Quantum Mechanics

great unwashed wrote: »

Can any of you give me the best description of charge you've ever heard without mentioning quarks?

Charge is not intrinsically related to quarks. Quarks just happen to have a charge. Some leptons also have a charge, for example the electron.

Charge is simply a way of quantifying how particles interact with photons, of quantifying the strength of the electric field surrounding a particle.

great unwashed wrote: »

Is there an explanation for the exclusion priniciple - why these numbers of electrons and not others?

Yes, and it's actually very simple (although somewhat mathematical).

We call particles which have an anti-symmetric wave function Fermions. Particles with symmetric wave functions are called Bosons. Bare in mind that both these particles are identicle.

So, for fermions, a two particle state is
1/sqrt(2) {|psi_1> |psi_2> - |psi_2> |psi_1>}
while for a boson it is
1/sqrt(2) {|psi_1> |psi_2> + |psi_2> |psi_1>}
where |psi_1> and |psi_2> are the two states, one of which is occupied by each particle, and the position indicates which particle is in that state.

So, if the two particles are in the same state, then |psi_1>=|psi_2> and so the Fermions are in the state 0. This is not a valid state, since that probability of measuring the particle to be in any state is 0 (probabilities always add up to 1), and so it is impossible for this to ever happen. Hence the Pauli exclusion principle.

great unwashed wrote: »

When did physicists start talking about particle spin and what use does it have?

Particle spin is just the quantum equivalent of angular momentum of the free particle. Each particle is like a spinning top, and the rate at which is turns is it's spin. This is what gives rise to the magnetic field around charged particles with non-zero spin.

It was first proposed in 1925, and so has been around almost as long as quantum mechanics.

bogwalrus wrote: »

When you say in Quantum Mechanics "the observer is the observed" what does this mean?

I'm afraid I don't know what Bohm was refering to. He has a rather unconventional approach to measurement in quantum mechanics, so I'm not sure if he was simply elluding to a specific interpretation, or was trying to capture something about quantum mechanics in general. It's a nice sound bite, but not very informative.

Kareir wrote: »

I've read various books, but i was wondering if someone could explain the basic points of Quantum Mechanics to me.

Not really, it is a rather broad subject. If you have specific questions I would be happy to answer, but I can't really see anyone teaching you how to do quantum mechanics in full via the forum.

waitinforatrain wrote: »

1. The "observer". From what I understand, the photon begins to behave like a particle when it is being measured. Therefore the "observer" changes the thing observed. But does this observer need to be a conscious entity? Does the photon counter count as an observer? Does the photon counter have a discrete state and value before I (the experimenter) observes it?

No, it's just an effect of open systems (systems which interact with a broader system). If a system is closed, the evolution of the wave function is unitary, but when it is allowed to interact with other systems it causes it to appear as if the wave function collapses.

So the observer is simply another system. It doesn't need to be an animal or person, simply a few electrons will do.

waitinforatrain wrote: »

2. How difficult would it be to replicate the double slit experiment for the average joe? This is something I've found surprisingly difficult to find out.

Well, you can do the Young's slit experiment fairly easily, but doing so with single photons is much harder.

waitinforatrain wrote: »

3. How can we be sure that only one photon is firing at a time?

Usually an attenuated laser is used, but single photons are detected. Another way to do it is to use a single photon source. There are plenty of candidates which work. As it happens, though, we can do far better than simply using photons. The double slit experiment has been done with neutrons, atoms and even molecules.

waitinforatrain wrote: »

Ah thanks, that really clarifies it for me.

A few more quick questions for you if you get the time: What do you mean by 'unitary' (I googled and all I got was "Having a magnitude of one" which doesn't help me to understand it). Are you just saying that the wave function behaves as a wave, but when another system depends on it, it converges to a discrete value.

Is this 'wave' function vibrating in physical space, or is it a representation of the probability of the particle being at a certain point when observed?

You say "causes it to appear", does it not actually collapse or is it just relative to the system that is observing/interacting with it? Could two different systems measure it as collapsing to a different point?

It's a little more tricky. Basically quantum systems in pure states are systems in a state such that we can write the state as a superposition of classical states. By a superposition I mean that each classical state psi_i is waited by some complex amplitude A_i, such that |A_i|^2 is the probability of measuring the system in that state, so the 'wave function' which is just a list of these amplitudes has nothing to do with physical vibrations in general. Unitary operations are operations which preserve this structure.

When a quantum system interacts with the environment it undergoes a non-unitary operation. The effect of this is to bring the system into a mixed state. A mixed state is simply a classical probability distribution of quantum states. When the interaction is strong enough, the system will loose the complex phase of A_i (called dephasing), essentially making the system classical.

waitinforatrain wrote: »

Do you have any book recommendations for this stuff (hopefully that avoids complex maths)?

The Feynman Lectures, perhaps.

Azelfafage wrote: »

Put an Irishman in a Box-room with a bottle of whiskey.

Is he drunk or sober before you open the door?

That shows you the sheer absurdity of Schrodinger's "experiment".

Look, Azelfafage, at this stage you're comments are degenerating into complete nonsense (and frankly offensive nonsense). If you want to go start denying the existence of quantum mechanics, fine, but if so you may want to stop using pretty much every modern piece of technology, including your computer.

Azelfafage wrote: »

Schrodinger was merely making an analogy about Quantum Physics.

He was astounded that anybody believed about the cat.

A cat has too many atoms to be relevent to Quantum Physics.

That's not correct. The number of atoms has nothing to do with whether quantum effects will be visible or not. Decohrence (and hence the transition to classical physics) is entirely determined by interaction with an environment.

onedoubleo wrote: »

So I have two questions, is there anywhere else in nature we are seeing quantum effects like tunneling play a major role? And other than verification of theory is there any other reason to get large scale object to show wave phenomenon?

Q1) Photosynthesis, your sense of smell and bird navigation are all believed to exploit quantum effects.

Q2) To build a quantum computer.

onedoubleo wrote: »

Sense of smell I can kinda get but how Bird Navigation?

Apparently they use an entangled state of two spins to sense magnetic fields.

If you can reconstitute a photon or atom or molecule with quantum effects you will eventually be able to reconstitute a cat. It is logical,just scaled up. The division of quantum world with physical world is probably just in our heads.