# Tag Info

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Let's be clear on what we call a particle. It is an object of which you can measure its physical properties like energy, momentum, charge or spin. None of those is space and for a good reason : space (or time) is not an intrinsic property of a particle. Space is a useful mean to describe the universe and the particles in it, nobody could deny that. But as ...

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There are no such things as particles in the physical world. The correct description of "small things" in classical mechanics is that the dynamics of the motion of the center of mass of an extended object is the only relevant physical quantity while internal degrees of freedom like rotation, vibration, magnetization, temperature etc.. can be ignored. That ...

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So, Quantum Mechanics implies that a particle has no trajectory whatsoever It depends what "whatsoever" means and what "particle" means and what "trajectory" means. All these words in physics depend on the framework. For distances larger than nanometers and energies larger than some kilo electron volts or so, the classical framework is what defines ...

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Quantum systems do not have a position. This is intuitively hard to grasp, but it is fundamental to a proper understanding of quantum mechanics. QM has a position operator that you can apply to the wavefunction to return a number, but the number you get back is randomly distributed with a probability density given by $|\Psi |^2$. I can't emphasise this ...

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Quantum physics has momentum. It's just not something fundamental. Saying that quantum physics doesn't have momentum because it's just the waveform evolving is like saying that an airplane doesn't have wings because it's really just a bunch of atoms.

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If you have a conserved probability (such as in nonrelativstic quantum mechanics), then you get a conserved current, the probability current. In almost all situations it will do for you what you want a velocity to do. Just don't try to get it to do more than you want by expecting it to be too classical. For instance the expectation value of momentum could ...

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completing the good Frank answer : Bell designed an experiment that cuts between forecasts of Einstein and Bohr. In a particular situation, Bohr's theory provides a result that other theories could not foresee. Technically, Bohr theory predicts $cos²(angle1-angle2)$ correlations where other theories could not expect better than a line sloping at 0.5. ...

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It all goes back to EPR experiment. In a paper published by Einstein, Podolsky and Rosen the authors argued that quantum mechanics is "incomplete". They argued that principles (such as the principle of locality) needed to be restored in order for it be a complete theory. The problem is, ultimately quantum theory is a "nonlocal" theory. What this means is ...

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WSC - Not all matter is made of fermions. Things such as helium-4 and carbon-12 are bosons, and there are are many other composite particles and molecules that are actually composite bosons. Any composite particle with an even number of fermions, and thus with an integer value of spin, is boronic, which to me seems a little moronic. I imagine that you are ...

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