A 20-year journey confirms a long-held theory of the formation of celestial bodies

The magnetorotational instability, a theoretical process in astrophysics that helps explain the formation of planets and stars, was just one of the topics that astrophysicist Jeremy Goodman of Princeton University brought up one April day in 2000.

The idea of the instability was first proposed in the 1950s and blossomed into widespread speculation in 1991 when Steven Balbus and John Hawley, theoretical physicists then at the University of Virginia, addressed it. They connected it with accretion discs that orbit celestial bodies and the formation of planets, stars, and supermassive black holes.

Goodman’s discussion of MRI caught the attention of physicist Hantao Ji of the Princeton Plasma Physics Laboratory (PPPL) and the scientists were soon emailing each other about an idea Ji had. Could the theory, he wondered, be confirmed? Neither thought the answer would take more than 20 years.

“I didn’t expect that it would be so difficult to find the MRI,” Ji said. “Sometimes we had doubts about whether it was really worth the effort. I'm glad we are here now looking into the future.” Goodman was equally excited. “After 20 years of effort, it’s really a thrill,” he said.

A month after the talk, Ji emailed Goodman with his idea for the experiment and Goodman was excited about it. This led to the start of the Magnetorotational Instability Experiment, which the scientists outlined in a paper in 2001.

The pair envisioned an experiment with two cylinders and end caps spinning at two different speeds to mimic the swirling of accretion discs around celestial objects. They built an early version of the device in 2004 using plastic components and water. Next came a version that replaced the plastic with steel and used galinstan, a liquid metal alloy that responds to a magnetic field.

By 2012 they achieved the conditions needed to produce an MRI but failed to see it in experiments. They added a layer of copper for better conduction and better diagnostics developed by PPPL physicist Erik Gilson, with improvements aided by German physicists at the Max-Planck Princeton Center for Fusion and Astro Plasma Physics. In 2016 PPPL theorist Fatima Ebrahimi began crafting computer simulations of the experiment.

It all came together last year when physicist Yin Wang, working from home during the COVID-19 pandemic, detailed the combined experimental, numerical, and theoretical confirmations. His findings led two recent papers, one in Physical Review Letters in September and a paper in Nature Communications in August. “This has been a group effort,” Wang said. “I am standing on the shoulders of my predecessors.”