What Are Gravitational Waves?

About a month ago, two specialized observatories called LIGO announced the first direct detection of what are called gravitational waves. This discovery represents another step in knowing that science is on the right track with this whole "relativity" thing Einstein figured out a century ago. I'll answer the two most common questions I've gotten from my friends when they've been kind enough to let me talk about gravitational waves for entirely too long.

"What are gravitational waves?"

Here is a great primer from Brian Greene:

Gravitational waves can be thought of like waves in the surface of a lake. When there is some disturbance in the lake, say a poodle jumps into the water, waves propagate across the surface. The same goes for gravitational waves, but rather than propagating through water, they propagate through space and time.

A bug on the surface of the water a little ways away from the poodle will travel in little circles, moving up and down in the wave (below [top]), while an object "caught up" in a gravitational wave actually gets stretched and compressed while staying still as space and time are altered around it. In the case of this circle (below [bottom]), you can see how it gets distorted and changes shape.

Note that the points in the circle get closer and further away as the circle flexes, this property is what we exploit in order to detect gravitational waves, but more on that a little later.

Going back to our poodle, the disturbance that a poodle jumping into a lake makes is pretty large relative to the water molecules. The opposite is true for gravitational waves, they are extremely small, though everything creates gravitational waves. Even by typing this sentence, my fingers are creating small gravitational waves, though they are immeasurably insignificant.

Gravitational waves produced by large, energetic events are they only types of waves we have a hope of detecting, because they will warp space enough for us to notice. Some common sources of large gravitational waves are supernovas, neutron stars, or black holes rotating around each other (above) and merging.

Let's move on to the specific gravitational wave the LIGO team detected in late 2015. A little before 3pm mountain time on September 14th of 2015, a gravitational wave swept over the earth, altogether lasting about a tenth of a second. The event that created this wave occurred about 1.3 billion years ago, when two black holes, each about 30 times the mass of our sun (one 36, one 29), began rapidly rotating around each other, then merged. Here are two short videos visualizing the event, one as if you were up close observing it with your own two eyes, and another showing the warping of the gravitational field around the event.

The distortion is caused by gravitational lensing, gravity strong enough to alter the direction of beams of light.

This merging of black holes created a cataclysm in the fabric of spacetime, and the rippling from this event has affected the Earth 1.3 billion years later by compressing it and stretching it by about a nuclear diameter. Here's a short video showing this effect, greatly exaggerated:

<sidenote>

Humans are not good at thinking about scale. Our brains have never needed to be able to comprehend a billion of anything, so evolution didn't set us up to be able to comprehend this sort of number. Thinking in analogies helps, so I came up with this: The same gravitational wave that stretched and compressed Earth by an atomic diameter stretched and compressed the entire solar system by the length of a single skin cell, and the Milky Way galaxy by the distance someone could run in about an hour.

</sidenote>

"How did we detect it?"

Aerial view of LIGO

...by looking very closely at two specific beams of light. The two LIGO observatories in Washington and Louisiana do not look up at the sky, but rather have a very unique setup designed to detect differences in the space between a few sets of mirrors.

Here's how to observatory works:

In the above image, the leftmost element is a really expensive laser pointer.

 - The laser pointer produces incredibly pure light of a specific wavelength (1064 nm).

 - The laser beam hits a half-silvered mirror that splits the beam by letting half the light through and reflecting the other half.

 - Each beam then travels 4km through a vacuum, bounces off a mirror, then travels  4km back to the half silvered mirror at the base.

 - The beam is then recombined and received at a very sensitive detector (H-shaped object near bottom)

A quick not about interference (the "I" in "LIGO"): Light, being a wave, can either "stack up"  or "cancel out." In the image below, you can see that where peaks line up with peaks, the beam multiplies and gets stronger (constructive), and where peaks line up with troughs, the beam cancels out (destructive).

As the mirrors move closer and further apart due to the warping of spacetime, the waves of light align and misalign, making the beam "turn on" and "turn off," as you can see in the animation above.

It's at this I must admit I lied to you. The beam of light pointing at the detector doesn't actually turn on and off, because the warping over a distance of 4km is merely 1/1000th the size of a proton. This is nowhere near enough to warp the mirrors enough to move from fully constructive to fully destructive interference. In actuality, the beam changes by an incredibly small fraction and the change in the brightness in the beam is exceedingly slight. As a result, the instruments that detect the light have to be very precise. While in operation, the LIGO team has had to take remarkable measures to create such a precise instrument. Among the factors that were caught up in the noise they recorded were: individual atoms of gas in the 4km long vacuum tubes, trucks driving on highways kilometers away, as well as quantum effects in the mirrors themselves. That's right, the fact that mirrors are made of atoms was something the team has to consider and remove from their data.

Here is the actual data. The two signals were received 7ms (speed of light delay) apart from each other, and matched predictions nearly perfectly. The confidence level was reported at 99.999994%

One last thing I'll mention.

While reading the paper about the detection, I was struck by this table:

Take a look at the first three items. These are the masses of the two black holes, and the resulting black hole after the merger (M_☉ is solar mass).

There are three solar masses missing.

Einstein figured out what's called mass-energy equivalence (E = mc²), which, at its simplest, states that a particular mass m, say an apple, can be converted into a particular amount of energy, E. Using this equation, we can figure out that our apple contains more than enough energy to form this crater:

Notice the parking lot near the bottom

Using this same equation, three solar masses is the amount of energy released in 5000 supernovae, or to use a common analogy, roughly one million billion billion billion Hiroshima bombs. That's the amount of energy required to make a tiny blip on the screen of a ludicrously precise instrument on earth 1.3 billion light years away.

Cheers,

   - Scott


P.S. - If you turn those waveforms above into audio, you get the sound of a gravitational wave, and it's fantastic:

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What Can We Learn From Twinkling Starlight?

When you look up at the night sky, the stars twinkle. This is interesting, but not what I am talking about today. If you are curious why the stars twinkle from here on Earth check out this video:

There ya go. Now, the 'twinkling starlight' I'll be mainly talking about is related to exoplanets, or planets orbiting stars other than our own. If you look carefully at stars from outside our atmosphere, or correct for atmospheric effects, the stars still twinkle, but for what I think is a much more interesting reason.

Do you want to know something interesting about starlight?

Great!

For the longest time, it was thought that our planet and our solar system were pretty unique. Aristotle laid down the thinking about many topics including astronomy for many centuries, and as it turns out he was wrong about a fair bit of it. While understandable for his time, by the 1600's, times were changing. In the early 1600's Galileo looked up at the moon with a telescope he made (didn't invent) and observed the moon's terminator, the area where light met dark. In the shadows he saw craters, bumps, and ridges; the moon wasn't a perfect celestial orb, it was its own world with its own unique features. Couple that with his discovery of moons orbiting Jupiter, and we were on our way to discovering other worlds, inferred from points of light.

*IC6.G1333.610s, Houghton Library, Harvard University

Both the cratering of the moon and the motion of the Jovian
moons were published by Galileo in 1610 in this pamphlet.

We have now sent spacecraft to nearly all of the thirty or so largest bodies in the solar system. With missions visiting the asteroids Vesta and Ceres, and the upcoming mission to Pluto, New Horizons, our curiosities about other worlds just took steps much farther afield.

Just as we could see other worlds in our own solar system, we can now look for worlds orbiting other stars using several methods of analyzing flickering starlight from their home star. With few exceptions, we cannot just take pictures of the planets because their star outshines them by many orders of magnitude, what we can see it the influence they have on their star.

Transit method -

One way to detect exoplanets is too find a planet that passes directly between its home star and us here on earth. A bit like a solar eclipse. When this happens, the planet blocks a little bit of the light, and the star dims. We can track the stars brightness and if it dims consistently and periodically we can tell that there is probably a planet orbiting that star. Here is what one of these dips looks like:

Image: Nikola Smolenski

The transit method is currently by far the most common way to detect exoplanets, but it has its drawbacks. Due to the fact that the planet has to pass between the star it is orbiting and the observer here on earth, it biased toward planets that orbit "edge - on" to us here on earth. Imagine flipping a coin, and taking a picture when the coin is exactly edge-on. Most of the pictures are going to show at least some of either the heads side or the tails side. This is roughly the same probability as a particular star system appearing exactly edge on to ours so the planet passes in front of the sun.

This method tends to be biased in finding large planets orbiting close to their stars. The larger and closer to its star that a planet is, the more likely it is to cross in front of the star and dim the light we see. These planets are known as "hot Jupiter" because they tend to be larger than Jupiter and closer to their star than Mercury is to our sun. This flies in the face of how we think planets developed, suggesting that hot Jupiters are quite rare. If this is the case, there could many, many more planets out there than we can currently find using this method.

It is important to note at this point that we cannot see the outline of the planet in front of the star. The only thing we can detect from here on earth is the slight dimming from a distant point of light.

Other Methods

Two more ways I'll briefly touch on on the radial velocity method and something called astrometry.

To describe the radial velocity method I first have to talk about the Doppler effect.

<sidenote>
I always thought "Christain Doppler and the Effects" would make a great band name
</sidenote>

There are plenty of great video about how this works, so I'll only go into it very briefly here. When a noise-making object approaches you, the sound waves "stack up" and compress on their way to your ears. This registers as a higher pitch. When the noise-making object moves away from you the sound waves "stretch out" and you register this as a lower pitch. This is why cars passing you make the characteristic "weeeee-yahhhhhh" sound.
The same goes for light. When an object is moving toward you, you register the compression of the waves as a "blueshift," the object literally looks a bit bluer. When the object travels away, the light looks redder, a "redshift."

TL;DR: Stars look bluer moving toward you, redder when they're moving away.

Alright, on to radial velocity. A large planet orbiting a star will cause the star to wobble a little bit, as seen below:

This is because the planet gravitationally tugs on the star, just as the star tugs on the planet. Notice how the star moves up and down. If we look at this star from earth, we can see it getting redder and bluer as it travels farther and closer to us, and from that, infer the presence of a planet by looking at the rate of the wobble.

On to Astrometry!

If you imagine looking at the system above from earth just as it is portrayed, you would see the star travelling in a little circle. If you look at both the foreground star as well as background objects, you can see the motion of the star and from that find out characteristics of the planet orbiting it.


This topic is difficult to convey through writing alone, so if you're interested check out YouTube for some great videos about exoplanets and exoplanet detection. Here are a few of my favorites:

Overall, it is truly amazing what we can discover merely by looking at twinkling starlight.


Cheers,

   - Scott



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