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Physics Forums Featured Threads

U(1) invariance of classical electromagnetism
This is an interesting question that popped through my mind. Some of us should know what is meant by „gauge transformations”, „gauge invariance/symmetry” and are used to seeing these terms whenever lectures on quantum field theory are read. But the electromagnetic field in vacuum (described in a specially relativistic fashion by the tensor... 
Automated tennis ball launcher for my dog
I thought I would post my thoughts/progress on designing and building an automatic tennis ball launcher for my dog. The little fella gets a bit bored during the day when we are at work, so I want to build him this so he has something to do other than push a basketball around the backyard. Any and all advice/criticism is appreciated. 
U.S. Solar Eclipse of Aug. 21, 2017
I've been waiting for this for a long time and it's just a little more than a year away now. This will be the opportunity of a lifetime for people in the U.S. The 2017 solar eclipse will be visible across the width of the entire U.S! The points of Greatest Eclipse and Greatest Duration are going to occur on opposite sides of the Kentucky  Illinois border. 
"probability of finding the electron"
I was trying to help (elsewhere) a student with some QM related problem and I realized something. When discussing QM we underline the fact that electron doesn't behave as a particle, it behaves as a wave. Yet when we explain wave function we say something like "square of the wave function is a density probability of finding the electron"  which seems to suggests to students the electron is a particle that can be found... 
A new exomoon candidate: Kepler1625b I
The authors looked for signals of potential moons in the Kepler transit data. There is some weak evidence that a group of smaller moons exists, but the measurements are not accurate enough to pin that down on a starbystar level. One particular planet, however, Kepler1625b, has very curious features in all three observed transits. The Hubble telescope will observe the next transit in October this year. 
First Human Embryos Edited in U.S.
"Researchers have demonstrated they can efficiently improve the DNA of human embryos. The first known attempt at creating genetically modified human embryos in the United States has been carried out by a team of researchers in Portland, Oregon, MIT Technology Review has learned. The effort, led by Shoukhrat Mitalipov of Oregon Health and Science University, involved changing the DNA..." 
Should Algebra Be Required At Community Colleges?
The chancellor of the California Community Colleges system believes that students who are not majoring in math or science should not have to take intermediate algebra to earn an associate degree. California has the largest community college system in the US, and what goes in California sometimes spreads across the country. What do you think? 
Is it possible to detect a single phonon?
Is it possible to detect a single phonon? If yes, can it be detected at a welldefined position? 
A Poor Man's CMB Primer. Part 5: Quantum Seeds
The CMB establishes a record of ancient acoustic oscillations in the baryonphoton plasma. We’ve been studying how these primordial sound waves evolve, and how to analyze the last scattering surface to learn about them. Now it’s time to confront their origin: what process composed the cosmic symphony? A few different proposals have been advanced over the years to explain the origin of the primordial perturbations. 
Great Magic Triangle Math Puzzle
"The missing square puzzle is an optical illusion used in mathematics classes to help students reason about geometrical figures; or rather to teach them not to reason using figures, but to use only textual descriptions and the axioms of geometry. It depicts two arrangements made of similar shapes in slightly different configurations. Each apparently forms a 13×5 rightangled triangle, but one has a 1×1 hole in it." Can you figure out an easy way to inspect it? 
A propagating Majorana mode
"In a discovery that concludes an 80year quest, Stanford and University of California researchers found evidence of particles that are their own antiparticles. These 'Majorana fermions’ could one day help make quantum computers more robust."  Stanford News 
Results shown at EPS
The EPS conference was a week ago, and many new results and future plans were shown. A good general overview has been collected by Paris Sphicas in his Summary slides. I had a look at some presentations and collected things I personally found interesting. Warning: The selection is heavily biased, I cannot cover everything, and I probably missed many interesting things. 
Super pBrane Theory Emerging from Super Homotopy Theory
What are even the principles of the theory? Two principles worth considering: The gauge principle: No two different things are ever equal, instead they may be related by a gauge euivalence. Consequently, no two different gauge transformations are ever equal, instead they may be related by a gaugeofgauge equivalence. And so on.Mathematically this is the principle of homotopy theory... 
An Integral Result from Parseval's Theorem
In this Insight article, Parseval's theorem will be applied to a sinusoidal signal that lasts a finite period of time. It will be shown that it necessarily follows that ## (\frac{\sin(2 x_o)}{x_o})( \frac{\pi}{2})=\int\limits_{\infty}^{+\infty} \frac{\sin(xx_o) \sin(x+x_o)}{(xx_o)(x+x_o)} \, dx ##. 
Hamiltonian in Quantum vs Classical
The Hamiltonian in classical mechanics is not always equal to the total energy of the system. I believe this is only true if there is only a potential field and no vector potential. However, in quantum mechanics for a particle in an EM field, even if a vector potential is used the energy operator E > H. Is this an inconsistency?
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