Above: What two black holes orbiting each other look like in light. Very different from the graphs and compression waves you saw above! Credit: The SXS (Simulating eXtreme Spacetimes) Project
The relationship between spacetime and gravity. Where there’s lots of mass, space gets pretty warped. Gravitational waves are compression waves through this green medium. Credit: T. Pyle/Caltech/MIT/LIGO Lab
What are gravitational waves? Gravity seems like a familiar enough concept, but what exactly is waving? Well, in order to answer that, let’s look at a concept called ‘spacetime’. We tend to think of space and time as two separate things; we move through space, and we notice the passage of time. As it turns out, space and time are actually interwoven with one another – they can’t be separated, which turns out to be really important in discussing Einstein’s famous theories of relativity (Einstein actually had two theories of relativity: special relativity and general relativity). The important point here is that spacetime itself, the fabric of reality, is what is ‘waving’; the distance between two points is being compressed and expanded. Spacetime is the medium through which gravity propagates, just like how air is the medium through which sound waves travel by compressing and expanding the air!
A rendition of how gravitational waves are released by two massive orbiting bodies, in this case white dwarfs. The waves are regions where space is compressed compared to normal. Credit: NASA
So how can we tell that reality itself is waving? Imagine two walls, placed some distance apart, and let’s say you can measure that distance really, really accurately. You go out once every hour and measure their distance. You measure the same distance every time except once, when you notice they’re closer together than they were before. You check to make sure your walls haven’t been moved, and they haven’t. In fact, an hour later, you measure the walls to be at their original distance again! That means that during that one weird measurement, space itself was compressed somewhere between your walls. The atoms in the air didn’t shrink, and there aren’t any fewer atoms (or maybe there are, but it doesn’t matter because you aren’t counting them). There was just some region of space itself which was compressed!
One of the LIGO stations where they detected gravitational waves. You can see their long tunnels where the lasers are used to measure the distance of the tunnel! Credit: LIGO
Well, that’s exactly what a gravitational wave is. Just like how a sound wave is made of regions of high pressure and low pressure, a gravitational wave is made of regions of compressed space and stretched space. As it travels through the universe, space compresses and expands!
The discovery of gravitational waves was announced on February 11, 2016 by the Laser Interferometer Gravitational Wave Observatory (LIGO). LIGO detected such a wave on September 14, 2015, produced by the merger of two black holes 1.3 billion light-years away. The scientists at LIGO didn’t use walls and measuring tape; they used mirrors and lasers to be extremely precise, and were very careful to rule out any other possible reason that their mirror distances seemed to ‘wave’ over a short period of time. By using lasers to measure the mirror distances, they were able to make measurements on very small time scales – the entire ‘wave’ was only strong enough to be detected for 0.2 seconds!
The LIGO signal. The top two graphs are the two LIGO stations’ detections, and the thin fitting line is what was predicted by theory. Credit: Caltech/MIT/LIGO Lab
Now, if these waves were flying all around, you’d think it would be pretty obvious when distances seem to shrink and expand out of nowhere. Unfortunately, it turns out that the scale of these waves is tiny; so tiny, in fact, that they compress space by 10-21 metres. That’s nearly a millionth of a millionth of the size of a hydrogen atom! Put another way, if the distance between the mirrors was the distance from here to the nearest star, the detected change in distance was only the width of a human hair!
So detecting them took quite a bit of effort and some really advanced technology, which is part of why everyone is so excited. Another reason why this is an exciting discovery is that these waves are predicted by general relativity, a theory put forth by Einstein over 100 years ago, and have only just been discovered! That’s a pretty big deal, for a theory to successfully live under constant scrutiny for so long. But the biggest reason why gravitational waves are big news is neither of these.
For all of human history, humans have only been able to observe astronomical events that radiate light. With the invention of the telescope, we have been able to peer deeper into the universe than ever before, and with radio telescopes and X-Ray telescopes, we can even see in wavelengths of light that the human eye cannot detect. But events that don’t radiate light, like black hole mergers, cannot be observed with telescopes – but they can be observed with gravitational wave detectors! It’s like being deaf all your life, and then suddenly being granted the ability to hear. There is a whole new realm of information to which we are now sensitive. It’s like the invention of the telescope, but a telescope that can ‘see’ gravitational radiation.