Quantum mechanics is over 119 years old and we have indeed come a long way. The theory has stood all the tests through time and it is, along with general relativity, on of the two pillars of physics. Yet, there are a few unsolved problems in this great theory that must be mentioned. We will discuss 5 most important ones in this last article.

### 5. Interpretation of Quantum Mechanics

In quantum mechanics, the fundamental parameter of a quantum system is its wavefunction. Unlike classical mechanics, a particle's position cannot be described exactly. You can tell where the particle probably is. To define a position of a particle without measurement, we use probability because it can be anywhere. However, when you measure a quantum system, the wavefunction or the probability spread collapses. What constitutes a "measurement" which apparently causes the wave function to collapse into a definite state?

Also Read: The Wave Function and the Schrodinger Wave Equation (Day 3)

### 4. The Physical Information Loss

Do black holes exist? Is the phenomenon of wavefunction collapse true? This is the puzzle in quantum mechanics. What happens to the quantum information that goes inside a black hole (if it exists). What information is lost when we measure a quantum system and collapse a wavefunction into a particular eigenstate.

### 3. Pauli's Exclusion Principle

What is the physical reason behind the existence of the exclusion principle? What can't two electrons (fermions to be specific) be in the same quantum state? We do know the mathematical reason behind this but the physical reason is still unknown, even after about 100 years. A rigorous mathematical reason given by physicists is: "The wavefunction of a fermion is anti-symmetric under the permutation operator". The real intuitive reason is still unknown. (Source: NASA Website)

Also Read: Pauli's Exclusion Principle and its applications in astronomy.

### 2. Magnetic Monopoles

This problem involves particle physics and quantum mechanics both. Magnetic monopoles are the magnetic charges that would give rise to magnetic field just like an electric charge gives rise to electric field. We know that when we cut a magnet into two pieces, we don't get a separate north and south pole. Instead, we get two smaller magnets with their own pairs of north and south poles. Did magnetic monopoles exist in the past when the energy of the universe was very high? If yes, where are they now? In fact, Paul Dirac showed the existence of such types of magnetic monopoles.

Also Read: 5 Hypothetical Particles, The Discoveries of Which Would Change The Course of Science.

### 1. Quantum Gravity

Can quantum mechanics and general relativity be combined into a fully consistent theory? Is the spacetime continuous or discrete? Would a new consistent theory involve graviton as a force carrier of gravity? These are a few questions that a theory on quantum gravity must answer.