Cambridge, Massachusetts, July 30, 2025 — In a quiet lab at MIT, a team of physicists has pulled off a historic feat—resolving one of the most iconic and enduring debates in the history of science. By reimagining the legendary double-slit experiment with cutting-edge quantum tools, they’ve delivered the most precise version yet of the test that famously challenges our understanding of light—and reality itself.
And in a twist that would’ve delighted Niels Bohr, the experiment’s outcome shows that Albert Einstein, despite his towering intellect, was mistaken in his assumptions about how light behaves at the quantum level.
Quantum Physics’ Most Famous Puzzle, Revisited
For over a century, the double-slit experiment has baffled and fascinated physicists. When a beam of light is shone at a barrier with two slits, the resulting pattern on a screen behind it is not just two bright spots, but an interference pattern—a ripple-like effect suggesting the light went through both slits at once, like a wave.
But when scientists try to observe which slit the photon passed through, that interference pattern disappears. The light behaves like a particle instead. This fundamental duality—light being both particle and wave—sits at the heart of quantum mechanics.
Einstein famously pushed back. He theorized that perhaps, if you were clever enough, you could detect the path of a single photon without destroying the wave-like pattern. Niels Bohr strongly disagreed, insisting that such a measurement would always disrupt the system. The debate never reached a clear conclusion—until now.
MIT’s Modern Twist: Atoms as the New Slits
Led by Nobel Laureate Wolfgang Ketterle, researchers at MIT’s Department of Physics, along with collaborators from the MIT-Harvard Center for Ultracold Atoms, designed an experiment that Einstein and Bohr could only have imagined.
Instead of traditional slits, the team used over 10,000 ultracold atoms cooled to near absolute zero and arranged them in a crystal-like lattice using finely tuned lasers. Each atom, spaced precisely apart, acted as an individual “quantum slit.”
“We essentially created the smallest slits possible—single atoms—and observed how individual photons interacted with them,” said Ketterle.
The team included lead author Vitaly Fedoseev and colleagues Hanzhen Lin, Yu-Kun Lu, Yoo Kyung Lee, and Jiahao Lyu. Their study, now published in Physical Review Letters, redefines how we study the particle-wave duality.
How “Fuzzy” Atoms Helped Settle the Debate
To truly challenge Einstein’s proposal, the team devised a way to measure how tightly an atom was held in place—essentially, how “fuzzy” its position was due to quantum uncertainty. The fuzzier the atom, the more likely it was to disturb the passing photon and reveal its path, causing the interference pattern to vanish.
This direct control over an atom’s confinement gave the researchers unprecedented precision in determining how light shifts from wave to particle behavior.
“In Einstein’s thought experiment, he imagined a photon rustling a slit like a bird brushing a leaf. We recreated that with single atoms, and we showed that it’s not about mechanical movement or springs—it’s about quantum uncertainty,” said Fedoseev.
No Springs Needed: Quantum Correlation Wins
Previous attempts to realize Einstein’s idea involved slits suspended on springs to measure force. MIT’s team eliminated this entirely. By briefly turning off the lasers that trapped the atoms—allowing them to float freely—they demonstrated that the results still held. The interference vanished only when the atoms were more uncertain in position, not due to any mechanical interaction.
“This proves that what really matters isn’t how the slits move, but how much we don’t know about their position,” said Fedoseev. “It’s a beautiful confirmation of Bohr’s perspective and a profound insight into quantum correlations.”
The End of an Intellectual Standoff
This achievement comes in the same year the world celebrates the International Year of Quantum Science and Technology—commemorating 100 years since quantum mechanics began reshaping our understanding of the universe. And now, with the help of MIT’s ultracold atoms and quantum finesse, a long-standing intellectual rivalry has found resolution.
“This isn’t just a tribute to a historical debate,” said co-author Yoo Kyung Lee. “It’s a leap forward in how we can manipulate and measure the quantum world.”
Why This Matters for the Future
Beyond its historical and philosophical implications, this experiment could influence how future quantum technologies are developed. The methods used to trap and manipulate single atoms and photons with such high precision are exactly the kind of innovations that drive quantum computing, secure communications, and ultra-sensitive sensors.
Moreover, it brings science education full circle. The double-slit experiment—long used in classrooms to demonstrate the strange nature of light—now has a definitive, real-world resolution that can be taught with newfound clarity.
A Milestone in Science, and a Reminder of Its Beauty
In a world grappling with complex questions—from AI to climate change—it’s refreshing to see a century-old scientific mystery resolved through patience, precision, and collaboration.
And as students across the globe continue to watch photons dance through slits on YouTube or in classroom labs, they can now know the outcome of the greatest physics debate of the 20th century: Bohr was right, Einstein was not.
But perhaps Einstein himself would’ve approved of the result. After all, he once said, “The important thing is not to stop questioning.”
Thanks to MIT’s physicists, the answers are now clearer—and the quantum world, a little less mysterious.
