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| wave-particle_duality [2021/02/08 20:33] – [Quantum Particles] admin | wave-particle_duality [2022/09/06 18:08] (current) – [Quantum Particles] admin |
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| We observe a wave-like interference between the two possible trajectories //only// if we cannot tell which trajectory was actually taken. Otherwise, we observe particle like properties, with no interference patter. | We observe a wave-like interference between the two possible trajectories //only// if we cannot tell which trajectory was actually taken. Otherwise, we observe particle like properties, with no interference patter. |
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| Niels Bohr introduced the principle of //**complementarity**/// to describe this: We need both particle and wave concepts to describe quantum systems, but we can only ever observe one of them at a time, depending on the experimental arrangement. It is not a case of either/or or both/and. They are complementary aspects of the same physical system. | Niels Bohr introduced the principle of //**complementarity**// to describe this: We need both particle and wave concepts to describe quantum systems, but we can only ever observe one of them at a time, depending on the experimental arrangement. It is not a case of either/or or both/and. They are complementary aspects of the same physical system. |
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| Bohr, along with many physicists, would say that it is meaningless to ask which trajectory a quantum particle takes when we are observing an interference pattern. Since you cannot observe the trajectory at the same time as observing the interference, it simply does not have a trajectory. Note that in popular science accounts, it is often said that the particle travels along //both// trajectories. But the conventional view is not //both//, not //either/or//, and not //neither//, but that the particle is in a new kind of physical state where the question itself is meaningless. | Bohr, along with many physicists, would say that it is meaningless to ask which trajectory a quantum particle takes when we are observing an interference pattern. Since you cannot observe the trajectory at the same time as observing the interference, it simply does not have a trajectory. Note that in popular science accounts, it is often said that the particle travels along //both// trajectories. But the conventional view is not //both//, not //either/or//, and not //neither//, but that the particle is in a new kind of physical state where the question itself is meaningless. |
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| The superposition principle is responsible for the wave-like interference effects we have been discussing. It holds because the equation of motion of quantum mechanics (the Schrödinger equation) is a linear, homogeneous differential equation, just like the wave equation in classical physics. | The superposition principle is responsible for the wave-like interference effects we have been discussing. It holds because the equation of motion of quantum mechanics (the Schrödinger equation) is a linear, homogeneous differential equation, just like the wave equation in classical physics. |
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| | {{:question-mark.png?direct&50|}} |
| | ====== In Class Activity ====== |
| | - In light of the double slit experiment, many physicists are inclined to say either: |
| | * Only the wavefunction exists. Other properties only come into existence when observed. |
| | * Nothing exists until observed. |
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| | Consider the experiment depicted below. |
| | {{ :einsteinsscreen.png?direct&600 |}} |
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| | In this experiment, which principle of physics would be violated by the "only the wavefunction" or "nothing exists" points of view? |
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