#298      30 min 16 sec
Curve your expectations: Observing planets and galaxies with the help of gravity

Cosmologist Dr Bart Pindor explains gravitational lensing, in which the curvature of space by gravity allows us to investigate galaxies and other astral bodies. Presented by Dr Shane Huntington.

"Because the dark matter doesn't feel electromagnetic forces so it doesn't feel pressure, it doesn't participate in this giant conflagration." -- Dr Bart Pindor




Dr Bart Pindor
Dr Bart Pindor

Dr Bart Pindor is a research fellow in the School of Physics at the University of Melbourne. He received his BSc from the University of Toronto and completed his PhD at Princeton University. His thesis work involved a search for strongly-lensed quasars in the Sloan Digital Sky Survey. He has also proposed a novel method by which future survey telescopes could discover new gravitational lens systems through serendipitous measurements of their time delays. His current research focuses on attempting to detect radio emission from the hydrogen gas which pervaded the universe during the formation of the first stars and galaxies. This work uses data from the Murchison Widefield Array, a pioneering low-frequency radio telescope situated in Western Australia. The Murchison Widefield Array is a precursor for the low-frequency Square Kilometer Array which is to be built on the same site over the coming decade.

Credits

Host: Dr Shane Huntington
Producers: Eric van Bemmel, Kelvin Param, Dr Dyani Lewis
Audio Engineer: Gavin Nebauer
Voiceover: Nerissa Hannink
Series Creators: Kelvin Param & Eric van Bemmel

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VOICEOVER 
This is Up Close, the research talk show from the University of Melbourne, Australia. 

SHANE HUNTINGTON 
I’m Dr Shane Huntington thanks for joining us.  Almost 100 years ago Albert Einstein published a paper describing the geometric nature of space, better known as General Relativity.  In this document Einstein described gravity in terms of the shape of space we find ourselves in. Near massive objects like planets and stars, space is distorted and we feel this distortion as gravity.  Einstein's ideas today are of great significance to our everyday lives.  Without General Relativity many of the things we take for granted, such as our GPS systems would not function with any acceptable degree of accuracy, but what about if we look further afield into the universe?  How do Einstein's predictions bear out over much larger distances, well beyond the reach of the solar system, or even our own galaxy?  Today on Up Close, we'll be speaking to astrophysicist Bart Pindor to explore these questions.  Dr Bart Pindor is a research scientist in the School of Physics at the University of Melbourne.  Welcome to Up Close Bart.

BART PINDOR:
Thank you very much Shane.

SHANE HUNTINGTON:
What exactly does Einstein's Theory of General Relativity actually tell us?

BART PINDOR:
Well if we think about gravity normally we think of it as something that is a force, so we think of it as something that makes apples fall from trees onto people's heads, or maybe as something that makes massive objects attract one another and that's what we think about gravity in everyday, and it's a very good description.  It lets us build massive bridges or skyscrapers or whatever, but Albert Einstein and his refinement of Newton's Theory of Gravity said that actually a more accurate way to think about gravity, as you said is to think about it as the curvature of space and time so whenever there are massive objects or equivalently whenever there's energy, that mass or energy causes the curvature or the geometry of space and time to change.  So if we think of it in that way then we can see why, in the case where gravity is simply a force, we wouldn't expect that light is affected by gravity at all because particles of light are photons, they're mass-less and so why should they be affected if gravity is simply a force that pulls on massive objects.  But if we go into Albert Einstein's Theory of General Relatively we now start seeing gravity as being the curvature of space or a change in the geometry of space then even light, which is mass-less is going to be affected by the fact that straight lines are in some no longer straight, and so when a ray of light tries to propagate it's going to follow a curved line instead of a straight one.

SHANE HUNGTINGTON
I suppose a lot of people would have the idea, when they think of space, as a relatively homogenous type of environment, but there is a lot of stuff out there that we can see.  Does that mean that the majority of our universe is distorted in this way?

BART PINDOR
Well yes it is.  Even hear sitting in the studio between you and me there's a microphone and when the light that's bouncing off me that allows you to see me, I mean it's also going to be gravitationally lensed by the microphone that's sitting between us, but the effect is so tiny that you could never perceive it or never even hope to measure it.  So in outer space, yes the effect is happening all the time but only in certain, particular situations are we able to measure it accurately enough that we can say oh, the effect is happening or draw some conclusions from that.  

SHANE HUNGTINGTON
So let's say I go and I look through a reasonable sized telescope, how does this effect actually manifest itself in what I'm seeing?

BART PINDOR
Well the earliest prediction of gravitational lensing which Albert Einstein made is that for instance, the sun will deflect the positions of stars which are near it so, in other words the light from distant stars passes near the sun and then it is essentially bent when it passes very near the sun and so the apparent position of the star is essentially moved.  So instead of being in one position it's slightly shifted to another position, and the way that people actually went out to measure this in the very first observational confirmation of General Relativity was by looking during a solar eclipse.  It's obviously hard to see the stars when they're right near the sun, that's why we call it the day time, but during the solar eclipse it becomes much darker near the vicinity of the sun and then people are able to see this a very subtle shift of the position of the stars.  So that's one example of gravitational lensing or one of the manifestations that it moves the apparent positions of objects on the sky, which are more distance than the object that's doing the lensing, in this case the sun.

SHANE HUNTINGTON 
Now when we talk about something like the sun and the stars that we're seeing we know their exact positions or we have an idea of what they should be and as you say, we use something like an eclipse to compare that to the scenario where there was no sun.  What about when we're looking a bit further afield, I mean how do I know if something's been changed in terms of position if I don't have something like that normal star field to compare it with?

BART PINDOR 
There are two examples or let's say regimes of gravitational lensing where you could have this kind of certainty.  So one case is the case where the object which is doing the lensing is actually moving in an apparent way on the sky, so we think of things as stationary in the cosmos but they're really not I mean, things move around, the planets move, the asteroids move and even stars on the sky move.  Over tens of thousands of years the constellations change because the stars are moving so if a star moves across our line of sight to another more distant object like a distant star, then we can see a gravitational lensing effect there.  Suddenly the alignment between those two stars is very good and we see a sudden brightening of the background star, that's called gravitational micro-lensing.  Another example is where the things aren't moving but the gravitational lensing effect is so large or so spectacular maybe, that it's clear that gravitational lensing is occurring.  For instance one of the manifestations can be that multiple images of the same background object can form. A very famous example is that there can be a background object called a quasar, which is a very massive, powerful galaxy.  It has a large black hole in the centre and as matter falls onto this black hole a great amount of radiation is released and we see this object as an extremely luminous object called the quasar.  If there happens to be a galaxy on the line of sight between us and such an object, and the alignment is extremely good, then often what we can see is actually two or even sometimes four images of the background quasar.  And then we can think well by process of elimination either there are four quasars very close to each other on the sky, or it's a gravitational lens.  In fact the case is even more compelling than that because one of the unique signatures of gravitational lensing is that, not only can it produce multiple images but like any optical system, the existence of multiple images implies that there are multiple time travel paths.  Actually if we saw this case where there was a distant quasar and we saw let's say four images of it, then those four images would all be seen at different times in the quasar relative to each other, so there would be a time delay we call it between those four images.  So if the quasar was for instance to suddenly become brighter than we would see in turn, the four images suddenly get brighter, not at the same time and that's sort of proof definitive that what we're seeing is a gravitational lensing effect and not simply the extremely unlikely alignment of four quasars in exactly the same place.

SHANE HUNTINGTON 
We have this incredible option when we look through telescopes of looking back in time but what you're talking about there is essentially like having a time lapse photography scenario going on with some of these objects.  Have we been able to determine various aspects of some of these distant objects as a result of that, that we wouldn't otherwise be able to see without gravitational lensing being in place?

BART PINDOR 
Yes, well I would say more so in this case of the gravitational lensing time delay.  The effect we learn is more about the lenssystem itself so for instance, one of the things that we can see - there's a very famous lenscalled the Einstein Cross which is a case where there are four images of a single quasar caused by a single galaxy and we can actually see the brightness of these images flicker as a result of the motion of the stars in the galaxy that's causing the lensing.  We can actually tell how fast the galaxy is moving across our line of sight simply by observing the variation and the brightness of the quasar images.  Another very, very interesting application of this time delay and one which is in some sense, the longstanding romantic project of gravitational lensing and why many people are interested in it, is that the time delay sets the absolute scale of the system that we're looking at or indicates to us the absolute scale of this system.  If the quasar was near to us and the galaxy was also near to us, or if it was far away from us and the galaxy was also far away from us, then the angles could be the same and we couldn't distinguish those two things simply by the positions of the galaxy and the quasar images.  However in the case where the system is small, the time delay would also be correspondingly small and in the case where the system is large, the time delay would also be correspondingly large.  So by measuring the time delay we can in some sense, measure the distances involved in this gravitational lensing system and measuring distance in astronomy is one of the most sort of fundamental problems we have.  We can't really run out with our ruler sticks and measure things and so we're always looking for better and better ways to try to measure distances and gravitational lensing offers sort of the prospect of a one shot measurement to objects which are really quite far away - what we call cosmological distances.  That's something that - people have attempted to do this many times or over the course of the last few decades since gravitational lensings have been observed. And I think it's something which, in the future people still think could be a very, very interesting and competitive method for making such distance measurements.

SHANE HUNTINGTON 
You're listening to Up Close, I'm Shane Huntington.  Today I'm speaking with astrophysicist Dr Bart Pindor about Gravitational Lensing.  Now Bart this idea of measuring the light obviously over the last few decades, we've had the extraordinary data coming in from the Hubble Space Telescope which I know has given us a lot of information in this area, but we're about to move to the James Webb Telescope in the coming decade and that doesn't use visible light.  Are the effects the same regardless of the part of the electromagnetic spectrum that we're looking at in terms of gravitational lensing?

BART PINDOR 
Yes they are so gravitational lensing is what we call achromatic so in other words, it doesn't separate the frequencies of light in the way that a prism that would separate the frequencies of light.  And so any electromagnetic radiation will be gravitational lensed and in fact, objects like quasars give out electromagnetic radiation all the way from gamma-rays to radio waves and we've seen gravitational lensed radio quasars.  Just very recently there was a report of the gravitational lensing of a gamma-ray burst so multiple times, the same gamma-ray burst was observed because of the gravitational lensing effect.  However as you go into different regimes you're essentially looking at different properties of the source so for instance in the infrared, quasars are much less concentrated than they are in the visible.  For instance the variability of quasars is generally much lower in infrared because the emitting region is much larger and so in some sense, if you were trying to study the properties of the source then you have access to different properties of the source when you're in different wavelength regimes.

SHANE HUNTINGTON 
When we think about these alignments that are required to give us this sort of effect one might imagine that it's pretty rare but given the vastness of the universe, how common are these sorts of gravitational lensing alignments?

BART PINDOR 
Well in the case that we're talking about where you have the spectacular gravitational lenswhere there are multiple images and when you look at the Hubble Space Telescope picture you'll be like wow, that's an amazing thing that's happening in outer space.  I now believe in general relatively because I can see it in a picture so that sort of thing is not terribly common.  In the case of, for instance quasars - approximately one in a thousand of quasars will be lensed in this way.  It depends essentially on how many galaxies there are between us and the quasar but there are many other instances where gravitational lensing is, in some sense inevitable.  So for instance, another very common and very spectacular form of gravitational lensing is lensing by clusters of galaxies, so a cluster of galaxies is a region of space where thousands of galaxies have fallen in by their mutual gravitational attraction and in that case, if we look at the galaxies which are again, behind the cluster - so it's sort of on the line of sight more distant than the cluster - then we see that some of them are very, very highly distorted by the gravitational field of the galaxy.  They are often turned into what are called giant arcs, so instead of looking like normal galaxies they're stretched out in these very eerie ghostly, almost filamentary structures.  In that case, any massive cluster that you look at, if you look at it closely enough with the Hubble Space Telescope you almost inevitably see strong gravitational lensing, just because the region of sky is inevitably going to have some galaxies behind it which are suitably aligned.  But gravitational lensing is also interesting in the case where it's not this kind of spectacular lensing and that's a regime called weak lensing where, as you mentioned at the very beginning, this effect happens all the time.  When we're trying to make an accurate measurement of things like the distribution of matter in the universe and so forth, the spectacular things are not always the most interesting or informative, and weak lensing – so let me just set up the scenario.  Let's suppose we now have our galaxy cluster but the background distance source galaxy is not so well aligned that it turns into one of these beautiful, spectacular giant arcs, but nonetheless its shape is still suddenly distorted by the gravitational field of this cluster.  If we add up the distortions which we think have happened to thousands and thousands of galaxies, which are in the part of the sky where the cluster is, then we can even more accurately map out the mass that's associated with the cluster.  That measurement is a very compelling measurement of how much mass there is in the vicinity of the cluster, and in fact we can even go to the next step and by simply trying to measure the distortions of all of the galaxies on the sky, and in particular the correlation of the distortions - in other words if the galaxies are all being distorted by the same mass then the direction in which they're distorted should be aligned - then we can even measure what is the total distribution of mass in the universe or more accurately, what are the statistics of the distribution of the total mass in the universe.  This gives a very, very powerful probe which is completely independent of electromagnetic radiation.  Normally when we try to count how much mass there is in the universe we do it by counting galaxies, but then we have to make an equivalence between how many stars there are in the galaxy - which are things that we see - and how much actual matter is there in that galaxy and that's a not perfect alignment.  It's hard to make that exact equivalence whereas in the weak lensing case, we're directly measuring the gravitational effect of all that matter, whatever it is.  So if we can accurately measure this effect over a large region of sky we can accurately determine what is the distribution mass of the universe which is kind of one of the fundamental issues in cosmology.

SHANE HUNTINGTON 
Bart when we use the analogy of a lens, one of the things that people would often think about is that they are used for magnifying objects.  Do we see this magnification effect in gravitational lenses, I mean essentially magnifying is being caused by a redistribution of the direction that light comes from, is the same thing possible in these cases and do we see magnified images of stars and galaxies and different objects?

BART PINDOR 
Yes absolutely, that's certainly one of the signature effects of gravitational lensing is magnification and we see it in many guises.  I mentioned with micro-lensing where there's simply the passage of a small object across the line of sight and we see a sudden brightening of the source object.  In that case it just simply looks brighter, because we don't resolve the background object so all we're seeing is the total integrated light from it, and we see that light suddenly get brighter and then fade away again once the alignment is no longer very good.  In the case of things for instance the quasar lenses where the alignment is not changing then yes, the quasars can be very highly magnified, sometimes in the case of a hundred times brighter.  And in the case again of these, for instance giant cluster arcs, the objects are very highly magnified so that we can see a faint galaxy in much more detail than we would otherwise.  And in fact, one of the earliest predictions if you like of gravitational lensing was by I think, a Swiss-American astronomer named Fritz Zwicky, and he suggested that cluster of galaxies could be what he called natural telescopes.  So by looking at clusters of galaxies we could attempt to see very faint galaxies which are further away and learn more about their properties.  That certainly proves to be true - he made that prediction I think in 1937, sort of 40 years or more before the first gravitational lenswas actually observed - so quite a prescient prediction.

SHANE HUNTINGTON 
Now you're interested in particular in quasars.  I'm going to ask you first to explain to us what a quasar is but why is there a particular requirement to use any sort of gravitational lensto study quasars?

BART PINDOR 
A quasar is a massive galaxy which has - what we call massive or astronomers like to call a super massive black hole in the centre - that's what we think a quasar is.  The super massive black hole has a disc of gas around it called an accretion disk and as this gas falls onto the super massive black hole it release energy in the same way that if you drop a penny off a skyscraper it releases a tremendous amount of energy, and don't try to catch it at the bottom.  But this is much, much more energy being released and so this energy has to go somewhere, some of it is swallowed if you like by the black hole but a lot of it just is simply radiates away in every direction and so we can see these quasars from very, very large distances.  They're some of the most luminous objects in the universe.  In some sense, the reason why quasars are intimately related with gravitational lensing is simply because they are so luminous, in gravitational lensing we require an alignment of a foreground object and a background object, and the background object has to be quite far away, so it has to be very bright for us to see it, so a quasars a natural source to observe with gravitational lensing.  And they're also variable so unlike, for instance, galaxies which if you integrate up the light from a galaxy and you observe it the next day and [unclear] again and again and again, the light from the galaxy doesn't change very much, simply because the time scales of the lensgalaxies changes is millions or tens of millions of years.  But because a quasar's being powered by this central engine of this super massive black hole, which is a region that's quite small - it's probably smaller than the solar system or of that scale - then changes in the intensity of radiation that's coming from the quasar can happen on the light crossing time, which in the solar system might be a day.  We can see the amount of luminosity coming from quasar change rapidly over time scales which are in the order of a day, that gives us the ability for instance to measure this time delay effect.  Which we couldn't do in the case of a galaxy being the background object because we don't have this variability to tell us oh, this image got brighter and this image brighter - if nothing's getting brighter then you can't make that measurement.  That said galaxies, as sources are also very interesting, they just allow us to study different parameters if you like, of the lens.

SHANE HUNTINGTON 
When we talk about quasars I mean obviously we've mentioned the Hubble Space Telescope that can only point in one direction at a time, what sort of equipment do you currently use to make these measurements?  Because these are far distant objects, some as you say, are relatively dim but some are quite bright and not exactly in our local neighbourhood, so how do we go about making these measurements?

BART PINDOR 
For instance, the work that I did for my PhD Thesis, we used a survey called the Sloan Digital Sky Survey.  It was developed at a time when the first large format, CCD cameras were available, so a - a CCD camera is the same kind of camera you have in your cell phone - and there came a time when all of a sudden it was possible to get lots of megapixels.  Everyone knows what a megapixel is now or thinks they know what a megapixel is now but essentially in that time, it was the late 1990s, let's say it was - it was very expensive to get a few thousand pixels so a megapixel was oh my God.  So astronomers were first able build these cameras which essentially allowed them to take images of a very large part of the sky with a single, if you like, exposure.  Then we were able to do this again and again and again and build up images of a large amount of the sky with these digital cameras which are much, much more sensitive and more accurate than traditional photographic film.  This survey was a multipurpose survey which allowed us to study essentially - well a great number of properties of the sky because it was simply observing a large fraction of the sky, photographing everything that was there.  One of the things that it was able to photograph, because these gravitational lenses were so rare, it was possible to search through literally a hundred million objects to try to find these very rare instances where this is happening.  As technology marches forward, as I just said, back then a megapixel was a big deal, now it's not such a big deal and so in the future, there's a planned observatory called the Large Synoptic Survey Telescope which is going to be built in Chile, and it's like the next generation of this project that I discussed.  And it will be able to essentially photograph the entire sky every few nights.  Then we will be in the regime where literally every object, well at least every object that's visible from Chile, will be observed many, many, many times and we will really be able to drill down and try to find the rarest objects which are there in the sky.

SHANE HUNTINGTON 
I'm Shane Huntington and you're listening to Up Close.  Today we're speaking with astrophysicist Bart Pindor about Gravitational Lensing.  Bart you mentioned that this lensing effect is caused by mass.  One of the really big interest areas over the last few decades has been that of dark matter.  What do we know in terms of the distribution of dark matter from this lensing effect?  Do we see the potential for these lenses giving us an answer around where dark matter sits, how much of it there is and so forth?

BART PINDOR 
Yes I mean, definitely gravitational lensing has a lot to say about dark matter.  One thing that you could simply say is that the observation of gravitational lensing has convinced many, many people simply of the validity of General Relatively.  Because when the dark matter sort of puzzle began to rear its head and people observed for instance, the rotation curves of galaxies were not correct, people naturally thought well perhaps our understanding of gravity is simply incorrect in a fundamental way.  For instance, there was a competing theory called Modified Newtonian Dynamics which essentially said that we don't understand the way that gravity and dynamics interact, that's why we infer the wrong thing about the rotation curves of these galaxies.  But General Relativity predicts not only these rotation curves of galaxies; it also predicts this gravitational lensing effect which no other theory at the time was able to also predict.  And since you can go out now and very routinely observe the gravitational lensing effect, it's a very compelling indication that our understanding of gravity within the regime of General Relativity is correct or in any case, that whatever way you want to modify it, it must be consistent with that same observation.  So gravitational lensing is definitely a very strong indication that you can't just fiddle around with gravity however you like, you have to reproduce many things to get the picture right.  In terms of making accurate measurements of dark matter, I mean gravitation I think has already done a lot.  Certainly these things we talked about with weak lensing where we're accurately measuring the distribution of mass in the entire universe and in clusters of galaxies has shown many sort of compelling examples that there is dark matter.  It's distributed in the way that we think it would be based on it being a particle.  There's a very spectacular demonstration from gravitational lensing that occurred a few years ago, the so called Bullet Cluster.  That's a system where two massive clusters of galaxies have collided with each other - that sometimes happens, things in the universe are always falling towards each other because of gravity - and in this case what has happened is essentially, as the name implies Bullet Cluster, one of the clusters of galaxies has just essentially rammed right through the other cluster of galaxies.  If you look at it in x-rays which reveal where the gas is, it looks just like a picture of a ballistics firing range where you see a bullet flying through and you see a shockwave and so it's all sitting there, the galaxies are on this giant conflagration which has happened from this collision.  However, when you do a weak lensing measurement of the same system, what you see are these two peaks of mass which are sitting quite - to either side of this giant conflagration - and what has happened is that as these two clusters of galaxies, which comprise of gas and galaxies and dark matter, have collided.  Because the dark matter doesn't feel electromagnetic forces so it doesn't feel pressure, it doesn't participate in this giant conflagration - this bow shock, not of that.  So it just happily goes on its way and the two clouds or concentrations or haloes as we call them, of dark matter - have just flown basically right through each other and they're flying happily off in either direction.  Meanwhile there's kind of like fighting cats and dogs of the gas and the galaxies in the middle, so that picture when you look at it and you understand what it's telling us, it's very hard to separate dark matter and galaxies.  But in this case, nature has done it for us in saying there's the dark matter, there's the gas in the galaxies, they're really distinct physical things.

SHANE HUNTINGTON 
I mean it sounds like  an a priori proof there that the dark matter exists.  I mean what about a scenario where you - you mentioned the rotation curves of galaxies so these are the speed essentially at which the rotation is occurring in a galaxy, dependent on its mass - have we got any examples from gravitational lensing where we can essentially determine the mass from the gravitational lensof a galaxy, then calculate what that speed would be, and does that then match up with what we're seeing?

BART PINDOR 
I would say not exactly, simply because the gravitational lensing effect in some sense of - where the stars are rotating, that part of the galaxy is often dominated by the baryonic matter.  Baryons are just like normal matter, like atoms or gas and because they have - they like to have electromagnetic interactions they're able to call down.  That means when something cools down in space its able collapse, if something is hot then it expands, if it cools it can fall into the centre and so in galaxies, the gas falls to the centre because it's able to cool down.   The dark matter isn't able to do that, it doesn’t have these electromagnetic interactions which would allow it to radiate away some energy and so it stays in its more diffused cloud.  And so generally in the centre of the galaxies, the mass is dominated by the baryons so it's harder to disentangle the effects of dark matter.  I mean you can try to make a measurement there, but it's not convincing. However, what you can do is look at individual galaxies and then again, apply this weak lensing effect and by stacking up many, many, many individual galaxies, you can trace out what is the distribution mass in the outer parts of the So, in that sense, although it's not exactly an equivalent measurement it's showing you that yes, there is a mass, there's gas, it just keeps going further and further away where there's no more stars, when we would have thought there's no more mass.

SHANE HUNTINGTON 
Bart the last area that I wanted to quiz you on was the area around the search for planets around stars other than our own sun, so these extra solar planets that exist outside of our solar system.  Can you use gravitational lensing to find these planets and have any been found using this particular technique so far?

BART PINDOR 
Yes we can and in fact, some planets have been discovered by the gravitational lensing effect so I talked about this effect call the micro-lensing, or this regime of gravitational lensing called micro-lensing where we have again, the fortuitous alignment of basically two stars.  So one star is more close to us and it passes almost directly across the line of sight to the more distant star, and we see a sudden brightening of that distant star.  Well if the star that is doing the lensing, that's passing between us and the more distant star has a planet around it, then what we can see is a secondary little bump which is caused by the mass of the planet.  I think at the moment something like 20 planets have been discovered in this way.  The thing that's quite interesting about this method of planet discovery - so obviously it relies on this coincident alignment so it's - you can't discover as many planets as through some of the other methods because you have to get lucky.  What's interesting about it is that of course it's completely independent of observing the star itself, and so you don’t have the problem where the star is much brighter and so you can't observe the planet.  And if you can make an accurate measurement you can probably detect the planets which are much less massive than with some of the other methods which rely upon measuring some properties of this - sort of a contrast if you like, between a star and a planet - so yeah, there is definitely application of that.

SHANE HUNTINGTON 
Presumably, extending on that you could - if they were large enough - you could observe moons in the exact same way of those planets?

BART PINDOR 
You certainly could.  I would say it would be difficult; it depends on how massive the moon is.  If you observed enough stars for a long enough time and in some sense - because there's a finite amount of sky it's - in some sense it's inevitable that we will do so then yes, you could just wait long enough and there would be a coincidental alignment that would be just good enough that you would see that.  But on the other hand its - it's not a great thing to necessarily wait for all your career [laughs].

SHANE HUNTINGTON 
Bart, thank you very much for being our guest on Up Close today.

BART PINDOR 
Okay thank you Shane.

SHANE HUNTINGTON 
Dr Bart Pindor is a research scientist in the School of Physics at the University of Melbourne.  If you'd like more information or a transcript of this episode, head to the Up Close website.  Up Close is a production of the University of Melbourne, Australia.  This episode was recorded on 10 April 2014.  Producers were Kelvin Param, Eric van Bemmel and Dr Dyani Lewis.  Audio engineering by Gavin Nebauer.  Up Close is created by Eric van Bemmel and Kelvin Param.  I'm Dr Shane Huntington, until next time, goodbye.

VOICEOVER
You've been listening to Up Close.  We're also on Twitter and Facebook.  For more information visit upclose.unimelb.edu.au.  Copyright 2014 the University of Melbourne.  END OF TRANSCRIPT


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