# Mark Mitchison

reputation
112
bio
website        www3.imperial.ac.uk/…
location       United Kingdom
age
member for     7 months
seen           2 hours ago
profile views  188

Currently doing a PhD in Controlled Quantum Dynamics at Imperial College and the University of Oxford.

|            |                  | bio            | visits                |             |          |
|------------|------------------|----------------|-----------------------|-------------|----------|
|            | 1,691 reputation | website        | www3.imperial.ac.uk/… | member for  | 7 months |
| 112 badges | location         | United Kingdom | seen                  | 2 hours ago |          |

# 343 Actions

|       |          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|-------|----------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| 2h    | comment  | Is every quantum measurement reducible to measurements of position and time?Also see the comments of Dan Stahlke and Jess Riedel for a slightly better explanation of what I was trying to say :)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
| 2h    | comment  | Is every quantum measurement reducible to measurements of position and time?But on matters of theoretical principle, it clearly should be possible to phrase the outcome of any measurement process in terms of final positions of particles, just by pushing the quantum/classical boundary back a few steps further. Often it only matters that one can do something in principle for a particular theoretical manipulation to be valid.                                                                                                                                                                                                                                                       |
| 2h    | comment  | Is every quantum measurement reducible to measurements of position and time?You make a good point about electromagnetic field intensities, but it is easy to shift the emphasis again to positions. After all, "reading off measurements with our eyes" involves conformation changes of certain proteins within retinal cells. These in turn stimulate an action potential as different ionic species are pumped across the membranes of neurones, and you can keep going like this all the way until you hit the question of what is consciousness anyway. All of these involves changes in positional degrees of freedom. Of course, in practical terms this view hugely complicates matters. |
| 2h    | comment  | Is every quantum measurement reducible to measurements of position and time?I'll admit this is a matter of opinion, but I think you have missed Feynman's point. The question is where do you draw the boundary between quantum and classical, or more precisely, how you fix the boundary conditions of the quantum path integral.  Depending on this choice you can say that the "outcome" was a set of positions, a set of field strengths, a set of spin orientations, whatever. As long as all of these results are consistent with the same physics between "observations", the choice is irrelevant - QM does not give a prescription for where exactly the observation takes place.      |
| 3h    | revised  | Can a diver swim a short distance in great depths without being physically crushed by the pressure?added 41 characters in body                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
| 3h    | comment  | Can a diver swim a short distance in great depths without being physically crushed by the pressure?Hence my "pretty much no risk" as opposed to no risk whatsoever ;) But yes, this is absolutely correct, thanks for the clarification. Not only would the time have to be short, but you only have a single lungful of air. Even if you could remain at depth long enough for the gas in your lungs to chemically equilibrate with your slowest tissues, I doubt that you would get bent on ascent. But I haven't done any calculations to prove it :)                                                                                                                                         |
| 4h    | revised  | Can a diver swim a short distance in great depths without being physically crushed by the pressure?added 12 characters in body                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
| 4h    | answered | Can a diver swim a short distance in great depths without being physically crushed by the pressure?                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
| 6h    | revised  | Is every quantum measurement reducible to measurements of position and time?added 79 characters in body                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
| 6h    | answered | Is every quantum measurement reducible to measurements of position and time?                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
| May8  | revised  | Slinky reverb: the origin of the iconic Star Wars blaster soundadded 2 characters in body                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
| May8  | asked    | Slinky reverb: the origin of the iconic Star Wars blaster sound                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
| May5  | comment  | Do we always ignore zero energy solutions to the (one dimensional) Schrödinger equation?                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| May4  | comment  | Do we always ignore zero energy solutions to the (one dimensional) Schrödinger equation?@BenCrowell But all the basis states $|x + 2n \pi\rangle$ with $n$ integer are physically equivalent. I would argue that the correct variance of the position is finite ($2\pi / \sqrt{3}$ if my algebra's right). Presumably the apparent conflict with the canonical position-momentum uncertainty relation is resolved by properly deriving the analogous expression on a compact space with periodic BCs. I'm going to bed but will try to formalise this tomorrow. Or leave it as a challenge...                                                                                                    |
| May3  | comment  | Coulomb interaction and conservation lawsYou're welcome, hopefully Emilio's detailed response has answered your question.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
| May3  | comment  | Coulomb interaction and conservation laws@DaPhil I disagree, in condensed matter physics the natural basis to choose is plane (Bloch) waves because the free Hamiltonian is invariant under (discrete) translations. In quantum chemistry the "free" problem is obviously not translation invariant and momentum is not conserved. Choosing a basis of momentum eigenstates to make the translation invariance of the Coulomb potential manifest would be a bizarre convention. Far more important is the nuclear Coulomb potential, which is not translation invariant (in the Born-Oppenheimer approximation), so molecular orbitals are a natural basis set.                                  |
| Apr27 | comment  | On the Aharonov-Bohm effect, and the reality of the classical fields                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
| Apr23 | answered | Ratio of Size of Atom to Size of Nucleus                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| Apr23 | revised  | Free 1d proton in magnetic fieldadded 194 characters in body                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
| Apr23 | revised  | Free 1d proton in magnetic fieldadded 427 characters in body                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
{"extraction_info": {"found_math": true, "script_math_tex": 0, "script_math_asciimath": 0, "math_annotations": 0, "math_alttext": 0, "mathml": 0, "mathjax_tag": 0, "mathjax_inline_tex": 3, "mathjax_display_tex": 0, "mathjax_asciimath": 0, "img_math": 0, "codecogs_latex": 0, "wp_latex": 0, "mimetex.cgi": 0, "/images/math/codecogs": 0, "mathtex.cgi": 0}, "config": {"markdown_headings": true, "markdown_code": true, "boilerplate_config": {"ratio_threshold": 0.18, "absolute_threshold": 10, "end_threshold": 15, "enable": true}, "remove_buttons": true, "remove_image_figures": true, "remove_link_clusters": true, "table_config": {"min_rows": 2, "min_cols": 3, "format": "github"}, "remove_chinese": true, "remove_edit_buttons": true, "extract_latex": true}, "english_confidence": 0.9168676733970642, "perplexity_flag": "middle"}