It is now almost 2023. The story goes back to early 1900. So, before the 1900s, we had a perfect understanding of the laws governing the physical universe. These laws were handed down by Sir Issac Newton.
His laws are still taught at high schools, called classic physics. And if you remember any of this from high school, some math formulas would work the details.
At the core of Newton's laws is the idea that when you specify how the world is now, especially when you define its current position and velocity and other detailed measured data, these formulas can accurately predict how the world will be at any moment - in the future.
So, for example, predicting the future where a pendulum would be while swinging or where a ball would land when thrown and consequently predicting and expecting where the moon would be in its earth orbit can be done precisely and accurately, and deterministically. Or another example is how orbital eccentricity is calculated;
Fast forward to the early 20th century. At this juncture, scientists gained the capacity to explore the micro world, the world of molecules of atoms and subatomic particles. Using this realm of physics, accurate predictions from Newton's math don't work. We need a new set if rules.
So, now we need new rules to understand the microscopic world, the atomic scale and even smaller; electrons and protons, photons, etc. Within just a few decades, a generation of scientists, including Albert Einstein, Max Planck, Niels Bohr, Werner Heisenberg, Erwin Schrodinger, Max Born, and many others, ushered in a new understanding called quantum mechanics. Quantum mechanics comes with its own powerful mathematical formulation. But again, even I, (a non-physicist) can grasp the central new idea of quantum mechanics without understanding the underlying math.
As we discussed, Mr. Newton said, tell me how the world is now, and I will tell you how the world will be tomorrow.
Quantum mechanics says:
Tell me how the world is now. And I'll let you know the probability that the world will be one way or the other tomorrow.
And according to quantum mechanics, these probabilities provide the most profound and detailed description of the physical world with the rigid certainty of the classical world.
We have heard Einstein's resistance to some Quantum Mechanic facts that were not entirely clear to him then. Much has been made of Einstein's resistance to the probabilistic nature of quantum mechanics, the famous and frequently quoted, "God does not play dice."
But Einstein did not deny that quantum probabilities provided a spectacularly accurate description of the microscopic realm. Instead, he firmly believed that quantum mechanics was only a provisional theory that would ultimately be replaced by a more profound understanding that would not rely on probabilities. The idea of probabilities didn't satisfy him.
And Toward this end, Einstein worked tirelessly to expose qualities of quantum mechanics that he hoped would be so obviously unacceptable, so counter to any reasonable person's expectation of how the world works, that everyone would have to agree with his view that quantum mechanics was not the final story.
And in 1935, with two colleagues, Boris Podolsky and Nathan Rosen, Einstein believed he had finally found the ultimate quantum mechanical Achilles heel, with a property inherent to quantum mechanics called quantum entanglement.
Briefly put, Einstein and his colleagues found that according to the math of quantum mechanics, if two objects interact and then widely separated, a subsequent measurement on one of those objects, revealing one or another quality, would have an instantaneous influence on the other object regardless of the distance between them.
Einstein called this strange quantum connection "he called it" spooky, spooky action at a distance, and it troubled him deeply because his straightforward, intuitive belief was that widely separated objects are independent of one another. But quantum entanglement seemingly denied that by providing an invisible "quantum connection" capable of linking the distant objects together, or, as the name says, entangling them. Einstein could not accept this quantum view of reality. And so he concluded that something had to give. What he was saying is that quantum mechanics could not be the complete and final story.
That's the upshot of 1935, the so-called EPR paper (see below.) Now, for years, no one paid much attention to the EPR result, mainly because quantum mechanics worked. And moreover, no one could see a way to test Einstein's speculation that quantum mechanics would one day be replaced by a more profound understanding that would not need probabilities.
That is, until 1964, when a physicist named John Stuart Bell asserted that if certain predictions of quantum theory are correct, then our world is non-local. "Non-local" here means that there exist interactions between events that are too far apart in space and too close together in time for the events to be connected even by signals moving at the speed of light. He basically says there is no spooky action and no entanglements.
A lot has happened since 1964.
It is now 2022. And this year's Nobel Prize in Physics has been awarded to Alana Spain (left), John Clauser (center), and Anton Zeilinger (left), whose collective works have carried out that test establishing, to most people's satisfaction, Einstein's conventional view of reality is rolled out that quantum entanglement is genuine. It is "real."
And more than that have shown that quantum mechanics and entanglement can be leveraged for applications ranging from quantum computing to quantum teleportation, spectacular results that have both changed our view of reality and whose technological implications are both exciting and profound.
EPR document:
The Einstein–Podolsky–Rosen paradox (EPR paradox) is a thought experiment proposed by physicists Albert Einstein, Boris Podolsky and Nathan Rosen (EPR), with which they argued that the description of physical reality provided by quantum mechanics was incomplete.