Experiment To Find Elusive “Chameleon” Fifth Force Suggests It Doesn’t Actually Exist

The universe is expanding faster and faster and the best explanation for this effect is the existence of an energy field that permeates all of time and space. We call that dark energy. We are not certain that dark energy exists and if it does, we don’t know what it is. One interesting theory, proposed in 2003, suggests the existence of a particle and associated force that can interact with matter but very weakly. This is the "chameleon" particle and hypothetical fifth force. But a new experiment has shown that this force is unlikely to exist.

The four fundamental forces of nature are gravity, electromagnetism, and the strong and weak nuclear forces. This chameleon force would scale with the surrounding density, so near Earth it would be very weak but in less dense regions, like intergalactic space, it would be felt. The term chameleon comes from the ability to change in different environments.

“Despite substantial efforts in astrophysical observations and laboratory experiments, previous tests are not sufficiently accurate to provide decisive conclusions as to the validity of these theories. One particularly compelling idea—the so-called chameleon theory—describes an ultra-light scalar field that couples to normal-matter fields and leaves measurable effects that are not explained by the four fundamental interactions, a so-called fifth force,” the authors, led by a team at Nanjing University, wrote in their paper published in Nature Physics.

To see if the mysterious interaction could be detected, the research team used a wheel with plastic films attached that spun past another film sitting on a levitating force sensor made of graphite. For the chameleon theory to be correct – the chameleon force would exert less force in a dense environment – the spinning films should exert a periodic force on the levitating film, pulling it up and down. 

The device, set to measure the displacement between the two masses, was in a faraday cage to block electric fields. But, despite being 100 times more sensitive than previous detectors, it could not measure any anomaly.

“We find no evidence for the fifth force predicted by chameleon models," the team explained in the paper. "Our results decisively rule out the basic chameleon model as a candidate for dark energy and demonstrate the robustness of laboratory experiments for unveiling the nature of dark energy in the future. The methodology developed here could be further applied to study a broad range of fundamental physics."

The findings rule out most versions of the chameleon explanation of dark energy, but the team hopes to improve the system by cooling it down to very low temperatures. This could be used to further probe the chameleon theory and other explanations for dark energy.