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More than 2,000 years ago the Greek philosopher Democritus
first coined the term atom, meaning, "uncuttable," something you can’t split into smaller pieces.
If you take a piece of metal, and cut it half, then cut it half again,
and do this over and over again, he figured there must be a smallest piece of metal that
cannot be cut any further. And so it must be the fundamental particle of the material – the atom.
This search of a deeper understanding of the fundamental nature of matter,
has brought mankind to the physics of today. And we now have an understanding,
that the ancient Greeks could not have even dreamed about.
So what do we know now that Democritus didn’t know? After 2000 years, we have today the
standard model of particle physics which is like the periodic table of fundamental particles.
These are the particles that almost everything in the universe reduces to.
But the question is what really are fundamental particles? Can we say what they are made of?
The answer, which may surprise you…is coming up right now…
If you ask 10 physicists what a particle is, it would not be
surprising to hear 10 different answers. In my previous video, I talked about how
light comes in discrete packets of energy called photons. We learned that these photons,
act as both a particle and a wave. This is the wave-particle duality of nature.
Quantum mechanics revealed, via the Schrodinger equation, that all quantum objects should not be
considered particles but more like waves smeared out in space. They are described by a mathematical
term called a wave-function. This function describes the state of the object. It doesn’t tell
us where a particle is, but only the probability of where it might be if we measured it.
The wave function tells us that what we call a particle is spread out over space…until
the moment we measure it. And as soon as we do, the wave function representing the particle
suddenly collapses, so that is not spread out any longer, but is localized around a distinct point.
The double slit experiment is the classical example of this collapse. If we send one photon
at a time through a double slit thousands of times, we see a wave patten, but if we stick
a detector at the slit to measure which slit the photon is going through, we get rid of the wave
pattern. The wavefunction has collapsed. Why does the wave function – which
is a mathematical expression describing the probabilities
of a particle, suddenly collapse upon measurement? Why or how this happens is not well understood and
is known as the "measurement problem of physics." So we have something that could best be described
as a spread-out wave, prior to measurement, but after measurement we get a delta function,
which has a spike like a localized wave. This is something resembling more like a particle.
So is a particle just a collapsed wave function? Perhaps, but this leaves us with more questions
than answers because we don’t really even know what wave function collapse means.
As you might have surmized, it’s not that simple to answer the question of what a particle is.
And if you ask a physicist, he might just go straight into the math
describing a particle, and the answer he may give you could be purely in mathematical terms.
But the world we are in is physical. Math is just a tool used to describe this physical reality.
If pushed for something more concrete, some physicists will just tell you that particles
are simply those object that we detect. But these answers are deeply unsatisfying.
So let’s not do that and instead let’s look at what our current best description is, based on what we know.
What we have learned about particles from quantum mechanics, is that
we can consider photons in terms of a wave packet. This wave packet can be looked at as
a kind of particle. And something similar can be done for describing other particles as well.
But their wave aspect smears out their properties like position and momentum.
The Schrodinger equation, works quite well in describing the wave nature of quantum objects,
but it has some limts. It does not account for special relativity. This happens specifically
when velocity is close to the speed of light. In order to account for relativistic effects,
space and time have to be treated equally. But If you look at the Schrodinger equation, you will see
that time and space are not treated equally. In the 1930’s scientists realized that
one could get an even better formulation of quantum mechanics than what physicists
like Schrödinger achieved, by considering quantum objects in terms of quantized fields.
When this is done, the equations of quantum mechanics could be modified to account for
special relativity by treating space and time equally. This led to quantum field theory.
In quantum field theory, what we call particles are quantized waves
in a field that spreads throughout spacetime. The idea of quantized fields was a necessary step
in order to describe objects like photons and other fast moving objects that demand to be
treated according to the rules of special relativity which starts to have significant
effects at, or close to the speed of light. Quantum field theory is nothing but a theoretical
framework that unites ideas from classical fields, special relativity and quantum mechanics.
The idea is like this: you start with a field or analogously picture yourself
a calm sea. This is an empty field without any particles. This is our starting point.
It may sound a bit weird, but stay with me here. Imagine that this field or
sea stretches throughout all of spacetime. Note that this would be in three dimensions, but for
visualization purposes, we are representing this in two dimensions, like the surface of the ocean.
Now imagine that there is a wave in the sea, this is a particle in our field. Then there can be
another wave, which represents another particle. In the field, each particle is an excitation
of the field, just like a wave in the sea. Because we are considering our particles as waves,
if they collide, then the particles collide like waves just as in the double slit experiment.
There is however at least one difference between the quantum field and our ocean,
our ocean is not quantum. For our ocean field to be quantum
we have two requirements. First, the waves in our sea must have some discrete magnitude,
because in quantum mechanics things have discrete quantizations. This can be represented
by the amplitude or height of the wave. Let’s say the wave can be 1 meter, 2 meters,
3 meters and so on. Each meter corresponds to the number of particles in any one place.
But there is second requirement too. A real quantum field is never calm. It always has
some minimum energy state. So we will say that the 1-meter waves represent this minimum energy state
of the field. This minimum energy of 1 meter is called the vacuum energy. It means that for a
real quantum field empty doesn’t mean nothing. There is still something - some minimum energy present.
Then each extra meter of amplitude or height is a real particle.
Now the interesting thing about quantum waves is that we can only create taller waves in increments
of whole meters. So if we don’t have enough energy to add at least one meter to the wave,
then we won’t have any change to the waves at all. In other words, there can be no 1.5 meter,
or 1.75, or even 1.9 meter waves. Only 2 meters, 3 meters, 4 meters, etc.
So, our quantum field is full of these minimum 1-meter waves, and each extra meter is then a
real particle. But we can only create these taller waves if we have enough energy to reach the next
meter. If there is not enough energy to reach at least 2 meters, no taller wave is created.
These one meter waves are from the quantum vacuum, they represent the non-empty minimal state.
These quantum waves slosh around and, in some places, there might be momentarily enough energy
to create a larger 2-meter wave or a particle. But then almost just as quickly as it’s formed,
the particle vanishes again in the sea of fluctuations. These are analogous
to virtual particles that come in and out of existence all around us, but are undetectable
because they last for too short a time. Since virtual particles cannot be directly observed, they
only exist mathematically. However, they do result in other effects which can be detected, namely the
Casimir effect, which happens due to a pressure difference caused by the virtual particles
between the inside of two plates and outside of the plates. This is a unique effect supporting the
idea that the universe consists of quantized fields, rather than just quantum mechanics.
The quantum field is like the sea – it is the background on which waves appear and
disappear. And just like energy can create waves in the sea, energy added to the field
generates particles which we can observe. So in quantum field theory,
we have a field that is never still, never empty. And we can create a particle if we have the exact
amount of energy needed. But still you should keep in mind that this is just a mathematical
construct, that appears to fit reality. If you expand this concept to other particles,
we have to imagine other seas on top of seas, representing a different field for each fundamental
particle that we know of. So there would be a sea representing the field for electrons, for example, a
field for photons, a field for quarks, etc. This concept led us to where we are now.
We have this general theory called the Standard Model in which we include
all known fundamental particles, and we assign individual fields to each type of particle.
Now, what does “fundamental” in the word fundamental particles mean?
From a mathematical point of view, it means that the particles of the standard model
are the smallest particles from which we can construct the world around us.
There is nothing smaller, that we know of, that these fundamental particles are made of.
This brings the circle back to Democritus, and the search
for the minimal unit or particle of the universe. That’s what we mean by fundamental. They are the
most fundamental particles, at least for now. You have to remember though that at one time,
we considered the atom to be the smallest fundamental particle. Then we discovered
the electron, and proton, and later even smaller particles like quarks that the proton is made of.
So maybe we shouldn’t be so confident that we have really reached the ultimate fundamental level.
What’s more, the word fundamental really doesn’t tell a whole lot about the particle. In fact,
we can’t even directly detect all these particles. For example, quarks can never
be observed as free particles. So, these cannot be confirmed with direct observation. And for
the particles we can observe, what we really see is an energy signal in our detectors – not the
particle itself. So should the definition of a particle be “an energy signal in our detectors?”
Perhaps. But I think we can do a bit better than that. As of now, the best description
would be something like this: A fundamental particle is an excitation in
a quantum field that is constantly in flux. There are at least as many fields as there are particles
in the standard model. Each particle can propagate in its field. The interactions of
these fields and exchange of energy results in particle creation and annihilation.
We must respect the fact that in the end, this is just what the math is telling us. To really
be satisfied, and to get at the heart of physics, we need to be able to understand
the physical story behind the math. So in this sense, physics is like a CSI crime scene
where physicists are the protagonists examining all the evidence, finding little clues that are
sometimes hidden from view, or sometimes in plain sight but unrecognized, eventually building up to
the story that tells us what really happened. If you want to learn quantum physics in detail
from scratch, I think it’s important to start with the basics. While I try
to show you visually and intuitively how quantum mechanics works, to really understand
the details you have to get into the math. One of the best math courses available online
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Along your learning journey, you’ll be guided by interactive lessons that not only
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be sure to click the link in the description. And if you have a question, please leave it in
the comments, and I will do my very best to answer it. I’ll see you in the next video my friend.