Particles Questions


  1. Concept: The big bang is thought to have created equal amounts of matter and antimatter, but CP violation created an imbalance.
  2. Concept: Messenger particles should be bosons (integer spin) so that it is possible to transmit as many as necessary without crowding issues in dense fields that fermions would experience.
  3. Definition: A spin 1/2 particle is elementary iff it satisfies the dirac equation.
  4. Concept: Weak interactions can change the elementary constituents, whereas the other three forces cannot. The strong and electromagnetic forces only cause rearrangements of bound states or creation/annihilation.
  5. Concept: All photons are off mass shell to some extent. The extent is determined when it is created and does not change. So a photon can be created that disappears after travelling one lightyear.
  6. Definition: Spectator quarks are quarks in a process that do not change during the process and are just carried along.
  7. Concept: The tau particle was discovered by observing the increase in the e+ e- -> e mu Z cross section due to the opening of a new channel going through tau+ tau-.
  8. Concept: The spin angular momentum of a bound object is equal to the sum of the orbital and spin angular momenta of the constituents.
  9. Fact: The charge conjugation eigenvalue for the photon is -1.
  10. Fact: Electric multipole transitions change parity because photon parity is -1. Magnetic multipole transitions do not change parity because photon parity is +1. So the parity of a photon depends on how it was created.
  11. Concept: If you accelerate color, you emit gluons, just as if you accelerate charge you emit photons.
  12. Fact: The mass of the gluon is zero.
  13. Concept: The coupling constant is really the product of two constants, one for the amount of momentum transfer per force carrier, and one for the probability (cross section) for interaction. Note that the functional form for momentum transfer will still be 1/q^2 regardless of the coupling constant, but the probability distribution can still be scaled.
  14. Fact: C and P operators commute
  15. Fact: If you apply a symmetry operator simultaneously to every particle in the universe then you would not notice a difference.
  16. Concept: I don't think there is any need for defining the concept of mass. It seems that everything I know about supports using energy in place of mass. Mass is basically an abstraction layer for looking at energy. You can ignore the finer details of the distribution of energy by wrapping up a bunch of it and calling it the mass.


  1. How do photons establish a static electric field?
  2. What determines the acceleration in a particle interaction? For example, how does a photon know whether to push or pull on the charge that it hits, does it have some internal information about the charge that created it?
  3. Why is the frequency of a photon proportional to its energy? I think mathematically it has to do with the fact that the electromagnetic wave is like a simple harmonic oscillator.
  4. Why is the transverse spatial extent of a photon proportional to its wavelength, which is a longitudinal quantity? Short answer: the uncertainty principle
  5. What causes vacuum fluctuations and what determines their rate?
  6. Why is the graviton spin 2? One explanation from Weinberg is that gravitational radiation must have helicity 2.
  7. Why does having spin 2 make the graviton couple to velocity squared?
  8. Why do infinite range forces need to have massless mediators? I think it is because all interactions are mediated by virtual particles, which disappear based on the time-energy uncertainty principle. This agrees with the explanation in Aithison & Hey section 2.2, but the question still remains: why are they using E=mc^2 for the energy rather than the full relativistic energy expression?
  9. How does the Higg's Mechanism work?
  10. How could you experimentally tell if the vacuum had a uniformly distributed electric charge?
  11. If the speed of light suddenly changed, would you be able to tell? How?
  12. How are photons generated and what determines the direction of propagation? More specifically, when a field of virtual photons causes a charge to accelerate, what fundamental process causes the radiation reaction?
  13. What is the connection between polarization and spin? Conceptually electron spin is the angular momentum of the electron magnetic field due to the circular polarization of the plane wave.
  14. What is the mechanism behing E=mc^2? How does gravity/inertia know the binding energy of particles?
  15. Why do only odd spin mediators allow repulsive forces?
  16. Why does spin tell you the number of indices in the potential?
  17. Why does the cross section go as E_cm^(-2) ?
  18. Before a neutron beta decays, what form of energy is the 1.3 MeV difference stored in? Nobody knows exactly, but it must be some combination of kinetic energy, EM potential energy, and maybe other forms of potential energy.
  19. Wouldn't there be a violation of conservation of energy if two photons with the same frequency, polarization, and phase overlapped? Yes, that's why it is impossible for this to happen. For example with an interferometer you will find interference fringes. I am still concerned about gravitationally steering two such photons towards each other along intersecting great circles orbiting a black hole. This won't cause perfect overlap, but it could be made arbitrary close.
  20. What determines the energy of bound states of particles, their kinetic energy?
  21. Why don't Cosmic Microwave Background photons just run away? If they were reflecting off Hydrogen, then it probably wouldn't be so uniform.
  22. Is gravity subject to gravitational lensing?
  23. How do you balance the force of gravity on a photon against the velocity of the photon in calculating the Schwartzchild radius of a black hole?
  24. How does the strong force between nucleons come from the color force between quarks?
  25. Why are quantum operators distributive over direct products of particle states? The answer may be in writing down the QFT expresssion for the multi-particle state using raising operators. Then you can insert the identity in the form of A^dagger A between the raising operators, which effectively distributes it to all individual particles.
  26. What stimulates radioactive decay? I recall that it is mediated by the weak force, which is important for neutrino interactions, so is the halflife dependent on the cross section for neutrino scattering?
  27. Are there only three generations of matter? My professor said there are good arguments from the big bang theory that there can be only three generations of neutrinos. I was also thinking that neutrino oscillations seem like good evidence since it solves the solar neutrino problem when they oscillate between three types of neutrinos.
  28. How does CP violation create more matter than antimatter?
  29. Has anyone ever witnessed neutrino annihilation?
  30. Why does isospin have to be a vector quantity, i.e. what does it need to do that can't be done with a scalar quantity?
  31. How can you unify the weak and electromagnetic forces if there is the qualitative difference that the weak force can change constituents?
  32. How do you know when you have achieved a unification? One answer is that you need to find a relationship between the coupling constants, but does this really imply "unification"?
  33. Why do we call it unification when all the strengths are the same, what is so special about strength that makes us identify forces when they have the same strength?
  34. Is it true that interaction probability increases when two particles are travelling at the same velocity? Only because this means that they will spend longer near each other.
  35. The number of colors has to be three because that is the number of generations. Why?
  36. Does the CMB blackbody distribution exhibit doppler broadening?
  37. Is the information content of the universe finite? Classical mechanics has an infinite amount of information in the location of a single particle and quantum mechanics has an infinite amount of information in the wave function of a particle, so the answer seems to be 'no', but this is by no means a proof.
  38. Is space discrete?
  39. Why is space three-dimensional?
  40. Are all forces pairwise? Maybe it's better to think of forces as individual particles interacting with the field generated by all other particles.
  41. Why is dynamics specified by acceleration as opposed to any other time derivative of position? Can this be related to the fact that the second spatial derivative of a smooth curve is the lowest to go to zero when zoomed in far enough?
  42. Why doesn't the uncertainty pressure keeping electrons from crashing into the nucleus constitute a force? Perhaps it is technically a "force" but not a fundamental "interaction"?
  43. Do strange/charm, top/bottom quark matter have different emission spectra?
  44. Does the weak force propagate at the speed of light?
  45. Why are phi and phi^* independent fields?

Is it possible?

  1. Is it possible that there a slight bias of charge in the universe that could be responsible for the expansion of the universe?
  2. Is it possible that uncertainty in spectral lines is due to Doppler shifting from the electron’s motion in orbit? According to my quantum professor, this has already been accounted for.
  3. Is it possible that oppositely charged particles, one being antimatter, could form a gravitationally bound pair that overlap in space so that they are completely non-reactive to electromagnetism? Well, if a third charged particle overlapped with this pair, then it would probably tear the pair apart since electricity is stronger than gravity, but this isn't a complete answer.
  4. Is it possible for two photons to overlap and combine their energy to form a single photon of higher frequency? Probably not because this would violate conservation of angular momentum, but could they redistribute their energy between the two?
  5. If a neutron in a nucleus beta decayed, under what circumstances would the electron be pulled back in by the positive charge of the nucleus? The binding in the nucleus is supposed to change the radioactive lifetime of the nucleus, but is there an isotope that has unstable neutrons, but is effectively stable due to pulling beta decays back in?
  6. Can black holes be used to prevent the heat death of the universe? According to my astronomy professor, the answer is no, but I am not fully convinced yet.
  7. Is it possible that the universe started with a charge imbalance? We think not because we think that the big bang was a vacuum fluctuation, which conserves charge.
  8. Is it possible that there are particles that cannot be created in pairs directly from an e+ e- collision? Hans Paar says he thinks not, so far we assume that all particles can be created in this way.


  • Can we think of neutrons as an electron confined within a proton? Although a neutron can decay into an electron and a proton and a neutrino, high energy collisions indicate that there are three charged quarks in a neutron and the internal charge structure of the neutron can be explained by the distribution of these quarks.
  • Why are all photons circularly polarized? Actually they aren't, its just that the eigenstates are circularly polarized. A photon can have any polarization since it can be in a superposition of the two eigenstates.
  • Is the weak force really a force in the sense of accelerating particles according to F=ma? Yes because it is responsible for neutrino/electron scattering, which can't be caused by electromagnetism because the neutrino has no charge.
  • Why do we call the weak force a force when it does more than just accelerate particles? The word force is used only for backward compatibility and for publicity, we should really call it the weak interaction.
  • Why is the speed of light so special, what if there is some undiscovered force that propagates faster than the speed of light? It has to do more with the fact that it travels at the same speed in all reference frames. The proof that only one speed can have this property is in my GR paper.
  • Why does a stronger force mean a faster decay? This comes from Fermi's Golden Rule.
  • Why does the Pi+ decay into a muon more often than an electron? Fermi's Golden Rule makes it more probable for lighter particles to come out, but there is another factor that dominates. The Pi+ has spin 0, so when it decays into two particles, they must have the same helicity to conserve spin, but the weak interaction doesn't like to do that. So in order to do the decay, it must boost the lepton enough that its helicity flips. For a given momentum, a muon requires a smaller boost than an electron because it has a higher mass, hence lower velocity.
  • Why is a photon a spin 1 doublet instead of a triplet? The problem is that the triplet derivation was implicitly assuming working in the center of mass frame. To get the S_z=0 state, you need to be able to Lorentz transform to the CM frame, but you can't do that for a photon.
  • Can a neutral fundamental particle with spin create a magnetic field? Kind of. For example, the Z boson does by temporarily creating electron positron pairs that orbit each other before annihilating back into the Z boson. The intermediate charged particles create a magnetic field that looks like a field of the Z boson.
  • What interaction do mesons and quarkonium decay via? Mesons decay via the weak interaction because it needs to change flavor. Quarkonium can decay via any interaction, so the strong interaction dominates, with three gluons being created.
  • Could neutrinos be responsible for activating "spontaneous" decays instead of vacuum fluctuations? Probably not because the neutrinos from the sun and from the big bang have much to low an energy for this.


  • Virtual particle analogy with lightning: virtual photons are like the electric field sensing the path of least resistance and real photons are like the lightning bolt that delivers a full quanta of energy.