Quantum FAQ

(1) What is the meaning of psi and psi*?
psi and psi* by themselves have no meaning. Only psi*psi has meaning, which is the probability.

(2) What is the difference between psi and ket (or psi* and bra)?
ket describe a state of a particle. psi is the wavefunction of that state. For example a ket |s> describes a particular state. is the wavefunction of that ket in terms of position.

(3) What do we mean when we sandwich something between a bra and a ket?
For example, we have . This means the expectation value or average value of A. A can be energy, position, momentum, etc.

(4) What are colour charges?
In electricity, we have positive charges and negative charges (2 quantities to describe the electronic charges).

For quarks, we have red charges, blue charges and green charges (3 quantities to describe colour charge.) It happens that the number of quantities to describe colour charge is 3.

(5) What is the difference between leptons and quarks?
Quarks are affected by all 4 fundamental forces(strong, weak, electromagnetic, gravity).

Leptons are affected by only 3 fundamental forces(weak, electromagnetic, gravity).

(6) How spin and anti-particles comes about when we marry QM with SR?
One equation that unifies QM and SR is the Dirac equation. There are 4 solutions to the Dirac equation (particle spin up, particle spin down, anti-particle spin up, anti-particle spin down).

(7) What are hadrons?
Hadrons are particles held together by strong force.
Hadrons can be baryons(3 quarks), anti-baryons(3 anti-quarks) and mesons(1 quark and 1 anti-quark).

(8) Matters are made up of fermions. Why helium is a boson?
The bounded system containing even number of fermions can be considered as bosons. E.g Helium contain 2 proton and 2 neutrons. Since the number of fermions is even, we can treat helium as boson.

(9) Why the sea of electron is considered the vacuum? i.e.we cannot "see" it.
The negative energies are all filled up by electrons.  Seeing it means that we can see the negative energies. However, negative energies are not  natural occurring. We have to artificially create it. This means that we need to promote the electron occupying in the negative energy state in order to “see” the negative energy.
(10) Why QED is easier than QCD?
 QED are easier than QCD. The reason is that the photons (messengers) come only as 1 type of particle. However, in QCD, the gluons (messengers) come with 3 colours. This makes the calculations more complex.

(11) Where are vertices in the Feymann diagrams? What are they?
The vertices are at the intersection points. The vertices represent specific interactions. Actually, at every vertex, there is an integration to perform.

(12) What is the meaning of "off mass shell"?
 Off mass shell means that virtual particles are involved. During those situations, the energy dispersion relation E^2=p^2c^2+m^2c^4 is not true any more.

(13) How to deduce D^0?
Since D^0 is a meson it contains an anti-quark and a quark. We are told that it has charm number=+1. This means that there is charm quark. So what we need to do is to identify the other anti-quark. D^0, the zero means it has no electronic charge. Since charm quark has electronic charge 2/3, the anti-quark must have electronic charge -2/3. There are only 2 possibilities u-bar or t-bar. By convention, we choose the lightest one that is u-bar. Thus D^0=cu-bar.

(14) Why quarks cannot be separated? (Dumb bell analogy? Quark Confinement )
Lets take an example that we want to separate a X quark from a Y quark. We are forcing the X quark to the left and the Y quark to the right.

Note that we need to input a lot of energies to separate them. However, when the energies are high enough, the energies input are converted to particles!

This means that on the left, the X quark is paired up by another Y quark(created by the energy we input). The Y quark on the right is paired up by another X quark(created by the energy we input).

(15) How many “particles” are there?

Case 1: Exclude electronic and colour charges; exclude anti-particles; exclude force particles àwe have 6 leptons and 6 quarks = 12 particles.

Case 2:Exclude anti-particles; exclude force particles -- we have 3 colours for each quarks. That is 18 quarks and 6 leptons = 24 particles.

Case 3: Exclude colour charges; Exclude force particles -- The particles in case 1 plus their anti-particles = 24 particles.

Case 4: Exclude force particles -- The particles in case 2 above plus their anti-particles = 48 particles.

(16) How many force particles are there?
If we ignore all charges(electronic and colour), there are just 4. If we want to include the charges there are 13 = 1 graviton + 1 photon + 8 gluons(rgb)+(W^+)+(W^-)+Z.
Gluons have 8 independent colours states(out of the scope this module).


(17) Are bosons forces particles?
All force particles are bosons but not all bosons are force particles. Eg, helium-4 is a boson but not a force particle.

(18) What is the purpose of R3 in lasing?
If we promote atoms to the excited state, they will decay to the ground state spontaneously and randomly. We cannot create laser using 2 states systems.

In order for a laser to work, we need an intermediate state R3(meta-stable). The excited atoms will be stored in this state. When the many atoms stored in this state starts to decay to the ground state at the same time, the laser will be created.

The strength of the laser depends on how many atoms the meta-stable state can store.

The inclusion of this meta-stable state allows us to re-derive the planck's law. This is the link of laser with quantum physics.

 (19) Why helium spin is zero?
Helium contains 2 protons and 2 neutrons. Protons and neutrons are fermions, spin=1/2. The total spin of Helium can be 1/2+1/2+1/2+1/2=2 to 1/2-1/2+1/2-1/2=0. i.e spin of helium can be 2, 1, 0. Thus helium being spin zero is only 1 of the 3 possible cases.
Since helium contains even number of fermions, it is a boson.

(20) Does Higgs mechanism applies only to massive bosons(W+,W-,Z)?
No, higgs mechanism gives mass to all fundamental particles.

(21) Is order of quarks in baryons/anti-baryons/mesons important?
No, the order is not important. Eg ddu=dud=udd.

(22) How do we name the supersymmetric particles?
All force particles will have their supersymmetric partner with name ending with "ino". eg Photino, Gluino, Winos
All quarks and leptons will have their supersymmetric partner with name starting with "S". eg Selectron, Smuon, Sup, etc

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