What is quantum mechanics explained You are currently facing one of the
most important equations of all time. It is called the Schrödinger wave equation.
Let me explain why it is so beautiful. First of all, as you can
see, it is really simple. Usually, in the Physics world, simple and elegant formulas are the most important ones.
Even Einstein had this strong belief: that the world and the universe could have
been described by means of some...cute formulas. A well-built theory is usually
visually good in terms of equations. As Einstein himself said:
“It can scarcely be denied that the supreme goal of all theory is to make the
irreducible basic elements as simple and as few as possible without having to surrender the adequate
representation of a single datum of experience.” Just think about it.
It's like when you want to express your emotions. Sometimes you don't think too much about them,
your words come out of your mouth and you produce a messy cascade of words. Your interlocutor can
still understand what you are saying, but it is kinda difficult to follow your thoughts.
But if you write your thoughts on paper, you will probably find a better way to express them.
A more efficient and concise way. This also holds for physics theories. We could have good ones, but
some of them are just more elegant than others. Of course, this is a hard job, and
our scientific inability to simplify is something we should always accept. But besides its formal beauty, the
Schrödinger equation tells us something more. It is the starting point for the understanding
of quantum mechanics. What is it? Follow me in this video: I will
explain it to you, keeping it simple. I could start and end this video by saying that
quantum mechanics is the study of very small things, but you would be very disappointed then.
So I will try to tell you something more. First of all, for “very small things” we mean
we are interested in stuff that exists in the real world, but has atomic-scale dimensions. We
are talking about atoms and subatomic particles. So we can say quantum mechanics deals
with the atomic and subatomic world. And if you take a lot of particles,
you have the macroscopic world. The macroscopic one, is the
world you, me, we are used to. In everyday life, we have to
deal with macroscopic objects. Your Moka is a macroscopic object. However,
it is made of atomic and subatomic particles. Now.
The macroscopic world is well described by Classical Physics, for
example, Newton gave us some laws that fit well what we observe in the everyday life.
Classical physics can help us understand why the Earth orbits around the sun, why do we
have seasons, how planes fly and much more. So it is really useful.
But at a certain point, at the end of the 19th and the
beginning of the 20th century, scientists realized that something was missing.
When they decided to study the atomic and subatomic world, they found classical
physics didn't work anymore. They needed another approach. This was
a huge issue, a problem that needed to be solved. In fact, Physics is no more
physics if it can't describe reality. The desire to resolve inconsistencies between
observed phenomena and classical theory led to two major revolutions in physics
that created a shift in the original scientific paradigm: the theory of relativity
and the development of quantum mechanics. The most important result is that light
behaves in some aspects like particles, and in others like waves. I know
what you're thinking: no way! And that's pretty the same thing physicians were thinking when they first approached the
undiscovered world of quantum mechanics. Let me explain it better.
Matter, the “stuff” of the universe, consists of particles such as electrons and atoms.
But it also exhibits wavelike behaviour. This phenomenon has been verified
not only for elementary particles but also for compound particles like atoms and
even molecules. For macroscopic particles, because of their extremely short wavelengths,
wave properties usually cannot be detected. Although the use of wave-particle
duality has worked well in physics, the meaning or interpretation has not been
satisfactorily resolved. Bohr called it the "duality paradox" and regarded it as a fundamental
or metaphysical fact of nature. A given kind of quantum object will exhibit sometimes wave,
sometimes a particle, character, in respectively different physical settings. He saw such duality
as one aspect of the concept of complementarity. Talking about this wave-particle
duality, Einstein said: “It seems as though we must use sometimes the one
theory and sometimes the other, while at times we may use either. We are faced with a new kind of
difficulty. We have two contradictory pictures of reality; separately neither of them fully explains
the phenomena of light, but together they do.” One of the most famous experiments that
allowed scientists to understand the dual nature of matter was the double-slit experiment.
It demonstrates, with unparalleled strangeness, that little particles of matter
have something of a wave about them, and suggests that the very act of observing a
particle has a dramatic effect on its behaviour. To start off, imagine a wall with two slits in it.
Imagine throwing tennis balls at the wall. Some will bounce off the wall, but some will travel
through the slits. If there's another wall behind the first, the tennis balls that have travelled
through the slits will hit it. If you mark all the spots where a ball has hit the second wall, what
do you expect to see? That's right. Two strips of marks roughly the same shape as the slits.
In the image below, the first wall is shown from the top, and the second
wall is shown from the front. Now imagine a light at a wall with two
slits. As the wave passes through both slits, it essentially splits into two new
waves, each spreading out from the slits. These two waves interact with each other, and
they are said to interfere with each other. The interference could be disruptive
or constructive, and in the first case, they will cancel each other out. I the
second case, they will reinforce each other, giving spots with the brightest lights.
So when the light meets a second wall placed behind the first, you will see a stripy
pattern, called an interference pattern. Now, if you do the same thing with a beam
of electrons, you would expect to see two rectangular strips on the second
wall, as with the tennis balls, because they are particles.
But what you actually see is that the spots where electrons hit replicate
the interference pattern from a wave. As you can see, this experiment suggests that
what we call "particles", such as electrons, somehow combine characteristics of
particles and characteristics of waves. And this is the real essence of the quantum world. Basically, everything can be described or
associated with a so-called wave function. In physics, we usually indicate a wave
function by means of the Greek letter “psi”: ψ And if you remember, the psi function
appears in Schrödinger's equation. In quantum mechanics, the Schrödinger equation
is a fundamental equation that determines the temporal evolution of the state of a system, for
example of a particle, an atom or a molecule. When it comes to quantum mechanics,
intuition is no more present. For example, if you hold a ball you will
notice it has some mass because of its weight. You can feel its weight, and if you take something
heavier you will notice. It is just something very intuitive, and the Classical Physics
world is built upon this kind of intuition. However, when it comes to quantum physics,
things get more complex, and we soon realize we can't predict exactly what is going to
happen to the motion of a ball, for example. Or we can't really tell, for example, if a cat is
black or white. One has to imagine reality as a set of possible configurations, and you don't
know a priori which configuration is going to be chosen. We can tell that there is a certain
probability associated with each configuration. Only when we make measurements, we
can see the chosen configuration, which we call state. Let's go back to the
cat. Let's suppose you want to pet him. We have a cat, but we don't know if it
is black or white or whatever colour. Quantum mechanics essentially states that the
only way for us to know which colour the cat is, is to “measure” it, and we do so by applying some
mathematical objects to the “state” of the cat, which is called “operators”: you have a wave
function, a state (which in this case is the colour of the cat), you apply an operator,
you get the result: black, white or whatever. If you repeat the experiment a high
number of times, you will end up with a distribution probability of colours, and this
distribution will have a peak on one colour, and that will indeed be the colour
of the cat you are want to pet. One of the requirements of quantum mechanics is
of course the description of macroscopic reality. In the “big number of atoms” limit, quantum
mechanics should reproduce the macroscopic world. That is, putting in the crassest possible
terms, if you see the cat is white, quantum operators applied to the colour configuration
should mainly give the “white” result. Many aspects of quantum mechanics are
counterintuitive and can seem paradoxical because they describe behaviour quite
different from that seen at larger scales. Do you know what quantum physicists
Richard Feynman said about quantum physics? "Before finding out the answer to this question,
be sure to like or dislike the video so that we can continue to improve and make these videos
better for you the viewer. Plus, be sure to subscribe to the channel clicking the bell so
that you don't miss ANY of our weekly videos." Feynman said quantum physics deals
with "nature as she is – absurd". For example, the uncertainty principle of quantum
mechanics means that the more closely one pins down one measurement (such as the position
of a particle), the less accurate another complementary measurement pertaining to the
same particle (such as its speed) must become. If you know very well the position of a particle, you will lose a lot of
information about its speed. And vice-versa, you know very well the velocity
of a particle, you will keep losing track of it. Another example is entanglement, in which a
measurement of any two-valued state of a particle (such as light polarized up or down) made
on either of two "entangled" particles that are very far apart causes a subsequent
measurement on the other particle to always be the other of the two values (such as
polarized in the opposite direction). Quantum mechanics teaches us an
important thing about nature: its description is essentially probabilistic.
Before quantum mechanics, we were used to thinking that the world is governed by some given laws
that give precise results. You act, you get precise results, and every action is followed
by a reaction that seems to be predictable. Instead, nature doesn't work like that.
The probability of an event—for example, where on the screen a particle shows
up in the double-slit experiment—is related to the square of the absolute value
of the amplitude of its wave function. We can just say, for example, that we have an
80% of chance that we'll find the particle in a given position interval, but we will know
where it is only when we will measure it. Another important thing quantum
mechanics tells us is that is not possible to know the values of all of the
properties of a system at the same time. This is of course due to the uncertainty
principle we discussed before. Last but not least, we know that quantum
mechanics is a good theory because, even if it studies the “very small things”, it closely approximates the classical description
of nature in the case of large systems. Well, I guess that's all for today. "This video ends here! Thanks for watching
everyone! Do you like quantum mechanics? Let us know in the comment below!
See you next time on the channel!”