What are gravitational waves? What are Pulsars and what does it mean to time a pulsar? How does this relate to past and future observations of gravitational waves? These are all fundamental questions to ask and this article seeks to answer them in simple and easy-to-understand terms for a general audience. In short by observing a coordinated array of 67 pulsars the International Pulsar Timing Array was able to observe coordinated motions of these pulsars which indicate gravitational waves passing of a wavelength comparable to the size of a galaxy.  It must be noted that discovery is a strong word, one could say observations that indicate such a background, and so forth.  Any observation is provisional until replicated by others.  This observation arises from the analysis of decades of data by many  scientists and technicians.  We can talk about dense details of metrics and wavelengths and complex math, but to understand this, one needs to recall one basic equation from middle school math: the Pythagorean theorem. Aside from math, one can think of an oversimplified model.

First and foremost, I want to congratulate the whole community of scientists, technicians, and others who worked on and supported the work of the international pulsar timing array community, which made the recently announced discovery. Their work will give all of us much to think about, discuss, learn from, and build upon.

As the IPTA collaboration describes their finding:

For the last 15 years, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) Physics Frontiers Center has been using radio telescopes supported by the National Science Foundation to turn a suite of millisecond pulsars into a galaxy-scale gravitational-wave detector. Millisecond pulsars are remnants of extinguished massive stars; as they spin hundreds of times each second, their “lighthouse-like" radio beams are seen as highly regular pulses. Gravitational waves stretch and squeeze space and time in a characteristic pattern, causing changes in the intervals between these pulses that are correlated across all the pulsars being observed. These correlated changes are the specific signal that NANOGrav has been working to detect.

NANOGrav’s most recent dataset offers compelling evidence for gravitational waves with oscillations of years to decades. These waves are thought to arise from orbiting pairs of the most massive black holes throughout the Universe: billions of times more massive than the Sun, with sizes larger than the distance between the Earth and the Sun. Future studies of this signal will enable us to view the gravitational-wave universe through a new window, providing insight into titanic black holes merging in the hearts of distant galaxies and potentially other exotic sources of low-frequency gravitational waves.

That's great but what are gravitational waves?  How can you dear reader understand this in simple terms?

Imagine observing a swimming pool in near total darkness.

Imagine looking at a swimming pool in the dark, with only highly reflective white ping pong balls or pool toys floating on the surface. There is just enough ambient light to see these ping pong balls. One could infer the presence of ripples on the water from observing those white balls. Sure, you may observe some ripples by their sound, but only ripples of a certain frequency will make a sound. You may observe other ripples by light reflecting off of them, but not all will be of a frequency where that can happen. Some you would only detect by their effect on these widely spaced highly reflective objects. In essence, this is what pulsar timing does for gravitational waves.

Pulsars are neutron stars which are spinning very fast.  These objects have strong electric and magnetic fields and when they spin they produce pulses of radio waves which we can detect. In this way they are like very precise clocks. By timing the pulses from a chosen set of pulsars, astronomers are able to take advantage of a large naturally occurring gravitational wave observatory. Such an array of pulsars, many light years in scale, will be sensitive to gravitational waves of a frequency that cannot be observed by any current or future man-made experiment. The closest we have come to doing gravitational wave observations on this scale was by observation of the cosmic microwave background’s B-Mode polarization.

Pulsar timing would not be the best way for observing gravitational waves of shorter wavelengths and higher frequencies. Experiments such as LIGO, VIRGO, KAGRA and others have observed gravitational waves from the mergers of black holes and neutron stars. The future LISA space-based observatory, on which I am one of many working scientists, will be best for observing extreme mass ratio inspirals, where one object is very massive and the other is relatively light, up to very massive black hole mergers, but at a different part of their inspiral than observed by LIGO, VIRGO, or KAGRA.

In More Technical Terms.

What does this mean in terms that are more technical but still understandable?

Take a right triangle and label each leg as a and b. The slanted side, the hypotenuse, label c. The square of the hypotenuse is equal to the sum of the squares of the other two sides: a^2+b^2=c^2. This simple, very old equation is the first and simplest example of what is known in the study of gravitational waves as a metric. It is an equation that determines the distance between points on a two-dimensional plane. To describe gravity, we need four dimensions: three of space and one of time. Then we also need to apply calculus to this, which gives us something called differential geometry. It is a subject so complex that while Einstein was able to set up the set of 16 coupled equations that would describe how space-time tells matter where to go and matter tells space-time how to curve, he was not able to solve them. Other people did solve those equations; the solutions, known as metrics, determine distance in 4D space-time. Curvature of space-time can be thought of in very simple terms, as the degree to which a metric varies from what we would expect from the flat, very Pythagorean theorem-like, Minkowski metric. We measure this variation using light and what is known as interferometry, the degree to which a ray of light will interfere with itself, to measure this variation. In the case of gravitational waves, this change from the background metric varies with time in a periodic manner.

The above is an example of a gravitational wave form from my own theoretical work.  Not derived from to the current observation in any way.  However, it illustrates the shape of a wave one might observe given general relativity and some special ingredients.  (It is also a picture I have the legal copyright to share.)  It is merely for illustration.

In the case of these gravitational waves observed by this international collaboration the gravitational waves are of a wavelength that would be a significant fraction of the whole pool in the above analogy. Like a swell on the ocean, or the sum of many many large people cannon balling into the deep end of a very large pool, in the dark.

By doing so, they have given us a sound in the silence that hints at new physics, and sharpens our understanding of fundamental physics. To quote Kelly Holley-Bocklemann a fellow LISA physicist they have “added a new instrument to the bass section of the cosmic symphony”.

It will take a while to digest this and say much more regarding the fundamental physics at play. Be wary of any overly extravagant claims made regarding theories of this or that type of field or string, or dark matter and such. Fundamental physics can be very exciting, and new discoveries are always provisional until they aren’t.  Don't get carried away.