# Quantum Entanglement and Bell’s Theorem Essay

Custom Student Mr. Teacher ENG 1001-04 6 November 2016

## Quantum Entanglement and Bell’s Theorem

In the early 20th century, physicists were in need of a new theory to describe the world of the atom and its components. Newtonian mechanics and Einstein’s theory of relativity worked very well at describing the motion of the planets and stars, but when these theories were applied to the atom, they completely broke down. Max Planck discovered that atoms exchange energy in individual packets of specific energy values. Planck called these energy packets “quanta”, Latin for “unit of quantity”, hence the name quantum theory.

Two pioneers of quantum theory, Werner Heisenberg and Erwin Schrodinger, devised mathematical formulas to describe the atom. Two fundamental principles of quantum mechanics emerged from their equations: the uncertainty principle and the principle of superposition. Superposition states that an atom exists in all possible states until it is measured. The uncertainty principle says that you cannot know a quantum particles location and momentum (momentum is a particles velocity,roughly) at the same time.

These principles are important because they reduce predictions of physical object’s position from an absolutes to only a range of probabilities. This is very different from the certainty of classical physics. The strangest phenomenon predicted, however, is quantum entanglement. It predicted that when a particle is split in two, it behaves as if it were still joined, no matter how far they are separated. Change one of the entangled particles and the other reacts instantly. These strange properties described by quantum mechanics were unacceptable to Einstein and many other physicists.

Einstein felt that quantum theory itself must be a flawed theory to produce such strange predictions. The bizarre behavior and properties of the atom and sub-atomic particles must be attributable to some other mechanisms, he reasoned. Niels Bohr, another pioneer of quantum theory, deflected Einstein’s criticisms and claimed that quantum theory was a sound theory. The problem, Bohr said, was that we need an entirely new set of words and terminology for the theory because the realm of the atom was so different from our everyday experiences.

In 1935 Einstein, along with Boris Poldolsky and Nathan Rosen, submitted a famous paper outlining their criticisms of quantum mechanics titled “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete? ”. The EPR paper, as it is known, included an idea for an experiment that would test and prove who was right, classical physics or quantum mechanics. The test, however, was not thought possible. For 30 years the debate between the classical and quantum views continued.

Physicist John Bell brilliantly devised a feasible experiment involving entanglement using individual photons, light filters, and photon detectors. He calculated two sets of equations that predict the results: one using classical mechanics, the other using quantum theory. The predictions of classical and quantum theories give very different results. The theory that matches the experimental data must be the correct theory. It would not be until 1980 that the technology existed to perform Bell’s experiment. I am going to greatly simplify how the experiment works for clarity.

When a photon is split, each photon retains complementary properties of one another. That is, if a photon starts as “AB”, the individual halves of the photon become “A” and “B “(“B” is complementary to “A” and vice versa). If we measure one of the split photons as being “A”, the other must be “B”. In the experiment, the photon is split and the individual photons race through a path in opposite directions. They each go through a filter that polarizes the photons. Simply put, polarization orients the photon in a certain direction.

Imagine the photon as a sphere with a pole through it marking as “north” or “south”. Polarization flips the direction of the pole. So, polarized light becomes either “up” (north) or “down” (south). In this case, the complement of “up” is “down” and vice versa. Our photons can be labeled “A up” or “B down”; “A down” or “B up” depending on how the filter polarizes it which is completely random. If we were to send a pair of photons on separate and opposite directions without a filter, no polarization happens and the detectors would register “A” on one and “B” on the other invariably.

Add the filters, and the detectors register “A up”,”B down”,”B up”, or “A down”. Since the filters completely randomize each photon’s polarization, one detector could indicate an “A up” and the other could detect an “B up” for the same set of split photons, right? The Bell tests show that when when one detector registers “A up”, the other detector shows a “B down”. It’s not surprising the “A’s” are opposite to the “B’s”, it’s that their polarizations are always complementary, or opposite. How does the other photon “know” what the other polarization will be and act accordingly?

Are they still connected somehow? If not, does one photon somehow send information about its state to the other photon so it can act accordingly? If the photons do somehow communicate, the information they send must travel much faster than the speed of light and violate a fundamental physical law. Whatever the case, it shows our understanding of the universe is incomplete. Bell was a proponent of Einstein’s view of reality and didn’t expect quantum theory to be proven right. After witnessing a confirmation of his theory he said “I have seen the impossible done”.

The phenomenon of entanglement has been demonstrated in experiment after experiment and progressively separating the photons at greater distances. Recently in Vienna, an even more stringent test was completed by Professor Anton Zellinger. The tests have sent split photons from one island to another many kilometers away and had the same eerie result. Our whole description of fundamental reality has to be revised. After the latest confirmation of quantum theory in Vienna, Dr. Zellinger and his colleagues posted a help wanted. They are seeking a philosopher to help understand the profound implications.

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• Subject:

• University/College: University of Chicago

• Type of paper: Thesis/Dissertation Chapter

• Date: 6 November 2016

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