Spooky Action at a Distance
Quantum Entanglement, the strangest phenomenon in quantum mechanics.
A man and a woman decide to get married. They tie the knot and are thus, “entangled” in a certain way. The wife becomes a widow if the husband passes away. Thus, the woman’s marital status is completely dependent on the man's life status. As soon as the man dies, the wife instantly becomes a widow.
In a sense, this is how quantum entanglement functions.
As discussed in a previous post, the key to understanding quantum physics are the fundamentals of superposition ie., quantum particles exist in several states concurrently until they are observed when the wavefunction collapses and they display a single state.
The spin states of the entangled electrons exhibit superposition as well. However, no 2 electrons can have the same quantum numbers. Thus, if one’s spin is observed to be anticlockwise, the other’s spin has to be clockwise.
This also demonstrates the non-locality of quantum physics ie., quantum particles are not only dependent on the conditions in their neighborhood. Even if the two electrons are separated by a huge distance (say, in different galaxies), as soon as you witness the spin of one electron, you will immediately know the state of the other entangled electron, meaning that one particle's observation influences the state of the other particle. This phenomenon occurs faster than the speed of light.
But nothing can move faster than the speed of light, according to Einstein's theory of relativity.
Einstein was really troubled about this occurrence. Together with Boris Podolsky and Nathan Rosen (EPR), he developed the hidden variables hypothesis to explain quantum entanglement without violating relativity. According to the hidden variables hypothesis, measurements or observations of one entangled particle's state do not affect the others since each particle has internal information about its state. The observed state of the other entangled particle has no impact on the particles; all that happens is that they physically exhibit the spin state information that they already had.
Neils Bohr vigorously disputed this in favor of his Copenhagen Interpretation. A future post on the Einstein vs Bohr debates will explore this in more detail. John Bell, however, proved Bohr right when his experiments effectively refuted the hidden variables theory.
So does this mean Einstein was wrong? Can we use quantum entanglement to achieve faster-than-light (FTL) communication? Unfortunately, the answer is NO.
Quantum entanglement only provides one of the observer's knowledge about the other state faster than light, it says nothing about the communication between the duo. If there is no communication between the two observers, altering the entangled particle's state itself won't help. Continuing with the widow analogy, even if the wife becomes a widow as soon as her husband dies she won't know it herself, unless she is informed of his death via communication that is slower than the speed of light.
To further illustrate the idea, consider the scenario in which both you, the reader, and I, the writer, receive one shoe from a pair in a locked box. The left shoe will go to one of us, and the right shoe to the other. You will be able to tell that I have a right shoe in my closed box as soon as you open yours and see a left shoe, but you will not be able to give me a message at the same speed verifying that I have gotten a right shoe. If the two shoes are entangled electrons, you would know my electron's spin as soon as you saw yours, but you wouldn't be able to tell if your electron's spin changed as a result of my observation since I wouldn't be able to send you my observation FTL.
Thus, as we can see Einstein had nothing to worry about quantum entanglement as it does not violate relativity since relativity essentially places an upper limit on the speed of information transfer which is not broken by this phenomenon.
An important point to note is that the entanglement between 2 quantum particles breaks upon measurement. Therefore, you cannot observe multiple entanglement phenomena between the same particles by repeating measurements.