Albert Einstein is often quoted as saying “it is harder to crack prejudice than an atom”. This is a very important idea to remember, especially for scientists. They must remember to put new evidence before old expectations. Theories which appear to be irrefutable for centuries can be overturned with just one disagreeing experiment. Scientists should be disciplined, and put proof before personal preference and bias .
The turn of the twentieth century demanded this of physicists like never before. Newton’s laws of mechanics and gravitation were not religion, but they had managed to withstand the wrath of time like never before . However, it became clear that they needed revision . Einstein’s innovative mind was essential for this process. Among his early contributions lie Brownian motion or the special and general theories of relativity. In particular, his description of the photoelectric effect in 1905 was fundamental for the initial steps of quantum theory, with its importance recognized by receiving the 1921 Nobel Prize for Physics [5,6].
Quantum mechanics required physicists to take distance from prejudice more than any other theory before it. Everything about it was new. Particles behave like waves, and phenomena thought to be wave-like also had a corpuscular manifestation . Physicists learned that a correct description of the microscopic world demands accepting that particles can be in multiple places at once, only to materialize at a given location upon measurement. Quantum physics was radically different to the world as Newton’s laws described it, both conceptually and mathematically. There was one aspect of the quantum world which, upon noticing it, made Einstein very uneasy with the theory he helped found, leading him to believe that it was incomplete. This phenomena was entanglement.
Quantum entanglement allows different parts of a physical system to be linked in such a way that affecting one part automatically affects the other. This is a relationship which persists until measurement. When different parts of a system are entangled, a measurement of a property of one of the parts also yields information about the rest. If the parts are moved apart by a great distance, quantum physics says that the parts are still related. Therefore, a measurement on one part will automatically yield information about the other, however far away it is. This troubled Einstein because in quantum mechanics a measurement is accompanied by something known as wave function collapse, which affects the state of the system. Entanglement, it seems, allows for remote action on a physical system .
Until the coming of quantum mechanics, physics strived to be local . What this means is that a given physical system can only (at least quickly) effect its immediate surroundings. Special relativity made the case for locality even stronger, as it introduced the cosmic speed limit, the speed of light. Nothing can propagate faster than light, be it a particle or information about someone measuring your entangled partner photon . Our previous thought experiment, though, contradicts this: even if the entangled photons are very far away, measuring one will effect the state of the other.
Einstein did not like non-locality in physics, and dubbed this effect as “spooky action at a distance” . Quantum entanglement seemed to manifest non-locality in how it connects distant particles. In 1935 Einstein, along with Boris Podolsky and Nathan Rosen, published a paper entitled “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?” . In this work they challenged quantum mechanics with a thought experiment which in their view indicated that quantum mechanics was an incomplete description of reality. They argued that entanglement (even though it wasn’t known as that yet), led to what was dubbed the EPR paradox, which allowed for violation of the Heisenberg uncertainty principle. They argued in favour of local realism: even quantum mechanical systems have well- determined properties. Entanglement can not cause a measurement on one particle to affect another further away. Instead, particles actually have well determined positions, velocities… However, the reality of these is obscured by an incomplete description by means of the wave function, and hidden variables are required to have total knowledge of the system.
The EPR paper catalysed and directed scientific thought towards this topic. Was quantum mechanics an incomplete description of reality? Had Einstein, Podolsky and Rosen come across a fundamental limitation of the theory? Does it require additional unknown variables to describe reality? Quantum theory was truly in danger. However, in 1964 John Bell published a paper titled “On the Einstein Podolosky Rosen Paradox” , in which he examined the consequences of hidden variable theories, as they had become known. In essence, his work found that such a theory could not be constructed and still give predictions consistent with quantum mechanics. And quantum mechanics was doing very well under experimental scrutiny. In fact, he found out that hidden variable theories would even be in contradiction with special relativity. It can only be speculated what Einstein would have thought of this had he been alive at the time.
Since then, experiments have time and time again been in agreement with both the predictions of Bell’s work and those of quantum mechanics. Among those predictions is quantum entanglement, which has long since been validated as a true phenomenon and is the most important resource of the rising fields of quantum information and computing . In fact, entanglement is one of the reasons why quantum computers are expected to overpower classical computers in the future, threatening encryption and our digital lifestyles [14, 15]. If Einstein were with us, he would have no choice but to break his prejudice and accept entanglement for what it is: one of the weird aspects of the quantum world, supported by empirical proof.
On a closing note, I would like to add one more thought. The EPR paradox debate showcased another fascinating aspect of science. When one of the great champions of science voices their thoughts, even if ultimately wrong, they cause revolutionary thought, debate and research, leading to incredible results. Even when they are wrong, they make science a little bit better!
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