Waves From the Universe: Scientists Uncover a New Way to Detect Gravitational Waves

While it’s a common misconception that waves are patterns confined only to the ocean, many different kinds of waves exist; there are waves that can occur in the fabric of space-time itself. Gravitational waves are undulations in spacetime that were first predicted by physicist Albert Einstein. Einstein showed that massive and accelerating objects, such as binary black holes, would alter space so that waves of space-time—defined as the continuum consisting of the dimensions of space and time—moving at the speed of light would ripple from the source. Currently, gravitational waves have been observed at high and ultra-low frequencies measured in units of Hertz by instruments such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), but not at middle-range frequencies. However, scientists have recently found a new way to detect gravitational waves in this middle range, known as the milli-Hertz frequency range. The milli-Hertz frequency range can be more than 3,000 times smaller than the lowest frequency range of the electromagnetic spectrum, radio waves. In order to avoid interference from noise from Earth’s natural vibration and gravitational field, programs to explore this frequency range have entailed going to space, but this method would avoid that as a result of the detector’s small size. 

A group of physicists led by Giovanni Barontini published this discovery in the scientific journal Classical and Quantum Gravity. The researchers, from the University of Birmingham and University of Sussex, have put forth a new method of detecting gravitational waves through optical cavity and atomic clock technologies, which can sense gravitational waves in this range. Optical atomic clocks are extremely accurate clocks that use optical cavities, a cell of mirrors with light ricocheting between them, to keep their lasers stable. At this specific frequency range, a detector could discover signals from various cosmological events such as merging black holes and white dwarf binaries. White dwarf binaries consist of two stars that orbit each other: a star like our sun and a white dwarf, which is a star that has run out of fuel and condensed to become smaller and denser. This would give researchers vital information about the mechanisms of our universe. 

The potential detector would take advantage of optical resonator technology that was created for atomic clocks to increase their precision. This technology takes advantage of optical cavities in order to stabilize the lasers of the atomic clock and can calculate very small phase shifts in laser light from gravitational waves. By measuring the phase difference of the laser beams, which is based on changes in the path length (the distance the laser travels when a gravitational wave passes by), gravitational waves can be detected. When a gravitational wave passes by, the phase difference of the lasers is changed, which is a measure of the effect of the gravitational wave, and this phase difference is measured, resulting in the detection of a gravitational wave. Each detector would be made up of two ultrastable optical cavities and an atomic frequency reference (a device that delivers a reliable frequency signal), which would allow for gravitational wave detection using multiple measurements of the phase difference. Optical cavities are often made of two highly-reflective mirrors that are separated by a spacer composed of ultralow expansion material and work well for sensing gravitational waves in the milli-Hertz range because of their stability. When a gravitational wave passes by the cavity, it alters the space-time curvature between the mirrors, changing the path that the light travels. However, the distance between the mirrors stays the same. Because of this, optical cavities can be used to measure how the path of the light between the two mirrors differs from the change in space-time curvature. Essentially, a gravitational wave will very slightly change the path of the light, so a measurement of this change will suggest a gravitational wave has passed by Earth. To carry out this method, there could be a network of detectors dispersed globally, and if two or more detectors sense gravitational waves, it would indicate a positive detection of a gravitational wave.

Several other methods of gravitational wave detection are currently being implemented. For example, LIGO is the land-based interferometer that detected gravitational waves for the first time. Interferometers are instruments that combine light sources, creating an interference pattern that provides information about what is being investigated. LIGO is the largest gravitational wave detector in the world. It is made up of two detectors, each with two steel vacuum tubes in the shape of an “L” that a laser travels through. Similar to the proposed detector, LIGO uses the phase difference of the laser to detect gravitational waves. Another method of finding gravitational waves is pulsar timing arrays, which can sense gravitational waves at ultra-low frequencies. Pulsar timing arrays are similar to interferometers like LIGO, except they utilize a pulsar’s electromagnetic pulse as opposed to a laser. Pulsars are the cores of neutron stars that collapsed, which, from Earth, appear to have a pulse that can be detected by radio telescopes. 

Stationing interferometers in space presents another method to detect gravitational waves in the milli-Hertz range. Technologies aboard space missions like the Laser Interferometer Space Antenna (LISA), which won’t begin until the 2030s, and Sun Yat-sen University’s TianQuin, which is also expected to launch in the 2030s, could locate gravitational waves from sources like neutron stars, binaries of white dwarfs, stellar-mass black holes, and more, but such projects take time to develop. LISA consists of three spacecraft that will form a triangle when in space, with each spacecraft being millions of miles away from the others. It will then measure how the distance between the spacecraft changes due to gravitational waves. This will allow LISA to obtain more precise information about those waves, such as their amplitude and direction. Additionally, the objects LISA would be able to find in this frequency range are heavier in wider orbits than those observed by LIGO, allowing for a greater quantity of objects to be detected. 

Unlike space missions like LISA and TianQuin, the University of Birmingham and Sussex’s proposed detector would be compact and able to be used immediately upon creation. By using these detectors, scientists would be able to uncover more information about these sources without having to wait for missions like LISA and TianQuin. The proposed sensor would be able to find objects like binary stars or merging black holes, helping scientists learn more about these mysterious cosmic events. Binary star mergers can lead to high-energy events such as gamma-ray bursts, and gravitational wave detectors have the potential to warn of these mergers, predicting such events. Although these instruments aren’t currently a reality, they serve as an example for future projects. In the future, gravitational waves could also potentially be detected at lower frequencies by combining the detectors with current atomic clock networks.