Can Planets Bend Light?

Because large objects in space have an effect on light, can smaller objects do the same thing? Can something the size of the Earth also influence light and make it bend?
This artist's concept shows OGLE-2016-BLG-1195Lb, a planet discovered through a technique called microlensing. Image credit: 

This artist's concept shows OGLE-2016-BLG-1195Lb, a planet discovered through a technique called microlensing. Image credit: 

Originally posted on Forbes!

Every object in the universe with mass has the potential to bend light a little; the harder part is being able to measure that change.

This behavior of light, taking apparently curved paths around very massive objects, was one of the earliest tests of General Relativity. General Relativity suggested that all light should travel in locally straight lines, but if space itself was being warped due to the presence of massive objects, then the direction a beam of light might find to be “straight” might not look so straight to an external observer.

In this image the light from a distant galaxy, nearly 10 billion light-years away, has been warped into a nearly 90-degree arc of light in the galaxy cluster RCS2 032727-132623. The galaxy cluster lies 5 billion light-years away.Image credit: NASA; ESA; J. Rigby (NASA Goddard Space Flight Center); and K. Sharon (Kavli Institute for Cosmological Physics, University of Chicago)

In this image the light from a distant galaxy, nearly 10 billion light-years away, has been warped into a nearly 90-degree arc of light in the galaxy cluster RCS2 032727-132623. The galaxy cluster lies 5 billion light-years away.Image credit: NASA; ESA; J. Rigby (NASA Goddard Space Flight Center); and K. Sharon (Kavli Institute for Cosmological Physics, University of Chicago)

An extreme case of the deflection of light exists as gravitational lensing. Gravitational lensing occurs when you have two objects lined up, one behind the other, and the light from the more distant object winds up curling around the closer, massive object, like water from a tap flowing around your fist. As a result, you can get multiple images of the same object; instead of a single jet of water hitting the bottom of the sink, the water splits, and hits the sink in a few places. The way that the light flows around the heavier object tells you about the exact alignment of the two objects, but it also tells you in great detail about the concentration of mass for the object in front. You can imagine that if you ran water over your fist, it would create a different landing pattern than if you ran it over a bowl, and different again if you ran water over a wine glass.

Typically we hear about gravitational lensing from studies of distant galaxies, where we’re able to observe an extremely distant object in great detail because gravitational lensing spread the light out over a wider area of the sky, or because the gravitational lens magnified the light from the object, making it brighter and easier to spot. But these galaxies aren’t the only thing we can observe using these methods; smaller scale objects work just as well.

Still of an animation illustrating how gravitational microlensing works. On the left, a diagram of how the brightness changes as the foreground star moves from right to left across the background star. On the right is a top-down image of the light path, and in the middle, a simulated view from Earth. Image credit: NASA's Goddard Space Flight Center Conceptual Image Lab

Still of an animation illustrating how gravitational microlensing works. On the left, a diagram of how the brightness changes as the foreground star moves from right to left across the background star. On the right is a top-down image of the light path, and in the middle, a simulated view from Earth. Image credit: NASA's Goddard Space Flight Center Conceptual Image Lab

In fact, this is one of the ways that you can find exoplanets. Say you’re looking at two stars which happen to be lined up; the light from the more distant one will bend around the mass of the star in front. If you have planets orbiting that star, the mass of the planet can bend the light a little further than you might otherwise expect. If the planet is massive enough, and all is very perfectly aligned, this additional distortion can wind up producing an extra image of the background star. Because the effect is relatively small, this method works best if the two stars are very closely aligned, which also means it works best if your planet is also very close in to its parent star.

The tricky part is measuring it! All the nearby stars are measurably in motion, and so the alignments between any two stars are fleeting. On top of that, you need extremely well calibrated data to be able to catch these small changes to a star’s light, as it passes behind another star. One such survey, the Optical Gravitational Lensing Experiment (OGLE) has been running for 25 years, and while its main goal isn’t finding exoplanets, it’s found a few through gravitational lensing measurements anyways. However, an upcoming NASA mission, the Wide Field Infra-Red Survey Telescope (WFIRST) has the detection of exoplanets by microlensing events as one of its main science goals, so when it launches in the 2020s, we should expect to be inundated with new exoplanets all over again - they're expecting to find at least 2,000 new planets.

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Could we see the earth in the past?

Would it ever be possible to see the Earth in the past? For example, if one had a powerful enough telescope and could point it at a mirror hundreds of light years away could one actually see the Earth. Also, if one calculated it just right, and once again had an impossibly powerful telescope, could you in fact use something like gravitational lensing to bend light and view the Earth millions of years in the past?
A portion of the Apollo 15 lunar laser ranging retroreflector array, as placed on the Moon and photographed by D. Scott.  Credit: NASA/D. Scott

A portion of the Apollo 15 lunar laser ranging retroreflector array, as placed on the Moon and photographed by D. Scott. Credit: NASA/D. Scott

Originally posted at Forbes!

From a purely geometric standpoint of the motion of the light involved, this could work. Reflected sunlight from our Earth could go out into space, be reflected off of a gigantic mirror, and back towards the Earth.

We actually do something like this currently (at a much smaller distance scale), because a few of the Apollo missions put mirror-like reflectors on the surface of the moon. They’re not specifically mirrors, but a special kind called a retroreflector, which directs light back the way it came from, which normal mirrors don’t do. By firing high powered lasers towards the surface of the Moon where the mirrors are situated, we can count the time delay for the round trip of the laser beam. This is primarily used to measure the distance between the Earth and the Moon to incredibly high precision, which is one of the reasons we know that the moon is receding from us by a little more than an inch every year.

For a laser, this round trip from the Earth to the Moon takes a relatively quick ~2.5 seconds to go the ~478,000 miles there and back. Even though we’re not dealing with large distances (on an astronomical scale) yet, we start to run into a problem already. We start to run out of photons.

Even for a laser beam, which starts out with all of its light focused into a very small beam, will spread out at larger and larger distances. Red lasers are more prone to doing this than green lasers, simply because the wavelength of light is shorter for green lasers, and this spreading is partially a function of wavelength. (Purple lasers would be even less prone to spreading out.) However, over several hundred thousand miles, even the highest wavelength lasers we can manufacture at the moment are going to spread out, and by the time this light gets to the Moon the laser is only able to faintly illuminate the surface of the moon, and only a tiny fraction of that light is going to be bounced off the reflector and back to Earth. The further away you put your mirror, the worse this problem gets, because light gets more and more spread out the further it’s traveled before it arrives at your mirror.

The Earth does reflect sunlight out into space, so we’re not in trouble there. However, this light begins to spread just as the laser light does, but it’s coming from a larger area to start with, so it’s never as compact as the laser light. By the time that we get to any distance away from the Earth, this reflected earthshine is very dim indeed. We can see the earthshine at night- the reason the dark part of the moon isn’t 100% black is that it’s getting some reflected light from the Earth. By the time this light has traveled several hundred light years, one can imagine that this light has become very, very diffuse. And then, of course, it would have to travel several hundred light years back, becoming even more diffuse on the return journey. (We’d also have to conveniently find a mirror out in space which has a clear light path between us and it – if we just put it out there now, we’d have to wait a few hundred years for anything to come back our way.)

But let’s say we managed to get a few photons back from our several hundred light year’s distant reflector, which we can arbitrarily make sufficiently enormous that this would happen – would we be able to identify them? Part of the reason we like using lasers for our moon experiment is because they’re all of a very particular color of light, so we can count up the returning photons at that color, relative to photons of any other color, which we know to be noise. The Earth is not a single color, and the atmosphere is incredibly complicated, so the set of photons that we would reflect would be a much more complicated set than the laser beam we’re firing at the Moon.

On top of this, when the Earth is showing the most reflected light, it’s because the angle between us and the sun is the smallest. So when the Earthshine is the brightest, we’re also most likely to be blinding our reflector with light from the Sun. The Sun, as you will recall, is really, really bright. Stars in general tend to be a big problem for taking direct pictures of planets around other stars, because they’re so bright that they swamp out any of the reflected light from a planet, and we have to get really clever with how we block out the light from the star without blocking out anything else.

Can you match each galaxy in the top row with its warped counterpart in the bottom row? Such galaxy warping occurs naturally in nature in a phenomenon called strong gravitational lensing. The gravity of matter in front of a more distant galaxy, either dark or normal matter, bends and twists the galaxy's light, resulting in wacky shapes and sometimes multiple versions of the same galaxy.  Image credit: NASA/JPL-Caltech/UCL

Can you match each galaxy in the top row with its warped counterpart in the bottom row? Such galaxy warping occurs naturally in nature in a phenomenon called strong gravitational lensing. The gravity of matter in front of a more distant galaxy, either dark or normal matter, bends and twists the galaxy's light, resulting in wacky shapes and sometimes multiple versions of the same galaxy. Image credit: NASA/JPL-Caltech/UCL

That’s just the mirror method. If you want to throw gravitational lensing in here, it gets really complicated really fast. Most forms of gravitational lensing work more or less along a straight line, so if light from the Earth were gravitationally lensed, that wouldn’t help us see it, since the light would still be heading out away from us. There are objects which cause more extreme gravitational distortions, where light can do a U-turn before leaving; these are either going to be black holes or neutron stars. However, if the light is being distorted that much, so is all the other light coming from every other source in the universe. Gravitational lensing is difficult to untangle even in the mostly straight line case; trying to pick apart which photons have come from what source around a black hole or neutron star is difficult to impossible, unless it’s very isolated from other objects. Many neutron stars are in pretty complex areas, since they can be surrounded by the remains of the supernova that produced them. Trying to send some earthshine that way, and hoping that the photons do a U-turn and come back in the same direction is an even harder aiming job than getting light to a reflector and back, since there’s only a narrow range of angles that will allow the photon to do that. On top of this, we’d still have the same problems with the Earth’s light getting overwhelmed by the light from the sun, making it extra hard to untangle the Earth’s light from that of our star.

So in the end, while this is geometrically possible, given a pre-existing reflector, and a hypothetically massive telescope, from a practical standpoint we’re unlikely to observe ourselves this way.



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How do you get Einstein Rings?

Can you answer why even a small distortion of the massive lensing object in gravitational lensing techniques can result in Einstein rings around the forefront object? Thanks :D

Gravitational lensing was something predicted by Einstein’s theory of relativity long before it was actually observed, and the theory behind it goes something like this.

Light always travels in a straight line. A laser pointer always travels from the exit of the pointer straight to whatever you’re pointing it at, which is quite handy for things like presentations or laser assisted guides, if you’re trying to draw or cut straight lines. However, if we know anything about general relativity, it’s that the space light must travel through is not totally flat. Space is curved and warped by the presence of massive objects, which means that we have to amend our rule of how light travels to “light always travels in a locally straight line”. Whatever direction is “straight forward” at a given point in space is the direction that light will go. If you’re an external observer watching a beam of light, the path the light takes may appear to bend around a massive object as it encounters the curve in space caused by gravity. But if you could ask the light particle which direction it was going, it would always say “straight forward,” no matter how curvy its path had seemed to you.

Similar to the way that a magnifying lens changes the path of light and magnifies it, the results of the gravitational ‘lens’, as it is technically called, can distort the image of the background object and magnify it. If you have a look at the objects along the edge of a photo taken with a fisheye lens, you’ll notice that things that would be straight lines (like walls or trees) are bent into a curve - the light travelling through the lens was distorted as it passed through. The fun thing about our gravitational lens is that it means that we can spot objects that would have been too distant and faint to see without the magnification that the lens was able to provide.

Usually, the gravitational heavyweight that is bending the light from the object behind it is a cluster of galaxies, or something similarly hefty, like a black hole. Occasionally, a single massive galaxy can serve as a heavyweight. In order for the gravitational distortion caused by the heavy object to be observed to be functioning as a lens, it has to have another object pretty close to directly behind it, from our line of sight - you need light from the background object to pass very near to the distortion in space in order to have a noticeable bend to the path of the light.

If everything is very exactly lined up, then the light from the background object will pass around the heavy object in front of it in an even way, like water flowing over a sphere. So, as we see it, there would be a perfect ring of light from the background object (generally a galaxy) around the heavy front object. This is what’s called an Einstein ring, because it’s the ideal case of gravitational lensing, as predicted from Einstein’s theory.

Most of the time, the background object, the heavy lensing object, and we as observers are very slightly out of line. This means that light doesn’t pass around the heavy lensing object in an even way. More light will bend around one side, leaving an empty space on the other side. If the background object offset enough, you just wind up with multiple distorted images from the background object; they’ll just look a bit stretched, like the fisheye lens effect. This is what was behind the Space Invader galaxy that popped up a little while ago.

Usually, the background object behind the heavyweight is another older, more distant galaxy, and it’s the light from this galaxy that is sheared out and stretched like silly putty. The foreground gravitational weight really just has to stretch space for this to work; but if you want a perfect Einstein ring, you just have to cross your fingers and hope for a cosmic alignment.

Something here unclear, or make you curious? Have your own questions? Feel free to ask!