On February 11, David Reitze spoke the words many of us have been waiting to hear since September of last year. “We have detected gravitational waves. We did it.” He said to a round of applause.
To understand the magnitude of this discovery we need to step back to when Einstein first proposed his General Theory of Relativity. In the simplest of terms, it said this: the three dimensions of space were fused with time to form a four-dimensional continuum. There is no such thing as space and time, only space-time. Einstein went on to say that massive objects distort space-time or cause it to warp or curve. A more massive object would cause space-time to warp more significantly than a less massive one. The movement of objects in space can then be explained as a result of this warping.
Kip Thorne compares how we have seen this curvature of space-time until now as looking at the surface of an ocean that is very calm. “We’ve never seen the ocean roiled in storm with crashing waves.” He said in the LIGO announcement. The gravitational waves detected by the LIGO were the violent storm in the fabric of space time. The cause for these gravitational waves was two colliding black holes*. To get some perspective on how violent this storm was here are some numbers.
The two black holes were about 150 km in diameter. That might not sound like a lot but imagine this. Imagine the mass of 30 Suns compressed in a diameter as small as a hundred and fifty kilometers. Then imagine this thing going at half the speed of light and then crashing into an almost identical mass going at about the same speed. Imagine that collision and imagine the kind of havoc it must have wreaked.
If it was that big of storm then detecting these waves shouldn’t be that hard, right? Right? Wrong. Why? Because this happened 1.3 billion years ago. 1.3. Billion. Years. Multi-cellular life was just evolving on Earth when these two black holes collided. The gravitational waves travelled through space-time for billions of years before hitting Earth and being picked up at the LIGO.
Gravitational waves can be thought of as perturbations in space-time. They contract space in one direction and expand it in the other. They cause something of a wiggle and LIGO was built to detect that wiggle. And now comes my second favorite part of the story: The Elegance of LIGO.
LIGO uses a deceptively simple method of measuring these distortions—an interferometer. For those unaware of the instrument I’ll link to a video at the end. But to give you a general idea, the most basic interferometer uses two mirrors, a laser and a beam-splitter (or a partially silvered mirror). Of course, the one used here was augmented with other equipment to get rid of noise, false signals and what not. But the experiment itself is breathtakingly elegant.
So what was LIGO looking for here? What were the length scales that we are working on? This is where the story goes from wow-that’s-pretty-neat to IS-THAT-EVEN-POSSIBLE-I-HAVE-TO-TELL-SOMEONE-ABOUT-THIS. When the gravitational waves reached Earth, the perturbation that they caused was a thousandth of the diameter of a proton. Roughly, about 10-18 m. Here’s a nice representation of that number.
That is how precise the LIGO is. If this doesn’t impress you, what is wrong with you?
And now for the showstopper. On September 14, 2015, LIGO at Livingston detected a blip that lasted for a few milliseconds. Seven milliseconds later, the same blip registered at LIGO Hanford. And the rest is history.
It took the scientists working there months to verify their findings. And then last week they declared victory.
When Einstein proposed general relativity in 1915 no one thought that we would detect gravitational waves within a hundred years, if at all. Now we’ve done it and just as Galileo pointing his telescope at the sky changed astronomy, this discovery might very well change the way we see and hear the Universe.
Until next time.
*How do we know that the gravitational waves were the result of a black hole merger? Because the output that we got is the same that we would get if we solve Einstein’s equations for a binary black hole merger.
Listen to the symphony of the Universe here: