Did Einstein get it wrong? Two new quantum experiments may finally settle one of physics’ greatest debates

A scientific dispute nearly a century old now appears to be nearing its conclusion, as the results of two new quantum experiments may clarify which of the opposing theories of Einstein and Bohr was correct, opening a new chapter in the history of quantum physics.

Albert Einstein is one of the greatest figures in modern physics, whose theories fundamentally reshaped our understanding of the universe and how the world works. According to an article in Popular Mechanics, however, it now seems that a long-standing debate between Einstein and Bohr over quantum experiments may finally have been resolved – and this time, Bohr appears to have been right.

The roots of a legendary debate

The 1920s are often regarded as a golden age of science. During this decade, Einstein’s theory of general relativity was confirmed, penicillin was discovered, the existence of other galaxies was proven, and the theoretical foundations of modern quantum mechanics were laid. This scientific revolution also gave rise to fierce debates, the most famous of which was the clash between Albert Einstein and Niels Bohr.

At the heart of the dispute was the so-called principle of complementarity. This theory states that a photon cannot simultaneously exhibit both particle-like and wave-like behaviour. In other words, we can either measure which path the photon took or observe the interference pattern it produces – but not both at the same time.

Einstein disagreed. He believed that, using a special experimental setup, both properties could be demonstrated simultaneously. His idea involved a spring-mounted device that would determine which slit the photon passed through, while the interference pattern would still reveal its wave nature. Bohr, however, argued that the quantum mechanical uncertainty principle made this impossible.

• The discovery of the quantum world could rewrite the well-known laws of physics

Modern quantum experiments may provide the answer

For decades, the question remained unanswered. Now, however, two consecutive quantum experiments – one at MIT and the other at the University of Science and Technology of China (USTC) – may finally bring the debate to an end.

MIT researcher Wolfgang Ketterle and his team created an “idealised” double-slit experiment. They used individual atoms as slits and applied extremely weak light so that each atom scattered only a single photon. The result was clear: the more information they obtained about the photon’s path, the weaker the interference pattern became. This is exactly what Bohr predicted – the two properties simply exclude each other.

At USTC, researchers trapped a rubidium atom using optical tweezers and controlled its quantum state with lasers and electromagnetic forces. They scattered light in two directions and again found that precisely determining the photon’s path erased any trace of wave-like behaviour. Once more, Bohr’s theory was confirmed.

Einstein was wrong – but what does this mean for the future?

Despite this, it is important to stress that Einstein’s apparent defeat in this debate does not make him any less of a genius. Science advances through doubt and questioning, and Einstein’s critical approach also contributed to a deeper understanding of quantum mechanics.

Chao-Yang Lu, one of the leaders of the Chinese research team, said it is breathtaking to observe quantum mechanics operating at such a fundamental level. As he explained, Bohr’s counter-argument was brilliant, but for nearly a century the thought experiment remained purely theoretical – until technology finally made practical testing possible.

The researchers are not stopping here. They plan to use these new experimental methods to explore lesser-known phenomena of the quantum world, such as the relationship between decoherence and entanglement, which could prove crucial for the development of future quantum computers.