This video explains Quantum Mechanics intuitively. Classical mechanics failed to describe how an electron could orbit an atom. An accelerating charge always creates electromagnetic radiation.
This means it would constantly lose energy and crash into the nucleus.
Only quantum mechanics could explain why this does not happen by showing that electrons exist in quantized orbits proportional to Planck's constant.
Louis de Broglie showed that they must be waves. And Erwin Schrodinger developed an equation to explain this wavelike behavior.
Max Born came up with the idea that the wave function in the Schrodinger equation should be interpreted as a probability.
So quantum objects have only a probability of being found at any particular location in space, which can only be determined once we measure it, not in advance.
Quantum objects are not like little basketballs. They are like waves because they create interference patterns like we see in the double slit experiment. The problem is that we only observe particles, not waves.
So the concept of measurement was introduced to account for what we observe.
The most common interpretation of quantum mechanics is that whenever a measurement is made, the wave collapses and becomes a localized wave, or particle.
What is a measurement?
Measurement is an interaction, and interaction of the quantum object with some kind of measuring device, more specifically an irreversible exchange of energy.
But there is a huge problem. No one can explain how or why this “wave collapse” occurs through measurement.
This is called the “measurement problem” in quantum mechanics. And since all our information comes from a measurement of some kind, we can never directly see this quantum world.
Everything we observe must go through this measuring process that seems to result in the conversion of quantum objects into particles.
So how this wave evolves according the Schrodinger equation, is never actually seen. This is a fundamental problem that we need to resolve. In Quantum mechanics, objects have wave-like behavior described by wave functions, which are abstract mathematical solutions to the Schrodinger equation.
These waves aren't localized but instead take up all of space. It isn't until you look for a particle that it becomes what appears to be a particle; before that, the particle is a collection of probability waves that theoretically extend out to the entire universe.
This has profound consequences. One is called the Uncertainty Principle, which states that you can never simultaneously know exactly where something is, and how fast it is going. More precisely we cannot know the position and momentum at the same time. This is not a limitation of our measuring devices. The universe itself doesn’t know the answer.
Why don’t we see this wave behavior in macro objects like a basketball? Well, actually all objects actually do have wave-like behavior! But their wavelength is so small, that you don't notice it. For example, the wavelength of a tennis ball moving 10 meters/second, is 10^-33 m. This is less than the width of a proton.
A second consequence of wave-like behavior is nonlocality. A wave exists over multiple regions of space. This nonlocality explains interference, but it also means that waves can add together to give complex interference patterns. This gives rise to strange correlations between such particles, called “Entanglement.”
Einstein called this, “Spooky action at a distance” because it appears to indicate instant communication between distant objects at faster than the speed of light, which is forbidden by Relativity theory. But while two or more objects are correlated, no communication is actually happening.
The wave behavior of electrons also means that the concept of circular orbits of electrons around the nucleus of atoms, that you commonly see everywhere, is wrong. A better picture is that they exist in a well-defined probability cloud around the nucleus.
The main point is that the Universe is quantized. Familiar quantities such as energy, momentum, electric charge, mass – possibly even time and space – are not continuous.
They occur in discrete quantum units.