"I think I can safely say that nobody understands quantum mechanics." That's what Richard Feynman said in 1965, and it is not getting any easier. Scientists are reporting in the journal Science that they have linked the uncertainty principle and “spooky” nonlocal interactions.

Quantum measurements are governed by the Heisenberg uncertainty principle. It states that measurements of pairs of physical quantities, such as time and energy or position and momentum, are linked such that the more you know about one, the less you do about the other. If you know exactly where a particle is, you know nothing about its momentum, and vice versa.

When the states of two quantum systems are coupled, they are said to be entangled. For example, if a particle decays into two with opposite spins, quantum mechanics states that each particle has a 50 percent chance of being spin up or spin down. If you measure one particle to be spin up, then you have collapsed the wave function and changed the probability of the second particle to be 100 percent spin down.

So what happens when the two particles have moved apart? Entangled interactions are referred to as nonlocal interactions when the measurement of one of the particles would have to travel faster than the speed of light to collapse the wave function of the second particle. Einstein really did not like this, and referred to these interactions as "spooky," because one particle seems to instantaneously know what was measured on the other.

Linking these two apparently separate phenomena is the concept of steerability. Steerability refers to the ability to influence the states of one particle with measurements made on another. If two people each have access to an entangled system, then they each have some idea of what measurements each other have made, and they can make educated guesses as to the states of each other’s systems.

What the researchers have shown is that the strength of nonlocal interactions is a tradeoff between steerability and uncertainty. The more influence one has on the system, the more uncertainty there is. And the more you decrease the uncertainty, the less you can steer the measurements. In a nutshell, the uncertainty principle puts a limit on the amount of information that an entangled system can hold.

Science, 2010. DOI: 10.1126/science.1192065 (About DOIs).