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Imagine taking a notebook, a plastic bottle, a toaster, and a glass container and burning
them in a fire. If the fire was hot enough, all the materials in these 4 things would
break down into a gaseous mixture of molecules and atoms.
If you make this fire hotter, all the atoms would break down into their component electrons
and neutrons.
If you go hotter still, these particles would break down into the fundamental particles
of the standard model, things like quarks and leptons.
And if you go even hotter, we are talking astronomical temperatures around 10^31 degrees
or about a million, trillion, trillion degrees Celsius, all the quarks and leptons and the
4 fundamental forces of nature, would break down into one entity – a kind soup where
all the fundamental particles and the underlying forces responsible for their behavior would
meld into one.
This is what is believed to have existed at the moment of creation, at the moment of the
big bang.
All the particles and forces of our reality came from this primordial soup. But this is
the story of the 4 fundamental forces of the universe that, as far as is known, control
everything in the universe – every movement, every phenomenon, and every process you can
possibly think of.
What is the nature of these forces? How do they work? And where did they come from?...that’s
coming up right now.
According to modern particle theory represented by the standard model of particle physics,
all matter is composed of 6 quarks and 6 leptons and their 12 antiparticle pairs.
But that’s not all, matter is subject to 4 fundamental forces that cause this matter
to behave in certain ways, and result in essentially every action that you see all around you and
everywhere in the universe.
These 4 forces are the strong force which binds the nuclei of atoms by holding protons
and neutrons together in their center, the weak force which is responsible for some kinds
of radioactivity, electromagnetism responsible for electricity and light, and is also the
root cause of chemistry, and finally gravity which binds us to the earth and keeps the
planets in their orbits around the sun.
Yet, the most interesting aspect of this picture is that at a fundamental level, scientists
believe that all the forces come from one underlying force or principle.
And astonishingly, not only that but according to current understanding, all 24 of the different
fundamental particles and the 4 forces are one and the same at some deep level. This
is the ultimate symmetry of the universe.
Perhaps the best way to understand this and how these forces emerged is to visualize what
happened at the very beginning of time, at the big bang, when everything was one.
Time began not at zero, but at the smallest measurement of time that can be represented
by our models of quantum mechanics and that is Planck time. This is not zero but pretty
close from our perspective, 10^-43 seconds.
This is not to say that nothing existed prior to this, but it is just the limit of our knowledge.
We are ignorant of anything that might have occurred prior to this 1st epoch of existence.
So this is aptly called the Planck Epoch.
All the forces and particles were one and the same in a point smaller than the size
of a proton. Gravity is thought to have separated from everything else, shortly after this time
period. So it was the first force to separate out from the other 3 forces.
The temperatures at this stage of the universe were as you would expect, astronomical , 10^31
degrees Celsius, and the energies were in the range of 10^19 billion or giga electron
volts. If we ever develop a theory of everything, then it would explain everything that occurred
up to this time. The strings of string theory and the loops of loop quantum gravity, if
those theories are correct, come into existence and apply here.
The next era, called the Grand Unified Epoch, lasts from the first Planck second 10^-43
seconds up to about 10^-35 seconds. Up to this period, the remaining three forces, the
Strong and weak force and electromagnetism were all unified. But shortly after this period
from 10^-34 to about 10^-32 seconds, the strong force separated from the other two - electromagnetism
and the weak force, which were united as one force called the electroweak force. Temperatures
were now around 10^26 degrees Celsius. And the energy was reduced to 10^15 Giga electron
volts.
If we ever discover the grand unified theory or GUT, which unites the strong force with
the electroweak force, we would need to model what happens at these energy levels and temperatures.
Somehow this separation of the strong force is thought to have resulted in or powered
something called cosmic inflation. This is a momentary expansion of the universe which
went from something tinier than the size of a proton to the size of a grapefruit. This
is huge increase in size. It is analogous to something as small as a tennis ball becoming
the size of the solar system in a very small instant.
At 10^-12 seconds, called the Quark Epoch, the electroweak force split into the weak
force and electromagnetism. These two now became separate forces. So at this point,
all the 4 forces became distinct. The temperature of the universe cooled to 10^15 degrees Celcius,
and energies are about 100 GeV. We know a lot about the universe up to this era because
such energy levels can be easily modeled in particle accelerators such as the Large Hadron
Collider. So our understanding of the electroweak force is fairly robust. The Higgs field exists
at this stage – as well. How do we know? because about 100 GeV was needed to create
the Higgs boson. And this can be done in the LHC.
Now, fast forward to today, 13.8 billion years later, where the average
temperature of the universe is -270 degrees C, and energy is on the order of 0.25 eV
Let’s look closer at the 4 forces, and lets start with the two we are most familiar with
- gravity and electromagnetism.
They are similar in that their mathematical formulas look nearly identical. Newtons law
of universal gravitation can be expressed as the following, were G is
Newton’s gravitational constant. Coulomb’s law of electric force between charged bodies
can be expressed as the following, where k is Coulomb’s constant.
The first formula tell us that the gravitational force between any two bodies is proportional
to the product of their masses and inversely proportional to the distance between them
squared. Notice that force extends infinitely far. This means that the earth has a gravitational
effect not just on the moon and other planets, but also some effect on every other massive
object in the universe. Mind you, this is a very small effect because the r in the equation
would be very large for things very far away. So the force would be very weak, but it is non zero. And since gravity affects
anything with masses, this effect is the most influential force on a cosmic scale.
But almost everything I just said, also applies to electromagnetism. The electrostatic force
between charges extends also infinitely far away. This means that a charged particle on
earth has a non zero effect on a charged particle near Proxima Centauri, our nearest neighboring
star, and charged particles everywhere else in the universe. And this force is much much
greater than the force of gravity. In fact the magnitude of electromagnetism is 10^36
times greater than the magnitude of gravity.
So why isn’t electromagnetism the most dominant force in the universe? The reason is because
on large scales electric charges of large objects tend to cancel each other out. Large objects
tend to be neutral.
So since a charged particle can only affect other charged particles, this force is mostly
non-existent at large scales. If large things were not electrically neutral, this force
would completely dominate the universe. But Electromagnetism still has a big influence
in our world. It is responsible for the nature of light. And it is the main force responsible
for all the biochemistry taking place in our bodies and the rest of the earth. It is the
basis of all chemistry.
So now you have ask, if electro magnetism is so strong, then what keeps multiple protons,
each of which are positively charged, bound in the nucleus of atoms? According to Coulomb’s
law, if you do the calculations, the two protons in a Helium atom experience a force equal to about 20 lbs.
For something as small as atoms, this is a huge force. They should repel each other and
fly apart instantly. The reason that protons don't fly apart is because they
are kept glued together with a force that is even stronger than electromagnetism – one
hundred times stronger. And this force is creatively called the Strong nuclear force.
It is the strongest force in the universe.
However, it extends only as far as the width of a proton. Remarkably, it is very small
or non existent beyond this length. This force has been described as Velcro. When two protons
are really close, this force keeps them strongly bound together, but when they are further
apart than the width of a proton, it is about non existent.
-------- This force is not simply the opposite of electromagnetism though
because it is responsible for holding electrically neutral neutrons in the center of atoms as well.
Even though this is a force we can’t directly experience, the world has experienced its
effect. How?...because you see, the release of this force is the energy behind nuclear
bombs. The fission and fusion of atoms releases huge amounts of binding energy from the nucleus
of atoms, which results from the strong force. The binding energy of this force, is in fact
responsible for most of the mass of any object, not the Higgs Field as you might have thought.
And when this binding energy is released, the mass of the atom is converted to energy
in nuclear reactions
Then we get to the weak force. This is responsible for a type of radiation called beta radiation,
which is the emission of electrons, or its antimatter equivalent, positrons. One of
the most important processes in nature is the beta decay of a neutron. This happens
for example when a neutron decays into a proton, an electron and an antineutrino. If this kind
of decay did not occur, then the universe would have been awash in a sea of neutrons,
and no atoms would have ever formed. And we would not have had life.
This process is also important in making larger atoms stable, because a similar decay, called
Beta plus decay a Proton is converted into a Neutron, a positron, and an electron neutrino.
It results in positron emissions. This process allows for nuclear stability.
Like the strong force this force is also only effective at really small scales, but the
range is even shorter than the strong force. It’s effective length is only about one
thousandth the diameter of a proton.
Now, I’ve tried to give you an overall perspective on the 4 fundamental forces. But if you’re
like me, you probably have a few questions. First, you might ask, how exactly does a force
between particles work? What is the underlying mechanism? What causes an attraction or repulsion?
And, why does electromagnetism and gravity have infinite range, but the strong and weak
force have such a small range.
The answer to these questions are closely related, and they get into some of the most
cutting edge and complex science involving quantum mechanics and particle physics. And
this will be the subject of my next video, so stay tuned. You won’t want to miss it.
In the meantime, I’d like the thank the sponsor of today’s video The Great Courses
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And if you enjoy subjects like this, then check out some of our other videos. If you
have a question, post it below, because I try to answer all of them. I will see you
in the next video my friend.