[ThTh1] On our last episode, I mentioned the damage that had been done to the Arecibo radio telescope in Puerto Rico and the NSF’s decision to decommission it. Earlier this week, however, the telescope collapsed. Or, more accurately, the beam-steering mechanism above the dish where delicate radio instruments picked up and recorded the radio signals focused there, fell onto the main dish, wrecking both. If you’ve ever seen Goldeneye, it’s like the end of that movie, only without the explosions and Sean Bean.1
Our friend Dennis will soon have a piece up about what the telescope meant to the people and the island of Puerto Rico. All I can comment upon is what it meant to the scientific community.
The above picture shows the iconic beam-steering mechanism that was suspended above the dish. Pictured are the broken 96-foot 430 MHz antenna (damaged by Hurricane Maria) and the massive Gregorian feed sub-reflector. When I visited in 1993, the Gregorian feed was not there. It was all long antennas dipping down, which ran along that track on buildings the size of trailers. We took the cable car up to the mechanism and jumped down onto them to go inside. For someone with acrophobia … it was an experience.
Arecibo is not a typical radio telescope. Most telescopes are parabolic in shape and focus light to a single point. You can then steer the dish toward whatever you want to study. Arecibo was a spherical reflector that focused the light onto lines that the antennae were then moved to cover. This allowed the observatory to cover large swathes of the sky while leaving the massive 1000-foot dish fixed firmly on the ground.
The dish itself, as you know if you’ve seen the movies it’s featured in, is like a colander — metallic and with lots of holes to allow rain to get through. These holes are smaller than radio waves (which range from centimeters to meters in size). So, they don’t affect the light collected by the telescope at all. This is sort of like the way your microwave has a cover for the window with little holes in it that allow tiny visible light waves through while the large microwaves are blocked. This allow you to see your burrito being cooked without you being cooked yourself.
Instead of cooking burritos, however, Arecibo studied the cosmos. Operating for over half a century, it studied everything from hydrogen gas in our galaxy to the gigantic black holes in the cores of distant galaxies. My own work was on pulsars, measuring the changes in their rates of spin. Arecibo was particularly good at pulsars, having discovered the binary pulsar PSR B1913+16 (work which won a Nobel Prize), the pulsar at the heart of the Crab Nebula, the first millisecond pulsar (one spinning 642 times a second) and the first planets beyond our solar system surrounding PSR B1257+12. The latter discovery was completely unexpected because we thought that the supernova explosions that create pulsars would destroy any planets. It was also involved in SETI’s search for extraterrestrial intelligence and sent the Arecibo Message to the globular cluster M13.
But the legacy of Arecibo was not in its most spectacular discoveries. It was in the day-to-day. For 57 years, this was a workhorse of astrophysics, observing day and night, year after year, making the slow steady contributions by which we come to glimpse the vast engines that grind the cosmos. There is no way to measure its impact. Just a search of the archive for abstracts with “Arecibo” in the title returned nearly 2000 refereed papers. I suspect the actual number of papers published using data from the observatory is many times that. And the number of studies on other telescope inspired by or informed by Arecibo data is even greater than that.
I consider myself part of Arecibo’s legacy2. I mentioned last time my only visit to the observatory and how it set me on my present career course which has seen me go from radio to optical to ultraviolet, from a 1000-foot telescope in Puerto Rico to a 12-inch telescope in space and drop about 100 scientific research papers along the way. And I have had my own students and collaborators I have helped along their paths. All of that got its start with me sitting in a control room in the middle of the night, watching that behemoth move around and thinking, “You know … this wouldn’t be a bad way to make a living.”
Right now, no one knows what will happen next. It is unlikely that the telescope will be rebuilt from its current state. It could be completely demolished and another one put in its place, though. The staff will still be there (they should be OK for a while since telescope itself is part of a larger observatory which houses a number of instruments). The site is still ideal. And there is precedent for replacement. 32 years ago, the 300-foot Green Bank telescope collapsed after wrapping itself around the support structure. Three years later, construction began on what would become the Robert Byrd Telescope, an even better instrument.
Regardless of what happens from this point, nothing can take away Arecibo’s legacy of scientific discoveries, illumination of the deeper mysteries and the careers it started and sustained. Ad Astra, you magnificent monster of a telescope. You will be dearly missed.
[ThTh2] One of the videos that every early physics student watches is the collapse of the Tacoma Narrows bridge. A version can be seen below:
The first version of the bridge was opened in July 1940 and immediately had problems with the wind causing it to oscillate. These motions were alarming but not necessarily dangerous as the bridge was built to withstand such stresses. On November 7 of that year, however, “Galloping Gertie” began to exhibit much more dangerous twisting motions and eventually collapsed. The only casualty was a dog who refused to leave a car stranded in the midst of the bridge. (You can watch a video here that gets into the physics and engineering issues).
Yesterday, this footage circulated of the Verrazzano Bridge in 60 mph winds.
If you were wondering why the Verrazzano Bridge was closed on Monday pic.twitter.com/W8AoCEUZkA
— Icculus The Brave (@FirenzeMike) December 2, 2020
What you’re seeing is good engineering. Any suspension bridge is going to respond to high winds. The Verrazzano Bridge is handling them just fine, moving up and down in response to the vibrations instead of resisting and taking damage from them. You wouldn’t want to ride on the bridge right now (indeed, it was closed). But there is unlikely to be any structural damage. And, more importantly, the design of the bridge prevents these high winds from creating the kind of twisting motion that destroyed Tacoma Narrows.
[ThTh3] Release the Kraken! No, not that one. The one on Titan.
[ThTh4] When our first astronauts went into space, they reported the existence of “fireflies” fluttering around the spacecraft. For a while, we had no idea what these were. There was even speculation that the Mercury astronauts had discovered a new form of life in space. Eventually, however, we figured out that it was condensation from the spacecraft’s cooling system. It has sometimes been claimed that this was astronaut urine, but Mercury didn’t have a system for urine dumps. But … what’s old is new.
[ThTh5] Touchdown.
YOU GUYS! HERE IT IS!!
Video of the descent and landing of the Chang'e-5 spacecraft, which touched down in the basaltic plains of Oceanus Procellarum on the lunar nearside yesterday.
Look at that landscape! It's craters all the way down!! 😍 pic.twitter.com/qiW9vNzKXE
— Paul Byrne (@ThePlanetaryGuy) December 2, 2020
[ThTh6] A huge amount of ancient art was just discovered.
[ThTh7] Maybe Thanos unleashed a turbulent flow.
Numerical simulations of a turbulently mixed cow
by my wonderful colleague Xiaoshan Huang @UVA pic.twitter.com/RksPUnGJkm— Enrico Ramirez-Ruiz (@astroenrico) November 25, 2020
[ThTh8] One of the remaining frontiers in astronomy is the origin of cosmic background light. Whenever we look at an object like a galaxy, there’s a certain amount of diffuse “sky” radiation – background light against which the galaxy or whatever is contrasted. That background mostly comes from dust in our Solar System or the Galaxy itself. But the New Horizons probe, now well past the orbit of Pluto, is able to ignore most of that light. And it appears the background glow is more prominent and less explainable than we thought. There are variety of explanations for it, including some more exotic explanations of Dark Matter, the invisible material that comprises about 85% of the mass in the Universe. Look forward to a lot more work on this subject, which has lain dormant for far too long.
[ThTh9] And speaking of Dark Matter candidates … there’s some more work being done on primordial black holes.
[ThTh10] If you look at the western sky in the late evening, you will see two bright points close to each other. These are the planets Jupiter and Saturn, relatively close to each other. In late December, they will be the most closely aligned in almost a millennium. A particularly good time would be around December 16, when you’ll have the new crescent moon in the same part of the sky. They were close in November and the view was spectacular.
Update Some footage of the collapse:
There was a drone inspecting the Tower 4 Cables when the collapse started so we get a close up on the cables breaking, check the second video segment. pic.twitter.com/Qw37Z5byWg
— Scott Manley (@DJSnM) December 3, 2020
Did the get the instrumentation out of the beam steering housing?Report
ThTh2 & ThTh7: Two fluids stories! The Narrows bridge is an excellent study in fluid structure interactions. Also, I wonder why people assume buildings shouldn’t flex? Clearly this is someone who hasn’t lived in an area with earthquakes, or every been at the top of a tall building with a motion damping system.Report
People often confuse “strength” and “stiffness”, even though those two terms have very different meanings in structural engineering. If you have a glass rod and a rubber rod, the glass rod is stiffer, in that you apply load and it doesn’t deform; but the rubber rod is stronger, in that you can apply more total load before the rod snaps.
Now, strength and stiffness are usually coincident; you generally don’t want something to deform significantly when service loads are applied, because “stay in the same shape when typical service loads are applied” is generally part of the requirement set for useful artifacts. And indeed, as we see from the Narrows Bridge, while the bridge is able to handle the structural loads, it isn’t staying the same shape, and this makes it unsuitable for its purpose of being a bridge…Report
I do hope you included that explainer for all the non-engineers on this site, right? And not for my benefit?Report
whoever it was for, it was helpful to me! I read it to my kids and they understood the principle. 🙂Report
Oh, good! Now let’s get into a discussion about plastic and elastic deformation, and yield, and fatigue, and how all of this gets way more interesting when we go from homogeneous materials to composites or heterogeneous materials!Report
yes, please do, using simple terminology, and I will devour every word of itReport
Oh geez, that’s a post in itself…Report
FYI to DDs mini-post below:
Metalic alloys are heterogeneous, but there is a new nanostructure material that is also called heterogeneous that is not technically a metallic alloy.
https://www.tandfonline.com/doi/full/10.1080/21663831.2017.1343208Report
That was…apparently the comment box did NOT like my editing of the comment. Could someone maybe delete that and we’ll try it again?Report
Now this paragraph is flexible, and this paragraph is stiff, and if you mix them together you get… well… that mess up there.
Thus materials science and language structure are not analogous.Report
OK let’s try this again.
Homogenous materials are like a bunch of gummy bears all mashed together. You can pull on the blob and it stretches, and eventually you pull hard enough that some of the gummy bears detach from the blob; that’s material failure.
How hard you have to pull before the bears detach is a matter of the surface area of each bear. (This metaphor is gonna get a little weird.) The surface of the gummy bear has a particular stickiness, and if you have bigger bears then there’s more surface area, so each bear sticks more strongly to the other bears. (We’ll assume for now that ripping a bear’s head off is a lot harder than pulling it away from the other bears.)
A heterogenous material — like an alloy — is as though you took a bunch of Jolly Ranchers, licked them, rolled them in gummy bears, and then mashed the resulting lumps together. The gummy bears stick to the licked Jolly Ranchers much more strongly than to each other, so each of the bear-Rancher lumps acts like a big gummy bear; and, as I said earlier, big bears have more surface area and stick more strongly because of it.
You might also ask “why not just make the whole blob out of Jolly Ranchers”, but the problem there is that Jolly Ranchers are very rigid and brittle; the gummy blob can flex and stretch more, and so if you haven’t made your blob exactly perfect (and if you aren’t pulling exactly straight) the gummies will shift around to line up with the actual load, rather than just sitting there. If you apply the load off-center to a rigid structure, you start bending it in addition to pulling or pushing at it, and bending can concentrate or amplify the applied load. This is part of the strength-versus-stiffness thing; it might take a lot of pulling to separate a gummy from the blob but the blob will stretch waaaaay out before that happens, whereas a Jolly Rancher brick won’t stretch even a little bit but will snap almost right away. But a mix of the two will have both more strength than pure gummies (larger individual blobs) and will be stiffer (the Jolly Ranchers make up part of the mass, and they don’t stretch) without being brittle (the gummies shift around to minimize load concentrations.)
Another option, if you have a bear of sufficient stiffness (stop that snickering!) is to use some advanced techniques to make a ginormous gummy bear. (Remember earlier when we said that ripping a bear’s head off is harder than separating bears?) When they talk about “single-crystal casting” things, picture someone pouring molten gummy into a mold and then slooooooowly cooling it so that it makes a single gummy bear thirty-five feet long and weighing approximately six hundred pounds…Report
From a materials standpoint, it surprised me how much the Tacoma bridge was able to flex before failure. Naively, I would have expected it to break apart from even a third of that amount of deformation.
The point: most of my materials intuition was formed by playing with legos! 🙂
When I was (much) younger, one of my roommates was a materials engineer, mostly metallurgy — Cal Poly guy. Anyway, he introduced me to Tacoma Narrows and explained how it all worked out. It was my first introduction to both materials engineering and fluid dynamics. Cool stuff. I learned a bunch of words and a little bit of math.Report
Had the bridge survived, it most likely would have needed serious repair, if not complete replacement. The cable stays and towers would have been fine (I think), but the span itself most likely exceeded the limits of elastic deformation and would no longer be sound. At the very least, the concrete road bed was a loss (concrete back then could not flex that much without cracking, although there are modern concretes that probably could).Report
For fun, another materials failure video:
https://www.youtube.com/watch?v=Ai2HmvAXcU0Report
The thing is that people might look at something flexing and swaying and say “that does not seem very strong”, and what they mean is that it’s not very stiff, and it might actually be quite strong…Report
Ah, so the explanation was regarding the confusion others would have over stiff and strong.
Which, as an engineer, I get.
People who don’t, however, need to grab a leaf or coil spring from a car and think about stiffness and strength while trying to bend or compress the spring.Report
For drinks, stiff and strong mean the same thing.Report
Damn if that isn’t as good an explanation for the confusion as any.Report
“as we see from the Narrows Bridge…” This was meant to refer to the Verrazzano Narrows bridge we see in the video, not the Tacoma Narrows bridge that collapsed. Obviously the Tacoma Narrows bridge was unsuitable for the purpose of being a bridge after it fell into the river!Report
The apocryphal story of Gertie as I was told in school, is that the Professor (you can see him on the film trying to save the dog) just happened to be on the bridge when the shaking started. He convinced the people on the bridge to drive/walk down the middle of the road since that’s the place with the most stability and probably saved some human lives, even though he was not able to save the scared dog. I don’t know if that’s entirely true, but it did give me an understanding of how that kind of wave works.Report
From what I recall, when bridges start moving like that, it is because some sort of harmonic mean is reached, where the movement spurs more movement. Or something? Is any of that right?
I’ve walked across the GWB several times and feeling it sway and move is a trip. It is also a really fun way to freak someone out who has never experienced it before.Report
Possibly. A flexible structure like a bridge has a natural amount of damping of vibrations. Exceeding that damping can happen either through a resonance, or simply through the constant increase of input energy.
So you could, say, just keeping increasing wind speed until the bridge fails because you’ve gone well beyond the physical limits of the structure.
Or the wind speed can stay constant, but hits the structure resonance, in which case the wave amplitude grows despite damping.
IIRC, modern bridges (etc.) have multiple damping modes, so you can’t hit a single resonance, or the design is such that the resonance frequency is so far beyond what could occur in nature.Report
My father’s masters thesis (1948) was on the aerodynamic characteristics of structural sections. Although it doesn’t actually mention bridges, he always told me that it was inspired by the Tacoma Narrows Bridge failure. If I remember his layman’s explanation correctly, as the bridge tilts, either towards or away from the wind, it catches the wind better, more tilt means even more force. And at certain speeds you could indeed get a resonance that would disastrously grow the amplitude.Report
Yeah, it’s not a resonance that we think of like shattering glass, it’s just a runaway aerodynamic force feedback, because even a flat plate with an angle of attack has lifting force. You can add small structures that will spoil that.
The wiki has a nice video of the FSI.
https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_(1940)#CollapseReport
No one, even the NSF (especially the NSF?) likes to pay for maintenance. What new things would have gone undone to keep Arecibo in good repair? Would it have attracted more money if more time had been spent looking for “rogue” asteroids — something I understand it was good at — and less on science? Can/will the USSF fund a replacement and let it be used for science some of the time?Report
Scientist have figured out what is killing salmon returning to urban waterways to spawn.
Tires.
More specifically, tire dust.
Even more specifically, an anti-corrosion/anti-oxidation agent added to tire rubber to keep the tire from corroding.
That’s gonna be a trick to fix.Report