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"Quantum theory, as first introduced by Plank in 1900, is based on the concept of using the quantum unit to describe the dynamic properties of subatomic particles and interactions of matter and radiation. It is a departure from the classical Newtonian mechanics in which certain physical quantities (velocity, acceleration, position) can only have individual discrete values assigned to them. Quantum mechanics gives a statistical description of the behaviour of physical systems.

Heisenberg's Uncertainty Principle discusses the calculation of the uncertainty in the measurements of momentum and position of a particle. A consequence of the principle is that any measurement of a system must disturb the system under investigation, with a resulting lack of precision in the measurement. Erwin Schrodinger proposed an illustration of the problem of viewing these particles in a "thought experiment":

"Suppose we put a cat in a cage with a radioactive atom, a Geiger counter, a hammer, and a poison bottle; further suppose that the atom in the cage has a half-life of one hour, a fifty-fifty chance of decaying within the hour. If the atom decays, the Geiger counter will tick; the triggering of the counter will activate the hammer, which will break the poison bottle, which will kill the cat. If the atom doesn't decay, none of the above things happen, and the cat will be alive. Now the question, What is the state of the cat after the hour?"

Schrodinger's thought experiment asks whether the cat is dead or alive after an hour. The most logical solution would be to wait an hour, open the box, and see if the cat is still alive. However once you open the box to determine the state of the cat you have viewed and hence disturbed the system and introduced a level of uncertainty into the results. The answer, in quantum mechanical terms is that, before you open the box the cat is in a state of being half-dead and half-alive.

This paradox arises because the atom, being microscopic, must be described by quantum mechanics. Therefore after one hour inside the box, before it has been observed, the atom is in a state of superposition of being both undecayed and decayed. Therefore, as the state of the cat in the box is directly related to the state of the atom, the cat must also be in a state of superposition being both dead and alive. This is known as a coherent superposition, many different solutions exist simultaneously. However the superposition breaks down as soon as the cat is observed as dead or alive."

What is the story on the cat and why is it still dead? Sounds like a crazy story...amuse me

From Cecil Adam's "The Straight Dope" newspaper column, when asked a rhyming question about Dr. S's cat:

Schroedinger, Erwin! Professor of physics!
Wrote daring equations! Confounded his critics!
(Not bad, eh? Don't worry. This part of the verse
Starts off pretty good, but it gets a lot worse.)
Win saw that the theory that Newton'd invented
By Einstein's discov'ries had been badly dented.
What now? wailed his colleagues. Said Erwin, "Don't panic,
No grease monkey I, but a quantum mechanic.
Consider electrons. Now, these teeny articles
Are sometimes like waves, and then sometimes like particles.
If that's not confusing, the nuclear dance
Of electrons and suchlike is governed by chance!
No sweat, though--my theory permits us to judge
Where some of 'em is and the rest of 'em was."
Not everyone bought this. It threatened to wreck
The comforting linkage of cause and effect.
E'en Einstein had doubts, and so Schroedinger tried
To tell him what quantum mechanics implied.
Said Win to Al, "Brother, suppose we've a cat,
And inside a tube we have put that cat at--
Along with a solitaire deck and some Fritos,
A bottle of Night Train, a couple mosquitoes
(Or something else rhyming) and, oh, if you got 'em,
One vial prussic acid, one decaying ottom
Or atom--whatever--but when it emits,
A trigger device blasts the vial into bits
Which snuffs our poor kitty. The odds of this crime
Are 50 to 50 per hour each time.
The cylinder's sealed. The hour's passed away. Is
Our pussy still purring--or pushing up daisies?
Now, you'd say the cat either lives or it don't
But quantum mechanics is stubborn and won't.
Statistically speaking, the cat (goes the joke),
Is half a cat breathing and half a cat croaked.
To some this may seem a ridiculous split,
But quantum mechanics must answer, "Tough @#&!
We may not know much, but one thing's fo' sho':
There's things in the cosmos that we cannot know.
Shine light on electrons--you'll cause them to swerve.
The act of observing disturbs the observed--
Which ruins your test. But then if there's no testing
To see if a particle's moving or resting
Why try to conjecture? Pure useless endeavor!
We know probability--certainty, never.'
The effect of this notion? I very much fear
'Twill make doubtful all things that were formerly clear.
Till soon the cat doctors will say in reports,
"We've just flipped a coin and we've learned he's a corpse."'
So saith Herr Erwin. Quoth Albert, "You're nuts.
God doesn't play dice with the universe, putz.
I'll prove it!" he said, and the Lord knows he tried--
In vain--until fin'ly he more or less died.
Win spoke at the funeral: "Listen, dear friends,
Sweet Al was my buddy. I must make amends.
Though he doubted my theory, I'll say of this saint:
Ten-to-one he's in heaven--but five bucks says he ain't."

--CECIL ADAMS

There's a lot more to it than implied by the poem, though. It's not just a matter of, "if we can't tell what it's doing when we're not observing it, why speculate" -- countless tests in quantum physics all seem to indicate that quantum events really *are* in a multiple superposition of possible states until we make a test (i.e. "observation") that "forces" the events to "choose" a specific result.

That's not such a big deal at the level of electrons or whatever, but as Schroedinger's proposal about an experiment using a cat demonstrates, we can set things up such that the subatomic weirdness can be caused to have effects in the human-level scale (i.e., whether a cat is alive or dead -- or both?). To further complicate the "huh?" factor, there's the question about whether the cat itself rises to enough of a level of "observer" to "collapse the wave function" before a human even opens the chamber to take a look, but that adds a further paradox -- there's only an observer if the cat isn't *dead*.