# Questions tagged [superposition]

A basic principle of solutions of *linear* differential (often wave) equations, ensuring that the sum ("superposition") of two solutions is automatically a solution as well. Conversely, solutions (amounting to quantum states in quantum mechanics, since the Schrödinger equation is linear) can be represented as a sum of two or more other distinct solutions, and so can be Fourier/eigenstate resolved to enhance mathematical tractability.

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### How do we know that superposition exists? [duplicate]

If I was a god that could see the state of a particle in superposition without touching it, wouldn't I know what the particle is at that moment? Wouldn't that make the superposition concept just ...
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### Question about Dirac notation (using the Braket notation) [duplicate]

How do I add two energies and calculate the expectation value given a set state in two $|nlm\rangle$ notation? $|\psi\rangle=\sqrt\frac{1}{2}|n_1l_1m_1\rangle+\sqrt\frac{1}{2}|n_2l_2m_2\rangle$ I know ...
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### Does QED evolve unitarily after the Schwinger limit?

If QED becomes nonlinear after the Schwinger limit, shouldn't QED no longer be unitary (above the limit) since linearity is a requirement of a unitary operator (and vice versa)? Does this mean that ...
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### 'Deriving' superposition representations for 3 particles

I'm trying to write up all the possible superposition states for 3 spin-1/2 particles (one spin-up, 2 spin-down). Lets denote $|\uparrow \rangle = |0\rangle, |\downarrow \rangle = |1\rangle$. ...
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### Can two perpendicular beams cancel each other out?

This question is related to the Michelson-Morley experiment. See below an illustration of the setup: When looking at this image, I am wondering where and how exactly the interference occurs. Is it ...
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### Has it been practically proven that quantum superposition exists ? If yes, how does it even work?

I was wondering if quantum particles do actually exists in two different states simultaneously and if it has been proven they do indeed exists in a superposition of states. How has it been figured ...
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### Understanding Huygens' principle: How is the direction of wave propagation determined? And why there is not destructive interference in every wave?

The Huygens' principle makes me a headache. It sounds ironic to me that many articles say that Huygens' principle helps understand the direction of propagation, and helps understand refraction. When ...
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### Can viruses be in a superposition? [duplicate]

I know that atoms and entire molecules and even sets of hundreds of molecules can be in superpositions of position eigenstates. Viruses are the smallest living organisms (if they can even be called ...
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### Superposition of Microwaves experiment? How does it work?

Please can someone help me out with this experiment and see if this is correct? So the experiment consists of one metal sheet that fully reflects microwaves (and is fixed in its position), and the ...
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### Amplitude of superposition states

I have a question about the following chart (for 3 and 4 particles case): What does $P_{cycl}$ mean? How's that relevant to the amplitude of superposition states and their symmetricity?
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### Superposition and Boundary Conditions in Standing Waves

A standing wave is formed due to superposition of two waves with same anplitude, wavelength and period but propagating towards opposite direction to each other. I would like to focus on how initially ...
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### What is a neutrino state if not a particle?

When reading about the 2015 Nobel prize and how this led to the possibility of the existence of sterile neutrinos I am told that: "(...) three active neutrinos $\nu_e$, $\nu_\mu$, $\nu_\tau$, are ...
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### Standing wave and energy conservation

Consider a one-dimensional standing wave formed on the interval $0 \leq x \leq L$. Let's say that this is an ideal model motivated by a string as a medium with both ends closed. Assume that there is ...
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### Why is quantum measurement not happening all the time? [duplicate]

I have a question which may be very naive yet I have no answer. I studied undergraduate quantum mechanics 4 years ago now and even if I studied more advanced stuff like QFT I feel like I don't ...
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### Proof of quantum superposition

My question is really very simple, how can one see that a spin 1/2 particle in a definite projection in z(say, up) is in a superposition of Sx states?
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### Time-dependent state probability harmonic oscillator [closed]

For my homework i am considering a harmonic oscillator which´s wavefunction at $t=0$ is the superposition of the eigenstates $\psi_n$. $$\psi(x,t=0) = \sum\nolimits_{n} c_n \cdot \psi_n(x)$$ Now i ...
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### Relations between symmetries in sources and fields

I've edited this question way more times that I like to admit. I'll do my best Relation between highly symetric charge distributions and $\nabla\times\mathbf{D}$: In electrostatics, for some charge ...
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### Quantum mechanically, is superposition a sum or a product?

This question may sound like a no-brainer, but I'm getting confused after watching this lecture (cf. the slide at minute 5:07). The context is to motivate the quantization of a field which, for the ...
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### Questions about Schrodinger's cat [duplicate]

While I was listening to my teacher present the basic idea's of Schrodinger's famous thought experiment, he said that because we can not SEE the cat, the cat exists in some sort of position in which ...
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### Is it ever possible to test whether an object is in a quantum superposition (versus a mixed state)?

Today I came across a paper, "Experiments testing macroscopic quantum superpositions must be slow," by Mari et al., which proposes and analyzes a thought experiment involving a first mass mA placed in ...
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### Representation of wavefunction as superposition of eigenstates

In Quantum Physics it is postulated that, any general state $\psi$ can be represented as superposition of eigenstates with constant coefficients corresponding to any observable. Say, I have all the ...
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### The superposition principle in quantum mechanics

This is a question parallel to this question The importance of the phase in quantum mechanics. In introductionary quantum mechanics I have always heared the mantra The superposition principle ...
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### Does QM talk about reality in itself or our observation of it? [closed]

I am not a physicist and I've recently started watching introductory lectures on QM on youtube (MIT, Stanford) and reading the Feynman lectures. I have a high-school level knowledge of Math so I'm not ...
Given the initial state: $$\Psi(x,t=0)=c_1 \psi_1(x)+c_2\psi_2(x)+c_yy(x)$$ where $\psi_1$ and $\psi_2$ are eigenstates of $\hat{H}$ and $y(x)$ is a normalizable function but is not eigenstate of \$\...