Prof. Disbrow listens in to the sounds of the Universe with gravity
Longtime readers of this column might remember that back in 2014 researchers running the BICEP telescope claimed to have observed “Gravitational Waves.” Unfortunately, just months later, the peer-review process began to poke holes in their research and, eventually, they had to concede that their results were bad. What they had thought were Gravitational Waves were a false signal caused by interstellar dust.
Jump to 2105, and rumors began to circulate that the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment had finally detected these waves. For real this time!
What are Gravitational Waves? Gravitational Waves are waves in spacetime that are caused as massive objects move through spacetime itself. The more massive the object, the bigger the waves.
To make this a bit more relatable, picture a boat on the water. When the boat moves, it sends out waves. Now, replace the boat with a black hole, and the water with space itself, and you get the general idea.
Like objects in the water, every object with mass creates Gravitational Waves. And, just like waves in water, more massive objects create bigger waves. In general, these waves are tiny…smaller than the diameter of an atom. And, like water waves, they dissipate with distance and eventually fade out.
Now, waves in the water will just displace things. Gravitational waves however, actually stretch and distort space! So, as a wave passes over a region of space, it might make the objects in that region longer or shorter as it goes through them.
So, the question is, how do you actually detect these tiny, tiny waves?
Well, first, you need a really big wave source. Something that will create waves big enough to detect, but, not so close that it will kill us all. Fortunately, the Universe is full of super-massive objects that are far, far away. (In this case, it was two colliding black holes, 1.3 billion light years away.)
Second, because you need to be able to detect ripples in spacetime that are less than an atom’s width across, you need the most exquisitely precise measuring instrument ever created.
The LIGO detector is basically a giant “L” shape, where both limbs of the “L” are the same length (about 4km). A laser beam is fired from the corner of the “L” and split, so that the beam travels down each limb. At the end of each limb is a reflector, that turns the beam around and shoots it back at the source. When they arrive, the two halves are recombined into a single beam.
Now, at this point, one of two things will have happened:
Each half of the beam returns to the source at the same time and they generate a well-known and predictable interference pattern. This means the beams traveled the same distance in the same amount of time.
But, if a Gravitational Wave has passed through the space occupied by one (or both) of the beams, the distance that beam has travelled will be slightly different than the distance travelled by the other beam. The beams will return to the source at different times, and the interference pattern generated will be slightly out of whack with what it should have been. Measure that discrepancy, and you’ve caught a Gravitational Wave!
And that’s what happened! Very shortly after LIGO was powered up in 2015, it began to show tiny differences in the beams, giving our first direct observation of Gravitational Waves!
Of course, the next question is, “What good is this?” Well, for our entire history, we’ve had to look at the Universe. And every time we’ve looked at a different wavelength of light (visible, infrared, x-ray, etc.) we’ve found something new and amazing.
Now, we can hear the Universe! Heck, this was our first observation and we’ve already heard the collision of two black holes! The longer we listen, who knows what we’ll hear?
Steven Disbrow is a computer programmer who specializes in e-commerce and mobile systems development, an entrepreneur, comic-book nerd, writer, improviser, actor, sometime television personality and parent of two human children.