The Journey of a Gravitational Wave II: GWs Get Bent

What happens to a gravitational wave between when it is produced and when LIGO can detect it?  It turns out not much, which makes it a key new medium in which to observe the Universe!

Last week, I began discussing what happens to a gravitational wave as it makes its way from its source to Earth; specifically that gravitational wave can travel through matter and come out the other side unchanged!  Today's post talks about how the gravitational effects of other masses in the Universe can deflect the gravitational wave from its otherwise straight path.


GRAVITATIONAL LENSING

Let's think again about what happens to light on its way to Earth.  We know from the last post that any matter light comes into contact with will reflect or absorb at least part of the light.  There is also another effect called gravitational lensing that causes light to bend around massive objects due to the massive object's gravitational influence.  This is caused by light following its natural path on curved spacetime (or light being bent by a gravitational field since we've previously established that the curvature of spacetime is a representation of the strength of the gravitational field there).  The first thing that pops into my mind that illustrates something following its natural path on a curved surface is miniature golf:

This is hole 13 at Safari Mini Golf in Vero Beach, FL.  This image is taken from a review of this course and can be read here.

Consider the example hole in the above image.  After you get your ball past the three bumps, there is a wonderful bowl-like curved portion behind the target hole (if you look very closely, you can see the hole directly after the last bump and in the center).  If you hit your ball into this area, the ball's trajectory will change from a straight path to a curved one.  If your ball begins its path on the left side of the bowl, it will curve right; if your ball enters the bowl from the right, it will curve left.  The same thing happens for light and gravitational waves that pass by a massive enough object to cause a significant depression in spacetime (i.e. a strong gravitational field):

The bending of light from a star that is really behind the Sun but appears to be to the side of the Sun.
[Credit: Ethan Siegel of Lewis & Clark College, OR]

The the light from a star, galaxy, or other source travels through the depression in spacetime made by a nearby massive object (in the image above it is the Sun).  The path that light takes is curved just like the golf ball in the miniature golf example above.  But our brains are wired to assume that any light that enters our eyes has come to us in a straight line (which is how light usually travels) so we perceive the location of the source to be directly behind where it appears to be.  Therefore, while the star is really behind the Sun (at point A in the image above), it appears to us to be to the side of the Sun (at point B).  This bending effect is called gravitational lensing and it applies to gravitational waves just like it does to light.


KINDS OF GRAVITATIONAL LENSING

The example of gravitational lensing given above was one of the first observational proofs that Einstein's general relativity was correct.  Before relativity, there was already a prediction of the bending of light due to Newtonian gravity (what we use in our everyday life) but Einstein predicted the bending effect should be twice that predicted without relativity.  In 1919, there was a total eclipse of the Sun which would allow those stars that are near the Sun to become visible.  Images of the eclipse were taken and it was seen that the shift in the position of stars near the Sun was indeed twice that of the shift predicted by Newtonian gravity.

There are also kinds of lensing that produce much more dramatic effects than shifting the position of stars!  Things like large galaxies and clusters of galaxies can cause the light from objects behind them to be split up to form multiple, separate, and complete images. 

[Image from NASA]

The image above shows gravitational lensing of a quasar and a galaxy by a distant galaxy cluster SDSS J1004+4112 (SDSS indicates that it was discovered by the Sloan Digital Sky Survey).  Each image of the quasar is of the same single object; the same is true of the galaxy! 

You may notice that each of the images are a little different from each other.  This is due to the distortion that a gravitational lens can cause.  This effect is illustrated well in the simulation below of a black hole creating a gravitational lens as it passes in front of a galaxy:

[Image from Wikipedia]

Note the circular distortion of the light from the galaxy as the black hole passes by.  When the black hole is directly in front of the galaxy, there is a circular halo of lensed light around it.  This halo called an Einstein Ring can can be caused by any extremely massive object (black hole, galaxy, galaxy cluster, etc.).

[Image credited within the image and retrieved from Wikipedia.]

HOW LENSING AFFECTS THE SEARCH FOR GRAVITATIONAL WAVES

Gravitational lensing affects both light and gravitational waves.  This produces spectacular images of objects using light, but LIGO will not produce images and the sources that produce gravitational waves are more point-like (a black hole, a star exploding, etc.) than large scale objects (like galaxies which are thousands of light-years across).  The effect that will most likely be seen in gravitational waves is their focusing; the bending of gravitational waves can produce more intense gravitational waves from the lensed source (similar to a magnifying glass focusing light to a smaller point).  This paper suggests that the galactic center of the Milky Way could increase the intensity of a source in our galaxy (bur behind the galactic center) up to 4000x.  Also, if a gravitational wave is emitted from a galaxy that has multiple images from lensing, then that gravitational wave will come from each image!

However, most sources will not have appreciable lensing.  While this is something we will always need to consider while conducting gravitational-wave astronomy, it isn't something that is likely to change the information contained on the gravitational wave noticeably (and that's a good thing)!

The Journey of a Gravitational Wave I: GWs Cast No Shadows!

What happens to a gravitational wave between when it is produced and when LIGO can detect it?  It turns out not much, which makes it a key new medium in which to observe the Universe!

In order to make this information more digestible, I will address one aspect of a gravitational wave's journey through space.  Today's topic discusses how the Universe is essentially transparent to a gravitational wave.  Future editions will discuss how matter can bend gravitational waves (gravitational lensing) and how the expanding Universe can stretch out (redshift) gravitational waves.


GRAVITATIONAL WAVES CAST NO SHADOWS:

First, let's think about what happens to light.  As light travels through the Universe, any time that it encounters other matter, some of the light is absorbed by the matter or reflected away from its original path.  The opposite happens for a gravitational wave; it can pass through matter and come out the other side unchanged (although there are some negligible effects)!  That means that there is no such thing as a gravitational-wave shadow and nothing can obscure our detection of a gravitational wave!


The Spacetime Explanation:

But why is this?  In a previous post, I described a gravitational wave as a change in the gravitational field moving out into the Universe.  This change in gravitational field is often illustrated as a ripple, or wave, on spacetime (where the steepness of the curvature of spacetime represents the strength of the gravitational field, or the gravitational force a mass would feel, there).  Let's look at what the Earth sitting on space-time looks like:

This picture isn't a perfect representation since the size of the Earth will affect the shape of the depression and this has no effect on real spacetime.  Also, this is a simplified 2-dimenional representation of 3-dimensional (or a snapshot of the 4-dimensional spacetime) space.  But if you were to imagine giving a corner of this flexible grid (spacetime) a swift shake, the Earth in the middle would be affected by it but it would not impede the wave.  So, this is an example of how matter doesn't interact with gravitational waves, but I am still somewhat unsatisfied with this since you may think that the Earth will bounce after a wave passes creating more waves of its own (here is another aspect where this representation of spacetime is not perfect).

FYI: A better 3-dimensional representation of spacetime is shown in this clip from the American Museum of Natural History's short documentary called Gravity: Making Waves (which can be seen in its entirety on my Viewing Fun! page).  This animation shows a grid-like scaffolding filling space in which there is a depression caused by mass.  While it still isn't a perfect representation of spacetime, it is much better than the trampoline approximation above.

The Lunar Eclipse Explanation:

Recently, I thought of another more intimate example that most of us can identify with: a total lunar eclipse (which I have also blogged about).  This is a situation where the Sun, Earth, and Moon line up in that order so that the Moon is completely in the Earth's shadow:

Image from Wikipedia

When the Moon is completely in the Earth's shadow (or the umbra in the diagram above), a viewer on the Moon would not be able to see any part of the Sun.  If the gravitational field from the Sun were blocked by the Earth, then the Moon's orbit would appear to change.  Since there is no change in the Moon's orbit during a total lunar eclipse, then the Earth does not block the Sun's gravitational field.  By extension, the Earth also would not be able to block any changes in the gravitational field (which are gravitational waves).  If you are familiar with physics, this is an application of the principle of superposition.


Great!  But Why do We care?

Since mass does not absorb or reflect gravitational fields, the Universe is transparent to a gravitational wave.  This is a huge advantage when using gravitational waves to make astronomical observations since nothing can block our view of a gravitational wave!  If you have ever seen the our own Milky Way galaxy in the sky on a clear, dark night, you've seen the billions and billions of stars that live in our "backyard": 

Fish-eye mosaic of the Milky Way galaxy as seen from Chile.  [Image from Wikipedia]

While this is beautiful, it also it almost impossible to see past all the stars and dust there to observe what is behind our "backyard".  We will be able to see right through the Milky Way with gravitational waves! 

P.S.  We can also detect gravitational waves from the other side of the Earth with LIGO since gravitational waves can travel through matter.  Read more about this in this previous post (under the subheading of "Why are there 2 LIGOs?")!

Total Lunar Eclipse

Tonight (really, very early tomorrow morning), there will be a total lunar eclipse viewable in the United States, Mexico and Canada.  Lunar eclipses are caused by the Moon passing through the Earth's shadow.  Now, if you sit down and think about it, you might ask why we don't have a lunar eclipse every month during the full Moon?  The reason is that the Moon doesn't orbit the Earth exactly on the plane (flat 'imaginary' surface) that the Earth orbits the Sun.  Instead, the plane on which the Moon orbits the Earth is very slightly tilted (about 5°).  These two planes intersect on two points of the Moon's orbit.  When the Moon is at (or very near) one of these points during a full Moon, a lunar eclipse results.

Below is a very nice animation from Wikipedia on what tonight's eclipse will look like (UT is Universal Time - subtract 5 hours for Eastern time, 6 for Central, 7 for Mountain and 8 for Pacific):

The two circles that you see show the two parts of the Earth's shadow.  The outside circle (black) is called the penumbra and the inside circle (gray) is called the umbra.  The penumbra is a region where only part of the Sun's light is blocked from the Sun and the Moon will have only subtle darkening.  The umbral region is where all of the direct light from the Sun is blocked, but the Moon has a red/orange shine to it.  This red illumination is caused by the Earth's atmosphere acting like a prism and separating the colors of light and bending the longer wavelengths (like red) toward the center of the shadow.  It is this bent light that causes the Moon appear red.  The illustration below shows how the Earth acts like a prism:

The top half of the image is looking down on both the Earth and the Moon and shows how the Earth's atmosphere separates the colors of light.  The bottom half of the image shows what the Earth may look like from the Moon during an eclipse.  (This beautiful image was also taken from Wikipedia.)

If you would like to observe the eclipse but it is too cold outside or there is cloud cover, you can watch it live on NASA streaming video here:

http://www.nasa.gov/connect/chat/lunar_eclipse.html

For those of us in the Central time zone (that would be me), the eclipse starts 11:27 pm tonight, totality begins at 1:40 am, the greatest eclipse (peak of totality) is at 2:17 am.  If you get up very early in the morning, you can see the last bit of the eclipse before 5:06 am.

Clear skies!