What is Quantum Physics?
What is quantum physics?
Why should we care about knowing what is quantum physics?
Do we NEED to know what quantum physics is?
How does quantum physics affects our daily lives?
These questions, and many more, come to mind when one thinks of quantum physics.
Which questions should we try to answer first?
We will first settle the issue of know what we are talking about: quantum physics.
Understanding the story of the birth of quantum physics will help us understand what quantum
physics is.
Quantum physics is the physics of the microscopic world; the world of molecules, atoms, and
sub-atomic particles.
It is a branch of physics that was born in the early decades of the twentieth century,
with its seeds germinating in the late decades of the nineteenth century.
Quantum physics was born out of a mixture of failure to understand certain experiments,
and also out of non-sense results when applying classical physics to atoms.
Yes, physicists were already familiar with the notion that matter is composed of microscopic
packets, the atoms.
This notion was initially put forth by the Greek philosopher Democritus who postulated
that everything is built from atoms and the void.
Later on, chemists starting in the eighteenth century with English chemist John Dalton,
and after carefully studying chemical reactions and the proportions of reagents that go in
and the proportions of the resultant chemicals, correctly inferred that matter is built of
individual packets, which he called atoms!
Dmitri Mandeleev in the nineteenth century classified atoms by their chemical properties
and arranged them into the periodic table of elements that we are familiar with today.
We will see as go on with our story that the chemical properties of atoms are a direct
consequence of quantum physics!
Physicists knew that matter is overall neutral: made up of equal of positive and negative
charges.
So after verifying that matter is made up of atoms, experiments focused on understanding
the architecture of the atom.
Basically, understanding how the negative (electrons) and positive charges (protons)
are arranged in atoms.
Enter New Zealander physicist Ernest Rutherford in the first decade of the twentieth century
who shot alpha particles, helium atoms that that have lost their two electrons, onto thin
gold foil.
Rutherford observed that the alpha particles (positively charged) were deflected from positively
charged point particles, of much higher charge.
The picture became clear now: atoms are made up of dense positively charged nuclei surrounded
by orbiting negatively charged electrons.
All this is fine and nice and a source of confidence, so much so that many physicists
at the beginning of the twentieth century were predicting the death of physics!
Their opinion went something along these lines: "It is only a matter of a few years when we
know all that we need to know about the fundamental physics of Nature.
We know everything.
It is now just a matter of applying our physics to the world around us."
And that is exactly what they did, but nasty surprises awaited them!
Follow our Youtube channel, Insane Curiosity, to learn about more nasty surprises awaiting
scientists even today!
Just a decade before Rutherford conducted his experiments, German physicist Max Planck
was studying radiation emitted by the so-called blackbody.
In physics a blackbody is an object whose peak emission wavelength, or in other words
color, is related to its temperature.
Imagine a piece of iron that you throw in a fire.
As it gets hotter it first glows red (long wavelength), and when it gets even hotter
it glows even brighter but its color will become blue (short wavelength).
This piece of glowing piece of iron is referred to by physicists as blackbody.
What Planck was pondering was what happens if one were to increase the temperature of
a blackbody even further.
The relevant physics known at the time, thermodynamics, predicted that the blackbody should glow yet
brighter but in the ultra-violet.
The color of the blackbody should not only shift toward shorter and shorter wavelengths,
but its brightness should become increasing higher very fast, so fast that it reaches
infinity!
But this is non-sense.
How can heating an object, basically pumping a finite amount of energy into it, make it
emit an infinite amount of energy?
This is forbidden by the law of conservation of energy.
Planck's success was postulating that radiation, or light, is QUANTIZED in individual packets
of energy, called photons.
The energy carried by each photon is inversely proportional to the wavelength of the radiation,
with the constant of proportionality now called Planck's constant.
None other than THE Albert Einstein used this result to describe the photoelectric effect
experiment (another important experiment of quantum physics), for which he later won the
Nobel prize.
Other physicists in the early decades of the 20th century applied their physics known to
them at the time - classical mechanics and electromagnetism - to the simplest of atoms,
the hydrogen atom (one electron orbiting a single proton).
Let us put our physics hats on.
Classical mechanics says that an object orbiting a center must experience centrifugal acceleration.
Electromagnetism says that an accelerating charge must give off radiation (energy).
Now, let us put one and one together.
The electron, a charged particle, as it orbits the proton in the hydrogen atom, will experience
acceleration so it MUST radiate.
So far so good.
However, and here comes the nasty surprise, the radiation emitted by the electron will
steal away its energy making its orbit decrease and spiral toward the proton in a fraction
of a fraction of a fraction of a second!
So basically, classical physics predicts that NO atoms should exist! … and by consequence,
we too should not exit!
This non-sense result was a slap in the face of early 20th century physicists that all
not is well with physics, or at least there must be something over and beyond the classical
physics known till then: classical mechanics, thermodynamics, electromagnetism, and optics.
This new physics became known as quantum physics.
Its cornerstone is that all physics quantities that could be observed, the so-called observables,
are all quantized.
That is they come in individual, discrete units instead of taking on a continuum of
values, as in classical physics.
Faced with the above "atoms should not exist debacle," Danish physicist Neil Bohr proposed
in the second decade of the 20th century his toy model of the hydrogen atom.
He postulated that the existence of stationary orbits in which the electron, if it satisfies
a certain condition, does not radiate its energy away.
That condition is related to its orbital angular momentum and it dictates that the orbital
angular moment of the electron in this orbit must be a definite multiple of Planck's constant.
This toy model was able to explain to a high degree the spectrum of emission of hydrogen
atoms observed in the lab.
Such work of theory and experiment going hand-in-hand was the way in which quantum physics was built.
Moreover, quantum physics was a group effort in which nearly all physicists working at
the time contributed to the endeavor, some more than others, of course.
This kind work continued well into the 20th century in which quantum theory was fully
development and compared against experiment.
Of course, needless to say, experiment (which is science's truth-o-meter) agreed with the
predictions of quantum physics.
Quantum theory is the most successful theory in physics.
It has also spawned new fields: materials science (think smaller and more power computer
chips, LED TVs, solar photovoltaic cells, etc), nuclear physics (think nuclear power
and nuclear medicine), and particle physics (theory to describe the sub-atomic world and
related to how the Universe came to exists – see other Insane Curiosity videos to learn
more).
Quantum theory at the utilitarian level is very powerful and leads to practical results
that permeate our everyday lives.
However, problems with quantum theory sprung up from the very beginning.
These problems are at the interpretation level: how do you interpret the story told by the
equations.
Keep in mind that the equations themselves are correct and self-consistent; they give
specific, well-defined predictions that turn out to be correct when tested experimentally.
The problems arise when you try to understand the equations!
Neil Bohr remains to the this day one of the giants of quantum physics and the father of
the most intriguing interpretations of quantum physics.
An interpretation so bizarre but also undeniably true that it gave Albert Einstein many a headache.
This interpretation is known as the Copenhagen Interpretation.
In a nutshell, the Copenhagen interpretation stipulates that if I don't see you then you
don't exist!
At the fundamental level, it states that reality depends on the observer!
The role of the observer is paramount in quantum physics.
The observer and the act of measurement, such as "seeing," itself affects the outcome of
the experiment.
The reason for this is relatively easy to understand.
Imagine going into a dark room holding an unlit flashlight.
Turning the flashlight on will shoot photons from it to an object in the room, a chair
for example.
These photons bounce on the chair back to your eye and that is how you see the chair.
Now, the energy carried by the photon is very small compared to the energy that could move
the chair and the chair remains in place.
So the act of measurement here did NOT change our system, the chair.
Now imagine that instead of a flashlight, you have a gun that shoots golf balls at very
high speeds.
You go into the room and start spraying with fast golf balls.
One or two golf balls are probably going to hit the chair and may be also probably move
it too.
From the sound of the golf balls crashing on the chair you will let you know where the
chair WAS located, BUT now the new position of the chair becomes unknown.
Therefore, the act of measurement here changed the system.
Of course, in quantum physics, all systems are microscopic and the measurement tools
are also microscopic leading measurements to messing with the system.
The Copenhagen interpretation takes this notion to the max and says that the very act of measurement
makes a system exist!
One might be tempted to say this is just a interpretation and it need not be correct.
However, recent experiments done at top quantum optics labs strongly suggest that the Copenhagen
interpretation is correct.
A vary graphic illustration of the apparent absurdity of quantum physics and the Copenhagen
interpretation is the Schrodinger cat paradox.
In this famous experiment, named after Irish-Austrian physicist Ewrin Schrodiner, a cat is put in
a closed box with a sealed vial of poison.
A radioactive source might decay at random and emit a radioactive particle that would
break the seal thus killing the cat.
To an outside observer, the cat has 50-50 chance of being dead or alive.
The way this is expressed in quantum physics formulas, the so-called state function, will
assign equal weights to the cat being dead or alive.
A physicist can do all calculations required and compare to experiments and all will be
fine, but when the physicist tries to interpret the state function formula, all hell breaks
loose: the equation seems to be saying that the cat is dead and alive at the same time!
But remember, who determines the truth?
It is the act of observation itself: the measurement.
The observer will have to OPEN the box to determine whether the cat is alive or dead.
Another weird aspect of quantum physics is the so-called particle-wave duality, or the
Complementarity Principle.
In our macroscopic world, matter behaves like particles (the ball is here, the bike is there)
and light behaves like a wave (cause of the rainbow seen on a DVD's data surface).
In the microscopic world governed by quantum physics, an entity (matter or light) will
behave as a particle or a wave, depending on the experiment (more support to the Copenhagen
interpretation above).
French physicist Louis de Broglie, author of probably the shortest doctoral thesis in
physics in which he presented the smallest formula that would later win him a Nobel prize
in physics, proposed in that same thesis in the nineteen twenties that a particle with
a given value of linear momentum, basically mass multiplied by velocity, will have the
wavelength of its complementary wave inversely proportional to its momentum.
The constant of proportionality is Planck's constant, yet again!
Experiments done on electrons showed that they behave like a wave upon passing through
very narrow slits, a behavior seen for light also.
Albert Einstein successfully explained the photoelectric effect, mentioned before, by
invoking the PARTICLE nature of light: in that experiment light will behave like small
particles, photons!
Other weird aspect of quantum physics, such as quantum entanglement, quantum teleportation,
quantum encryption, and quantum computing will probably be dealt in future videos on
our channel, Insane Curiosity!