# What is quantum mechanics?

1. Jun 13, 2012

### Patridor

i'm a high school student and a amateur astronomer as well i have also been intrested in physics and i am wanting to become a theoretical physicist and i am trying to write equations so basically i'm wanting to know what is quantum mechanics .

patridor

2. Jun 13, 2012

### Mark M

3. Jun 13, 2012

### edpell

This may not be very educational but I will answer the question as I see it. Quantum Mechanics is mechanics that takes into account the phase of the particles involved. I would add Feynman's observation that a given particle can take all paths and the net result is the sum taking into account the phases.

The other thing is non locality. If we have two entangled photon and one goes one light year to the left and the other one light year to the right. If we then measure the spin of the left along the x axis it seems that instantly the joint wave function of the two collapses and the photon two light years away "knows" what axis the first photon got measured on. (Bell's Inequality and Aspect's Delayed Choice Experiment). In my mind this is the hottest topic in current day physics. But few work on it because no one knows how to.

To simplify the two slit experiment basically covers all the features of QM. If you can understand that you are in great shape.

4. Jun 13, 2012

### fissile_mop

Quantum mechanics is a mathematical framework for understanding physical phenomena that fail to be described sufficiently by classical physics: the work of Newton, Maxwell, and others. You might ask, 'Why can't we use classical methods at that point?' We're unable to sufficiently probe atoms and subatomic particles with classical instruments at that scale. The simplest instruments that we can use at that scale, like probing with an electron or photon, are either too energetic or don't have the right properties to measure what we want to measure. This doesn't just mean that WE are unable to measure it, but that, and this is what makes QM weird, physics doesn't allow us to learn any more about it. So instead of being able to watch particles zooming and blipping and popping about like we are able to watch a ball on a playground, we have to use statistical methods to determine where some particles might be and where it might be going. This is the very broad description of how QM works.

Wikipedia has a good historical intro which will provide you with who discovered what, when, and where, and how QM came to be essential for furthering our knowledge about the small-scale stuff. Unfortunately I lost my old account and can't post a link for you until I reach 10 posts again. But google 'intro to quantum mechanics' and it should take you straight there.

Last edited: Jun 13, 2012
5. Jun 14, 2012

### Staff: Mentor

QM is based on the principle of superposition - it is its fundamental axiom and its essential mystery - it's what Feynman meant when he said the double slit experiment contains the essential mystery - it is this weird state of affairs when a particle goes through both holes that is its essence. Feynman explained it beautifully in his QED lectures:
http://vega.org.uk/video/subseries/8

But IMHO no one explained it better than Dirac in the first chapter of his famous book:
http://www.scribd.com/doc/30407433/Dirac-Principles-of-Quantum-Mechanics [Broken].

The probability stuff follows from Gleasons Theorem:
http://en.wikipedia.org/wiki/Gleason's_theorem

Schroedingers Equation etc follows from Galilean Invariance the details of which can be found in Chapter 3 of Ballentine - Quantum Mechanics.

All the other stuff is basically gloss and dross - very important gloss and dross - but not the essential mystery - that is and always will be the Superposition Principle. Why the Superposition Principle - check out: