Why did Einstein never wear denim jeans?

Wait, Einstein wore jeans?

As far as anyone knows, Einstein never wore denim jeans—or, at least, he never allowed himself to be photographed in them. Which is a little ironic in a cosmic kind of way, because few things had as much of an impact on the twentieth century and the modern world as the Theory of Relativity and this humble, workaday garment.

We can trace their strange entanglement to the mid-1800’s. In 1848, the discovery of gold deposits in California brought a literal wealth of change to the United States’ fledgling economy. It brought on a boom in the manufacturing and service industries, led by the sudden spike in demand for food, clothing, and transportation for the sudden deluge of immigrants.

Among these new arrivals was a young entrepreneur named Loeb Strauss, who sought his fortune not in gold mining but in one of the numerous industries that popped up to support it. In 1853, he set up shop as an importer of dry goods, which he distributed to small stores across California. He quickly became a successful businessman and man-about-town, thanks to his dealings in everything from raw textiles to finished umbrellas.

Strauss might not have been aware of it, but he owed no small part of the safe passage of both himself and his goods to a device that was perfected over a century earlier: the marine chronometer, invented by the Englishman John Harrison in 1735.

Before then, astronomical tools such as the astrolabe and sextant enabled seafarers to accurately determine their latitude based on the positions of celestial bodies relative to their location.

But longitude was another matter. As anyone who’s hopped on an international flight today knows, moving East or West puts you in a different time zone from where you started. So, by knowing the local time at your point of origin and comparing this to the time at your location, you can figure out how far you’ve gone in either direction—your longitude.

Harrison’s new timekeeping technology was so reliable and accurate that it paved the way for safer and more reliable journeys across the oceans, facilitating international travel and global commerce.

Accurate timepieces were invaluable on land as well. As the California Gold Rush spurred development on both sides of the continental United States, connecting railways were built and with these also came the need for accurate watches to keep trains on schedule and to avoid collisions. If a watch was off by even just four minutes, it could mean disaster—as it did in the Great Kipton Train Wreck of 1891.

Moreover, though not everyone could afford a railroad-grade pocket watch, the industrial revolution enabled the mass production of cheap and reasonably accurate pocket watches. For the first time in history, people could set their watches to a standard time and could rely on their watches to be faithful to the time no matter what.

Pocket watches had become so ubiquitous that, when Straub partnered with Jacob Davis, a tailor from Nevada, in 1872 to patent and sell rivet-enforced denim “waist overalls” on the East Coast, the garments featured a small front pocket for carrying—you guessed it—a pocket watch.

That same garment eventually came to be regarded as the quintessential working man’s pants. It became practically synonymous with the name that Straub had adopted when he migrated to the States: we know them today as Levi’s 501 denim jeans.

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The design has changed little over the years. The oldest surviving pair of Levi’s jeans, dating from 1879, could easily be mistaken for a modern pair—right down to the rivet-enforced watch pocket.

But more than influencing fashion well into the next century, pocket watches also reinforced the concept of “universal time:” if you synchronized all the clocks in the world, you would have the same time no matter where you go.

Alas, as Einstein showed in his Special Theory of Relativity in 1905, time is relative: clocks slow down or speed up depending on an observer’s frame of reference. This mind-boggling conclusion, which flies in the face of our everyday experience, was the death knell of classical Newtonian physics and of our common-sense faith in the universality of time.

Still, even though Einstein wasn’t known for wearing jeans, he did adopt an almost signature getup at the height of his fame in the 1930’s: he appeared in numerous photographs as well as on TIME Magazine’s April 4, 1938, coverin a brown leather jacket which his Princeton colleague Leopold Infeld described as “funny” but solved Einstein’s coat problems “for many years.”

The brand of the jacket? Levi, Strauss & Co.