## "Anyone who is not shocked by quantum mechanics has not probably understood it"

quantum mechanics for dummies, Quantum mechanics for dummies

# Quantum mechanics for chemists

Matrix mechanics | Dirac Equation | Quantum electrodynamics# Theory of relativity

Special theory of relativity | General theory of relativityquantum mechanics for dummies, this page is about quantum mechanics for dummies

### Mathematical priliminaries

**Kinetic energy **

Kinetic energy of a moving body is its mass times velocity squared. So mathematicall it is :

E(ke) = 1/2mv^2 = p^2/2m where p = mv = momentum of the body

When dealing with wave function ψ(r,t) we use Δ operator to find the kinetic energy of the moving particle. We use momentum operator ih(d/dx) to find momentum. There are various other operators which are used in Quantum mechanics. Operators do the same thing as variables (x, p) do in classical physics. One thing that is different is that in Qunatum mechanics operators have expectation values.

Potential energy of a system is another kind of energy which varies due to a change in the configuration of the system. It is a function of position in Newtonian gravity. In electrostatics it is a function of distance R from charged body.

The remarkable property of potential energy is that it is conservative. That is to say it is path indepenent. in Classical mechanics mass of charge create potential and its associated force. When you study quantum field theory you will see quantum fields create such kind of potential and forces. In quantum field theory which is somewhat complicated theory due to crazy integrations. Quantum field is like matraces which can be perturbed to create energy in them.

### Philosophical thoughts

The atomicity of atom is a hypotheses, as old as the Greeks and no way distasteful to our mental
habits. The theory that atom is composed of electron and proton is beautiful throughout its
success, but it is not difficult to imagine. It is otherwise, with the form introduced
by the theory of quanta. This might not possibly have surprised Pythagoras, but it would definitely
have astonished every latter man of science as it have astonished every man of our own day. It is necessary to
understand the general principles of the theory before attempting a modern philosophical account of it. But there
are still unsolved physical problems connected with the theory , which makes it improbable that any satisfactory philosophical
account can yet be constructed. But we must do what we can.

Everybody knows that the revolution began when Plank gave his hypotheses on the observation
of black-body radiation. Plank showed that when we consider the vibration of a body , the frequencies of the
vibration do not arrange themselves in all possible ways accordinng to usual law of frequency or frequency distribution.
But they do so in a certain way. Let e be the energy of any radiation and v be the frequency. Then e/v will always be equal
to certain amount, i.e plank constant h, or 2h or 3h and so on. We can not have the ratio (e/v) to be a fraction of plank constant.
This happens to be so in all occurences involving quanta. Nobody know why this is the case but it seems to be the fact. There is no
reason known for its non-occurrance , which remains , so far, the nature of brute fact. At first it was an isolated fact . But now Plank's
constant has been found to be involved in various other kinds of phenomena ; in fact, wherever observation is sufficiently minute to make it possible to
discover whether it is involved or not .

A second field for the quantum mechanics was found in the photo-electric effect. This can be explained in the following way:

If high frequency sunlight falls on a metal surface, the electrons bound in the atom of the surface seems to produce a potential at the two far ends of
the surface. This phenomena has been shown quite exclusively with photo-electric effect. It is the emission of electron by the incident of the light. The minimum energy of the light which
set the electron
free from the surface is called the threshold energy of the surface. Below that frequency no electron gets emitted. Einstein discovered this
unique phenomena relating to metal surface. He was awarded novel prize for
this discovery.

The explanation of the above phenomena is that four-fifth of the energy hv is absoved by the electron , which makes it possible to liberate the electron
from the atom. Other one-fifth of hv is absorbed by the atom. So total amount of energy absorbed by the electron and atom is one quanta hv.

The most interesting application of quantum mechanics is the explanation of Bohr's atomic model. Electron
jumps from one orbit to another while emitting or absorbing radiation in the form of quanta. When an electron
jumps from higher orbit to lower orbit it emits light. When it jump from lower to higher orbit it absorbs
radiation. The amount of energy is exactly the same as predicted by Planck's law. It had been found that lines in the hydrozen
spectrum which were known had frequencies obtained from the difference of two terms according to the formula :

Where R is Rayleigh constant and n, k are small integers , usually corresponding to the orbit. It is apparent that the equation above
does not say that the line of hydrozen spectrum is not connected to a single orbit where electron involves in periodic vibration. But it
correspond to the transition connected with a change from a state defined by an integer to a state defined by another. This also suggests that
orbits of electron is not a simple one as described by Newtonian mechanics but orbit designated by integral "quantum number" with a multiple of
h.

At this point we do not know what causes electron to jump from one orbit to another. Our knowledge of the atom is only statistical. But we know of course , when an electron
jump from lower to higher orbit , electron absorbs energy from incident light. We do not know , in a large number of atoms , of the electrons which are not in minimum orbits ,
some jump at one time and some other time just as we do not know some radioactive elements disintegrates and some do not. Nature seems to be full of revolutionary occurences as
to which we can say that , if they take place, they will be one the many possible kinds, but we can not say they will
take place at all or if they will , at what time. This way we enter into the physics which must be modeled with probability.

The probability of an event is the ratio of the number of ways the event can happen to the all possible events that can occur during an experiment. So if x
is the number of ways an event A can occur and n is the total possible events then the probability of A confusing is P(A) = x/n .

Previously De Broglie developed his wave-particle duality hypotheses. According to his hypotheses the electron wave wrapped around the nucleus has integer number of wavelengths.

All the quantum phenomena that happen are the special case of a more general law known as generalized quantum principle. There are generalized coordinates and momentum associated with them. First thing is to observe that energy is not fundamental entity but action is. Action is energy multiplied by time. Now suppose we have a system that depends upon several coordinates , periodic with respect to each. The coordinates are conditionally periodic. The momentum p(k) associated with the coordinates q(k) will be d(E)/d(q'(k) where q'(k) is the derivative with respect to time. E is the kinetic energy of the system. Now we have the quantum principle

Integral[p(k)dq(k) = nh , where h is planck constant.
The motion of the earth around the sun is a system which depends on several coordinates i.e (earth spins on its own axis as well as orbits around the sun). When applied to quantum system we divide the motion into smaller parts and take associated momenta and coordinates. Then we multiply them together and sum each individual bits. In the limit the sum will give total amount which is exactly equal to plank constant h or multiple of h.

Integral is to be taken over one complete period. There
is quantum law which can be derived using this general principle mentioned above. Classical physics thus differs from Quantum physics in a very profound way.
What causes the transition from Quantum world to classical world is very hard to find out but a large number of quantum phenomena can average out to give classical behaviour.
In the atomic realm space and time , no longer, have usual properties like smoothness and continuity. Space and time appear to have
discreteness and discontinuity, at least, inside the atom.

** "the man who did the waking buys the man who was sleeping a drink and the man who was sleeping drinks it while listening to a proposition from the man who did the waking.. Savvy" **

## Quantum mechanics

Mysterious talks of quarks, spin, and cats in boxes have given quantum physics a reputation as the scientific theory that no one really
understands. The practical applications of quantum physics are all around us in daily life. Were it not for quantum physics, computers would not
function, kettles would not boil, and power stations would not heat our homes.

To many people mathematics represents a significant barrier
to understanding of science. Certainly, mathematics has been the language for physics for the last four hundred years and
it has become impossible to progress in physics without mathematics. Why is that? it seems like nature is governed
by cause and effect
of changes. Mathematics is able to describe this relationship of cause and effect very precisely. One example is the differntial law where
the law is necessary to be written in differntial equation. For example a man is walking across a road and his dog is in the field beside it.
When the man whistles the dog it follows his master. The dog follows a curve which describes the dog's velocity exactly towards his master at each moment
of time. This is a kind of curve which can be described by a differential equation. Similarly many laws of nature are necessary to be written in the language of
calculus.

** "Don not fool yourself, you are the easiest person to fool.." **

Quantum mechanics (QM; also known as quantum physics, quantum theory, the wave mechanical model, or matrix mechanics), including quantum field theory,
is a fundamental theory in physics which describes nature at the microscopic scales of energy levels of atoms and subatomic particles.
Classical physics, the physics existing before quantum mechanics, describes nature at ordinary (macroscopic) scale.
Most formulationas in classical physics can be derived from quantum mechanics as an approximation valid at large (macroscopic) scale.
Quantum mechanics deviates from classical physics in that energy, momentum, angular momentum and other
quantities of a bound system are constrained to discrete values (quantization);

Objects have characteristics of both particles and waves (wave-particle duality);
and there are limits to the precision with which quantities can be measured (uncertainty principle).

Quantum mechanics gradually arose from theories to explain observations which
could not be reconciled with classical physics, such as Max Planck's solution in 1900 to the black-body radiation catastrophy,
and from the correspondence between energy and frequency in Albert Einstein's 1905 paper which
explained the photoelectric effect.
Early quantum theory was pdeeply re-conceived in the mid-1920s by Erwin Schrödinger,
Werner Heisenberg, Max Born and others. The modern theory is formulated in various specially developed mathematical formalisms.
In one of them, a mathematical function, the wave function, provides information about the probability amplitude of position, momentum,
and other physical properties of a particle.
Important applications of quantum theory include quantum chemistry,
quantum optics, quantum computing, superconducting magnets, light-emitting diodes,
and the laser, the transistor and semiconductors such as the microprocessor, medical and
research imaging such as magnetic resonance imaging and
electron microscopy.
Explanations for many biological and physical phenomena are rooted in the nature of the chemical bond, most notably the macro-molecule DNA.

**"The best thing about me is that there is so many me's"**

### Introduction

In the beginning there was continuous flow, and then Max Planck came along and proposed discretization. Quantization basically just implies, that instead of being continuous, things such as EM radiation, can only take multiples of certain values. It’s a little bit like having a tube of smarties. The whole tube represents a beam of light. Inside it you have collection of smarties.
You can divide the tube, so you can have less number of smarties in there, or you can get another tube and have smarties, but you have to have a whole number of smarties, because they can’t be split (if anyone e-mails me suggesting I squash/crush/split a smartie, I will hunt them down and make them pay!).

Planck came to this conclusion when solving on the “Ultraviolet Catastrophe”. According to principles of classical electromagnetism, the number of ways an electromagnetic wave can undulates a in a 3-dimensional cavity, per unit frequency, is proportional to the square of the frequency. This means that the power you would get out of per unit frequency should follow the Rayleigh-Jeans law, which again , means that the power would be commensurate to the frequency squared. So if you put the frequency up higher and higher the power would be limitless. Planck said that electromagnetic energy did not follow the classical description. He said that it could only be emitted in discrete packets or chunks of energy proportional to the frequency

E=hω

where \hbar (pronounced “h bar”) is h/(2\pi). These equations mean the the radiation eventually goes to zero at infinite frequencies, and the total power is finite. Planck called these packets of energy “Quanta”. The value of h is 6.626\times 10^{-34} J·s and the value of \hbar is 1.06\times 10^{-34} J·s.

**"Science is the manipulation of nature"**

### Little Packets of Light

If you shine a light onto a metal body for long enough the surface of the will heat up. This must mean that the light is transferring energy to the metal, so in theory it is possible that if you shone a light on a surface for long enough, sufficient energy would be transferred to liberate an electron from an orbit. Even with a weak light you should be able to wait long enough for the energy to build up and an electron to be emitted from atom. So physicists tried the experiment. It failed horribly. For some metals specific light would cause electron separation, for other metals the same light source wouldn’t, no matter how long it was left. And it was found that the electrons came out with higher energies depending on the colour of the light, not the intensity. The problem of the photoelectric effect was resolved in 1905 by Einstein, and was what he won the Nobel Prize for in 1921. Einstein applied Plancks theory of Quantization to light and said that the light is not a continuous stream of energy but rather bundle of little packets of a certain energy value that depended on its wavelength. This explained why no matter how long you left the light on the surface there would be no emission unless the individual photons had enough energy. This also explained why different colours gave the emitted electrons different energy values. The energy was shown to be related to wavelength by Plancks equation. Einstein also showed that the energy of the emitted electrons would be equal to [E = hf - ψ] where φ is the energy needed to get the electron from inside the metal to just outside the surface, and is called the “Work Function”.

# Schrodinger equation

Schrodinger equation describes the behavior of subatomic particles which are associated with a wave function Ψ. Wave function Ψ(r,t) is basically function of space and time, which solve
Schrodinger equation when some potential energy that influences the particle is given. Schrodinger equation is the quantum mechanical version of newton's force (F=ma) equation. If we know the position of a particle and force acting on it, we can find the position of the particle at another time. This simply involves solving an equation describing the time evolution of the particle with mass m. Similar thing happens in case of Schrodinger's equation. In this scenerio, the solution gives the probability of finding the particle.
It also says that energy of the particle associated with the wave function is conserved. The derivation is simple

The equation described below is time independent equation where the parameter t is absent.
h is the plank constant. Schrodinger equation can be interpreted as an operator known as Hamiltonian (H) that acts on some wave function Ψ(r,t) and produces an corresponding eigenvalue (E) of eigenvector
|Ψ(r,t)> . Hamiltonian contains the dynamics of a quantum system. Schrodinger equation is second order linear differential equation. It is not a covariant equaton , i.e it does not hold for relativistic partcle. Space and time are not given equal footing in the equation. So theory of relativity is not consistent with the equation of Schrodinger.

Operator Hamiltonian (H) acts on the wave function and gives the value of an observable E for wave function Ψ(r,t) . Another operator is the momentum operator P which can be defined like :

Momentum operator thus acts on wavefunction to give observable p(momentum). In quantum mechanics we are always interested in quantities that can be observed. Wave function itself has no physical properties. So it needs to be acted upon by some operator which can give us observable associated with the wave function. This, consequently becomes an Eigenvalue equation where the observable serves as the eigenvalue

It is one of the most successful foundation of modern quantum mechanics that Schrodinger discovered. It is tantamount(equivalent) to Heigenberg's matrix mechanics. The symbol |> enclosing the wavefunction is called ket vector which is a column vector. Bra vector <| is a row vector. This two vector notation is known as braket notation which Dirac introduced. The bra <| or ket |> vector is the representation of state vector i.e |ψ>. A state vector is a state of an isolated quantum system. Mathematical interpretation of state vector is related to the probability of outcome of certain measurement. More specifically state vector assigns probability distribution for the value of each observable( position, momentum). Quantum state is a set of integer, that evolves toward another set of integers denoting another state. That is why states are usually represented by bra or ket vectors. In schrodinger picture the state vector evolves with time.

**"we are middle children of history man, no purpose and no place. We have got no war and no great depression, our great war is the spiritual war. Our great depression is our lives"**

The wave function describes the probability of finding a particle at a certain position, which Schrodinger was not aware at the time when he baked his revolutionary equation. Max born interpreted this wave function as the probability that an electron can be found at certain place at a certain time. The squares of modulus of wavefunction represent the apllitude of such probability. Given specific physical condition there is always a finite , though very meager , probability that an electron can be found anywhere in the entire universe. That is only slight craziness that the quantum particle can show. If you are not still shocked , there are many more absurdities to come to make you that. When we perform a measurement we can find electron at specific place. The wave function collapses at the moment of measurement. We force the electron to be in some definite state by doing the measurement.

A typical wave function that solves the Schrodinger wave equation is interpreted above. Each individual state has a probability amplitude |a{j}| (squared) associated with it. Total amplitude is unity.

Schrodinger equation is a linear differential equation. So a linear combination of multiple solutions is also a solution to the wave equation. This creates a superposition of states. A quantum system can exist simultaneously in multiple states. But this seems a bizarre idea which is interpreted in many ways. One of these is the copenhagen Interpretation. According to Copenhagan interpretation quantum system does not possess definite state prior to measurement. It is the measurement that causes the system to have one of many possible states. Another interpretation is the many-worlds interpretation. In this case the quantum system exist simultaneously in different states each of which resides in a unique universe known as parallel universe. A collection of many such parallel universes is called multiverse.

**"uncertainty is what make quantum physics very interesting.."**

### Schrodinger Cat

Thus a wave function representing a particular ket vector can be viewed as a column vector of complex quantity. The wave function is thus a complex quantity.

The wave function cab be thought as an element of Hilbert space of N dimension. There are N number of components in vector representation of wave function. Each has an individual probabilty measure according to quantum mechanical interpretation. And each component corresponds to particular state of the system. Momentum space representation of wave function can be found by taking Fourier transform of wave function over all space. This integrates wave function taken inside three dimensional space. That is why volume integration is used in the following equation. The inverse transform will give the original wave function

The time evolution operator describes the evolution of quantum state and and is thus a function of time.

When Schrodinger equation is applied to electron inside atom , the wave function perfectly describes atomic orbitals of Hydrozen.

This is a separable, partial differential equation which can be solved in terms of special functions. The normalized position wavefunctions, given in spherical coordinates are:

When everything is derived using Schrodinger equation, various eigenstate of hydrozen atom can be found accordingly.

**"The only things that matters is what a man can do and what he cannot.."**

### Some useful quantum mechanics equations

Poisson bracket is an important binary operation in Hamiltonian mechanics, playing a central role in Hamilton's equations of motion, which govern the time evolution of a Hamiltonian dynamics. For generalized coordinates q(i) and two functions f and g , the poisson bracket takes the form :

### Double slit experiment

"all the mystery of quantum mechanics is rooted in the young's double slit experiment"

In modern physics, the double-slit experiment is a demonstration that light and matter can show characteristics of both classically
defined waves and particles; moreover, it displays the fundamentally probabilistic nature of quantum mechanical phenomena.
The experiment was first executed with light by Thomas Young in 1801. In 1927, Davisson and Germer demonstrated that electrons show the same
behavior, which was later extended to atoms and molecules.

Thomas Young's experiment with light was part of classical physics well before quantum mechanics, and the concept of wave-particle duality.
He believed it illustrated that the wave theory of light was right, and his experiment is sometimes cited to as Young's experiment or Young's slits.
In the basic version of this experiment, a coherent light source, such as a laser beam, illuminates a plate pierced by
two parallel slits, and the light passing through the slits is observed on a screen behind the plate. The wave nature of light causes the
light waves passing through the two slits to interfere, creating bright and dark bands on the screen — a result that would not be expected
if light consisted of classical particles. However, the light is always found to be absorbed at the screen at separate points,
as individual particles (not waves), the interference pattern appearing via the varying density of these particle hits on the screen. Furthermore, versions
of the experiment that include detectors at the slits find that each detected photon passes through one slit (as would a classical particle), and
not through both slits (as would a wave). However, such experiments demonstrate that particles do not form the interference pattern if
one detects which slit they pass through. These results demonstrate the principle of wave–particle duality.

### Physics and philosophy

Physics has been a supreme branch of science and engineering. This is probably because a lot of mathematics are needed in physics. Electricity and other forces certainly make physics very interesting. Quantum mechanics raises some profound questions about reality. There was a matter of great controversy around the question "Does the moon exist if no one else is looking at it? ". So is the question "Does a tree in a lonely forest make a sound when no one else is hearing?". Electron and elementary particles do not have definite location. These only have tendency to exist. We can not say with certainty that an electron is located at a particular point in space. They behave like wave. What happens inside an electron? This raises certain problems when we consider Rutherford type electron.
Heigenberg's electron removes the difficulties. According to new theory, electron is not in a definite place. An electron is essentially a collection of raditions
emitting from the place in question, which are observable at other places than that where the electron would formally have been. These radiations become
weak as the distance increases. This reduces electron to a law as to certain occurrances in a region. We can not say electron is a point or a region but
having certain properties of different logical type.

Philosophy is good at answering these kinds of puzzling questions about reality. Einstein was a philosopher too. Some other scientists became to be philosophers at later parts of their lives. Philosophy is the study of world and life at large. It falls in the intermediate way between religion and science. If you want to understand you have to be some kind of a philosopher. Philosophy has the freedom of speculation. Philosophy is what we do not know contrary to science which means what we know.

**Quantum theory of atom**

An atom is composed of electrons and protons, the latter being inside the nucleus, the former partly in the nucleus (except in hydrogen) , partly
planetary. The number of the protons in nucleus gives the atomic number. When the atom is unelectrified the number of planetary electron is the same as the
atomic number. If the quantum theory is correct , an atom has ceratin number of characters, each measured by integers called quantum numbers, which are allways small.
It has also a property called energy, which is a function of the quantum numbers; and in connection with each of the quantum numbers there is periodic process which is subject
to quantum rules. Each quantum number is capable of changing suddenly from one integer to another integer. When the atom is left to itself, the change will be such as
to diminish energy. But when it recieves energy from outside the change may increase the energy. All this , however, is more or less hypothetical. What we really know is the
interchange of energy between the atom and the surrounding medium; here there are simple laws as to form the radiant energy will take (classical laws). There are at present
no definite laws determining when quantum change will take place in the atom.

The theory of atom had made it possible to develop periodic table where all the elements are arranged in specific order. The position of the elements depends on the number of electron they contain. These elements take part in chemical reactions and form various kinds of other compounds and mixtures. The chemical properties of the atoms can be deduced using quantum mechanical principles. A relatively new branch called "molecular quantum mechanics" has been developed. All the elements and compounds are basically replica of simple structure like hydrogen or helium. Combining multiple numbers of hydrogens and heliums we can create other atoms. The only difference lie in the number of protons, neutrons and electrons. What are these electron, proton and neutrons made of? These , in turns, are composed of quarks , gluons and other tiny particles which particle physics deal with.

How did atom come into existence in the first place? How did nature make a well-organized, symmetric and miniscule structure like atom? This is the question which should probably be addressed in TOE(theory of everything). All the three forces are in effect in the atom. If we analyze quantum characteristics inside the atom we find these fundamental forces there. In nucleus strong and weak nuclear forces exist and between proton and planetary electron electromagnetic force works. But in principle
gravitational force also exist inside the atom. Proton, electron and neutron have certain mass. All the forces exist inside the atom. Atom is a tiny universe so to speak.

### Quantum tunnelling

Another bizzare consequences of quantum mechanics is quatum tunnelling. A classical object can not pass through a rigid barrier but an electron has always some chance to be found on the other side of the a potential barrier.

**"time is the most precious commodity in the whole universe"**

### quantum entanglement

Quantum entanglement is a physical phenomena which occurs when pairs or groups of particles are produced or interact or share spatial proximity in a way such that the measurement of states of each particle can not determined independently of the state of others. In such a situation the particles are said to be entangled.
quantum pure states are rays or elements of Hilbert space (H). Hilbert space is a generalized Euclidean space where inner product between two elements are
well defined. A point in Hilbert space can be seen as a state of vibrating string , which can be projected on to coordinate axes representing different
overtones or harmonics. In general Hilbert space is the infinite dimensional analog of Euclidean space. Pure states or rays are usually represented by ket vector.
Particles in these pure state can be separable. But there are states where such separation is not possible. These states are entangled states which can be
modelled mathematically.

Quantum entanglement should better be classified as an example of non-locality. Classical physics is always concerned with local measurement or it only enables us to compare local quantities. Non-locality violates causality . This is weird but non-locality seems to be inherent in quantum mechanics.

This quantum entanglement permits faster than light communication which is also known as spooky action at a distance. Suppose two electrons are entangled so that when we measure one electron's spin then the spin of other is
determined with certainty. Spin states can be either up or down. So when we fix one of this , the other is also fixed. This will happen no matter how far apart the two electrons are. This is quite mysterious yet experimentally verified.

### quantum mystery

Sometimes, if you want to understand how nature truly works,
you need to break things down to the simplest levels imaginable.
The macroscopic world is composed of particles that are — if you divide them until they can be divided no
more — fundamental. They experience forces that are determined by the exchange of additional particles
(or the curvature of spacetime, for gravity), and react to the presence of objects around them.

At least, that's how it seems. The closer two objects are, the greater the forces
they exert on one another. If they're too far away, the forces reduces to zero,
just like your intuition tells you they should. This is called the principle of locality,
and it holds true in almost every instance. But in quantum mechanics, it's violated all the time.
Locality may be nothing but a persistent illusion, and seeing through that facade may be just what physics needs.

Imagine that you had two objects located in close proximity to one another. They would attract or repel one another based on
their charges and the distance between them. You might picture this as one object generating a field that affects the other,
or as two objects exchanging particles that impart either a push or a pull to one or both of them.
You'd expect, of course, that there would be a speed limit to this interaction:
the speed of light. Relativity gives you no other way out, since the speed at which
the particles responsible for forces propagate is constrained by the speed they can travel, which can never exceed the speed of light
for any particle in the Universe. It seems so straightforward, and yet the Universe is full of surprises.

We have this notion of cause-and-effect that's been hard-wired into us by our
experience with reality.
Physicists call this causality, and it's one of the rare physics ideas that actually conforms to our intuition.
Every observer in the Universe, from its own point of view, has a set of events that exist in its past and in its future.
In relativity, these are events contained within either your past light-cone (for events that can causally affect you)
or your future light-cone (for events that you can causally effect). Events that can be seen, perceived, or can otherwise
have an effect on an observer are known as
causally-connected or causal. Signals and physical effects, both from the past and into the future,
can propagate at the speed of light, but no faster.
At least, that's what your intuitive notions about reality tell you.

But in the quantum mechanical Universe, this notion of relativistic causality isn't as straightforward or universal as
it would seem. There are many properties that a particle can have — such as its spin or polarization — that are
fundamentally indeterminate until you make a definite measurement. Prior to observing the particle, or interacting with it in
such a way that it's forced to be in either one state or the other, it's actually in a superposition of all possible outcomes.

Well, you can also take two quantum particles and entangle them, so that these very same quantum properties
are linked between the two entangled particles. Whenever you interfere with one member of the entangled pair,
you not only gain information about which particular state it's in, but also information about its entangled partner.

This wouldn't be so bad, except for the fact that you can set up an experiment as follows.
1.You can create your pair of entangled particles at a particular location in space and time.

2.You can transport them an arbitrarily large distance apart from one another, all while maintaining that quantum entanglement.

3. Finally, you can make those measurements (or force those interactions) as close to simultaneously as possible.

In every instance where you do this, you'll find the member you measure in a particular state, and instantly "know"
some information about the other entangled member.

In every instance where you do this, you'll find the member you measure in a particular state, and instantly "know"
some information about the other entangled member.

What's puzzling is that you cannot check whether this information is true or not until much later,
because it takes a finite amount of time for a light signal to reach from the other member.
When the signal does arrive, it always confirms what you'd known just by measuring your member
of the entangled pair: your expectation for the state of the far-removed particle agreed 100% with what its measurement indicated.

Only, there seems to be a problem. You "knew" information about a measurement that was taking place non-locally,
which is to say that the measurement that happened is outside of your light cone. Yet somehow, you weren't entirely unknowledgeable about what was
going on over there. Even though no information was transmitted faster than the speed of light, this measurement describes a troubling truth about quantum physics:
it is fundamentally a non-local theory.

There are limits to this, of course.

It isn't as clean as you want: measuring the state of your particle doesn't tell us the exact state of its entangled pair, just probabilistic information about
its partner.

There is still no way to send a signal faster than light; you can only use this non-locality to predict a statistical average of entangled particle properties.

And even though it has been the dream of many, from Einstein to Schrödinger to de Broglie, no one has ever come up with an improved version of quantum mechanics
that tells you anything more than its original formulation.
But there are many who still dream that dream.

One of them is Lee Smolin, who cowrote a paper way back in 2003 that showed an fachinating link between general ideas in
quantum gravity and the fundamental non-locality of quantum physics. Although we don't have a successful quantum theory of gravity,
we have established a number of important properties concerning how a quantum theory of gravity will behave and still be consistent with the known Universe.

When you attempt to quantize gravity, by replacing the concept of curved spacetime with an exchange of particles that mediate the gravitational force,
huge violations of locality will arise. If you look at the consequences of those violations — which Smolin and his coauthor,
Fotini Markopoulou, did — you find that they are capable of explaining the non-local behaviors of quantum mechanics via new, non-local, non-observable variables.

Imagine that you pass an electron through a double slit. If you don't measure which slit it goes through, you wind up concluding that it must pass through both slits simultaneously, interfering with itself as it does. That's how you get the interference pattern on the screen behind it. But then, you ask, what happens when you try and measure its gravitational field?

Does the gravitational field display an interference pattern? Or does it follow a single particle-like trajectory, passing through one slit alone?

### Stern-Gerlach experiments

"When we measure something we are forcing an undetermined, undefined world to assume an experimental value. We are not measuring the world, we are creating it."

You have to realize that this is something very subtle, but that is indisputable. There are experiments you can perform that show you the world behaves
differently if you do or don't measure it.

For example, there's an experiment you can do called the Stern-Gerlach experiment, where you put an electron through a magnetic field oriented
in a particular direction. This could be along, say, the x-axis. Electrons that spin in one direction will split in the positive direction,
electrons that spin in the other direction deflect in the negative direction.

The act of determining this result along the x-axis destroys any information along the y-axis or z-axis. If you set up another Stern-Gerlach experiment in the x-axis,
the particles that deflected positively will still deflect positively; those that deflected negatively will still deflect negatively.

But if you throw in another experiment in the y-direction, for instance, you'll not only see a split in that new direction, you'll destroy any information about
the x-direction. It's messed up, but it's experimentally real.
An illustration between the inherent uncertainty between position and momentum at the quantum level. There is a limit to how well you can measure these two
quantities simultaneously, and uncertainty appear in places where people often least expect it.

this is another aspect of quantum physics that is very real: fundamental quantum uncertainty. There are certain combinations of properties that cannot be simultaneously known to better than a certain accuracy, combined. Position and momentum, energy and time, or even (as we just illustrated) spin in two mutually perpendicular directions, cannot be known to arbitrary accuracy. "Why is it this way?" We don't know! That's the problem: there's no governing principle that underlies it. This is the principle.

### How do measurements collapse quantum wave functions?

In the strange realm of electrons, photons and the other fundamental particles, quantum mechanics is law.
Particles don't behave like tiny balls, but rather like waves that are spread over a large area.
Each particle is described by a "wavefunction," or probability distribution, which tells what its location, velocity, and other
properties are more likely to be, but not what those properties are. The particle actually has a range of values for all the
properties, until you experimentally measure one of them — its location, for example — at which point the particle's wavefunction
"collapses" and it adopts just one location. [Newborn Babies Understand Quantum Mechanics]

But how and why does measuring a particle make its wavefunction collapse, producing the concrete reality
that we perceive to exist? The issue, known as the measurement problem, may seem esoteric,
but our understanding of what reality is, or if it exists at all, hinges upon the answer.

### Reference materials:

Mathematical Quantum physics for layman (pdf)Mathematical Quantum mechanics for student (very interesting)

Mathematical Quantum mechanics by feynman

Mathematical Grand Design by Stephen Hawking

Mathematical Higher Engineering Mathematics ( PDFDrive.com ).pdf

quantum mechanics for dummies