Today, the future is now— for fans of the Back to the Future trilogy, at least. October 21, 2015 marks the day when Marty McFly and Emmett “Doc” Brown touched down in their DeLorean time machine, flying in from 30 years in the past.

It wouldn’t be a proper celebration of Back to the Future Day without tie-in merchandise, or speculation about whether the producers will correctly predict the Cubs winning the World Series. Or, yes, talk of hoverboards.

Today, though, I’m interested in another gizmo from Back to the Future‘s vision of 2015, the one that made the sequels’ time travel possible: Mr. Fusion. Recall that when Doc came back from the future, he’d replaced the plutonium fueling the DeLorean’s time-travel circuits with a conveniently travel-sized fusion reactor. Alas, though 2015 has brought wearable-tech glasses similar to those featured in the film, fusion power remains out of reach.

What’s taking us so long?  Today is Back to the Future Day. Where’s my Mr. Fusion?

To get some answers, I spoke with Egemen Kolemen. He’s a specialist in the control of fusion plasmas at Princeton University, where he is an assistant professor with joint appointments at the Andlinger Center for Energy and the Environment, the department of mechanical and aerospace engineering, and the Princeton Plasma Physics Laboratory.