Matt_W
4413
May be a bit sensational. Here’s today’s press release from Fermilab:
While there have been persistent hints from DES and several previous galaxy surveys that the current universe is a few percent less clumpy than predicted — an intriguing find worthy of further investigation — the recently released results are consistent with the prediction.
How do you do a Dark Energy survey?
“Mr. Energy, would you answer a few questions?” “Yes?” “Great!”
"How do you feel about being called “dark energy?” Please rate your feelings on the scale of 1 to 5, with 1 being “Pissed off” and 5 being “No biggie.”
Seems Ingenuity had some stability(?) issues on latest flight…
https://www.youtube.com/watch?v=umjQXZemWz4
What questions? There’s at least some chance I can answer them. :)
RichVR
4417
Scientists are interested in these structures because they suspect that gravity may behave very differently inside them.
Astronomers are able to map the existence of dark matter by looking at light travelling to Earth from distant galaxies; if the light has been distorted, this means there is matter in the foreground, bending the light as it comes towards us.
These statements seem to be contradictory. They are using light bent by gravity to map the area. But gravity may be behaving differently there. So how accurate could the measurements actually be?
Yeah, I’m not sure what’s meant by that first statement either. Our understanding of general relativity, which is the basis for mapping matter concentrations based on lensing effects, is pretty well tested and well-verified at this point.
Yep, gravity is gravity as far as we have been able to test, which is pretty darned far. Maybe it turns out to be a low resolution map that is enhanced later on by some very advanced discoveries, but it would still be useful right now.
RichVR
4420
If I’m not mistaken, and I often am, Einstein first came up with gravity lensing and an eclipse proved it.
True that, and for some reason Einstein never really came up mistaken. Dude was a freak. He handwaved some shit he didn’t really get but he didn’t lay into it with another contradictory theory, which I appreciate in a scientist.
Edit: Imagine if you had Einstein and Tesla working together with modern instruments.
Matt_W
4422
The DES produced a couple of dozen papers this year, only one of which I could find discusses cosmic voids:
From what I understand, this paper is discussing a variety of techniques collectively called “weak lensing” that the DES is using to produce mass maps of the cosmos. They’re validating the weak lensing techniques by cross correlating with other data sets, and presenting the results they get from using them.
Weak lensing is described in this paper:
There is no generally applicable definition of weak lensing despite the fact that it constitutes a flourishing area of research. The common aspect of all studies of weak gravitational lensing is that measurements of its effects are statistical in nature. While a single multiply-imaged source provides information on the mass distribution of the deflector, weak lensing effects show up only across ensembles of sources. One example was given above: the shape distribution of an ensemble of galaxy images is changed close to a massive galaxy cluster in the foreground, because the cluster’s tidal field polarises the images.
. . .
All these effects are quite subtle, or weak, and many of the current challenges in the field are observational in nature. A coherent alignment of images of distant galaxies can be due to an intervening tidal gravitational field, but could also be due to propagation effects in the Earth’s atmosphere or in the telescope. A variation in the number density of background sources around a foreground object can be due to a magnification effect, but could also be due to non-uniform photometry or obscuration effects. These potential systematic effects have to be controlled at a > level well below the expected weak-lensing effects.
Cosmic voids (areas of statistically low matter density as determined by various other surveys, including optical ones) provide a potentially interesting lensing situation (that I don’t really understand) because they represent underdense areas surrounded by overdense areas. They’re able to correlate the voids they find with weak lensing solutions to voids found in other surveys and the two correlate. All of this is in service of validating and constraining the current ΛCDM (Lamda Cold Dark Matter, where lamba is the cosmological constant standing in for dark energy) model of the cosmos.
A good quick read that relates to all this:
Matt_W
4424
He initially got the same result that you get using Newtonian mechanics and the equivalence principle (inertial mass is equal to gravitational mass), which had been derived a hundred years earlier. But by the time he published his theory of general relativity, he had the right angle and predicted the correct bending of light around the sun that Eddington measured during the 1919 eclipse. It was that validation of GR over against the Newtonian prediction that made Einstein famous.
Bah, been too long since my GR class. Thanks for the correction.
Matt_W
4426
Well you’re way up on me, I’ve never taken a course on GR, though I wish I could. (I really should just plow through the initial math in one of the many YouTube courses available.) But I have watched way too many PBS Spacetime videos :)
I’ve lately been interested in reading about the continuity of physics at the end of the 19th century. Einstein definitely represents a paradigm shift, but more in the sense of tipping the balance over rather than leading a revolution. If he hadn’t written the 1905 paper on special relativity, someone else would have before long. The pieces were all there. The naturalists of the 19th century were just as ingenious as any modern physicist and in many ways were better at using tools and conjecture than we are to extract information from the natural world.
Those tend to be excellent treatments of their respective topics, so it’s a good choice.
Neat topic, I confess I don’t know as much about the history of my field as I’d like to. Good resources can be a bit hard to come by, unfortunately.
Matt_W
4428
I really liked this paper on how Einstein’s development of SR was more dependent on 19th century investigations into the phenomenon of stellar aberration and Fizeau’s measurement of frame dragging effects in water than on the Michelson-Morley experiment:
https://www.pitt.edu/~jdnorton/Goodies/rel_of_sim/index.html
Norton has a bunch of other easy-to-read papers here discussing Einstein’s thought in the context of the science of his day.
And it’s super interesting to read about 19th century investigations into the nature of the luminiferous ether, the frame-dragging hypotheses, and what each of the seminal experiments (e.g. Bailey’s discovery of stellar aberration, Fizeau’s experiment, measurements of aberration in different media, etc) of the time revealed about it. Even just reading Einstein’s 1905 paper reveals that it’s far more about electrodynamics than about clocks, traincars and thought experiments–Einstein was trying to resolve the discord between Maxwell’s equations and Galilean relativity, i.e. it’s easy to construct an apparatus where a stationary observer sees pure electric fields, but a moving observer sees both electric and magnetic fields. Lorentz actually resolved the conflict between Maxwell and Galileo using math several years before Einstein’s conceptual revolution, which is why we don’t talk about the Einstein transformation in Special Relativity.
And this video from PBS Spacetime about the Michelson-Morley experiment is really cool too:
Very nice. We obviously teach Michelson-Morley, Fizeau etc but when I was a student, at least, they weren’t great about drawing the sort of connections between those experiments and the genesis of Einstein’s ideas. I always did want to put together a “history of physics” course to do exactly that job.
Also let me just emphasize that for a layman you’d extremely well informed - most folks have no idea that magnetism is a relativistic effect. :)
Matt_W
4431
If you ever do, I’d love to see your materials. I minored in Physics for my EE degree, which mostly meant I just took Optics and a one credit “intermediate lab” course. But the lab course was actually amazing. There were only 2-3 of us in the course and we did it over the summer. We measured G using a torsion balance and two kg weights. We measured the speed of light using a rotating mirror apparatus. And we did a few of the measurements that Thomson did to determine the e/m ratio. It was really cool to actually perform these classic experiments and see how sensitive they were. The other part of the class was doing complete error analysis on all the results and writing them up in paper form, which was maybe one of the most useful things I did during my undergraduate degree. It was really time consuming for a one-credit summer class, but felt as close to actual science as I got in school. (Weirdly enough, I work at a plasma physics research facility now and am surrounded by physicists ever day, but feel more like an industrial instrumentation and controls engineer than a science guy.
It’s hard to imagine the department ever being so well-staffed that they don’t need me to spend all my time on astro and the intro physics sequences, but maybe someday. If so, I’ll let you know.
Ah, so not a layman at all, leastwise when it comes to matters of E&M! And yes, the “advanced physics lab” as it’s usually called is where all the fun stuff is. We did all the ones you mention along with the Hall Effect, photoelectric effect, some interesting radiation experiments, all kinds of good crunchy physics.
Cool! My masters research involved mapping the spectrum of hydrogen plasma (as a model for stellar atmospheres) and the doctorate is on interstellar plasmas (albeit more observational than theoretical).
