It's the one aspect of reality we all take for granted: an object exists in the world regardless of whether you're looking at it.
But theoretical and quantum physicists have been struggling for years with the possibly of a "many worlds" interpretation of reality, which suggests that every time two things could happen, it splits into new parallel realities. Essentially, they think you're living in one branch of a complex multiverse — meaning that there are a near-infinite number of versions of you that could have made every conceivable alternate choice in your life.
Physicist Sean Carroll from the California Institute of Technology deals with this problem in his new book "Something Deeply Hidden." In a new interview with NBC, Carroll makes his stance on the matter clear: he thinks the "many worlds" hypothesis is a definite possibility.
"It's absolutely possible that there are multiple worlds where you made different decisions," he told the network. "We're just obeying the laws of physics."
So if there are multiple worlds, how many are there?
"We don't know whether the number of worlds is finite or infinite, but it's certainly a very large number," Carroll claimed. "There’s no way it’s, like, five."
And he goes further, into a metaphysical view of the universe in which physical reality has much to do with the observer.
"Before you look at an object, whether it's an electron, or an atom or whatever, it's not in any definite location," Carroll told NBC. "It might be more likely that you observe it in one place or another, but it's not actually located at any particular place."
Carroll isn't the only one that has examined the possibility of many alternate realities. The likes of Stephen Hawking and Erwin Schrödinger have suggested that many other parallel worlds exist as well.
In his most recent work, Hawking suggested that thanks to quantum mechanics, the Big Bang supplied us with an endless number of universes, not just one.
As for the ability to visit other parallel universes, a topic that's come up countless times in science fiction, Carroll is not hopeful.
"[Alternate universes] don't interact, they don't influence each other in any form," he said. "Crossing over is like traveling faster than the speed of light. It's not something that you can do."
More on parallel universes: Parallel Universes Could Solve One of the Biggest Mysteries in Physics