Imaginary numbers could be needed to describe reality, new studies find

Imaginary numbers are crucial if traditional quantum theory remains true.

To test how important imaginary numbers were in describing reality, the researchers used an updated version of the Bell test, an experiment which relies on quantum entanglement. (Image credit: Jurik Peter via Shutterstock)


Two new research claim that imaginary numbers are required to adequately depict reality.

When you square the square root of a negative number, you get imaginary numbers, which have long been utilised in the most essential equations of quantum mechanics, the branch of physics that deals with the realm of the very small. Complex numbers are formed when imaginary and real numbers are added together, allowing physicists to state quantum equations in simple terms. However, it has long been debatable whether quantum theory requires these mathematical chimaeras or just exploits them as useful shortcuts.


In fact, even the pioneers of quantum mechanics were concerned about the consequences of using complex numbers in their equations. Physicist Erwin Schrödinger — the first to introduce complex numbers into quantum theory with his quantum wave function (ψ) — remarked in a letter to his friend Hendrik Lorentz, "The use of complex numbers is unappealing, and it is directly to be opposed to. is unquestionably a genuine function."

Later scientists have done the same with other sections of quantum theory. Schrödinger did develop ways to represent his equation using only real numbers, along with an additional set of rules for how to utilise the equation. However, given the absence of clear experimental data to decide on the predictions of these "all real" equations, a debate has lingered: Are imaginary numbers an optional simplification, or does working without them render quantum theory incapable of describing reality?

Now, two studies published in the journals Nature and Physical Review Letters on December 15 have disproved Schrödinger's theory. They demonstrate that, if quantum physics is right, imaginary numbers are a necessary feature of our universe's mathematics through a relatively easy experiment.

Lead author Marc-Olivier Renou, a theoretical physicist at the Institute of Photonic Sciences in Spain, said, "The early founders of quantum mechanics couldn't discover any method to understand the complex numbers emerging in the theory." "Having them [complex numbers] works well, but there is no clear way to identify complex numbers with a realistic element."

The authors of the first study created a variation on the famous quantum experiment known as the Bell test to see if complex numbers were genuinely important. The test was first proposed by physicist John Bell in 1964 as a technique to show that quantum theory needed quantum entanglement — the strange connection between two far-apart particles that Albert Einstein derided as "spooky activity at a distance."

The physicists created an experiment in which two independent sources (named S and R) would be put between three detectors (A, B, and C) in an elementary quantum network in an updated version of the original Bell test. The source S would then emit two light particles, or photons, in an entangled form, one to A and the other to B. The source R would also emit two entangled photons, which would be sent to nodes B and C. The photons arriving at detectors A and C would not need to be entangled if the universe were characterized by a typical quantum mechanics based on complex numbers, but they would in a quantum theory based on real numbers.

The researchers of the second study conducted an experiment in which they beamed laser beams onto a crystal to evaluate this configuration. Some of the crystals' atoms received energy from the laser, which was later released as entangled photons. By examining the states of photons arriving at their three detectors, the researchers discovered that the states of photons arriving at detectors A and C were not entangled, implying that their data could only be explained using a quantum theory based on complex numbers.

The finding makes logical sense: photons must physically interact to become entangled, thus those arriving at detectors A and C shouldn't be. However, the researchers highlighted that their experiment only rules out theories that do not use imaginary numbers assuming the current quantum mechanics norms are valid. Despite the fact that most scientists are certain that this is the case, this is a crucial caution to remember.

According to Renou, the conclusion shows that the mathematical ways we may represent the cosmos are far more confined than we might have anticipated.


"We can rule out many alternative descriptions just by watching what comes out of particular trials," Renou added, "without making any assumptions [on the] dependability of the physical instruments employed in the experiment." In the future, this could mean that physicists will only need a minimal number of experiments to arrive at a comprehensive quantum theory based on fundamental principles.

The researchers also stated that their experimental setup, which consisted of a basic quantum network, could be beneficial in laying down the principles on which a future quantum internet would be based.

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