Time sits at the center of a perfect storm of unsolved scientific mysteries involving free will, consciousness, and the unification of relativity and quantum mechanics. There’s two views.

One is called presentism; that’s the intuitive view—the view that only the present is real, and the past is no longer real and the future is not yet real. And that we can take actions in the present and modify or change or shape the future. That’s certainly been the dominant view throughout history. Now in modern physics, going back to Einstein, that view began to change.

Now there’s a tension between neuroscience and physics, with many physicists and philosophers saying that the laws of physics suggest that the presentist view is wrong, and that the correct view is eternalism, also called the “block universe” view. In that view, “now” is to time as “here” is to space. We have no problem saying we both exist in space, even though we’re far apart; you’re in Toronto and I’m in LA. Under eternalism, the same thing is true of time. There would be other “versions” of you in the past or in the future that all coexist.

I think the most intuitive way for people to understand this notion—that all of time is laid out in this way—is through the concept of time travel, that is, the Hollywood version of time travel. Under presentism, time travel is 100 percent impossible, because you cannot travel to moments that don’t exist. So all our favorite time travel movies, whether it’s The Terminator or Predestination, are non-starters.

So, this is a bit of a roundabout answer, but under eternalism, our subjective sense of the flow of time is hard to explain, and some physicists and philosophers view it as an illusion. In other words, since time isn’t flowing in the normal sense [in the physical world], then it must be an illusion imposed by the brain. And this is what causes this tension between neuroscience and physics.