While we currently have a very good model of the world around us, there is not a single physicist content with the Standard Model. Every physicist realizes that there are cracks in the model. But, as of today, no new theory or model can patch up the cracks without introducing new cracks someplace else. We stand today on humble ground; we have a pretty good model of the elemental particles and forces, i.e. we can model most of the elemental interactions under simplified conditions, but we lack a model with a sense of gravitas, i.e. with a sense of “oh, that makes sense.”
So, here’s a list of the some of the remaining problems.
Problem #1: Gravity has not been framed as a quantum field theory. Einstein updated Newton’s theory of gravity to account for relativity, i.e. (1) that space and time are not separate entities, (2) that an accelerating reference frame cannot be differentiated from a reference frame in a gravitational field, and (3) that mass and energy are the same thing. However, general relativity has not been framed as a quantum field theory. In other words, gravity has not been reconciled with quantum uncertainty principles.
Problem#2: We don’t know why the particles have the masses or charges that they have. As I’ve mentioned in previous posts, there is an eerie coincidence lying within the Standard model. There’s a group of three of each of the particles. For example, one group of three is the set of electrons, muons, and tauons. Each group has a distinct charge, and each particle in the group has a different mass. The values of charge for these particles (and their anti-particles) are -2/3, -1/3, 0, 0, 1/3, 2/3. This seems way too coincidental, as if there were some underlying structure that is not included in the Standard Model of particles physics.
Problem#3: The Standard Model rests on roughly 28 parameters that must be calculated experimentally, and which can’t be predicted from the current theory of electro-weak or strong forces. This includes the 12 masses of the fermions, 4 coupling constants of the 4 forces, 8 mixing angles, 1 vacuum angle, Higgs mass and coupling, and the Vacuum Energy.
Problem#4: Why is the weak nuclear force the only force that is not symmetric with respect to reflections in space-time and reflections of particles into anti-particles? Part of this problem will be addressed if the Higgs Boson is determined to be the particle found at 125.3 GeV. But we still need to understand the following: (1) why 125.3 GeV for the Higgs Boson? (2) Why does it couple only to the electroweak force carriers, and not to gluons? (3) Is the mass of the force carriers of the electroweak force the cause of the lack of symmetry with respect to reflections in space (P for parity reflection), reflections in time (T for time reflection), and charge (C for charge reflection of particles into anti-particles? Hence, what is the connection between the Higgs boson and the time asymmetry of the universe? (4) Why is the mass of the Higgs Boson nearly half the mass of Fermi’s constant for the weak nuclear force (n energy/mass units, it is 246 GeV) and nearly half of the sum of the masses of the three carriers of the weak nuclear force (250 GeV)?
Problem#5: It has been known for over 40 years that CP symmetry is violated in the weak nuclear forces, but CP symmetry is valid for gravity, E&M and the strong nuclear force. Is CPT symmetry a valid symmetry for all forces in the universe? And if so, why is CP or CT or TP not valid symmetries for the weak nuclear force?
In this post, I’d like to discuss the future of research in particles physics, and in particular, I’d like to address those topics that I find most fascinating: Why is the weak nuclear force not symmetry with respect to reflections of space-time and charge? And why is the weak nuclear force likely symmetric with respect to the combined CPT symmetry? (i.e. a combined reflection in time, space, and charge.)
If you are interested in this topic, check out the following presentation that discusses some of the most precise tests of whether CPT is a valid symmetry for the weak nuclear force. http://www.phy.bnl.gov/~partsem/fy11/etw_bnl_seminar.pdf
This presentation presents experiment results that strongly suggest that CPT is a symmetry of the universe (including the weak nuclear force) to within the accuracy of their measurement device. It should be noted that if the weak nuclear force were not CPT symmetric, then the weak nuclear force would violate Lorentz symmetry (i.e. special relativity.) If the weak nuclear force violated special relativity, we’d have to make some major modifications to the Standard Model of Particle Physics. Luckily (or not luckily if you enjoy developing new theories of the universe), it appears that the weak nuclear force is symmetric with respect to the combined CPT reflection. The ramifications of CPT symmetry are the following: particles and their respective anti-particles will have the same mass, lifetimes, and magnetic moments.
But it’s been known for over 40 years that the weak nuclear force is not symmetric with respect to CP reflections. This has profound implications for the world in which we live. The fact that the universe is not completely CP symmetric means that there are ways to tell the difference between left-and-right, forward-and-backwards time, and particles-and-anti-particles. This means that we can communicate with alien civilizations and determine in advance whether they are made of particles or anti-particles. We won’t visit those alien planets made of anti-particles (if any such worlds exist.)
It appears that the weak nuclear force that is the source of space-time asymmetry in the universe. If there were no weak nuclear force, it’s not clear how we would differentiate between the past and the future, or differentiate between left and right. But the question remains: why are three of four forces of nature are symmetric with respect to CP reflections, but one of them is not symmetric? And what exactly is the connection between the weak nuclear force and the second law of thermodynamics (which states that the entropy of the universe only increases or stays the same)? [If you’re interested in the physical foundations for the second law of thermodynamics, I suggest reading an article this week by New Scientist magazine that discusses a proof of why removing the quantum uncertainty principle would violate the second law of thermodynamics.]
So, to wrap up this post on what the future of research in particle physics might look like, I’d like to discuss the fact that CERN is not finding evidence for supersymmetric partner particles. As the accelerator at CERN goes to higher and higher energy levels, it is slowly making the theory of supersymmetry seem less and less likely. Supersymmetry is a theory that posits an addition symmetry in the universe (i.e. the symmetry between bosons and fermions), and posits the existence of pair-partners for every particle that we’ve found so far. However, there is no evidence for these supersymmetric pair-partners, and as the CERN accelerator produces collisions of higher energies and finds no evidence of super-symmetric particles, the probability that supersymmetry is a symmetry of the universe becomes less likely.
This also has ramifications for String Theory because most versions of String Theory assume supersymmetry as a valid symmetry of the universe. So, not only is the LHC at CERN starting to confirm the Stand Model of particle physics (by possibly confirming the existence of the Higgs Boson), but LHC at CERN is also starting to make String Theory and Supersymmetry seem less and less likely.
This is both good and bad news. The bad news is that String Theory addresses many of the remaining problems listed at the beginning of this post (such as coupling gravity with quantum field theory and predicting many of the values of the 28 experimentally measured constants, such as the masses of the particles.) The good news is that the Death of String Theory & Supersymmetry should oblige the field of particles physics to start developing new theories of the universe.
In conclusion, while the possible discovery of the Higgs Boson strengthens the case for the Standard Model of particle physics, there are still a lot of unanswered questions in this field. We are still far away from developing and testing a Grand Unified Theory of the Universe that explains the mass of the particles, that explains the charges of the particles, that explains the strengths of the forces, and that explains why the weak nuclear force is not C, P, T, CP, CT, or PT symmetric?