium-cloud study led by Josiah Sinclair and the 2024 negative-duration experiment led by Daniela Angulo Murcillo and Steinberg—cannot be explained away by simple pulse reshaping. Instead of clocking when the peak of a wave arrives at an exit detector, the team used the rubidium atoms themselves as a quantum clock to measure the dwell time: how long transmitted photons actually spend interacting with the atoms inside the barrier. When the researchers filtered for the photons that passed all the way through the cloud unabsorbed, the net duration of atomic excitation was negative. Arrival time and interaction duration—two quantities that must agree for any everyday object—split into conflicting timescales for the same event.
Why can a quantum clock run below zero without violating relativity? Because photons are massless quanta of the electromagnetic field, not miniature billiard balls with individual identities and fixed trajectories. Between emission and detection, a photon’s path through matter is a superposition of probability amplitudes that interfere with the atoms; the negative dwell time is an interference effect in how those probabilities weight the atoms’ history.
Moreover, relativity tells us that because a photon travels at the speed of light, the proper time elapsed along its worldline is always zero. Photons do not have clocks or experiences, of course, yet the geometry is exact: from the “perspective” of light itself, emission from a distant star and absorption years later in your eye happen in the same instant, and the distance between them contracts to nothing. Time is not a highway that light travels down; time is what happens to massive things when light interacts with them.
Retro-causality and the Block Universe
Once we let go of the assumption that events at the quantum scale march to a single ticking clock, the puzzle of quantum entanglement looks different too. Physicists and philosophers such as Emily Adlam suggest that what looks like “spooky action at a distance” across space may be better understood through time. In a retrocausal account, a measurement made in the future constrains the state of the particles back at their earlier entanglement event. Because fundamental physical laws are nearly all time-symmetric, influences that run along worldlines in both temporal directions can account for quantum correlations without requiring instantaneous spatial signals.
In our ordinary picture of a moving present, retrocausality sounds like an invitation to paradox. Yet as Adlam argues in her work on retrocausal models, that objection vanishes once we place quantum mechanics inside the ontology that relativity already implies—the block universe, or eternalism: the view that past, present, and future exist equally within a single four-dimensional spacetime structure.
Special relativity abolished the idea of an absolute, universe-wide “now.” Because observers in relative motion disagree on which distant events are simultaneous, there is no objective knife-edge of the present slicing reality into a dead past and an unborn future. Every event occupies its four-dimensional coordinates on the same ontological footing. Inside a block universe, neither retrocausality nor negative dwell time requires backward travel against a moving river of time; instead, physical laws act as four-dimensional constraints across the structure as a whole.
The View from Inside Time
The further we follow these discoveries, the more counterintuitive they become; at every turn, we run up against our most ingrained models of space and time. Human intuition would tell us that when a physical result defies common sense, it must be a mere mathematical trick. But we forget that our common sense and our grammar were built for three-dimensional orientation and survival, not for the architecture of the cosmos. In Lacanian terms, quantum physics repeatedly confronts our Symbolic order—the human world of language, tense, and sequence—with the Real: the hard kernel of reality that resists symbolization.
Still, adopting the block universe does not dissolve the mystery of time; it relocates it into the heart of human experience. If past, present, and future already exist on equal terms, we are forced to confront who we are inside that geometry:
- The Experience of Passage: Later brain states contain memories of earlier ones, while earlier slices hold no trace of the future. The feeling of “flow” is what it is like for a remembering creature to be strung across spacetime.
- The Arrow of History: Even if fundamental equations are time-symmetric, the macroscopic world is governed by thermodynamics. Entropy increases, glass shatters without reassembling, and actions leave irreversible traces.
- Agency and Responsibility: No human being lives outside the block. From our embedded perspective inside spacetime, the future remains open, and our choices are the causal threads that bring later events into being.
What, then, does this physics mean for us and for human history?
It suggests that we are creatures living at the intersection of two radically different orders of time. At the fundamental level of light and spacetime geometry, the universe has no single moving present, a single event can hold multiple durations, and a photon crosses the cosmos in no time at all. Yet human history belongs to the world of mass, thermodynamics, and consciousness—the world where things take time, where causes leave scars, and where the future must be made before it can be known.
Physics confirms Kant’s suspicion that our clocks do not measure the heartbeat of “things in themselves,” while freeing us from his belief that our mental architecture is fixed forever. Just as human consciousness once expanded from the flat plane of the cave wall to the three-dimensional space of the Enlightenment, quantum time invites us to expand our construct again—moving from a rigid, ticking line to a four-dimensional awareness where we no longer mistake our habits of mind for natural necessities. For a photon, emission and absorption are the exact same instant; for a human being, everything that matters—memory, grief, justice, and the entire drama of civilization—unfolds in the delay between them. Learning to live awake inside that interval, knowing that we are the authors of how we inhabit it, has an old name: presence.