[EDITOR’S NOTE: The author of this essay has been placed on leave by the Arizona State University as it investigates allegations of sexual misconduct, which he denies hold any merit. The Post published this piece before becoming aware of his employment status.]

I’ll never forget the first time I realized how deeply Stephen Hawking had affected people around the world: It was in the late 1980s, shortly after his bestseller, “A Brief History of Time: From the Big Bang to Black Holes,” was published, and we were together in Aspen for a gathering of physicists.

I told him about a place I knew that was, at the time, a kind of cowboy bar in Woody Creek, a small town nearby but culturally far removed — a place where Hunter Thompson used to hang out. I asked Stephen if he wanted to stop by, and, ever the adventurer, he said yes. I called the bar and asked whether it was wheelchair-accessible, was told it was, and off we went.

Thirty minutes later, we arrived, got out of Stephen’s van, went up a short ramp and went inside. The bar was rustic, with a pool table in the back and several tables up front. The bartender looked up at us and immediately exclaimed: “Well, if you had told me it was Stephen Hawking coming, we would have built a ramp if necessary!”

Until then, short of coming in with Albert Einstein, I would have been reasonably certain that no other scientist in the world, even Carl Sagan at the height of his fame, would have been so instantly recognizable. Thirty years later, it makes complete sense. Stephen’s mega-stardom only continued to rise. But in 1988, before his book appeared, it was almost unfathomable that a physicist could be so widely recognized, admired or sought after.

Stephen’s science was certainly one reason for his popularity. Working in the same area as Einstein, Stephen had, in many senses, revitalized interest in the general theory of relativity among many physicists by directly confronting what has become the most vexing outstanding problem in theoretical particle physics: How to make two of the towering achievements of 20th-century physics — general relativity and the quantum mechanics theory — consistent with each other. Stephen’s discovery, known as Hawking radiation, held that quantum effects around black holes could cause them to radiate particles and presents a challenge to classical theoretical physics, which had traditionally held that nothing can escape a black hole. This new phenomenon has implications that, so far, no one has been able to fully address.

But as fascinating as that is, that is not what electrified the public. Stephen did. The image of a man in a wheelchair, whose mind could roam the mysteries of the universe, was so compelling that this image, on the cover of his book, made it irresistible to millions.

Even among his admirers, those who didn’t know him personally had little idea just how extraordinary a human being he was. To start, it’s easy to forget just how difficult every single day was for him. Things most of us take for granted — breathing, talking, eating — Stephen had to work at. That he was able to face each morning with the determination to take everything he could from life and make an enduring impact took courage, determination and, yes, even stubbornness, beyond most people’s reach. In the best way, he was too stubborn to let the world interfere with the things he wanted to accomplish.

He also was funny. He could look at his own circumstances, and those of the world around him, with sufficient appreciation for the cosmic absurdity of our existence, so that he didn’t view himself as a victim but rather as an active participant in a universe that doesn’t care if we are happy, fulfilled or healthy. He’d be damned if he wasn’t going to get everything out of life, including the life of the mind, that he could, no matter what. It made him a pleasure to be around.

I like to tell jokes, and watching the twinkle in his eye when I told one he enjoyed made the awkwardness that inevitably accompanied any long conversation — there could be, at times, five-minute silences while he composed his thoughts on his computer — not just tolerable but enjoyable. His playfulness infused his writing, as well. When he agreed to write the foreword for my book, “The Physics of Star Trek,” I didn’t know what to expect.  What I got was a delightful mix of serious discussion of the importance of imagination in both science and science fiction and a wonderful story about his poker-playing scene with Einstein, Isaac Newton and Data on the USS Enterprise — a game he won on the show but one for which he was never able to collect his winnings.

Stephen pushed every boundary he confronted, in science and in life. He enjoyed breaking the rules, in part because he knew he could. One time when I was lecturing at Cambridge, I had to dress up for a fancy official dinner, and Stephen saw me in my formal get-up. The next day I was giving a seminar, missed my train and arrived on campus just in time, dressed in jeans and a T-shirt. That day, when Stephen saw me, he invited me to join him at high table at his college. I told him I wasn’t appropriately dressed, but he said, with a twinkle in his eye: “It’s okay. You’re with me.” That night, when I arrived at his college, an attendant took one look at me and wasn’t going to let me in, till Stephen arrived, that is. Throughout the dinner, with the rest of the guests in suits, and the students in robes, I know Stephen got a kick out of both the inappropriateness of my attire and that fact that we both knew how awkward we were supposed to feel.

When Stephen was determined to do something, it was difficult for anyone or anything to get in the way. More than once, trying to get from point A to point B, he’d simply turn his wheelchair into traffic, daring motorists to hit him. When he wanted to ride NASA’s “vomit comet” to experience the sensation of weightlessness, it required not only that Stephen go aboard but also his caregivers — which meant a lot of nauseated people on the flight.

Stephen’s fearlessness, combined with his charming impertinence, was vital to his success as a scientist and as an individual. Despite the difficulties, he traveled the world more than most of my colleagues, and I can’t recall a time when he said no to trying something new: He once agreed to be tied to a gurney and then slide down the entryway to a submarine so that he could go beneath the surface for his first time to view the ocean floor.

From the bottom of the ocean to the edge of space, Stephen forced his body to accompany him. His mind knew no limits. Throughout his career, he addressed truly fundamental questions about the cosmos, helping spur many of the rest of us to join him on the journey.

Leading by example, he encouraged us not to fear an uncertain future, nor the unknown mysteries of existence. We are poorer for his absence, but the memory of this remarkable man enriches us all.