33,189 reputation
345108
bio website inspirehep.net/…
location Duchy of Grand Fenwick
age 43
visits member for 3 years, 9 months
seen 16 hours ago

Experimental nuclear and particle physicist. These days I'm teaching, but I've spent a lot of time on nucleon structure in fixed target electron scattering and neutrino oscillations using reactors and beam sources.


Aug
23
comment why are the square of the components of velocity equal?
The velocity components of individual particles at any given time are generally not equal. An appeal to an average (either across many particles or in time) is essential to the argument. Notice the use of angle brackets ($\langle \rangle$) in some of the answers---they denote averages.
Aug
23
comment Can a human size object move so fast that it ceases to be observable?
"kgf"? I have to assume that is "kilograms-force", and all I can say is for FSM's sake NO! I can understand the historical reason for engineers working is pounds-force, but SI has had the proper distinction between mass and force all along. Just use Newtons already!
Aug
23
comment Is “microbunching” in a free electron laser limited by the Pauli exclusion principle?
I don't know what current beam densities are in electron machines, so it's hard to write a precise answer. It's just that achieving densities (for anything) where degeneracy comes into play is very difficult unless you just want to point at an atom. It is managed for BECs by making them very cold.
Aug
23
revised Is “microbunching” in a free electron laser limited by the Pauli exclusion principle?
edited tags
Aug
23
comment Is “microbunching” in a free electron laser limited by the Pauli exclusion principle?
I can't imagine that we are anywhere near degenerate density/pressure conditions in a free beam. By many orders of magnitude. I'd look for the primary constraint from space-charge effects or beam cooling.
Aug
22
comment Does the zero energy universe hold true on some mathematical grounds besides observations?
I always find these questions awkward. It doesn't matter. In fact, it matters less than not at all because ground fact trumps math no matter how beautiful it may be.
Aug
21
comment Is the flow in a ducted fan really incompressible?
There has been a minor comment edit to reduce abrasiveness. Try to remember to "Be nice."
Aug
21
comment Is the flow in a ducted fan really incompressible?
Notice the caveat "at low speeds"? That's a hint that there is an approximation at work here. The next question to ask yourself is 'what sets the scale of "low"' and from that to deduce what physics is being neglected.
Aug
21
comment Reading experimental results
The usual advice is (1) read the abstract (2) examine all the figures and plots and read the captions (3) only then start trying to make your way through the paper. You may find certain historically significant papers easier to manages because (a) they are not so far ahead of you and (b) if they changed the understanding of physics it is often the case that they changed the way physics is taught and the important concepts trickle down earlier in the curriculum.
Aug
20
comment Capacitor with big space of insulation and polymer inside
Every introductory physics text I've seen that does any E&M talks about the dependence of capacitance on geometry and the dielectric constant of the material in them. And so does the wikipedia article.
Aug
19
comment Experiment demonstrating interference patterns of neutrons
@NikosM. Ah. Diffraction is the go-to term that a pro would use to describe a n-slit type experiment.
Aug
19
comment Experiment demonstrating interference patterns of neutrons
@NikosM. Er...did you read the abstract?
Aug
19
comment Experiment demonstrating interference patterns of neutrons
Jon edged you out. Great minds or something.
Aug
19
comment Experiment demonstrating interference patterns of neutrons
Nice. That must be a tour de force.
Aug
19
comment Experiment demonstrating interference patterns of neutrons
Protons interact strongly by electromagnetic means. Neutrons are neutral and interact mostly by the residual strong force which is reasonably modeled as a contact interaction. That makes the scattering cross-section proportional to the size of the area of the nucleus rather than the square of the distance between atoms.
Aug
19
comment Experiment demonstrating interference patterns of neutrons
Neutrons are much harder to scatter than electrons (the interaction cross-section is 5-7 orders of magnitude lower), so you want a lot of chances to bounce them---two just isn't practical.
Aug
19
comment Experiment demonstrating interference patterns of neutrons
Diffraction does demonstrate the wave-particle duality. It only works (gets the diagnostic pattern as opposed to a out-of-focus blob) with waves, but the neutrons are still detected as particles. It's the same physics.
Aug
19
answered Experiment demonstrating interference patterns of neutrons
Aug
19
comment Doubt in a certain equation of a research paper
Toward Danu's comment. We have the MathJax rendering engine active on the site which means that you can display very nice equations by writing them in a LaTeX-alike language and putting them between $s.
Aug
19
comment Does this count as moving faster than light?
Just a comment about "However, they are about adding velocities together, which isn't quite the same as in the case I was describing.". They are exactly the same. You have situated "yourself" on the middle object (call it D) and asked the question from there, but the situation described is identical. D moves forward relative A and B moves forward relative D. Same thing. And you'll note that Floris gives the same answer.