I spent the weekend of June 16-19 at Carleton College. The occasion was my 50 year class reunion, for which I was a member of the gift committee, which in turn was a part of the overall planning committee. Mia (my wife) and I had a room on the third floor of Watson Hall, “3rd Watson” was how we would have referred to it back in the day. I don’t know if the current students refer to campus locations like that. I lived on “6th Watson” my senior year.
e– + e+ → γ + γ
I had always assumed that the inverse reaction
γ + γ → e– + e+
was also possible because of time reversal symmetry. Apparently so had everyone else, but it has only recently been observed: Matter arises from light? We finally know the answer to this question!. Even now there is a caveat about “virtual” as opposed to “real” photons.
As I wrote previously , Mia (my wife) and I spent last weekend at Capricon, a Chicagoland science fiction convention. We went to most of the Capricons in the ’80’s and ’90’s, but in our first years in Minnesota the pressures of parenthood prevented us from going. Those have eased somewhat and we have been to most of the Capricons (and Windycons) since 2009. While most Capricons have been in the Chicago suburbs, this year the convention was downtown, at the Sheraton Grand Hotel.
Black holes are not totally black! They will evaporate by Hawking radiation. This is required by Thermodynamics and Quantum Mechanics. All properties of a Schwarzschild Black are determined by its mass, so if you know the mass the lifetime and other properties follow automatcally. Or you can start with the lifetime and determine the initial mass. Or the Schwarzschild radius, or the temperature, or the entropy, etc. For black holes comparable in mass to “normal” astronomical objects this lifetime is much longer than the current age of the universe. Viktor Toth’s Hawking radiation calculator is a convenient tool for such calculations. Here are some results:
I have vaguely known about renormalization since the 1970’s, but had never seriously studied it. Out of curiosity I watched Renormalization and envelopes on YouTube Thursday evening. This was the final lecture of the Asymptotics and perturbation methods course by Prof. Steven Strogatz of Cornell University. I had watched the first two lectures of the course, but none of the others until this one. Fortunately, there were relatively few explicit dependencies on them, so I was able to follow this quite well. Here is the description:Continue reading
An email from Cosmoquest led me to find Physicists Narrow Range of Potential Masses for Dark Matter Candidate articles. Technical details at Theoretical bounds on dark matter masses and on the Arxiv. The mass range is 10−3eV≲mφ≲107eV. Thus we have a range from neutrino masses (still very uncertain, but nonzero) up to about 1/10 the mass of a muon. This appears to exclude WIMPs, which were already in trouble. This also suggests that finding dark matter is not a justification for building a more powerful particle accelerator than the LHC; 107eV is well within the range of the LHC and other current devices.
We still do not have a complete theory of quantum gravity, but apparently enough is understood to make such calculations possible. I am looking forward to seeing what other theoretical physicists say about this.
Tumblr User Explains Why Elves’ Eyes In Lord Of The Rings Shouldn’t Look The Way They Do, but in fact there is no problem.
In The Two Towers Legolas claims to clearly see the horsemen of Rohan at a distance of 5 leagues. How is this possible when you consider the curvature of the Earth? You can come up with some bizarre ideas about Elven anatomy which are not suggested in the books or seen in the movies. However, there is a an elegant answer based on the nature of Arda, Tolkien’s world. It was flat, not round, until the end of the second age. At that time the Valar made it round so that mortals could not access Valinor. But Elves could still go there. For them the Earth was still flat and they could sail the straight path to Valinor. Hence for Legolas the curvature of the Earth and the horizon did not exist and hence he could accurately see the horsemen 5 leagues away.
Christopher Matthew, A Storm of Spears: Understanding the Greek Hoplite at War. The most important conclusion from this is how the Greeks wielded their spears. Despite all the pictures, they did not hold them over their heads with a back-handed grip. Instead, they held them with the butt of the spear tucked under their armpits. This allowed for much greater reach and strength. Lots of good experiments with re-enactors.
When hipsters are too slow in detecting the trends, they will keep making the same choices and therefore remain correlated as time goes by, while their trend evolves in time as a periodic function.