In the last episode we have discussed about spins, directions, and why the Z and X axes are fundamentally incompatible measurements. If you haven’t read it yet, I strongly recommend starting there (read here).

we ended the episode with a deceptively simple question:

And the answer was: No.

This is not a technical limitation. It is not about imperfect equipment. It is built into the structure of quantum mechanics.

So probability — true indeterminacy — appears to be at the heart of the theory.

But here’s the crucial question:

That is where Einstein enters the story.

Einstein was deeply uncomfortable with the idea that physical properties do not exist until measured. He suspected something else was going on.

What if the spin actually does have definite values in every direction — Z, X, and all others — but we simply don’t know them? Like the image shown below, it is pre-determined for every direction but we just don’t know.

If this were true, then measurements would merely reveal pre-existing properties. The randomness is only because of our ignorance.

That sounds reasonable. In fact, it sounds very classical.

But here’s the problem.

Suppose we first measure the spin in the Z direction and obtain “up,” as if that result had been predetermined, just like in the figure above. Next, we measure the spin in the X direction and obtain “right,” again matching what the figure suggests. Up to this point, everything seems consistent.

Now we measure the Z direction once more. If the spin had been predetermined, we would expect to get “up” again. However, when scientists performed this experiment, that is not what they observed. Instead, the result was sometimes up and sometimes down, appearing completely random.

From this, we can conclude:

Okay, we’ve seen that the spin is not pre-determined. But then, what’s the real problem? Why was Einstein so determined to believe in determinism?

The following is a simpler version of his thought experiment—read it very carefully, because it reveals the core of his argument.

Now imagine a special situation.

Two particles, A and B, are created together in such a way that their total spin is zero.

This means:

Now comes the key move.

Suppose we carefully separate particles A and B and take them very far from each other. We do not disturb their spins. The total spin remains zero. If it were not zero, it would violate fundamental conservation laws of physics. Therefore, it must be zero.

Now we measure particle A.

Here’s the striking fact:

But notice something subtle and profound:

We are free to choose which direction to measure A — X or Z. We can decide at the very last moment. Yet whichever direction we choose, B’s result is guaranteed to be opposite.

You might wonder why this is such a big deal. Recall the question we asked at the end of the last episode (or at the beginning of this one). We showed that it is impossible to prepare a spin so that its outcome is known in advance for both Z and X measurements (see Episode #25).

So here is what Einstein thought: Spins cannot be predetermined in advance, as we argued above. They also cannot be prepared so that they give definite outcomes for both X and Z measurements. On top of that, spin B is very far away and has no way of knowing which direction we choose to measure spin A.

And yet, whenever the measurements are performed, spin B always ends up giving the result opposite to spin A. Somehow it always “knows” what the result should be, even though it has no information about what measurement was done on A.

So he asked the most naive question:

This seemingly innocent question is actually testing the very foundations of physics that have stood for centuries!

So what are the options?

We are forced into a difficult choice:

Measuring A somehow instantly affects B, no matter how far away it is.

Einstein called this

This seems impossible, because it conflicts with the theory of relativity and causality. If signals can really travel instantly, it would lead to absurd scenarios where effects could happen before their causes—for example, it would be like being born before your grandfather. So this is impossible.

The whole problem actually begins with probability and measurement outcomes.

In our example, We already established that a single spin cannot have definite values for both X and Z simultaneously. The structure of quantum mechanics forbids it. More generally, Quantum theory assumes that the state of a spin is not determined until it is measured, and this very assumption is the root cause of all these puzzling effects.

This was Einstein’s preferred explanation.

So, Einstein, together with Podolsky and Rosen, finally summed it up by asking the following question:

All three together published a paper with the same title as the question above, you can read it here. The conclusion is as follows:

The theory works—its predictions are correct—but it cannot be the full story. There must be some hidden parameters that we are missing. If these were included, the whole picture would be complete: everything would be predetermined, there would be no probability, and yet the predictions would still match reality.

There is also a Wikipedia article on this topic, commonly known as the EPR Argument:

This was so important that it even made the headlines of The New York Times.

A few months later, Niels Bohr published another paper with exactly the same title, arguing that quantum mechanics is complete as it stands. He claimed that measurement unavoidably affects the system, so it is meaningless to demand predetermined values for all properties. However, many still found this explanation unconvincing. You can read the original paper here.

Let’s summarize the logical flow carefully:

Therefore, either:

That was the EPR challenge.

And for decades, it remained an open philosophical debate.

The EPR argument is not about technical details. It is about something far more fundamental:

Einstein could not accept that performing a measurement here could influence reality somewhere arbitrarily far away.

So Einstein bet on incompleteness. He believed neither that God plays dice, handing out probabilities, nor that there is a ghost in the atom, instantly knowing what happens at the other end.

History, however, had a surprising twist in store — and that story awaits in the next episode. Don’t even think about missing it!