Ben Crowell

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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.

	6,103 reputation	bio	website	lightandmatter.com	visits	member for	1 year, 10 months
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May 19	comment	Is every quantum measurement reducible to measurements of position and time? @DanStahlke: "If a degree of freedom does not connect to our experience in any way at all, then does it really even exist?" But spin degrees of freedom do connect to our experience.
May 19	comment	Can we measure “wavefunction” of quantum particles? The downvote wasn't mine, but this only says how to measure the square of the wavefunction, not the wavefunction itself.
May 19	comment	Can Earth's Magnetic Field Create Electricity? "The Earth's magnetic field is quite homogeneous over short distances though so the coil would need to move fast and very far to generate much." You can just spin a coil. The issues are that: (1) the earth's field is weak, and it's easier to use the field of a stronger permanent magnet; and (2) as with any generator, it requires an input of energy to turn the crank.
May 19	comment	what is the magnetic quadrupole operator? It's not true that higher multipolarities have never been measured. They've been seen in yrast traps, which are states that can only decay by the emission of high-multipolarity photons, because the only states lower in excitation energy have much lower spins. These states tend to be very long-lived. E.g., there's a 12+ state in 52Fe that decays by emission of an E4 photon.
May 19	asked	In coordinate-free relativity, how do we define a vector?
May 19	comment	Can we measure “wavefunction” of quantum particles? For a photon, the wavefunction is simply the electromagnetic field, which is measurable.
May 19	comment	Can we measure “wavefunction” of quantum particles? "If you are able to produce multiple copies of the same pure quantum state..." How do you do that without violating the no-cloning theorem?
May 19	comment	What is the magnetic quadrupole moment of a nucleus in cylindrical coordinates? Oops, I missed the fact that the question was about a magnetic rather than electric quadrupole moment, so my second comment doesn't apply.
May 18	comment	What is the magnetic quadrupole moment of a nucleus in cylindrical coordinates? en.wikipedia.org/wiki/Tensor
May 18	comment	What is the magnetic quadrupole moment of a nucleus in cylindrical coordinates? In general, the static quadrupole moment of a nucleus, in the body-fixed frame, can be found just by using the nucleus's deformation and the classical definition of the quadrupole moment. E.g., many nuclei in their ground states are nearly ellipsoidal, so you can use the classical equation for the quadrupole moment of a uniformly charged ellipsoid. No quantum mechanics is required.
May 18	comment	What is the magnetic quadrupole moment of a nucleus in cylindrical coordinates? This doesn't sound right to me. A quadrupole moment can be expressed as a tensor, and a tensor can't be zero in one set of coordinates and nonzero in another. If you can express a tensor in one set of coordinates, you can always reexpress it in some other set of coordinates just by using the tensor transformation law.
May 18	revised	Radio antennas that are much shorter than the wavelength added 177 characters in body
May 18	revised	Radio antennas that are much shorter than the wavelength added 78 characters in body
May 18	asked	Radio antennas that are much shorter than the wavelength
May 18	comment	What are some novel physics demonstrations that can be done cheaply? What topics are being covered in the class? Are you a student? A teacher doing a demo as part of a job interview?
May 18	comment	Widom-Larsen Theory This answer doesn't address the question and also doesn't make any sense on its own, since it doesn't make any logical connection between cold fusion and Van de Graaff generators.
May 18	comment	Widom-Larsen Theory "observed phenomenon--- heat production, He production, and elemental transmutations in Pd/d cells with a very wide spectrum of atomic weights." None of these phenomena are actually observed. They have been thoroughly debunked.
May 18	comment	Extended Rigid Bodies in Special Relativity Nice answer, +1. But given Kitchi's comment explaining what aspect of rigidity he/she really wanted to know about, I think it's also helpful to introduce the concept of Born rigidity, as explained in my answer.
May 18	revised	Extended Rigid Bodies in Special Relativity added 301 characters in body
May 18	revised	Extended Rigid Bodies in Special Relativity added 308 characters in body