Physicists of the 19th century assumed that space was distinct from time 鈥� and two researchers now suspect they were correct to do so. Their conclusion, which comes from considering the behaviour of qubits, questions the now-dominant idea that four-dimensional space-time is the fundamental fabric of physical reality.
A qubit is an object that has two possible states 鈥� for example, two different spins. Because it is quantum, a qubit can also exist in combinations of those states that any familiar object could never take on 鈥� a phenomenon known as a superposition.
For years, physicists have found the mathematics of qubit states to be 鈥渆xtremely suggestive鈥� of some deeper connection to the geometry of space, says at the University of Oxford. Now, he and at Hainan University in China have made a mathematical argument for how the geometry of space may be encoded in a qubit鈥檚 behaviour in time.
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They started with a mathematical model for a single qubit that an experimenter can subject to a sequence of measurements over the course of a given period of time. Within this model 鈥� and without assuming anything about the qubit鈥檚 initial state 鈥� they analysed what the correlations between the outcomes of such measurements would be when considered across different time intervals. The process is a little like analysing whether what the qubit is doing today is related to what it was doing over the preceding 24 hours, or to what it was doing over the preceding 48 hours, and so on.
They found that the structure of these correlations was mathematically similar to three-dimensional space. Specifically, from a qubit鈥檚 behaviour through time the researchers retrieved a formula for measuring distances in space 鈥� the so-called 鈥淓uclidean metric鈥�.
Vedral says the geometry of space that we live in is more complex than the version they uncovered through their calculations of the qubit鈥檚 behaviour through time. But retrieving the Euclidean metric from such a minimal set-up and with no prior knowledge of the qubit could still be an indication that space is related to time and quantum information. 鈥淚t鈥檚 interesting that a single qubit suffices to actually get fully three-dimensional Euclidean space,鈥� he says.
But there is another tantalising implication in the work: that time is somehow separate from space, because the geometry of the latter can be derived from it. Space and time are typically considered to be components of a four-dimensional continuum we know as space-time, which underlies our physical world. Tearing them apart would violate the laws of Albert Einstein鈥檚 special relativity, and as such, it is a controversial idea among physicists.
There are, however, other researchers who argue that space and time should be separated. For instance, at the Perimeter Institute in Canada says that in his view, time is more fundamental than space. However, he doesn鈥檛 think of time existing in a way that can be captured with the equations in the new study. His hypothesis, which is also not mainstream, is that 鈥渢ime is not something that is frozen or needs structure鈥�, but should be understood as a succession of present moments that occur one after another 鈥� with no physically meaningful, or knowable, past or future.
at the Institute for Quantum Optics and Quantum Information in Vienna, Austria, says it may be intuitive to think that time is somehow different from space simply because we experience it as such. But a thorough mathematical understanding of what that means in the context of qubits is still rather elusive. For instance, Galley points out that the new study, while interesting, does not elucidate an exact mechanism by which space would emerge from the qubit and time. Moreover, the proposition that 鈥渜ubit plus time equals space鈥� may not be unique, as it may turn out to be possible to swap the qubit for a more complicated quantum object and still extract a Euclidean metric, says Galley.
Vedral says it may be possible to test some of these theoretical ideas through experiments in the future. Ultracold quantum objects can assume superposition states like the qubit in the new work, as can physical qubits similar to those used in quantum computers. But it鈥檚 likely that many mathematical questions will remain 鈥� which means physicists may still be debating how we should think about space-time for years to come. 鈥淚t seems to me that space-time may well be a fiction, in the sense that it鈥檚 a useful, convenient way for us to talk about things that happen in the universe, but in the final analysis, you won鈥檛 really need it,鈥� says Vedral.
arXiv
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