I have known about Electron–positron annihilation for at least 52 years, from my course in Atomic and Nuclear Physics at Carleton College in the spring of 1970 if not before. The reaction is
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.
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 units 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 found How Washington Got Hooked on
Flying Saucers to be fascinating if somewhat depressing.
“There is nothing new under the sun.”
This is a subject I have been watching from a safe distance for well over half a century, when I first read Martin Gardner’s
Fads and Fallacies
in the Name of Science (Few books have influenced me more than this one).
I met J. Allen Hynek in my last year of high school, 1967-68, when I was a student in the
(Still around although in a different
format) at Chicago’s Adler Planetarium. This program was
organized and run by Hynek. Hynek was a pleasant and interesting
speaker, and a good teacher, but he never spoke to us kids about UFOs.
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
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
I was reminded tonight of the Gaia hypothesis. It was quite a thing in the 1970s.
The Gaia hypothesis, named after the ancient Greek goddess of Earth, posits that Earth and its biological systems behave
as a huge single entity. This entity has closely controlled self-regulatory negative feedback loops that keep the conditions
on the planet within boundaries that are favorable to life. Introduced in the early 1970s, the idea was conceived by chemist
and inventor James E. Lovelock and biologist Lynn Margulis.
This had a natural appeal in the early days of the environmental movement. I was skeptical back then, thinking of it as new age wishful thinking,
and impossible to test. I was wrong: Its origins are far darker. Here is the abstract of Gas
Guzzling Gaia, or: A Prehistory of Climate Change Denialism:
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
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.
Approach to Scanning Historic Glass Plates Yields an Astronomical Surprise. Technical details at
Precise Photometric Measurements from a 1903 Photographic Plate Using a Commercial Scanner.
Professional astrophotography used to be done on emulsion-coasted glass places. That was how astromical discoveries were made for nearly a
More than an estimated 2.4 million glass plates are out there in collections in North America alone. These were taken starting in
the 1890s right up until the 1970s, when CCD (Charged Couple Device) detectors started to come online for astronomy. Of these, only an
estimated 400,000 plates have been digitized to research quality
The team in this article has found a much cheaper way to proceed with this process, using
I went (via Zoom) to a great lecture last night. Serafina Nance spoke to the
The Calgary Centre of the Royal Astronomical Society of Canada on
Tracing the Lives, Deaths, and
Explosions of Massive Stars.
Supernovae are cosmic events of gigantic power. Their explosions can shine as bright as a galaxy, a pinprick of extraordinarily bright light in the night sky. What is less well-understood, however, is which stars reach the point of explosion and how they evolve to their deaths. Interestingly, their explosions provide astronomers with key tools to uncover fundamental aspects of our Universe. While we know that the Universe is expanding at an accelerated rate due to dark energy, the rate of the expansion of the Universe is not well-constrained. Supernovae provide us with independent ways to measure this expansion and work to resolve one of the most pivotal questions in astronomy: How fast is the Universe really expanding?
I found this post on Facebook: Why is it
important to know so many digits of pi?.
As someone who started computing with log tables and slide rules, the first question I ask is
how many significant digits do the other variables in your calculation have? The smallest such number tells you how
many digits of pi you need. With electronic devices there is no harm in using more in your calculation, as many as
your device has, but do not let that give you a false idea of the precision of your result.
I learned about significant figures in my high school chemistry in 1967-68. (Thank you, Mr. Wheeler!).
Use of appropriate significant figures, also from a chemistry
class, clearly explains the concept and its use in practice.
I only first saw Star Trek (TOS) after high school, in reruns. Thanks to that chemistry class I gag every
time I hear Mr. Spock reporting some calculation to an absurd number of decimal places. His input data could
not possibly be that precise!
It’s confirmed. There is water on the Moon. OK,
what’s the big deal? Water is very common in the cosmos
and has been seen on the moon before. The
difference is that previously it had been been only in craters near the lunar south pole, perpetually in darkness. Now, as the updated NASA announcement said,
NASA’s SOFIA Discovers Water on Sunlit Surface of Moon,
which will be much more accessible to future explorers.
How significant this is remains to be seen, but it has provided Katie Mack with some