4 FUNDAMENTAL FORCES( INTERACTIONS): WHAT GOVERNS OUR WORLD?
Have you ever experimented gravity? For an example, when trying to ride a bike for the
first time and falling again and again. Or have you ever played with magnets as a child? I’m sure
that the first time you saw those little metallic objects attracting each other you got really
surprised. Usually, when children are totally driven crazy by an event or a natural effect
they start to ask a lot of questions repeatedly in a funny way. They continuously say: “why?”.
Although this might seem a childish approach to reality, it turns to be very deep when trying to
investigate a phenomenon. As a matter of fact, physicists tend to use this method to build
appropriate mathematical models that can describe how objects behave in different situations.
They need to build up laws that can predict the outcome of an experiment, and if they find an
event that doesn’t match their theories, they start to correct their beginning principles.
And what do we have discovered thanks to this endless research? Where did this method brought
us to? What governs our world? How can we resume all the existing sciences in just a handful
of different types of forces? These puzzling questions are the topic of this video that will
present you the four fundamental interactions that describe why our world is like the one we observe.
Do you want to know more about this? Stick with me and I’ll tell you everything in a moment.
Today we are talking about the deepest theory in the entire history of modern science:
Standard Model. What is that? Actually, it’s the best model we have to describe nature, basing on
four main interactions between particles. In order to make everything easier and,
probably more interesting, we need to take a little step back forgetting about particles.
Let’s try to introduce each force in a simplified version and then we’ll discuss about bosons,
fermions and other incredible things. Firstly, each one of us can experiment
gravity daily. If not, well you are probably an astronaut orbiting around
Earth in the International Space Station, but I think this is very improbable. However,
gravity is the interaction that attracts us to the ground and even the same that makes planets orbit
around stars, like the ones in our Solar System that seem to endlessly revolve around the Sun.
What is the easiest description of gravitation? Well, Newton may help us saying that gravity is
an attractive interaction that happens between objects with a defined mass. The gravitational
force is inversely proportional to the square of the distance between the two objects, and
directly proportional to the masses through the well-known Gravitational Constant that is around
6,67x10-11 (I avoid to focus on units of measure). What is the second force everyone will surely
know? Electric force. To be more precise, electric interaction alone is not a fundamental force:
in physics we have discovered that electricity and magnetism are two sides of the seam coin,
so we call it electromagnetic interaction. Therefore, don’t worry, we’ll discuss it later.
Electric force has nearly the same proportionality of gravity when dealing with distances,
but the constant, also known as Coulomb constant is way bigger (nearly 9x109). Moreover, electric
interction doesn’t depend directly on masses, but on charges (electrons have negative charge,
neutrons have zero charge and protons have positive charge). As a result of that, this force
can be either attractive (charges of different sign) or repulsive (charges of the same sign).
This interaction is what makes electrons orbit around the nucleus of protons and neutrons in
atoms. Probably, fluctuate would be a better term because quantum mechanics has shown that
we can’t know the exact orbit of an electron revolving round the nucleus, but this is
another story we don’t need to deal with right now, so we’ll use this simplified description.
However, electrons and protons have a small but not null mass. So,
an interesting question might be: what influences atomic particles more? Gravity or electric force?
Since the Coulomb constant is way bigger than the Gravitational one,
electric interactions are billions of billions stronger than gravitational ones.
A pivotal application of this interaction is obviously electricity that has revolutionized
the entire world, changing our lifestyle and boosting technological research for experiments.
But do you know what shocking phenomenon happened in the 19th century?
Physicists observed that electric wires interacted with each other and slight changes in current made
magnetic compasses turn around. Briefly speaking they understood that electricity
and magnetism were two different aspects of one unique interaction: electromagnetism.
In the second half of the century, James Clerk Maxwell collected in just four incredible
equations, the entire knowledge about this interaction. Surprisingly, from these four
laws we can derive the existence of electromagnetic waves like UV rays,
radio waves and light. Yes, light. The entire optic theory firstly developed by Newton was
just a consequence of Maxwell Equations. At this point physicist thought that all
possible phenomena can be explained with either the gravitational mechanics and
dynamics of Newton or the electromagnetism of Maxwell. But as it always happens in Science,
some problems emerged in the description of effects like the photoelectric one (a radiation
that is able to free up the electrons of a metal) and the black body radiation, that brought to the
establishment of the most revolutionary theory of Physics: Quantum Mechanics.
We don’t need to deep in this topic, let’s just say that Einstein understood that electromagnetic
waves could just “carry” discrete values of energy and called them “photons”.
The intensity of radiation is linked to the number of photons emitted, whereas the energy
of a single photon depends on the frequency (red light is less energetic than blue light).
Quantum Mechanics has clarified that electromagnetic waves and particles
in general can be considered both as waves and particles, depending on the situation.
What is the name of the third fundamental interaction? Strong interaction. Yes,
it’s funny to hear, but this name is appropriate and now we’ll understand why.
It is responsible for the strong force between neutrons and protons. As a matter of fact, very
precise calculations have shown that the repulsion between protons should make them move away and
destroy the nucleus, but fortunately this doesn’t happen. Why? Because the interaction between
neutrons and protons is nearly 137 times stronger than the electromagnetic repulsion among protons.
It is the most intense interaction, indeed! However, this is just one possible way in
which we can see the strong force. To deep more into this topic we must study better the nucleus.
At school you surely have learned that electron, proton and neutron are fundamental particles.
Well, if this is true for the first one, it’s definitely not correct for the other two.
They are composed by quarks, which are fundamental particles. Protons are made out of two quarks “up”
and one quark “down”, while neutrons are made out of one quark “up” and two quarks “down”.
The strong nuclear force between neutrons and protons is just a small remnant of
a much stronger interaction between quarks. So, the strong interaction allows the existence
of protons and neutrons and thus of atoms. Finally let’s move towards the last,
but not the least, fundamental force that governs our world: the weak force.
Like before, the name is not randomly chosen. In fact, the force is termed weak because its field
strength over a given distance is typically several orders of magnitude less than that of the
strong nuclear force or electromagnetic force. This interaction acts at very small distances
on subatomic particles and is responsible for nuclear fission and radioactive decay of atoms.
Initially proposed by the famous Enrico Fermi in 1933, this interaction was successively
unified with the electromagnetic one in the so-called electroweak theory.
Further developments are expected from the GUT (theory of great unification) that aims to unify
the electroweak and the strong interaction in a single electronuclear interaction.
And it doesn’t end here, a bigger task is that of The Theory of Everything,
a name that you will surely remember from the famous movie about Stephen Hawking.
The aim of this Model is to collect electronuclear force and gravitation
in a single interaction that describes the entire world.
So, we have seen that the four fundamental interactions can be reduced to three (and
probably less) and we have made a little highlight on their main characteristics. Now, we need to
understand how the Standard Model catalogues all of them together with elementary particles.
What are elementary particles? They are the smallest known building blocks of the
universe. They are thought to have no internal structure, meaning that researchers think about
them as zero-dimensional points that take up no space. Electrons are probably the most familiar
elementary particles, but the Standard Model of physics, which describes the interactions of
particles and almost all forces, recognizes two different groups: fermions and bosons.
The first ones are characterized by half-integer spin. What does it mean? What is spin? It’s really
difficult to define this quantity, but in a very simplified way we can imagine it as an
intrinsic angular momentum of each particle. It doesn’t have a classical counterpart and
even if has the same units of measure of an angular momentum it’s not associated with a
real rotation of the particle around its axis. Fermions are divided in other two subcategories:
leptons and quarks. Do you know what is the most common lepton of all? The electron.
They participate in electroweak interactions and they are commonly organized in three generations.
What does it mean? Electrons have two “cousins” with exactly the same charge and spin,
but a heavier mass. These particles are called muon and tau. The possibility of observing the
last one of the two is very rare, whereas the first one is usually produced when cosmic rays
hit on the polar atmosphere of our Earth generating exotic bunches of particles.
Going on with leptons, for each type of negative charged particle,
there is a neutral one and it is called neutrino.
What are neutrinos? Since it has zero charge, it just interacts through weak force and gravity,
but its mass is so small that for a long time scientists have thought that it was zero.
We have a neutrino for each particular type of negative charged particle (scientifically speaking
we say that there are three different flavors of neutrinos) and so we call them electronic
neutrino, muon neutrino and tau neutrino. These particles are probably the most difficult to
observe as they rarely interact with matter and they can even change their flavor. As an example,
the Sun usually emits a lot of electronic neutrinos due to nuclear reaction in its core,
but a part of them changes in tau neutrinos or muonic neutrinos in the journey to Earth.
To complete the family of Fermions, we need to describe quarks. Obviously, we have six quarks
divided in three couples, each one linked to a particular negative charged particle.
We have quark up and down for electrons, quark charm and strange for muons and
finally quark top and bottom for taus. According to these definitions, we can
organize Fermions in three different generations respectively linked to either electrons, muons
or taus. Each generation has a negative charged particle, its linked neutrino and two quarks.
Practically speaking, they differ just for mass and we are still not completely
able to explain why the matter we normally see is based on the first generation since
atoms are formed by electrons and protons and neutrons made of quark up and down.
As a matter of fact, the heavier leptons are not so stable. For example,
the muon has an average time of decay of just 2.2x10-6 seconds.
Obviously, you’ll have heard about antimatter. What is it? It’s matter formed by antiparticles.
Each fermion has a counterpart with the same values of mass and spin, but opposite
Furthermore, we need to deal with bosons. What are they? Their name was coined by the genius Paul
Dirac and they are the force-carriers of the four fundamental interactions with a spin equal to 1.
What is a force carrier? It seems really difficult, but if you have paid attention,
you’ll notice that we have already mentioned an example of force-carrier. Do you know what it is?
The photon. In fact, this particle with zero mass is the carrier of electromagnetic interaction.
When a UV ray hits an atom, it’s the photon that is absorbed giving a great energy to the
electrons that may get into an excited state and return to their fundamental one after few
seconds emitting another photon. Together with this one we have other three vectorial bosons:
gluons (which are classified into 8 different types) that carry the strong interaction,
W (that can be W+ or W-) and Z that are the intermediators of weak interactions.
It’s difficult to explain why in a few lines, but this whole theory “needs” a scalar boson
with zero spin called Higgs boson that “gives” mass to all other particles. The evidence of
the existence of such a boson was definitely proved in 2012 at Cern with the CMS experiment
after the first theorization in 1964 by Peter Higgs. The researchers working in Geneve, Peter
Higgs and Francois Englert were given the Nobel Prize in 2013 for this exceptional discovery.
Therefore, the existence of one last tensorial boson (with spin equal to 2) called graviton,
carrier of gravitation, is what could complete the model. However,
we have no evidence of such a particle and further research must be made in order to
give a proper structure to the Standard Model. Great problems such Dark Matter and Dark energy
still need to be solved and we must elaborate more and more models that
can try to explain such phenomena, even if this means going beyond the famous Standard Model.
Thanks for watching everyone! Are you amazed by the four fundamental
interactions? Do you want to know more about a particular type of particle them? Have
you ever heard about fermions and bosons before? Let me know in the comments below,
be sure to subscribe and i'll see you next time on the channel.