What is the name for a phenomenon where the presence of the observer changes the nature of the observed?

Asked by: Lee Pritchard


The link between reality and observation is based on what has been called the 'Copenhagen Interpretation' of quantum mechanics because it was proposed by Niels Bohr, Werner Heisenberg, and other physicists working in that city. A more colorful and memorable reference, however, is probably one based on a thought experiment. That experiment puts a cat in a box with a device triggered by a single particle's quantum behavior. The device, if activated, kills the cat. Since quantum theory says that the particle's behavior is indeterminate until its probability wave 'collapses' upon observation, the cat can be considered both alive and dead at the same time until the box is opened and one or the other condition is observed.

That animal in the box is known as 'Schrodinger's Cat', named for the physicist who proposed the scenario. So the 'Schrodinger Cat phenomenon' might be another name you'd see associated with this phenomenon.

This view of nature is obviously speculative, and raises questions about what kind of observer is necessary...need it be a human, could it be another cat, or an earthworm? There are other interpretations of 'quantum weirdness', but they all have strange aspects to them because of the unexpectedly bizarre nature of the quantum universe.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor

This phenomenon is called the Heisenberg uncertainty principle. It states the we cannot precisely know both the position of a particle and its momentum (mass times velocity). Its easy to consider this example. Imagine a dark room with a particle in it. In order to find it, we have to turn on a flashlight. When the flashlight hits the particle, we will for an instant know the position but the photons in the light will hit the particle changing its velocity, so we can only know one aspect of a particles behavior with certainty at a given time. This is why we always refer to electrons existing in a probability cloud, since we know where they are likely to be, but aren't sure of an exact position.
Answered by: Mike Perkins, Physics/Astronomy Major, Penn State

The cat in the box! Have you ever heard of the cat in the box? Well, have you ever heard of Erwin Schroedinger? He is the man who first understood that the act of observing affected what was observed. He came up with this idea of putting a cat in a box.. Along with the cat is a device with an atom that could decay (radioactive decay) and cause a poison to be released and kill the cat. Or, the same atom could decay and not cause the release of a poison and the cat would live. It all depends on the nature of the atom's decay. Since the probability of the atom decay one way or the other is 50-50 we have no way of knowing which way the atom decayed until we opened the box and looked at the cat. There is nothing unusual about this experiment until you ask about the cat before the box is opened. By the way, this experiment was never done! This is what is called a thought experiment.

OK, do you want to ask the question? Before the box is opened, is the cat dead or alive? In the answer to this question lies the whole key to quantum mechanics. The answer is ...... are you ready? The answer is....... Both! Both! The cat is both dead and alive. It is only at the moment in time that we actually look at the cat that one or the other reality becomes the reality upon which the next question about the cat can be asked.

From here we can go to two places: One, the act of observing determines what is observed. This is sort of like observing a date. If an adult scientist observes the behavior of teenagers on a date and then reports what he observes no one who has ever been a teenager will believe what he reports! Two, both realities occurred at the same time and we are living in the universe in which our observation is the one that leads to the next set of questions. But, there is another universe in which the second option was observed and in that universe that is the reality that leads to the next set of questions.

I like what Einstein said: 'God does not play dice with the universe.' But there are enough people who think enough of this multiuniverse idea to have made a TV show based on it. If you get the Sci-Fi channel look for a show called 'Sliders'.. The premise is that people have found a way to get from one universe to another but they are lost and cannot find their way back to the one they came from. There is also a great book by Greg Egan called 'Quarantine'. The premise here is that the creatures who live in the universe are sick a tired of us observing them and thereby making that reality their reality. They prefer to live in there quantum state wherein they are aware of all the possibilities and they can make their choices according to there de But that is fiction. Here is something even stranger. Have you heard about the quantum computer? How about a computer that uses the quantum state of an atom as its memory? Or how about a computer that can be aware of all the possible outcomes of a problem and knowing this can choose the solution that is correct for our reality? For example, suppose it is looking for a code. It can be aware of all the possible codes at once and choose instantly the correct one without having to try each one individually.

This sure is strange stuff. Imagine how strange it would be if we actually understood it.
Answered by: Tom Young, B.S. Science teacher, Whitehouse High School