ENLIGHTENED: HOW LIGHT HELPS US UNDERSTAND THE COSMOS

Table of Contents

Introduction

The universe is huge, and despite its tremendous size, we know a lot about it. We can say that the whole universe is expanding, we can identify the chemical composition of objects very far away from us in space without ever going there, we can determine whether objects extremely far away from us are rotating and/or revolving, and so much more. And we can do a lot of these things thanks to light. Today is the international day of light, and to celebrate its importance in our lives let us dive deep into understanding how light enables us to explore and understand the universe.

The basics of light

To understand how light helps us expand our knowledge of the universe, let us first try to understand light itself. Through hundreds of years of theoretical and experimental developments in physics, physicists concluded that light is electromagnetic radiation that behaves both like waves and particles (photons) of electromagnetic field traveling through space (Beiser, 2002). Like all waves, light has a wavelength and speed. Wavelength is the distance between two peaks of a wave and its speed is the rate at which it travels through space. Through experimentation and the assumption that light travels with the same speed in all directions, we know that its speed whilst traveling in a vacuum is approximately 299,792,458 meters per second (Veritasium, 2020). The speed of light changes as it travels through denser media; this can be observed by noticing how the angle of a straight straw changes when we place it in a clear container filled with water. Simple calculations of speed and wavelength allow us to calculate other important physical properties of light like its frequency and energy.

Figure 1

Light Wave

Note by Wikimedia Commons, n.d.-c, an illustration explaining various components of light

Using the aforementioned fundamental properties of the light wave, we can determine a lot of things about its nature. The range of all different wavelengths of light is represented by the electromagnetic spectrum, where visible light only occupies a small portion, i.e., approximately 400 to 700 nanometers. A Light wave’s wavelength is inversely proportional to its frequency and energy. EM radiation with a shorter wavelength than visible light is called ultra-violet, while one with a larger wavelength is called infrared. Figure 2 classifies different blocks of the electromagnetic in higher detail and provides colloquial terminology used for these blocks. This huge spectrum of invisible light helps us understand the universe more and communicate the information we obtain at literal lightning speeds.

Figure 2

EM spectrum properties

Note By Wikimedia Commons, n.d.-a, an illustration explaining different types of EM waves

Light and Communication Systems

The collection and sharing of information are of the most essential aspects of any form of scientific development, as they are critical to collaboration. Modern wireless technologies have helped us develop communication systems like the internet and telephone, which have increased the pace and quality of communication to unprecedented levels. And we were able to come about such systems through our increased understanding of light.

As mentioned before, although only a small portion of the EM spectrum is visible to us, EM waves come in all sizes of wavelengths that have wide applications. Radio waves are EM radiation of the largest wavelengths, i.e., more than 10 cm. They can travel over large distances and overcome physical obstacles, hence we can use them for transmitting information by building systems that can produce and receive different types of radio waves. This technology was first implemented in the telegraph system which could send binary information (in morse code) over large distances. Over time, with scientific research and sophisticated engineering, systems that can send more complex types of information have been designed. AM and FM are among the most common types of communication systems we use. AM refers to Amplitude Modulation; this system sends and receives information through changes made in the amplitude of an EM wave of a specific wavelength or frequency. FM refers to Frequency Modulation; this system sends and receives information via modulating the wavelength of EM waves over a small range. FM radio waves relatively pick up less noise and have higher quality, while, AM radio waves can travel over larger distances and can even reflect off the atmosphere. Higher frequency radio waves are used in technologies that require higher transfer rates and bandwidth like Wi-Fi and cellular data networks (Giancoli, 2014). Hence it is safe to say almost all modern communication is possible due to Radio wave technologies.

The Redshift and the Expanding Universe

Modern communication methods like Radio waves have enabled us to send our satellites up into space and observe the Cosmos without the atmospheric filter of earth. This has helped us make many staggering discoveries about our universe, one of the most essential ones being the Redshift observed in the cosmos, or in other words, the fact that our universe is expanding, which consequently has helped develop the big bang theory.

To understand the Redshift properly, we first need to understand another fundamental property of traveling waves. As waves move away or get closer to a stationary observer, the observer notices an enlargement or shortening of that wave. This is called the doppler effect. One can observe these changes in sound waves while noticing the pitch of the sound of a fast car as it passes by them. Light behaves similarly. As light moves away or gets closer to a relatively stationary observer, its wavelength increases or decreases respectively. Hence, as objects emitting light move away from an observer, the light appears to be more reddish, similarly, as they get closer it appears more bluish. This shortening and enlargement of EM waves are respectively called Blueshift or Redshift.

Figure 3

Doppler effect animation

Note By Flipping Physics, n.d., an animation representing the Doppler effect. The change in wavelength in sound implies a change in the pitch of that sound. In light, the same phenomenon leads to a change in color.

The concept of Redshift revolutionized our understanding of the universe as astronomers started observing the light from stars and galaxies very far away from us and observed that the light emitted from these astronomical bodies is Redshifted. Moreover, the Redshift observed was proportional to the distance from the astronomical object, which led to the conclusion that our universe is expanding. This observation later evolved into the big bang theory, which states that all the universe was enclosed in a small extremely hot amount of space that started expanding after a huge explosion, like a bang, hence the name ‘Big Bang'.

This model about the nature of the universe was later confirmed through some astronomical observations. (Nobel Media, n.d.)

Spectroscopy

Our telescopes and satellites can look deep into space and record the astronomical bodies, the redshift can tell us how the universe is expanding, but there’s another question that anyone who wishes to observe the universe would ask. What are stars, planets, and other matter in space made of? As you may have guessed at this point, light will help us again answer this question, through spectroscopy.

To understand how spectroscopy works one needs to dive deep into the atomic scale. We generally think of atoms analogous to a solar system,i.e., a nucleus in the middle (like the sun), and electrons revolving around it. However, the actual model, especially the movement of the electrons is much more complex. To understand spectroscopy, one can use an alternate simple model. The electrons are allowed to occupy very specific volumes around the nucleus of an atom. And these specific volumes are different for different elements. One can think of these specific volumes as steps, and an electron can move up and down these steps by absorbing or emitting energy, respectively. This energy is emitted or absorbed by the atom in the form of light. And since the steps are different for each element, the light it emits can be used as an identifier of that element. (Figure 4 can help one understand this phenomenon better)

Figure 4

Emission of Light

Note By Encyclopædia Britannica, 2011, an illustration depicting emission of light as an electron moves to a lower energy level (or step according to our analogy)

A spectrometer is a device that uses the same concept to identify the composition of matter based on the light it emits. This device separates the EM radiation emitted by an object and provides a spectrum of its electromagnetic frequencies, that can be used to identify its composition. When used in telescopes, spectrometers can detect the composition of astronomical objects.

Figure 5

Emission Spectrum of Hydrogen

Note By Wikimedia Commons, n.d.-b, showing Emission Spectrum of Hydrogen. If a spectrometer-enabled telescope retains this while looking at an in space, one can conclude that the object is made up of Hydrogen.

Conclusion

The study of light has helped us uncover the nature of our universe and study its origins. By meticulously observing the light released by astronomical objects, we can identify the composition of matter far away from our physical reach. We have developed advanced techniques to obtain more information, for instance, about the rotation of an astronomical body and the revolution of any planets or moons around it, by the light it emits. Moreover, a lot of historical information we obtain about space also comes from the light from distant matter, this is because the light that reaches us is older. In a way, light acts like our own time machine into the past.

There are many other ways in which light plays a key role in understanding the world around us, that are beyond the scope of this article. However, we can evidently say that we understand the Cosmos because we understand light and everything we know about it, we owe to light.