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Femme_physics
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Does the wave–particle duality apply only to electrons and photons?
Femme_physics said:I wonder, is there an exact certain size where classical ends and quantum begins?
So a particle left to itself in a vacuum will act quantum, and with other particles and electromagnetic radiation, will act more "classical"?Every physical system is always interacting to some degree with the
environment it's immersed into (even if the system is in space, as
the 'vacuum' is actually filled with radiation). This interaction tends to
diminish the purely quantum effects (like interference or 'spreading')
associated with the 'wavy' nature of quantum objects.
For simple systems
like small molecules, single atoms or free electrons, the effect is generally
rather weak, but for more complex systems like large molecules and up,
it is much more noticeable, so these tend to behave much more classically.
For macroscopic systems, the effect is exceedingly strong and fast.
alxm said:'Wave' and 'particle' here are classical concepts. The quantum-mechanical viewpoint is more that they're neither, but depending on the context they can act more 'particle-like' or 'wave-like'. This applies to everything. To make this all a bit less mysterious, you can define 'particle-like' behavior as something which has a definite location in space, and 'wave-like' behavior as something which does not. A quantum-mechanical particle does not have a specific location in space. It only has a certain probability of being at a given location. But: Once you measure it, that probability distribution disappears. It's now at that location, and isn't exhibiting 'wave-like' behavior, for the time being at least.
alxm said:What happens after that, if you leave the particle alone, is that the probabilities of where you may find the particle spread out over space. It 'smears out' again.
alxm said:There are some situations where quantum effects become larger. You can deduce them from the conditions above: When particles aren't interacting a lot with their environment, such as when you have low temperatures, low pressures, or rigid materials (all amounting to fewer interactions/random bumping and jostling around). It's also in those kinds of situations you see quantum behavior on the larger scale, such as superfluids, superconductivity, Bose-Einstein condensates, etc.
sci-guy said:Isn't that somewhat true at a classical level too? If you have a water wave, it obviously has wave-like properties, but if you measure it at any point in time, it's at a specific place (like taking a photo of it).
My question here applies mainly to QM. My understanding is that the act of measuring changes the wave-particle. Is that only in the sense of reducing the wave state down to a point, or does it mean that, once measured, the wave changes in some way AFTER the measurement.
What about laser light, which (as far as I know) doesn't involve low temperature. What allows the macroscopic quantum behavior?
sci-guy said:Re effects of measurement on the system, I found the answer: measurement of the quantum state DOES affect the future evolution of the system, so that IS a difference between QM and classical (discounting the more common "observer effect" that may occur).
alxm said:A coherent state in quantum mechanics isn't the same thing as coherence in classical optics and wave mechanics.
Laser light is highly coherent, in the classical, in-phase sense. It also corresponds to a coherent state of the light field. The EM field isn't a macroscopic object, or even an object at all, really. The electrons emitting the light aren't in a coherent state, and aren't macroscopic either.
ArjenDijksman said:In quantum measurements, the probes (photons, electrons,...) with which we observe the system are in the same size range.
sci-guy said:I just looked into wavefunction collapse and the so-called "measurement problem," which IS (if this debated topic is proven true) an example of the act of measuring (i.e. "the physicist's mind") altering the outcome of the quantum state.
So that's distinct from what I pointed out in the previous post? Am I understanding correctly?
ArjenDijksman said:...mathematically, the best way we can describe it is as a linear combination of the allowed states. These states are represented by rotating vectors (just arrows). A rotating arrow has a position and a momentum (particle nature of the quantum system) and a periodically evolving phase (wavy nature). Its projection on an axis is a wave-function.
sci-guy said:Now I have some kind of a picture of what a wavefunction represents. One question though:
I had thought the wave part of the equation was that we don't know the exact position of the particle, but you seem to suggest that it pertains to the particle's evolving phase in time -- or were you just over-simplifying?
The wave-particle duality is a concept in quantum mechanics that states that all particles, such as electrons and photons, have both wave-like and particle-like properties.
Yes, the wave-particle duality is a fundamental principle in quantum mechanics and applies to all particles, including electrons and photons.
The wave-particle duality is important because it helps us understand the behavior of particles at a microscopic level and allows us to make accurate predictions about their interactions.
We know that the wave-particle duality applies to electrons and photons because of numerous experiments, such as the double-slit experiment, that have shown the dual nature of these particles.
While the wave-particle duality applies to most particles, there are some exceptions such as larger particles like atoms and molecules, which exhibit more classical behavior. However, they can still exhibit wave-like properties under certain conditions.