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Ahra Kwon
Physicists aren’t known for playing fast and loose with the rules of reality. But when it comes to the concept of space, some take a remarkably permissive attitude. For most of us, it would seem there are just three dimensions of space. Yet string theory, for instance, postulates there are at least nine.
The same attitudes don’t tend to apply to time. For almost all physicists, time has just one dimension, period. Yet some dare to wonder if there might be more to time than meets the eye.
It is a dangerous game – mess with time, and you can end up scrambling the laws of cause and effect. But then again, we know physics has plenty of problems. Maybe a drastic step is exactly what is needed to solve them? “Everything in reality can come out from a theory with two times,” says Itzhak Bars at the University of Southern California, who has been wondering about this for decades.
And with his ideas and those of others now hinting at what might lie inside black holes and even suggesting a possible solution to one of the biggest mysteries in quantum mechanics, perhaps this wild idea deserves the time of day.
The edifice of modern physics is supported by two great pillars. Quantum theory describes the world of particles and three of the four fundamental forces, while general relativity explains the remaining force, gravity, as the curvature of space-time. Yet nearly every deep problem in physics is a crack along the same fault line: these theories paint incompatible pictures of how reality works.
Broadly speaking, physicists suspect a deeper picture underlies both theories. Trying to work out what that is has led them to ideas like string theory, with its many extra dimensions of space. But few think we are anywhere near the true answers yet.
So, what about time, then? At first blush, Albert Einstein’s ideas about relativity do at least teach us that time is far stranger than everyday experience suggests. We know that distances and durations aren’t fixed properties of the world, but depend on how fast you are moving. Two observers whizzing at different velocities close to the speed of light can disagree about how far apart two events were and how much time elapsed between them, without either being wrong.
Even so, Einstein insisted they had to share an objective reality – which he hinged upon the speed of light. Every observer, he argued, must measure light travelling at exactly the same speed in a vacuum, and for that to work, space and time couldn’t behave independently. A change in one had to be compensated for by a change in the other: the faster you move through space, say, the slower you move through time.
The upshot is that you can’t just bolt on another dimension of time without risking some pretty disastrous consequences. Add a second temporal dimension, and that compensation can break down; instead of keeping the theory in balance, the extra time contribution can lead to outlandish predictions such as negative energies and events that have a less-than-zero chance of happening. Physicists call such nonsense predictions “unphysical”, and they can be difficult to exorcise from ideas with more than one time.
Even worse, causality can suffer, too. With two temporal directions, there may be routes through time that let you evade the ordering imposed by the other. Events that seemed safely arranged into before and after can become connected in unexpected ways, creating loops or allowing an observer to bypass part of history. In principle, you could “go around any event you wanted to avoid”, says theoretical physicist Chris Hull at Imperial College London. “Biology as we know it would be very hard to envisage in such a theory.”
The master of two times
It is safe to say that messing with time isn’t for the faint-hearted. But some are unfazed by the challenge – and no one has spent more, well, time thinking about these ideas than Bars. In 1998, he unveiled “two-time physics”, a framework with four dimensions of space and two of time.
Bars says the idea grew out of thinking about two of the most basic things about an object: its position and momentum; in other words, where it is and how it is moving. At first, the two seem quite different. One tells you where an object is; the other, what it is doing. But quantum mechanics hints that they are intimately linked. According to a foundational rule known as the uncertainty principle, you can never know both with arbitrary precision. Pin down an object’s position more exactly, and its momentum becomes less certain, and vice versa.
Bars wondered whether the connection went deeper still. After all, knowing an object’s momentum allows you to predict where it will be later. The history of physics, too, has vindicated many efforts to unify once-disparate ideas – electricity and magnetism forming electromagnetism, for example.
Just as one object can cast several different shadows, a single object in higher dimensions could appear in different forms in the reality we see
Shutterstock/Magic cinema
By treating position and momentum as merely alternative ways of describing the same underlying reality, Bars predicted that reality could have extra dimensions, including a second dimension of time. One way to picture this strange second time is to imagine shining a light on a cylinder. From one angle, its shadow is a circle, yet from another, it is a rectangle. They may look completely different – the same way momentum and position do – yet both can come from the same object. “Think of four space and two time dimensions as a room, and our three space and one time [dimensions] as its walls. On each wall, you see a different shadow of the same reality,” says Bars.
Crucially, treating position and momentum as interchangeable descriptions of reality also avoids the problems that plague more naive attempts to add a second time. Once you choose one way of looking at the system – in terms of position rather than momentum, say – one space and one time dimension effectively drop out of the physics, leaving us with the familiar picture of space-time and eliminating the usual causal paradoxes.
This may all be mathematically workable, but that still leaves a more basic question: what does it actually buy us? “There needs to be a clear sense of where it’s going and what kind of progress it might lead to,” says Emily Adlam, a philosopher of physics at Chapman University in California.
Inside a black hole
In 2013, Bars began applying his ideas to cosmology with physicists Paul Steinhardt at Princeton University and Neil Turok, now at the University of Edinburgh in the UK. But before long, the work began producing some rather unsettling results.
Treating position and momentum as interchangeable may be a simple idea, but it has surprisingly far-reaching consequences. A quantity that looks fixed in one four-dimensional shadow may take on a completely different value in another. This means that things we normally regard as constants, such as particle masses or the strength of gravity, could potentially vary.
And this is exactly what Bars and the others saw when they considered a universe collapsing towards a “big crunch”. The model predicted that particle masses dwindled towards zero as gravity became overwhelmingly strong. Then, instead of ending at the crunch, the universe passed through a brief phase in which gravity became repulsive, before emerging into another big bang.
“When I first saw that, I was afraid. I thought maybe something is wrong with my theory,” says Bars. The result ruffled him enough that he largely put two-time physics aside for more than a decade – retreating instead to the comparative comforts of string theory and theoretical particle physics. “But finally, I decided that either I give up my theory or I accept that it says something interesting.”
In 2025, Bars returned to the idea and asked whether the same machinery could tackle a similar situation: the one inside a black hole. A universe collapsing into a big crunch and matter falling into a black hole both head towards points, known as singularities, where quantities such as density or the curvature of space-time shoot towards infinity and equations stop making useful predictions. Most researchers take that as a sign that the physics is incomplete. But Bars argues that a falling particle has no reason to care that our equations have failed. “Why stop?” he says. “Something should happen.”
In his six-dimensional picture, it does. Rather than merely ceasing to exist, a particle falling into a black hole keeps going. Echoing Bars’s earlier cosmological results, its path carries on through the point where ordinary physics breaks down and into another region of space-time on the far side. There, gravity flips sign and becomes repulsive – an antigravity patch within the black hole.
In some versions of two-time physics, the crushing interior of a black hole could give way to an “antigravity” region on the far side
Science Photo Library/Alamy
It might sound wild, but Bars says it could offer a new angle on an old problem, known as the black hole information paradox. Quantum mechanics says information can’t be destroyed. Yet Stephen Hawking, in the 1970s, showed that black holes should slowly radiate away their energy, apparently leaving no trace of anything that fell inside. In Bars’s model, the information doesn’t disappear. It merely passes through the singularity, emerging on the far side, where gravity becomes repulsive.
“The topic and techniques introduced in this paper are highly innovative,” says Federica Muscolino, a theoretical physicist at the University of Milan-Bicocca in Italy. “It resolves the problems posed by singularities in the universe and provides a solution to the black hole information paradox.”
Could we ever know if any of this is right? In principle, yes. Astronomers can look for signs that supposedly fixed quantities have changed over cosmic history. Two-time physics predicts that such “constants”, including particle masses, could shift in extreme environments. Although finding such a variation wouldn’t immediately prove Bars right, it could offer a rare glimpse of the hidden structure he thinks lies beneath ordinary physics.
Quantum entanglement through time
However, one thing to remember about Bars’s hypothesis is that the extra time dimension isn’t something we can travel through or experience. That makes some physicists suspicious. “It’s a nice, interesting idea. It’s a nice way of looking at the equations,” says Hull. “But it’s mathematics rather than physics.”
Even so, the idea of two times has got its hooks into other physicists. Marco Pettini at Aix-Marseille University in France is among those trying to introduce a more physical version of a second time without falling into its usual pitfalls. His route into the idea began with a conversation with Nobel laureate Roger Penrose about quantum entanglement, a strange effect linking the outcomes of measurements on two particles, even when enormous distances separate them. Measure one, and you can immediately infer something about its partner, even though no ordinary signal appears to pass between them.
That sits uneasily with locality, the principle that an object can be influenced only by what is nearby, or by something that has had time to reach it. Quantum mechanics predicts these correlations perfectly, but doesn’t explain how the particles are linked.
Quantum entanglement mysteriously links particles across space, but a hidden dimension of time might offer another way to understand their connection
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“Entanglement is really astonishing because it has no place in our space-time,” says Pettini. In 2025, he suggested taking that literally. If this type of connection couldn’t be seen in our familiar space-time, perhaps it occupied another dimension. Extra space wouldn’t help, he argued, because information would still need to travel through it. “So, the only possibility is the extra time dimension,” he says.
To picture what he means, imagine crossing London’s Trafalgar Square, when everything around you suddenly freezes. While the rest of the world remains pinned in ordinary time, you slip into another temporal dimension and drag one of the bronze lions across the square. When normal time resumes, nobody will have seen the journey. The lion has simply vanished from one spot and appeared in another.
Pettini proposes something similar to explain entanglement. The particles aren’t connected by a signal racing through space. Instead, the information linking them extends through a hidden second time dimension. From inside our own time dimension, the connection appears instantaneous.
There is an obvious problem. If ordinary matter could move through this extra time, bronze lions really might start teleporting. To prevent that, Pettini borrows a mathematical device from string theory known as a warp factor, which can stretch or compress dimensions. He arranges things so that the particles and forces of our standard model of particle physics remain trapped in the four-dimensional world we know, while information associated with entanglement can extend into the hidden temporal direction.
It is fair to say this is all bold stuff. Yet, crucially, Pettini says the idea can be tested. In a paper from June that hasn’t yet been peer-reviewed, he proposes using two independent pairs of entangled particles of light, known as photons. Photons in each pair would be measured by detectors that are placed far apart. Under standard quantum mechanics, the two pairs of photons have nothing to do with one another, so the results from one measurement experiment should bear no relation to those from the other.
His model predicts otherwise. In Pettini’s picture, all entanglement reaches through the same second time dimension. That means two photon pairs created entirely independently could still interfere with each other’s measurements, producing a tiny amount of “crosstalk” – and a small correlation between the results of both detectors. “This is a falsifiable statement,” says Pettini.
Sören Petrat, a mathematical physicist at Constructor University in Bremen, Germany, welcomes the attempt. “It’s a bit of an exotic suggestion,” he says. “But from a scientific perspective, it looks like he has done everything right: suggest a solution to a deep problem, discuss its consequences and propose experimental tests.”
If a correlation appeared in such a test – and that’s a big if – it would be tempting to see this second time dimension as a potential new route through reality. Pettini has already allowed himself to consider the outrageous possibility that we could somehow harness it. “For fun”, he is applying the idea to a possible warp drive, a hypothetical device that would move faster than light by distorting space-time around it.
For now, though, none of this means that a second dimension of time really exists. It may ultimately prove to be no more than a useful mathematical device – a way of revealing connections that are difficult to see in the four-dimensional world we experience. But what once looked impossible may one day explain physics’ remaining mysteries.
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