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?

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