This is an excerpt from the Lost in Space-Time newsletter. Each month, we showcase intriguing concepts from around the globe. You can Click here to subscribe to Lost in Time and Space .

Adolf Hitler’s death is recorded as April 30, 1945. At least, that’s the official narrative. However, some historians contest this, suggesting he escaped war-torn Berlin and lived in secrecy. Today, this alternate theory is largely viewed as a conspiracy, yet no rational historian can deny that, regardless of the available evidence, the “facts in question” existed. Hitler was either deceased that day or he was not. It’s nonsensical to suggest that he was both alive and dead on May 2, 1945. But if we replace Adolf Hitler with Schrödinger’s renowned cat, the historical “facts” become quite muddled.

Schrödinger is recognized as a foundational figure in quantum mechanics, the most successful scientific framework to date. It serves as the backbone for many fields, including chemistry, particle physics, materials science, molecular biology, and astronomy, yielding remarkable technological advancements, from lasers to smartphones. Yet, despite its successes, the essence of quantum mechanics appears perplexing at its core.

In our daily lives, we operate under the assumption that an “external” real world exists where objects like tables and chairs possess clearly defined traits, such as position and orientation, independent of observation. In the macroscopic realm, our observations merely uncover a pre-existing reality. Conversely, quantum mechanics governs the microscopic domain of atoms and subatomic particles, where certainty and clarity dissolve into ambiguity.

Quantum uncertainty implies that the future is not entirely dictated by the present. For example, if an electron is directed toward a thin barrier with a known speed, it can either bounce back or tunnel through, emerging on the opposite side. Similarly, if an atom becomes excited, it might remain excited or decay and emit a photon a few microseconds later. In both scenarios, predicting outcomes with certainty is impossible—only probabilistic estimates can be offered.

Most individuals are comfortable with the idea that the future holds uncertainties. However, quantum indeterminacy similarly applies to the past. The process is not yet complete. When scrutinized at a minute scale, history transmutes into a blend of alternate possibilities, a state known as superposition.

The hazy picture of the quantum microcosm sharpens during measurements. For instance, localizing an electron may show it at a specific location; however, quantum mechanics asserts that this doesn’t imply the electron previously existed in that state. It is already there. Observations merely disclose the specific location prior to measurement. Rather, measurement transforms the electron from a state without a defined location into one with a defined position.

So, how should we conceptualize electrons prior to observation? Picture an abundance of semi-real “ghost electrons” dispersed in space, each denoting a distinct potential. The reality dwells in an indeterminate state. This notion is sometimes explained by stating that an electron occupies multiple locations simultaneously. Moreover, measurements serve to convert a certain “ghost” into tangible reality while eliminating its counterparts.

Does the experimenter have control over the outcome? Not if they opt for the prevailing ghost. The process hinges on randomness. Yet, a layer of choice is present, which is vital for grasping quantum reality. If, instead of measuring position, the experimenter decides to assess the electron’s speed, the fuzzy initial state resolves into a distinct result. This time, instead of locating electrons, measurements yield electrons with velocity. Interestingly, it appears that electrons with speed exhibit wave-like properties, distinct from their particle nature. Thus, electrons embody both wave and particle characteristics, contingent on the measurement approach.

In summary: the behavior of electrons—as waves or particles—is dictated by the type of measurement the experimenter chooses. While this may seem bizarre, the situation grows even stranger. What has transpired to atoms before measurement relies on the experimenter’s selections. In essence, the properties of electrons—wave or particle—are contingent upon one’s choices, suggesting that something may have retroactively influenced the “external” world prior to measurement.

Is this time travel? Retroactive causality? Telepathy? These terms are often overused in popular quantum physics discussions, but the clearest explanation comes from John Wheeler, who coined the term black hole: “The past exists solely as recorded in the present,” he asserted.

While Mr. Wheeler’s assertion is thought-provoking, is there an actual experiment that validates it? Over breakfast at the Hilton Hotel in Baltimore in 1980, Wheeler mentioned a curious inquiry: “How do you suppress the ghosts of photons?” Recognizing my bewilderment, he proceeded to elaborate on a unique twist he devised for a classical quantum experiment, applicable to light, electrons, or even entire atoms.

This experiment traces back to the British polymath Thomas Young, who in 1801 aimed to demonstrate the wave properties of light. Young established a screen with two closely placed slits and illuminated it with a pinprick of light. What transpired? Instead of the anticipated two blurred light bands, Young observed a series of bright and dark stripes known as interference fringes. This phenomenon arises because light waves passing through each slit disperse, where they amplify and create brighter sections through constructive interference while canceling out in others, resulting in dark patches through destructive interference.

The conversation surrounding quantum mechanics began with scientists debating whether light consists of waves or particles called photons. The resolution is that it is both. Thanks to modern advancements, we can conduct Young’s experiment one photon at a time. Each photon produces a minuscule dot on the second screen, and over time, multiple dots accumulate, forming the characteristic striped pattern unearthed by Young. This situation raises questions: if a photon is a minuscule particle, it should clearly pass through either slit or the other. Yet, both slits are necessary to create the interference pattern.

What occurs if an astute experimenter wants to determine the slit a particular photon travels through? A detector can be placed near a slit to achieve this. Once that occurs, the interference pattern vanishes. The act of detecting effectively causes the photons to assume a particle-like behavior, obscuring their wave characteristics. The same principle applies to electrons; one can either pinpoint which slit the electrons traverse, resulting in the absence of interference stripes, or obscure their pathways and observe stripes manifest after numerous electrons have produced the pattern. Thus, experimenters can dictate whether photons, or electrons for that matter, act like waves or particles when they hit the detection screen.

Now, let’s discuss Wheeler’s twist. The decision to observe or not doesn’t need to be premeditated. Photons (or electrons) can pass through a slit system and remain until reaching an imaging screen. The experimenter can even opt to glance back in time to see which slit a photon originated from. Known as a delayed choice experiment, this setup has been executed and yielded anticipated outcomes. When the experimenter decides to observe, the photons fail to coalesce into a striped pattern. The essence of the phenomenon is that the reality that It was—whether the light behaves like a wave traversing both slits or a particle going through one—is contingent on the later choice of the experimenter. For clarity, in real studies, the “selections” are automated and randomized to prevent biases, occurring more swiftly than human response times.

In delayed choice experiments, the past remains unchanged. Instead, without experimentation, multiple pasts exist, intertwining distinct realities. Your measurement choice narrows down this history. While a unique past remains elusive, the number of possibilities can be reduced. Thus, this experiment is frequently referred to as the quantum eraser experiment.

Although the time used in actual experiments is merely nanoseconds, in principle, it could reach back to the dawn of the universe. This is what lay behind Wheeler’s intriguing query regarding retaining the ghost of a photon. He envisaged a distant cosmic light source being gravitationally lensed from our view by an intervening black hole, with two light paths bending around opposite sides of the black hole before converging on Earth. This scenario resembles a two-slit experiment on a cosmic scale, where a photon’s ghost may arrive via one path while another, possibly longer, route carries a different one. To execute such a cosmic interference experiment, like Young’s original experiment, the first ghost must be preserved, or “held,” allowing the waves to overlap simultaneously, awaiting the arrival of the second ghost before they merge.

Einstein claimed that past, present, and future are mere illusions. In this case, he erred in specifying “the”. A While the past is recorded in today’s history, it comprises myriad interwoven “ghost pasts,” collectively creating unique narratives on a macroscopic level. Nevertheless, at a quantum level, it transforms into a mosaic of blurred partial realities that exceed human comprehension.

Paul Davies is a theoretical physicist, cosmologist, astrobiologist, and bestselling author. His book, Quantum 2.0, will be published by Penguin in November 2025.