# Can you count “collapses”? How many collapses in the observable universe?

If that’s too hard, how many collapses in 100 cc’s of boiling water in one second?

In biology, the very first robin that is scientifically described is preserved and called the “type robin”. The “type robin” for collapse was described by Einstein in 1905, and won him the Nobel prize. It is called the photoelectric effect. (Later collapse was formalized by von Neumann as a mathematical projection.)
A very similar effect is the building up of an Airy circle in a telescope. Another is the point by point emergence of an interference pattern in a two-slit experiment, perhaps very slowly. In these examples, “collapses” can be counted. When one photon is absorbed and detected, one collapse happens. When a second photon is absorbed, a second collapse happens.
The collapse is caused by the photon hitting the detector. (Or the collapse is caused by the silicon atom absorbing the photon.)

Collapses like these are countable and it makes sense to ask and answer how many there are in a given 4-volume of spacetime.
So, how many?

-
Read "collapse" as "decoherence event" and this is a meaningful but difficult question - difficult because decoherence is a matter of degree, so one would need to talk about thresholds and other technical details. – Mitchell Porter Aug 28 '12 at 1:15
Are you assuming an objective collapse? – user11714 Aug 28 '12 at 8:56

## 1 Answer

No, as what counts as a collapse depends on how you separate your system from the environment.

Note that detecting photons is not a collapse of the photon wave function in von Neumann's sense, as the photon is afterwards not in a position eigenstate, but completely disappeared.

However, for certain simple systems, collapses (quantum jumps into eigenstates) can be experimentally observed, however, and then counted. See the references in the section ''Are there quantum jumps?'' of Chapter A1: Fundamental concepts in quantum mechanics of my theoretical physics FAQ at http://www.mat.univie.ac.at/~neum/physfaq/physics-faq.html . See also the section ''Observable collapse'' of Chapter A4: The interpretation of quantum mechanics in this FAQ.

-