300-year-old experiment could become world’s best dark matter detector

A centuries-old experiment could help accelerate the search for new and exotic particles, including those that make up dark matter.

In 1773, British scientist Henry Cavendish set up a simple experiment aimed at discovering the nature of electromagnetism. This involved measuring the electric potential on the surface of two interlocking metal shells to discern how the charged particles influence each other inside them.

Now, Peter Graham of Stanford University in California and his colleagues say that reviving the Cavendish experiment could help reveal an even more mysterious feature of our cosmos: the particles that make up dark matter.

Although dark matter makes up a much larger part of our cosmos than ordinary matter, we don’t know what it is made of. There are many theoretical proposals for what this might be, and experiments to find out range from using particle colliders to developing underground detectors.

Graham and his colleagues focused on a dark matter candidate called millicharged particles (mCPs), which, as their name suggests, have unusually small charges. The property of being charged makes it a good choice for the centuries-old Cavendish setup.

The team proposes to replicate its nested shell design, apply voltage to the larger outer shell, and then measure the voltage difference between it and the inner shell. Since mCPs are electrically charged, this measurement would reveal whether or not there are any in the experiment.

To achieve this, the new experiment would include an accumulator device that would suck up all charged particles from the room like a vacuum cleaner, bringing all potential mCPs into the facility, says team member Harikrishnan Ramani from the University of Delaware.

The design is simpler and more affordable compared to other mCP research, with an estimated cost of less than $1 million, or one-thousandth of the cost of running a particle accelerator for a year. The researchers’ calculations also show that it could be more sensitive than some particle accelerator experiments that come online in the future.

Kevin Kelly of Texas A&M University says that some of the estimates in the researchers’ calculations are likely conservative, so the proposed experiment could ultimately be between 100 and 10,000 times more sensitive than previous methods, allowing them to detect mCPs with even smaller charges than expected.

“This technique might be better than some of the things I and others use [already] “, says Christopher Hill of Ohio State University. Like Graham and Ramani, he believes the experiment could be built and completed much more quickly than, say, an experiment with a particle accelerator, thereby significantly reducing the time needed for a potentially huge discovery. “It would be a big step forward in understanding what much of the universe is made of and how it works,” says Hill. He says he plans to build a similar experiment with his own team.

The team is currently fine-tuning construction details and securing funding for the experiment. If it works, which Ramani says could happen in two or three years, it will have an added benefit: mCPs could be extracted from the Cavendish device and studied afterwards. “You could store and give people millicharged particles,” he says.