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In quantum mechanics, the term “observer” is used in a specialized manner. It has a meaning quite different from that of the regular English word.1While the observer has an important role in some interpretations, it does not figure in others. I am speaking here in the terms of the original interpretation of quantum mechanics,

In quantum mechanics, an observer is anything that detects a quantum particle. Physicists say that an observer measures the properties of a quantum particle. Observation is also called “measurement.” Understanding the role of the observer depends upon understanding the special role of measurement in quantum mechanics. If you’re not familiar with this role, please see the article on measurement.

According to the Copenhagen Interpretation and some other interpretations, an observer (red arrow) transforms quantum waves to physical universe particles. [Image source:
Modification of David Chalmers and Kelvin McQueen, “Consciousness and the Collapse of the Wave Function” http://consc.net/slides/collapse.pdf]

Here’s an example of the role of the observer. A photon is emitted from the sun and hits the retina of a boy whom I’ll call “Felipe.” There is something that we need to understand about this photon. Prior to hitting Filipe’s retina, its position is not definite. We can’t identify its position, but, in addition, it actually has no specific position. It is a quantum wave.

The mathematical description of the wave2The mathematical description of the quantum wave is an equation called a wave function. It is derived from a quantum mechanical equation such as  Schrodinger's Wave Equation or the Dirac tells us only about probabilities that we will find it in particle form in specific positions. But the quantum  wave specifies no definite position. Upon hitting Filipe’s retina, the photon takes on a position. At that moment, it becomes a localized particle. The photon is absorbed by a specific electron in the retina and gives the electron additional energy. The energized electron creates an electrical impulse in Filipe’s brain and he has the experience of seeing. This, of course, is an oversimplified description of sight which leaves out quite a bit.

Observation and Information Theory

Observation requires a transfer of energy between the observer and that which is observed. In our example, the photon from the sun gives energy to the electron in Felipe’s retina. One way to think about the term “observer” is that the interaction is called observation because it is the moment at which the photon becomes observable. At this moment, it has created a physical change in our universe. In this way, it is observed and recorded in the history of our universe. An information theorist would say that the photon has created information.

Must the observer be conscious?

The use of the term “observer” in quantum physics can be confusing. It seems to imply it’s a human being or maybe a particularly astute cat. In the 1920’s and 30’s, pioneering quantum physicists like John Von Neumann and Eugene Wigner postulated that human consciousness collapses the wave function. But, today, most physicists reject this idea.

Consciousness, however, may play another role in quantum mechanics. Recently, physicists and philosophers have proposed that our universe is a simulation. This is the idea that we’re living in a virtual reality very much like characters in a video game. One version of this idea, proposed by philosopher, Nick Bostrom, is that we’re living in a computer simulation created by an advanced race of beings.

Another version of the simulation hypothesis, however, harkens back to an ancient Buddhist idea: Our physical universe is dream, a highly-lawful dream. (See, for example, the interviews and writings of cognitive psychologist, Donald Hoffman and of quantum physicist, Henry Stapp). In an updated version of the Buddhist idea, life is like a video game, a virtual reality. It runs in accordance with mathematical equations and is possibly fed data by the players of the game. In this view, the equations are fleshed out into an experiential reality by our consciousness. The virtual reality runs like a movie, a highly organized dream, in the mind. Possibly, it’s running in one all-encompassing mind that all of us players are plugged into.

How does this version of the simulation hypothesis relate to quantum mechanics? Quantum mechanics is a set of equations which underlies physical reality. How do we get from equations to the physical reality that we experience? Stephen Hawking asked the question this way: “What is it that breathes fire into the equations...?” One possible answer is consciousness.

Not all interactions result in observation. 

Not all interactions represent an observation and the creation of information. The observer must exchange energy with a particle for an observation to occur. Here’s an example of an interaction where there is no exchange of energy: when particles entangle with each other. The particles do not act as observers for each other. Instead, they retain their wavy quantum state. Later, however, if one of the particles is observed, it adopts properties in physical reality.

Light polarized first vertically, then horizontally. No light would be expected to exit the second polarizer. Two polarizers oriented perpendicularly to each other would block all light. [Image source: https://physics.stackexchange.com/questions/154121/non-polarised-light]

For example, consider an entangled pair of photons that are created when an ion passes through a crystal. The ion passes through the crystal and out shoot two photons. They were created at the same moment in spacetime. This causes them to be entangled. But they exist in the wavy quantum state. Then, one of the photons hits a detector screen in a physics lab. It energizes an electron in the screen. An observation has occurred—the screen is the observer. At the moment of observation, the photon takes on a particular direction of polarization. Should its entangled partner be detected and its polarization measured, it will be found to have a correlated polarization. This is the meaning of “entanglement”; the properties of the two particles are correlated despite any distance between them.

Any physical object can be an observer.  

Of course, the observer needn’t be a particle in a detector screen or even the retina of a human eye. Any particle—any atom, subatomic particle, or molecule—can act as an observer. So long as the observer interacts energetically with a particle, the particle loses its wavy quantum state and transforms into a particle. This is an observation (measurement).

FOOTNOTES
1While the observer has an important role in some interpretations, it does not figure in others. I am speaking here in the terms of the original interpretation of quantum mechanics, the Copenhagen Interpretation. The explanation provided here also has some application to other interpretations such as the Transactional Interpretation. In the Many Worlds Interpretation and the DeBroglie-Bohm Interpretation, for example, the observer does not play the role described here.
2The mathematical description of the quantum wave is an equation called a wave function. It is derived from a quantum mechanical equation such as  Schrodinger's Wave Equation or the Dirac Equation. Mathematically, the moment of observation (measurement) is the moment in which the wave function collapses. In wave function collapse, the probability of finding the particle (in this case a photon) where it was detected goes to 100% and the probabilities of finding it in all other positions falls to zero.

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