View by:







Schrödinger’s Cat and the Meaning of Quantum Theory

One of the basic problems of quantum theory is the relationship between measurement and reality. This is illustrated by the EPR Paradox. A good way to picture the problem is via a famous thought-experiment involving a hapless cat. The cat is in a box together with a canister of poisonous gas connected to a radioactive device. If an atom in the device decays, the canister is opened and the cat dies. Suppose that there is a 50-50 chance of this happening. Clearly when we open the box we will observe a cat that is either alive or dead. But is the cat alive or dead prior to the opening of the box?

Quantum orthodoxy (Copenhagen interpretation)

The dominant view in quantum mechanics is that quantum probabilities become determinate on measurement - that the wave function (see The Schrödinger Wave Equation) is collapsed by the intervention of classical measuring apparatus. This means that the cat is neither alive nor dead until the box is opened. The cat is in an indeterminate state. It merely has some specifiable probability of being alive, or, on the other hand, dead.

This interpretation is usually allied with a tendency to extreme instrumentalism. On such a view the probabilities generated by the Schrödinger Wave Equation do not correspond to any physical reality. There simply is no reality to be described until an act of measurement collapses the wave function. Quantum mechanics is merely a useful calculating device for predicting the possible outcomes of such acts of measurement.

In spite of its dominance in the textbooks, this interpretation is hardly satisfactory. To begin with, it may be regarded as proposing a dualism in physical reality: two worlds - an indeterminate quantum world and a determinate classical world. Then there is the problem of what constitutes classical measuring apparatus. At what level does the wave function actually collapse?

The act of measurement that collapses the wave function cannot be limited to scientific instruments. After all, why should we assume that our scientific measurements are solely responsible for collapsing the wave function? This would give rise to a most peculiar world - one that was indeterminate until the evolution of hominids.

Other interpretations

Some physicists, e.g. Wigner and Wheeler, have identified the classical measuring apparatus of the Copenhagen interpretation with consciousness. If so, they must be using a much broader definition of consciousness than is usual. What level of consciousness would be needed to make something determinate? Is the cat sufficiently conscious to determine the outcome of the experiment? Would earthworms do? What about viruses? The effect of pursuing this line of inquiry is to move towards a form of panpsychism - the doctrine that every part of the natural world no matter how humble is in some sense conscious!

An alternative might be to ask does God collapse the wave function? (Click on this related topic to see the problems with this point of view.)

Returning to the classical measuring apparatus, perhaps we should put the emphasis on ‘classical’ rather than ‘measuring’ - stressing not so much our intervention in the system as a transition from the world of the very small, in which quantum principles operate, to the everyday world of classical physics. This ‘neo-Copenhagen’ interpretation has the merit that it avoids the absurdities of the consciousness-based approaches. However, we are still faced with the difficulty of identifying an acceptable transition point. One suggestion is that we choose the level at which physical phenomena become so complex that they are irreversible.

Two other interpretations are the hidden-variable theory of David Bohm and the many-worlds interpretation.

Email link | Feedback | Contributed by: Dr. Christopher Southgate
Source: God, Humanity and the Cosmos  (T&T Clark, 1999)

Topic Sets Available

AAAS Report on Stem-Cells

AstroTheology: Religious Reflections on Extraterrestrial Life Forms

Agency: Human, Robotic and Divine
Becoming Human: Brain, Mind, Emergence
Big Bang Cosmology and Theology (GHC)
Cosmic Questions CD-ROM Preview...
Cosmic Questions Interviews

Cosmos and Creator
Creativity, Spirituality and Computing Technologies
CTNS Content Home
Darwin: A Friend to Religion?
Demystifying Information Technology
Divine Action (GHC)
Dreams and Dreaming: Neuroscientific and Religious Visions'
E. Coli at the No Free Lunchroom
Engaging Extra-Terrestrial Intelligence: An Adventure in Astro-Ethics
Evangelical Atheism: a response to Richard Dawkins
Ecology and Christian Theology
Evolution: What Should We Teach Our Children in Our Schools?
Evolution and Providence
Evolution and Creation Survey
Evolution and Theology (GHC)
Evolution, Creation, and Semiotics

The Expelled Controversy
Faith and Reason: An Introduction
Faith in the Future: Religion, Aging, and Healthcare in the 21st Century

Francisco Ayala on Evolution

From Christian Passions to Scientific Emotions
Genetic Engineering and Food

Genetics and Ethics
Genetic Technologies - the Radical Revision of Human Existence and the Natural World

Genomics, Nanotechnology and Robotics
Getting Mind out of Meat
God and Creation: Jewish, Christian, and Muslim Perspectives on Big Bang Cosmology
God, Humanity and the Cosmos: A Textbook in Science and Religion
God the Spirit - and Natural Science
Historical Examples of the Science and Religion Debate (GHC)
History of Creationism
Intelligent Design Coming Clean

Issues for the Millennium: Cloning and Genetic Technologies
Jean Vanier of L'Arche
Nano-Technology and Nano-ethics
Natural Science and Christian Theology - A Select Bibliography
Neuroscience and the Soul
Outlines of the Science and Religion Debate (GHC)

Perspectives on Evolution

Physics and Theology
Quantum Mechanics and Theology (GHC)
Questions that Shape Our Future
Reductionism (GHC)
Reintroducing Teleology Into Science
Science and Suffering

Scientific Perspectives on Divine Action (CTNS/Vatican Series)

Space Exploration and Positive Stewardship

Stem-Cell Debate: Ethical Questions
Stem-Cell Ethics: A Theological Brief

Stem-Cell Questions
Theistic Evolution: A Christian Alternative to Atheism, Creationism, and Intelligent Design...
Theology and Science: Current Issues and Future Directions
Unscientific America: How science illiteracy threatens our future
Will ET End Religion?

Current Stats: topics: >2600, links: >300,000, video: 200 hours.