How the Present Can Impact the Past: Retrocausality, Explained

Klenance
12 Min Read

In grammar, we learn about the past, present, and future tenses from an early age. This shapes our understanding of time as a one-way arrow. Eggs break, but don’t un-break; we grow older, but never grow younger; you can form a snowman, but it will inevitably melt back into a pool of slush again. In physics, this concept is called the “arrow of time.”

But a fascinating idea, known as “retrocausality,” complicates our conception of time. Instead of time functioning like a one-way train track (past → present → future), time could work more like a single timeline or block. In this backward-in-time effect, tenses don’t follow one another; they exist as different slices of the block, all at once. From this perspective, the present—and perhaps even the future—could influence the past in subtle ways.

And some scientists even believe that your own consciousness may follow the rules of retrocausality, meaning thoughts, feelings, or decisions you have or make today might influence events in your past. Proponents of the retrocausality theory even suggest the future is not something that “unfolds,” but is rather already present in the structure of the universe; in a sense, it has already happened—or is happening.

Put another way, what you experience as the linear passage of time might be more like a stable shadow of reality rather than fundamental reality itself, suggests Matthew S. Leifer, Ph.D, an assistant professor of physics at Chapman University. “We only see shadows cast on a wall,” Leifer explains, much like the prisoners in Plato’s Allegory of the cave, described by the ancient Greek philosopher over 2,400 years ago. These shadows represent our limited perceptions, while the world outside the cave symbolizes a deeper, more profound reality—the full 3D shapes behind them.

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As Socrates writes in Book VII of Plato’s Republic, circa 380 B.C., the Allegory of the Cave is all about our limited grasp of reality. “And now look again, and see what will naturally follow if the prisoners are released and disabused of their error. At first, when any of them is liberated and compelled suddenly to stand up and turn his neck round and walk and look towards the light, he will have pains; the glare will distress him, and he will be unable to see the realities of which in his former state he had seen the shadows; and then conceive some one saying to him, that what he saw before was an illusion, but that now, when he is approaching nearer to being and his eye is turned towards more real existence, he has a clearer vision, —what will be his reply? And you may further imagine that his instructor is pointing to the objects as they pass and requiring him to name them, —will he not be perplexed? Will he not fancy that the shadows which he formerly saw are truer than the objects which are now shown to him? ”

“But one thing we’ve learned from fundamental physics over and over again,” Leifer continues, “is that direct observations—what manifest reality looks like on the surface—are often a poor guide to what’s going on at the fundamental level.” In this fundamental realm, there could be backward-in-time influences, and time may not flow in the symmetrical, orderly way we see in classical physics.

And what is this fundamental reality? It’s the quantum world—a realm in which particles can exist in multiple states at once, where entanglement binds objects across vast distances, and where effects might ripple backward as well as forward in time. In this strange, non-intuitive domain, cause and effect may behave differently from what we expect in everyday life.

To explore the puzzling nature of time in the quantum world, Leifer and his team posed a provocative dilemma in a paper published on the pre-print server arXiv. The scientists argued that either time symmetry is fundamental to reality—where the laws of physics work the same forward and backward in time, making retrocausality (the future influencing the past) necessary—or time symmetry isn’t fundamental, and retrocausality isn’t required. Their conclusion? Fully embracing time symmetry in quantum mechanics might demand accepting the controversial idea of retrocausality.

Recent studies have begun bridging the gap between theory and practice. In a study published in Physical Review Letters in October 2023, Nicole Yunger Halpern, Ph.D.—a physicist at the National Institute of Standards and Technology and an adjunct assistant professor of physics at the University of Maryland—demonstrated a method to improve magnetic field measurements using quantum mechanics. Her co-authors entangled a quantum particle (the “probe”) with a twin particle. By adjusting the twin after the measurement, they effectively “sent” the ideal setup back in time, making it seem as though the probe was perfectly prepared from the start.

“What we showed is that you can perform a protocol in the lab with a quantum system that is mathematically equivalent to sending quantum information backward in time to enhance a metrology experiment,” says Yunger Halpern. (Metrology refers to the science of accurate measurement). However, she is quick to clarify: “That we have a protocol doesn’t necessarily mean information actually goes backward in time. It just means that, in our protocol, it’s as though information went backward in time.”

“But we don’t make the philosophical statement that information necessarily went backward in time,” Yunger Halpern continues, noting that she can certainly feel the flow of time herself. Leifer also feels the flow of time. He distinguishes between phenomenal time, the time we experience, and the concept of time in physics, though. “My way of thinking about time aligns with the block universe theory,” he says.

The block universe theory (or eternalism), rooted in physics and philosophy, sees spacetime as a four-dimensional block, with three dimensions of space and one of time. Described by Einstein’s theory of relativity, this perspective holds that the past, present, and future exist simultaneously, each occupying their own separate slice of the cosmological grid. In this 4D reality, there’s no need for a “start” or a “flow”: the past, present, and future are all equally real, coexisting within the Einsteinian space. Events don’t unfold in our familiar linear sequence. Instead, they simply “are,” fixed in their specific positions.

But how does this idea relate to consciousness? A speculative hypothesis called retrocausal consciousness says that your thoughts, feelings, or decisions today might influence events in the past—not by physically changing them, but by subtly shaping how they unfolded, like ripples flowing backward in time. Say you regret skipping a major opportunity years ago: your emotions today could subtly nudge “past you” to feel braver or hesitate less. Or, if you think about your grandmother and the bedtime stories she used to tell, your sentimental pull now could spark her to feel a little more inspired while telling those stories.

Though fanciful—or even heartwarming—this idea provokes profound questions about the mysterious interplay of time and consciousness. And at the moment, retrocausal consciousness remains a purely theoretical idea without empirical support. Simply put, it lacks scientific legs. Mainstream science says that in the brain neural activity happens first, then conscious awareness follows. This aligns with the traditional cause-and-effect flow—future decisions or thoughts can’t jump backward to change what’s already happened. Also, consciousness needs to fit into how life evolved; if it only influenced events after the fact, it would have no survival value and wouldn’t help us adapt or thrive.

Still, some studies in consciousness come up with intriguing possibilities. For instance, certain theories compare the brain to a quantum computer that uses both past and future information to make decisions in the now. Others say the universe is not made of “things” but shaped by connections between events across time, your thoughts or feelings today possibly tugging on threads intertwined with the past and the future. Alternative views even discuss the idea of the brain having some sort of “time loop” mechanism that lets it connect to the future and past on the tiny, subatomic level.

Leifer takes a different path altogether. For him, the four-dimensional block universe is simply “what it is”—a realm devoid of agents, decisions, or consciousness embedded within it. He believes that physics, including quantum mechanics, cannot fully explain consciousness, because it deals with “inert” things—entities that neither feel nor experience anything.

“The hard problem of consciousness is something unsolvable in terms of physics,” Leifer says. No matter how many physical theories of consciousness we develop, our explanations fall short in explaining how we get the rich imagery and mental activity from the gooey, sticky entanglement of meat that is our brain.

“We need something new,” Leifer says. “One way to find something new is by exploring the less-traveled paths.” Retrocausality, he believes, is one such path—but its mysteries are best suited to quantum mechanics. Consciousness, however, lies beyond the dominion of physics. It’s a puzzle of an entirely different kind, Leifer says, and we may need not just new theories and off-the-beaten path approaches, but an entirely new language to comprehend it.

Headshot of Stav Dimitropoulos

Stav Dimitropoulos’s science writing has appeared online or in print for the BBC, Discover, Scientific American, Nature, Science, Runner’s World, The Daily Beast and others. Stav disrupted an athletic and academic career to become a journalist and get to know the world.

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