Schroedinger's Cat

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In NetHack, Schroedinger's Cat is the name of a housecat that is generated in the possession of a quantum mechanic.


Main articles: Quantum mechanic and Housecat

Quantum mechanics have a 5% chance of being generated with a special large box.[1] This box will contain a housecat that is either alive or dead; the state of the cat is not fixed until it is observed by opening the box. When the box is opened, there is a 50% chance the game will attempt to create a peaceful live housecat named "Schroedinger's Cat" as close as possible to the player. If the 50% chance fails (or housecats are genocided or the level has no square free for a monster), the box will contain a named housecat corpse instead.[2]

Besides its name and unusual origin, Schroedinger's Cat is just a normal housecat; you may tame it with food, kill it for experience, or even change its name with the call command. If Schroedinger's Cat is generated as dead, you can revive it, though it will be hostile.

The state of the cat is not fixed until it is observed in any fashion, either by opening the box or by tipping it. Before the box containing Schroedinger's Cat is opened, it is technically empty, but its weight is increased to account for the weight of the cat.[3] In particular, neither telepathy nor monster detection will show a live cat in the box, nor will food detection show its corpse; however, a wand of probing will say that the box seems empty. Ascension with an unopened box containing Schroedinger's Cat will reveal whether or not the cat was alive or dead.


In NetHack 3.4.3 and earlier versions, including some variants based on these versions, zapping the box containing Schroedinger's Cat with the wand of probing will simply show the box as empty. This is bug C343-263, and was fixed in February 2007 via commit 7abf238.


"Schrödinger's cat" refers to a famous thought experiment in quantum mechanics involving imagining locking a cat in a box with a mechanism that has a 50% chance of killing the cat, depending upon the final state of a quantum system - for example, whether an unstable nucleus has decayed within a certain time.

The orthodox Copenhagen interpretation of quantum mechanics asserts that the cat in the box is in a superposition of possible outcomes, in half of which the cat is dead, and half of which it is alive. Only when the box is opened and "observed" does the quantum wavefunction collapse, and the fate of the cat become determined. Schrödinger constructed the thought experiment in order to assert that this, and by extension the Copenhagen interpretation, was absurd; as of this writing, physicists are still divided on this matter.


The housecat inside the box is still alive!
Schroedinger's Cat is alive - a housecat is generated in the square nearest to the large box.
You think something brushed your foot.
As above, but you cannot see the housecat.
The housecat inside the box is dead!
Schroedinger's Cat is dead - a housecat corpse is in the large box.
The large box seems empty.
You probed the large box.

Encyclopedia entry

Imagine a sealed container, so perfectly constructed that no physical influence can pass either inwards or outwards across its walls. Imagine that inside the container is a cat, and also a device that can be triggered by some quantum event. If that event takes place, then the device smashes a phial containing cyanide and the cat is killed. If the event does not take place, the cat lives on. In Schroedinger's original version, the quantum event was the decay of a radioactive atom. ... To the outside observer, the cat is indeed in a linear combination of being alive and dead, and only when the container is finally opened would the cat's state vector collapse into one or the other. On the other hand, to a (suitably protected) observer inside the container, the cat's state-vector would have collapsed much earlier, and the outside observer's linear combination has no relevance.

[ The Emperor's New Mind, by Roger Penrose ]