BREAKFAST in my house is a causal affair. The kettle boils because I have switched it on. The toast acquires its golden crust because I put it in the toaster. The butter makes its way to the table because I removed it from the fridge. For all the weirdness that the universe throws at us, these are simple truths that we can take for granted. The past is the past. The present precedes the future. Cause comes before effect. Except when it doesn鈥檛.
Physicists have started to realise that causality might not be as straightforward as we thought. Instead of cause always preceding effect, effects can sometimes precipitate their causes. And, even more mindbogglingly, both can be true at once. In this version of events, you would be opening the fridge because the butter was already on the table, and your toast would be perfectly golden both before and after you put it in the toaster. You wouldn鈥檛 just be making breakfast 鈥� your breakfast would also be making you.
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Playing fast and loose with causality does more than make for confusing mornings. It could shake physics to its very foundations. No longer having a definite order of events goes against the picture of the universe painted by general relativity, and even hints at a reality beyond quantum mechanics, the best model we have of the subatomic world. Allowing causality to operate in both directions could allow us to combine these two theories into a single framework of quantum gravity, a goal that has eluded us for over a century. The end of causality as we know it could be a cause for celebration. Or vice versa.
Until now, we have largely bumbled along in just one direction: forwards. 鈥淭he arrow of time has a huge impact on our lives,鈥� says physicist Julian Barbour. For that, according to prevailing wisdom, we have the second law of thermodynamics to thank. It states that the universe has been getting more and more disorderly over time, providing a clear direction for everything that happens in it. The second law explains why you can鈥檛 unbreak the egg you just fried, for example, and also suggests why we can鈥檛 reverse the big bang 鈥� getting back to that highly ordered early universe would be impossible from where we are now. The arrow of time has been fired, and there seems to be no stopping it.
That doesn鈥檛 mean its path is always smooth. In the early years of the 20th century, Albert Einstein鈥檚 theories of relativity added a complication to our picture of time. It turns out that time runs slower for observers travelling at higher speeds, as well as for those in the presence of enormous gravitational fields. For example, if one of a pair of twins spent five years 鈥� by their watch 鈥� in a spaceship travelling at near light speed, upon return to Earth they would have aged a lot less than their sibling. And events that appear simultaneous to one observer can appear sequential to another. There is one important condition, however. Even if two events appear to take place simultaneously, they can only be causally connected if there is time for one to influence the other. As information can鈥檛 travel faster than the speed of light, that produces a hard limit on which events might cause each other. Because it takes 8 minutes for light to travel between the Earth and sun, for example, the sudden explosion of the sun would take 8 minutes to have any consequences on Earth.
鈥淧laying fast and loose with causality could shake physics to its foundations鈥�
That seemed to be about as complicated as time could get. But then, a few years later, quantum mechanics got involved. Among its weirdest predictions is the notion of quantum superposition: the idea that an object can be in two different states at the same time.
This is often taken to mean that an event can be said to have happened and not happened, or that a cat 鈥� to borrow the most famous example 鈥� can be both alive and dead until observed. But as weird as these are, some things were thought to be off limits. The order in which events take place, for example, was thought to be exempt from this quantum weirdness. 鈥淭hus far we assumed that temporal order was well defined,鈥� says Caslav Brukner at the University of Vienna, Austria.
In a , Brukner shattered this assumption. He proposed that the temporal sequence of two events, just like the positions of a particle or the path it took, could also exist in superposition. That would mean that the arrow of time could have abrupt kinks in its trajectory.
This radical revision of time turned out to be more than a wild imagining: it appears to be backed up by real experiments. In , Philip Walther at the University of Vienna and his colleagues saw a photon pass through two gates, A and B, in an indefinite order, meaning that it was impossible to tell whether it went through gate A and then B, or through gate B and then A 鈥� its path was a superposition of both.
There are tantalising hints that a similar picture might hold for causality. Walther and his team followed up the work in 2017 with a more complex version of the 2015 experiment that incorporated a measurement designed to test the causal order (see 鈥�Which came first?鈥�). This is where it got tricky to implement. 鈥淵ou have to build the device in such a way that until the very end of the entire process you鈥檙e not allowed to know or to extract which result you got,鈥� says Walther. If you learned the result of the measurement during the experiment, the superposition would collapse and causal relationships would resume as normal. 鈥淚t was the next level of experiment to say, look community, we really had this,鈥� says Walther.
But the fiddliness of the design meant that the experiment hasn鈥檛 proved totally convincing. 鈥淭his is really cool stuff,鈥� says Ciar谩n Gilligan-Lee at University College London, but he sounds a note of caution. 鈥淚t鈥檚 not at the stage where we鈥檝e had conclusive or even good experimental evidence that it鈥檚 really out there,鈥� he says.
In 2019, Brukner published . He wanted to build a picture of causality that reflected the full complexity of the world, merging the notions of temporal superposition from quantum mechanics with general relativity鈥檚 prediction that time seems to pass more slowly in stronger gravitational fields. His thought experiment imagines a scenario in which two spaceships 鈥� operated by sworn enemies we shall call Alice and Bob 鈥� synchronise clocks before readying their photon cannons to fire. Then, at precisely 1200, each of them fires a photon at the other鈥檚 ship. But there is a plot twist: Bob鈥檚 spacecraft is docked near a dense planet. According to general relativity, objects such as this with strong gravitational fields would cause nearby clocks to slow. So, time should run slower for Bob, and he would get Alice鈥檚 photon before his clock shows 1200.
鈥淭his radical revision of time appears to be backed up by real experiments鈥�
So far, so classical. But, Brukner asks, what if you could put that massive planet into a quantum superposition state, so that it is close to both Alice and Bob, and affects both of their clocks? In that scenario, the impossible seems to happen: a superposition state is created where Alice鈥檚 photon arrives at Bob鈥檚 spaceship before he sends his, but Bob鈥檚 photon also reaches Alice before she sends hers.
Giving an object the size of a planet the full quantum treatment may seem impossible, but physicists are working diligently to put ever-bigger objects into superpositions. In 2019, for example, molecules made up of 2000 atoms each were put into two simultaneous states. Several labs are also working on putting small spheres, a few nanometres across, into a superposition, says Walther, although he says he doubts whether these will be massive enough to bring about the effects seen in Brukner鈥檚 thought experiment.
Further weirdness awaits. In its present form neither Alice nor Bob have the power to make a choice about their actions. But if they did have that power, then the causal orders of their choices would also be intertwined, says Brukner. 鈥淚f Alice would change something it would influence Bob,鈥� he says.
Even as a thought experiment, adding in this element of choice will be tricky because it requires one or both parties to perform a measurement 鈥� and measurements cause superpositions to collapse. But Brukner鈥檚 co-author Magdalena Zych at the University of Queensland in Australia hopes to find a way to extend the experiment to causal order in the future. After all, she says, if temporal order can be entangled, then causal order shouldn鈥檛 be far behind.
If that thought experiment can be replicated in a lab, it means that Brukner鈥檚 picture of causal order in superposition can be reconciled with general relativity. Assuming that it is possible to put a sufficiently large mass into a quantum superposition, Brukner and his team have shown that the causal wibbly wobblyness extends outwards from the points in space-time where the object might be. 鈥淚n a sense, the time order between some events in one region of space can be entangled with the time order of events in another region of space-time,鈥� says Zych. That means that everything that Alice and Bob do, as well as anything else that happens on their ships, happens both before and after anything done by the other. It would mean that we live in a world where not only can we not know what order the events happened in, but that they fundamentally have no set order at all.
鈥淚t is simply astounding,鈥� says Gilligan-Lee. He isn鈥檛 the only one who thinks this is big news. 鈥淭his is something astonishing even for those well acquainted with the bizarre features of quantum theory,鈥� says Ana Bel茅n Sainz at the University of Gdansk, Poland.
Rerouting time鈥檚 arrow
Physicists have messed with causality before. Earlier theories have suggested that causality can be reversed, or that the arrow of time flows in the opposite direction. But quantum causality goes a step further than just flipping or shifting the order of events, allowing multiple orders to essentially exist at once. 鈥淲e can think about A influencing B, and B influencing A, but there is also something else that cannot be understood with only these two terms,鈥� says Brukner. This coexistence of causal orders is a new quality that makes the causal structure in quantum theory richer, he adds.
If this new quantum property does hold up, there is a chance we could use it to our advantage. Quantum computers can theoretically tackle more complicated calculations than classical ones, and work faster. Whereas ordinary computers store memory in binary bits 鈥� 0 or 1, say 鈥� which can exist only in classical states, quantum bits or qubits can exist in a superposition, allowing them to be manipulated in a more complex way. Brukner and his colleagues have shown that into a superposition can make a specific computational task more efficient. There are also hopes that such superpositions could make for clearer communication channels by reducing sensitivity to noise.
But the real benefits could come from a new picture of the universe. For decades, quantum mechanics and general relativity have been irreconcilable frameworks: one describes the enormously small, the other the very big. And there is as yet no satisfying quantum treatment of the workings of gravity, which remains the purview of general relativity. Combining the two into a single theory of quantum gravity is therefore one of the outstanding challenges in theoretical physics.
While the bare bones mash-up of quantum mechanics and general relativity used in Zych and Brukner鈥檚 thought experiment isn鈥檛 itself a theory of quantum gravity, the mathematical framework they have developed could provide a useful testing ground for features that such a theory might possess. 鈥淚f we can understand those features really well, then maybe that will guide us towards how to actually build a quantum theory of gravity,鈥� says Gilligan-Lee.
In fact, the idea that the causal structure of events can be in a superposition is something that physicists looking into theories of quantum gravity already use routinely in their work, says Francesca Vidotto at Western University in Ontario, Canada. 鈥淭his is really at the core of a quantum gravity theory.鈥�
Those physicists expect causality to break down under extreme conditions where physics as we currently understand it falls apart, like in the early moments of the big bang or inside black holes, which are thought to have infinite density. Recent work on quantum causality means we could reproduce such quantum gravitational effects in experiments on Earth, making them far easier to follow. 鈥淲ith the development of new techniques, we can think about probing quantum gravity not only with astrophysics, but also in our labs,鈥� says Vidotto.
Although she says she doesn鈥檛 think we will be seeing a tabletop version of the quantum planet thought experiment in the next few years, we shouldn鈥檛 rule out physicists being able to manage something approaching it. 鈥淚 think it鈥檚 something that will be doable at a certain point in laboratories,鈥� she says.
Even more exciting for some, work on quantum causality is pushing the boundaries of quantum mechanics itself. For theorists like Gilligan-Lee, quantum theory isn鈥檛 necessarily the last word in the physics of the very small. In order to move beyond it, however, to some newer, shinier school of thought, we would need to find ways of relaxing the requirements of causality. Brukner鈥檚 work represents exactly the kind of relaxation that would be needed. 鈥淭hat could be one way to go beyond quantum theory and get a deeper theory of nature,鈥� says Gilligan-Lee. 鈥淚t鈥檚 the type of research that makes me most excited, currently, about the foundations of physics.鈥�
Letting go of our intuitive idea of causality may seem like a radical step, but it could lead to a clearer picture of how the universe really works. That is something to ponder over breakfast.
