Ep. 465: Exploiting Interfering Light

Electromagnetic radiation, also known as “light” is pretty handy for astronomers. They can use it to directly and indirectly observe stars, nebula, planets and more. But as you probably know, light can act like a wave, creating interference patterns to teach us even more about the Universe.
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Show Notes

Visible Spectrum of light
Single Slit interference explainer video at Khan Academy
Double Slit experiments
How DVD and CD drives work
LIGO detections
What is inteferometry?


Transcription services provided by: GMR Transcription

Fraser: Astronomy Cast, Episode 465: Exploiting Interfering Light. Welcome to Astronomy Cast – our weekly fact-based journey through the cosmos. We’ll help you understand not only what we know, but how we know what we know. My name is Fraser Cain. I’m the publisher of Universe Today. With me is Dr. Pamela Gay, the Director of Technology and Citizen Science at the Astronomical Society of the Pacific, and the Director of CosmoQuest. Hey, Pamela, how are you doing?
Pamela: I’m doing well. How are you doing, Fraser?
Fraser: Good. We need to apologize in advanced for your audio quality.
Pamela: Yes.
Fraser: Your proper microphone isn’t with you, and you are recording with a headset microphone, but you’re practicing your professional mic control training to make the best possible sound that you can, so kudos.
Pamela: You know, sometimes the cable isn’t with you, literally and figuratively, and you just make due.
Fraser: It’s true. You’re suffering from both today. The Force, though, is with you. Any announcements to make this week?
Pamela: I don’t think so. We are getting into the holiday season, so we will probably be recording strange days and hours, off and on. We are going to do our best to recorded on time next Friday – for those of you who like to watch the live episodes, but I will be at a meeting in Washington DC, so it will be hotel internet.
Fraser: Uh-oh. Maybe is someone in Washington DC has a super-fancy internet connection –
Pamela: Yes.
Fraser: I have an announcement to make.
Pamela: You do.
Fraser: Mm-hmm.
Pamela: What’s your announcement?
Fraser: We’re writing a book.
Pamela: Congratulations.
Fraser: Yeah. Now, I’m not writing the book. I am helping the book get written. This is a super-early announcement. As we get closer to it, I’ll give more info, but the gist is Dave Dickenson – who is the Visual Astronomer on our team at Universe Today – is going to be writing a book, and it’s going to be like a comprehensive guide to amateur astronomy. It’s Universe Today’s Guide to the Skies. I forget the exact title that we’re using right now, but it’s going to have how to choose a telescope, and what things to see, but we’re going to try to make it a more modern book for the modern age. So, the things like accessing online telescopes, and the online communities that are out there, and ways that you can do citizen science with your telescope and stuff.
And, the other cool part is we have a great community of amateur photographers out on Instagram, so we’re going to be showcasing a lot of that photography. So, I hope it’s going to be like an astronomy book that nobody has ever seen before, and it’s going to be coming out in about a year from now.
Pamela: That’s awesome.
Fraser: Yeah. Dave is working hard on it right now, and as we get closer to what it’s actually going to look like, we’ll get your help, and get peoples’ help beta testing it, and stuff. So, that’s just a really early initial announcement. Let’s move on to the show. Here we go. Electromagnetic Radiation – also known as Light – is pretty handy for astronomers. They can use it to directly and indirectly observe stars, nebulae, planets, and more. But, as you probably know, light can act like a wave, creating interference patterns that teach us even more about the universe. Alright, Pamela. Interference.
Pamela: Yes.
Fraser: Go.
Pamela: So, basically, waves can interfere with one another. If you’ve ever been to the ocean, you’ve probably seen this. You have two waves that go between piers; go between boats; and, come out the other side, maybe with a slightly different angle. When the high points of the wave combine you get a bigger wave. When the high point and the low point of the waves combine you get flat, still water. And, what we’re used to experiencing with water also happens with light. And, in this case when you get the waves synched up so that the highs and lows are all aligned, you get beautiful, bright, coherent light.
When the highs and lows are exactly out of sync, you get darkness, lack of light, and in between you get all manner of different intensities, and the way the intensity varies tells us exactly how much the light is and isn’t out of sync, and allows us to measure things – which is a cool and awesome thing to get to do.
Fraser: Alright. So, let’s provide people with some examples. What is the main use? I mean, I know that we’ve talked about this in the past – about the double slit experiment, and things like that – but, you can make light, essentially, interfere with itself, and by doing so you get peaks and valleys like, as you said – like waves of water. Pretend that light doesn’t act like a particle right now. So, what is the classic use of this phenomenon of light?
Pamela: So, there is the classic use in terms of the way we abuse it in science, which is doing single slit and double slit experiments to see how light does and doesn’t interfere. But, then there are all of the practical uses – which is why I wanted to do this episode on exploiting light interfering. This is where we start to get into things like CD players, Blu-ray players; DVD players.
All of these devices have inside of them lasers that are bouncing off of the engraved surfaces of our media, and the reason that each progressive kind of player has been able to store more and more data is because we’ve been able to do more and more high resolution measurements of lasers of increasingly short colors that allow us to pack the data more closely together to get more information onto a surface.
So, when you’re dropping that CD into your CD player, all that’s really happening is instead of a needle – like a record player going around, and going up and down as it goes up, up and down in the groves of the record – what’s happening is laser light is going across the surface of the CD, and how it bounces back is or isn’t interfering in a way that tells us whether or not it’s hitting a pit or a peak on the surface of the CD.
Fraser: So, you have – we all have, as long as we have a CD player or a DVD drive in our computer – we have a device that depends on interfering light to function.
Pamela: Exactly. So, we now rely on interfering light for our storage media, and that’s just something that’s kind of cool to think about where this basic physics – that when we learn it in school we’re like, “How am I ever going to use the fact that laser light interferes?” – Well, it’s how you play your games on your Xbox.
Fraser: Right. Well, now people care. Now they’re listening. Now you have their attention.
Pamela: You just have to put things in the right language.
Fraser: Right. That’s a great way to explain how we use this in a practical sense, but how do astronomers use it?
Pamela: For astronomy – and, part of the inspiration for doing this episode – is we see interfering light in things like LIGO, where we’re making very, very precise measurements of the distance between two objects. So, the detections with LIGO were made because a variety of different beams spread out across the planet detected changes in the length of the beam, with offsets in time, that corresponded for how long it would take something traveling at the speed of light to get between these various places. So, as the gravitational wave swept across the world, it changed the laser path distance by just enough to change what was being detected from the laser going up and down the laser paths.
Fraser: Right. So, let’s just understand this. You have this laser emitter. You have this mirror at the other end of this big, long detector machine, and then the laser pulse is sent down, and it bounces back and forth between the emitter and this mirror at the end of this big, long hallway. And, then it bounces back and forth, and back and forth, and back and forth – so that it’s essentially traveled a very long distance.
And, then it gets captured again, and it is – essentially, they can see the interference pattern of what this laser is doing, and it’s a way to sort of make sure the laser is perfectly aligned, so that if that gravitational wave passes over, and stretches the length of the pathway, then it gets out of alignment, and they can measure precisely how out of alignment that is – giving you that measurement of a gravitational wave – which is just amazing. And, I guess they’re going to be taking that and scaling it up when the future – like the Lysa, and some of the gravitational wave systems work. Then they are going to be traveling potentially millions of kilometers, and still be able to tell the interference.
Pamela: And, they key to making this work is they have two different paths that the light can take. So, you start with a single laser. You send the light down two different paths; bring it together; and, you tune the device so that it interferes in a known way – preferably creating a bright point of light – and, then you look for changes in that brightness that indicate that the light is no longer interfering the way you would like it to.
Fraser: Why is interference the technique that they use for this level of precision? What is special about interference that is different from some other way of measuring the light?
Pamela: It’s because we can start to use the light to be able to measure things at distances down to the nanometer – which is a meter 00000001 of a meter.
Fraser: Right.
Pamela: And, so that nanometer size differences – variations – that we’re able to get. So, if you’re doing interference with a red laser, you’re looking at being able to measure changes of a wavelength – which in this case is 635 nanometers. If you’re able to precisely use a blue laser – where blue has an even shorter wavelength – you can start to measure things down to just 445 nanometers – which is part of the blue in Blu-ray. And, by having this wavelength of light level precision, we may not know exactly how many wavelengths there are between two things, but once it’s set up, and once it’s tuned, we know that there is exactly an integer number of these wavelengths.
And, if something goes from being the bright pattern of positively interfering to the blackness of destructive interference where the peak and trough line up perfectly – we know that we’ve had a shift of half a wavelength, so 222.5 nanometers if you’re using that blue laser. And, the only way we can precisely measure that kind of a variation in real time is with lasers and interference.
Fraser: Right. So, I’m just imagining – there are scientists, say, working at LIGO, and you’re looking at that interference pattern, or detecting it, how big is the interference pattern? How well can it been seen? Can you see with your eyes, or is that interference pattern as small as the wavelengths of the light?
Pamela: So, the interference pattern itself – I have to admit, I haven’t gone and stood in LIGO, or looked at pictures in LIGO of exactly how it looks on their detector, but from the interferometers interference system that I’ve set up – And, any of you who have a CD and a laser pointer can do this for yourselves – What you do is you bounce the laser off the grid on the CD. If you still have an AOL CD kicking around, they’re perfect for this.
Fraser: They have no other purpose, yeah.
Pamela: And, it’s true. It’s true. The groves in the CD are essentially a grading, and when the laser light bounces off of this, it will create a pattern on the wall with a bright central point, and then a series of additional points that are all of the places where you have positive interference, and the spacing between these points on the wall, and the distance from the wall to your CD allows you to backwards calculate what is the size of the pits on your CD – which if you’re feeling really industrious means you can actually use a laser pointer like you use to torture your cat, and an old AOL CD to calculate how much data can be stored on a CD, which is a cool experiment that I used to make my students do.
Fraser: Right. So, thanks to, essentially, the physics of light, the precision of the wavelengths of the light, you can turn something that would be microscopic and moving at the speed of light – so you wouldn’t be able to detect it in any way, or be able to measure it – to something that you can measure.
Pamela: Yes.
Fraser: It’s just mind-bending. So, you’ve explained one example, which is in the LIGO detector, but that’s like a recent use of the interference of light. Astronomers have been doing this for a long time, right?
Pamela: It’s true. So, one of my very first jobs in astronomy was in radio astronomy. I worked at Haystack Observatory in Massachusetts, and one of the things that we had to do there was combine the recordings of signal coming into a variety of different radio telescopes scattered around the world. And, what’s fabulous about radio telescopes is you can combine the data after the fact with optical interferometry. We’re not that good yet at being able to record individual wavelengths of light, so when we do optical interferometry, it has to be done physically, in the moment.
But, with radio, you record the signal; you ship all of the data to one location; and, then you do what’s called Finding Fringes. Which is you work on adjusting the timing of the data on each of the different recordings – the start point.
The reason you have to do this is it turns out in 1992 – when I was doing this prior to GPS – we didn’t have the most accurate of clocks. So, we’d be able to line up the data to get all of the wavelengths in sync to allow us to do high-resolution radio imaging with very long baseline interferometry by combing the data to make sure that we had that constructive interference pattern.
Fraser: This idea of interference is, I think, something that’s fairly counter intuitive to people, but is one of the most powerful techniques that astronomers can use these days. If you imagine that you had two radio telescopes side by side, and they were both gathering radio waves from some point source, but you weren’t trying to interfere them, what would you get, and how is that different from it actually being interfered?
Pamela: I love the way you phrased that. So, what you want to do is have the spacing of your detectors such that each of them receives the light, and it then gets combined in integered differences of the wavelength of the light.
Fraser: But, I guess I’m saying, what is the difference? Why do you want to interfere it, and not just have the gathering capacity of the two separate radio dishes?
Pamela: Okay. Sorry. I misunderstood it. So, with interferometry, we can essentially – with, like, with the very large baseline array in New Mexico that was built to do this all of the time – with the various networks that have been set up with telescopes spread out across the planet, we are able to treat all of these individual dishes as a single telescope, and get the resolving power that corresponds to the diameter – which could be the diameter of the planet of the separation between the furthest apart of those telescopes.
Fraser: Right. So, in other words, if you have a radio dish on one side of the earth, and you have a radio dish on the other side of the earth, and if you can interfere the light that both of them are perceiving of the universe, you don’t get double the resolving power because you have twice as much telescope; you get a telescope the size of the earth. Sort of.
Pamela: Yeah. It’s one of these things where how faint we’re able to observe. So, how much light we can gather is directly related to the area of a mirror. So, if you double the size of the mirror, you collect four times as much light, and are able to see that much fainter of an object. With resolution, it goes strictly as the diameter across the longest side of your telescope.
And, since we’re used to thinking of circular mirrors, we don’t normally think of it in terms of being anything other than a circle, but where it gets different is where you have things like the very large telescope down in Chile – that are combining the light from four different massive mirrors, and a variety of little satellite 1 meter mirrors – and when all of this gets combined, you can have different resolutions along different axis based on how big the telescope is in these different directions. And, that’s just weird and awesome.
Fraser: Right. And, it’s easier to do in radio waves because the wavelength is so long that you can be rough. As you said, you can find those edges. You can find those parts and line up the radio waves.
Pamela: It also has to do with how we’re able to record it. So, when the radio waves get detected, you have a kilometer long wave. You’re sensitive to that wave. With light, the wavelength itself, we can’t get the resolution in our recording to sample across that wavelength. So, we have to physically combine the light, rather than combine the recordings of the light later. It’s tricky.
Fraser: But, you can imagine, right? Imagine you have something that’s going at the nanometer scale. The wavelengths are moving past you with a nanometer, and you have to align those wavelengths to the individual wavelength. Say you had a video from one telescope, and a video from the other telescope, and then you tried to line up with precision, right to the exact right wavelength when each one is – as you said – a 0 followed by eight more 0s – that is really tough, while when your wavelength in centimeters, meters, it gets more feasible.
But, the ones that do it in real time are the ones that – down in Chile – where they’re just combining the light simultaneously, they can do it because essentially they’re just turning the knob until they get to the exact right fraction of a nanometer.
Pamela: And, that’s pretty much what they’re doing. I hadn’t been down to the VLT in Chile, but I got to visit the US Navel Observatory a few years ago where they’ve been doing experimental work on defining best practices, and creating infrared interferometry. That’s pretty much where we’re still confined – is down in the infrared part of the no-longer-radio telescopes, but now optical telescopes – optical outside of what we can see with our eyeballs.
What they do is they have the light come in, and they have all of these different chambers that they adjust the length of so that things are getting reflected; fiber optics are getting used to make sure that the path length is the exact right distance to have things combine down to these 100 nanometer scales.
Fraser: That is like the area – and, this is all still fairly new in terms of actually being able to do it instrumentally. The technology at – say, the Very Large Array down in Chile – is amazing, and fairly cutting-edge, but now there’s a whole new class of telescopes that are coming out. Things like the Giant Magellan Telescope, which has, I think, seven separate big mirrors that will be combined together. And, there are a bunch more more. So, this technique – because the limit is the size of the telescope –
Pamela: Yes.
Fraser: You know, before they started to get all mushy because of gravity, right. So, separate telescopes.
Pamela: And, part of how we’re able to focus these multi-mirrored telescopes is actually using interference patterns, again, where if you have, for instance, the Hobby-Eberly Telescope down at McDonald observatory, you have a mirror that has a number of different segments down in the primary, and each of these segments has to be aligned perfectly to be able to get the light into the detector at that top.
If you’ve ever seen pictures of the Hobby-Eberly Telescope, or the Sult Observatory down in South Africa for the South African Large Telescope – both the Hobby-Eberly and Sult have your typical telescope dome, and next to it they have a large tower with a dome on top of it, and it kind of looks like a little robot dude sitting on top of a mountain, in both cases. And, the reason they have this configuration is you can open up the dome from the main telescope; open up the little arm with the dome on top of it, and shine lights down onto the mirror, and use interference patterns to precisely line up all of these mirrors so that you get the perfect interference pattern of the laser light coming off of all of these mirrors.
Fraser: Wow. That’s really cool. So, are there any other ways that astronomers use interference?
Pamela: Well, at the end of the day, everything that we do involving spectroscopy is just based on the principals of interference where we have light that is getting scattered and reflected off of gradings. It’s getting scattered by going through slits in particular ways. And, the way we build a lot of our precise filters that allow us to look at the sun; to only look at narrow colors of light – these are also often built in a way that uses the interference.
So, for instance, a really good solar filter that takes advantage of interference is one that only allows constructively interfering light to go through a series of films so that light that has the wrong wavelength ends up bouncing out light that has the exact right color is able to pass through because of the interference that happens between the films.
Fraser: Yeah. A lot of the times when you see pictures that are taken from spacecraft from NASA, and Hubble Space Telescope, and things like that, they’ll say that the nanometer wavelength that was used for one of the filters of the image. So, a lot of the times when you see pictures that came from Cassini, for example, back when Cassini used to take pictures and was still alive – you would get such and such nanometer for one infrared filter, and then a different nanometer for a different infrared filter, and then a third one. And, then they would just, in computer, turn one into red, one into blue, and one into green, and turn that into what looks like a full color image, but the reality is it’s just that perfect – and, they’re right down to 1 nanometer off for each of these colors – each of these filters that they’re using.
Pamela: And, the majority of filters that get used in astronomy are more broadband filters where there’s a certain percentage of light let in at this color, slightly different percentage at this color, and 0 at this color. So, you have this curve that defines the amount of throughput as a function of wavelength for the given filter.
Any of you who have used the common Johnson or Cousins filters to do photometry have encountered these broadband filters, but then there are very specific interference filters that can take you down to have a hard cutoff of everything – Well, not everything. It’s never everything. Most of the light at colors A through B gets through, and then nothing on either side of that. We shall not let you pass. And, it’s this ability to get these hard edges in what does and doesn’t get through that is so awesome about the interference filters.
Fraser: What about planet-hunting? I guess you kind of covered everything in spectroscopy, but with the radio velocity method, they use spectroscopy for that.
Pamela: There, it’s not quite the same thing because, yes, it requires you to have a spectroscope, but once you have the spectroscope doing its thing, there’s no added interference just because you have a planet. It’s simply a matter of where the star is changes from day to day due to having that planet.
Fraser: Right. Would it be possible, do you think, to get interference through gravitational waves?
Pamela: Yes. But, I don’t know if we’ll ever be able to measure it.
Fraser: Right.
Pamela: Waves interfere. It’s what they do. So, the question then becomes: What do we have the technology to measure?
Fraser: Right. Amazing. Cool. Well, thanks a lot, Pamela.
Pamela: My pleasure.
Male Speaker: Thank you for listening to Astronomy Cast, a non-profit resource provided by Astroshpere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at AstronomyCast.com. You can email us at info@astronomycast.com; tweet us at Astronomy Cast; like us on Facebook; or, circle us on Google Plus. We record our show live on YouTube every Friday at 1:30 p.m. Pacific, 4:30 p.m. Eastern, or 20:30 GMT. If you miss the live event, you can always catch up over at Cosmoquest.org, or on our YouTube page.
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[End of Audio]
Duration: 29 minutes

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