Interferometers are research instruments used in a wide variety of scientific and technical disciplines. As a result of their ability to combine two or more light sources to produce an interference pattern that can be measured and studied, they are known as interferometers. Interferometers produce interference patterns that reveal details about the object or phenomenon under investigation. They are often used to take incredibly small measurements that are impossible to do otherwise. LIGO's interferometers are very effective at detecting gravitational waves, as a result of their ability to measure a distance of 1/10.000th the width of a proton!
From the mid to late 1800s, many scientists, including Hippolyte Fizeau, Martin Hoek, Éleuthère Mascart, George Biddell Airy, and Eduard Ketteler, believed that light could be used in a variety of media, particularly moving media (air and water were the main media used for this, followed by others). He invented interferometers to measure speed. (like running water). This work was a component of research to understand how the medium through which light passes affects the wave properties of light.
Scientists of the time suggested the existence of the "luminous ether," an amorphous substance that permeates everything and only serves as a medium for the propagation of light waves. This idea was supported by the notion that all waves need a medium to propagate. This hypothesis was the main hypothesis.
Interferometer-based experimental testing on this hypothesis primarily focused on it. American physicists Albert Michelson and Edward Morley, who created the Michelson-Morley Interferometer, were among the international experts working on this problem. The results of their tests, published in 1887, are cited as the first concrete experimental evidence against the existence of ether, that light (actually all electromagnetic radiation) propagates without a medium, namely in vacuum. This finding, one of the cornerstones of Einstein's special and later general theories of relativity, allowed him to trace the curvature of space-time by describing the vacuum paths through which light travels.
The proprietary interferometric setup used by Michelson and Morley was considered a natural fit for the detection of gravitational wave strain in space-time in the late 1960s, almost a century later, given the precise measurement of the phase change of light traveling along two perpendicular arms. Consequently, the optical arrangement of the Michelson interferometer is an essential component of all current gravitational wave interferometric detectors, including LIGO.
What kind of device is an interferometer?
Interferometers are available in a variety of sizes and formats due to their wide range of applications. They're used to measure everything from tiny changes in an organism's surface to the structure of vast fields of gas and dust in distant regions of the Universe, and are now used to find gravitational waves. All interferometers have one thing in common: whatever their unique shape or use, they superimpose light rays to create an interference pattern. On the right is a picture of a Michelson laser interferometer in its basic configuration. A photodetector (black dot), a laser, a beam splitter, a set of reflectors and a photodetector are all components.
What is the Interference pattern in Physics?
To understand how interferometers work, it's helpful to have a deeper understanding of "interference". The interference is familiar to anyone who has thrown a stone into a flat, glassy pond or pool and observed the results. When stones hit the ocean, concentric waves are created that move away from the source. Also, these concentric waves collide with each other where two or more of them intersect. The result of this interference may be an ascending wave, a descending wave, or no wave at all. It is only an "interference" pattern that can be seen where the waves collide.
The basics of entanglement are easy to understand. At least two waves converge. The resulting wave is the "interference" pattern created by adding the heights of the waves in contact. Total constructive interference and total destructive interference are two different types of interference shown in the picture to the right. The perfect intersection of the crests and troughs of two (or more) waves results in total positive interference. When two smaller waves add up, the size of the larger wave is equal to the sum of the heights (and depths!) of the two waves at each location where they physically interact. Total destructive interference occurs when the crests of several waves coincide with the troughs of a single wave.
When they come together, they balance each other, or in other words, they "destroy" each other.
In nature, the crests and troughs of one wave do not always coincide exactly with those of the other wave. Conveniently, the height of the wave due to interference is always equal to the sum of the heights of the converging waves along each point with which they physically interact, no matter how synchronized they are when they converge. Therefore, partially constructive or destructive interference can occur when the waves collide slightly out of phase.
The black wave is produced by adding the heights/depths of each wave at each location as they pass through each other. A full height range is experienced from twice as high/deep (total constructive interference) to flat. (total destructive interference). The interference pattern can be seen in this example as a dark wave. (pattern resulting from continued interference of red and blue wave). Observe how it changes as the red and blue waves interact.
What are the Similarities with Light?
Luckily, light waves behave exactly like water waves. Depending on how well the light waves align when they come together, two beams of laser light can also create an interference pattern. Similar to water, fully destructive interference occurs when the peaks of the waves of one beam exactly meet the troughs of the other. As a result, there are no waves in the water. There is no light in light after all! On the other hand, full constructive interference occurs when the vertices of the two rays exactly coincide.
Again, the height of the wave emerging in the water is equal to the sum of the heights of the two waves; in the case of light, the result is a beam of light of intensity equal to the sum of the intensities of the two separate rays. If we use this comparison to its logical conclusion, waves in water can interact with each other in various ways, causing both constructive and destructive effects. (bigger wave, smaller wave, no wave). The result is a full spectrum of luminosity, ranging from total beam intensity in light to complete darkness.
Going back to LIGO's interferometers, the distance the beams travel before they merge determines how well the beams align when they merge. The light waves from the beams will align exactly and if they travel the same distance they will cause complete destructive interference. However, if for some reason the lasers do not travel the same distances, the light waves will no longer be in phase as they merge, resulting in either no light reaching the photodetector or only a slightly brighter light than the first laser beam. If the arms are lengthening or shortening over time, the rays experience various interferences depending on how they come together at any given time, which causes vibration.
How does LIGO's interferometer respond to gravitational waves?
As a result of gravitational waves, space stretches in one direction and simultaneously contracts in the opposite direction. As long as he's kidding, LIGO experiences this as one arm of the interferometer lengthening and the other shortening, then vice versa, back and forth. Because the arms change length simultaneously in opposite directions or differentially, this movement is technically referred to as the “Differential Lever” movement or differential displacement.
As mentioned earlier, the distance traveled by each laser beam varies according to the lengths of the arms. The scenario changes as the arms alternate between being longer and shorter; the beam in a shorter arm reaches the beam splitter before the beam in a longer arm. Because they arrive at different times, the light waves no longer combine well in the beam splitter. Instead, as they coalesce, they move or align in and out of phase, causing the wave lengths to oscillate. Simply put, this causes a flash of light to come out of the interferometer.
While the concept may seem almost simple in theory, in reality this vibration is not easy to spot. The effect of a gravitational wave on limb length can be as small as 1/10.000 the width of a proton, or as little as 10-19 meters! Also, discovering a gravitational wave tremor amongst all the other tremors LIGO has encountered (caused by earthquakes or anything that could shake the mirrors like traffic on local roads) is a different story. The detailed filtering process that LIGO uses to find the distinctive light "flicker" produced by the gravitational wave is described in LIGO Technology.
📩 31/03/2023 12:36