It is often said that scientists do their best work while young. With Albert Einstein this certainly seems to have been the case. Before the age of 40 he developed special relativity, laid the groundwork for quantum theory by explaining the photoelectric effect and in his greatest achievement, developed his elegant theory of gravity, general relativity. However, it was a paper he wrote with two colleagues in 1935-when Einstein was nearly 56 years old-which stands out as his most cited scientific paper. In fact, it may well turn out to be one of the most significant scientific papers of all time.
This is of course the “EPR” paper, written with his colleagues Boris Podolsky and Nathan Rosen. Following a decade of vehement arguments with the great Neils Bohr about the meaning of quantum theory, this paper stands out as Einstein’s “parting shot” in the debate-his last ditch effort to prove that quantum mechanics could not be a fundamental theory. The paper-titled “Can quantum mechanical description of reality be considered complete?”-uses quantum mechanics to demonstrate that particles which interact in someway become entangled, in a loose sense meaning that their properties become correlated. As we’ll see in a moment, this is not an ordinary correlation in any sense of the word. It implies that there exists a strange connection between the particles that persists even when they are separated by great distances. In some sense, this connection is instantaneous, putting it in direct conflict with the special theory of relativity. It was this strange connection that led Einstein to the phrase “spooky action at a distance”.
Quantum Entanglement
The EPR paper is based on the following thought experiment. Two particles interact and then separate. Furthermore, we imagine that they separate such that they are a great distance apart at a time when measurements on the particles can be made. EPR focused on two properties in particular-the position and momentum of each particle.
These properties or variables were chosen because of the Heisenberg uncertainty principle. The uncertainty principle tells us that the position and momentum of a particle are complimentary, meaning that the more you know about one variable, the less you know about the other. If you have complete knowledge of a particles position, then the particles momentum is completely uncertain. Or if instead you have complete knowledge of the particles momentum, then its position becomes completely uncertain. Intermediate ranges of accuracy are possible, the lesson to take home is that you cannot measure one variable without introducing some uncertainty into the value of the corresponding complimentary variable. The amount of uncertainty is quantified precisely by the uncertainty principle. The uncertainty of quantum mechanics never sat well with Einstein, he felt the theory, which is statistical in nature, is statistical because there exist some unknown or “hidden” variables in the microscopic world we are not yet aware of.
We now imagine that two particles interact and then move off in different directions. Because they have interacted, they become entangled. When two particles are entangled, the state of each particle alone has no real meaning-the state of the system can only be described in terms of the whole. In terms of elementary quantum mechanics, there is a wavefunction which describes the two particles together as a single unit. The wavefunction, being a superposition of different possibilities, exists in a ghostly combination of possible states. The Copenhagen interpretation tells us that the properties of the particle, position or momentum, don’t exist in definite values until a measurement is made.
When a measurement is made, and we can choose to make a measurement on one particle or the other, the wavefunction “collapses” and each particle is found to be in a definite state. The measurement results obtained for entangled particles are correlated. So if we make a measurement result on particle A and find its momentum to be a certain value, we know-without making a measurement on particle B-what its momentum is with absolute certainty. As EPR put it, by making a measurement of momentum on particle A, using momentum conservation tells us that pA + pB is an element of physical reality. In other words the wavefunction has collapsed and the variables have definite values-the ghostly superposition of possibilities is gone. The crucial point is that even though no measurement has been made on the distant particle B, the observer at the location of particle A has learned the value of B’s momentum. Somehow the wavefunction has collapsed instantaneously across a spatial distance-presumably in violation of the speed of light limit set by relativity.
The situation can be made even more interesting by noting that we can choose instead to measure the position of particle A. Again, using conservation principles, we will learn the value of the position of particle B, and the quantity qA - qB assumes physical reality.
Notice that the observer at position A can choose, by making different measurements that he or she desires, which properties of particle B assume definite values-or assume physical reality in the terminology of EPR. They can make this choice at a later time without any prior agreement with an observer in possession of particle B. This is another aspect of spooky action at a distance. The observer at A makes a measurement choice-presumably chosen using the free will of the mind-and forces particle B into a definite value instantaneously.
The interpretation of these results is still in debate, some believe that the wavefunction only represents our state of knowledge about the system. However it seems that it would be difficult for anyone who believes this to examine diffraction images from electron scattering and deny that the wavefunction is a real physical entity.
In summary, it appears that the position or momentum of each member of the EPR pair is determined by measurements performed on the other, distant member of the EPR pair. The effect seems to be instantaneous, leading Einstein and his colleagues to refer to the phenomenon as “spooky action at a distance”. The effect is non-local and appears to be instantaneous, but can anything useful come out of it? Can we exploit this to communicate faster than the speed of light? It turns out that as things are currently understood, the answer is no.
