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 Feb 23 comment Cosmic radiation cutoff at LOW energies? Below 1GeV particles aren't generally considered relativistic and therefore, when they bombard our atmosphere from outer-space, they're just not normally refered to as cosmic rays. Furthmore, sub-GeV particles would very quickly thermalise in our atmosphere (Ie exhibit Brownian motion) and therefore retain no information about from where they came. Feb 23 comment Cosmic radiation cutoff at LOW energies? From where did you here there was a low energy cut-off? Technically, there isn't. Stable particles (alphas, protons, neutrinos etc) can have arbitarily low energies. Of course there comes a point when either; we would tend not to refer to them as cosmic rays (most would be solar wind for example), or when detector technolgy prevents us from seeing them. However, these don't really constitute as a 'cut-off' in the traditional sense of the word. Feb 22 comment Cerenkov light - a practical calculation @zephyr, could you perhaps type up your calculation into an answer? Allbeit approximate, I think it will be sufficient for my purposes and will therefore duly accept it. Feb 20 comment Cerenkov light - a practical calculation Isn't it okay to assume that the muon would be travelling at $c$ throughout it's passage through the glass? It is only 10cm afterall. For a $5keV$ muon: $1-v/c=2.4\times10^{-5}$. Either way, my attempt at the calculation gave $4\times10^{-47}J$ which I really don't think is correct. Basically I'm still stuck :( Feb 20 comment Cerenkov light - a practical calculation Hi zephr. Of course your right if the integral were carried out over the full range of frequencies. In practice however, I'll be using something like this[1] to detect ring patterns and it is sensitive only to wavelengths in the range 300-650nm. [1]: sales.hamamatsu.com/index.php?id=13199716&language=2&; Oct 11 comment Very basic question: When to use $s=vt$, $s=1/2vt$, $s=at$ and $s=a/t^2$? Hi @dmckee: the equations should read $s=ut+\frac{at^2}{2}$, $s=\frac{(v+u)t}{2}$, $v=u+at$, and $v^2=u^2+2as$. Or at least they are the ones I was forced to remember some 10 years ago ;-) Aug 4 comment Particle physics plots Hi Jim: if you search through some of the older posts of this blog you'll find many lucid and pedagogical explanations of particle physics graphs: blog.vixra.org Jul 29 comment What does scalar phi represent in spacetime? Can't decide whether I like the clarity or the humour more in this answer: +1 ;) Jul 28 comment Roughly how many atoms thick is the layer of graphite left by a pencil writing on paper? Tim: In light of your above comment defending your answer I have decided it warranted my accepting it. Thanks also to @TheSheepMan for his experimental perspective. Jul 28 comment When will the Hubble volume coincide with the volume of the observable Universe? Hi @Ned Wright, thank you kindly for taking the time to answer. I would like to just clarify the conclusion that appears to be drawn from your calculation, namely - that the observable Universe is destined to increase indefinitely, albeit with increasing redshift? This appears to contradict what I read in the aforementioned book by Penrose. On a side note, I opted for the "photist approach" purely as we're a long way from building a high-resolution neutrino or gravitational wave telescope (well, perhaps just decades for the former.) Jul 27 comment Roughly how many atoms thick is the layer of graphite left by a pencil writing on paper? Thanks @TheSheepMan, that sounds like a good method to establish an answer experimentally. Perhaps you would like to transcribe it into an answer and add to it a theoretical estimate that improves upon Tim's effort? ;-) Jul 26 comment When will the Hubble volume coincide with the volume of the observable Universe? @lurscher: Whilst I don't think it answers you query there was another independent varification of DE published recent. Here is a sciam article about it and here is the paper on which it is based. Jul 26 comment Does the Big Bang need a cause? I was considering offering an answer and then I realised I could just direct you to a lecture entitled 'A Universe From Nothing' by theorist Lawrence Krauss: richarddawkins.net/videos/… Jul 21 comment What conservation law corresponds to Lorentz boosts? Nice warning at the start ;-) (+1) Jul 21 comment Is apparent horizon curvature lesser due to refraction of light in the atmosphere? See the comments by @Philip Gibbs in this question: physics.stackexchange.com/questions/11021/… It's not exactly the same situation but the relevant physics is the same. Jul 21 comment What conservation law corresponds to Lorentz boosts? You can answer your own question @Warrick. In fact, I think there's even a badge for that ;-) Jul 20 comment Decay of massless particles @Bebop: Good point, not sure why I only considered decay to massive particles. I'll edit the answer accordingly. Jul 20 comment Decay of massless particles I agree that the word "radioactive" is superfluous in the question's title. For clarity I think a better title would be 'Can massless particles decay?' but of course the choice is yours. Jul 19 comment How to determine the mass of a quark? I hereby promise to upvote a comprehensive, "enormously long answer" alluded to in @Genneth's comment. ;-) Jul 18 comment How to determine the mass of a quark? +1 for the links to the pdg.