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Hi! So you want to know the basics of
quantum mechanics?
Great; you're in the right place!
What is quantum mechanics?
Quantum mechanics attempts to explain
the behaviour of subatomic particles
at the nanoscopic level; it is one of the
most successful branches of physics
and there are countless examples of 
scientific experiments
confirming predictions made by the laws of quantum mechanics.
What is a particle?
One of the particles you will be most
familiar with
will be the electron. These orbit the nucleus of atoms
and the nucleus is made up of two
other particles; protons and neutrons.
The electron is an elementary particle,
one of the fundamental
constituents of the universe.
Scientists have found that protons and
neutrons are made up of other particles
called quarks. Quarks are also
elementary particles. Quarks are held
together by gluons. Gluons are also

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particles but they are different from
the quarks
they are the particles that produce the
strong force.
that holds the quarks together. Gluons are
also behind a strong force which holds
the protons and neutrons together
within the nuclei of the atom. Gluons
do this by mediating the strong force
between the quarks or between the
protons and neutrons
as the case may be. The types of
particles like electrons and quarks we
can think of
as creating matter and the types of
particles like gluons
we can think of as creating the forces.
There are four forces that we know of, the
strong force, the electromagnetic force
the weak force, and gravity.
The standard model elementary particles.
The standard model of
elementary particles describes how all
the elementary particles and forces
in the universe behave, apart from
gravity. How gravity works in the quantum
realm is actually not very well understood.
We have the matter particles of fermions
which are divided

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into quarks and leptons. There are six
quarks;
up, down, charm, strange, top and bottom.
There are six leptons;
electron, muon, tau, electron neutrino,
muon neutrino and tau neutrino. We then have
the force carrier particles, otherwise
known as gauge bosons.
The gluons we have discussed already, the
photons are particles of light
and carry the electromagnetic force,
which holds the electrons in atoms.
The W and Z bosons carry the weak
nuclear force
which is involved in some forms of radioactivity and
plays a role in how the sun burns.
Owing to the discovery of the Higgs boson
in 2012
that particular particle has achieved
something of a celebrity status.
Countless Higgs Bosons make up the Higgs field
and it is the interaction with this field
that gives other particles their mass. For
example, the top quark is interacting with
the Higgs field and this is what is giving
the top quark it's mass.

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And the final boson on we turn our
attention to
is the graviton, which is a hypothetical
particle
that mediates the gravitational force.
Quantum leap
In the early 19th century it was
theorized
that at the sub-atomic level, energy can
only be released
and absorbed in discreet indivisible units
called quanta.
This means electrons have fixed orbits around the nucleus of the atom
as their energy comes in discrete amounts.
When the election gets excited
or de-excited
they will absorb or emit a specific quanta of
energy which will mean they leap from
one orbit
to another with out inhabiting the space
in between,
this is called the quantum leap. In
essence,
there are places within the atom that the
electron will be likely to be
and other places where they won't, as
energy is being absorbed
and released in discreet units.
Particles
behave like waves
There's a famous experiment in quantum
physics called
the Double slit experiment which exposed something
about particles

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that still surprises us today; particles
display both particle-like
and wave-like behaviour.
In a version of the experiment on a
larger scale we have a gun that shoots
tennis balls one by one at a detector,
which will register where the tennis balls land.
In between the gun and detector we place
a barrier with two slits
which leave openings for any tennis
balls to go through.
Over a period of time and after many
tennis balls a pattern emerges
on the detector indicating where the
tennis balls have landed,
the results show that the balls which
have not been blocked have landed
directly behind the slits, in the barrier.
If we replicate this
experiment but
on the subatomic scale and use electrons
instead of tennis balls
we expect similar results but this was
not the case.
Scientists found that when the gun
shoots electrons one by one
toward the detector and past the double
slit barrier the pattern that emerged on
the detector look like this:
The electrons landed not
just behind the two slits

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but in narrow strips
 across the length of the detector
and a significant number of electrons
even landed straight behind the middle
of the barrier.
The pattern that emerged is an interference pattern
and is associated with the behaviour
of waves.
Imagine we had two waves that interact,
the peak of the waves
will combine to form a higher peak and
troughs will combine
to create a deeper trough. And when a peak and
trough meet
they will cancel each other out. So if we
imagine that are electrons
are less tennis ball-like
and more wave-like, then what happens?
This is a wave from above
the black lines represent a peak and the
spaces in between
are the troughs. When an electron goes
through the double slit
it's wave is split in two and these waves
then interact.
The peaks meet here and reinforce each
other creating higher peaks
and the troughs meet here also reinforcing
each other creating deeper troughs.
And here a peak and trough meet
cancelling each other out.
The interaction between the waves results
in the interference pattern

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at the detector screen. Where the waves
are most intense we find more of the
electrons on the detector screen
and where they cancel each other out
there are no electrons on the detector screen.
Erwin Schroedinger came up with an
equation for the electron's wave function,
and using this equation we can find out
the probability of the electron being
in a particular location. Think of the wave
as a bundle of probabilities and the
size of the wave in any location
predicts the likelihood
that the electron will be found there
if it is looked for. That is why on the
detector screen we observe most of the
electrons landing in the places where
the electrons wave is at it's most intense.
It seems as if the electron is not in a
fixed position but has different
probabilities of being in many
different places at once.
The act of measurement
To observe the electron's wave function
going through both
slits at the same time, detectors were placed
next to the slits
to capture this activity. But when this
was done something strange happened,

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the electrons stopped behaving like waves and
went through one or other of the slits
and landed on the detector screen to
form the two-striped pattern
rather than the interference pattern. It seemed as
if the act of measuring did something to
collapse the wave function
The superposition principle.
The superposition principle, states that while we do not measure the electron for it's position
it is in all the possible positions it
could be in, at the same time
and when we observe it the superposition
collapses.
So in this illustration, when our detector
is off,
our electron is in all of the possible
positions or states
it could be in simultaneously but when
we switch our detector
on, the superposition collapses and
electron gives up
all of it's possible states to choose just
the one,
that is why we were not able to observe
the elections wave function going
through the double slits.
The very act of attempting to observe,
made the electron's wave function collapse.
The electron gave up it's superposition and
chose just the one state to be in.
i.e, the electrons actually reverted back to
particle-like behavior

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and that is why instead of going through both
of the slits, the electrons that landed
on the back detector
chose to go to through either one 
or the other of the slits.
Let's use one of the most famous thought
experiments to further illustrate what
quantum mechanics is saying about the
electron and the superposition,
we turn our attention to Schrodinger's cat.
Schrodinger's cat.
Erwin Schrodinger's described a thought
experiment where  a cat
was placed in a covered box with a
radioactive sample that has a 50
percent chance of decaying and killing
the cat.
While the box is covered we have no idea
if the cat is dead or alive
and only once we open the box will we
know if the cat made it or not.
So if the cat were similar to an electron,
using the superposition principle
we would say that while the box was
covered and the cat was not being observed
the cat was both alive and dead at
the same time in order for it to be in
all the states it could possibly be in.
Only when we lifted the cover to observe
the cat, did it's superposition collapse
for it to be either alive or dead.

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Obviously this feels intuitively wrong
and the reason why this thought experiment
was employed, was to illustrate how
odd the laws of quantum mechanics are when
it comes to
describing the behaviour of particles.
However countless experiments have
corroborated the results predicted by
quantum mechanics.
It does indeed seem as if particles have
a wavefunction
and that particles are in all the states
it could possibly be in simultaneously
until it is observed.
But why do we see
evidence
of wave-like behaviour from particles and
not cats, after all
cats are made up of particles. Well, the
reason is, the bigger the object the
smaller it's wavelength
and at the size of a cat, the wave is simply
too small to be detected.
Time traveling electrons
Another way scientists have tried
to observe the electron's superposition
in the Double slit experiment, was to
observe them after they had passed
through the slits,
so they placed detectors in between the
barrier and the detector at the back.
The reasoning was that as the electrons
passed through the barrier
their waves would have split as there was
no measuring happening

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while they went through and then when
they were measured past the double slit
the electrons will be found across the
length of the area
behind the double slit barrier
and in correspondence with the final
interference pattern.
But that didn't happen, in fact when they
did the experiment
the electrons were found only in the
areas directly behind the slits
and no interference pattern emerged on
the back detector.
Stranger still when the detectors were
switched off
the interference pattern returned.
When scientists weren't looking
for the electrons
they continued to exhibit their
wave-like properties and land on the
back detector to produce the
interference pattern
but when the electrons were observed
they collapsed back to a particle
and eliminated all evidence of the
superposition by not being detected
anywhere but behind the slits. So
the scientists attempted to outsmart the
electrons and decided not to have the detectors
switched on
until after some of the electrons had 
passed through the slits.
But even when they did that electrons somehow
knew that the
scientists were going to switch the

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detectors on, and the superposition collapsed.
There was no interference pattern
and electrons
were only found behind the slits.
Every time the scientists decided to
leave the detectors off
for the whole of the experiment the
interference pattern emerged.
And every time the scientists decided to
turn the detectors on
midway through the experiment
the interference pattern no longer
appeared. So it was almost as if the electrons
were going through the slits
with their wave function intact and as
soon as they realised
that the detectors were going to be switched on
they went back in time, reverted back
to particle like behavior
and erased all evidence of the
superposition.
So what's actually going on here? Are
the electrons actually time traveling?
Well, quantum mechanics says that the
electrons did not go back in time but
that they did in fact go through both of the slits
at the same time, but that as soon as the
detectors were switched on
the electrons collapsed their wave functions,
choosing to be in line with one or other
of the slits,
and erasing all evidence of their
superposition.

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The Many Worlds theory
So now we know
that measuring appears to have an effect
on the behavior of particles and there
have been many theories to explain this,
a few notable ones are the Spontaneous
Collapse Theory,
Bohemian Mechanics or Pilot Wave Theory
and the Many Worlds theory. Of all the
theories the one that seems to have
captured popular imagination
is unsurprisingly the Many Worlds theory so  
we will touch upon that briefly.
The Many Worlds theory states
that anything that can happen does happen.
Before we attempt to measure
the electron quantum mechanics says
it is in many states at once and act of
measuring collapses it
to only the one state. The Many Worlds theory
states that in fact when the
measurement happens the electrons will collapse
to all of the states
but different worlds will observe
different results, so in fact instead
one world there are many different worlds
and in world A you may see the
electron turn up in position A but in world B
your clone will see an electron turn up
in position B and so on and so forth.
The implications of this are
mind-boggling;

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every time there is one or more possible 
option available for the universe
the universe splits and creates copies
were all possible outcomes come true.
So for example there could be a world
where Hitler won World War 2,
a world where mankind never made it to
the moon and a world where penicillin
was never discovered.
But there could theoretically be worlds
that are much more advanced than us,
a world where we have glimpsed further
and deeper into the workings of the universe
and have been able to achieve
immortality; if that was the case
then every person has achieved quantum
immortality in one alternate reality or another.
Introducing quantum
entanglement
Quantum entanglement occurs when two
particles become connected
in such a way that when the property of
one particle is changed an
instantaneous change in the property of the other particle occurs.
Entangled particles have the opposite properties
or states. Particles have a property
called spin

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and the particle will either be spin up
or spin down in any given direction.
When two particles are entangled and
their spins measured in the same direction
one particle will be spin up and the other
particle will be spin down.
According to quantum mechanics a pair of entangled particles could be separated
by an entire universe
and when the state of one is measured and it's
superposition collapsed
it would immediately collapse the
superposition of the other particle
and measurement would indicate that it
had the opposite state of it's entangled partner.
So in this example particle A and B are
entangled
and separated by an entire universe. They are
not measured and have the superposition
of being both spin up and spin down at the
same time.
When particle A is measured it's
superposition is collapsed
and it indicates spin up and as particle B
is entangled with particle A
it's superposition is also collapsed and
it indicates spin down.
Quantum mechanics seems to say that entangled
particles can communicate with
a speed faster than the speed of light,
something that Einstein's theory of
special relativity ruled out.

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Einstein called this phenomena
Spooky Action at a Distance.
Einstein was not impressed by the
explanation given by quantum mechanics
to describe the behaviour of entangled particles
and he actually offered up his own
alternative theory;
he said that when the particles were
entangled it was decided what states they
would each have when measured in any
given direction
and there was no communication happening
between the two particles
when they were measured, after they had been separated.
According to Einstein the particles
didn't have some mysterious superposition
of being both spin up and spin down
until measured
but rather at the point of entanglement
it was decided what spin one would
have and what spin the other would have
in any direction it was measured.
Einstein's explanation versus Quantum mechanics explanation
Professor John Bell actually came up
with an experimental way to test whether
quantum mechanics
or Einstein's more classical explanation
worked when it came to explaining the
results entangled particles gave when
measured for their spins.

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What emerged was that Einstein's
explanation broke down
and predictions made by quantum
mechanics were consistent with experimental results
The experiment proved that the spin of
the particles were not defined specifically
in all directions of measurement when
the particles were entangled
as Einstein had theorized.
Introducing
the quantum tunneling
In a phenomena called quantum tunneling
in has been observed
that particles have the ability to cross barriers
they shouldn't be able to, the reason for
this is down
to the particle's wavefunction.
Let's use an example, this particle here
shouldn't have the energy to be able to
cross this barrier,
but it can and that's down to it's wave
function.
Even though the probability of finding
the particle undecided the barrier is low
it is not zero and the particle can indeed
tunnel its way to the other side of the barrier.
Quantum tunneling is responsible for
nuclear fusion in our sun.
In the Sun hydrogen atoms fuse together
to create helium and other heavier elements,
releasing huge amounts of energy in the
process.

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The hydrogen nuclei consist of positively charged protons
and as like charges repel each other
fusion only occurs when a huge amount of
heat is applied to overcome this barrier
and force the protons together, however
the Sun is not hot enough to give the
protons the energy required to overcome
their repulsion,
the reason why the proteins can overcome
the repulsion between their positive charges
is down to quantum tunneling.
Owing to the proton's wave fucntion there is a small probability
that some of them will tunnel across the
barrier and fuse with the other proton
creating heavier elements and thus fueling the Sun.
Why do we accept
quantum mechanics?
The predictions of quantum mechanics
have proved so
reliable that we cannot ignore the
experimental evidence to corroborate the theory.
The entire electronics industry is built
on using quantum theory,
those principles have led to the
invention of lasers,
transistors and the integrated circuit.
The Future of Quantum mechanics - the Quantum Computer

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The first quantum
computers are currently being developed
harnessing a particles ability to be
in many states at once
means that multiple processes can be
executed simultaneously
increasing our computing power
exponentially. Our lives have already
been changed dramatically by technology
and when the power of quantum computers
is realized it will herald the dawn of a
new era in technological advancement
The future of Quantum mechanics -Teleportation
Using quantum entanglement scientists
have been able to teleport particles.
Two particles are entangled and
separated by large distance,
a third particle particle T is brought
in and this is the particle that we want
to teleport.
This particle T interacts with particle A
and we learn how the quantum state of
particle T relates to particle A. This information is
then sent across
to where particle B is kept. As particle A
and B are entangled
this information about how the quantum state of Particle T

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relates to particle A will also reveal
how the quantum state of particle T
relates to particle B. Particle B will
then be manipulated to replicate the
quantum state of particle T,
becoming an exact copy of particle T.
Meanwhile
the original particle T is destroyed as
its information was extracted,
and sent across.
This method of teleportation has only
ever been done on particles,
and a single human being contains a huge
amount of particles
and that would be mean an immense amount of data would need to be transferred
for human teleportation.
Transferring this amount of data using the
means we have today would take
upwards of a quadrillion years. At
the same time
it opens up a philosophical debate of
whether the teleported particle T is actually
the original particle T or just a
precise copy
of the original particle T. Imagine that
human quantum teleportation
became a reality, say Alice wants to
travel from London to Tokyo.
Two chambers of entangled particles
would be in each city.

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Alice would step into a scanning
device and the quantum state of each
individual particle
would be measured in relation to the
chamber of particles in London.
This information will be relayed to Tokyo
where the chamber of particles would
be manipulated to replicate the quantum
state of each of Alice's individual particles,
creating an exact replica. Meanwhile the
original Alice
in London has been destroyed. The
question is, as the original Alice dies
in the process of teleportation
but is then re-constructed at her
destination in Tokyo,
is that still Alice?
Tokyo Alice is identical to the last
atom
to the London Alice, and is in full
possession of London Alice's knowledge
experience and memories; and Tokyo
Alice even believes
she is the London Alice while the
original London Alice
no longer exists to debate the issue
with her. It's a philosophical
and ethical dilemma, but it may not be one that
we will need to face,
human teleportation would be extremely
difficult,

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so though it is not impossible it is
improbable that it would ever become a reality.
Quantum mechanics and General Relativity
incompatibility
Quantum mechanics and Einstein's General
theory of Relativity
are two of the most successful theories in
physics,
but they're incompatible. Quantum
mechanics describes
space and time as being quantized
whereas general relativity
describes space and time as a smooth
continuum.
String theory attempts to resolve the
incompatibility,
and is an attempt to find the elusive
theory of everything
that explains all the matter and forces
in our universe;
that in itself is a whole other video.
This video has attempted to explain quantum mechanics
for the layperson, who wants to know what
it's all about.
It is a simplified explanation,
and can be used as a springboard for
further study.
As always be hope you find this video
enjoyable and informative.
Make sure to subscribe to LondonCityGirl
for more interesting videos,
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