# Ben Crowell

reputation
1234
bio
website        lightandmatter.com
location
age
member for     1 year, 10 months
seen           11 mins ago
profile views  935

I teach physics at Fullerton College, a community college in Southern California. I have an undergrad degree in math and physics from Berkeley and a PhD in physics from Yale. Back when I was doing research, my field was experimental low-energy nuclear physics.

|             |                  | bio     | visits             |             |                   |
|-------------|------------------|---------|--------------------|-------------|-------------------|
|             | 6,002 reputation | website | lightandmatter.com | member for  | 1 year, 10 months |
| 1234 badges | location         |         | seen               | 11 mins ago |                   |

# 983 Actions

|     |          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|-----|----------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| 4m  | comment  | What causes the permittivity and permeability of vacuum?@KoenVanDamme: I don't think the interpretation given in your comment is correct. Materials don't slow down electromagnetic waves by partially blocking them. When an EM wave encounters a material, the charges in the material oscillate and produce a secondary wave. The superposition of the two waves is a wave that moves at less than $c$.                                                                                                                                                                                                                             |
| 15m | comment  | What counts as a measurement?                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
| 21m | comment  | Is every quantum measurement reducible to measurements of position and time?@DanStahlke: The "ink" argument is cute, but the nontrivial issue here is that particle spins live in their own vector space, which has no obvious connection to the t-x-y-z vector space of the spacetime that classical physics describes and that we experience in daily life. There are two steps: (1) showing that it is possible to connect the spin vector space to our experience in any way at all, and (2) showing that any such experience reduces to a position measurement. The ink argument takes care of #2, but it's #1 that's nontrivial. |
| 2h  | comment  | What are units actually?You can actually get away with taking the log of something that has units. Changing the units just introduces an arbitrary additive constant. In a context where an additive constant doesn't matter, there's no problem with doing this.                                                                                                                                                                                                                                                                                                                                                                      |
| 2h  | revised  | Doubts about the definition of massadded 83 characters in body                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| 3h  | revised  | Doubts about the definition of massadded 101 characters in body                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
| 3h  | answered | Doubts about the definition of mass                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
| 3h  | comment  | Why distinguish between row and column vectors?                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
| 19h | comment  | Is every quantum measurement reducible to measurements of position and time?@Lagerbaer: Beat me to it!                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| 19h | comment  | Is every quantum measurement reducible to measurements of position and time?I don't see how it applies to spin measurements. Maybe the assumption is that you can impose an external field, and then do something like Stern-Gerlach?                                                                                                                                                                                                                                                                                                                                                                                                  |
| 19h | comment  | How can I determine whether the mass of an object is evenly distributed?The non-rigid version is possibly useful in practice, but doesn't seem likely to lend itself to a definite answer that could be given on this site. BTW, it's not obvious to me how to generalize this to relativity, since you basically can't have rigid objects in relativity.                                                                                                                                                                                                                                                                              |
| 20h | comment  | How can I determine whether the mass of an object is evenly distributed?In GR you could spin the sphere. Then Birkhoff wouldn't apply, and I think you'd get geodetic and frame-dragging effects that might be different in the two cases. Even in SR, the rotational dynamics are different, and in any case it's not possible for it to be perfectly rigid as assumed.                                                                                                                                                                                                                                                               |
| 20h | revised  | How can I determine whether the mass of an object is evenly distributed?added 102 characters in body                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
| 20h | comment  | How can I determine whether the mass of an object is evenly distributed?I'm construing the question to mean that we assume the object is rigid. Otherwise we could shake it, probe it with ultrasound, or find out that it contained gyroscopes or Mexican jumping beans.                                                                                                                                                                                                                                                                                                                                                              |
| 21h | comment  | How can I determine whether the mass of an object is evenly distributed?@Mike: By the parallel axis theorem, you can't get any additional information that way.                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
| 21h | revised  | How can I determine whether the mass of an object is evenly distributed?simpler, better example with same shape                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
| 21h | comment  | How can I determine whether the mass of an object is evenly distributed?Nice. This suggests a way of simplifying my own example, which I'll do.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
| 21h | comment  | How can I determine whether the mass of an object is evenly distributed?This doesn't work. The buoyant force only depends on the weight of the displaced water.                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
| 22h | comment  | How can I determine whether the mass of an object is evenly distributed?@Manishearth: The boxes can be made to have the same uniform density as the uniform object. I've edited the answer to show this explicitly.                                                                                                                                                                                                                                                                                                                                                                                                                    |
| 22h | revised  | How can I determine whether the mass of an object is evenly distributed?modified to clarify issues raised by Manishearth                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
{"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": 1, "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": 0.9315721988677979, "perplexity_flag": "middle"}