Pulsar Timing Arrays: Explained
Using Spinning Neutron Stars to Detect Gravitational Waves
Okay. We gotta admit that it's quite incredible how the universe provides astronomers with the tools to unveil her mysteries. Since its detection back in 2015, gravitational waves have been a topic of discussion among the scientific community around the globe. That is partially because gravitational waves were actual theoretical predictions of Einstein about a hundred years ago before its actual detection!
The 2015 detection of gravitational waves was made possible through laser interferometry, a technique based on the principle of light's basic property - interference. Although that was a pretty genius set-up to catch the very faint gravitational waves that come our way, astrophysicists have now shown us that there is another smart way to detect the spacetime ripples. And unlike the previous technique, this method doesn't require sophisticated instruments, but rather, a certain type of star!
Long story short, astronomers just used spinning neutron stars to detect gravitational waves. And yes, we're talking about a whole BACKGROUND of gravitational waves that lingered in the spacetime. Not only did this method detect a hum of gravitational waves, but it did so by utilising a number of pulsars that were located at different parts of our galaxy - which is why we call the entire set-up Pulsar Timing Array!
The whole setup of the pulsar timing array is quite simple. All astrophysicists had to do was to keep a number of pulsars as observation points, and keep track of the time of pulses given off by each of the pulsars.
One important feature of these rapidly spinning neutron stars, or pulsars, as we call it, is that their pulses of radiation reach us (Earth) at very precise time intervals. In fact, in the case of millisecond pulsars, the pulsar timing can be accurate up to 100 nanoseconds! This means that pulsars can act as universal light houses that shine at regular intervals of time. What this also implies is that if, in any case, the radiation pulse from the pulsar changes, even to a slight extent, there is some variation that occurs to the distance between the pulsar and the Earth. Or in simpler words, the spacetime between the two objects is getting wobbled.
This seems quite logical though, because we know that speed = distance/time; and since both the speed of light and time are constant in this case, the only other possibility is for the distance to vary. And of course, there is no other who can disrupt the void of space between objects than the mighty gravitational waves itself.
That being said, observing this array of pulsars was not a joke. It took more than 15 years of observations from various radio telescopes around the world to capture the pulse times of these pulsars, and it is these years worth of data that was released a few months ago, which in turn revealed that there was something unusual going on in our galactic neighbourhood.
Now the question may arise: What is so special about this recent observation of gravitational waves?
Of course, the previous 2015 detection of gravitational waves was a breakthrough mainly because it was the first-ever detection of these spacetime ripples and the first direct observation of the aftermath of a supermassive black hole merger. The interferometry technique used back then was only capable of detecting high-frequency gravitational waves, with wavelengths in the range of kilometres.
However, in the pulsar timing array, because each pulsar in the array is located lightyears across from one another, it enables the detection of the faintest and longest gravitational waves that go past our cosmic neighbourhood. This is why the findings from the pulsar timing array are so crucial - it detected a background hum of gravitational waves that dawdled like symphonies in spacetime.
Though we are no strangers to finding gravitational waves, those waves emerged from a single source, like a black hole merger or a neutron star merger. However, this observation of a gravitational wave chorus definitely did not seem like they belonged to the same source. In fact, astrophysicists are still trying to understand the different possible sources of these faint background gravitational waves frolicking in and about the interstellar void, which may even lead to development of new hypotheses on gravitational physics. This may even shed light on the early universe and its evolution, definitely some exciting stuff.
Anyway, all of this gravitational waves business in the scientific domain has opened up a lot of speculations on our current understanding of the nature of spacetime and the universe itself. This all just proves that we're inching closer to unveiling the mysteries of the universe, from gravity to black holes to dark matter, we're preparing ourselves for a new era of physics!
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