Hey it's professor, Dave let's talk about
quantum gravity.
When we looked at the standard model of particle
physics, we learned about fermions and
bosons. Fermions are the particles that
comprise ordinary matter,
while bosons are the particles that
mediate the four fundamental forces,
which in quantum theory we no longer
view as field forces, but instead as
interactions between bosons and fermions.
Electromagnetism is mediated by photons,
the weak nuclear force by W and Z bosons,
and the strong nuclear force by gluons.
These particles have all been confirmed
experimentally, and are quite well
understood. We also understand that after
the Big Bang billions of years ago, the
electromagnetic force and the weak
nuclear force became distinct forces
after existing for a short duration as a
singular force, called the electroweak
force. For a short time before that, this
electroweak force and the strong nuclear
force were also one even more
fundamental force, but that leaves one
more step to go. We have not as of the
time of the writing of this course been
able to verify the existence of the
graviton, the hypothetical particle that
should mediate the gravitational force.
Being able to describe such a quantum
field theory for gravity, called quantum
gravity, could allow us to show how for
the briefest instant after the Big Bang
all four fundamental forces were
actually one singular force, and the
theory that governs this force would
therefore be a theory of everything, a
theory from which all the forces and all
the particles could be derived. This is
obviously an area of great interest in
physics today, and there are many
competing theories attempting to succeed
in this monolithic task. Such a theory
would completely reconcile general
relativity with quantum field theory and
offer a complete description of the
universe that
currently eludes us. So why can't we do it?
Well the tricky part is that general
relativity and quantum theory, while they
both enjoy mountains of empirical
evidence supporting their validity, are
completely incompatible with one another
as currently formulated. Because general
relativity looks at space-time on the
grandest scale, and quantum theory looks
at the tiniest particles, their realms
are vastly different, and we currently
use them in isolation as the situation
deems appropriate. If things are very
tiny, they are quanta, and we use quantum
theory. If things are very big and
massive, they warp space-time, and we use
relativity. But there are certain
situations in physics where we need to
talk about things that are both very
tiny and very massive. A black hole is
such a situation, as it can be regarded
as a dimensionless point, with a mass of
anywhere from a few solar masses to a
few million solar masses. Another example
of such a situation is found in early
universe cosmology, where the first
fraction of a second after the Big Bang
is extremely mysterious to us. The Big
Bang involved the formation of the
universe from a single point, meaning
that all of the energy in the universe
was for an instant contained in an
unimaginably tiny volume at an absurdly
high temperature, which then cooled as it
expanded. Through increasingly
sophisticated experiments in particle
accelerators we have been able to
recreate conditions in the early
universe at successively earlier and
earlier times, or epochs, which is how we
came to understand how the forces must
have been unified and subsequently split
apart. But as we said, in that first
instant before gravity broke away from
the other three forces, the universe was
so dense that our current understanding
of physics is unable to describe it.
While this seems like a problem so
daunting that we may never figure it out,
we must remember that humanity will always
progress. In the past century we have
made so many astonishing discoveries
that have transformed our understanding
of the universe. Quantum gravity will
indeed be developed, it's not a matter of
if, only when, and by whom. Maybe it could
be you! Or if quantum field theories are
not your cup of tea, there are lots of
other problems in physics that need to
be solved.
Exotic concepts like dark matter and
dark energy are popular topics of
research. If the names sound mysterious
it's for good reason, because while we
have a lot of evidence that dark matter
exists, we don't have the faintest idea
what the stuff is. But a true discussion
of dark matter has to be rooted in the
observational astronomy that brought
about its discovery. We look out into the
universe with telescopes, see what's out there,
and try our best to make sense of it.
So if you want to know more about stars and
black holes and dark matter, you'll have
to wait for the upcoming astronomy
course. But astronomy and physics are
close bedfellows, so it's a good thing
that you've watched these modern physics
tutorials first. I hope that you have
been able to get a basic grasp of the
incredible advancements that have
occurred in this field over the last
century or so, and that you are inspired
to learn more about this and other
related topics. Until next time.
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