Shedding Light on Gravitational Waves: Part Two

Before we get started, if you haven’t read the first post of the two-part series: check it out!

Last time, I outlined a research project I completed last year on gravitational waves, inspired by the Nobel Prize in Physics. We covered the basics, and at this point in my research, the articles were getting progressively longer and more complex. But, the topic still interested me, so after spending a long time reading, I’ve come up with an explanation that is easier (at least in my opinion) to absorb.

Here goes.

So, How are Gravitational Waves Detected?

Here’s a diagram of a Michelson Interferometer, the device that the gravitational wave detector was based on.

Michelson interferometer with labels (c)Krishnavedala, CC BY-SA 4.0

The Michelson Interferometer works by splitting a laser beam into two halves with a beam splitter (a half-silvered mirror), and using regular mirrors to reflect the beams back so that they interfere. One of the mirrors (either M1 or M2) is adjustable to demonstrate the range of interference patterns. Since light travels in waves, the two light beams create an interference pattern when they meet each other at the detector.

The blue and green lines in the gif below represent two different light waves. The red line represents the resulting light wave.

Wave Interference (c)Wikimedia CommonsCC BY-SA 4.0

When the crests of one light wave align with the troughs of the other, there is no resulting interference pattern (figure 1). On the other end of the spectrum, when the crests of both light waves match up, the resulting light wave will be amplified (figure 2).

Changes in the resulting light wave directly correspond to changes in the interference patterns of the two initial light waves. But while someone must manually adjust the mirrors in a Michelson interferometer to change the interference pattern, gravitational waves warp the length of the interferometer arms in the LIGO interferometers that actually detect gravitational waves.

Wait, what? Gravitational waves WARP THE LENGTH OF PHYSICAL OBJECTS? Yes, in fact, they do. However, this warping is incredibly slight which is why you don’t see it happening in real life, and why LIGO Interferometers need to be so sensitive and precise.

So as a recap:

How is the LIGO Interferometer able to detect Gravitational Waves?

Here’s the breakdown:

  • When the LIGO interferometer is at rest, the two light beams cancel each other out (figure 1).
  • Because gravitational waves are ripples in space-time, they stretch or compress time and space by minuscule amounts wherever they go.
  • Gravitational waves stretch or compress one arm of the interferometer as it passes through, therefore changing the distance light travels. A laser beam travelling down a longer arm takes longer to meet with the other laser beam, changing the interference pattern. Thus, a resulting light can be seen.
  • Computers detect the emitted light and analyze the interference patterns to check whether a gravitational wave has been observed.

Unfortunately, because of the interferometer’s sensitivity, an earthquake or even a car driving near the interferometer can disturb the detection process. Thus, physicists compare results from interferometers all over the world to determine whether the discrepancy came from local disturbances or a gravitational wave. 

With all that being said, I hope that you walk away from this post with more comprehensive knowledge of complex physics and a greater appreciation for the world around us.

Until next time,
👋

 

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