Testing Quantum Gravity with Merging Black Holes

2.9 billion years ago, when life on Earth was still in its infancy, two black holes merged and sent ripples of gravitational waves throughout the universe. Only now have astronomers been able to detect these ripples, vibrations less than that of an atom’s nucleus, using lasers and mirrors. It’s the third detection of such an event, expanding the field of gravitational astronomy.

These waves, named GW170104, were a result of two stellar-mass black holes, one 19.4 solar masses, and the other 31.2 solar masses, merging to create a 48.7 solar mass black hole and releasing two solar masses worth of energy in gravitational waves. Similar to the previously detected mergers, having a third event proves that a stellar mass black hole of more than 20 solar masses can exist. Additionally, the event provides evidence of a medium black hole or a black hole between the stellar mass size and the supermassive size.

Furthermore, this black hole merger occurred twice as far as any other detected before it, nearly 3 billion light-years away. The distance is important because it allows astronomers perform new tests of Einstein’s theories regarding gravitons. A graviton is a name for the force between particles in gravity studies, like gluons in nuclear physics, photons in electromagnetism, and field quanta in quantum theory. Because, in general, gravity is a weak force, we haven’t had a chance to observe gravitons. However, the theory of relativity predicts that gravitons will be massless, like photons, and therefore should spread at the speed of light.

The initial and final masses of known black hole mergers. / Credit: LIGO/Caltech/Sonoma State (Aurore Simonnet)
The gravitational waves of three confirmed black hole mergers, and one tentative merger. / LIGO/B. Farr (U. Chicago)

A theory that can be tested by observing their dispersion properties. When waves are coming from the same source travel at varying speeds, this is known as their dispersion. An example of dispersion is the refraction of atmospheric water through sunlight, creating a rainbow. The different waves of light travel at different speeds through the water and the visual effect of this the breaking of white light into colors of the rainbow. The measurement of a waves dispersion can tell scientists many things about its properties or the make up of the space it traveled through. For example, light that’s traveled through charged particles will show dispersion because of the strong interaction of plasma and light, but it will show no dispersion if it has traveled through unionized gas, which it does not react with.

So, based on the Einstein’s prediction, gravitons, if they exist, would be massless and weak, thus should not disperse while making the 3 billion light year journey through space. Luckily, the theories of general relativity stand, yet again, since the gravitational waves from this third detected black hole merger shows no signs of dispersion. Although this latest detection is a boon for gravitational astronomy, there is still much more work needed. As of right now, there are not enough detectors to pinpoint where these mergers are happening, and therefore they cannot be connected with any visual observations that can be made from watching the skies. The more information that could be gathered through new detectors would increase our knowledge and understanding of black hole activity and therefore test the limits of general relativity more. The new and expanding field of gravitational astronomy may have just begun but is already making its mark with new detections like this latest one.

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