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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If light can't escape a black hole, how does a graviton escape?

If the intense gravity field of a black hole establishes an escape velocity at or above the speed of light, and if gravitons exist, how do they escape a black hole to establish the gravity field?

One of the most important things to keep in mind when thinking about the next Big Theory that people are developing is that physical theories are like nesting dolls. As we progress, we go to a bigger and bigger nesting doll, which is able to explain more and more things, but at its core, each theory has to explain the same set of phenomena.

So let’s take gravity - our first theory of gravity came from Newton. Newton’s laws are very basic, but they did an excellent job of explaining the way that things fall to the ground, and the way that the planets orbit the sun. These laws pretty much explain most everything we encounter in our every day lives.

However, Einstein’s theory of general relativity went a bit further, and explained that Newton’s equations could be understood as a simple way of looking at a much more complex system. Einstein’s new equations could be simplified to obtain Newton’s laws, in the small size, not very massive limit of General Relativity, but General relativity’s way of thinking about the universe provided explanations for a much wider range of objects. Many of these objects had not yet been observationally discovered, but the math Einstein put forward allows for their existence. Black holes are one of these objects. General relativity permitted objects to exist which are so dense that even light cannot escape, and many years later we found quite a bit of evidence showing that they’re real.

Gravitons come into this equation as an attempt to understand how gravity works. Gravity is kind of a mess, theoretically speaking- it just doesn’t play well with the other fundamental forces - it’s far too weak relative to the rest. We know that gravity can be explained as a curvature of space as a result of the presence of mass, but that doesn’t explain how gravity propagates. Most other forces of nature have what are called “force carriers” - little particles (or packets of energy) that obey quantum rules (in that they can pop into and out of existence) and go between two different particles as the particles interact. The force carrier particles are also called ‘virtual particles’ and, because they obey quantum rules instead of classical ones, can serve as a handy “get out of jail free” card when the classical rules seem a little restrictive. But these force carriers are predicted as part of what’s called the Standard Model.

Gravity is not part of the Standard Model. We can’t figure out how to get it in there. (We’re trying - this is how we wound up with string theory.) The quantum world and the general relativity world seem to be fundamentally incompatible. But if we’re trying to come up with a quantum-world version of gravity, then the assumption becomes that since every other force has a carrier quantum particle, gravity should have one too. And so we’ve called it the graviton, even though there’s no evidence so far that it exists, and very little information on how we think it ought to behave. If it does exist, it should at least play a role in how gravitational waves propagate.

Two more things before we can answer this question: force carriers are active at a quantum scale. This means that they will exist for extremely short periods of time, and the uncertainty principles will play a role in their behavior. Second: force carriers are only needed when there is a transfer of energy. So if a black hole were losing energy somehow (perhaps by colliding with another black hole), then the gravity waves that would be given off would involve the graviton.

But if you just had a black hole that was not losing energy, then the black hole would not be producing gravitational waves. It doesn’t change the fact that the mass of the black hole is enormous. At distances large enough to be outside of the event horizon of the black hole and scales above the quantum level, any graviton-based theory of gravity should simplify to the general relativity situation, where we can understand the mass of the black hole by the curvature of space. This curvature appears like a force because external particles respond to the local geometry of the universe. As far as gravitons go, it’s hard to understand precisely how a theoretical, highly chaotic quantum particle would behave in a region of space that has always been understood in the framework of general relativity, but however it behaves, it must result in the same solutions as general relativity.

Something here unclear, or have your own question? Feel free to ask!