But in new experiments, physicists at MIT and Harvard have now created a new form of light, demonstrating that groups of photons can be made to interact with each other, slow down and gain mass.

The new study builds on the team's earlier research into making "photonic molecules," which involved coaxing pairs of photons into interacting with each other. If that kind of unexpected interaction could take place between two photons, the team reasoned, could it happen between three or more?

"For example, you can combine oxygen molecules to form O2 and O3 (ozone), but not O4, and for some molecules you can't form even a three-particle molecule," says Vladan Vuletic, lead researcher on the study. "So it was an open question: Can you add more photons to a molecule to make bigger and bigger things?"