20/03/2024

Folha: The most profound discovery in science?

Reproduction of Marcelo Viana's column in Folha de S. Paulo.

An electron and a positron are created from a photon and move towards two very distant observers, Alice and Bob. The spin of each can be +1/2 or -1/2, with a probability of 50-50. But, since electrons and positrons are antiparticles, their spins along any direction must be symmetrical: if Alice measures that the electron's spin is +1/2, this "instantaneously" informs the positron that its spin along that direction is -1/2. How can we explain this bizarre behavior?

In quantum mechanics, the common origin caused the two particles to become linked to each other ("entangled"): they became part of a single entity, even though they may be in completely different locations in the universe.

It's not surprising that Einstein had difficulty accepting this. For him, the explanation would be that the two particles "know" from birth what their spin is and carry this information with them: the values appear random only because the information is hidden from quantum mechanics, an incomplete theory. It would be a bit like a coin toss: the result – heads or tails – seems random only because we don't know how to calculate the coin's movement to determine how it will land.

So, in 1964, the Irishman John Bell proved a theorem that pointed the way to resolving the dispute and which was called "the most profound discovery in science". The idea is that Alice and Bob should measure the spins of electron-positron pairs along different directions! Let's suppose, for example, that they choose directions with an angle of 45º. Then the value of the electron's spin still "influences" the value of the positron's spin, but without giving absolute certainty of the result: according to quantum mechanics, there is a 15% probability that the two spins are equal.

Well, Bell mathematically proved that if particles carry the value of their spins from the beginning, then the probability that they will be equal is 25%! So all Alice and Bob have to do is measure the spins for a bunch of electron-positron pairs and check how often the values obtained are equal: if it's in 15% of cases, Bohr's quantum mechanics wins; if it's in 25%, it's a point for Einstein's local realism.

These experiments were conducted starting in the 1980s, initially in France by the team of physicist Alain Aspect, and quantum mechanics always won by a landslide. The universe is much stranger than Einstein wanted to accept…

To read the full text, visit the newspaper's website.