Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?

Quantum Theory and the Nature of Reality

The Unfinished Quantum Description

Quantum theory describes the behavior of tiny particles like electrons. It uses "wave functions" to predict outcomes of experiments. But does the wave function give a complete description of reality? Einstein, Podolsky, and Rosen said no. They argued quantum theory is incomplete.

Some related key questions:

- Is quantum theory correct in its predictions?

- Does it fully describe reality?

- What are the basic elements of physical reality?

To call a description complete, they said every element of reality needs a counterpart in the theory. But strange quantum effects seem to defy a common sense view of reality.

A Criterion for Physical Reality

What defines an element of reality? They proposed this criterion:

If we can predict the value of something with certainty, without disturbing the system, then it is real.

For example, knowing the wave function means we can predict the momentum of a particle. So momentum has reality. But we can't know position, only probabilities.

The Central Paradox

Here is the key quantum paradox:

If two quantities are related in a certain way called "noncommuting", knowing one precludes knowing the other. Measuring one disturbs the system and ruins knowledge of the other.

So it seems either:

1) Quantum theory does not fully describe reality

OR

2) The two quantities cannot have simultaneous reality.

But they devised a thought experiment that shows: If 1) is false, so is 2). This means quantum theory is incomplete in its description of reality.

Entangled Particles Reveal the Paradox

They imagined two interacting particles. When the interaction ends, the particles stay "entangled".

Measuring one particle affects the probabilities for the other. This lets you predict two contradictory things about the other particle with certainty, violating the idea they can't both be real.

So quantum theory must be incomplete, because its wave function can't describe the full reality. There must be more underlying variables it does not capture.

Conclusions on Quantum Theory's Limits

Einstein, Podolsky and Rosen presented a powerful philosophical argument exposing inconsistencies between quantum theory and our intuitive notions of physical reality.

Their paradox forced theorists to dig deeper into foundational questions on the meaning of measurement, entanglement, and the nature of reality under quantum mechanics.

Key References

- Bohr's response arguing complementarity resolves the paradox - Schrödinger's introduction of entanglement - Bell's theorem ruling out local hidden variables - Experiments confirming quantum entanglement

This remains an active area of fundamental research in physics.