Quantum Physicists Discover Evidence of Negative Time

3 February 2025
5 min read
Hayk Tepanyan
Co-founder & CTO
Negative time

Negative time may sound like something straight out of a sci-fi movie, but the peculiar results of a recent experiment show that it evidently exists in real life. This is what a team of scientists at the University of Toronto discovered. What has long been a mere theoretical idea is now a tangible part of reality. Published in arXiv, the discovery raises many questions about how we see the physical world. Does it mean time travel is possible? How does it challenge our understanding of causality? And, of course, what implications does it have in regard to the fundamental laws of quantum physics?

Maksim Sokolov (maxergon.com), CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

The Experiment: How Did Scientists Find Negative Time?

This discovery came as a result of a two-year-long quantum physics experiment. The experiment involved observing interactions between light and matter. Some photons (light particles) that pass through atoms are absorbed by those atoms and then re-emitted. This puts the atoms in a higher energy or “excited” state before they return to normal. When the team, led by Daniela Angulo, measured how long the atoms remained in that state, they found that the time was negative. 

A Clock That Moves Backward

Doctoral student Josiah Sinclair explained the surprising result:

“A negative time delay may seem paradoxical, but what it means is that if you built a ‘quantum’ clock to measure how much time atoms are spending in the excited state, the clock hand would, under certain circumstances, move backward rather than forward.”

The experiment showed that photons can appear to leave a material before entering it.

The Apparatus and Key Findings

To investigate further, the team used a specially designed apparatus that had taken three years to develop. It shot photons through a cloud of extremely cold rubidium atoms and used the cross-Kerr effect to measure atomic excitation.

This led to two surprising findings:

  • Some photons passed through the cloud unaffected, yet the atoms still entered an excited state as if they had absorbed them.
  • When photons were absorbed, they seemed to be re-emitted almost immediately—long before the atoms returned to their ground state.

Sinclair noted that their results raise questions about how photons travel through absorptive materials and challenge the traditional understanding of group delay in optics.

Theoretical Explanation and Next Steps

To make sense of their findings, the team collaborated with Howard Wiseman, a quantum physicist at Griffith University. The theoretical model suggested that the time spent by transmitted photons as an atomic excitation was exactly equal to the expected delay the light acquired.The researchers then designed a follow-up experiment to test this idea further. They recognized that a photon could be transmitted in two ways:

  1. It could pass through the rubidium atoms without interacting at all.
  2. It could be absorbed and then re-emitted.

So, What Is Negative Time?

Simply put, negative time refers to a segment of time that is less than zero. In classical physics, where time is exclusively positive, particles move forward in time as they travel. In quantum physics, however, time doesn’t always behave the way we expect it to. 

But what does this mean for our understanding of reality? To be clear, it has nothing to do with the actual passage of time. Rather, it’s a way of explaining how particles travel and how their phases change, according to theoretical physicist Sabine Hossenfelder, who criticized the research

In quantum mechanics, certain equations remain the same as time goes forward or backward. In other words, reversing time brings results that are equal to those with forward time. As a matter of fact, some theories point out that time can flow symmetrically, which is why the laws of physics look the same regardless of the direction of time. This makes the idea of negative time plausible—at least mathematically. 

Negative time is, in some ways, linked to antimatter (matter made of antiparticles). When two particles interact, their behavior can appear like they are rewinding their path. This notion of antimatter can be traced back to the late 1900s when physicist Richard Feynman talked about antiparticles being regular particles traveling back in time. While it’s only an interpretation of quantum field theory, it helps connect the idea of negative time with practical quantum phenomena.

What Does This Discovery Mean?

The discovery of negative time has the potential to reshape our general understanding of how the universe works. It directly challenges the assumption that time only moves forward.  

To be clear, this breakthrough doesn’t necessarily indicate the possibility of time travel. It does allow for interpreting time as bidirectional or even multi-dimensional. This can help explain phenomena like the Big Bang and black holes, potentially suggesting that the timeline of the universe isn’t fixed but part of a wider structure we may not understand yet. 

On another note, the concept of negative time interestingly goes hand in hand with certain interpretations of quantum mechanics, such as the transactional interpretation, which assumes that particles exchange waves traveling both forward and backward in time. The research provides an experimental basis for these theories, offering a new perspective on phenomena like quantum entanglement and retrocausality. According to Sinclair and Steinberg, the discovery can answer questions about why light doesn’t always travel at a constant speed. 

When it comes to quantum computing vs classical computing, understanding how photons interact with atoms can improve control over quantum systems, which rely on photons to store and transmit information. It also allows researchers to create quantum circuits that benefit from these unusual behaviors to optimize the performance of quantum computers. 

One area where this discovery may have an impact is quantum data loading, a critical process for efficiently transferring information into quantum systems. While still theoretical, insights from negative time could contribute to a better understanding of how quantum states evolve and interact. This, in turn, might help researchers explore new methods for optimizing data input processes in quantum computing.

Looking to the Future

The discovery of negative time by the team at the University of Toronto created more questions than answers. Nevertheless, it was nothing short of a transformative milestone, paving the way for understanding the deepest mysteries of the universe—from the origins of time itself to the nature of quantum theory. Going forward, researchers may want to investigate similar phenomena in other atomic systems or with different forms of light. Quantum technologies like BlueQubit will certainly play a major role in such breakthroughs. 

While the general implications of the discovery remain elusive, Steinberg hints that the work is far from over. "I'll be honest, I don't currently have a path from what we've been looking at toward applications," he admits. "We're going to keep thinking about it.”

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