Chapter 7 – QM and the real world

The real world

Quantum mechanics says things don’t have a position until observed so why do we see things in expected places in the real world? The answer is that wavelength is proportional to 1/mass

The uncertainty in an electron is small which makes the uncertainty in things we interact with insignificantly small

The size of things in the real world are so large that any uncertainty is undetectable

The measurement problem

The measurement problem asks how and why does the wave function collapse when measured? The Copenhagen Interpretation doesn’t answer this – it just says it happens when observed!

There are other interpretations that try to address this but none are broadly accepted so the measurement problem is considered unsolved. The other interpretations have implications for the role of QM in the real world.

Spontaneous collapse interpretation

In this interpretation, wave functions spontaneously collapse and the collapse is not due to observations.

For single particles, collapse is extremely rare, so standard quantum behavior is seen. The probability of collapse increases as more particles are involved in an entangled state.

Instead of a measurement event causing collapse, it is the interaction of a complex system that causes rapid collapse. Any measuring device has a massive number of particles that all become entangled with the state being observed – that causes the wave function to almost instantly collapse.

Schrödinger’s cat is complex so it won’t be both alive and dead!

Many worlds interpretation

The Many worlds interpretation does not have wave functions collapse at all. Instead, the quantum states keep branching when different outcomes are possible. The result is an extremely large wave function for the universe that encompasses all possible outcomes.

The measurement problem is avoided because there is no wave function collapse – all outcomes exist in parallel

The universal wave function has all possible quantum states of the entire cosmos. It would be an unimaginably complex, shifting, and entangled landscape of probabilities, where every possible reality exists at once. Observers only see a single result because they are only experiencing one branch of a much larger, encompassing superposition

Schrödinger’s cat is alive and dead in different branches but we only see one branch