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Quantum Mechanics
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from http://www.quantumphysicslady.org/what-does-measurement-mean-in-quantum-mechanics/
In quantum mechanics, when physicists measure a property of a quantum particle, like a photon, they are really saying that they detect one of its properties. The key to understanding why “measurement” is of such consequence in the quantum world is understanding that detection creates a fundamental change in a particle’s condition.
For example, let’s take the detection of a photon. Here’s a highly simplified experiment: we hold an extremely dim lamp in front of a photographic plate. It’s so dim that it emits only one photon at a time. The photon exposes the plate in one spot and makes a mark there. Once the photon hits the plate, we know the position of the photon—it’s a certain distance from each edge of the plate. So, we have detected it and, specifically, we have detected its position. Quantum physicists would say that we have “measured” its position.
This might seem just like measuring anything in the macroscopic world. For example, let’s say that a biologist uses a microscope to detect the position of a living bacterium in a living human cell on a slide. She measures the distance of the bacterium from each edge of the cell.
Here’s the difference: prior to the biologist measuring the position of the bacterium, the bacterium was happily swimming about the cell. After the measurement, same thing: happily swimming bacterium.
What was the photon doing before the measurement, before it hit the photographic plate? In our physical universe, it wasn’t doing anything because it hadn’t yet made an appearance in our universe.
from http://www.quantumphysicslady.org/glossary/wave-function/
In Newtonian physics, physicists are able to calculate exact positions of objects, given sufficient data. The inability to calculate aspects of the collapse of the wave function and why a measurement instrument should cause this collapse is called the “Measurement Problem.”
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