What are Gravitational Waves?
Gravity is the result of the curvature of spacetime by matter. Non-symmetric acceleration of matter produces gravitational waves that ripple across the universe at light speed.
In other words, all accelerating objects with mass curve spacetime, and that effect is what we call gravity. When one or more massive objects “wobble” (by not accelerating in a symmetric way), it produces waves that radiate outward from the center of the mass.
Waves differ by amplitude, frequency, wavelength, and speed depending on the behavior of the matter, and it’s interactions. Most of these gravitational waves are weak and pass through matter and other waves. Only the biggest of waves are detectable by our modern technology.
Here is a quick visual of “matter curving spacetime”. The concept is simple once you get it. The videos below are much “smarter”, but this will help you get the concept quickly.
TIP: Gravity can suck in (acceleration + mass = curved spacetime = gravitational force). Gravity can also radiate outward as gravitational waves (when the acceleration is non-symmetric).
Where Do Gravitational Waves Originate? A Matter of Symmetry, Mass, and Acceleration
All systems of one or more particles have a property called mass, and all things with mass curve spacetime to some extent. The denser the mass, the more spacetime is curved.
When an object accelerates, if that acceleration is not perfectly spherically symmetric (like an expanding or contracting sphere) or cylindrically symmetric (like a spinning disk or sphere), the resulting “oscillation” produces gravitational radiation as waves that ripple across spacetime.
Typically waves are produced when two or more objects orbit each other or interact, but “a single spinning non-axisymmetric planetoid, say with a large bump or dimple on the equator, will radiate” and an exploding supernova will as well (unless the explosion is symmetric). See more examples of the source of gravitational waves here.
Waves from the interactions of supermassive objects can be “heard” by instruments similar to the way sound waves can.
Have Gravitational Waves Been Discovered?!? | Space Time | PBS Digital Studios. This video describes gradational waves, the video below the followup video that describes the confirmation of gravitational waves by LIGO.
FACT: In February 2016, LIGO (Laser Interferometer Gravitational Wave Observatory) announced that they had detected gravitational waves from a black hole collision (black holes are supermassive, these black holes were orbiting each other, and the collision caused waves strong enough to measure).
LIGO’s First Detection of Gravitational Waves! | Space Time | PBS Digital Studios.
NOTE: In Einstein’s theory of General Relativity gravitational waves transport energy as gravitational radiation. The confirmation of gravitational waves was the last piece of empirical evidence needed to prove Einstein’s theories.
Gravitational Waves Example: Like A Ripple in a Pond
If you are still having trouble grasping the concept, here are some examples (if you already get it, skip to the LIGO stuff below).
Gravitational waves are analogous to how a drum head ripples when it is struck with a mallet, or how a pond ripples when a pebble is dropped in it. If you imagine the universe as a stretched out blanket as spacetime, then imagine an object with mass as a blowing ball as matter, you can see how the matter would curve spacetime causing what we call gravity.
Now if we factor in momentum as “spin” (which all objects down to elementary particles have) we don’t just get a dip in spacetime, if the spin is not “symmetrical”, we get a wobble resulting in spinning pulsing waves rippling away from the object through spacetime.
Gravitational waves emitted from an object can travel forever at the speed of light if unimpeded. It turns out that those waves are so weak that they pass through everything. This made them difficult to find; they were not detected until 2016.
Jim Hough explaining the history of detecting Gravitational waves. A 2012 interview with Jim Hough, who has a long history of working on detection devices, this interview predates the LIGO findings by about four years. Learn more about gravitational waves here.
LIGO and the Detection of Gravitational Waves
Gravitational waves are very weak. Gravity is the most tenuous of the four forces; it is effective for pulling planets and stars together, but not ideal for measuring. Although Einstein proved that gravity curved spacetime with general relativity 1916, it was only confirmed true after rigorous testing and technological advances100 years later, in 2016. Scientists at LIGO authenticated the existence of gravitational waves in a paper published on February 11, 2016.
The first wave found by LIGO was in September; a lot of testing was done before the results were published.
An explainer showing how LIGO detects waves.
FACT: Gravity is one of the four fundamental forces, there are only four, learn about the four forces here.
You can see an introduction to gravitational waves from LIGO here. See the links below to explore LIGO more and importantly, listen to an example signal of gravitational waves translated as sound.
- Listen to an example signal of two neutron stars merging.
- Listen to the same signal mixed in with noise. Noise in the LIGO detectors is mostly caused by vibrations from the local environment.
- There are four main sources of gravitational waves caused by different kinds of motion and changing distributions of mass – continuous, inspiral, burst, and stochastic.
How to Think of Mass-Energy and Gravitational Waves
All matter has measurable properties called mass and energy. Particles with mass curve spacetime, particles without mass don’t. However, mass-energy equivalence shows us that we can think of mass and energy together. So systems of particles with mass-energy get their “mass” from both mass and energy. Thus we can say, mass-energy curves spacetime, or be basic and say mass curve spacetime, or just be very simple and say “matter” curves spacetime… so we don’t have to give this explainer. Learn more about mass-energy.
If you made it through the rest, check out this fun physics video about gravity and time with a young Michio Kaku.
TIP: At this point, you might be asking yourself, “how can time and space curve, and what does that mean?” Great questions, the answers are in Special and General Relativity. Given we have now proved Einstein’s theories, why not have a look at the wacky world of time dilation and length contraction.