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1.
A theory that developed from Max Planck's quantum principle and Werner Heisenberg's uncertainty principle.

The quantum principle states that waves (light, infrared, X rays, etc.) are emitted from any body in "packets" known as quanta, rather than arbitrarily. Each quantum would have more energy depending on the frequency of the waves.

The uncertainty principle states that the more accurately you know the position of a particle, the less accurately you can know its velocity, and vice versa.

Heisenberg, Erwin Schrodinger, and Paul Dirac created a theory called quantum mechanics, which was based on the uncertainty principle. Particles were now thought to have a quantum state, a combination of position and velocity.

In a nutshell, it brought randomness into physics, which many physicists (Einstein, for example) objected. Still, many others accepted it.
Even after reading all that, does anyone actually understand what quantum mechanics is?

Quantum mechanics is complicated.
by SRG September 23, 2004
 
2.
A branch of physics in which the measurable qualities (position, momentum, energy) of a system are described by "observables." The system, upon measurement by the given observable, after being in a superposition of multiple states, collapses into one of a variety of states allowable by the Schrodinger equation. The exact state the system collapses into, is probablistically determined.
X|s1>=x1|s1>

In this example, |s1> is the system being "measured" by the observable X (X denoting the position observable, allowing us to see where the system is within space.) We obtained |s1> as a single state, from a previously superposed (|s> = |s1>+|s2>+|s3>+...)state through an earlier measurement, which collapsed |s> into |s1>, and allowed us to measure the value x1 for its position. In quantum mechanics, all of this information is encoded in the language of vector spaces, as well as linear algebra, and expanded into whats known as a hilbert space (can be thought of as an infinite dimensional vector space).
I'm probably going to get all thumbs down for this, but that's cool, you guys are all the same audience that jerks it to youtube, and probably added a new definition for "poop" or something stupid like that on this site.

by JP2269 October 19, 2006
 
3.
A topic in physics that will fuck with your mind over and over again.
After trying to understand quantum mechanics for 10 hours, I decided to write a bullshit definition for it.
by ADD.i.kt February 04, 2010
 
4.
A concept in physics concerning the counter-intuitive motion and mechanics of subatomic particles. Despite not applying to anything larger than an electron, quantum mechanics is often lamely invoked by pseudoscientists to justify whatever ridiculous phenomenon they are proposing.
"Psychic abilities are made possible by quantum mechanics"
by Zamzara October 09, 2007
 
5.
Ones who attempt repairs on busted stuff,
often utilizing second-hand or
remanufactured parts.
I bought a new Dodge Quantum.
When something went wrong,
I took it to the Quantum Mechanics
at the dealership, and they patched-up
the busted thingamabobber.
by alteregoboy December 18, 2006
 
6.
"I bought a new dodge Quantum.
when something went wrong,
I took it to the Quantum Mechanics
at the dealership,
and they patched-up
the busted thingamabobber."
by alteregoboy January 07, 2007
 
7.
why did i choose a degree in physics???
by SR May 06, 2005