Whispers in the cosmos: gravitational waves

Chances are good that you’ve thought about the concept of gravity once or twice. If you’ve ever taken a high school physics class, you might have heard that gravity is an invisible force that is responsible for keeping you planted on the earth. At the beginning of the 20th century, the young Albert Einstein was also interested in gravity. The accepted theory of gravity at that time had first been put forth by Isaac Newton in the 17th century, and it had seen great success in physics for centuries.

But Einstein was bothered by one of the key foundations of Newton’s gravity: that space and time are both independent, absolute entities. In his Principia Mathematica, Newton stated that

“Absolute, true, and mathematical time, of it self and from its own nature, flows equably without relation to anything external… absolute space, in its own nature, without relation to anything external, remains always similar and immovable”.

In the early 20th century, physicists were just beginning to understand electricity and magnetism, and while carefully scrutinizing these developments Einstein came up with a new idea: that space and time are not distinct, absolute quantities as Newton said, but rather that they are intertwined in a very special way.

Putting the dimensions of space and time together, into what we now call spacetime, turned out to be necessary to avoid paradoxical outcomes in electricity and magnetism. But the concept of spacetime also leads to some very strange outcomes. A new theory of gravity, called General Relativity, is one of these outcomes.

In General Relativity — Einstein’s theory of gravity — gravity is the curvature of spacetime itself. Physicists often say that spacetime is the “fabric of the cosmos”, but it’s not exactly made up of “stuff”, so how can it be curved?

This is difficult to conceptualize, but one can use an analogy to understand a little better. If you were to place a baseball on a spandex sheet of fabric, the ball would distort the sheet by bending it. A bowling ball would also bend the sheet, even more than the baseball. If you took a marble and added it to the sheet, the marble would follow the spandex surface, curving around the bowling ball (an orbit).

In this image, two masses both bend spacetime. [You can ignore the yellow lines, which are part of a bigger image not included.] This is the essence of Einstein’s gravity: massive objects bend spacetime, and in turn, spacetime tells matter how to move. Now we’re ready to talk about gravitational waves.
In this image, objects with different mass bend spacetime. The heaviest object, in yellow, bends spacetime more than the other two. Image credit: ESA – C. Carreau
This is the essence of Einstein’s gravity: massive objects bend spacetime, and in turn, spacetime tells matter how to move. Now we’re ready to talk about gravitational waves.

Imagine for a moment that you rotate the bowling ball about its vertical axis on the spandex sheet. The bowling ball has a smooth surface and is round, so there isn’t any effect on the sheet. However, if we took a pair of bowling balls and rotated them around each other, ripples would begin to spread outward on our spandex sheet. Similarly, if we took a bowling ball that wasn’t quite round (maybe with a lump of cement stuck to a side) and tried to rotate the ball, the lump would “catch” on the spandex and create ripples.

These ripples, produced either by two objects orbiting each other (we call this a binary system), or a lumpy object rotating about its axis, are gravitational waves. They propagate radially outward from the objects that produce them and travel at the speed of light. As you can imagine, this means that gravitational waves are being produced all the time, all over the universe, from all kinds of systems: the moon and Earth in their orbit; a pair of ice-skaters spinning while holding hands; a football wobbling from a poor throw; a distant rotating planet with mountains.

When the gravitational waves produced by any of these examples hit matter like you and I, the effect they have is to stretch and squeeze it. All the stretching and squeezing happens in the direction that’s perpendicular to the gravitational wave’s travel path (using more technical physics lingo, we would say that gravitational waves are transverse).

The amount of stretching and squeezing is extraordinarily small because gravity is actually fairly weak [just think: you can defy the entire gravitational pull of the earth just by jumping or using a refrigerator magnet to pick up a paper clip!]. To produce “big” gravitational
waves, we have to look for waves that are produced by something called compact objects.To explai
n what a compact object is, imagine that you have a large round bread roll. The roll is made up of ordinary atoms and has mass; we could weigh it on a kitchen scale, and measure its diameter. Now imagine squashing the roll with your hands until it forms a dense lump of bread. If we place the squashed bread roll on our scale, it would have the same weight as it did before. It contains the same number and type of atoms it had before we squashed it. But now, the roll occupies a much smaller region of space, and the bread is more dense than it was before. It is now a compact object.

This figure shows how spacetime is curved for for objects with the same mass, but increasing compactness from left to right. You can see that the most compact object, on the far right, distorts spacetime in a much more violent way than the others. Image credit: National Geographic

Compact objects bend spacetime in a much more extreme way than other objects, and as a result, the gravitational waves they produce stretch and squeeze matter enough that we can actually detect it. Don’t get too excited about the amount of stretching, though. The most compact astrophysical objects in the universe (that we know of) are black holes and neutron stars, which are dead remnants of very massive stars. If a “big” gravitational wave produced by a pair of orbiting black holes or neutron stars were to hit you head-on, your height would only change by one thousandth the diameter of a proton!

Detecting gravitational waves is thus a very tricky business; the detectors we use must be capable of measuring changes in length that are smaller than the atoms the detectors are made of! We will discuss the methods that we use to construct such detectors and conduct searches for gravitational waves in a future blog post.

But before you go, it’s important to understand why any of this matters. One reason we’re interested in detecting gravitational waves is to test Einstein’s gravity. Although we have significant experimental evidence that supports General Relativity, there are feasible contending theories that are similar and vary only from Einstein’s in the way that gravitational waves behave. So far, our detections of gravitational waves have continued to support Einstein’s theory, but it is important to continue to test this.

The second reason to search for gravitational waves is to learn from them. Every telescope ever built has relied on light of some form (x-ray, gamma ray, infrared, visible, radio, etc.) to observe the universe. Gravitational wave detectors are completely different; they use gravity instead of light observe the universe. This allows us to study systems, such as black hole binaries, that can’t be directly studied using light. We now have the potential to unlock mysteries about black holes, neutron stars, stellar evolution, the Big Bang and much more.

And that’s not all. Any time our species has found a new way to observe the universe around us, we find unexpected things, things we didn’t even know that we didn’t know. There is no reason to suspect that this will be any different. We’re standing on the cutting edge of a new observational experience for mankind, and there are all kinds of beautiful, bizarre and unexpected things to be discovered!

Stay tuned to hear more about gravitational waves from the IGC. In the meantime, if you’ve got questions, I love to talk science! Feel free to email me.