The *measurement problem* in quantum mechanics appears because making a measurement of a quantum system causes the quantum wavefunction to collapse from a superposition of probable states into single state, but the theory itself doesn't explain how that collapse happens. In fact, whether the collapse itself actually does happen is potentially even called into question.

**Measurement Problem Examples**

Consider the double slit experiment that demonstrates wave-particle duality.

In this case, when the beam of light, electrons, or other particles is sent through the slits but no measurement is made about which slit the particles go through, then the result is an interference pattern on the screen, which demonstrates that the light or particle beam is described as behaving in a wave-like way, with the wave passing through both slits (because that's the only model that results in an interference pattern).

However, if a detector is set up so that it measures when a particle (such as a single photon, for light) goes through a slit, this act of measuring causes the entire situation to turn out differently. The interference pattern is completely gone. Instead of the particles going through both slits as a wave, the act of looking at the slits means that each particle has to "pick" which slit it goes through, and the wave behavior vanishes completely. Instead, it's like firing bullets toward two slits in a bulletproof wall ...

you end up with just two clusters of bullets on the other side.

In the first situation, you have a probability wave passing through both slits. In the second situation, with the measurement, you have actual particles passing through individual slits. The measurement problem asks: *What about the act of measuring causes this change?*

Another classic example of this is the Schroedinger's Cat thought experiment, in the decay of a radioactive isotope (an event governed by quantum probability) results in the release of a deadly poison in a sealed-off box, which kills a cat. In other words, a quantum event results in a non-quantum result ... but until the measurement is made by an external human observer, the non-quantum result is said to remain in a state of quantum uncertainty, contrary to all of our conventional logic. Again, we have situation where the act of measuring takes something from a probabilistic quantum system into one in which there is a definitive outcome, and that's the measurement problem at work!

**Resolving the Measurement Problem (or Not)**

When quantum physics was first being developed, a group of physicists in Copenhagen (led by quantum pioneer Neils Bohr) addressed this problem with the Copenhagen interpretation of quantum physics ... and they did so by not really addressing it. Instead, they just accepted that the act of measuring causes the collapse of the quantum wavefunction as a simple empirical fact which didn't require additional explanation. This behavior was accepted as part part of the methodology of doing quantum physics experiments and calculations.

This was not particularly satisfying (and inspired the aforementioned Schroedinger's Cat thought experiment as criticism)

The main resolution to this among modern physicists is to invoke the concept of decoherence, but decoherence does not actually resolve the measurement problem either, it just gives you a mathematical reason for ignoring the measurement problem and moving ahead with the calculations (see: Decoherence and the Measurement Problem).

Both the Copenhagen interpretation and decoherence accept that the wave function collapse actually takes place, but an ingenious alternative was proposed by the physicist Hugh Everett: that the wave function never actually collapses, but that all of reality exists as a superposition of possible states, continually unfolding in all possible permutations.

This is the heart of the Many Worlds Interpretation of quantum physics.

Regardless of the approach invoked, nearly a century after it first came up in the development of quantum physics, there is still no clear and satisfactory resolution to the measurement problem. The need to explain this foundational problem of quantum physics remains one of the greatest difficulties within science, and made Lee Smolin's list of the five great problems in theoretical physics.