Physics at the atomic scale obeys the rules of quantum mechanics, which tell us that a particle can be in two or more states at the same time.  It can be in a coherent superposition of states.  Quantum mechanics has been very successful in describing physical systems at the microscopic level.  Applying quantum theory to our macroscopic world, however, reveals the weirdness of quantum mechanics and poses conceptual problems.  Schrödinger formulated his famous cat paradox in 1935, just to show how absurd the consequences of quantum mechanics are if applied to macroscopic or even living objects.  Schrödinger proposed a thought experiment with an unfortunate cat lingering in the twilight zone between life and death.

Schrödinger's Cat (in his own words)

• "One can even set up quite ridiculous cases.  A cat is penned up in a steel chamber, along with the following diabolical device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small that perhaps in the course of one hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid.  If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed.  The first atomic decay would have poisoned it.  The Psi function for the entire system would express this by having in it the living and the dead cat (pardon the expression) mixed or smeared out in equal parts.  It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation.  That prevents us from so naively accepting as valid a "blurred model" for representing reality.  In itself it would not embody anything unclear or contradictory.  There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks." -- Erwin Schrödinger
  Translation by John D. Trimmer

This thought experiment raised questions about whether quantum mechanics breaks down when the system is complex enough.  We know that superpositions of states exist at the microscopic level, because they produce observable interference effects.  We also know that the cat in the box is either dead, alive, or dying and not in a smeared out state containing all the alternatives.  When and how does the fog bank of microscopic possibilities transform itself to the blurred picture we have of a definite macroscopic state?  Schrödinger's cat is a simple and elegant example of the quantum measurement paradox.

The paradox of Schrödinger's cat has provoked a great deal of debate among theoretical physicists and philosophers.  Although some have argued that the cat actually does exist in a superposition states, most contend that superposition only occurs when a quantum system is sufficiently isolated from the rest of its environment, i.e. if it only minimally interacts with the rest of the world (the observer).  The question is, at what scale do the probabilistic rules of the quantum realm give way to the deterministic laws that govern the macroscopic world?  Every real system, whether microscopic or macroscopic, is in contact with an external environment.  This coupling between a quantum system in a superposition and the environment in which it is embedded leads the system to 'collapse' or decay over time into one state or another.  This process is known as decoherence.

The modern view of quantum mechanics states that Schrödinger's cat, or any macroscopic object, does not exist in a superposition of states due to decoherence.  A pristine quantum state, undisturbed by observations, can be coherent.  But Schrödinger's cat is not a pristine quantum state, its is constantly interacting with other objects, such as air molecules in the box, or the box itself.  Thus a macroscopic object becomes decoherent by many atomic interactions with its surrounding environment.  Decoherence explains why we do not routinely see quantum superpositions in the world around us. It is not because quantum mechanics intrinsically stops working for objects larger than some magic size.  Instead, macroscopic objects such as cats are almost impossible to keep isolated to the extent needed to prevent decoherence. Microscopic objects, in contrast, are more easily isolated from their surroundings so that they retain their quantum secrets and quantum behavior.

The rate of decoherence depends on the size of the quantum system. Physicists can now create and maintain quantum particles such as atoms or single photons of light in superpositions for significant periods of time, if the coupling to the environment is weak. For a system as big as a cat, however, comprised of billions upon billions of atoms, decoherence happens almost instantaneously, so that the cat can never be both alive and dead for any measurable instant.  It is rather like a juggler trying to keep billions of balls in the air at the same time.