Size Matters - Gravitational Waves and More

A few weeks ago, the scientific community announced a truly amazing discovery: the observance of gravitational waves. And while it made the headlines for a day or two, I didn't really hear anyone talking about it, and interest from the lay community seemed to fade pretty quickly... But to call this a shame is the understatement of the century. For reasons I am about to describe, the discovery (and how they did it) is easily the most incredible scientific achievement since the splitting of the atom.

Here's why: first, what are gravitational waves? Put simply, gravitational waves are a theoretical (well, not anymore) ripple through spacetime. They are released by hugely energetic cosmological events in which a large amount of mass is converted to energy, like the combining of binary stars, supernovae, or (hypothetically) any time where a large amount of mass clumps together. You can think of it as a shock wave moving through space at the speed of light. Now that's an oversimplification because this shock wave is in the very fabric of spacetime itself. As the ripple passes through you, space itself distorts and your very atoms shrink and expand, as does your perception of time (time would also move more and less slowly as the ripple passes through you). And these waves are passing through us all of the time.

Now you're probably thinking, "if this is happening constantly, why haven't we noticed it before?" Because gravity is exceeding weak compared with the other fundamental forces of nature (the electroweak force and the strong nuclear force), and because of that, gravitational waves are itty bitty. They are so incredibly unimaginably tiny that  many people (recently) thought we would never ever be able to observe them. In comes LIGO (the Laser Interferometer Gravitational-Wave Observatory). You just knew the answer had to involve lasers, right?

LIGO was specially designed to pick up these tiny tiny changes in length that you would see as a gravitational wave passed through you. Basically, LIGO is a 4 km (2.5 miles for those stuck in customary units) long concrete tube that the researchers watched day and night. They watched and waited for the length of the tube itself to contract and expand. They could do this by measuring (very precisely) the amount of time it takes for light to bounce back and forth from one end to the next. Since the speed of light is the same everywhere (even with a wave passing through), by measuring the time, they are effectively measuring the length. Easy right? Wrong. Even with a tube that length,  the amount of change that they were expecting to see with a even a decent sized gravitational wave would be lost in the noise using regular optics like I described above.

What do I mean by that? I mean that even with a 4 km tube, the length change from a gravitational wave would be completely obscured by quantum mechanical fuzziness. Remember, on very very small scales (like smaller than the size of an atom), matter can't really be pinned down. A subatomic particle's "location" is more of a probable set of locations. The nice crisp version of atoms that we all have in our heads doesn't really exist as such. So the scale of change that they were looking for was soooooo small, that the ends of the concrete tubes of LIGO at that scale would have looked more like an indiscernible cloud rather than a solid wall with definite edges.

How did they get around this? They had to be clever. Very clever. They relied on a technique known as interferometry: a type of quantum optic technique that makes a light beam interact with itself. The resultant interference pattern (where the name comes from) can then be analyzed for minute changes that would correspond with differences in distance traveled by the two split beams. So how small a difference am I really talking here? Well, the precision they achieved in measuring these waves when they finally did arrive would be like seeing details on an individual molecule of hemoglobin on Pluto... from Earth. That's why this is such a technical feat. It is almost impossible to even comprehend how precise they had to be.

So... why do we care? Since the Greeks first looked up to the skies, almost everything we -- as a species -- have ever learned about the universe has been from studying electromagnetic radiation: different forms of light. We study radio waves and gamma rays and visible light -- but all of this is light. And as such, we've been limited by our singular tool of observation. It's like having a tool box with only different types of hammers in it -- very complicated hammers, but all hammers nonetheless. And we just invented the wrench.

This is huge. Now instead of just watching the universe, we can listen to it too. Here's one practical application: the Big Bang. For the first part of the universe's existence, the universe was so dense and hot that charged particles flew around on their own. Space was completely filled with ionized plasma -- no neutral atoms (electrons bound by an atomic nucleus) existed. Because free-flying charged particles scatter and absorb light so easily, it wasn't until the universe cooled enough for electrons and atomic nuclei to come together that the universe became "transparent" to light. This was a cosmological epoch known as "decoupling" and it happened about 380,000 years after the Big Bang. Therefore, the oldest light that we can see (and the earliest that we can observe) is still almost half a million years into our universe's existence. This ghostly "cosmic microwave background" gives us a peak at the structure and make up of the universe from the moment it became "transparent," but using traditional techniques, that's as far back as we can go. 

 The cosmic microwave background: a snapshot of the universe at the time of decoupling

The cosmic microwave background: a snapshot of the universe at the time of decoupling

The CMB Cold Spot" (CC BY 2.0) by  NASAblueshift 

However, gravitational waves are woven into the fabric of spacetime itself and are not bound by these limitations. So one can imagine putting our ears to the skies and hearing back before decoupling to discern the very first moments of structure formation in the universe. We could learn more about the origins and nature of dark matter and dark energy. And maybe, one day, be able to hear the Big Bang itself.