# Tag Info

### Why is it impossible to measure position and momentum at the same time with arbitrary precision?

When someone asks "Is it really impossible to simultaneously measure position and momentum in quantum theory?", the best preliminary answer one can give is another question: "what do ...
Accepted

### The importance of the phase in quantum mechanics

When people say that the phase doesn't matter, they mean the overall, "global" phase. In other words, the state $|0 \rangle$ is equivalent to $e^{i \theta} |0 \rangle$, the state $|1\rangle$ is ...
Accepted

### Why do eigenvalues correspond to observable quantities?

Suppose we don't know quantum mechanics yet and we want to calculate the expecatation value of an observable $A$. Could be momentum, spin whatever. It is given by $$\mathbb E(A)=\sum_i a_i\,p(a_i)$$ ...
Accepted

### Why are wavefunction collapses instantaneous?

It’s important to remember that quantum mechanics is a tool that we use to describe the world — it is not the same as the world. For all that we love to talk about wavefunctions, it’s not clear at ...

### Do we or do we not observe (measure) superpositions all the time?

To answer the question, you have to specify what observable’s eigenstates the “superpositions” you’re talking about are superpositions of. If you measure the energy, you always get one single energy ...
Accepted

### Why do we use Eigenvalues to represent Observed Values in Quantum Mechanics?

One of the postulates of quantum mechanics is that for every observable A, there corresponds a linear Hermitian operator A^, and when we measure the observable A, we get an eigenvalue of A^ as the ...

### Are observables in QFT actually observable?

A quantum mechanical secret kept very well in plain sight is that the word "observable" when defined as "self-adjoint operator on a Hilbert space" does not actually mean "you ...
Accepted

### Does the collapse of the wave function depend on the observer?

The measurement problem is one of the most relevant open problems of quantum mechanics. What is a measurement? What constitutes an observer and what doesn't? Is the wavefunction a physical object (...

### The importance of the phase in quantum mechanics

The global phase does not matter. In your example $\lambda(\vert\psi_1\rangle+\vert\psi_2\rangle)$ has the same physical contents as $\vert\psi_1\rangle+\vert\psi_2\rangle$ but this will be in ...
Accepted

### Why is it impossible to measure position and momentum at the same time with arbitrary precision?

You can't measure precise values at the same time because precise values for both don't exist at the same time. All the properties of, say, an electron and be inferred from the electron's wave ...
Accepted

### Confusion about the description of the uncertainty principle

The first one is correct, the second is not. The second definition1 is actually describing the observer effect. Explanations written by non-experts often mix the two up. But one key difference is ...

Accepted

### Is the Copenhagen interpretation still the most widely accepted position?

This is very hard to quantify, particularly because it's hard to define the population that should be under consideration. The best(-known) attempt to do this is reported in the paper A Snapshot of ...