Ben Crowell

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bio	website	lightandmatter.com
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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.

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

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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? duplicate of physics.stackexchange.com/questions/27/…
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 mass added 83 characters in body
3h	revised	Doubts about the definition of mass added 101 characters in body
3h	answered	Doubts about the definition of mass
3h	comment	Why distinguish between row and column vectors? There are many practical, ordinary situations where you don't have a metric. See this answer for some examples: physics.stackexchange.com/a/62564/4552 . When you don't have a metric, there is no way to convert back and forth between vectors and covectors. Also, vectors and covectors transform differently under a change of coordinates, e.g., a rescaling does opposite things to them.
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