Month: February 2016

Confirmed: Gravitational Waves are a Thing!

If you dedicate any amount of time to following science these days then you WILL have heard about the recent detection of gravitational waves. Science media truly went mental when the discovery was announced on February 11th, and it’s not surprising when you consider what this means for the fields of physics and astronomy. But before we get started with all that jazz, we should probably look at the specifics regarding what the hell these waves are and how the discovery was made.

What they are is a disturbance in the fabric of space-time, much like how dragging your hand through a still pool of water will produce ripples that follow and spread out from it. Why is this a valid comparison? Well Einstein described the universe as made from a “fabric” hewn from both space and time. This fabric can be pushed and pulled as objects accelerate through it, creating these ripples. A similar distortion is also the cause of gravitational attraction, which is nicely demonstrated in the video below.

Almost any object moving through space can produce gravitational waves provided they are not spherically or cylindrically symmetrical. For example, a supernova will produce some if the mass is ejected asymmetrically, and a spinning star will produce some if it’s lumpy rather than a spherical. Unfortunately, the vast majority of sources produce waves that have dissipated long before they get anywhere near us, with only incredibly massive objects producing some that we have a chance of detecting.

Okay! Now that we have at least some idea of what these gravitational waves are, we can look at who and what detected them. This discovery can be attributed to the great minds and machinery involved in the LIGO experiment, which aims to detect gravitational waves by observing the effect they have on space-time. But how would they do this? Space-time isn’t even something we can see! Well my friends, the answer is very clever indeed.

It all involves a machine known as an Interferometer (Figure 1). This device starts by splitting a single laser beam into two, which then shoot off in lines perpendicular to each other. These beams travel exactly the same distance down long vacuum tubes, bounce off mirrors located at the end, and return. Since both beams have travelled the same distance they will still be alignment when they return to the source. They will then destructively interfere with each other and no light will reach the detector.

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Figure 1: Diagram of a basic Interferometer design. Source: http://www.ligo.org/science/GW-IFO.php

However, a passing gravitational wave, with its space-time distorting powers, can actually change the distance that one of the beams travels.  This would mean they are no longer in alignment when they return to the source and won’t cancel each other out. Some light would therefore be able to reach the detector.

And voila! A gravitational wave has been detected… or has it? Well, it actually has in this case, but the point I’m making here is that this amazing machinery is incredibly sensitive to noise. If a gravitational waves were to pass by, it would only change the beam’s distance by about 1/10000th the width of an atom’s nucleus, which is a size I have trouble comprehending.

To pick up such a teeny-tiny change LIGO has to filter out any and all sources of noise, which can include earthquakes and nearby traffic. In fact, to test the research groups ability to distinguish a genuine gravitational wave from noise, senior members of the team secretly inserted “blind injections” of fake gravitational waves into the data stream. While it does seem a bit cruel, it seems their training paid off.

Now we move on to the understandably common question of why this matters to people who aren’t hardcore science nerds. Well, beyond the fact that this discovery will almost certainly win a Nobel Prize this year and that it confirms the final prediction made by Einstein’s general theory of relativity, it could also have a huge impact on the field of astronomy.

Similar to how we use various electromagnetic wavelengths like visible light, infra-red, and x-rays to study a wide range of things, gravitational waves could act as a new analytical tool. Scientists would listen to these waves to learn more information about the objects producing them, which include black holes, neutron stars, and supernovae.

So, while this discovery won’t exactly change your life, it’s easy to see how big of a discovery this was for the field of physics, giving us both a new way to observe the cosmos and further cementing the theory of relativity. Once again, Einstein has been proven right many decades after his death. That’s a feat that very few people have achieved.

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