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Happy Earth Day 2017!
Time to think about what you can personally do to be a good steward of our home Earth. What activities are you doing today? I will pick up trash I find in my community during my dog walk. I will also be planting some flowers. Oh and patching some holes in my lawn with white clover. It's good for the bees! 
Math Challenge by Andrewkirk #1
A triangle has vertices ##A##, ##B## and ##C##. Mark points ##A',B',C'## on edges ##\overline{BC},\overline{CA},\overline{AB}## respectively so that each is one third of the way along the edge, as shown in the diagram. The lines ##\overline{AA'},\overline{BB'},\overline{CC'}## intersect at points ##A'',B'',C''## and form a triangle inside triangle ##ABC##... 
b > s µµ decays: Current status and updates next week
Something curious is going on with these decays. LHCb gives a seminar talk Tuesday next week, a livestream will be available. I'll summarize the current status here. I tried to keep most at the advanced (I)level, but I don't think that always worked. (B)level summary: We might have found signs of new unexpected physical effects, but the situation is still unclear... 
Concordance cosmology paper
What do people think of this paper? If I understand correctly, they are saying that the need for Λ disappears if we do Nbody simulations that properly take account of GR. I think they are saying that past simulations have used Newtonian approximations that don't properly account for density fluctuations. Can this be? If true, it seems like a major advance. Comments? 
Entanglement Entropy  Part 1: Quantum Mechanics
Quantum Mechanics(QM) is one of the greatest intellectual achievements in human history. Not only because it describes the world at the microscopic level and in turn provides us with the technological advancement that we enjoy today, but also because it shows us how little we know about the world we’ve been living in for so long.... 
Relativity using the Bondi kCalculus
In the early 1960s, Hermann Bondi advocated a presentation of special relativity (called the kcalculus [which involves no calculus–just simple algebra]) that is advertised to be a “simple logical extension of wellknown Newtonian ideas, without any of its mathematical trappings.” 
Impossibility to have fever
I'd like to know if it's physiologically possible for a human being to be unable to have fever. A related person to me claims that he has never experienced fever in his life, even during influenza, pneumonia and other infections. Is there any similar case of immunodeficiency registered in the medical literature? 
Interpretation of the photoelectric effect
The photoelectric effect is usually presented as an example disproving classical electromagnetism as viable model for interaction of light with matter and as evidence of quantization of energy in the electromagnetic field, i.e. the existence of photons. I would like to discuss a thought based on nonrelativistic quantum mechanics w/o and relation to Planck, Einstein etc. showing  imho  why this is not so straightforward. 
Inflation, horizonproblem and thermal equilibrium
In this paper the following arguments are given why the horizon problem is a fake problem. It raised by me the following question: Why is it assumed in the formulation of the horizon problem that the observed thermal equilibrium from the CMB has to be explained by thermalization right after t=0? 
So You Want to Be a Physicist: 22 Part Guide
One of the most frequent questions we get is the process and background of being a physics major. Often, we have students asking what are the requirements of obtaining a physics degree, and what can one do with such accomplishments. I am hoping that, in a series of postings on this topic, we get to go over and demystify the whole process of what one can expect... 
Cancer causes: circa 67% are Replication error
"Most textbooks attribute cancercausing mutations to two major sources: inherited and environmental factors. A recent study highlighted the prominent role in cancer of replicative (R) mutations that arise from a third source: unavoidable errors associated with DNA replication." 
Tiny Tweak in Teaching Basic Circuits
I'm sure we have all seen the frequent questions from students who don't understand the difference between potential and voltage in a circuit. They seem to think that zero voltage has physical significance. Even using the phrase "voltage drop" is not sufficient, because they say, "What happens when it drops to zero," leading to the concept that voltage is a consumable... 
Why does formation of snow flakes not violate entropy law?
Snow flakes are highly structured, they form from water vapor which has very little structure. I must be misunderstanding entropy, my interpretation of it is that isolated system must evolve into more chaotic less structured states, which is not what happens with snow flakes. Can someone explain? 
LHCb observes 5 new particles (excited hadrons)
Plain old standard model baryons, but 5 at the same time, and with crystal clear peaks in the decay to ##\Xi_c^+ K^##. Each peak in the figure is a particle never seen before, and the significances of those peaks are about 20 sigma (10 sigma for the 5th one). A broad 6th state might hide around 3200 MeV... 
Math Challenge by Erland #1
A rectangle is divided into a finite number of subrectangles. The sides of the subrectangles are all parallel to sides of the large rectangle. Each subrectangle has at least one side with integer length. Prove that the large rectangle also has at least one side with integer length.
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