THE OBSERVER

The Protium Atom under scrutiny.

According to the Heisenberg Uncertainty Principle, there is a limit to the amount of information that you can define for a quantum particle.  The more precise you measure one attribute, the less you know of the other. This established the concept of  complementarity as one of the basic principles of quantum mechanics.  Niels Bohr maintained that quantum particles have both "wave-like" behavior and "particle-like" behavior, but can exhibit one kind of behavior only under conditions that prevent exhibiting the complementary characteristics. This complementarity has come to be known as the wave-particle duality of quantum mechanics.

In the double-slit experiment, the common wisdom is that the Heisenberg Uncertainty Principle makes it impossible to determine which slit the photon passes through without at the same time disturbing it enough to destroy the interference pattern.

There have been many experiments trying to circumvent the issue of disturbance due to direct measurement of a photon.  The delayed choice quantum eraser experiments "found a way around the position-momentum uncertainty obstacle and proposed a quantum eraser to obtain the 'which-path' or particle-like information" without disturbing the wave function.  They could then choose to 'see' the path that the particle took later, or erase it.

It was found that there is no interference pattern when which-path information is recorded, even if this information was obtained without directly observing the original photon, but that if you somehow "erase" the which-path information, an interference pattern is observed.  The total pattern of signal photons at the primary detector never shows interference, so it is not possible to deduce what will happen to the idler photons by observing the signal photons alone, which would open up the possibility of gaining information faster-than-light (since one might deduce this information before there had been time for a message moving at the speed of light to travel from the idler detector to the signal photon detector) or even gaining information about the future (since as noted above, the signal photons may be detected at an earlier time than the idlers), both of which would qualify as violations of causality in physics.

This has lead many to remark that the observer is somehow important to the results.  This gives the observer some control over reality, or implies that reality can not exist without the observer (tree falls in the forest concept).  The conciousness causes collapse interpretation attributes the process of wave function collapse (directly, indirectly, or even partially) to consciousness itself.

The other important weirdness to note is that time is somehow held in suspension until the observation is made.  This is most graphically shown by quantum entanglement.  Entangled particles can be at opposite ends of the universe and still 'pass' information to the entangled twin. These particles are entangled in space-time, meaning that they are also entangled in time, according to S. Jay Olson and Timothy C. Ralph of Australia's University of Queensland.  The time component may play a larger role in understanding the quantum process than previously thought.  The entangled particles may be linked in time, rather than through space.