Physicist Richard Feynman once supposedly said: 鈥淚f you think you understand quantum mechanics, you don鈥檛 understand quantum mechanics.鈥� The same could probably be said of time, which seems simple to explain because everyone experiences it, yet we don鈥檛 really know what it is or where it comes from. So in 2024, two teams of physicists tried to combine the two, turning to quantum theory to attempt to glean the essence of time.
Both zeroed in on quantum entanglement, when quantum particles are so inextricably linked that interacting with one always reveals something about the other, even if they are incredibly far apart. at the National Research Council of Italy and his colleagues investigated the idea that any object that we see as changing over time must always be entangled with a clock of some kind.
This idea originated in the 1980s and has one striking consequence: if the passage of time only arises because of entanglement, then any observer who could step outside of the entangled clock-and-object system would see a universe forever frozen in one unchanging moment. Coppo and his colleagues put this theory through a series of mathematical tests and found that a world where time is an illusion created by quantum entanglement would in many ways follow the same physics laws as the one that we experience.
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at the University of California, San Diego, and at the University of Surrey in the UK took a different approach. They investigated how time may be defined in reference to quantum entanglement at the scale of the whole universe. Their calculations suggest that in the very early universe there was little to no quantum entanglement, but as our cosmos matured, the amount of entanglement increased. Because this increase is tied to an irreversible process called 鈥渄ecoherence鈥�, the researchers defined a quantum arrow of time that directs it to only ever flow in one direction.
Keming Chen says that a notion of an arrow of time already exists in thermodynamics 鈥� the physics of heat, energy and entropy 鈥� where it points from past states of the universe that have low disorder to future states that will be highly disordered. His work implies that the early universe would have both low disorder and low entanglement, so he has been studying whether the two arrows, the quantum and thermodynamic ones, would push it into the more entangled and more disordered future at the same rate.
The existence of the thermodynamic arrow of time may also have implications for what sorts of quantum states objects in our universe can have, which could, in turn, change their entanglement, he says. So, there could be connections, and complications, between his work and Coppo鈥檚. Entanglement between clocks and objects, which is at the centre of Coppo鈥檚 theory, may also have to keep increasing, says Keming Chen. What that in turn means for time is unclear.
For Coppo, there is also the issue of connecting space and time, since physicists understand the two as parts of a single entity we call space-time, often referred to as the fabric of physical reality. He and his colleagues want to determine whether their theory is consistent with gravity, which we know arises from space-time鈥檚 structure, but which has so far eluded all attempts to be seamlessly combined with quantum theory.
at the University of Oxford says that both of the new studies play with well-known ideas about quantum physics and time, but don鈥檛 yet bring us radically closer to understanding time鈥檚 essence. 鈥淭he fact that entanglement features in both is perhaps not so surprising given that, in quantum physics, all things non-trivially quantum must ultimately lead to entanglement,鈥� he says.
In addition to thermodynamics and quantum entanglement, there is one more factor that we may have to consider before we crack the problem of time, says at Chapman University in California. 鈥淚 am particularly interested in seeing further research on quantum time which explores the nature of temporal experience,鈥� she says.
Quantum theory already suffers from the so-called measurement problem, where the act of observing a quantum object seems to change it, but it is unclear how that change happens. New insights into how observers end up experiencing quantum time may shed light on this problem while also elucidating the nature of that time itself, says Adlam.
Keming Chen feels similarly. 鈥淢aybe it鈥檚 not just the quantum part. We have new ideas about quantum [time], but we also need to think about what it means to explain our temporal experiences, so we may need to bring in cognitive science and neuroscience. I鈥檓 sometimes optimistic about this,鈥� he says.
