The Past, Present, and Future Exist All at Once, Some Physicists Say. Does It Mean Your Fate Is Fixed?

6 min readHere’s what you’ll learn when you read this story:Some physicists are considering the idea that all of time exists all at once.Both the block universe interpretation and a concept called retrocausality suggest that the future could affect the past.Researchers are exploring what a version of the block universe might mean for our idea of free will.What came first, the chicken or the egg? Normally we’d expect the explanation to run one way—the chicken laid the first egg, or the egg hatched the first chicken. But what if a future chicken could influence the egg before it hatched? That nonintuitive idea has weight when you consider a concept that some physicists are mulling over: that all of time exists all at once.Welcome to the “block universe,” where the future influences the past, everything that will happen already has, and the nature of free will is debatable.The way we perceive the cosmos normally is that time flows forward. We get older, our planet revolves around the sun, the universe moves inextricably forward from the Big Bang. But in the block universe interpretation of reality, we don’t actually need to invoke time in this way to explain how things work.“There’s no flow of time in the block universe,” says Emily Adlam, PhD, an assistant professor of the philosophy of quantum mechanics at Chapman University. “Time is just another dimension, like all the dimensions of space.” It’s akin to the directions “left” and “right” we’re all familiar with, and which both exist simultaneously. Time’s directions—the past and the future—likewise both exist simultaneously, she explains.Imagine looking into our solar system and first seeing Earth on one side of the sun, then on the other. Would that be proof that time has passed, or would it simply show that our planet’s orbit was carving a path through spacetime? The block universe argues the latter, and there’s a reason for that.“Our understanding of the flow of time is flawed,” says Rod Sutherland, PhD, a theoretical physicist at the University of Sydney’s Centre for Time. “If you slowed time down to a half, or a quarter, or an eighth, it would be unobservable. It starts to throw doubt on what it means by the flow of time.”The idea of a block universe dates back more than a century, but in recent years scientists have increasingly explored how a version of it might resolve some of the deepest puzzles of quantum physics.In particular, Adlam and others have been studying a concept called retrocausality, where events in the future can influence events in the past. It turns out that this is a surprisingly good way to explain some of the peculiarities of the quantum world, which we still don’t understand. “There’s a number of paradoxical aspects of quantum mechanics that become a lot more understandable if you bring this in,” says Sutherland.For example, in the quantum world two particles can be entangled and seem to share some properties across great distances of space. When they are both measured, they seem to have impossible correlations, implying that there must be some sort of “spooky action at a distance” between them, a term coined by Albert Einstein.But how does this actually work? One possibility might be retrocausality, where the effect of something can precede the cause. “When you prove there must be spooky action at a distance, you assume the future can’t possibly affect the past,” says Ken Wharton, PhD, a professor of physics and astronomy at San José State University. “If the future could affect the hidden past, those theorems fail.”Take an example where Alice and Bob are measuring two entangled particles, in two different locations. Under retrocausality, Alice can “cause some past variable, back where the particles were first entangled, and that variable would have also been sent to Bob,” says Wharton. “Some people call it zigzag causality. Then you don’t need spooky action at a distance. It’s just spooky retrocausal action.”Wharton has also been theoretically exploring retrocausal models of quantum circuits (these are models for quantum computation where information flows along a timeline). That way, he can treat simpler cases as arbitrarily complicated systems. And it matters for us all, he says, in understanding exactly what our place in the universe is.“It would be weird to have our choices affect the hidden past,” he says. “That would blow our minds. You get all of these weird consequences.” But it just so happens that these weird consequences “look a lot like quantum physics,” he says. “You could say that’s why quantum physics seems so counterintuitive, because retrocausality is so counterintuitive.”There are two main conceptual ways to perceive retrocausality, says Adlam. One is the two-arrow case, “where you have two distinct and independent arrows of causation going in opposite directions,” she says. “The other scenario is a sort of all-at-once style of retrocausality, where we think of the laws of nature applying all together to the whole of history.”While retrocausality is intertwined with the block universe idea, the latter could exist without the former being true. Adlam says the block universe interpretation focuses more on time than on the other laws of the universe. She compares the all-at-once idea of retrocausality to a game of Sudoku. “The rules of Sudoku apply to the whole grid, and just tell you whether a solution is valid or invalid,” she says. “The laws of nature could work like that. They could constrain all of spacetime.”That does raise the question, though, of why we perceive a present—a here and now—in both the block universe and retrocausality concepts. Adlam says that might merely be a matter of perspective. In the same way that looking at the universe from another planet might look different, so too is looking at the universe from our given place in time.“I am viewing the universe from a location here in California, and you are viewing the universe from some other location elsewhere in space,” she says. “Likewise, there’s a version of me viewing the universe from the 27th of October, and then tomorrow there’ll be another version of me viewing the universe from the 28th of October. They’re different perspectives on this big temporal block.”It’s worth noting that the design of retrocausality does not allow for time travel. That’s because, if it were possible to send a signal into the past, it would result in a causal paradox where someone could prevent the action and its subsequent effect from taking place at all, says Adlam. That means that any possible retrocausation must be restricted to the hidden past, making it hard to measure.Retrocausality and the block universe are understandably somewhat controversial ideas, and not everyone is convinced that they really exist. Not only do they bring up some potentially paradoxical scenarios but they also raise questions about whether the future is already fixed, and what that means for free will.“One of the longstanding philosophical worries about the block universe is that everything is kind of fated and there’s no free will,” says Adlam. “In some sense it’s true,” she adds, but it “doesn’t necessarily mean that we don’t have free will. You have to think more carefully about what you would understand free will to be.”For example, an idea called compatibilism says that we do have free will “as long as our desires and goals are the causes of our actions,” says Adlam. “It doesn’t matter if all of that was predetermined by physics. As long as I am the cause of my actions, I am acting freely.”Perhaps future research will tell us more about whether we really live in a block universe, or if an effect can precede a cause. Or maybe that research has already been completed in another part of spacetime, in a cosmos where everything has happened—and is yet to happen, too.Download Pop Mech Digital IssuesJonathan O'Callaghan is an award-winning freelance space journalist from the UK, currently based in Bangkok. His work has appeared in The New York Times, the BBC, Scientific American, New Scientist, and many other publications. He writes about astronomy, astrophysics, and space exploration, and sometimes appears on TV or radio as a space expert. He enjoys football, running, padel, spending time with his children, and exploring the world.