Time flows in one direction in everyday life. If Tony Soprano lights a cigar, the flame must touch the tip before it ignites. Cause always comes before effect. However, at the quantum level, time does not follow such clear rules. Scientists have discovered puzzling behaviors that challenge our understanding of past, present, and future.

Time in Reverse?

Imagine a different version of The Sopranos opening. An FBI helicopter watches Tony Soprano emerge from the Lincoln Tunnel before he enters. Even more strangely, their watches show he spent a negative amount of time driving through. In reality, this is impossible. However, physicists recently measured something similar—a duration of “negative time.” Light pulses, sent through a barrier, appeared to take less than zero time to pass through.

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This discovery is not the only strange phenomenon. Some researchers believe that particles might even alter the past from the future. This concept, known as “retrocausality,” raises even more questions about how time works in the quantum world.

The Mystery of Negative Delay

Quantum mechanics will soon celebrate 100 years. The United Nations has named 2025 the International Year of Quantum Science and Technology. Over the past century, physicists have studied bizarre behaviors such as entanglement, superposition, and uncertainty. One lesser-known discovery involves light passing through barriers in unexpected ways.

In the 1990s, researchers observed that when they sent photons as a “wave packet” through a barrier, the peak of the packet emerged before it entered. This seemed impossible. It looked as though light traveled faster than itself. Earlier theories had predicted this effect, called “negative group delay,” but proving it experimentally was shocking. The sequence of events appeared out of order.

“We had to find a new way to reconcile that with our ideas of causality,” says Aephraim Steinberg from the University of Toronto. Over the years, Steinberg and his team proposed an explanation. The photons were not truly traveling faster than light. Instead, their wave packet reorganized itself, creating the illusion of time running backward.

To illustrate, Steinberg compares it to a line of cars moving from New York to New Jersey. If 100 identical Tony Sopranos drive bumper-to-bumper, they resemble photons in a wave packet. The middle of the line enters the Lincoln Tunnel at 1:30 PM. Normally, it should emerge a few minutes later. However, in this case, it appears outside the tunnel at 1:25 PM—before it even entered.

In reality, not all photons make it through the barrier. Some get absorbed or ejected by atoms. This alters the structure of the wave packet, making it look as if the peak has exited before it entered. While the concept is complex, it does not break causality. It simply means the light pulse reorganizes itself in a way that seems paradoxical.

A New Twist: Negative Duration

Recently, Steinberg’s team found something even stranger. Earlier, they had measured a negative delay—light appearing to exit a barrier before entering. Now, they measured a negative duration—light spending less than zero time inside.

At first, delay and duration seem like the same thing. If a flight is delayed, it means waiting for a specific duration. However, at the atomic level, these concepts do not always match. “Quantum mechanics says there might be one process—one event—described by multiple time scales,” Steinberg explains. In other words, when scientists measure when a photon arrives and how long it stays, they do not get the same answer.

The negative duration is even harder to explain. In the car analogy, Tony Soprano would spend less than zero time in the tunnel, which cannot be solved by wave packet reshuffling. “The weirdness was explored in the 1990s, and people made sense of it,” says Josiah Sinclair from MIT. “But looking deeper, the mystery now seems irrepressible.”

Physicists tried to measure the time photons spent in the barrier by analyzing atomic excitations. Since photons do not have fixed positions like cars, scientists had to infer timing from how atoms reacted to their presence. Surprisingly, the results showed a negative time measurement.

“We understand mathematically why it’s happening, but we don’t know its physical meaning,” says Sinclair. “There’s no clear connection to the world we experience. Quantum mechanics are just different.”

Can the Future Change the Past?

As if negative time were not strange enough, some scientists propose an even more unsettling idea—retrocausality. This theory suggests that future events can influence the past. While there is no direct experimental proof, the idea is taken seriously because it could help solve another quantum puzzle: spooky action at a distance.

In quantum entanglement, two particles share a connection regardless of distance. If one is measured, the other instantly takes on a correlated property, even if it is light-years away. Einstein called this “spooky action.” The problem is that information seems to travel instantly, which should be impossible.

A possible solution is retrocausality. Instead of instant communication, information might travel backward in time to the moment the particles became entangled. Then, it moves forward to the present. In The Sopranos analogy, if Tony orders veal in New Jersey, his twin in California orders veal at the exact same moment. However, their choices were not predetermined. Instead, the decision somehow traveled back to when they were in the womb and then forward to the present.

“Mathematically, we know these correlations cannot be explained by a common cause in the past,” says Emily Adlam from Chapman University. “It looks like an instant signal is passing between the particles. But physics tells us signals should not travel faster than light.”

A Different View of Time

If retrocausality is real, time may work differently than we imagine. One possibility suggests two timelines—one moving forward and another backward—exist side by side. However, Adlam finds this view problematic. “It easily leads to inconsistencies and paradoxes,” she says.

Instead, she prefers the “block universe” theory. This idea suggests that all moments—past, present, and future—exist together as a four-dimensional structure. If true, retrocausality does not involve time travel. Instead, all events are simply part of a larger, unchanging reality.

If this is correct, the implications are unsettling. Like Tony Soprano in The Sopranos, we may believe we control our fate. However, in a block universe, our entire story—from birth to death—is already written. No matter where we are in life, our final scene may already be set.