How Christopher Nolan recreated the Trinity Test atomic bomb explosion in 'Oppenheimer' without any CGI

• Christopher Nolan's "Oppenheimer" is about the Manhattan Project, and features an atomic bomb test.
• To film the test, he used an old Hollywood trick and made a bomb in miniature instead of using CGI.

He's blown up a 747, filmed fight scenes in zero gravity, and now, he's made a bomb. Well, a mini bomb.

For Christopher Nolan's latest blockbuster, which follows the development and aftermath of the atomic bomb by following its creator, J. Robert Oppenheimer, Nolan chose not to use computer-generated imagery, or CGI.

So the "Oppenheimer" team had to figure out how to make something that mimicked an atomic explosion when filming their recreation of the Trinity Test, which was the first time a nuclear weapon was ever detonated in 1945.

To do so, they relied on an old movie-making trick known as forced perspective.

By making a smaller version of the explosion but filming it up close, this optical illusion tricks the viewer into thinking the reaction is larger than it really is.

"We do them as big as we possibly can, but we do reduce the scale so it's manageable. It's getting it closer to the camera, and doing it as big as you can in the environment," the film's special effects supervisor Scott Fisher told SYFY.

Fueled by gas, not by nuclear fission

But even though it was smaller, the team did build an actual bomb. This one was just fueled by petroleum instead of atomic energy.

Fisher and Andrew Jackson, the film's visual effects supervisor, concocted a secret recipe to imitate the bright flash and subsequent vibrant, roiling red plumes of an atomic explosion. It included gasoline, propane, black powder, aluminum powder, and magnesium flares, Fisher told SYFY.

After filming the explosion, the team layered the frames they took over one another in the editing suite, to create an effect faithful enough to the atomic explosion, Movie Web reported.

The film also features a life-sized replica of an atomic bomb that the actors could articulate and interact with while filming, which was built by Fisher and production designer Ruth De Jong, according to an Entertainment Weekly panel.

Nolan has made it clear in previous interviews that he intends for this film to portray the terrifying implications of an atomic bomb. CGI wouldn't feel real enough to scare the audience, he told The Hollywood Reporter, because it's "comfortable to look at."

"'This can't be safe," Nolan told THR he said to Jackson when creating the film. "'It can't be comfortable to look at it. It has to have bite. It's got to be beautiful and threatening in equal measure.'"

Correction: An earlier version of this story misstated Oppenheimer's name. It is J. Robert Oppenheimer, not Robert J. Oppenheimer.