What are gravitational waves? - Amber L. Stuver
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Gravitational waves are changes in the pull of gravity caused by mass moving around. Isaac Newton is credited with discovering gravity, but humans have always known that we are being pulled towards the Earth. What Newton discovered is that all mass, everywhere in the universe, has gravity pulling it together. This is called universal gravitation. There are no gravitational waves in Newton’s gravity theory because the change in the pull of gravity is felt everywhere in the universe at the same time. Therefore, there is no wave to move away from the source.
Einstein refined our understanding of gravity by developing the theory of general relativity. Part of this theory states that nothing can travel faster than the speed of light, even gravity. So, when mass moves, the change in the gravitational pull moves at the speed of light away from the source like a ripple on a pond. This is what gravitational waves are.
Another part of Einstein’s relativity is a concept called space-time. Space-time is abstract, meaning that it isn’t something that is physically there. It’s an idea that makes it easier for us to think of something in a different way. Here, space-time lets us think of the strength of gravity as how steeply a surface is curved. Mass is able to make depressions on space-time; the more mass you have, the more you “dimple” space-time. The steepness of curvature at any point tells us how strong gravity is there; the steeper the curvature, the stronger the pull of gravity towards the mass making the “dimple”. You can take this abstract idea and try it at home. Take a thin blanket or sheet and have some friends pull on the sides so you have a nice flat surface. If you roll a lightweight ball over it, you will notice that the ball goes in a straight line (you may even see the dimple it makes in the blanket). If you set a heavy ball on the sheet, the dimple is very noticeable. If you roll the same light-weight ball over the sheet again, it isn’t going to travel in a straight line, it is going to curve towards to the heavy ball just like if gravity were pulling it in. If someone goes under the blanket and jiggles the heavy ball from below, you will see ripples on the blanket (which is representing space-time). These are like gravitational waves! Scientists often refer to gravitational waves as ripples on space-time.
Observing gravitational waves have created a new field of astronomy called gravitational-wave astronomy. This is different from most other kinds of astronomy because it doesn’t need to observe light coming from space. LIGO and its international partners are observatories that don’t use any telescopes! Instead of seeing the universe, we “feel” the stretching and compressing effects as a gravitational wave as it passes by. We do this by measuring the length of each arm of an “L” shaped detector called an interferometer. If space is being stretched or compressed by a gravitational wave, the lengths of these arms with change compared to one another. We measure this using a laser. We now know about each of the letters in LIGO: L: Laser, I: Interferometer, G: Gravitational-Wave, O: Observatory. With LIGO, we will be able to observe how mass moves even if it doesn’t produce any light at all, like a black hole. Wonder what a gravitational wave would feel like? Click here.
There are other kinds of astronomy as well. When using light to observe the universe, there are many different kinds of light: microwave, radio, infrared, visible light, ultraviolet, x-ray, gamma ray, etc. Each of these different kinds of light interacts with the universe in different ways and allows us to see it in different ways too. There is also a kind of particle called a neutrino that is produced by nuclear reactions like the ones that take place inside of stars. We can detect these coming from our own Sun and from other stars when they undergo a supernova explosion.
Combining different kinds of astronomy is called multi-messenger astronomy. Gravitational waves tell us about how mass moves around, light can tell us all sorts of things like the temperature of a star or how far away it is, and neutrinos can tell us about when a star blows up. If we can observe the same thing happening in space in multiple ways, we can combine the different information communicated by the different observing methods to learn more than if we just made a single observation. This is the future of astronomy and gravitational waves are a big part of it.