Does time actually exist?
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The passage of time may be nothing more than an illusion that emerges from the quantum interactions between different parts of the universe – at least, this is true in a toy model of the cosmos. The experiment may offer hints as to the nature of time in our own universe.
Giovanni Barontini at the University of Birmingham, UK, started thinking about time while watching his 6-year-old son play. “He was building his own small universe, and I was thinking that that’s pretty much what we do also in our labs, when we build an ultracold-atoms system,” he says. “But then I was starting to think that this is also quite boring as a universe, because there’s not much going on there, and if nothing happens, it’s like if time is not passing by.”
To investigate whether time is truly an illusion in such systems, Barontini used lasers and electromagnetic forces to cool around 20,000 rubidium atoms to temperatures close to absolute zero. He divided the atoms of this toy universe into two sectors, one labelled “bright” and the other “dark”, in analogy to dark matter.
This initial universe was essentially timeless and unchanging, but then Barontini used lasers to coax the two sectors into exchanging atoms and thus interacting on a quantum level. This changed the entropy, or disorder, of the universe, and we know that in our universe, time flows in the direction of increasing entropy. Consequently, Barontini could define an internal time for the toy universe. What’s more, he could use this new time in the Schrödinger equation, which describes how quantum systems evolve, to calculate the atoms’ quantum states, finding that it matched with the results of the experiment.
There is precedent for this view of time as something that arises from quantum correlations or interactions, rather than being a given: in atomic physics, the idea was first floated by physicist Nevill Mott in the 1930s and has been explored theoretically ever since. It wasn’t until 2013 that Marco Genovese at the National Metrology Institute of Italy and his colleagues offered the first proof of its feasibility in an experiment with entangled particles of light. Here, too, a sense of time came from quantum correlations.
“The present work further elaborates on this idea with some significant progress,” says Genovese. In particular, the cold-atom universe is more complex than the one made using light, and Barontini was able to make the Schrödinger equation work with the system’s internal time, which hadn’t been done before.
Claus Kiefer at the University of Cologne, Germany, says that this toy universe experiment connects to the larger problem of how to combine gravity and quantum theory into a single framework that could apply to our universe at all scales. This question remains open, but some physicists have suggested that such a theory would be marked by the absence of time at the most fundamental level, he says. The new experiment mimics this situation, but Kiefer says that there are also differences – for instance, as ultracold atoms move between sectors, they don’t interact in complex ways that are expected in a bigger universe.
But Carlo Rovelli at Aix-Marseille University in France says that these types of experiments can’t discover something new about time because they are constructed based on physics that we already understand. Yet having them as mimics of big open problems might provide inspiration on how to treat unknown physics just like the infamously elusive issue of quantum gravity, he says.
For Barontini, the new study is an experimental confirmation of ideas that have been around for a long time and, as such, a demonstration that they aren’t fully out of contention. But it isn’t a confirmation that this is how time actually works at all scales, he says.
Cosmologists, who study the whole universe rather than lab-made toy models, are likely to have objections to the work, says Barontini. Yet he wants to explore the ultracold miniverse further, for instance by using lasers to create regions that atoms cannot move away from, similar to the pull of a black hole.
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