Things that get me excited #2: LIGO

LIGO stands for Laser Interferometer Gravitational-Wave Observatory. Now, if you’ve never heard of LIGO, or are saying to yourself “I will never know what any of this means”, then this article is for you.

However, before we get to the beast that is LIGO itself, it is important to have a bit of more-than-general knowledge about a few other things. Let us start with that first.

Gravity.

When you hear the word ‘gravity’ what probably comes to your mind is an apple falling on the head of one Issac Newton. Well, that or the John Mayer song. Anyway, even before Newton, we had some idea of what gravity was, thus negating the belief of my 5-year-old self that shit moved upwards until Newton came along. No, that wasn’t the case. You can read more about it here. But it wasn’t until Newton hypothesized the inverse-square law and related it to the orbits of celestial objects that we got a much much clearer picture. Newtonian mechanics(or classical mechanics) helped predict the existence of Neptune based on the motions of Uranus that couldn’t be explained by the other planets. Things were good and Newtonian mechanics held supreme. Until the end of the 19th century.

Precession

If you have heard of Kepler’s law of orbits, you know that planets don’t have a circular orbit around the sun. Instead, they have an elliptical orbit.

However, this isn’t a perfect representation either. As it turns out, the planets around the sun go through something called perihelion precession, due to the gravitational forces exerted by other planetary bodies, which causes them to trace out a ‘flower-petal’ like shape like this:

By WillowW – Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=3416065

Big words, but easy to understand. This is predicted by Newtonian mechanics as well. But at the end of the 19th century, we detected a small, how shall I say it, booboo in the data.

French astronomer and mathematician called Urbain Le Verrier (Same guy who’s calculations led to the discovery of Neptune by Johann Gottfried Galle) discovered that Mercury’s precession was slower than expected, and it couldn’t be explained by Newtonian mechanics unless there was another planet(or dust) that’s even closer to the sun than Mercury that we hadn’t detected yet.

However, nothing of such sort was observed and it couldn’t consistently fill the data.

Enter: Einstein

This is where things get really interesting. In 1915, Albert Einstein put forth the famous Theory of General Relativity. Now, this theory deserves an article of its own(probably multiple), and if you’re interested I will write one in the near future, but for now, let’s keep it relatively simple.

One of the tests of this theory was that it could correctly explain the slow precession of Mercury by adding a term of A/r^4. Other tests included bending of light in gravitational fields, and Gravitational Redshift.

Yet another thing the theory of relativity predicts is the existence of gravitational waves.

 

I would like you to pause here for a second and take everything in. Take a breather for a few seconds and recall everything you just learned.

Done? Good.

Now watch this video.

This should give you a workable intuition about gravity and space-time

(However, while this is a good analogy, I kinda dislike the fact that it uses gravity to explain gravity, but I’ll let that go)

Anyway here are some simple things you should know to continue further:

  1. Mass tells space-time how to curve
  2. Curved space-time tells mass how to move.
  3. Gravity travels at the speed of light

The first two things you probably understood from the video above, but the third thing is a curious one.

Consider this. You already know that the light from the sun takes a little over 8 minutes to reach the earth on average, right? That means that if the sun were to disappear right now, for 8 minutes the sky would still be bright where it’s day-time. After that time it’d be blackness and it’d get cold really quick and we would just sorta die.

The same would be true for the sun’s gravitational effects. We would continue to be in orbit for a full 8 minutes until the earth realizes that there is no sun. After which it’ll likely go off on a tangent to the orbit hurling through space.

Apocalypse. Fun.

Anyway, let’s get back to Einstein and the theory of relativity. As I mentioned previously, the theory of relativity predicts the existence of gravitational waves. What gravitational waves do is that they stretch and squeeze space-time. Kind of like a wave in a spring. Since every theory needs to be thoroughly tested it would only make sense to build a gravitational wave detector to see if there are indeed gravitational waves, right? Okay. Let’s think about how you’re gonna do this.

Can you take a ruler and measure a rod and see if it stretches and expands and contracts? No. Because while the rod would stretch and expand, so would your ruler, and so would you.

We’re gonna need Lasers for this one.

LIGO -Laser Interferometer Gravitational-Wave Observatory

1.3 BILLION years ago, two black holes, each weighing more than 30 times the mass of the sun locked into a spiral. They spun around each other over 100 times per second before merging into one. The energy released in this event was more than the energy released everywhere else in the universe TIMES 50.

Most of this energy was released in gravitational waves, which caused everything in its path to stretch and squeeze. 1.3 BILLION years later, on September 14th of 2015, these waves hit the earth and stretched and squeezed everything by…wait is this right? 10^-21 meters? That’s a million times smaller than the width of a proton. How in the world do we measure this?

Well, for one thing, the arms LIGO are 4kms long to bring the sensitivity down to 10^-18m. That’s still a thousand times smaller than the width of a proton.

Here’s a diagram of the LIGO apparatus

 

How it works:

A laser beam is split into 2 parts which travel down 2 separate interferometer arms, hits the mirrors and are reflected back to combine in such a way that when merged, they cancel each other out (Crest meets a trough, trough meets a crest) and no light is detected by the photodetector. When a gravitational wave passes by, the light beams are slightly out of the perfect sync they were designed to be in, since the interferometer arms stretch and squeeze, and a little bit of light is detected by the photodetector.

But here’s the thing.

  1. The laser needs to be incredibly precise. If it keeps changing the wavelength then the results wouldn’t make sense.
  2. The stretching and squeezing we are measuring is a trillion times smaller than the wavelength of the light we use in the laser. So what we are actually looking at is- dark to infinitesimally-bright.
  3. The light can be disturbed by air and particles in the air, all the air needs to be pumped out of both the 4km long arms. This took 40 days to accomplish. After which the arms were heated to expel any excess air that might still be in the arms. This made it the 2nd most precise vacuum ever created after the Large Hadron Collider.
  4. The mirrors LIGO uses are the smoothest mirrors ever created. They absorb only one out of 3.3 million photons, weight 40 kilograms and are suspended by silica threads not much thicker than a human hair.
  5. The LIGO laser has a megawatt of power. A MEGAWATT. That’s enough to power a thousand homes. In a tiny beam of light. Yeah. This is to reduce the uncertainty in the measurements.
  6. Since the detector is extremely sensitive, it can be affected by seismic activity, traffic, or a wide range of other things. SO THEY BUILT TWO OF THEM. There are 2 LIGOs in the US 5044 miles or 8117km away from each other. This is done so that they can filter out the results from the noise.
  7. The laser also gets stretched. Fortunately, the stretching and squeezing caused by the gravitational wave is slow when compared to the speed of light, and the insane one-megawatt laser is fired continuously.

 

Other cool things:

Physicists often say that detecting gravitational waves is like hearing the universe. This is because when converted to sound, it sounds like a “WHOOP”

That WHOOP sound actually tells us a lot. It tells us that the black holes weren’t spinning around themselves, just around each other. If the black holes were spinning around themselves it would’ve sounded like a vibrato WHOOooOOooP.

Isn’t that something.

If you want to know more about LIGO, I suggest heading on to https://www.ligo.caltech.edu/

You might also like to watch this TED talk by MIT’s Allan Adams: https://www.youtube.com/watch?v=jMVAgCPYYHY

 

If you have any questions leave them down in the comments below and I will try my best to answer them.

 

 

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