As far as we know, there are 4 fundamental forces of nature. Gravity, responsible for
keeping you planted to earth and the moon in orbit, the strong nuclear force responsible
for keeping protons and neutrons bound together in the nucleus of atoms, the weak nuclear
force responsible for some kinds of radiation, and electromagnetism which is responsible
for light and chemistry.
But remarkably, at the moment of the Big Bang, all these 4 seemingly different forces were
thought to be one and the same. But as temperatures cooled, and energies lowered, the forces separated
into distinct interactions of their own.
Gravity is thought to have separated first at some of the highest energy densities and
temperatures. Then the strong force separated at cooler temperatures. What remained after
that, at relatively lower energies and temperatures, was electromagnetism and the weak force, as
one combined force called the Electro-weak force.
Well, it turns out that the energies at which the electroweak force exists is something
we can simulate in particle accelerators such as the Large Hadron collider in Geneva. And
modern particle physics a has fairly robust theoretical basis that describes the nature
of this combined force.
But how could these two forces be one and the same at a fundamental level, because at
the temperatures and conditions we live in today, they seem utterly different and utterly
incompatible.
For example, electromagnetism is all around us in the form of light that we can see, and
chemistry that we can observe. It has infinite range and is relatively strong.
The weak nuclear force on the other hand, is pretty weak, and something we can’t directly
see. It’s effective at ranges smaller than the diameter of a proton.
How is it that these two seemingly incompatible forces are one and the same at a fundamental
level? It turns out that the key to uniting these two forces, is something that was only
recently confirmed to exist – and that is the Higgs boson.
How the heck did we figure this out? The incredible story of the electroweak theory is coming
up right now…
Before I talk about the electroweak theory, let me tell you what inspired me to make this video.
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The best theory we have about the make up and functioning of the universe is represented
by the standard model of particle physics. Think of this as the periodic table for elementary
particles.
These particles along with their antimatter partners, represent all the elementary particles
we know about.
I want to draw your attention to the red particles, these are called gauge bosons. And they represent
the mediating particles that carry 3 for the four fundamental forces of nature. Gluons
mediate the strong nuclear force, photons mediate the electromagnetic force, and the
W and Z bosons mediate the weak nuclear force. Gravity is the 4th force, that does not currently
fit into this model.
You should be aware that there are actually two W bosons combined into one on this chart.
There is a W+ and W- boson, which represents their electric charge.
You might ask, what is a mediating particle? The exchange of the virtual versions of these
particles confer the appropriate force. So for example, when two electrons are near each
other, their repulsion is due to the exchange of virtual photons. The simple analogy is
illustrated here. The basketballs represent virtual photons, the boat represents similarly
charged electrons. The exchange of momentum is what results in a repulsion, similar to
the way the exchange of momentum of the basketballs result in the two boats moving apart. Attraction
is also mediated by photons, but it is analogous to the exchange of boomerangs, and brings
the two boats closer together. This is a simple analogy of how charged particles
attract or repel each other. In either case, the mediating particle is virtual photons.
The weak nuclear force also works via the exchange of a virtual W or Z boson. Let’s
take a look at perhaps the most important weak interaction. And that is the beta decay
of a neutron. A neutron is composed of two down quarks and an up quark. An isolated Neutron
that is not part of a nucleus is unstable. On average, within 15 minutes, one of its
down quarks turns into a up quark by emitting a W- boson. This turns the neutron into a
proton.
This W- boson almost immediately decays into an electron and an anti-neutrino. So what
we detect in this decay is the electron and the antineutrino. The virtual particle (W- boson) is
not detectable.
The weak force is unique in this respect in that it is the only force which can change
the identity of an elementary particle. In this case a down quark becomes an up quark,
resulting in a neutron turning into a proton. This is called a beta minus decay.
Another similar decay, called a beta plus decay, turns a proton into a neutron.
This kind of weak force decay is important in making atoms stable, and plays a big role
in nuclear fusion reactions that power the sun. The sun wouldn’t shine without it,
and we wouldn’t exist.
Let’s go back to the chart of the standard model. You’ll notice that the mass of particles
are expressed in terms of energy, using the mass-energy equivalence, E=MC2 championed
by Albert Einstein.
You’ll notice one big problem in uniting the electromagnetic force with the weak force
– and that is the fact that photons are massless, but the W and Z bosons are massive…very
massive – their masses are about 80 and 90 Giga electron volts or GeV. This is over
80 times the mass of a proton, which has a mass of about 1 GeV or 1000 MeV.
So given the differences between these particles, and the fact that the weak force appears so
different than electromagnetism, how did scientists realize they were controlled by the same guiding
principles?
Scientists, including American Physicist Sheldon Glashow had noticed some clues indicating
that the weak force and electromagnetism could be related. For one thing, even though electromagnetism
is millions of times stronger than the weak force at large scales, the theoretical strengths
of both forces appeared to be identical at very small lengths – about 1/1000th the
width of a proton, suggesting a point where the two forces may merge as one.
In addition, theoretical models predicted an additional gauge boson, named the Z boson.
This weak force boson would need to be neutral, like photons are.
Glashow showed that an acceptable theory for the unification of the weak and electromagnetic
interactions could be found. But the two forces had to be treated as one – as
an electro-weak force. And his model predicted 4 bosons that would mediate this force. Those
bosons were: the W1, W2, W3, and B.
There were two problems with his model. First it predicted a third weak force neutral Boson,
now called the Z boson, which was not predicted by any theory at that time. The first evidence
of this particle only came much later in 1973, with the detection of something called weak
neutral currents.
In addition, Glashow’s model only worked if all 4 Bosons were massless. It put both
the weak force and electromagnetism on equal footing as both would have massless particles.
One can call this masslessness, a kind of symmetry.
But ironically, this was a big problem because this did not fit observations. Something had
to break this symmetry because the weak force was weak, and interacted at only at very small
distances, indicating that its mediating bosons had to be massive.
Glashow could not find a mechanism for making the 4 massless bosons from his
electroweak theory into the 3 heavy weak nuclear force bosons and the massless photon.
You might ask, why do these weak gauge bosons have to be massive? Why does mass make a difference
in the distance that the forces can act?
This has to do with the Heisenberg Uncertainty equation, which says that for virtual particles,
the change in energy times the change in time has to be less than h/4pi.
If we substitute MC^2 for Energy, we can see that the greater the mass, the shorter the
time of the virtual particle’s existence. So the short range of the weak force can be
explained by the fact that massive virtual particles just don’t exist for very long.
Whereas for electromagnetism, since photons have no mass, they can exist theoretically
for a long time, and thus interact over long distances.
So, how do we reconcile the idea that photons and the W and Z bosons are ultimately similar,
and have a common origin, if there is such a huge discrepancy in their masses? This remained
a mystery.
Glashow is even quoted to have said, “It’s a stumbling block we must overlook.” In
other words, he thought his idea was right, even though it did not fit observational data.
Three years later in 1964, Robert Brout and Francois Englert in Brussels, and Peter Higgs
at the University of Edinburgh, developed a mechanism whereby mass could be given to elementary
particles while maintaining a meaningful theory. The Brout-Englert-Higgs or "BEH mechanism"
employed the properties of a field, what is now called the Higgs field, to break the symmetry,
and it predicted another massive particle, the Higgs boson.
In 1967 and 1968, Pakistani physicist Abdus Salam and American physicist Steven Weinberg
independently took the ideas of the Higgs mechanism, and combined them with Glashow’s
ideas to show how Glashow’s 3 weak force mediating particles could gain mass, while
the photon of electromagnetism could remain massless, in the same theoretical framework.
This is the basis of what is now the Electroweak theory. Glashow, Salam, and Weinberg were
awarded the 1979 Nobel prize for their efforts.
So now the question is, how does the Higgs mechanism give rise to masses in the 3 weak
force particles, but not the photon.
Two illustrate this, I am going to show a diagram that depicts the Higgs potential.
The Higgs mechanism can, in a very simplified way be understood by considering the Higgs
potential. The Higgs potential gives the Higgs field the possibility of changing its effects.
At energy scales above 160 GeV, the Higgs potential looks like this. This does not do
anything, because at the origin, Higgs field interactions do not result in mass, and all
symmetries are intact.
However, at energies below about 160 GeV, the point at which electroweak interactions
break down into two distinct interactions, the weak and electromagnetic, the Higgs potential
begins to look much different. It looks more like this graphic. This is referred to as
the "Mexican hat potential" because it looks like a Sombrero.
The Z axis is energy, and the X and Y axes indicate the vacuum expectation value. This
vacuum expectation value becomes non-zero below 160 GeV. Vacuum expectation value is
like a value of the Higgs field. This non zero expectation value is what gives particles
that interact with the Higgs field, mass.
As I’ve said before, all particles prefer to be at their lowest energy state. At energies
of more than 160 GeV, the minimum of the Higgs potential is at the origin where the vacuum
expectation value is zero. In other words, everything is massless.
But at energies below 160 GeV, the minimum vacuum expectation value is not at the origin.
It is not zero, thus particles fall into the new minima. In other words, they obtain mass.
You might ask, ok why aren’t all the 4 particles including the photon massive, if this Mexican
hat potential applies to all four Glashow particles?
Well, to put it very simply, this has to do with the way the 4 particles interact with
the Higgs potential. The simplistic explanation is that the photon remains at the bottom groove
of the Mexican hat and does not have any other movement. It can freely move around this groove
without needing any energy. And as Einstein’s equation E=mc^2 says, 0 energy means 0 mass.
The other three particles however interact with the Higgs potential differently. They
oscillate around the low point of the potential. This interaction with the Higgs potential
requires energy as they try to climb the potential. Again, energy is equal to mass, and this is
how they gain mass.
So why is the electroweak force important? You have to realize that the reason we are
where we are today in our understanding of particles and forces is due, in no small part,
to the search for the Higgs Boson which was needed to explain the Higgs mechanism, that
united the weak force with Electromagnetism, even though the two forces seemed utterly
different.
And this gives us hope that the next step in the unification, which would be to unite
the strong nuclear force with the Electroweak force may also be possible, no matter how
different these two forces appear to be right now. This unification would be the Grand Unified
Theory (GUT).
And when we achieve this, the only missing piece of the puzzle towards a complete understanding
of all forces would be a quantum level understanding of gravity – called quantum gravity. And
when we unite the Grand Unified Theory with quantum gravity, we will have the biggest
prize in all of physics, and probably all of science, the holy grail – the mother
of all theories – the Theory of Everything! Let’s keep going until we get there.
I’d like to thank my generous supporters on Patreon and Youtube. If you have a question,
leave it in the comments, and I will make every effort to answer it. I will see you
in the next video my friend!