Karlie Kloss declares her love for longtime boyfriend Joshua Kushner, Jared's younger brother

Love is a science. Not convinced? Take it up with Karlie Kloss.

The supermodel declared her love for longtime boyfriend Joshua Kushner this week in the form of some scientific poetry: "My atoms love your atoms. It's chemistry," she wrote on Instagram, quoting poet Atticus.

"Happy birthday to my best friend and partner in crime," she continued. "The past 5 years of our adventures together have been some of the best memories of my life. I love you @joshuakushner."

Joshua is the younger brother of Jared Kushner, son-and-law and adviser to President Donald Trump. He and Ivanka Trump were already married when Karlie and Joshua began dating in 2012.

After five years together, many have speculated that marriage is on the horizon for Karlie and Joshua. While he's of typical marriage age -- 32 -- Karlie is only 24, which may be why they're taking things slow.

Karlie, a computer science activist, and Harvard Business School grad Joshua make for a not only attractive, but super-smart pair: In 2015, Joshua and Jared co-founded a tech platform that helps clients invest in real estate. The company has since raised more than $250 million in capital.

That same year, Karlie created a scholarship fund called "Kode with Klossy" benefiting girls interested in computer science and software engineering.

Although their relationship is no secret, the couple keeps a relatively low profile, only occasionally sharing photos of each other on social media. Joshua did post a faraway shot of his girlfriend in an unidentified (but exotic-looking) location on his birthday.

See photos of Karlie Kloss through the years