Thursday, July 12, 2012

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.


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?")!