Introductory Materials

You can learn more about the people and science behind NANOGrav through our ongoing podcast series. Subscribe through the Apple iTunes store or download the media files yourself by clicking the links below. You can also subscribe with this direct link using the media player of your choice.

What is gravity?

When most people think of gravity, they think of a force that keeps keeps things together: it keeps people on the surface of the Earth, it keeps the Earth in orbit around the Sun, and it even keeps/ the stars in entire galaxies together. This way of thinking about gravity — as a long range force of attraction — was firmly established in the 17th century by Isaac Newton. Newton's law of gravity is a spectacular example of how some simple mathematical rules can accurately explain what we observe in nature. But it isn't the end of the story. By the end of the 19th century, people had found several situations in which the classical physical laws didn't quite work, including Newton's law of gravity. The theory wasn't totally wrong, but it was incomplete. Few people realized just how profoundly a more complete law of gravity would change our view of the Universe, but that is exactly what happened after Albert Einstein weighed in.

In 1916, Einstein published his general theory of relativity, a completely new way of thinking about gravity. In general relativity, or GR, we think of gravity as a distortion, or curvature, of the fabric of space and time itself (called space-time). In this context, space means the distance between two objects, or the shortest path you could take to get between point A and point B. This is not the same thing as "outer space" — every thing in the Universe exists in space-time, including the Earth and everything on (and in) it. The time of space-time refers to that which is measured by clocks.

What does this all mean? You can think of space-time as a sheet of fabric that is pulled tight (it isn't a perfect analogy, but it is the simplest one). Now imagine placing a heavy object like a bowling ball on this sheet. The sheet curves into a well around the bowling ball, and according to GR, this curvature is gravity. A smaller object like a golf ball placed on the sheet will naturally fall towards the larger object, and if you give the golf ball a little push, it will circle the bowling ball just like a planet orbiting a star (friction eventually causes the golf ball to hit the bowling ball, but in space this doesn't happen). Anything and everything with mass (from stars to actual golf balls) will cause this curvature of space-time, and hence create a gravitational field. This may all sound pretty wild, but GR is the most elegant and complete description of gravity ever, and it has been passed every test that scientists have ever put it to. In other words, it works, and it works well.


What are gravitational waves?

One of the predictions of GR that has not been directly observed is the existence of gravitational waves. Gravitational waves are ripples in the fabric of space-time that are caused when massive objects move in a certain way. These ripples actually cause objects to shrink and stretch as the wave passes through them, but the effect is tiny — even a very strong gravitational wave will cause an object to shrink and stretch by one part in a quadrillion (1 quadrillion = 1,000,000,000,000,000)! Although we have indirect evidence for the existence of gravitational waves, we have never directly observed them.

Why do we care about gravitational waves?

But why are we so interested in detecting gravitational waves to begin with? Well, for the whole of human history, almost everything we have learned about the distant Universe (everything outside our Solar System) has come from studying things that emit light. Some very important contributions have also been made by studying things that emit sub-atomic particles, but for the most part light is our main window into the Universe. Of course, not everything emits light, or at least light that we can easily detect. Some examples are black holes, white dwarfs (a type of dead star), and neutron stars (more on them below), as well as hypothetical objects called cosmic strings. It is also impossible for us to detect light from when the Universe was younger than about 379,000 years old. But all of these things (specifically pairs of black holes, white dwarfs, and neutron stars orbiting eachother) are predicted to emit gravitational waves. A direct detection of gravitational waves will open an entirely new window on our Universe and allow us to learn things that we couldn't learn about otherwise. The things we will be able to study using gravitational waves are some of the most fascinating and extreme things in the entire Universe. And in the past, whenever we have opened up new frontiers in astronomy, we have discovered things we never even imagined. So we will almost certainly be surprised by what we learn from gravitational waves. And this is where NANOGrav comes in.

What is NANOGrav?

NANOGrav is a collaboration of astronomers, physicists, and engineers from across the United States and Canada, whose common goal is to directly detect gravitational waves using objects known as pulsars. Pulsars are a type of neutron star, the remains of a dead star that was more massive than our Sun, but not massive enough to form a black hole. Pulsars are extreme, fascinating, and just plain cool objects in and of themselves — a teaspoon of neutron star material on Earth would weigh as much as the entire human race! And pulsars have magnetic fields so strong that they would erase every credit card and computer hard drive on Earth...even if the pulsar was as far away as the Moon! But from the point of view of NANOGrav, what makes a pulsar so interesting is that it can be used as a very precise clock. That is because pulsars send out a beam of radio waves, and because they spin rapidly. Each time the beam of a pulsar points towards the Earth, we see a pulse of radio waves (hence the name pulsar). So pulsars are a lot like cosmic light houses. The fastest-spinning pulsars spin once every few milliseconds (that is hundreds of time in a single second), and so are called millisecond pulsars, or MSPs. The key idea here is that the pulse of a pulsar acts just like the tick of a clock, and MSPs approach atomic clock in their precision. A change in the rate at which the pulsar "ticks" tells us something about the environment that the pulsar is in, or about something that happened to the pulse as it traveled to Earth.

NANOGrav uses pulsars like a sort of cosmic global positioning system (GPS). The GPS that you use in your car or on your phone works by communicating with satellites in orbit around the Earth, and can determine your position very accurately. NANOGrav is using MSPs to do the same basic thing: to look for tiny changes in the position of the Earth that are due to the shrinking and stretching effect of passing gravitational waves. Just like GPS uses several satellites spread throughout the sky, NANOGrav uses an array of MSPs. This is known as a pulsar timing array. And since gravitational waves have such a small effect, only a handful of the most stable MSPs that make the best clocks will work.

NANOGrav astronomers use the world's most sensitive radio telescopes to observe these MSPs: the Arecibo Telescope located in Arecibo, Puerto Rico, and the Green Bank Telescope located in Green Bank, West Virginia. Arecibo is 305 meters in diameter, the size of 3 football fields, making it extremely sensitive. But because it is so big, it can't be moved around, and so can only look at the sky directly overhead, and a little bit off to the sides. The Green Bank Telescope is 105 meters in diameter (as big as one football field, so still huge), and can look anywhere in the sky. Without both of these exquisite and one-of-a-kind telescopes, detecting gravitational waves would be a lot harder.

NANOGrav hasn't detected gravitational waves yet but we believe that we will be successful within the next five to ten years. To meet this goal we need a few things: 1) access to the world-class Arecibo and Green Bank Telescopes, 2) as many clock-like MSPs as we can get our hands on, 3) a good understanding of our MSPs so that we can be confident that they really do make great clocks, and 4) time. This last need is important because our pulsar timing array becomes more sensitive to gravitational waves as we observe the pulsars over many years.

NANOGrav isn't the only experiment trying to detect gravitational waves using a pulsar timing array. There are similar projects using telescopes in Europe (the European Pulsar Timing Array) and Australia (the Parkes Pulsar Timing Array). It will probably be necessary for these different projects to cooperate and share their data in order to be successful, and so in 20?? they came together to form the International Pulsar Timing Array. There are also other experiments using totally different, non-pulsar based methods to detect gravitational waves. Two examples are LIGO, and its European counterpart VIRGO. But NANOGrav and the IPTA are hoping to be the first to detect gravitational waves. And you can help us find more MSPs through the Einstein@Home project. This is your chance to be a part of a ground breaking discovery!

Get Involved

One of the ways to get involved in the search for gravitational waves is to participate in the distributed computing project Einstein@Home. Einstein@Home uses your computer's idle time to search for weak astrophysical signals from spinning neutron stars (also called pulsars) using data from the LIGO gravitational-wave detectors, the Arecibo radio telescope, and the Fermi gamma-ray satellite. It's possible that a pulsar found in the project could be used in a pulsar timing array to detect gravitational waves!

If you are a teacher looking for information about gravitational waves, this site hosted by Penn State University's gravitational wave research group has a great collection of articles and projects.

Members of NANOGrav support three major outreach projects: the Arecibo Remote Command Center, the Pulsar Search Collaboratory, and the Mid-Atlantic Relativistic Initiative in Education. These projects are helping to get middle and high school students involved in the real work of using and analyzing data from two of the world's best radio telescopes.