As a scientist, I love
questions...the more the better. When I play a question
game with my kids, extra points are awarded
if the questions are tricky and a special bonus is
given if the questions are about something entirely
familiar, but with very deep consequences. In fact, I almost always
lose the competition, as I find that young
children have some of the very best questions. Let me give you some ideas of
the kinds of questions I mean. Why is it that I can't put
my hand through this table, but I can wave my
hand through air? How can it possibly
be that steam, water and ice are
actually the same thing? They seem to have totally
different properties. Just what is fire? And what makes it glow? Essentially, the
questions can all be boiled down to what are the ultimate
building blocks of reality and what are the rules
that govern them? Questions like these have
perplexed humanity for as long as we've kept records. And, of course, with
questions have come answers, with varying degrees
of sensibility, from the four elements of
fire, water, air and Earth, to the more modern
ideas of chemistry. However, in the last
50 years or so, we have made some
very rapid progress. Indeed, our modern understanding
of the underpinnings of the universe can explain
phenomena from the behavior of atoms, to how stars burn. We have a name for
this understanding. It is called the Standard
Model of particle physics, or just the Standard
Model for short. To understand what goes
into the Standard Model, we need to recall some ideas we
might have learned in school. If you've ever taken a chemistry
class, you've heard that all of the matter of the universe
is made of about 100 elements. However, even if you
never studied chemistry, you've probably heard that
all matter is made of atoms. You've even probably seen
this little logo for an atom, which shows a tiny nucleus, with
electrons swirling around it. Atoms like these are
the smallest examples of the various elements and you
could reasonably think of them as the universe's ultimate
building blocks However, nearly a century ago,
physicists realized that this wasn't the final word. We discovered that the
nucleus of the atom was made of varying numbers
of two particles, called protons and neutrons. This was a substantial
simplification in our understanding
of the universe. Rather than 100 chemical
elements, we now realized that with a mere three subatomic
particles, called protons, neutrons and electrons, we
could, in principle at least, construct an entire cosmos. And that is a pretty
impressive achievement. However, during the
1940s through the 1960s, physicists discovered many
more subatomic particles in experiments using
particle accelerators. Rather than the simple
model of three particles, literally hundreds of subatomic
particles were discovered. Clearly, another simplifying
insight was in order. The mid 1960s was when
our modern understanding of the subatomic realm
began to develop. Physicists realized
that the familiar proton and neutron were made of
smaller objects still. These smaller objects
are called quarks. We now know of six
types of quarks. They have kind of silly names,
which are: up, down, charm, strange, top and bottom. Up and down quarks are found
inside the proton and neutron, while the others are necessary
to explain vast number of discoveries made in
particle accelerators. In addition to the quarks,
there is another class of subatomic particles
called leptons. The most familiar lepton is
the electron, although it turns out that there are
six leptons as well. Three of these leptons
have electrical charge. These are the electron,
the muon and the tau. The other three are neutrinos,
which are electrically neutral. These quarks and leptons include
every particle that we know of. The up and down quarks and the
electron are the building blocks of the cosmos. The other nine particles
have all been observed in our accelerators. However, while the building
blocks of nature are important, we have forgotten
an important point. This important point is force. Without forces, these particles
would wander around the cosmos, not interacting with each other. And that would be bad. If something didn't stick the
quarks and leptons together, there would be no atoms
and consequently no us. Physicists know of
four different forces. The most familiar
force is gravity. It keeps us firmly attached
to earth and governs the path of the stars and
planets in the sky. It turns out that gravity is
actually a very weak force and we don't understand how
it works in the quantum realm. However, the three other forces
are very well understood. The next most familiar
force is electromagnetism. Electromagnetism is responsible
for electricity and magnetism of course, but it is the
reason why light exists and, in the context of
building matter, its most important attribute
is that it is the force that binds the electrons to
atomic nuclei and makes atoms. The electromagnetic
force is responsible for all of chemistry. The other two forces
are less familiar. The first is the strong nuclear
force, and it is this force that ties quarks together
inside protons and neutrons and other particles
physicists have discovered. The weak force is responsible
for some types of radioactivity and plays a role in
how the Sun burns. These four forces have
very different properties. Gravity and electromagnetism
have a very long range, like the gravity from the
Sun affecting the path of distant Pluto In
contrast, the weak and the strong nuclear forces
only have an appreciable effect over distances smaller
than the size of a proton. At distances bigger
than an atom, these nuclear forces
essentially don't exist. This is kind of like
Velcro, where if two pieces of Velcro are touching, they
are strongly tied together, but when they are pulled apart,
they feel no force at all. The strength of the forces
is really quite different. If we call the strength of
the strong force to be 1 unit of strength, like 1 mile or
1 hour, then the strength of the electromagnetic force
is about 100 times smaller. The strength of the weak force
is about 100,000 times smaller. And the strength of the
puny force of gravity between two particles is a 1
followed by 40 zeros smaller. This weakness of gravity
is why we can't study at particle accelerators
and is huge mystery. We don't understand why
gravity is so much weaker than the other forces. Gravity is currently not
part of the Standard Model. How do these forces work? In the realm of the super small,
we need to have a different way of thinking of forces. At the quantum scale, forces are
caused by exchanging particles. To understand how this works,
imagine standing in a boat and having someone
throw you a heavy sack. Your boat would move as
if it had felt a force. Similarly, if you throw a
heavy sack off the boat, the boat would move. All the subatomic forces work by exchanging a different
kind of particle. The particles are the gluon
for the strong nuclear force, the photon for the
electromagnetic force and the W and Z bosons for the
weak nuclear force. Physicists speculate about a
particle called the graviton for gravity, but this has
not been demonstrated. So that's the Standard Model. Twelve particles of matter,
governed by three forces that are caused by the
exchange of four particles. From these building blocks,
with the right recipe, we can build the universe. Experiments with particle
accelerators have completed our understanding of the Standard
Model with amazing precision. Now I don't want to
leave you with the idea that there are no
mysteries left to solve. While the Standard Model is
the most successful theory ever devised, there are still
frontiers to explore. For instance, the Standard Model
includes a particle called the Higgs boson which is
thought to give mass to the other particles. We still have a lot to learn
about the origin of mass. Further, we don't understand
why there are twelve matter particles and why the quarks
and leptons are different. We don't know why
there are four forces and where gravity
fits into the picture. There are plenty of
mysteries to solve. These are great questions, just like the ones we
started this video with. It's an awful lot of
fun to think about them. And there's no reason why we
scientists should have all the fun. So I invite you to join
my colleagues and I by reading up on these ideas. You could become a
subatomic adventurer like us, exploring the quantum frontier.
