I have read this question: In effect, the formerly spacelike direction inwards becomes timelike, and the singularity is in the future (and just as unavoidable as the future is for us). For someone "inside" the shell, the shell is in the past, and trying to go back to it is as futile as trying to...

In my physics notes there is a section on line spectra, and describing how absorbance line spectra can by used by astronomers to find out what gases are present in a star, should it be emitting white light from its core. I understand the concept of the absorbance by the atoms of photons of only s...

Often in condensed matter physics literature, one encounters a Hamiltonian that goes something like : $$ H = \sum_{i=1}^{n} J_{i}\ S_{i}^{z} S_{i+1}^{z}, $$ where $J_{i}$ are the coupling constants, $S_{i}^{z}$ representing the spin operator $S^{z}$ acting at $i^{\text{th}}$ site and $n$ being th...

A continuous wave signal of 'f' kHz will appear on a receiver plot tuned in at f kHz and nothing else, ie it is 1 Hz wide. However, apparently this is not the case with pulsed signals and I'm struggling to understand why. Apparently, even if the transmitted pulses all have a consistent frequency,...