'Oppenheimer' doesn't get Einstein's relationship with the Los Alamos director quite right. Here's what they really thought of each other.

• Albert Einstein plays a significant role in the Christopher Nolan film, "Oppenheimer."
• In reality, Einstein and J. Robert Oppenheimer knew each other but weren't friends until much later.
• The real Einstein wouldn't have helped Oppenheimer with top-secret calculations about the atomic bomb.

For someone who wasn't involved in the race to develop the first atomic bomb, Albert Einstein plays a surprisingly significant role in the Christopher Nolan film "Oppenheimer."

The movie focuses on J. Robert Oppenheimer, who led the assembly and testing of the first-ever atomic bomb at Los Alamos in New Mexico.

In several scenes, the physicist, who is becoming the "father of the atom bomb," goes to Einstein for advice during and after the secret initiative, code-named the Manhattan Project.

But in reality "Oppenheimer and Einstein were not friends," nuclear-weapons historian Alex Wellerstein told Insider. "They knew each other. They worked in the same place after the war. But Oppenheimer saw Einstein as kind of the old guard."

Einstein once wrote that he didn't believe in quantum physics, which would become Oppenheimer's field of study. The younger scientist later called Einstein "completely cuckoo."

It wasn't until the last decade of Einstein's life, after the bombs had already dropped, the war had ended, and both physicists were at Princeton, that the two came to be "close colleagues and something of friends," Oppenheimer wrote in 1965.

"I saw the relationship between them as very much one of the master who'd been supplanted and whose work had been taken over by the younger," Nolan told the New York Times.

The movie captures some of the real-life tension and, later, the camaraderie between the two influential physicists. But some parts of it are fiction.

Einstein would not have helped with top-secret calculations

In the film, as in history, a Manhattan Project physicist named Edward Teller calculates that the atomic bomb they're building could set off an unending chain of reactions that ignites the entire atmosphere.

Facing the possibility that his experiment will end all life on Earth, Oppenheimer rushes to Einstein to double-check the numbers. That's pure fiction.

"He didn't go to Einstein and ask him to check calculations. That did not happen. Einstein wouldn't have been any good for that anyway," Wellerstein said. "It's the wrong kind of science."

Even if Einstein had been the man for that job, it's unlikely that Oppenheimer would have gone to him with such confidential calculations.

In reality, Oppenheimer consulted Arthur Compton, who was directing the University of Chicago efforts of the Manhattan Project.

"I shifted that to Einstein," Nolan told the Times. "Einstein is the personality people know in the audience."

Einstein and Oppenheimer disagreed on a key issue: the government

Einstein wasn't invited to join the Manhattan Project, partially because of his socialist leanings, but it's possible he wouldn't have accepted such an invitation anyway.

The scientist was a staunch pacifist. Out of fear the Nazis would develop and use a nuclear weapon, Einstein wrote the letter that convinced President Franklin D. Roosevelt to launch an atomic-bomb program.

He later regretted it, saying: "Had I known that the Germans would not succeed in developing an atomic bomb, I would have done nothing."

Einstein was a refugee in the US, having fled the Gestapo raids and Hitler's rise. His distrust of the government doesn't feature prominently in the movie. Instead, there is a glimpse of it in his eyes as he passes Senator Lewis Strauss in one of the film's most critical scenes, beside the pond at Princeton.

In the movie, Strauss is convinced that Oppenheimer said something to turn Einstein against him. But Einstein turned against politicians long before that.

"The trouble with Oppenheimer is that he loves a woman who doesn't love him — the United States government," Einstein once said, according to the book "American Prometheus," which the movie is based on.

That's an "Einstein-level burn," Wellerstein said.

Einstein did encourage Oppenheimer to turn his back on the US

After all his work, and after the war was over, Oppenheimer's life came under a bizarre national security investigation, which is the subject of the movie's third act.

Einstein did, in fact, tell Oppenheimer to give up his security clearance and walk away from government work. That scene in the movie is based on true events.

"There's a fundamental difference between the two of them that I think is exposed in this. Einstein did not think that Oppenheimer owed the government anything or the country anything like that," Wellerstein said.

Oppenheimer, however, couldn't let it go.

Einstein the outsider, Oppenheimer the disgraced insider

After the government decided to revoke his security clearance, Oppenheimer stopped working on nuclear issues altogether. His career virtually ended.

"Even though he knew a lot of things and had a lot of opinions, he basically felt like, if you didn't have a security clearance, you were not capable of being an important person," Wellerstein said.

"Somebody like Einstein would think that's nonsense," he added, calling the legendary scientist "a perpetual outsider."

Wellerstein concluded, conversely, that "Oppenheimer starts as kind of an outsider, becomes this really key insider, and then gets kicked out and never can recover from that."