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Last week we talked about how new telescopes and techniques are allowing astronomers to explore the shortest wavelengths of light. This week, we go to the other end of the electromagnetic spectrum, and explore the longer radio waves which are now accessible to astronomers.
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VIDEO: Ep. 602: The New Colors of Gamma Rays – Getting Shorter (Astronomy Cast)
Tibet Observatory Confirms Existence of Galactic PeVatrons (Sky & Telescope)
Electromagnetic Radiation (Swinburne University)
Radio Spectrum (NASA)
How to Measure the Speed of Light With a Bar of Chocolate and Your Microwave (Popular Mechanics)
Speed of Light in a Microwave (with Marshmallows!) (Physica Mechanica)
The Hydrogen 21-cm Line (Hyperphysics)
Fast Radio Bursts (Swinburne University)
What is a neutron star? (EarthSky)
Lyman alpha systems and cosmology (UC Berkeley)
Very Large Array (NRAO)
Very Large Telescope (ESO)
Adaptive Optics (ESO)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast Episode 603, New Colors of the Radio Spectrum. Welcome to Astronomy Cast, our weekly facts based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of Cosmo Quest. Hey Pamela, how are you doing?
Dr. Gay: I’m doing well, how are you doing, Fraser?
Fraser: Doing great. Weather is perfect, I cannot complain. Really, that’s clearly half of my happiness, is how the weather’s doing.
Dr. Gay: I appreciate that deeply. It can’t decide today if it wants to be grey and ooky or sunny and the moments that it’s sunny, it is glorious.
Fraser: Yeah. My fault for choosing to live in a grim coastal rain forest, where the rain starts in October and it doesn’t give up until the end of March. But yeah, it’s so beautiful and everything’s turning green, and all the plants are out, and all the flowers in the meadows, and everything’s just popping, and the bees are out, and it’s great. It’s great. I love it. All right. So, last week we talked about how new telescopes and techniques are allowing astronomers to explore the shortest wavelengths of light. This week, we go to the other end of the electromagnetic spectrum and explore the longer radio waves which are now accessible to astronomers.
We’ll get to that in a second, but first let’s have a break. And we’re back. All right, Pamela. Last week, as people recall, what was the name for the super high energy gamma radiation?
Dr. Gay: PeVatrons.
Fraser: PeVatrons, which I do not like, but it’s not up to me. Let’s go to the other end, then, and let’s talk about the longest – as we mentioned, visible light, infrared, ultraviolet, gamma radiation, radio waves, it’s all the same thing. It’s all just photons of varying wavelengths. So, let’s go to the other end. Where does the radio spectrum begin?
Dr. Gay: Pretty much where your microwave starts cooking your food. As soon as the wavelengths of light get bigger than a piece of hair, it becomes easy to observe them with big dishes and it starts to become possible to combine the wavelengths of light, kind-of after the fact, with interferometry. And how we do science fundamentally changes at these longer wavelengths.
Fraser: Now you’re saying, the width of a human hair is where the radio spectrum starts?
Dr. Gay: Right around there. So, the way to think about it is if you have an optical telescope, its surface needs to be without flaws. A grain of dust on it would be the biggest defect on it, and you’d still be sad faced about that grain of dust. And this is because wavelengths of light are so tiny that that piece of dust is visible and causes issues.
Dr. Gay: As the wavelengths get longer and longer, your mirrors can have bigger and bigger defects, and in fact, you can go from using fancy high polished mirrors to essentially using chicken wire once you start looking at things that have a long enough wavelengths of light.
Fraser: That’s crazy.
Dr. Gay: It’s that millimeter wavelength where you start having the – you worry about that piece of hair instead of that piece of dust.
Fraser: And so, when people talk about the submillimeter, that’s less than a millimeter, that’s moving towards the hair. Then you get into – so, are microwaves smaller? They’re in the submillimeter, right, the microwaves?
Dr. Gay: Yeah.
Fraser: Although apparently you can use your microwave to see the wavelengths in chocolate and things like that. You can put chocolate in your microwave, you can turn on the microwave, and then you’ll see lines of heat in parts that are melted in a chocolate bar.
Dr. Gay: So, I didn’t know about that but a standard experiment done with university students is, basically you put out a call, who has a microwave they no longer want? You remove that rotating tray, you line the bottom of the microwave with marshmallows, turn on the microwave, and the marshmallows expand along where the waves are interfering positively inside the microwave, and it’s fabulous.
Fraser: Wow. That’s amazing. Yeah. So, it’s kinda crazy. We can’t see them with our eyeballs, but we can see them indirectly with our instruments. In this case, our instruments are marshmallows.
Dr. Gay: Exactly.
Fraser: Which I think is great. Okay. So, we know that we’re looking at these much longer wavelengths. So then, how big can they get?
Dr. Gay: This is the amazing part. Your FM radio gets down to about 10 megahertz, which is about 30 centimeters. We actually start to go past that with some of the upcoming detectors that are going to be starting to look for half meter, meter wave light. And the physics in our modern universe of what’s generating these changes as we go to the longer and longer wavelengths. But what’s amazing is, because the way the universe is expanding, the most distant parts of the universe, their wavelengths are getting shifted redder and redder and redder. So, we can only see cool things like the 21-cm line in hydrogen by going longer than 21 centimeters if we wanna see the distant universe.
Fraser: And so, what do we see at these longer – what does looking in the radio spectrum show us that we don’t see in other parts of the spectrum?
Dr. Gay: At the most simplistic level, there’s a lot of different molecules that have their energy states, when they transition between one level to another, are cropping up in the radio. So, you see water vapor. Water vapor is something that angers many a radio astronomer, and in optical in some places, too. You start to be able to see much more complicated molecules. Formaldehyde becomes visible once we leave the visible spectrum. And all of this chemistry of what is in giant molecular clouds, that chemistry is all coming from radio astronomy. And what’s amazing is because as we talked about last week, the really long wavelengths don’t scatter as easily as the short wavelengths.
These long wavelengths can also pass through the disc, the dusty, dusty disc of our Milky Way can penetrate from the core to where we are now. So, the universe opens up, becomes more transparent and reveals its chemical makeup just by stepping into the radio.
Fraser: The classic example, the one that’s super productive for astronomers, is that idea of the 21-cm line?
Dr. Gay: Yes. And the 21-cm line is one of those things that we see from the coldest, most boring areas of the universe that still have stuff in them. That’s the key. There’s boring places out there that normally look completely transparent, except they actually have a diffuse gas of hydrogen. And these hydrogen atoms have a spin/flip transition in them, where just this change in orientation withing the atom releases a bit of energy at that 21 centimeters. This is a fine line transition, and if the atoms are heated up, they don’t have a chance to have this rare transition. Leave the gas alone.
Dr. Gay: And you see 21-cm light. So, we can trace out cool, otherwise invisible material from the hydrogen that’s out there, otherwise being transparent.
Fraser: All right. So, we’re gonna talk more about these longer wavelengths in a second, but first let’s have a break. And we’re back. Right. Okay. We talked about the 21-cm line, we think about that’s like, .2 of a meter. That’s getting into those longer and longer wavelengths. So, the question I really wanna know is, how low can we go?
Dr. Gay: Well, we are going to find out with the Square Kilometre Array that is currently being planned out to be constructed in southern Africa and Australia. It is planned to go to meter resolution. And what is really amazing about these super long wavelengths is, as you go from optical where you have these perfect mirrors or lenses, to microwave where you have these reflective surfaces like satellite dishes that are still very well put together, to Arecibo, which was at the tens of centimeters and basically chicken wire. As you get to these meter wavelengths, you essentially have stabby bits coming out of the ground.
This is because what you want is for your antenna to be a multiple of your wavelength. And if you’re looking at something that has a one meter wavelength, you are essentially fine using a one meter stick or a multiple of a one meter stick coming out of the ground.
Fraser: Little Christmas trees.
Dr. Gay: It’s a radio antenna. And so, there’s gonna be a lot of stabby bits in the outback and spread across southern Africa.
Fraser: Yeah. They’re a pretty clever design, these – I call them Christmas trees, you call them stabby bits, because they have all these different little prongs coming off of them that allows them to essentially point in different directions without having to move the antenna. You just figure out which parts of the Christmas tree you’re going to be using to receive these signals from the universe. So, what do these extreme wavelengths start to tell us, and what is even theorized? We talked about the 21-cm line, which is very valuable because it tells you where the universe’s uncooked hydrogen is.
Things like formaldehyde, alcohol, there’s all kinds of really amazing molecules that we can sense. As we get longer and longer, is there some stuff that we suspect is out there that we just haven’t been able to see yet?
Dr. Gay: Well, this is one of those, how do you say what you can’t see? And on one hand, it’s going to allow us, as I said, to see things that are red shifted into those longer wavelengths. On the other hand, it’s going to start to allow us to put limits on objects that we know about but don’t understand. In this case, fast radio bursts is one of my favorite examples because what is now turning out is as you shift the wavelengths longer and longer, it takes longer and longer for us to receive the light. And it’s not that the light is traveling at a different speed, it’s that the light is being produced somehow with a delay built in as a function of wavelength.
We don’t fully understand fast radio bursts. We are currently blaming neutron stars for them. We don’t understand what material around the fast radio burst might be causing this delay in what we see at different wavelengths. We just know this is something we’re seeing. And any explanation that we come up with for fast radio bursts is going to have to incorporate this delay as a function of wavelength. And that’s kinda cool, and also – oh dear, that’s very complicated.
Fraser: I think the other thing is, there are objects like stars, things like that, that do blast out radio waves. But when you put on top of that the expansion of the universe and the fact that things are at incredibly long redshift, these get harder and harder to see because they’re pushing. What was once visible light is now in the microwave, which was once in the microwave or once in the radio is now into the really big radio. And so, to be able to have these longer wavelengths visible, there’s kind-of no limit. This idea that in the far, far future, millions, billions of years from now, future astronomers won’t have as much of a sense of the history of the universe because it will have expanded over the cosmic horizon. But the thing that will always be there is going to be the longer and longer wavelengths of radio.
Dr. Gay: And we’ve taken advantage of this for a long time. The visible line that we rely on in moderate distant galaxies is actually the Lyman-alpha line, which if we were to look at in a laboratory is blue-ward of what we can see with our eyes. But that too blue to see line gets brought redder and redder and redder so that when we look out at distant objects, we’re seeing Lyman-alpha with our visible light telescopes. We’ll keep shifting the suckers farther and farther away, and now you’re starting to look at them without a comma, eventually with bigger and bigger telescopes that remarkably allow us, because it’s possible to do interferometry with them, allow us such high resolution images.
Fraser: All right. So, we’re gonna talk about interferometry more in a second, but first let’s have another break. And we’re back. Now, you opened up this show talking about this idea of using these radio telescopes to collaborate, to work together in a way that the shorter wavelengths just can’t do. So, explain this technology.
Dr. Gay: Interferometry is the ability of multiple dishes, multiple telescopes, to collect light from the exact same sources, and that light to then get combined so that it acts like one significantly larger telescope. And the resolution of what we’re able to see is directly related to how many times does a wavelength fit from edge to edge. The mirror doesn’t have to be complete. The reflector surface doesn’t need to be complete. So, when we use the Very Large Array in New Mexico, those dishes spread out over miles of land add together to a telescope that has the resolution of a single dish miles across.
Now, we’re gonna go from having something spread over the countryside in New Mexico to having something spread over the southern part of the largest continent on the planet and the outback of Australia.
Fraser: Yeah, two continents.
Dr. Gay: Yeah. Now, the two sets of dishes do work differently. This is why they were able to split the child across two continents. But with all of these little detectors spread out over these great distances, they’re able to take these long wavelengths, which otherwise it’s just like shoot, how do I build a telescope big enough to get anything of reasonable resolution? They’re able to spread out the antennae and synthetically get a significantly higher resolution system. So, we’re gonna have high res images of distant objects using really long wavelengths of light.
Fraser: And the classic example that I think everybody is quite familiar with now is this idea of the event horizon telescope that took images of the super massive black hole event horizon at the heart of M87 and in theory has taken a picture of Sag A star, but we haven’t seen the picture yet. But people always ask me, why can’t we just use visible light telescopes as well? So, this idea of what makes interferometry with radio waves more feasible than interferometry with shorter wavelengths?
Dr. Gay: It’s that, actually the size of the wavelength. For interferometry to work, you need to take all the wavelengths that are coming together towards you and shift them so that even though each of your different detectors is receiving that same wave front at a different time, because they’re literally different distances from what they’re looking at, you have to shift the light so it appears to arrive at the exact same moment. With the Very Large Telescope in Chile where they have their four mirrors that work in the infrared doing this, they’re literally using fiberoptics to combine the light so that the light travel time to the detector is actually the same for all four of the systems.
It’s basically a physical shift. Now with radio light, because it can be recorded so differently, we can literally record each individual wavelength which we can’t really do with optical. We can take radio recordings, shift them in software after the fact, and line up those wavelengths. This is how Event Horizon Telescope worked, is everyone individually recorded what they could see, mailed the hard drives, and then they got put together after the fact.
Fraser: Yeah. When you think about with visible light, you’ve got things that are, say, 590 whatever nanometers, and you’re trying to line up these wavelengths perfectly so that they are all – you’re getting to the exact right wavelength when each wavelength, as I said, is so teeny tiny. And yet, when you’ve got these wavelengths that are a meter across, half a meter across, you just start your clock and go okay everybody, it’s close enough, and the computer can time everything together. And so, the advantage is the resolution but not the light collecting power, right?
Dr. Gay: Exactly.
Fraser: So, you can see an object that’s bright.
Dr. Gay: And so, there’s two factors that we worry about with taking images. The first one is, how small a thing can you discern? And when Hubble was launched 31 years ago prior to adaptive optics being a thing used by scientists, it had the highest resolution of anything that we could work with because it was above the Earth’s atmosphere. We now have the ability to effectively remove the atmosphere from our images in some cases. And this means that we can get significantly higher resolution images from the surface of our planet, but we run into the problem of the faintness. How dim an object you can see is directly related to the size of your mirror.
We see this with our own eyeballs to a certain degree. If there’s bright light, your pupil locks down. If there’s low light, your pupil opens up. Animals that work at night, they have huge pupils to allow massive amounts of light to come in. The bigger your telescope is, the more light it allows in, the fainter the object you can see. And today, with the Very Large Telescope, we can see things fainter than what Hubble can see with not identical but close to the same resolution because we’re correcting for the atmosphere. This combination is amazing. With radio, we just build a ton of tinier dishes, spread them across the world.
Fraser: Right, yeah. And that’s why you can have a telescope the size of the Earth. Just imagine what the future might hold when we can have a telescope the size of the solar system. Put one radio telescope on one side of the Earth’s orbit and one on the other, record, send signal home, crunch the numbers on a computer, and you’ve got a telescope that is as big as the Earth’s orbit, or bigger.
Dr. Gay: And ironically, you’re going to need a massive radio dish to receive those signals.
Fraser: Right, yeah. As big as the Earth. All right. Well, thanks Pamela, that was awesome.
Dr. Gay: My pleasure.
Fraser: Do you have some names for us this week?
Dr. Gay: I do. As always, this episode is brought to you by you. We are supported through our amazing Patreons, and this week I would like to thank Dean, Steven Coffey, Frodo Tennebo, Kseniya Panfilenko, David Gates, Alex Raine, Shannon Humber, Neuterdude, The Air Major, Corinne Dmitruk, Jean-Francois Rajotte, Gabriel Gauffin, Abraham Cottrill, Eran Segev, Daniel Loosli, Jeremy Kerwin, Claudia Mastroianni, Justin Proctor, Joe Wilkinson, Tim Gerrish, Arthur Latz-Hall, Matthew Horstman, J. Alex Anderson, Michelle Cullen, John, Aron Tannenbaum, Roland Warmerdam, Omar Del Rivero, Dustin A Ruoff, Brent Kreinop, and William Lauer.
Thank you all so very much, you make what we do possible and allow us to pay for our software, our sites, and most importantly the people behind the scenes that keep us on the straight and narrow. This would be Beth, Ally, and Rich.
Fraser: Thank you everybody.
Dr. Gay: Thank you.
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