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Oct
8
comment Does the commutator of anything with itself not vanish?
Interesting, but here $[\cdot,\cdot]$ is an anticommutator. I wonder if it is possible to meaningfully define something using the regular commutator such that $XX-XX\neq 0$ (or $XY-YX \neq 0$ for $X\overset{\scriptscriptstyle\wedge}{=}Y$).
Jun
20
comment What happens when some asteroid comes in the way of warp drive?
This might seem like SciFi/fringe physics, but it's actually pretty interesting. What kinetic energy does a spaceship have in Alcubierre's and similar schemes? It's probably tricky to even define $E_{kin}$ in warped spacetime. There are two extremes I could see happen: the asteroid strikes the ship with extreme energy like in a kinetic bombardement; or the asteroid enters the area of curved space and then just sits in front of the ship. Maybe it is even deflected, because the geodesics do around the ship? It would be instructive to see how to calculate the kinetic energy and the geodesics...
Apr
12
comment Decay of massless particles
@JSchwinger Right, the infrared divergences occur at the other end of the energy scale, so to speak. It's interesting that they also occur in QED, I wasn't aware (am just an experimentalist ;-)). It seems that they are not as big of an issue in QED though, because photos neither have self-interaction nor hadronize, right?
Apr
10
comment Kolmogorov-Smirnov test vs Chi-squared test
That you should really take to cross-validated if you want an accurate definition :-). We often blur the distinction between both terms in everyday work. What we often mean is we have some theory which gives us a formula, or a Monte Carlo program, etc., and that spits out an expected, "ideal" result in form of a probability density.... To your question, a hypothesis is an assumption (either true or false). A model is a theory where you have several "knobs" (parameters) to turn. A hypothesis could be "model A is true with x=0.5", and then you could test if that is consistent with data.
Apr
10
comment Kolmogorov-Smirnov test vs Chi-squared test
I think this should probably really be on cross-validated, so just as a comment: I'm not really sure how to give a physics-centered answer, but I'll try. You use a chi-square test to test the goodness of a fit, for example of a curve to data points, and when your goal is to minimize statistical errors. You use a KS test when you have a hypothesis (probability density) and some data, and want to see if it is plausible that the data was sampled randomly from the PDF (=is consistent) or not. Looking for something in that direction?
Mar
23
comment Why is my restaurant silverware magnetized?
I also heard they might heat the silverware through induction in the dish washer, which magnetizes it. Don't know if that is true, though.
Mar
20
comment What is special about a 126 GeV Higgs mass?
@JohnRennie: I thought about including a "disclaimer", a la: please understand this as within the rules of the site, I'm not looking for guesses and discussion, please explain the vacuum stability issue in more detail, and give formulas for the higgs mass in the MSSM, etc. pp.. But I assumed 1) that was silly as we are all grown up and know the context of the site, and 2) I figured I keep it a bit more open -- If I'd ask directly for e.g. a formula for $m_H$, people might answer literally and I might miss out on important insights.
Mar
12
comment What does library number 539 mean in physics books?
I think it could be really useful to have a physicists.stackechange.com to ask questions that come up in the life of a physicist, but are not directly physics. Alternatively one could think about allowing them on meta.physics.stackexchange.com, which is right now only meta Q&A for the site itself.
Mar
8
comment Prerequsites for Zee's QFT
Zee's book is fairly accessible, and I think QM and special relativity should be enough to get started. I haven't worked through the whole book, though.
Mar
4
comment Propagator of a scalar in position space
That makes sense, thanks! Is it true that you can go from the massless propagator back to the massive case by summing over intermediate interactions (like in this picture, I haven't found a better source online quickly)?
Feb
28
comment Renormalization and the Hierarchy Problem
@MitchellPorter: I think the argument goes: We expect the SM to break down at the latest at $M_P$, since that's where quantum gravity comes into play. Choose $\Lambda = M_P$ $\Rightarrow$ hierarchy problem $\Rightarrow$ BSM physics (e.g. SUSY). But if you don't go up that far, and say new physics comes already at $M_{BSM} = 1$ TeV, then there is no hierarchy problem (or you solved it, depending on how you look at it), but in any case $\Rightarrow$ BSM physics. Am I right? I've always found this argument oddly circular. (Nevertheless, I'd like to see the reasoning for why exactly M_P too)
Feb
28
comment Renormalization and the Hierarchy Problem
So... I don't understand why people are surprized that choosing a value for Λ implies a hierarchy. I mean you explicitly put in the hierarchy by saying Λ = Mpl... I also don't understand why the bare mass should have a meaningful (measurable) value, since it is not observable... or is it?
Feb
28
comment Renormalization and the Hierarchy Problem
Thanks, the repetition is OK, I'm trying to reaffirm what I basically should know :-). But some confusion remains: I understand, if you insert a finite value for Λ, you have to adjust $m_0$, and get the hierarchy $|m_0| \ll |\delta m|$. But if you leave Λ open, with the intent to let it go to infinity, then $m_0$ must cancel the divergence (not just a large number!). Then $m_0$ is just symbolic, and has no well-defined value of its own. I thought the main purpose of renormalization was eliminating the divergent Λ depencency from the equation! ...
Feb
25
comment Reference for the renormalization of a scalar field's mass
@Danu: Thanks, that looks helpful.
Feb
19
comment Does a muon detector on Earth's surface correctly measure the mean lifetime of a muon?
Nice question. I supervised a muon decay lab course, and when a student asked me this for the first time, I had a really hard time finding the answer. Now I like to ask my students this question to give them something to think about :-).
Jan
15
comment How to find SUSY with near-degenerate masses?
@annav: Thanks, I forgot to take the random boost of the initial state into account! That could help, even without additional emitted particles. Still, just from experience, there are parameter ranges areas where its not enough and a e.g. multilepton search is just not sensitive.
Jan
15
comment How to find SUSY with near-degenerate masses?
@anna v: True, that's why I wrote comfortably detect. The problem is for one the trigger, you have a minimal $p_T$ threshold to trigger on single electrons or muons, and if it's too low, you have to run prescaled (I think at the moment at ATLAS the lowest is at 10 GeV, and then 18 GeV, but I'm not sure). The other thing is that (assuming your event triggered on something) you don't know the trigger efficiencies very well below 15 or 10 GeV (because they were determined e.g. from $Z\rightarrow\ell\ell$. So very often, you can't use those soft leptons as you'd like.
Jan
6
comment Moving Between Degenerate Vacua?
What I was trying to say was, I think the symmetry breaking only implies that you are anywhere in the circle, not that you are in a certain fixed point. I don't know how you move around in the valley, or if there is a physical meaning to the "angular position" but I'd like to know, too.
Jan
5
comment Moving Between Degenerate Vacua?
I guess you're asking whether the state moves around in the valley, or if it stays fixed ("remains broken" in the same orientation), and if it stays fixed, what keeps it there? ... I'm not sure, so I just leave this as a comment. But I think you can have a different value of the field (position in the valley) at every point in space, and it doesn't matter which one it is, because you can't measure the field directly. What matters is the form of the potential - the position of the minimum gives you the higgs VEV, the curvature (oscillations up the walls) creates the gauge boson masses.
Dec
13
comment String Theory- Are strings the end? What are they made of?
@MBN: I guess they think it's settled that they are made of strings.