Right now, on the top of Mount Everest, time is passing just a little bit faster than it is in Death Valley. That’s because speed at which time passes depends on the strength of gravity. Einstein himself discovered this dependence as part of his theory of relativity, and it is a very real effect.

The relative nature of time isn’t just something seen in the extreme. If you take a clock off the floor, and hang it on the wall, Ye says, “the time will speed up by about one part in 1016.”

That is a sliver of a second. But this isn’t some effect of gravity on the clock’s machinery. Time itself is flowing more quickly on the wall than on the floor. These differences didn’t really matter until now. —New Clock May End Time As We Know It

So much of what we think of as “real”—that is, what is objectively true in the physical, even scientific sense—turns out to be the product of human thinking rather than a concrete reality. Tom O’Brian, “America’s official timekeeper” at the National Institute of Standards and Technology, nails it: “We can measure time much better than the weight of something or an electrical current, but what time really is, is a question that I can’t answer for you.”