Ok, Johandri Barris is asking us:
If the Higgs boson explains the mass of particles
how is its interaction with familiar objects of different sizes, like a pencil or an armchair?
Well, as he explains, the Higgs boson explains the mass of fundamental particles,
the smallest components of matter.
So, to understand how the Higgs boson interacts with familiar objects, like an armchair or a pencil,
we must ask ourselves what are the fundamental particles
making up a pencil or an armchair.
For that, let me start with the structure of the atom
to see the elementary particles making up atoms.
Atoms, you know, are made up of protons and neutrons,
which are located inside the atomic nucleus,
with electrons moving around them.
These electrons are elementary particles,
and acquire their mass via their interaction with the Higgs field.
But this is a very small mass,
the interaction is feeble and the electron mass
is some 2000 times lighter than the proton mass.
So the mass of the atoms, the mass of the armchair or the mass of the pencil,
basically concentrates on the protons and neutrons composing the atomic nucleus.
Protons and neutrons are fairly similar particles,
the proton has positive electric charge, and the neutron has no electric charge, but otherwise they're very much alike
The neutron mass is slightly larger than the proton mass,
but just slightly, they differ in one part in a thousand.
To give you an idea of the amounts we are discussing,
in a kilogram of mass
there are 10^{27} proton masses.
Protons and neutrons, as opposed to electrons, are not elementary particles,
they have substructure, they are composite.
They are made up of particles known as quarks and gluons,
and the theory describing the behavior of these elementary particles,
quarks and gluons, is known as the strong interaction,
described by Quantum ChromoDynamics, QCD for short.
QCD is a close cousin of another theory we know very well,
electromagnetism.
To get some intuition about QCD,
quarks would be the analogues of electrons in electromagnetism,
and gluons would be analogous to the particles of light, the photons, in electromagnetism.
In nature there are six different kinds of quarks,
but to understand the structure of protons and neutrons,
it's enough to consider only two,
known as the up quark and the down quark.
The proton is composed of two "u" quarks and one "d" quark.
The neutron is instead made up of two "d" quarks and one "u" quark.
These quarks are hold together inside the proton by some "glue",
motivating the name "gluon"
for the particles gluing quarks together.
These particles are all inside this box which is the proton.
Ok, so to understand how the proton interacts with the Higgs field,
we have to check
the interaction of the Higgs field with the particles making up the proton,
the elementary particles composing the proton
The gluon has not direct interaction with the Higgs field,
the gluon is massless, it doesn't talk directly with the Higgs field.
The "u" and "d" quarks do have interaction with the Higgs field,
but it's very feeble, almost as feeble as that of the electron.
In fact, the masses of the up and down quarks...
well, they would add up to at most 1% of the total mass of the proton.
Let me consider a thought experiment,
in which we turn off
the interaction of the Higgs with the up and down quarks,
so that we take these quarks are not interacting with the Higgs field,
and are therefore massless.
We physicists can carry out these "experiments" in our computational clusters,
and if we carry out this "experiment" in which all particles making up the proton are massless,
and we compute the mass of this "fake proton",
we obtain that the mass is almost identical to the actual mass of a real proton.
More than 90% of the mass of a real proton
comes from this fake world in which particles have no interaction with the Higgs field and are massless.
So the armchair and the pencil interact with the Higgs field through their elementary particles,
but their mass, the mass of the armchair, has little to do
with the way the particles interact with the Higgs field.
So, where is the mass coming from?
To explain that, let me use an intuitive picure
that we physicists like very much,
although it's just a picture.
Let's think that the proton is an elastic bag
with all those particles inside.
These particles move freely inside the proton...
almost freely, because if they try to escape, the elastic bag stretches but pulls them back inside.
We say that they are confined to move inside this little bag which is the proton.
These objects moving there have energy
precisely because they are moving inside the proton, bouncing off the bag,...
and this energy, via a famous formula you all know,
which is Einstein's formula, is equivalent to a mass.
So the mass of the proton has nothing to do with the interaction with the Higgs field,
it's a mass coming exclusively, or almost exclusively,
from the energy of the confined particles moving inside the proton.
So the mass of the proton is essentially energy,
energy from the strong interactions,
of gluons and quarks with strong interactions inside the proton.
Ok, this answers the question of
how the armchair and the pencil interact with the Higgs field;
there's interaction through their elementary particles,
but their mass is not determined by this interaction.
Their mass has a different origin.
What I'd like to do next is to explain
howe this interaction is nevertheless very important
for the armchair to be as it is, the pencil to be as it is, and the universe to be as it is.
For that, I'm going to
use this tiny energy difference between the neutron and the proton.
I've told you that the neutron is slightly heavier than the proton.
This is due to the different composition of the neutron and proton in terms of quarks.
The proton has two up quarks and one down quark,
the neutrons has two down quarks and one up quark.
And we know that the interaction with the Higgs field is such that
the down quark mass is slightly larger than the up quark mass.
A consequence of this is that the neutron is slightly heavier than the proton.
What happens in this fake world I made up,
in which the up and down quark masses are taken to be zero?
In this fake world, the proton is slightly heavier than the neutron,
and that's because it has electric charge,
and the electromagnetic interactions increase its internal energy, its mass.
The fact that the proton is slightly heavier than the neutron
changes the world completely.
It in fact changes the structure of the hydrogen atom.
You know the hydrogen atom is composed
of a single proton in the middle, the nucleus,
and a single electron dancing around it.
These two objects are bound, forming the hydrogen atom,
due to the electromagnetic interaction between the two charged particles.
Now if the proton is heavier than the neutron, the proton is unstable.
Nature like to roll to states of minimum energy.
So the proton decays into a neutron,
and the neutron is neutral, as its name says, it has no electric charge,
and so cannot keep the electron in the hydrogen atom.
The electron flies off the atom, and hydrogen atoms just don't exist!
There are many consequences of the way elementary particles
interact with the Higgs field.
Another very interesting one is what happens with the electron.
I told you the electron acquires its mass from its interaction with the Higgs field.
If we turn off the interaction of the electron with the HIggs field,
and we make the electron massless,
the electron is a particle moving at the speed of light,
and atomic nuclei cannot keep them inside atoms,
and the consequence
is that atoms don't exist in Nature as we know them.
To conclude, regarding the question asked by Johandri,
armchair, and pencils, and ordinary matter,
interact with the Higgs field through their elementary particles.
This interaction is not responsible for the mass of armchairs,
but is is responsible for the armchair to be as it is, and in general for the universe to be as we know it.
If it weren't for the Higgs field, the world would be very different form how we know it.
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