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\viewkind4\uc1 \pard\sa200\sl276\slmult1\f0\fs22\lang9 On
the third planet orbiting an average yellow dwarf star a curious creature evolved. Humans.
So curious in fact, they couldn\rquote t stop themselves from trying to figure out how everything
worked. Although their simple brains were evolved for a hunter gathering life, somehow
they were not content with what they could see with their eyes, and feel with their hands.
So they invented telescopes to peer into the depths of space, and particle colliders to
smash matter into its constituent parts. And eventually they discovered how everything
in the universe actually works.\par \par
Although they didn\rquote t actually manage did they? Not yet anyway. Excuse me.\par
\par So here\rquote s the problem. We\rquote ve
got two fundamental theories that describe the universe, but we don\rquote t know how
to join them together. On one end we describe how the fundamental particles work and interact
with each other with quantum field theory which explains electromagnetism and the nuclear
strong and nuclear weak forces. And at the large scale, the theory general relativity
explains how objects influence each other due to the curvature they make in space-time,
otherwise known as gravity.\par \par
These two theories are the most complete description we have of how the universe works, and both
have been tested and verified to incredible precision. But, they don\rquote t go together.
Gravity isn\rquote t described in quantum field theory, and general relativity says
nothing about the quantum world.\par \par
Big deal, you might say. If they are each doing their own bits okay then there\rquote
s no problem. But there is a problem. In between these theories live two of the biggest mysteries
in physics: black holes and the big bang. In these extreme events quantum mechanics
and general relativity meet, and we won\rquote t ever be able to understand them until we
develop a theory of quantum gravity, the fundamental theory of everything.\par
\par We have made some attempts. The most promising
are string theory or M-Theory and loop quantum gravity, but neither of these have experimental
evidence to back them up, for example string theory predicts new fundamental particles
called supersymmetric particles which we haven\rquote t observed. So for now these theories remain
hypothetical.\par \par
I should mention that, in practice, we don\rquote t only use quantum field theory and general
relativity to model everything in physics. Often they would be far too cumbersome, so
we actually use a whole suite of other theories that are perfectly good approximations for
those specific situations. But quantum field theory and general relativity are special
because they are the most fundamental theories.\par \par
So why has it been so long since we\rquote ve had any progress on quantum gravity? General
relativity was published in 1915, and quantum field theory was completed in the late 1970\rquote
s. Why has there been very little progress since then? Well there is one simple reason.
The force of gravity is so much weaker than the other forces, and so making an experiment
where something significantly feels all of the forces at the same time is very very difficult,
perhaps impossible.\par \par
Gravity only becomes strong when you are dealing with very large amounts of matter. For example,
it takes the whole Earth to keep you on the ground. And for stuff this big like humans
or planets or stars, quantum effects just aren\rquote t noticeable.\par
\par The place where quantum physics is important
is down at the scale of atoms and subatomic particles, and these things are so small and
light that the force they feel due to gravity is negligible compared to the other forces.
For example, two electrons centimetre apart would feel a repulsion from their electric
charges but attract each other due to their mutual gravity. But the force from the electric
charge is ten to the power of twenty four times higher than the attraction they feel
from gravity. That is 24 orders of magnitude bigger, so any effect of gravity is lost down
in the twenty fourth decimal place of the electrostatic force, an incredibly small effect.\par
\par So the only test bed of quantum gravity are
places where you have a huge amount of matter squeezed into very small volumes, which are
always incredibly high energy situations, like black holes or the big bang.\par
\par And unfortunately these are not things we
can create experimentally given our current level of technology. To get to the right energies
we would need a particle accelerator like CERN, but the size of the solar system with
detectors the size of Jupiter to get to the right kind of energies. And to create enough
energy to test quantum gravity, the experiment would actually end up turning into a black
hole.\par \par
So currently our best bet for testing our theories of quantum gravity are to look at
those natural experiments that have happened for us: black holes and the big bang. We do
that by using gravitational wave astronomy and measurements of the cosmic microwave background.\par
\par The cosmic microwave background shows us the
very first light in the universe, released about 380,000 years after the big bang, covering
the entire sky. But this light isn\rquote t even in every direction, it contains a fuzziness
which you can see here. These little fluctuations are actually an imprint of quantum fluctuations
from the big bang when the universe was smaller than an atomic nucleus. Think about how wild
that is. It is literally Heisenberg\rquote s uncertainty principle painted across the
sky. The large scale structures of the universe, clusters of galaxies and galactic voids were
seeded by quantum physics in the deep past.\par \par
So this is a clear place where gravity and the quantum world met, so people are studying
the cosmic microwave background in great detail, trying to find patterns in the data to point
them in the right direction. But these patterns are incredibly subtle, and we still don\rquote
t know if our current measurements have the resolution we need to see a signature.\par
\par We need to apply the same cautious optimism
to our gravitational wave experiments. They are really interesting because if there is
a theory of quantum gravity then there should be a gravity particle called a graviton which
is a tiny little packet of gravity: a quantization of the gravitational waves.\par
\par Unfortunately they are way too small to be
seen by our current gravitational wave detectors LIGO and VIRGO, which have done an incredible
job to just detect gravitational waves themselves, but to see a graviton you\rquote d need to
detect miniscule changes in that wave. This is similar to how it is easy for us to see
light waves, but very hard for us to detect a single photon.\par
\par And I can\rquote t overstate how hard it would
be to detect a graviton. In fact they may never be observable because we\rquote d need
to measure distances smaller than the Planck length, which is impossible according to quantum
mechanics. So, instead astrophysicists are looking very closely at the gravitational
waves from black hole collisions in the hope that there is some small departure from general
relativity. And if there is, then it\rquote ll be a tiny clue which we can follow to see
what direction to take with quantum gravity.\par \par
Until we get some new evidence of a departure from our existing very successful theories,
we\rquote ll never get closer to having a grand theory of everything.\par
\par Okay let\rquote s take a look at our best
contenders for a theory of quantum gravity: string theory and loop quantum gravity, although
I\rquote m not going to spend lots of time on them because they are highly theoretical
and very complicated and I don\rquote t really understand them.\par
\par Quantum field theory says that we have got
a field for each of the fundamental particles which all lie over each other in space. And
particles themselves are excitations of those fields. And in general relativity, gravity
is the curvature of space-time.\par \par
Now string theory treats spacetime as another quantum field and so tries to unify gravity
with the other forces in one framework, whereas loop quantum gravity doesn\rquote t unify
the forces, it attempts to work out what the quantum nature of space-time is.\par
\par So string theory hypothesizes that the fundamental
particles and their properties are the result of different vibrational modes of one dimensional
strings that exist in an 11-dimensional space and one of these vibrational modes corresponds
to the graviton. So it reduces all the particles in particle physics down to a single entity
which is a string. The theory has had some theoretical successes but has been criticized
for not actually describing the real world. Which is an issue.\par
\par Loop quantum gravity starts with general relativity,
but attempts to model the quantum nature of space time at the very short distances of
the planck length, and it implies that there is a minimum possible distance, kind of like
a space-time pixel.\par \par
These are the most popular proposals for quantum gravity, but there are others. And it is important
to state that neither theory has an experimental observation where they make a prediction of
something that isn\rquote t already covered by quantum field theory or general relativity.\par
\par Let\rquote s zoom out to the big picture again.
What\rquote s the point of discovering a correct theory of quantum gravity? What change would
it have to the world?\par \par
Well being able to understand black holes would be very cool. Currently general relativity
completely breaks down when spacetime becomes infinitely curved. And so a theory of quantum
gravity would hopefully solve the many mysteries of black holes. Do they contain other universes?
Are they the gateway to wormholes to other universes or other places in our universe?
What happened to the quantum information of objects that fall into them which quantum
physics tells us is not able to be destroyed? And it might give us an idea about what existed
before the big bang, and ultimately where the universe came from.\par
\par That's all very interesting, but how would
it affect us directly?\par \par
Truth is, I don\rquote t know, we would never know until we have the new theory. But looking
into the history of science every single paradigm shift in fundamental physics has led to new
technologies. For example, understanding quantum mechanics led to the invention of the computer
and the whole information age. And to get to a theory of quantum gravity, we know for
sure that we\rquote ll have to abandon at least one of our fundamental laws of physics.
So no doubt when we have a theory of quantum gravity it will open up possibilities that
we can\rquote t even imagine right now.\par \par
And most of all, for me, it would be cool to just to understand how the universe actually
works.\par \par
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Hey, thanks for watching.\par }