Random Musings: Observer Effect

One of my favorite subjects is the observer effect: when making a measurement on a system, the act of measurement alters the state of the system.

I first learned about the observer effect through the lens of quantum mechanics, where the concepts are mind boggling and the math is a bitch. More specifically, quantum superposition: an electron exists in every possible state until a measurement is made on that electron, causing its wavefunction to collapse into a single state (e.g. measuring the location or spin of an electron in orbit around the nucleus of an atom)

The observer effect is demonstrated by a relatively simple experiment: double-slit interference, or Young's Experiment (..sort of)[1].

A home-made double-slit interference experiment is possible, although it may take some patience. The basic procedure:

• Cut two parallel slits in a piece of cardboard (preferably black), separated by a millimeter or so (doesn't have to be exact). 
• Place the cardboard in front of a screen and shine a high intensity beam (e.g. laser pointer) onto the cardboard (may have to adjust the distance from the slits to the screen).  
• The resulting pattern on the screen is a series of alternating light and dark spots, like this photo from an MIT Physics Technical Services Group experiment:

Or just watch the experiment to see how it is done w/ fancy lab equipment.

And if you really like math, check this out! 

Double-slit interference is due to both the particle- and wave-like properties of light. When light waves pass through a small slit, they diffract, or bend. The direction of the light waves changes as depicted in the photo below.

The bent light wave emerging from each interfere with each other. When the light waves are in series (e.g. a wave crest hits another wave crest), they add together to produce a bright spot, known as constructive interference. When the two light waves are out of phase (e.g. a wave crest hits a wave trough), they cancel each other out and produce a dark spot, known as destructive interference.

Light waves, and all electromagnetic waves, are carried by photons (massless elementary particles). Photons transport the energy of the laser beam to the cardboard, through the slits and to the screen where they generate the interference pattern. The higher intensity (aka brightness) of a light beam, the more photons are being emitted.

Ok, so back to the Observer Effect!
Take a double-slit interference experiment and put in light detectors spanning each slit. Since photons are particles, the detector records when each quanta of light passes through the two slits. This data allows us to calculate how many photons pass through each slit. Logically, we expect that approximately 50% of the photons pass through one slit, while the other 50% pass through the other slit. Seems reasonable.

Well, it doesn't happen. The interference pattern disappears. When there are detectors recording the path of the photons, all of the photons pass through a single slit. Remove the detectors, and the interference pattern reappears.

Note that the observer doesn't necessarily have to be a "conscious being"; a computerized detector works just as well as our eyeballs and subsequent electrochemical reactions. What is so insane to me is that somehow the photons are aware of the detector. My best understanding is that the measurement device, or the "observer," forces the photon to choose a slit to go through, rather than passing through both slits at the same time (literally being in two places at the same time, aka quantum superposition).

The more I learn about the how the universe works, the more I am seriously blown away.