## Violations Of Bell’s Inequality

Local hidden variable theories fail, however, when measurements of the spin of entangled particles along different axes are considered. If a large number of pairs of such measurements are made , then statistically, if the local realist or hidden variables view were correct, the results would always satisfy Bell’s inequality. A number of experiments have shown in practice that Bell’s inequality is not satisfied. However, prior to 2015, all of these had loophole problems that were considered the most important by the community of physicists. When measurements of the entangled particles are made in moving relativistic reference frames, in which each measurement occurs before the other, the measurement results remain correlated.

The fundamental issue about measuring spin along different axes is that these measurements cannot have definite values at the same timethey are incompatible in the sense that these measurements’ maximum simultaneous precision is constrained by the uncertainty principle. This is contrary to what is found in classical physics, where any number of properties can be measured simultaneously with arbitrary accuracy. It has been proven mathematically that compatible measurements cannot show Bell-inequality-violating correlations, and thus entanglement is a fundamentally non-classical phenomenon.

## Special Relativity And Electrodynamics

Early attempts to merge quantum mechanics with special relativity involved the replacement of the SchrÃ¶dinger equation with a covariant equation such as the KleinâGordon equation or the Dirac equation. While these theories were successful in explaining many experimental results, they had certain unsatisfactory qualities stemming from their neglect of the relativistic creation and annihilation of particles. A fully relativistic quantum theory required the development of quantum field theory, which applies quantization to a field . The first complete quantum field theory, quantum electrodynamics, provides a fully quantum description of the electromagnetic interaction. Quantum electrodynamics is, along with general relativity, one of the most accurate physical theories ever devised.

The full apparatus of quantum field theory is often unnecessary for describing electrodynamic systems. A simpler approach, one that has been used since the inception of quantum mechanics, is to treat charged particles as quantum mechanical objects being acted on by a classical electromagnetic field. For example, the elementary quantum model of the hydrogen atom describes the electric field of the hydrogen atom using a classical â ) /} Coulomb potential. This “semi-classical” approach fails if quantum fluctuations in the electromagnetic field play an important role, such as in the emission of by charged particles.

## What Is Quantum Mechanics

Quantum mechanics is the branch of physics that deals with the behavior of matter and light on a subatomic and atomic level. It attempts to explain the properties of atoms and molecules and their fundamental particles like protons, neutrons, electrons, gluons, and quarks. The properties of particles include their interactions with each other and with electromagnetic radiation. So below mentioned are those two pointers one should know necessarily before tackling quantum mechanics.

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## Quantum Mechanics: An Incomplete Universe

So then how should we make sense of quantum mechanics? Here’s one way to visualize what this means:

#### Quantum Mechanics: The Glitch in the Universe

Imagine that you are playing a video game. Inside the game, there’s a town where you walk around. In the town are houses that make up the village, but you can’t actually go into the houses. Because you can’t go inside of the houses, you don’t know what’s inside of them. Who lives there? What kind of furniture is in there? What are the floors like?

But imagine that there’s a glitch in the video game that lets you go inside the house. Now, you can learn what’s inside the house. However, you discover that the programmers of the game didn’t think you would ever go into the house, so they didn’t actually put anything in it. You find nothing but empty space in the house. No one lives inside, there’s no furniture, and nothing on the floor.

In this case, it’s not that we can’t know what’s inside the house. The glitch lets us go inside. However, there’s nothing inside the house, because it wasn’t programmed. This means that we know what’s in the house, but the stuff inside the house doesn’t make sense.

If the universe were a video game, **quantum mechanics would be like discovering a glitch in the universe.**We can look at what’s happening at the smallest level, but what we find there doesn’t make sense.

**It’s not that we can’t make sense of quantum mechanics; it’s that quantum mechanics shows that the universe doesn’t make sense!**

## Dual Behaviour Of Matter:

This is mainly because the behaviour of macroscopic objects is practically particle in nature, they do have wave nature but it is negligible because of their huge masses; whereas on the other hand the atomic level particles have very little mass and hence both particle and wave nature is prevalent in them. This dual behaviour of displaying both particle and wave nature is known as dual behaviour of matter and for every particle, the particle nature comes from its mass and the wave nature comes from its matter-wave defined by De-Broglie relationship which is given by,

= \

= wavelength of the matter

h = planks constant

m = mass of the matter

v = velocity of matter

Classical Physics hasnt been able to explain the dual behaviour of a matter and Heisenbergs uncertainty principle, according to which the position and momentum of a sub-atomic particle can be calculated simultaneously with some degree of inaccuracy. Hence, there was a need for a new theory that could explain the behaviour of atomic and sub-atomic particles.

So, this led to the birth of quantum physics It is a branch of science that explains the physical phenomenon by microscopic and atomic approach and takes into account the dual behaviour of matter. It is theoretical physics and it specifies the laws of motion that the microscopic objects obey. When quantum mechanics is applied to macroscopic objects the results are the same as those from classical mechanics.

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## Two Applications That Use Them

Reduced density matrices were explicitly calculated in different spin chains with unique ground state. An example is the one-dimensional AKLT spin chain: the ground state can be divided into a block and an environment. The reduced density matrix of the block is proportional to a projector to a degenerate ground state of another Hamiltonian.

The reduced density matrix also was evaluated for XY spin chains, where it has full rank. It was proved that in the thermodynamic limit, the spectrum of the reduced density matrix of a large block of spins is an exact geometric sequence in this case.

## Is Light A Wave Or A Particle

Einsteins hypothesis posed a bit of a problem. There was an already well-established body of evidence in favour of a *wave* theory of light. The key observation is called the double slit experiment.

Push light through a narrow aperture or slit and it will squeeze through, bend around at the edges and spread out beyond. It diffracts.

Cut two slits side-by-side and we get *interference*. Waves diffracted by the two slits produce an alternating pattern of light and dark bands called *interference fringes*. This kind of behaviour is not limited to light such wave interference is easily demonstrated using water waves.

But waves are inherently *delocalised*: they are here *and* there. Einsteins hypothesis didnt overturn all the evidence for the delocalised wave-like properties of light. What he was suggesting is that a complete description somehow needs to take account of its localised, particle-like properties, too.

So, light acts like both a wave and a particle.

In 1923, French physicist Louis de Broglie made a bold suggestion. If light waves can also be particles, *could particles like electrons also be waves*? This was just an idea, but he was able to use it to develop a direct mathematical relationship between an electrons wave-like property and a particle-like property .

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## Six Things Everyone Should Know About Quantum Physics

Quantum physics is usually just intimidating from the get-go. It’s kind of weird and can seem counter-intuitive, even for the physicists who deal with it every day. But it’s not incomprehensible. If you’re reading something about quantum physics, there are really six key concepts about it that you should keep in mind. Do that, and you’ll find quantum physics a lot easier to understand.

**Everything Is Made Of Waves; Also, Particles**

Light as both a particle and a wave.

There’s lots of places to start this sort of discussion, and this is as good as any: everything in the universe has both particle and wave nature, at the same time. There’s a line in Greg Bear’s fantasy duology , where a character describing the basics of magic says “All is waves, with nothing waving, over no distance at all.” I’ve always really liked that as a poetic description of quantum physics– deep down, everything in the universe has wave nature.

Of course, everything in the universe also has particle nature. This seems completely crazy, but is an experimental fact, worked out by a surprisingly familiar process:

**Quantum Physics Is Discrete**

These oscillations created an image of “frozen” light.

Ultra-precise spectroscopy can also be used to look for things like dark matter, and is part of the motivation for a low-energy fundamental physics institute.

**Quantum Physics Is Probabilistic**

**Quantum Physics Is Non-Local**

A quantum teleportation experiment in action.

**Quantum Physics Is Very Small**

## Schrodingers Cat Is Either Alive Or Dead Surely Not Both

Macroscopic bodies lose their quantum behavior very fast. This was never well understood by the scientists of that time. This happens because of the regular interactions the body would have to endure. Quantum mechanics has been exceptionally successful in explaining microscopic phenomena in all branches of physics.

Stay tuned with BYJUS to learn more about quantum physics, and much more.

Put your understanding of this concept to test by answering a few MCQs. Click Start Quiz to begin!

Select the correct answer and click on the Finish buttonCheck your score and answers at the end of the quiz

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## Max Planck And Black Body Radiation

Have you ever wondered why fire sometimes looks blue? This happens because of something called **Black Body Radiation**. Physics tells us that colors are caused by different wavelengths of light. **Black Body Radiation **describes how something changes color as it gets hot. When things get hot, the light tends to change in this pattern:

- Red
- White
- Blue

Now because of the law of conservation of energy, physicists generally thought that the light would keep climbing the hotter something gets. If things change color the hotter they get, you’d think that blue fire would turn into ultraviolet, right?

#### The Ultraviolet Catastrophe

As it turns out, the wavelengths undergo a huge drop after reaching the blue spectrum. This was called the “ultraviolet catastrophe.”

Max Planck could only make sense of this one way. He hypothesized that the energy ** jumps **from one discrete wavelength to the next. So basically, this says that the energy moves from position 1 to position 2 without passing through positions 1.3, 1.4, 1.7 and so on.

#### Planck’s Constant: Energy Jumps from Place to Place

## What Is A Correct And Simple Definition Of Quantum Physics

Is it correct to define Quantum Physics as the study of Physics in sub-atomic scale? Does Quantum Physics studies something else other than sub-atomic phenomena?

Quantum physics is a probability theory where probability amplitudes appear. Any phenomenon where probability amplitudes appear is a quantum phenomenon, whether it is microscopic or macroscopic. The probability amplitudes unify the particle and wave classical limits into one object, and they probably apply to all objects, microscopic and macroscopic.

The unique way the human kind has got to determine if an effect needs quantum mechanics to be described consists in calculating the action and comparing it with Planck’s constant. If it is much bigger, there is no need of quantum mechanics; if it is on the same order, you will need QM to describe that effect.

The action being much bigger than Planck’s constant does not mean that QM cannot be used. Bohr used to think that the macroscopic and microscopic worlds were fundamentally different, but such examples as quantum levitation suggest that describing fundamentally was a synonym of describing with QM.

Is it correct to define Quantum Physics as the study of Physics in sub-atomic scale? **No.**

Does Quantum Physics studies something else other than sub-atomic phenomena? **Yes.**

Take artificial atoms or superconducting flux qubits as examples.

**Short answer:** It is the physics of small scales or high energies.

That is the simplest way I can define quantum physics.

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## S Of Creating Entanglement

Entanglement is usually created by direct interactions between subatomic particles. These interactions can take numerous forms. One of the most commonly used methods is spontaneous parametric down-conversion to generate a pair of photons entangled in polarisation. Other methods include the use of a fiber coupler to confine and mix photons, photons emitted from decay cascade of the bi-exciton in a quantum dot, the use of the HongOuMandel effect, etc. In the earliest tests of Bell’s theorem, the entangled particles were generated using atomic cascades.

It is also possible to create entanglement between quantum systems that never directly interacted, through the use of entanglement swapping. Two independently prepared, identical particles may also be entangled if their wave functions merely spatially overlap, at least partially.

## Who Developed Quantum Mechanics

As scientists gained the technology to measure with greater precision, strange phenomena was observed. The birth of quantum physics is attributed to Max Planck’s 1900 paper on blackbody radiation. Development of the field was done by Max Planck, Albert Einstein, Niels Bohr, Richard Feynman, Werner Heisenberg, Erwin Schroedinger, and other luminary figures in the field. Ironically, Albert Einstein had serious theoretical issues with quantum mechanics and tried for many years to disprove or modify it.

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## What You Will Learn About On This Course

This course is for anyone who has ever wanted to understand quantum physics but who has been daunted because the predictions it makes are so strange that**;even Albert Einstein found them disturbing**.

Course participants might want to prove to themselves that they are up to the challenge of understanding the basic principles of quantum theory.

Alternatively, course participants might be curious to know exactly how quantum physics has contributed to the development of an astonishing variety of modern technologies, from laptops to MRI scanners to Maglev trains.

Some participants might even be considering taking a quantum physics university course: **this beginners course offers a solid introduction to some of the themes that would be covered in a quantum physics degree programme**.;

This course is relevant to all such participants, and covers:

**The history of quantum physics****Matrix and wave mechanics****The development of quantum theory, including tests for entanglement****Quantum encryption and quantum computing****The interpretations physicists use to bridge the quantum/classical divide****Quantum gravity**

## Book Free Download For Android What Is Real: The Unfinished Quest For The Meaning Of Quantum Physics Fb2 9781541698970

“A thorough, illuminating exploration of the most consequential controversy raging in modern science.” New York Times Book Review Every physicist agrees quantum mechanics is among humanity’s finest scientific achievements. But ask what it means, and the result will be a brawl. For a century, most physicists have followed Niels Bohr’s solipsistic and poorly reasoned Copenhagen interpretation. Indeed, questioning it has long meant professional ruin, yet some daring physicists, such as John Bell, David Bohm, and Hugh Everett, persisted in seeking the true meaning of quantum mechanics. What Is Real? is the gripping story of this battle of ideas and the courageous scientists who dared to stand up for truth. “An excellent, accessible account.” Wall Street Journal “Splendid. . . . Deeply detailed research, accompanied by charming anecdotes about the scientists.” Washington Post

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## Relation To General Relativity

Even though the predictions of both quantum theory and general relativity have been supported by rigorous and repeated empirical evidence, their abstract formalisms contradict each other and they have proven extremely difficult to incorporate into one consistent, cohesive model. Gravity is negligible in many areas of particle physics, so that unification between general relativity and quantum mechanics is not an urgent issue in those particular applications. However, the lack of a correct theory of quantum gravity is an important issue in physical cosmology and the search by physicists for an elegant “Theory of Everything” . Consequently, resolving the inconsistencies between both theories has been a major goal of 20th- and 21st-century physics. This TOE would combine not only the models of subatomic physics but also derive the four fundamental forces of nature from a single force or phenomenon.

Another popular theory is loop quantum gravity , which describes quantum properties of gravity and is thus a theory of quantum spacetime. LQG is an attempt to merge and adapt standard quantum mechanics and standard general relativity. This theory describes space as an extremely fine fabric “woven” of finite loops called spin networks. The evolution of a spin network over time is called a spin foam. The characteristic length scale of a spin foam is the Planck length, approximately 1.616Ã10â35 m, and so lengths shorter than the Planck length are not physically meaningful in LQG.

## Why Is Probability Important In Quantum Physics

One more thing. That theres a 50 percent probability that a tossed coin will land heads simply means that it has two sides and we have no way of knowing which way up it will land. This is a classical probability born of ignorance.

We can be confident that the coin continues to have two sides heads and tails as it spins through the air, but were ignorant of the exact details of its motion so we cant predict with certainty which side will land face up. In theory, we could, if we knew exactly how hard you flipped it at exactly what angle, and at exactly what height you would catch it.

Quantum probability is thought to be very different. When we toss a quantum coin we might actually be quite knowledgeable about most of the details of its motion, but we cant assume that heads and tails *exist* before the coin has landed, and we look.

So, it doesnt matter exactly how much information you have about the coin toss, you will never be able to say with any certainty what the result will be, because its not pre-determined like in a classical system.

Einstein deplored this seeming element of pure chance in quantum mechanics. He famously declared that: God does not play dice.

And then, in 1927, the debates began. What is the wave function and how should it be interpreted? What is quantum mechanics telling us about the nature of physical reality? And just what is this thing called reality, anyway?

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