questions contributed by the modern physics class of 2009

questions we will attempt to answer in class

tuesday: Why do muon neutrinos transform into tau neutrinos?


the neutrino states that we detect (electron, muon, and tauon) are linear superpositions of 3 other neutrino states (the "mass" or "stationary" states) that are more fundamental (i.e, in the sense that they are eigenvalues of the Schroedinger equation) ....

we will review
    how neutrinos are produced
    how neutrinos are detected
    how the transformation behaves

       we will answer this question by working through problem 5(22)

wednesday thursday: What exactly is the electroweak force?  How are the force carriers the same?
Why are the Electric and Weak force unified?

hmmm.... this means that you -- the students -- could tell me exactly what the electromagnetism force is! (I can't wait to hear the answer on thursday!!)
why are the electric and the magnetic force unified?  (you should be able to answer this one a bit more easily)
how are the force carriers (of electric and magnetic) the same?

and what exactly does it mean to  ask "what exactly is the _______ force?" anyway?

section 5(11) of our text has a little related to this question

thursday: How does the Higgs field interact with Higgs bosons to give us mass as we know it?
How could a Higgs Boson be created, and how does it fit into the standard model?

the Higgs Field is to the Higgs boson as the Electromagnetic Field is to the Photon (aka the Electromagnetic Boson)
(except that the Higgs boson is not massless whereas the photon is)

the Higgs Field does not interact with the Higgs Boson to give particles mass; the Higgs field (or boson) interacts with EACH particle (electron, neutrino, ...) to give it mass

what is dark matter?

if we knew, we would have given away the Nobel Prize for that discovery!!

(we haven't!  if only you had included a proposal for such with your college app!  or, perhaps, something to do for miniterm next year?)




questions we already answered in class or the text

How do protons change into neutrons, that is, when they exchange say a pion, do they lose quarks? 
                                                                                                                             
writing out the quark compositions of p/n/
p answers the question:      p  -->   n  +  p+       (uud  -->   udd  +   ud)
                                    or                                                                                         n  -->   p  +  
p-       (udd  -->   uud  +   du)

(here, an underline indicates an antiparticle)

If quarks are not conserved, how do they change and create new particles?

we have seen a number of examples in class of non-conservation of quarks:
1) for example, quarks and antiquarks can be created (or destroyed) in pairs (see question above for p/n transformation examples);

2) in addition, a quark species can change into a different kind of quark by interacting with electrons/positions or neutrinos/antineutrinos
(as we also discussed during the presentation of evidence for quarks, gluons, and color:     
ne  +    u   -->    e+    +    d )


more about resonances in particles?

 everything there is to know is in our textbook.  really.

short answers to complicated questions

If we understand the physical laws governing particles, why can't we predict chemical reactions starting from the interactions of the particles without learning chemistry laws?

for essentially the same reason that knowing the laws of physics cannot predict the angle of scattering of an alpha particle impinging on a nucleus....
there are things that we cannot know (instantaneously) on the atomic scale about a sample of particles that contains an Avogadro's number of particles (or even a trillionth of that number)

instead, when we do have a sufficient number of particles and we know the statistical rules that apply to them  (e.g., Maxwell-Boltzmann, Fermi-Dirac, Bose-Einstein), we can then define macroscopic variables (such as temperature and pressure) that measure ensemble averages of physics properties possessed by individual particles (such as kinetic energy and momentum) in order to make sense of the situation....

the closer we are to dealing with individual particles, the more we have to use individual properties such as momentum, kinetic energy, and spin....
the  farther we are away from dealing with a few particles, the more useful are the macroscopic properties such as temperature, pressure, and volume

For a given nuclear reaction, each combination of products has some probability of occurring.
How can we determine the probability for a particular combination?


now that we have an idea of what the wavefunction means...
the probability of a particular combination happening is the product of the wavefunction of the initial state with the wavefunction of the final state (times the interaction strength) integrated over all spacetime....

easy to say, perhaps not so easy to calculate...


The end of chapter 15 talked a little bit about string theory, but I really want to know why string theory was created.
How would string theory help us or be beneficial to making new discoveries in physics?
Would proving string theory change our understanding of physics greatly?
I would really like to know whether string theory is viable or would it improve on our current theories.


The hope (by string theorists) is that string theory is a viable path to quantum gravity (a unified theory of quantum mechanics and gravity) and/or a grand unified theory of forces (including gravity).  By modeling particles as strings (as opposed to points), the hope is that many of the infinities (due to a divide by zero as one approaches a point particle) will disappear.
At present, string theory is unable to make any prediction that is testable by any current experiment. 
(There are many string theories and they do make predictions, just not testable ones.)

Until it is able to produce a testable prediction, it belongs to the fields of philsophy and religion.

(A string theorist would NOT give such an answer, however.  But he/she would be incorrect.)


Questions about material not really directly relevant to the course,
covered in other courses, miniterms, etc,
or that require a significant background in other fields of physics

in quantum physics


How does quantum entanglement work?

quantum miniterm 2009

how can light behave as a particle and wave?

our textbook, chapters 3 - 6

but please, NEVER say that particles behave like waves...

in astrophysics

What evidence is there for dark matter? 

the Astrophysics course spent approximately 1 week on the quite substantial and varied evidence for dark matter:
microlensing and macrolensing of background objects;
the flatness of spiral-galaxy rotation curves at large distances from their centers;
the trapping of hot intergalactic gas in clusters of galaxies

available in any decent astronomy textbook

when we were discussing Black Holes, you told us that Einstein's Theories of General and Special Relativity did not apply to black holes because they predicted acceleration either up or down from the black hole (and thus our discussion on quantum gravity, string theory, etc).  I was curious if that, with our knowledge of Special Relativity from Modern Physics, you could explain why Einstein's theories incorrectly predicts this.  
Or in other words, what part of Einstein's theories about black holes is wrong?


sorry, but I never said any such thing!  Not only do Einstein's General and Special Relativity theories make any number of predictions about the behavior of matter, space, and time in the observable region around a black hole or any other compact massive object, the predictions are all borne out correctly (!!) by current observations!  We spent days in G&C discussing both the predictions and the observations that indeed agreed!

What I did say is that General Relativity is unable to make a prediction for future behavior about a point particle that encounters a point singularity.  General Relativity is therefore incomplete in that regard. 
(sigh.... did i really become a teacher so that students could take things i say and re-quote them blatantly out of context???)