Imagine two pianists about 680 meters apart playing the same key for a moment at exactly the same time and repeating it every second. A third person, a listener, who is near one of the pianos, hears a first chord shortly after a musician presses the key. You will hear the sound of the other piano later because it takes a little longer for the sound of the further away piano to reach your ears.
Imagine the listener moving until they hear the sound of the two pianos at exactly the same time. He will notice this because he will hear a single note at the same time with greater intensity. If the listener could accurately measure this change in intensity and simultaneity, he could find himself at a point where both would reach their maximum. At this moment you could claim that you are the same distance from the two pianos. For example, it could be 340 meters away from everyone, with the sound of each piano taking one second to reach it (which corresponds to a speed of sound in air of 340 meters per second).
Once this is understood, it is easy to identify different possibilities from here. A listener located at a greater but equal distance from each musician would still hear the two sounds at the same time, although they would have taken longer to reach and would have been heard more quietly. Another listener could be twice as far away from one piano as from another (or three times, four times, etc.) and would hear a sound with greater intensity, but not at the same moment, but once before and once after.
In physics it is said that the union of these two repeating sounds, which constitute a wave in interference, has maximum coherence when the waves coincide exactly and are connected in a way that we call constructive. The listener receives the two tones simultaneously, and we say that the received waves are in phase.
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An individual listener cannot distinguish whether he is at the same distance from each musician or at a distance that is an integer multiple of the other, unless he knows the original power of each piano and can measure the power received. But if we had multiple listeners spread out all over the place and they could talk to each other and know exactly when each was receiving the two sounds and what their coherence was, then we could pinpoint the position of the pianos. The more listeners, the more precise the positioning would be. The further apart the listeners are from each other the better, because if they are very close together they will hear more or less the same thing at the same time and there won’t be much additional information.
Imagine not two pianos, but entire orchestras in two different locations, each playing multiple, even different, notes. By examining the coherence of sound in many places, the position of all instruments can be determined. To do this, the listeners must be perfectly synchronized, have a very precise common time reference and be able to compare sounds and times as perfectly as possible.
This section is about astrophysics, so let’s take the cosmic leap. The musicians could be various components of a distant galaxy, such as a supermassive black hole that ejects jets of matter at speeds close to the speed of light. These jets of matter usually contain large numbers of electrons that slow down and emit waves. These are not waves created by the vibration of air molecules or by the impulse created when playing a piano key at a frequency of once per second, but rather the electrons cause a periodic variation of the electromagnetic field at frequencies typically in on the order of ten or hundreds of billions of times per second. They would be so-called electromagnetic waves, commonly known as light.
The audience would be astronomers with multiple telescopes observing the star at the same time. Imagine a telescope in Grenada and another in Hawaii observing this distant galaxy. The telescopes should be connected to each other and have extremely precise timing control. If we consider light as a particle, and the light reaches both telescopes at the same time (or almost, one observatory will be closer to the galaxy than the other), we can assume that one photon arrived in Granada and another independent one in Hawaii. But we can view light as a single wave, so we would have to interpret that what reaches Hawaii and Grenada is the same photon (just as two listeners would hear the same sound), and use detection by two telescopes to learn more about it to find out how and where it was created.
It would involve studying the coherence of the waves that reach each telescope using a device called a correlator that compares the signals from each observatory, each radio telescope, to look for this coherence that provides spatial information about the star, which we have observed. These are telescopes that supposedly carry out interferometry of electromagnetic waves. Correlators must compare on the order of thousands of electromagnetic waves (each quite complex, not a single pulse like in our piano analogy) hundreds or billions of times per second; The computing power required (and the mathematical and technological tricks) are considerable.
Interferometric radio telescopes are relatively small telescopes, but in large numbers and separated from each other (but synchronized) by a certain distance. They allow us to obtain information, especially spatial, equivalent to a gigantic telescope of up to thousands of kilometers. The more telescopes observe at the same time and the further apart they are (perhaps on the moon at some point!), the greater the accuracy. Like in the example of pianos.
A century ago, interferometry made it possible to measure the radius of a star like Betelgeuse. More recently, interferometry has also achieved the spectacular feat of providing images of supermassive black holes, the core of the M87 galaxy and our Milky Way, more specifically the matter that surrounds them and how the magnetic fields they create around the event horizon around determine the orbits of this material. And interferometry of electromagnetic waves (produced by powerful lasers) is the basis for gravitational wave telescopes and their initial discovery in 2015, which has opened a new window for exploring the cosmos.
We owe all this (and in addition, interferometry is used in biology, medicine, etc.) to the better understanding of the nature of light and the progress in fundamental science in the search for insights into the (physical) foundations of reality. Today we cannot decide whether light is a collection of indivisible particles (massless!), like those of Democritus, Newton or Einstein, or waves, as Descartes, Huygens and finally and in much more detail Louis de Broglie told us. , 100 years that will be fulfilled next year. We also owe it to coherence, something so unused in society and that can even have light. Although it shouldn’t be idolized, it really does give a lot of information about the universe (and people).
Cosmic emptiness It is a section in which our knowledge of the universe is presented qualitatively and quantitatively. The aim is to explain the importance of understanding the cosmos not only from a scientific perspective, but also from a philosophical, social and economic perspective. The name “cosmic vacuum” refers to the fact that the universe is mostly empty and there is less than one atom per cubic meter, although paradoxically in our environment there are trillions of atoms per meter cubic, which invites us to wonder about our existence and to reflect on the presence of life in the universe. The section is composed Pablo G. Pérez GonzálezResearchers at the Astrobiology Center, and Eva VillaverResearch Professor at the Institute of Astrophysics of the Canary Islands.
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