Undergrad Significance of double slit experiment?

[LightningInAJar]

What is the significance of the double slit experiment? When I first learned about it I thought the human observer decided the outcome. But I guess a human observer isn't even needed and therefore a conscious mind is just as irrelevant. What does it prove that an outcome is neither one thing or the other until it is?

 

[phinds]

Typing your question into Google got as good an answer as I could give:

In modern physics, the double-slit experiment is a demonstration that light and matter can display characteristics of both classically defined waves and particles; moreover, it displays the fundamentally probabilistic nature of quantum mechanical phenomena.

 

[LightningInAJar]

Well everything is an odds game. Does it suggest the universe is less deterministic? Does it say anything regarding living observers?

 

[vanhees71]

According to quantum theory, which is the most comprehensive theory ever, which just means that there's not a single observation which contradicts it, and it was tested very carefully even in its most astonishing aspects, which all have to do with "entanglement", describing on the one hand "randomness" on a fundamental level (i.e., observables on parts of an entangled quantum system do not take determined values) but on the other hand also "stronger correlations than classically possible" when these observables are measured (the violation of the so-called Bell inequalities and related predictions of socalled "local realistic hidden-variable theories", which are realized by the classical, i.e., non-quantum, description of Nature).

The double-slit experiment is an example for the fact that certain aspects of the behavior of particles (but also macroscopic bodies) cannot be described in any way within classical physics. According to what was discovered in connection with quantum theory in 1926, e.g., an electron is neither correctly described as a classical point particle and the laws of (Newtonian or relativistic) mechanics nor as a classical field, but one needs in a sense both descriptions. In the old quantum theory (discovered by Planck, Einstein, Bohr, and others in 1900-1925) this was dubbed "wave-particle dualism", but it was always clear that this is not a consistent picture but just a set of more or less empirical rules. With modern quantum theory, discovered in 1925 by Heisenberg and then worked out in terms of three different mathematical descriptions by Born, Jordan, and Heisenberg ("matrix mechanics"), Schrödinger ("wave mechanics"), and Dirac ("transformation theory"), the inconsistencies have been resolved by Born's probability interpretation of what's called a "quantum state".

According to quantum theory it depends on how the particles are prepared initially (defining its quantum state before measurement) and what is measured in a given situation. This has nothing to do with consciousness of the observer, and no esoterical mechanism of realizing measurement results by a conscious being is needed, but just the knowledge about the interactions of the measured system with the measurement apparati, and this is part of the general physical laws as described by quantum theory.

 

[LightningInAJar]

According to quantum theory, which is the most comprehensive theory ever, which just means that there's not a single observation which contradicts it, and it was tested very carefully even in its most astonishing aspects, which all have to do with "entanglement", describing on the one hand "randomness" on a fundamental level (i.e., observables on parts of an entangled quantum system do not take determined values) but on the other hand also "stronger correlations than classically possible" when these observables are measured (the violation of the so-called Bell inequalities and related predictions of socalled "local realistic hidden-variable theories", which are realized by the classical, i.e., non-quantum, description of Nature).

The double-slit experiment is an example for the fact that certain aspects of the behavior of particles (but also macroscopic bodies) cannot be described in any way within classical physics. According to what was discovered in connection with quantum theory in 1926, e.g., an electron is neither correctly described as a classical point particle and the laws of (Newtonian or relativistic) mechanics nor as a classical field, but one needs in a sense both descriptions. In the old quantum theory (discovered by Planck, Einstein, Bohr, and others in 1900-1925) this was dubbed "wave-particle dualism", but it was always clear that this is not a consistent picture but just a set of more or less empirical rules. With modern quantum theory, discovered in 1925 by Heisenberg and then worked out in terms of three different mathematical descriptions by Born, Jordan, and Heisenberg ("matrix mechanics"), Schrödinger ("wave mechanics"), and Dirac ("transformation theory"), the inconsistencies have been resolved by Born's probability interpretation of what's called a "quantum state".

According to quantum theory it depends on how the particles are prepared initially (defining its quantum state before measurement) and what is measured in a given situation. This has nothing to do with consciousness of the observer, and no esoterical mechanism of realizing measurement results by a conscious being is needed, but just the knowledge about the interactions of the measured system with the measurement apparati, and this is part of the general physical laws as described by quantum theory.

Thank you very much.

 

[DennisN]

What is the significance of the double slit experiment?

In addition to the other answers above I would say it's one of the most important (both historically and scientifically) and well known experiments in physics. It's also a quite simple setup when you use light. It can also be made with massive particles, but this is a far more complicated setup.

As an example of its significance try searching for "double slit experiment" on this forum and you will get an amazing number of threads where it is discussed.