He threw the sphere and, without waiting to see if it had a safe arrival, wedged his foot under one of the trolley’s castors and heaved. The contents slid and fell, collecting in a blocky heap on the fifties lino.

He righted the trolley and looked around for what he needed. “Power supply there,” he pointed, and McNeil scurried to get it. “That bundle of leads there. Multimeter, any, doesn’t matter. And the Mukhanov book.”

The other student was frozen in place, holding the sphere like it was made of crystal. Dominguez, that was it. Had problems pronouncing his sibilants.

“You all right with that?”

Dominguez nodded dumbly.

The quantum gravity textbook was the last thing slapped on the trolley, and Petrovitch took the handle again.

“Right. Follow me.”

McNeil trotted by his side. “Dr. Petrovitch,” she said.

And that was almost as strange as being married. Doctor. What else could the university do, but confer him with that title as soon as was practically possible?

“Yeah?”

“Where are we going?”

“Basement. And pray to whatever god you believe in that we’re not over a tube line.”

“Can I ask why?”

“Sure.” They’d reached the lift. He leaned over the trolley and punched the button to go down.

“Okay,” she said, twisting a strand of hair around her finger. “Why?”

“Because what I was doing before wasn’t working. This will.” The lift pinged and the door slid aside. Petrovitch took a good long look at the empty space before gritting his teeth and pushing the trolley inside. He ushered the two students in, then after another moment’s hesitation on the threshold, he stepped in.

He reached behind him and thumbed the stud marked B for basement.

As the lift descended, they waited for him to continue. “What’s the mass of the Earth?” he said. When neither replied, he rolled his eyes. “Six times ten to the twenty four kilos. All that mass produces a pathetic nine point eight one meters per second squared acceleration at the surface. An upright ape like me can outpull the entire planet just by getting out of a chair.”

“Which is why you had us build the mass balance,” she said.

“Yeah. You’re going to have to take it apart and bring it down here.” The door slid back to reveal a long corridor. “Not here here. This is just to show that it works. We’ll get another lab set up. Find a kettle. Stuff like that.”

He pushed the trolley out before the lift was summoned to a higher floor.

“Doctor,” said Dominguez, finally finding his voice, “that still does not explain why we are now underground.”