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When Albert Einstein’s friend Michele Besso died in 1955, Einstein, who himself would die soon after, wrote to Besso’s family: “Now he has departed this strange world a little ahead of me. That signifies nothing. For us believing physicists, the distinction between past, present and future is only a stubbornly persistent illusion.”

They were more than just words of consolation, but an understanding that the laws of physics are “time-symmetric,” as described in a new article in Quanta. “That is,” writes Dan Falk, “the physics they describe is the same, regardless of whether the variable called ‘time’ increases or decreases. Moreover, they say nothing at all about the point we call ‘now’—a special moment (or so it appears) for us, but seemingly undefined when we talk about the universe at large. The resulting timeless cosmos is sometimes called a ‘block universe’—a static block of space-time in which any flow of time, or passage through it, must presumably be a mental construct or other illusion.”

Other physicists aren’t buying it, claiming that the universe is not static, and that the passage of time is physical. “I’m sick and tired of this block universe,” said physicist and philosopher Avshalom Elitzur. “I don’t think that next Thursday has the same footing as this Thursday. The future does not exist. It does not! Ontologically, it’s not there.”

Meanwhile, what’s a block universe? Marina Cortes, a cosmologist at the University of Edinburgh, explains: