WAVES – PHYSICAL CHARACTERISTICS
I will mention only a few physical characteristics of waves, which cannot by bypassed, as they are an important part of things, which make the universe.
A wave can be generally defined as an oscillation that propagates through a solid, liquid, air or space. Some waves can propagate only in a medium. An example is sound waves. They propagate through air, water and solid. All of these are mediums through which sound waves propagate. In this situation, the medium itself does not move but only the disturbance, which waves create, moves through the medium as a ripple. The best example is a wave created on the surface of water. If we have a leaf on the surface of this water, the leaf will move up and down the surface of the water as the wave passes beneath it but will not go away from this location as the medium, water in this case, does not move. As opposed to sound or water waves, which require a medium, electromagnetic waves do not require a medium and can travel in a vacuum. These waves are not created as a result of disturbances going through the medium, but as an oscillation of an electromagnetic field. As such, they go through an empty space unlike sound waves. So in space we will not be able to hear each other, as it is almost a vacuum with no medium. Light as electromagnetic waves, however, can travel through it.
Whether waves are sound, water or electromagnetic, they all have the same physical characteristics as follows:
Waves can travel as a singular wave, called a monochromatic wave, or as a number of waves, which are superimposed on each other. Examples are a light, which contains many different waves with different wavelengths, and frequency, which are superimposed on each other. This is called a spectrum.
Waves are composed of crests and troughs. Amplitude is the depth of a wave, or its precise size in length, from the base to the tip of the crest or trough of the wave.
Wavelength is the distance between the top of one crest to the next. Frequency is the number of waves in a unit of time.
The speed of a wave is calculated as a product of wavelength and its frequency. The speed of light is a constant and unchangeable. If we look at the equation of the speed of a wave which is:
Wave speed = wavelength x frequency
…then it is understandable that if frequency increases, the wavelength will decrease and vice versa. If we use numbers, it is easily demonstrated as follows:
20 = 20 x 1
20 = 10 x 2
20 = 5 x 4
When waves reach the surface, they can reflect. The angle of refraction is equal to the angle of incidence. They can enter the new medium at a different angle from the angle with which they strike the surface. This is called refraction. This is due to the change of the speed of the wave in a different medium.
A spectrum of light when it hits a prism separates into its colours, of which it is composed. This happens as waves with longer wavelengths (red) refract or bend more than shorter wavelength waves (from red going towards a violet colour).
One very important effect, which can be observed from the standing point when waves approach us or go away from us, is the so-called Doppler effect.
What that means is what I hope I will be able to explain in the next few paragraphs.
When waves travel through the space it has constant wavelength and frequency. We can observe this if we travel along the wave with the same speed, the wave goes next to us or if the wave goes from right to left or left to right in our vision field. We will also observe the constant, unchangeable wavelength of a particular wave if it comes to us from a source which does not move.
We can imagine a boat, which has its engine switched on, producing a wave on the surface of the sea. If this boat does not move, we will observe wave after wave reaching us with the same distance between waves. In other words, the same wavelength.
The situation will, however, very much change if the boat is moving towards us. In that situation, the wavelength we observe will be shorter. The reason for this is that the first wave that left the boat will be followed by the next wave which will leave the boat at a position closer to us than was the position of the first wave leaving the boat. It is because the boat is moving towards us. As a result, the wavelength of these waves reaching us will be shorter and the faster the boat moves towards us, the shorter the wavelength will become.
The opposite will happen if the boat is moving away from us. Similarly, the first wave will be followed by the next wave which will be created by the boat further away than the first wave was created as the boat has meanwhile moved away. The faster the boat moves away, the longer the wavelength will be.
The Doppler effect applies to all waves involving light. The light spectrum consists of superimposed waves with different wavelengths from a blue colour being the shorter wavelength towards a red colour having the longer wavelength. When light reaches us from stars, which are moving towards us, we see light moving more to a blue colour, as the waves we see are shorter due to the Doppler effect. If light reaches us from stars which move away, then light will be perceived as a red colour or towards a red colour. This is called blue or redshift respectively.
At the beginning of the 20th century, it was noticed that galaxies are seen to be in redshift spectrum, indicating their movements away from us.
In 1929, Edwin Hubble, an American astronomer, published a paper which demonstrated the relationship between redshift and the distance of galaxies observed, showing this relationship to be linear. It means that if a galaxy is twice as far away as another, its redshift is twice as large. Therefore, it was established that redshift is directly proportional to its distance.
The Doppler effect can tell us if stars are moving away or approaching. If we perceive light from a star as blueshifted on one side and redshifted on the other, that means that this star or celestial body we observe is rotating.
There is a very important part of study of light within physics called Spectroscopy.
Spectroscopy studies the relationship between matter and radiation energy. It is particularly important for astronomy as it helps to analyse the structure and composition of stars, which are far away and out of reach. It can help to determine the temperature of the star as well.
It was Gustav Kirchhoff, a German physicist, who made a significant contribution to the understanding of spectroscopy. He identified three types of light spectrum:
1.A continuous spectrum, which is produced by a hot solid object. He called it black body radiation in 1860.
2.Emission spectrum characterised by spectral lines produced by hot gas. These lines will be in light spectrum in the particular colour of the spectra depending on the energy levels of the atom in gas.
Basically, when gas is heated up then it gets energy impute or photons for its electrons to jump at higher energy levels or shells. Electrons then go back to ground state and emit light, which comes as a line within the spectrum. The lines are at a particular part of the spectrum, which corresponds to a specific energy level of this gas, or atom of this gas. Therefore, the composition of that gas or what elements are present there can easily be identified. (Oxygen will give a different line in the spectrum than nitrogen, for example.)
3.Absorption spectrum are black lines in the spectrum, which we can see when a continuous spectrum from a hot solid black body goes through cold gas which is between a black body which radiates continuous spectrum and us who observe. In this situation, the cold gas absorbs the part of the spectra that corresponds to the energy levels of the atom of the gas. In doing so, it leaves black lines on the spectrum as this part of the spectrum is now missing, being absorbed by gas.
Absorption spectrum can help us to identify what sort of elements are present on the surface of the Sun, for example. Namely, the core of the Sun, which is very hot, emits a continual spectrum of radiation which goes through gas on the surface of the Sun on its way to us. Also, the surface of the Sun is hot; it is cooler compared to its core. The g
as or elements making its gas absorb part of the spectrum of this radiation in a particular place which corresponds to a particular energetic level of the atom of a particular element. These will appear as black lines in the spectrum as part of this spectrum is missing. It is absorbed by atoms where election jumps to a higher energy level. This is specific for every element which is helpful to identify the composition of stars despite their distance, which is out of our reach.
At this point, I should be elaborating on a particular kind of radiation, which comes from black body. It is important as thanks to this type of radiation Albert Einstein and Max Planck have come to the idea that energy carried by electromagnetic waves is delivered in packets of energy called photons. Precisely, Einstein came to this idea when he studied the photoelectric effect, while Max Planck came to this idea when studying black body radiation. This opens a new chapter in physics where quantum mechanics was developed which enhances progress in science and technology.
Black body radiation was also crucial in our understanding of the origin of the universe. It was thanks to the physical behaviour of black body radiation that predictions were made about microwave background radiation whose existence supports the Big Bang theory. It was George Gamow who predicted cosmic microwave background radiation in 1948, but it was not discovered before 1965 when this finally consolidated the Big Bang model as the correct theory of universe origin.
I will leave this for now, as it would be more appropriate to talk about black body radiation within a chapter where I try to explain or rather give my version of how I understand the Big Bang theory from the point of view of a layman such as I.
Finally, just to touch once more on conservation of energy and mass in light of mass energy equivalence as expressed by Einstein’s equation:
E = mc2
At the end of the first chapter, I outlined that mass can be changed into energy and vice versa using an example of binding energy. In such cases, we are talking about a slight variation of the mass or mass reduction. However c squared is a big number, which shows that even with a very small amount of conversion of matter to energy, we get release of a huge quantity of energy. That is why release of nuclear energy is so enormous and powerful.
In the case of fusion of hydrogen nucleus in nucleus of helium, the binding energy, which was released from the fusion of protons, is so powerful outward that it balances strong gravitational forces of the enormous mass of a star which moves inwards. It is the reason the star is not collapsing on itself. The release of energy in nuclear fusion will continue to take place until iron is created. Any element after iron cannot be created by nuclear fusion within the star as in this process we now need to bring energy in for a new heavier atom to be created.
In a process of fusion, where the new element is created, the mass of nucleus of the new element or atom is slightly smaller than the sum of masses of nuclei of atoms of elements which were fused. This is due to binding energy, which is released when fusion takes place and which comes from reduction of masses of atoms involved in fusion. This process is repeated until iron is formed. From this moment, a further fusion process cannot take place, as energy needed to fuse these iron atoms is higher than binding energy relisted to bind protons and neutrons in an atomic nucleus. The mass of the newly created atomic nucleus will now be higher than the sum of the masses of the iron nuclei that took part in nuclear fusion. In that sense, the energy would not be released but it will be absorbed in order for further fusion to take place. That is why fusion of iron in a star is the moment when the star comes to the end of its own existence.
To go back to the law of mass-energy conservation, it can be stated that mass-energy is constant in the universe where it is possible to achieve some slight changes in a mass where some energy can be released from the mass, reducing the amount of mass but increasing energy which can be again incorporated or absorbed by mass locking this energy away in mass by increasing this mass again, but altogether giving a constant amount of mass-energy within an isolated and closed system.
As already mentioned, only a very small amount of mass can be converted to energy unless we have matter combined with antimatter where all mass involved is converted into energy.
4
DARK MATTER
Dark matter makes up about 23% of the universe. Its existence is confirmed by an indirect method as dark matter does not absorb or reflect light and, as such, cannot be seen. It does have a mass and, as such, it is affected by gravitational forces as well as exerting this force. It is thanks to this that the existence of dark matter was identified.
Before going to this it would be good to explain how we see things around us.
First of all, we see nothing in a closed dark room with no light on. To see things, we need light or photons. But what is specific in light that allows us to see things around us?
To answer this, just recall that light is composed of various electromagnetic waves with a different wavelength, which are superimposed on each other. If we defined only one line of electromagnetic radiation, which has a particular wavelength, for instance, length, which is in a spectrum or red colour, as one light beam, then daylight is composed of various beams superimposed on each other in one light beam. Each such light beam has a different wavelength from the shortest one within a visible light spectrum which is violet colour and blue colour and going via blue, green, orange, yellow, to red colour with the longest wavelength within the visible spectrum of light.
We know again that electromagnetic waves are delivered in packages called photons. Travelling at the speed of light, photons after photons are experienced as a continuum in the way of a light beam. It is the same as a jet of water where so many droplets of water go at such a speed one after another that this is experienced in a continuum in the way of a water jet.
One chunk of beam which corresponds to the size of a photon has insight itself superimposed various number of photons, with each one having a specific wavelength representing a particular colour of the light within a visible spectrum of the light. We know that many photons, which are superimposed on each other, can do that as they are bosons and, as such, are not subject to the Pauli principle of exclusion. In other words, it can be as many as possible number of photons at that same place and the same time.
We are now coming closer to understanding how we see things around us.
When beams of light or plenty of superimposed photons heat the surface of an opaque material, then plenty of these photons are absorbed by opaque material. How many and what kind of photons will be absorbed depends on the atomic structure of a particular material. If the atomic structure of material has electrons arranged in shells around the atom in such a way that there is the possibility left for electrons to absorb photons with a particular wavelength (say red colour), to jump on the outer shell, then that photon will be absorbed. If the atomic structure of an atom allows photons of all colours to be absorbed except the green colour, then that photon will bounce off or reflect and come to our eyes. As all the other colours are absorbed, we will detect this material or object as green-coloured, having only the photon of this colour being reflected and coming to our eyes. If all light is absorbed, we will detect this object or see it as black- coloured. If the colour of all light is reflected back, we will see this object as white-coloured. Fully opaque objects are mirror and black body . A mirror completely reflects light while a black body completely absorbs light.
Glass, however, has an atomic structure where the arrangement of electrons does not allow any space for a possible absorption of any light photon as there is no outer shell on which to jump, where a photon could be absorbed. It does allow photons of light to go through it completely with no interference or no scattering, making glass transparent.
Photons can scatter or hit or collide with matter and free electrons when they come into contact with them. If photons, which come in matter, scatter with electrons within the matter and
do not come out, then this material is perceived as opaque. That was the case with the universe when the temperature was so high that matter existed in a plasma state with protons and electrons being separated. The electrons were moving at relativistic speed, close to the speed of light. They were on their way to photons, causing them to collide with the electrons or scatter all the time. That is why the universe was opaque then. When the temperature reduced to about 3000 K around 38000 years after the Big Bang, then the speed of electrons reduced enough for electrons to be captured by the atomic nucleus in atoms. Photons did not scatter with electrons any more and were free to go away in all directions. The universe then became transparent.
Photons do not scatter or hit each other as in this case we would not be able to see.
Obviously, the above explains physical phenomena as to why we can see things as we see; it would not be enough if we did not have eyes. Trees cannot see as plants do not have eyes.
Dark matter does not absorb, does not reflect light and it does not have any interaction with ordinary matter. That is why it cannot be seen. It can, however, be detected indirectly due to the mass it possesses which is subject to gravitational force.
Journey Through Time Page 8
