In our last episode, we talked about what it’ll take to navigate across the Solar System. In this episode we scale things up and speculate how future civilizations will navigate to other stars and even other galaxies.
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Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit astronomy.swin.edu.au for more information.
Preston: Hi, folks. It’s your friendly Astronomy Cast editor Preston here. While recording this episode, Fraser and Pamela experience technical issues, so at times the quality may suffer. We’ll plan on recording another episode on this topic next year. Thanks for understanding.
Fraser Cain: Astronomy Cast, Episode 414: Navigating Far.
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. My name is Fraser Cain; I’m the publisher of Universe Today. And with me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville, and the director of Cosmoquest. Hi, Pamela; how you doing?
Dr. Pamela Gay: I’m doing well. How are you doing, Fraser?
Fraser Cain: Good. A little exhausted from the second show that we’ve done today; but, this is what we have to do to deal with both of our busy schedules. Actually, last week I spent a big road trip traveling around Banff, Jasper; just an amazing place to be.
Dr. Pamela Gay: All the cool geology.
Fraser Cain: Yeah; the geology, and the wildlife. We saw bears – grizzly and black – elk; it was a really good time. And my wife, who comes from Texas, had never seen anything like this. So, if you have a bucket list somewhere, put this on the bucket list.
Dr. Pamela Gay: That sounds absolutely amazing.
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Fraser Cain: Okay, so, in our last episode, we talked about what it takes to navigate across the solar system. In this episode, we scale things up and speculate how future civilizations will navigate to other stars, and even other galaxies. So just to catch people up, Pamela, last episode we talked about how when you’re here on Earth, we want to be able to know where we are, and we use the stars, and now we use GPS satellites, but it’s a fairly simple thing – relatively speaking – because we’re moving around on a two-dimensional surface, and sometimes you go to three dimensions as you go up and down in altitude, but it’s relatively straightforward.
To navigate inside the solar system, spacecraft use signals to and from the earth using the Deep Space Network to calculate their distance from the earth, and then will use other signals to be able to calculate their velocity as they move forward; and eventually you can sort of a state of your position of where these spacecraft are, and zip around.
We can imagine some far future where there’s transponders attached to every asteroid in the solar system, on lots of planets, on – even just floating in space. And you use this Deep Space Network proper to be able to navigate around the solar system. So, Pamela, let’s take this whole concept and scale it up one notch. What’s it gonna take for us to be able to navigate from star to star?
Dr. Pamela Gay: When you’re traveling between the stars, there’s two different things that you need to worry about. One is: where am I? And one is: where am I going? The “where am I” you get at in a lot of ways, just like they talk about on Star Trek with their astrometrics lab. You look around you and you take – hopefully on a regular updating basis – detailed images of the sky all around you. As you move through space, the nearest stars will gradually shift in position, and the most distant stars, the most distant galaxies, won’t appear to move at all.
So, just like when you’re in a car and you see the trees whipping by while the most distant mountains don’t appear to move until you’ve been moving for a long, long time; well, it’s the same effects as you move through space. You can get at your exact position by looking at how the stars around you appear to move relative to one another on the sky. You can’t see in three dimensions, but for getting at where you are, this is pretty much good enough, as long as your measurements are good enough. But that’s a longer story. Now, the “where are you going”; that one is a whole lot easier because of pulsars.
There are pulsars out there that are spinning amazingly fast, and as they spin they let out pulses of light and it turns out that if we’re moving toward a pulsar or away from the pulsar, the amount of time between those individual pulses appears to change ever so slightly. And the amount that it changes is directly related to how fast we’re going.
To get at that three-dimensional velocity, you need to look at well, this one over here on the left, it appears to have its time shifted this amount. This one, over here to the right, it appears to have its amount shifted this other amount. You need to get at least three different pulsars, preferably in three very different directions around the sky; and using those three different pulsars, you can get at your three-dimensional velocity.
Fraser Cain: Right. And so pulsars, right, are of course these dead stars, neutron stars rapidly rotating; they’re putting out these beams of radiation that are coming from them, and they’re so – I guess they’re like these clocks, these really precise clocks. You can sense those signals and get that Doppler shift from them. So you can tell your distance to all of these pulsars based on – you tell your velocity with the Doppler shift? Right, okay.
Dr. Pamela Gay: So this gives you your three-dimensional spatial motion. And you need to have at least three of them, because if you only use two, that tells you what plane of the universe you’re in; plane is useful, but there’s a whole lot of space in the other direction of space. So use three different ones to get at your three-dimensional location; or three-dimensional velocity, rather.
Fraser Cain: So do we do this right now?
Dr. Pamela Gay: Well, it’s part of our research on pulsars. We can study how they’re moving relative to us, how we’re moving relative to them. Some really neat research comes out of looking at these very high precision Doppler shifts where you have to start taking into account things like – rock has tides too. So the tops of mountains will move up and down, carrying observatories with them, with the tides.
It’s not a lot, but it’s enough you have to add it into your calculation. We have to subtract out the earth’s movement around the sun. All of these different motions add up, but if you remove them correctly, it also allows us to do things like find masses orbiting around pulsars, that are pulling the pulsars to and fro.
This is actually how the first planets – and by planets I mean small chunks of dead stuff – how the first planets orbiting other stars were found. It was found through the timing errors in pulsars, which should not be able to have timing errors. So you work it backwards, and you get at – these are Doppler shifts created by these small planet-like-ish things, yanking to and fro on the pulsars.
Fraser Cain: But aren’t we getting to a place where – I know like with, was it Dawn or maybe it was one of the previous missions to that – it actually had the capability to orient itself to find all of these objects in the sky, and know where it was. So I guess that’s –
Dr. Pamela Gay: – what it can do. This is, again, getting at that other dimension which we talked about in the last episode, where spacecraft like Dawn that are in our solar system can use radio signals to figure out their distance from Earth, and that puts them on a surface of the sphere that has that radius. But then to figure out exactly where they are on the sphere, they take pictures of the sky, and those pictures of the sky give them that other dimensional orientation.
So with pulsars, once – and pulsar observations are much harder than taking a simple image and figuring out your orientation – once they go through and they measure the Doppler shifting multiple pulsars, it would give a spacecraft its three-dimensional velocity. But then to start to get at its “where am I” in space, that’s where you go back to taking images. But you have to take amazingly precise images that allow you to make out the slight variations in positions of the stars that are closest to you, and the lack of motions of the stars that are most distant from you.
So you have to have a working three-dimensional model; which is something we’re actually working to slowly build up with the Sloan Digital Sky Survey, with Gaia upcoming, with Hipparchus in the past; we put all of these different pieces of information together, along with the actual velocities of the stars, to build a model that we can someday use to move through space. And say we can simulate moving through space – and things like Worldwide Telescope.
Fraser Cain: Right, right. Do you think that building the capability to spot and track a pulsar signal into a spacecraft is going to be something that’s fairly complicated? Or is it, are they fairly bright signals?
Dr. Pamela Gay: There are some that are fairly bright. And if you think about it, if you have a spacecraft that you plan to travel between the stars with, it should have the capabilities that – a not too complicated radio telescope that Jocelyn Bell Burnell built in graduate school had.
So that’s the kind of technology that we’re looking for, is the type of stuff that Jocelyn was building before we were born, and using it to detect pulsars. It’s smaller now, it’s easier to do; it’s something that some lab classes do. It’s just more complex than taking a picture. You have to turn your radio signal, listen for a little while, process that signal. It’s something we can do today, but we don’t need to do it today, with our spacecraft.
Fraser Cain: What kind of accuracy can they get?
Dr. Pamela Gay: For the velocities?
Fraser Cain: Yeah; yeah.
Dr. Pamela Gay: It all depends on how fast the pulsars are rotating. But you should be able to get down – if you have good enough detectors – down to the kilometers per second; and potentially meters per second. We don’t currently need to do meters per second, but that’s something that we do when we’re detecting planets around massive stars, and so there’s also the – if you’re really wanting to get deeply engaged – there’s all sorts of different ways to get at this information.
Fraser Cain: Right. I guess when you’re moving from star to star and maybe it’s going to take you years to make that journey; you’re going to have a long time to be able to sort of better zoom in, and really figure out the accuracy of where you are.
Dr. Pamela Gay: Right. And the more samples you take, the better your data is going to be.
Fraser Cain: So that’s getting around the local area; the local star system. How far would that be able to take you?
Dr. Pamela Gay: Well, you need to have pulsars all around you for this to be successful. So you need to be able to get those three not-too-far-away pulsars. By which I mean, you’re embedded within the system. So within our own galaxy, as long as there’s old population of stars within sight, this works beautifully. But once you get into the space between galaxies, the pulsars aren’t all that bright. So that’s going to be a problem.
But also, you’re going to have all the pulsars in that galaxy that’s now behind you. You may not be able to see the pulsars in the galaxy that’s in front of you. So once you start exiting galaxies, you need to find that new standard source that you can use as your Doppler shift, basically network of distant objects.
Fraser Cain: I love how you casually talk about how you’re going to be leaving your galaxy; being able to see, then you have to focus in on the next galaxy. It really sounds like we’ve somehow reversed roles here, because suddenly we’re a type III civilization; moving from galaxy to galaxy, spreading out into the entire Hubble volume of space that we can reach. So that’s –
Dr. Pamela Gay: But this is tractable. This isn’t pie-in-the-sky impossible; this is the kind of thing that once we figure out how to download intelligence into computers, and we aren’t worried about only living for 70 to 100 years, this kind of exploring the universe becomes much more interesting.
Fraser Cain: Right; once we’ve moved into our robot bodies.
Dr. Pamela Gay: Exactly.
Fraser Cain: So, but with the quasars – using quasars as navigational aids, you’re not – they don’t act in the same way that a pulsar does. It’s not like they’re putting out pulses on a regular basis, right?
Dr. Pamela Gay: No. But with quasars, you have this distant background. So when you’re within our own galaxy, you’re moving through these pulsars. So you might go from using one set of five – because five is better than three – to, as you drop one, you pick up another. Well, as you start moving between the intergalactic space, quasars have extremely bright emission lines.
These emission lines that you get are a concrete line of this is exactly where the atom is giving off light in a specific wavelength that’s a little bit smeared out because things rotate. And you look for that line to change how its Doppler shifted. Using information about the expansion rate of the universe, the distance to these objects, you know their starting point for how their Doppler shifted. And then you can get at how their apparent Doppler shift changes, due to your own velocity, and eventually due to that changing in distance between you and them.
Fraser Cain: Now we’ve talked about using natural sources to be able to navigate yourself around the galaxy; and eventually around the universe. But, as with the solar system, we’re going to want to put in some kind of artificial system. So are there methods that we could use – I’m thinking like, neutrinos, or could we have – the equivalent of transponders, but at a star system level?
Dr. Pamela Gay: So, the thing is, that while within our own solar system, we really do want to eventually add these beacons – these transponders with known frequencies, with known orbits. The universe really does provide, once we get beyond our solar system. The pulsars – we know how pulsars work. We know how they evolve with time. They don’t have any hiccups or weirdnesses, in general, with their rotation. So they are beacons. They simply are.
Now quasars, you do run into the periodic problem, and Nathan Lowell wrote a great little short story on this. You do run into the periodic problem that quasars shut off, and occasionally new ones turn on. So occasionally your beacons die, and occasionally you get a new one; and you have to keep track of these things.
But again, quasars – atomic spectra, they don’t change. Hydrogen is perfectly happy to have its Balmer series, it’s Lyman series; and by keying in on those emission lines, those are our beacons. We don’t need to build any intergalactic megastructures for navigation; we can just happily build them to live in. Because why not?
Fraser Cain: Now, we’ve already actually sort of used this navigation system with the Voyager spacecraft, right?
Dr. Pamela Gay: We’ve used it to help people understand how to find us later.
Fraser Cain: Right; where to send the invasion fleets.
Dr. Pamela Gay: Exactly. So things like neutron stars: we know their size; we know how much light they’re capable of giving off as a function of temperature. And so if you look out at a series of pulsars, and you measure their timings, and you rough out their distance – well, this image on the plate that was on Voyager, marks out where we are relative to a series of different pulsars.
So theoretically the alien who someday scoops up this plate and has interstellar travel will be able to come visiting. I’m not sure how I feel about that, but for better or worse, over the fullness of time we do get carried away from these pulsars. Quite literally, we get carried away through orbits.
Fraser Cain: Right. But as we know, the earth has been broadcasting the existence of life with its atmospheric gases for hundreds of millions of years. So any aliens capable of finding, tracking down, finding the golden record on the Voyager spacecraft and then being able to use that to trace its way back to us – this is going to be old news to them. As if it’s an old timey record. But it’s great that they used that; that was a Sagan suggestion, right? Carl Sagan suggested that they build this record; and it’s great, because it’s just got this pulsar map that tells you which star is the Sun, and where to come to steal our water.
Dr. Pamela Gay: Go look at that third rock, and that’s where we are. So we’ll be safer once we get to Mars.
Fraser Cain: Right, right. They won’t even know; they’ll go to Earth. They won’t even see us; they’ll go to Mars; they won’t even see us here on Earth and we’ll be set. So, what about quartz corrections and things like that? Because the speed of communicating – as we understand it right now – is really limited by the laws of physics.
So if you’re going to attempt to communicate with your spacecraft which is moving along this vector, the spacecraft may know where it is, but as that spacecraft gets further and further away, you’re going to have a longer time to communicate with it. And if it’s going faster than relativistic, time delays start to come into play. So how will that all come into the picture?
Dr. Pamela Gay: This is something that Orson Scott Card – say what you will about other things – he got the science right in Ender’s Game, where characters who were moving at near the speed of light who moved between solar systems essentially skipped through time. And you’d send out a burst of communications and then wait days, weeks, months, to receive that slow message that then your computer reprocesses to normal speed.
Fraser Cain: If I remember, they used – in his books they used a method of communicating faster than light speed.
Dr. Pamela Gay: That came in later, where they developed the ansible. But initially when they were speaking to their general, it’s all through horrific time lag.
Fraser Cain: Right. So you would be getting slow motion or sped up, depending – or I guess both people would see; I’m trying to remember how this works. But you would see slow motion from the other person because there is a difference in your velocities, and then you would have to then speed up the video and the audio to have it look normal.
Dr. Pamela Gay: Right. So the person who does the accelerating will appear to be significantly slowed down. Then the people who didn’t do the accelerating, or back on Earth, will appear to be extraordinarily accelerated. So if we’re here on Earth listening to this signal from the astronaut, it will be this high-speed machine on – sorry, it will be this greatly slowed down, like your dying-batteried Walkman from the ‘80s; whereas the signal that they’d receive from us is this this high speed buzz of a machine on fast forward.
Fraser Cain: Yeah; and then I guess the further you get, you have to take into account the transmission times to be able to reach them. Then the amount of time based on relativity; on how fast they’re going compared to you, to be able to understand and formulate a response to it, or to make a course correction, or whatever; and time to communicate back.
Dr. Pamela Gay: And once you start getting far enough from the planet Earth, once you start reaching cosmological distances, you have to also take into account the rate at which the universe is accelerating. So you’d have their velocity, you have the velocity of the universe carrying them away, and all of this adds up to math. It all adds up to doing lots and lots of math. But, it’s stuff we already know how to do.
Ned Wright has an excellent website that explains a whole lot of this, and has Ned’s cosmological calculator on it, that can do these kinds of calculations in a fairly straightforward manner. So we have all of these different things; we know how they work. But the real issue is, you end up with a separation of societies, separation of culture, as the two societies – the accelerated and the non-accelerated – drift apart through time, just as they drift apart through space. This also came up in the movie last year that Kip Thorne advised on, where they got far too close to a black hole. I’m blanking –
Fraser Cain: Oh, Interstellar?
Dr. Pamela Gay: Interstellar; it also came up on Interstellar, where you drift apart in time. Now, we do have some hopes for faster than light travel; it’s unsure if we’ll ever be able to actually harness them. But the entanglement of different particles that are generated together, we know that if you mess with one particle you’re also messing with the other, even if they’re separated by vast distances. We don’t know if there’s a way to control that, but it’s currently our only hope.
Fraser Cain: Right. Yeah, that’s the way – I mean if I understand right, the entanglement causes it to collapse instantaneously, but you still require the speed of light to be able to communicate with the other party, to find out what they saw. And so, at the end of the day, you’re both kind of stuck by, still the speed of light.
So it’s funny; we could do the calculations now to control these fast space fleets and redirect the Earth-defense fleet to Alpha Centauri as needed; except that we don’t have the technology to make the Earth-defense fleet go, nor necessarily the power to be able to send those kinds of communication transmissions those distances to moving targets, and so on. So we can run the calculations, and then we can just wish that we had an Earth-defense fleet.
Dr. Pamela Gay: But you know, given the choice of having the technology and not having the math, or having the math and not having the technology; we’re doing things in the right order. And it gives us room to dream; to imagine; to drive us forward with pathways of, “We just need to develop the tools to do this thing.” Science gives us a direction to head in. And then the engineers and technologists, they’re the ones who eventually get us there.
Fraser Cain: Yeah, yeah. So is that it? This was a two-parter; navigating around the solar system. I guess if we find some kind of idea about parallel universes, then we’ll have to figure out – then we’ll come back with a third show.
Dr. Pamela Gay: I think we do need to add one more thing in. So we talked a lot about how we can know our three-dimensional velocity, but knowing how fast and in what direction I’m running only helps if I know where I start from. Google Maps needs to know my starting location or it really can’t get me anywhere. So when it starts coming down to figuring out where you are in space, we hinted briefly at, you take very careful images, measure the positions of the stars, look for the parallax shift of the nearest stars, and the lack of shift of the most distant stars.
This is Star Trek’s astrometric lab; but you can also start to get information from things like, we know certain stars have certain absolute brightnesses. We measure the differences in their apparent brightnesses; and just like we can get at how far away a motorcycle is by how bright its headlight appears to be, we can get at how far away the stars are.
Now you can extend this out, and use some of the brightest stars in the universe to get between your nearest galaxies. You can use supernovae, however fleeting they may be, to nail down, “This galaxy is in this place; this galaxy is in this place.” So it’s much harder to get at the exactly where I am once you exit our galaxy, but we have starting points of how to do it.
Fraser Cain: Well, I think until we can actually get those parallel universes figured out, I think we’ll have to wrap up this and make this surely a two-parter. It won’t be a three-parter yet, but –
Dr. Pamela Gay: That’s okay.
Fraser Cain: Yeah. Cool. Well, thanks as always, Pamela, and we’ll talk to you next week.
Dr. Pamela Gay: It’s my pleasure.
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Duration: 29 minutes