Sabrina Shah
Tutor: Louise Dash
Word Count: 547
Why Doesn’t Quantum Entanglement Violate Relativity?
Quantum entanglement has been an area of interest due to the seemingly faster than light
communication between particles. Commonly known as ‘action at a distance’, quantum
entanglement is where particles are linked together in such a way that if the quantum state
of one of the particles is measured and becomes known, the quantum state of the other
particle can also be immediately determined. For example, quantum particles have an
internal property known as spin. If a particle with quantum spin zero decays into two other
particles each with spin 1/2, one particle must have spin +1/2 while the other has spin -1/2
in order for the total spin to be zero – the two particles are entangled. Therefore, if you
know the spin of one, you immediately know the spin of the other. The Copenhagen
interpretation of quantum mechanics states that a particle exists in a superposition of all its
possible states at once until one makes a measurement, at which point the wave function
collapses and the particle is forced to choose a state. (Faye 2002) If one of the entangled
particles has its quantum state measured so it becomes certain, the quantum state of the
other particle simultaneously also becomes certain, even if it is miles away; there appears to
be some kind of instantaneous communication between the two particles that occurs
regardless of whether they’re a millimetre apart, or on opposite ends of the universe.
(Kaiser, 2014) If you were to also measure precisely the momentum of one of the particles,
by the uncertainty principle the measurement of the particle’s position would become less
precise. Curiously, the measurement of the position of the entangled partner particle would
also become less precise. (Gribbin, 1984, 182) Results from experiments have suggested
that the interactions between the particles can happen at 10,000 times the speed of light.
(Ghose, 2014)
Einstein’s theory of special relativity has the consequence that nothing is able to travel
faster than the speed of light in a vacuum. This is due to the well-known equation E=mc2,
which depicts that energy and mass are interchangeable. Mass increases as one gets closer
to the speed of light; at the speed of light ones mass would be infinite and an infinite
amount of energy would have been required to accelerate to that speed. (Hawking, 1988,
21) Hence there is an apparent discrepancy between the faster than light communication
that occurs between entangled particles, and special relativity. This was viewed as a paradox
and one suggestion for resolving this was to assume that the particles had some prior
knowledge of the quantum state of their entangled partner particles before measurements
were undertaken. These are known as hidden variables, and they do away with the need for
the particles to communicate. (Tuloch, 2014) However, experiments were conducted which
were inconsistent with the existence of local hidden variables, and instead supported the
standard interpretation of quantum mechanics.
Another way of overcoming the paradox was to notice that no external information is
actually being transferred between the particles, merely their internal quantum state.
Sabrina Shah
Tutor: Louise Dash
Word Count: 547
(Francis, 2012) So in conclusion, while it may seem like the particles are communicating
faster than the speed of light, this is not the case and the rules set by relativity still stand: no
laws are violated.
Bibliography
Kaiser, D. (2014). Is Quantum Entanglement Real?. [online] The New York Times. Available
at: http://www.nytimes.com/2014/11/16/opinion/sunday/is-quantum-entanglementreal.html?_r=0 [Accessed 22 Nov. 2014].
Tuloch, David. (2014). Do hidden variables exist for quantum systems. [online] Science
Clarified Available at: http://www.scienceclarified.com/dispute/Vol-2/Do-hidden-variablesexist-for-quantum-systems.html [Accessed 22 Nov. 2014].
Ghose, Tia. (2014). Loophole in Spooky Quantum Entanglement Theory Closed. [online] Live
Science. Available at: http://www.livescience.com/28808-spooky-quantum-entanglementloophole-closed.html [Accessed 22 Nov. 2014].
Francis, M. (2012). Quantum entanglement shows that reality can't be local. [online] Ars
Technica. Available at: http://arstechnica.com/science/2012/10/quantum-entanglementshows-that-reality-cant-be-local/ [Accessed 22 Nov. 2014].
Faye, J. (2002). Copenhagen Interpretation of Quantum Mechanics. [online]
Plato.stanford.edu. Available at: http://plato.stanford.edu/entries/qm-copenhagen/
[Accessed 22 Nov. 2014].
Stephen Hawking, 1988, A Brief History of Time: From the Big Bang to Black Holes. New
York. Bantam Books.
Gribbin, John, 1984, In Search of Schrodinger’s Cat. New York. Bantam Books.