SPCS 2015 Frontiers of Physics
30 July 2015
Quantum Entanglement Isn’t so Spooky!
In the 1930s, Albert Einstein was upset with quantum mechanics, he proposed a thought
experiment, where according to the theory an event at one point in the universe could
instantaneously affect another event arbitrarily far away. He called this spooky action at a
distance because he believed it to be absurd. It seemed to imply faster than the speed of light
communication. Now this superluminal communication result did not correspond with any of the
works circulating at the time and was not supported by the concepts that defined Quantum
Mechanics. Something his theory of relativity ruled out. This spooky action at a distance stood
The EinsteinPodolskyRosen Paradox
The EPR Paradox is a thought experiment intended to demonstrate an inherent paradox in the
early formulations of quantum theory. It is among the bestknown examples of quantum
entanglement. The paradox involves two particles which are entangled with each other according
to quantum mechanics. Under the Copenhagen interpretation of quantum mechanics, each
particle is individually in an uncertain state until it is measured, at which point the state of that
particle becomes certain.
But nowadays we can perform this experiment and see this apparent spookiness. In order to
understand it, we must first understand spin. All fundamental particles have a property called
spin. It is an analogy used to convey its angular momentum, or the propensity and orientation in
space, rather than action. The spin of a particle can be measured in accordance with its direction.
This measurement can have only one of two outputs. Either the particle spin is aligned with the
direction of measurement, which is known as spin up. Or it is opposite the measurement, which
is known as spin down. After the measurement, the particle retains its spin.
The experiment Einstein proposed can be carried out using two particles . However; these need
to be prepared in a certain way. For example, generated spontaneously from energy.
Conservation laws state that the total angular momentum of the universe must remain constant, it
is easy to follow that if one particle is measured with spin up, the other measured in the same
direction must have spin down. Emphasis put upon the fact this works only when the two
particles are measured in the same direction that their spins must be opposite.
Einstein was a proponent for the idea that each particle was created with a clearly defined spin.
However; this would not work. Imagine their spins were vertical and opposite. If they are both
measured each particle has fiftyfifty chance for spin up. There would thus exist a fifty percent
probability that both measurements would yield the same result and this would violate the law of
conservation of angular momentum. According to quantum mechanics, these particles do not
have a well defined spin. They are entangled, which means their spin is opposite that of each
other. At the point of which the particle and its spin is measured, the other entangled particle’s
measurements immediately become known and clearly defined. This was checked thoroughly
and repeatedly by experiments. The angle of the detectors or the distance between them does not
affect the experiment. This is especially peculiar given the nature of locality. The principle of
locality is violated when it states that an object is only directly influenced by its immediate
surroundings.Therefore, the principle of locality implies that an event at one point cannot cause a
simultaneous result at another point. If these fundamental principles hold true for the rest of the
quantum mechanic world it would seem fair to assume that no two particles can relay an
immediate effect on one another across vast distances.
Quantum Entanglement: A Description
This phenomenon occurs in what can be described
as quantum entanglement. Two particles light years
away with undefined spins, where the
measurement of one particle instantaneously
influences the other to have the opposite spin. An
anomaly that suggests that information has been
relayed faster than the speed of light. Though some theorists interpreted the results this way,
Einstein did not. Einstein was a big proponent of predictability and determinism and proposed
that the information of what the spin’s direction would take was always there, but the hidden
information was not previously known until it was measured. There is an analogy suggested by
Dr. Brian Greene, of Columbia University to describe this alternate explanation. “Pretend that
you and a friend buy a pair of gloves. You place one glove inside one box and the other glove
inside another box. You take one box and travel to one side of the universe. Your friend takes the
other box and travels to the other side. You open your box, and find the left glove. You know
immediately that your friend is going to open the box and find the right glove. You don’t need to
call your friend on the telephone. Nor do you need to see inside the second box to confirm this
fact. The gloves are, in a sense, entangled. One glove can tell you all you need to know about the
other.” (Maliszewski, 2014)
A similar analogy is that of what Bohr suggests. Given two spinning wheels with a fifty percent
of landing on two different colors. The wheels would land on opposing colors in each instant.
The event is described to represent the “spooky” aspect of quantum entanglement.
Take Away Points
Although Bohr is correct in asserting that their is no predictability factor in quantum
entanglement, his representation of how quantum entanglement works is misleading. There is no
information transmitted or signal is sent. When the box was opened, you knew the information of
the other. The opening of the box alters the description of the “quantum state,” the math
describing the entangled system containing the two particles. The math is the collapse of the
wave function by the act of measurement. The wave function for the system of particles is a
nonseparable wave function, so interfering with particle y through measurement modifies the
wave function for particle x as well. Two entangled particles share the one wave function. It is
nonseparable because the two particles share at least one fundamental property, for example
mass, spin, energy, momentum, etc. There is no “spooky” action at a distance, rather it is
RealLife Applications: Namely Teleportation
Quantum entanglement has applications in the emerging technologies of today. Among these
quantum teleportation seems most enticing for its allure and convenience. The concept of
quantum teleportation uses entangled particles to transmit information. Quantum teleportation
can be achieved through the use of three photons:
Photon A: The photon to be teleported
Photon B: The transporting photon
Photon C: The photon that is entangled with photon B
“If researchers tried to look too closely at photon A without entanglement, they'd bump it, and
thereby change it. By entangling photons B and C, researchers can extract some information
about photon A, and the remaining information would pass on to B by way of entanglement, and
then on to photon C. When researchers apply the information from photon A to photon C, they
create an exact replica of photon A. However, photon A no longer exists as it did before the
information was sent to photon C.” (Bonsor, 2015)
In practice, physicists at the University of Geneva have succeeded in teleporting the quantum
state of a photon to a crystal over 25 kilometers of optical fiber.
This is only skimming the surface of the potential for quantum entanglement. The technology is
already underway and results are promising considering they follow predictions. I urge the
populus to put effort towards exploring this idea as it may well be our next best means of
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