Hello,
It’s time for yet another dose of science knowledge, straight from my keyboard, to your optic nerve. This month I will be discussing the gravitational wave discovery that was announced on February 11, 2016 and why they are so important. I will also be discussing how they were detected and verified beyond any reasonable doubt and the techniques they used to do so.
First off, I think it would be useful to explain what a gravity wave is all about. I have explained a bit about what gravity itself is, by explaining warping of Space-Time by massive objects, like the sun and our planets and how it affects the orbit of massive objects and can even warp visible light and radio signals by observing the difference in time for a radio signal from a distant space probe to reach Earth while passing by a massive planetary object as opposed to it following a straight path. But gravitational waves take this to the extreme by the events and objects that create them to such a huge disturbance that we can detect them with ground based observatories.
Even though these are large scale events that produce these waves, the percentage of the original detectable signal that reaches the Earth based observatories is tiny. Think about it this way….you are standing on the shore of a small pond and you throw a rock into the middle of this pond and in doing so create waves from the point that it enters/disturbs the water. In this example, the water can be analogous of space-time fabric, the rock is the source of the gravitational wave event, and the waves are the actual gravitational waves. The waves gradually become weaker and weaker as they get further and further from the source of the event, to a point they are almost unnoticeable compared to the other background noise (i.e. wind disturbances, fish, nearby swimming humans, etc) by the time it reaches the shore of the pond. Now supersize this example to the scale of the universe, now you can see the challenge that is presented in detecting these events in other galaxies, even in relatively closer galaxies, by the time it reaches us, the signal is super weak.
vs 
vs 
(In order, 1. Pond
creating waves with rock thrown in the middle, 2. Model of a
gravitational wave producing event of the merging of two black holes and 3.
The vast distance these waves must travel to reach observatories on Earth.)
What creates these gravitational waves in the first place? We’ll simply put, any large (cosmically) scale gravitational disturbances of space-time can create gravitational waves. The challenge is that said event has to be large enough to create waves that their signal are strong enough to be detected on Earth to be noticed in the first place. So there are lots of possible sources, but they are beyond detectable range. The best chance we have at detecting gravitational waves with current detector technology that was used to detect the gravitational wave that I will explain below was from the merging of two black holes orbiting each other, also called a “binary black hole”.
You can even see the gravitational lensing of light around this example binary black hole as light is bent as it passes around the black hole….well at least the amount of light that doesn’t fall inside the event horizon that is as we learned from my last email. This is an actual telescope image still, which is just amazing to see. The gravitational waves the black holes give off orbiting each other’s massive gravity wells follows a pattern, so it does not disturb the space-time fabric enough to be detected. But when the orbit decays enough and the black holes are forced to merge into a single black hole, this event is what creates the massive gravity waves that can be detected by Earth based observatories.
The first ever detected gravitational wave was exactly this type of event, the merging of two massive black holes into one. Keep in mind this is not the only type of gravitational wave producing event, but is the most common and most feasible at being detected.
(Ligo Obervatory near Richland, Washington and the Observatory in
Livingston, Louisiana and the distance between them (note that each arm
of the observatories are 4km long each))
Now the next question that needs to be answered, is how do we detect these events in the first place? With LASERS! Yes, since gravitational waves warp space-time, lasers (which are photonic light) are warped by this disturbance. By using lasers, we can build a very precise and relatively easy way to detect gravitational signals. These instruments are so sensitive they can detect the change in a laser’s signal down to ten thousandth of the width of a proton, proportionally equivalent to changing the distance to the nearest star outside the Solar System by one hair's width. This project is called “Laser Interferometer Gravitational-Wave Observatory” (LIGO), and has two operating observatories. One in Livingston, Louisiana and one near Richland, Washington. There is a very good technical reason for two different observatories separated by over 3,002 of distance between them. This allows local disturbances like audible noise, surface noises (cars driving nearby, loud airplane engines, trains) and even subterranean noises like minor tectonic plates movements which are detected by these instruments to be ruled out by comparing these signals to the partner observatory. If they are not observed, the signal can be omitted for more detailed analysis and assumed to be local noise. Also, when a plausible valid signal is confirmed, by using two observatories with that large distance between them, there is a 10 millisecond light speed delay between the two observation sites, which allows for triangulation of the source event.
LIGO uses a pair of lasers two cancel each other out. When there is a gravitational disturbance or vibration noise nearby, the lasers move ever so slightly out of sync with each other and the difference of the out of sync signal can be sensed and plotted as shown in the diagram below:
When the two LIGO’s sites detected the Gravitational Wave event on 09/14/15, the signal looked like this:
The two graphs show without a doubt that the same signal was received separated from noise by over 3,002 km’s of distance and arrived 7ms after one another at each site given the angle of the two site and the angle of the incoming signal at time of detection. In mathematical terms, the signal was confirmed with a significance of over 5.1 sigma or a confidence level of 99.99994%. The calculated black hole masses using redshift data and the above signals together that created this event was estimated to be about 36 and 29 times respectively each the mass of our own Sun, resulting in a final merger mass of about 62 solar masses (+/- 4) at a distance of over 1.3±0.6 billion light years away. Let’s put that into perspective for just a second by comparing the size of Earth to our own Sun below:
Now take this and scale it up to 62 times the size of our own Sun and then compare it to Earth again. We wouldn’t even amount to one Pixel at that scale. That is one truly massive black hole merger that created this event.
But, you may be thinking the whole time reading this, why is this so important and why have scientist invested so much time, money and effort into this project? Firstly, that’s a difficult question to answer, and it’s not as simple as just saying, “Because we can”. Detecting these gravitational waves allows a whole new field of astronomy to emerge and for us to gain a deeper insight into the current and early working of our Universe. The upside of gravitational wave observation is that it doesn’t rely on any charged mass objects (infrared/visible light astronomy) and it passes straight through any matter, so it doesn’t have to contend with interstellar dust which would hinder observations of other cosmic detectors. Further observations of future gravitational wave events will also help to create more precise models of the history of the expansion of the universe and the nature of the dark energy that influences it.
Furthermore, detection of the gravitational waves was a further agreement of the theory of general relativity and further validates the predictions set forth by Einstein’s predictions. Though this signal was limited in its scope to provide more complex interactions to further this study into general relativity of gravitational waves, further detections of these signals should present more opportunities to study these more complex interactions and delve deeper into this theory. Data from these gravitational events will also allow us to determine if the graviton particle (the particle which imparts the force of gravity) carries any mass. This will be important for other studies as this will need to be confirmed to allow more accurate equations in other experiments where precise control of constants including gravity must be determined.
I hope you enjoyed this and other science emails that I have written. As always, if you have any questions or comments, please let me know, I enjoy a good discussion. I’m looking for suggestions on my next science topic, so if you got a topic you want to learn more about pass me some ideas and I’ll see about making it happen.
Thanks again for letting me impart some science knowledge
on you and forming new neural connections mappings,
-Tyler W.