For millennia, humanity has tried to grasp the understanding of time. From clock tick to cosmos-wide tracts of historic depth, time is by and large uncomplicated in day-to-day life: past, present, future.
However, at some deeper level of quantum mechanics, things are much less straightforward. Bizarre experiments imply that what is happening now may indeed go back and alter the past. That’s not a metaphor. It’s an experimental result.
Fault Lines in Our Conceptualization of Time
Physicists have known for a long time that time is relative. More than a century ago, Albert Einstein proved that time can warp and stretch and run slower near heavy things or moving fast.
Astronauts flying around Earth, for example, technically age only a tiny bit slower compared to those who are on the ground, owing to relativity’s effects. GPS satellite signals need to be calibrated for these warps, or else they would provide us with directions that are utterly absurd.
Already, we can see from this evidence alone that time doesn’t tick the same everywhere.
But quantum mechanics escalates weirdness to an entirely new realm. Instead of a continuous timeline, quantum science proposes a cosmos of moving probabilities in which particles never decide their direction until observed.
As measured by one of the 20th century’s most prescient physicists, Richard Feynman, once said with respect to the double-slit experiment of quantum mechanics: it contains “the only mystery” worth considering in all physics.
That experiment, and its brain-bending variant, the delayed-choice quantum eraser, compels us to reconsider what it is for something to occur in time at all.
The Double-Slit Experiment: Where It Began
The double-slit experiment is straightforward to set up but mind-blowing in its implications.
Picture shooting small particles, e.g., photons, at a two-slitted screen. With one slit open, particles behave as we’d expect: going through and into a dense clump on the screen.
With two open, however, something surprising occurs. Instead of two clumps, particles form an interference pattern, like waves on water, indicating they behaved as waves.
Apparently, each photon passes through both slits simultaneously and interferes with itself. At least until someone makes the decision to see through which slit it really passed. The interference disappears when a measurement is instituted.
The photon now “decides” it takes one route and not another, as if it knew all along someone was observing it.
This isn’t just weird; it’s fundamental. As the New York Times once put it, quantum mechanics allows particles to exist “in a murky state of possibility, anywhere, everywhere, or nowhere at all, until clicked into substantiality by a laboratory detector or an eyeball.”
The reality that it seems to change based on whether it is observed or not is raising profound philosophical questions.
Do particles exist as waves of potential until we observe them at all?
Is the universe simply biding its time until it makes up its mind how things will unfold?
Some scientists believe that the wave function, the mathematical account of all possible outcomes, is reality itself. Others believe it’s only a predictive tool and not an account of what’s really going on.
Either way, we see from the experiment that by common-sense standards, our picture of the physical world simply doesn’t apply at all at a quantum level.
Reality on Hold
Physicists such as Andrew Truscott at Canberra’s Australian National University have gone further still. Experiments in his group revealed that “reality doesn’t exist unless we’re observing it.”
That is, observation itself is not merely to reveal reality but to help bring it into being.
That notion is close to science fiction but is based on iterative experimental evidence. It implies we are not passive participants in the universe. We are not silent observers of things happening through time.
We are participants who actively determine what events exist at all.
Step aside, Delayed-Choice Experiment
The delayed-choice experiment, first proposed by physicist John Wheeler in 1978, goes one step further. Here, you wait until after the particle has already passed through the slots to determine whether to measure it.
Strangely enough, you discover that whether it behaved like a wave or particle in the past is tied to that latter choice you make later. As if the universe waits until you make a choice now before it decides it did later.
Wheeler once challenged us to picture a photon being sent out by a star billions of years ago and being deflected around a galaxy to finally arrive at Earth.
By measuring today with equipment to see on which side it went by, the act of measurement causes it to have selected one side. By measuring for an interference pattern instead, the photon behaves as if it went by both sides at once.
That is to say, our selection today determines which path it went by billions of years ago.
It’s not merely a thought experiment either. French physicists proved in 2007 that later decisions could alter the behavior of photons retroactively.
The group found that after an event appeared set in history’s pages, it was possible to change it by actions occurring later.
“Quantum effects mimic not only instantaneous action-at-a-distance, but also… influence of future actions on past events, even after these events have been irrevocably recorded,” wrote physicist Asher Peres in Science back in 2007.
Bending Causality with Experiments
This is not conjecture. Experiments have been conducted at laboratories around the planet. In Australia, Truscott’s group at Griffith University has published in Nature Physics as recently as 2015 to show that Wheeler’s delayed-choice thought experiment is upheld at the level of reality.
In China, groups have conducted variations using entangled particles, with choices made on one particle being found to apparently alter its partner retroactively. Each time, it’s the same message: time is not the one-directional sharp arrow we picture it to be.
The implications are staggering. If the present can influence the past, what does that say about time itself?
Some physicists, like Max Planck, believed consciousness plays a central role. Quantum experiments lend unsettling support to this view.
The act of measurement, done by a person, a detector, or perhaps even the universe itself, seems to collapse possibilities into a single outcome. Without observation, reality remains in a fog of probabilities.
And those who propose that there is no necessary task for consciousness to perform, any interaction with the environment, present or absent, with observing humans, suffices to bring about the collapse of the wave.
That version has its queries, though: what is a “measurement” precisely?
Where do possibilities end and actualities begin?
And if perception can stretch back through time today, why can’t our linear notion of time simply be a convenience?
This has led some to wonder whether time itself may not function as we think it does. Instead of a river flowing from past to present to future, time may simply be like a landscape with everything happening all at once, and our consciousness flowing through them.
Beyond Physics: What It Is to Us
Of course, these findings are not permission to reverse personal history or undo mistakes. No one is claiming you can reverse having eaten that third piece of pizza or reverse who won last night’s ballgame.
The experiments do, however, test at its very basic level our intuitions about time and cause and effect. They are suggesting things are less fixed, and perhaps stranger than we live through ordinary life.
Even at a cosmological scale, the experiments are valid. A photon billions and billions of years old doesn’t “choose” its trajectory until we observe it today.
Our decisions ripple back, and we have to wonder whether the past is really determinate at all and whether it isn’t formed in real time by the present as well.
The philosophical implications are vast. If the past can be altered with respect to actions in the present, what exactly is “history”?
Might there exist any number of possible pasts as there are possible futures according to quantum mechanics?
Though these questions are left unanswered, they illustrate how quantum mechanics extends its scope beyond physics to philosophy, metaphysics, and even spirituality.
The Only Mystery
Feynman was maybe correct. Quantum mechanics is not only filled with mysteries but also contains the mystery itself, that which shakes the very basis of reality.
Time, instead of being a fixed arrow, is maybe closer to being a canvas upon which past and present, and future intermingle and interlock in fashions we are only starting to see.
And if that is the case, why then, each tick of the clock might not only bring us one step closer.
It might bring us instead one step further back, redoing whatever has come before it, in increments and beyond imagination, and distorting and warping our notion of reality itself.
No Comments