Why do we remember the past, but not the future? It seems like a silly question, but for some scientists, it’s a deep mystery wrapped up in physics and perception.
The mystery takes another twist in a study appearing in the same journal that published Albert Einstein’s theories of relativity more than a century ago.
In October’s issue of Annalen der Physik (Annals of Physics), two researchers say the phenomenon known as the arrow of time depends on observers like us as well as the clocks and other things we observe.
“Our paper shows that time doesn’t exist ‘out there,’ ticking away from past to future, but rather is an emergent property that depends on the observer’s ability to preserve information about experienced events,” co-author Robert Lanza, who is the head of Astellas Global Regenerative Medicine, told GeekWire in an email.
The paper plays off a few key concepts in physics. Physicists say that at its most fundamental level, the workings of the universe are time-symmetric. In other words, the cosmic filmstrip of causality should run as easily backward as it does forward.
But it’s obvious to even casual observers that everyday reality is time-asymmetric. You can’t unscramble an egg and put the yolk back in its shell, for example. This view is enshrined in physics’ second law of thermodynamics, which says the total entropy of a system increases over time.
So why does entropy, which is most easily thought of as disorderedness, move in one direction? Some physicists have suggested that it has to do with quantum mechanics.
The general idea is analogous to the outcome of the famous Schrödinger’s Cat thought experiment: If you open the box and find out the cat is dead, you can’t close the box and try to revive it somehow in a state of superposition.
One explanation of this phenomenon, known as decoherence, relies on gravity as the mechanism. But in their paper, Lanza and Harvard Medical School’s Dmitriy Podolskiy say gravitational decoherence isn’t effective enough to explain the progression of entropy and time.
Lanza and Podolskiy say time moves only forward in part because that’s how our brains are built.
“We observers have memory, and can only remember events that we’ve observed in the past,” he said. “Trajectories ‘future to past’ are associated with erasing of memory, since any process which decreases entropy leads to the decrease of entanglement between our memory and observed events. In other words, if we do experience the future, we’re not able to store the memories about such processes. You can’t go back in time without this information being erased from your brain.”
“The idea that gravitational decoherence is related to the arrow of time is certainly not accepted wisdom. In fact, I’d argue that it’s obviously wrong,” Carroll told GeekWire in an email. “The fact that this one mechanism doesn’t work doesn’t imply that others don’t. … At the most generous possible reading, it doesn’t seem very newsworthy.”
Another theoretical physicist, Luboš Motl; goes even further. He’s long said that all the fuss over the arrow of time is rooted in a misunderstanding of how physics works on the macroscopic as opposed to the atomic level.
“Most of the 21st-century debates about the arrow of time are profoundly irrational,” he wrote back in 2007.
Lanza is used to facing skepticism – not only when it comes to biocentrism, but when it comes to his principal field of regenerative medicine as well. “Sometimes the mind-sets in the scientific community are not all that dissimilar,” he said.
Whether or not this latest paper makes a splash, Lanza is convinced that the mysteries of time and causality will continue to challenge the scientific community.
As examples, he points to reverse-causality experiments that highlight some of the weirder implications of quantum mechanics. (One such experiment, conducted by the University of Washington physicist John Cramer, ended appropriately in a quantum muddle.)
He also notes that mind-bending perspectives on perception vs. reality continue to inspire tales that resonate with the public, ranging from “Slaughterhouse Five” to “Back to the Future.” (An upcoming alien movie called “Arrival” promises to touch on equally intriguing themes.)
The way Lanza sees it, this is how a paradigm shift begins.
“It’s usually imposed from the outside, not from the inside,” Lanza said. “These movies are intuiting that this is the reality.”
Podolskiy and Lanza are co-authors of “On Decoherence in Quantum Gravity,” published in Annalen der Physik. A version of the paper titled “On Decoherence in Non-Renormalizable Field Theories and Quantum Gravity” is available via the ArXiv pre-print server.