In the late 1970s, physicist John Wheeler began to ponder a question that continues to intrigue scientists to this day: at what moment does the universe ‘notice’ that we are observing a quantum process? And does it really matter? The answer turned out to be so unexpected that it challenged conventional views on the nature of reality.
It all started with the classic double-slit experiment. If you shine light at a screen with two narrow slits, a distinctive interference pattern—alternating bands of light and shadow—appears on the opposite wall. This is typical wave behavior, and a similar pattern can even be observed on a seashore if waves pass through two narrow gaps.
But if you reduce the intensity of the light so much that photons pass through the slits one at a time, things get even more interesting. Each photon seems to behave like a particle, landing at a specific spot on the screen. However, if you collect enough of these ‘single’ hits, the same wave pattern reappears. This suggests that even a lone photon is capable of interfering with itself.
Detectors and Choice
Further experiments introduced a new element—detectors that register which slit the photon passed through. Once it becomes possible to know the photon’s path, the interference pattern disappears. Only individual dots remain on the screen, with no sign of interference. It turns out that the very act of measurement forces the photon to behave like a particle, rather than a wave.
Scientists have concluded that it is impossible to observe both the wave and particle nature of light simultaneously. The experimenter must choose which aspect they want to measure. But Wheeler proposed taking it a step further and making the task more challenging.
Delayed choice
What happens if the decision to measure a photon’s path is made after it has already passed through the slits? Wheeler suggested a thought experiment involving a distant quasar, whose light travels toward us for billions of years. Some of this light comes directly, while some bends around a massive object acting as a gravitational lens. Both beams reach Earth at the same time, and we can choose which property to measure—the wave or the particle nature of light.
The results were astonishing. Even if the type of measurement is decided at the last moment, the photon ‘responds’ to that choice. It behaves as if it already knew what we were going to do. It almost seems as though a future decision can alter the particle’s past behavior.
Quantum eraser
Later, the experiment was refined and became known as the ‘delayed-choice quantum eraser.’ In this version, the photon first passes through the slits, and then the experimenter decides whether to preserve or erase information about its path. If the information is preserved, no interference occurs. But if the data is erased, a wave pattern appears on the screen—even though the photon reached the screen long before.
The key point is that the decision is made after the event has already taken place. This completely overturns our usual understanding of cause and effect. Quantum mechanics seems to suggest: we shouldn’t think of photons as something that exists between the start and end of the experiment. What matters is what we ultimately measure.
Rethinking Reality
Wheeler insisted that it makes no sense to talk about photons “in flight.” Only measurement results exist; everything else is just our attempt to interpret what’s happening. The order of events and the details of the experiment are irrelevant. A photon demonstrates either a wave-like or particle-like nature only at the moment of measurement—never before.
This paradoxical conclusion forced physicists to reconsider the very essence of reality. Quantum objects do not follow the familiar rules of logic. They exist outside time and space as we typically understand them. Only the act of observation determines which aspect of reality we perceive.
If you didn’t know, John Wheeler was one of the most influential physicists of the 20th century, the author of the term “black hole,” and a mentor to many Nobel laureates. His ideas about quantum mechanics and the nature of measurement remain a subject of debate in the scientific community. Experiments with the quantum eraser have become classics of modern physics and continue to inspire new generations of researchers in their search for answers about the fundamental laws of the universe.
