In the quantum world, our intuitive grasp of past, present and future may not apply. Richard Fisher explores the discombobulating concepts of "negative time" and "retrocausality".

Tony Soprano is smoking a cigar, driving home. His journey begins in Manhattan, where he enters the Lincoln Tunnel, headed towards New Jersey. There's no traffic, so he passes through in a few minutes, emerging into the daylight.

So begins the familiar opening credits of The Sopranos TV show.

In the physical world of mafia bosses, one event follows another. So if Tony strikes a match to light his cigar, this needs to happen before the tip ignites. Such causality appears to be fundamental to how we experience the Universe.

However, at the quantum level, temporal sequencing is not so clear or intuitive. In recent years, physicists have been exploring some seriously head-scratching behaviours at very small scales – some of which can be explained; some of which seem to throw our understanding of past, present and future into question.

To get a flavour, imagine The Sopranos opening credits featured an FBI helicopter watching Tony emerge from the Lincoln Tunnel – but they can't make sense of what they're seeing. From their perspective, the mafia boss leaves the tunnel before he enters. And when the confused FBI officers check their watches, he has spent a negative amount of time driving between Manhattan and New Jersey.

That of course is impossible. However, recently physicists were in the headlines for measuring a duration of "negative time". In quantum experiments, they sent light-pulses through the equivalent of a tunnel – but like Tony Soprano's puzzling drive, the pulses apparently spent less than zero time travelling through.

And that's not the only baffling example of temporal strangeness at very small scales – other theorists think it's conceivable that some particles could even change the past from the future, via an effect called "retrocausality". In the quantum world, it seems our familiar understanding of time quickly becomes, well, disordered.

This year will mark 100 years since the development of quantum mechanics, and the UN has designated 2025 as the International Year of Quantum Science and Technology. Over the past century, physicists have explored all sorts of unusual behaviours in the quantum world: entanglement, superposition, uncertainty and more.

One of the lesser-known examples is a puzzling way that light tunnels through barriers, such as clouds of atoms. In the 1990s, physicists fired photons through a barrier as a "wave packet" (a bundle of waves that describes both the particle and wave nature of light). Puzzlingly, the packet's peak apparently emerged before they entered – like a car leaving a tunnel before it drove in.

Earlier theory in the mid-20th Century had predicted the effect – now known as a "negative group delay" – but observing it experimentally was another thing, because it should be impossible. It suggested that light could travel faster than itself, which is nonsensical. Moreover, events in time were apparently occurring out of order.

"We had to find a new way to reconcile that with our ideas of causality," says physicist Aephraim Steinberg of the University of Toronto.

In the intervening years, Steinberg and his fellow physicists proposed what could be happening, without violating known physical laws. In short, they argued that the wave packet was not time-travelling but reorganising itself to give the appearance of effect coming before cause.

To understand, imagine a line of cars driving between New York and New Jersey, says Steinberg. We might picture them as 100 Tony Sopranos, each driving bumper-to-bumper. These represent photons in a wave packet.

The line of Tonys depart Manhattan at 13.00. At around 13.30, the midpoint of this car-train enters the Lincoln Tunnel. This is the peak of a wave packet. You would expect this peak to emerge a few minutes later, right? However, the peak has already left the tunnel – at 13.25. Tony Soprano, apparent time-traveller.

What may actually be happening at the quantum level, inside the barrier, is that not all the photons are making it through, says Steinberg. In the car analogy, some Tonys are getting pulled over or turned back; in the experiment, they are absorbed or ejected by atoms within the barrier. When this happens, the forward tail of the wave packet reshapes itself into a new peak. It looks uncannily like the one entering.

Naturally, it's more complex than that – photons don't act like queuing cars because........

© BBC