Was that really President Joe Biden calling thousands of Democratic voters the day before the New Hampshire primary and telling them not to vote? Was that really President Trump tossing bags of garbage out of a window at the White House? Were those real nude photos of ninth graders circulating around a high school in Pennsylvania, or images that a classmate had generated with a free app? Was that really Tom Hanks advertising an obscure dental plan to his fans? Was that the C.E.O. on the Zoom call asking for a wire transfer of $25 million, or a North Korean impersonator? Was that a real gun in Alex Pretti’s hand, or just a shadow? Was that really a 12-year-old daughter on the phone, screaming for help, saying that she’d been kidnapped?

“I miss the days when it was a grainy video of a shark swimming up the street,” Farid said one night, as he sat on the back deck of his house with his wife, Emily Cooper. He put down his phone and poured a whiskey. “The technology is getting so good. It takes me to a dark place.”

“Because you can’t tell just by looking anymore?” Cooper asked.

“Because nobody can,” Farid said. “I don’t trust anything. Every image I see, I’m drawing lines for shadows and doing geometry in my head, trying to figure out what I’m looking at. It’s over. Within a year or two, our whole visual system will be utterly useless.”

“And then what? You give up? You retire?”

“I don’t know,” he said.

They had met 15 years earlier when Farid came to lecture at the University of California, Berkeley, still confident he had the solutions. His father had worked for 50 years as a chemist at Eastman Kodak, and Farid had grown up visiting the darkroom, watching photographs develop in chemical baths and harden into evidence. He went on to help design a digital fingerprint that found child pornography hiding on the internet — a technology that led to more than 30 million abuse reports each year, hundreds of arrests and several rescues. As deepfakes began spreading online, he built software to catch the moment a speaker’s mouth moved out of sync with the audio. He co-founded a company, GetReal Security, and helped invent tools to measure lighting, shadows and vanishing points, checking online visuals against the physics of the real world.

Cooper was a leading vision scientist at Berkeley. She researched how humans perceive reality, while her husband investigated how that reality could be faked. They’d collaborated on studies about deepfakes, but in the last months that research had begun to follow them home. Instead of dealing with one case every few weeks, Farid was working as an adviser and an expert witness, juggling up to a dozen cases each day. For the first time in his career, he’d become not just an analyst but also a victim, when someone spoofed his cellphone number and used A.I. to clone his voice. The hacker made calls to one of Farid’s colleagues on a sensitive case, impersonating Farid and pressing for confidential information. Now Farid and Cooper had decided never to take their identity for granted. They invented a safe word to confirm they were real at the start of any sensitive phone call.