or faster-than-light communications is a go.
Quantum entanglement stands as one of the strangest and hardest concepts to understand in physics.
Two or more particles can interact in a
specific ways that leave them entangled, such that a later
measurement on one system identifies what the outcome of a similar
measurement on the second system - no matter how far they are
separated in space.
However, one possible explanation for
entanglement would allow for a faster-than-light exchange from one
particle to the other. Odd as it might seem, this still doesn't
violate relativity, since the only thing exchanged is the internal
quantum state - no external information is passed.
In other words, quantum entanglement
cannot involve the passage of information - even hidden, internal
information, inaccessible to experiment - at any velocity, without
also allowing for other types of interactions that violate
The standard explanation for this behavior involves what's called nonlocality:
That idea is uncomfortable to many
people (including most famously Albert Einstein), but it preserves
the principle of relativity, which states in part that no
information can travel faster than light.
Many of these fall into the category of hidden variables, wherein quantum systems have physical properties (beyond the standard quantities like position, momentum, and spin) that are not directly accessible to experiment. In entangled systems, the hidden variables could be responsible for transferring state information from one particle to the other, producing measurements that appear coordinated.
Since these hidden variables are not
accessible to experimenters, they can't be used for communication.
Relativity is preserved.
But could non-instantaneous,
faster-than-light hidden variables theories still work?
Obviously, one way to test it would be
to increase the separation between the parts of the entangled system
to see if we can detect a delay in apparently instantaneous
correlation we currently observe. Sufficiently fast rates of
transfer, however, would still be indistinguishable from nonlocality,
given that real lab measurements take finite time to perform (this
assumes that both experiments happen on Earth).
Next, they allowed the state of the
hidden variables to propagate faster than the speed of light, which
let them influence the measurements on the separated pieces of the
This is a violation of the no-signaling
condition, and causes serious problems for the ordinary
interpretations of quantum physics.
Once again, it would seem that local
realism and relativity are incompatible notions in the quantum