1. Particle Age

SOCRATES: The Brahmaputra originates on the northern slopes of the Himalayas from some glaciers like Chemayungdung and Angsi, which are only 71 km east of Manas Sarovar. Many rivers emerge from these glaciers and merge into one stream to form the Tsangpo River. As difficult as it is to find the source of a river, it is at least 24 times more difficult to find the source of the universe.

RABI: I understand that you want to focus only on Angsi to make things easier. But simplifying the description of the particle age of the universe will not be so straightforward. This cannot be understood without understanding the standard model of particle physics mathematically.

SOCRATES: The ancient Indians said that Brahma is the foundation of the universe, Manas Sarovar is created from the mind of Brahma, and Brahmaputra is the son of Brahma. Since Juno could not understand the mind of Brahma after so many visits to Manas Sarovar, I don’t think we can understand the mathematical form of the Standard Model, no matter how much we talk to you.

JUNO: With that out of the way, everyone, let’s go to the supposed source of the Brahmaputra. Sitting on the ice and talking will hopefully reduce Rabi’s fire a bit.

SOCRATES: Where is this ‘supposed’ source?

HERMES: Latitude 30.348, Longitude 82.045, meaning 30 degrees north of the equator, and 82 degrees east of the prime meridian passing through Greenwich, England. It should be remembered that the distance from the equator to the north pole above is 90 degrees, and if you go around the whole earth along the equator from the prime meridian and return to Greenwich, a total distance of 360 degrees is covered. So 30 degrees is one-third of ninety degrees, and eighty-two degrees is about one-fourth of 360 degrees.

[Everyone flies to the source of the Angsi River while listening to Hermes’ useless speech. Sitting on the ice on the banks of Angsi, everyone listens to Rabi about the particle age.]

SOCRATES: The big bang theory, the standard model of cosmology, says that our universe, meaning all of us, came from an almost infinitely small point. Do you remember the comedy of Borges with two point-dwellers? From a point about fourteen billion years ago, spacetime began to expand, an event called the Big Bang. This expansion is still going on due to the explosive energy of the Big Bang, our universe is still getting bigger day by day. But when I go to tell this to the gods of Olympus, they ask, what was before this big bang or how did this big bang happen? If the gods ask such questions, what will happen to mortals?

RABI: If spacetime was born at the time of the Big Bang, then the question of what was before, or beyond, the Big Bang is meaningless. But the question is, can there really be no spacetime except our spacetime created from the Big Bang? Could it even be that nature is actually a multiverse with many or an infinite number of four-dimensional universes? Is there nothing beyond the four dimensions we are thinking of with three dimensions of space and one dimension of time? Could our four dimensions be born from another world of higher dimensions? We still don’t know the answer to these, many mathematical theories have been made as an attempt to know, but which theory or model is correct has not yet been proven through observation. It is very common to use many dimensions in string theory.

SOCRATES: I understand that we know what happened after the Big Bang, but the cause or mathematical basis of the Big Bang itself is still unknown. Are mathematical theorems part of the universe, or laws imposed on the universe from outside?

RABI: Good question. The debate started by your student Plato and his student Aristotle is still going on. This figure made by Roger Penrose can explain the matter. Three worlds can be imagined in nature or reality: mathematical, physical, mental. There are countless things in the mathematical world (everything in mathematics), but only a few of them (eg, the fundamental theories of physics) make up, or govern, the entire physical world, shown by the arrow marked ‘1’ in the figure. Similarly, there are countless things in the physical world (from gorillas to galaxies), a few of which (eg, our brains) make up, or control, the entire mental world, shown by ‘2’. And there are countless things in the mental world (all the thoughts of all beings), only a few of which (some thoughts of theorists) can contain the mathematical world, shown by ‘3’. Now the question is which world is the first in this cycle, from which world the other two worlds were born or started, or do all three exist together? According to Plato everything starts from the mathematical or formal world, according to Aristotle the physical world first, and according to Bishop Berkeley everything is born from the mental world.

SOCRATES: That means Plato thought that all theories would exist even if there were no universe, because theories exist outside the universe in a separate mathematical world.

RABI: Yes. And according to Aristotle, theory is a human-made model to explain various phenomena of the universe. If there is no universe, there will be no theory.

SOCRATES: But Aristotle’s materialism seems the most correct for our universe.

RABI: That is your own choice. I am a Platonist. One who understands the meaning of the beauty and theory of Math cannot think of anything other than Math as the source.

SOCRATES: Well, then these three worlds together what can be said to be the basic structural elements of reality?

RABI: Definitely STEMIC, meaning space, time, energy, matter, information, and consciousness.

SOCRATES: Where is your math?

RABI: Everything in math is inside that information.

SOCRATES: What was the need to separate consciousness? Everything mental is information.

RABI: Many cognitive scientists think that consciousness is another type of thing, not information. But that is another debate. We should first focus on just the ‘STEM’ of STEMIC, meaning space, time, energy and matter.

SOCRATES: These four together can be explained by relativity, but only on a much larger scale. Relativity does not apply at very small atomic or subatomic scales, where energy and matter have to be explained by quantum theory. If a theory can be made combining general relativity with quantum theory, it is possible to explain STEM at all scales together. Many physicists are still trying to make a Theory of Everything (TOE), aren’t you one of them?

RABI: I still think some future version of string theory will unify everything. But surely you will not understand that. Rather, we should now see what it means to unify, and what is this ‘everything’ in TOE?

SOCRATES: Then tell us what is meant by Everything, and what is meant by unifying.

RABI: Currently there are four types of energy in our universe. Each energy interacts differently. The interaction of energy is called a force. Then there are also four forces associated with the four energies: strong, electromagnetic, weak, and gravitational forces.

SOCRATES: Yes, it appears so in this figure of yours. But why do you start with energy, leaving space, time, matter of STEM?

RABI: Because, Socrates, after the Big Bang there was only spacetime and energy in the beginning, matter was then created from energy; It can be explained by Einstein’s $E=mc^2$, if $E$ means energy and $m$ means matter, $c$ is the speed of light. We’ll get to the matter later. As seen in the figure, four types of energy or force have come into existence independently within the first 1 picosecond of the universe’s history. A picosecond is one trillionth of a second.

SOCRATES: Matter doesn’t matter even if we don’t understand it now, but will you go directly to energy without giving an account of the birth of spacetime?

RABI: We cannot explain something called zero time. At Planck time, all our theories fall apart. Planck time is 1 quattuordecillionth of a second (45 zeros before one), that is $10^{-45}$ seconds. Since Planck time, there has been spacetime, and with it only one unified energy and force. It’s called the TOE force, because if a theory of everything is ever discovered, that theory can explain this force.

RHEA: Wait, I don’t understand the time and temperature thing in your figure. Accustomed to the cultural age, I have a hard time digesting such a small number. What was the age and temperature of the universe at the time of this tow force?

RABI: Remember, all forces are born in the first picosecond. The TOE force existed much earlier, when the universe was less than 10 trdecillionth of a second. If you go to the number article you will get a list of names of small and large numbers. Meditating on this list from time to time will allow you to experience such numbers.

RHEA: From what I see in the list, 1 tredecillion means 42 zeros after one, then 10 tredecillion means 43 zeros after one. Just twelve zeros after one makes one trillion, and here we have to put 43. It is impossible for any human being to feel the terrible shortness of time when one second is divided by such a large number. And I don’t even want to try to feel the temperature.

RABI: It’s cool to sit in this ice and wonder what the temperature of the universe was at that time. Think about it. The source of the Brahmaputra is very cold, the temperature increases as you go towards the estuary (Bay of Bengal). The opposite is true of the universe. The temperature was the highest during the Big Bang, and since then, as the universe has expanded, both its density and temperature have decreased, and are still decreasing.

RHEA: If we look at this figure and the list of numbers, we can say that when the age of the universe is 10 tredecillionths of a second ($10^{-43}$ seconds), and when the temperature is 100 nonillion Kelvin ($10^{32}$ K), then by breaking the TOE force, two separate forces were born, gravity and GUT force. What does ‘GUT’ mean?

RABI: ‘GUT’ stands for Grand Unified Theory. Abdus Salam, a scientist exiled from Pakistan for belonging to the Ahmadiyya community, received the Nobel Prize for the discovery of this theory, along with Glashow and Weinberg. This is the closest theory to TOE that has been proven so far. When the age of the universe is 100 decillionths of a second, and the temperature is 1 octillion degrees, two independent forces arise from the gut force: the strong nuclear force and the ‘electroweak’ force. And finally, at a picosecond age, at a temperature of one quadrillion degrees, the electroweak force breaks down to form the last two independent forces: the electromagnetic force and the weak nuclear force.

RHEA: On the left side of the name of the four forces, I see the names of some particles, and on the right side some numbers and pictures. What does this mean?

RABI: This is a basic introduction to these four forces for the benefit of those who may not understand anything through math. Every force works by exchanging certain particles. The particles of the strong force or interaction are the gluons, the photons for the electromagnetic force, and three types of bosons (for example, Z boson) for the weak force. No such particle has been found for gravity so far, but just as people decide on a name before having a baby, scientists have decided on the name ‘graviton’ in the hope of future discoveries. The strong force is the strongest, with a strength of 1, and the rest are compared with this using the numbers on the right side of the figure. The electromagnetic force is 100 times weaker than the strong force, the weak force is 1 million times weaker, and gravity is 1 duodecillion times weaker.

RHEA: But the most important thing is to understand what the forces really are.

RABI: That is shown in the four pictures on the far right. Strong and weak forces act only inside the nucleus of an atom. The strong force binds three quarks together to form a proton. The weak force can convert an up quark of this proton into a down quark by converting the proton into a neutron, thus giving rise to radioactivity. Electromagnetic forces hold atoms together by creating an attraction between electrons and protons. And gravity bends the space around the earth and forces the moon to revolve around it.

RHEA: But you didn’t explain one thing. At one picosecond in the middle of your figure it says ‘electroweak symmetry broken’, what does that mean?

RABI: That is the most important thing. I mentioned the sequence of birth of the four energies or forces, but did not explain why it broke from one into four in this way. The reason is called symmetry breaking in physics. A theory in physics that has symmetry means that the theory applies equally in all space and time. The theory of gravity is as true for the apple tree as it is for the Andromeda galaxy, and is as true today as it was yesterday, and will be as true tomorrow. There is a more extreme form of this symmetry, where even the identity of the particles becomes symmetric, meaning that there is no theoretical difference between one particle and another. For example, there was no difference between photons and Z bosons before the electroweak theory’s symmetry was broken. Similar symmetry-breaking has occurred twice before, although we could not detect them in the lab.

RHEA: I understand, but don’t you think, Socrates, that Robbie’s description is too theoretical, and we can’t visualize the whole thing yet?

SOCRATES: Right.

RABI: Time for a metaphor then. The reason for symmetry breaking at a certain point is actually phase change. For example, water is a gas at 200 degrees Celsius. Lowering the temperature won’t do much until 100 degrees, but at 100 degrees the water will actually turn from a gas to a liquid. Not at any other temperature, but only at 100 degrees water can change phase like this. Then if the temperature is lowered further, nothing big will happen again until 0 degrees, but exactly at 0 degrees the water phase will become solid from liquid, we will get ice. These two phase transitions of water can be compared to the last two phase transitions of the universe. As the temperature of the universe decreases to 1 octillion degrees when it is 100 decillionths of a second old, the symmetry of the GUT force is broken, and its particles undergo a phase transition; metaphorically if the former is a gas, the latter will be a liquid. Then when the temperature drops further to 1 quadrillion degrees at a picosecond age, the symmetry of the electroweak force is broken, causing another phase transition of its particles, metaphorically from liquid to solid ice.

RHEA: Now I understand better. But the inflation you wrote down at an octillion degree age needs a little explanation.

RABI: Just as heat or energy is required to boil liquid water into steam, energy is released when steam is converted into liquid water. Similarly, every phase transition or symmetry breaking in the universe releases a lot of energy. At one octillion degrees, the energy release was so high that the universe suddenly became very large. This phenomenon is called inflation, but its definite proof is not yet available.

RHEA: We need to get to know these phase-changing, symmetry-breaking particles better.

RABI: That’s why we have to keep an eye on the figure below. The seventeen particles that have so far held their place of honor in the Standard Model of particle physics are shown below in two ways: on the left is the mass, charge, and spin of each particle, and on the right are the same particles arranged differently to show the beauty of the model.

RHEA: Here we also see four particles known as carriers of the three forces: gluons, photons, Z bosons, and W bosons, each with spin 1. Strong and electromagnetic forces are similar in that their particles have no mass and no charge. Two particles of the Wick force are very heavy, one has a mass of 91 and the other has a mass of 80 GeV, but what is the meaning of this unit of mass? I understand that GeV stands for giga electron-volt, but that is supposed to be a unit of energy.

RABI: I have already said that energy and matter (mass) are the same thing with Einstein’s equation. The electron-volt is the unit of energy, but dividing that by the square of the speed of light gives the mass. But for us comparison is more important than actual mass. That’s why you need to remember the meaning of mega and giga: mega means million, and giga means billion. Rhea, so will you describe the whole model once?

RHEA: Let’s look at the left figure first. Here on the left are 12 particles of matter whose common name is fermion, named after Enrico Fermi of Italy, and on the right are 5 particles of energy whose common name is boson, named after Satyendranath Bose of Dhaka University. Energy particles have spin 1, matter particles have spin 1/2. Particles of energy are called carriers of interaction or force. The Higgs boson is called a scalar boson, and the other four are called gauge bosons, or vector bosons. Even if I understand scalar and vector, I don’t need to understand the meaning of these names. The Higgs is the heaviest of the bosons, while the gluon and photon are massless. Matter particles are of two types: six quarks and six leptons. The six quark names are pretty cool: Up, Charm, Top, Down, Strange, Bottom. The charge of the first three is 2/3, the charge of the next three is -1/3. The names of the six leptons are electron, muon, tau, electron neutrino, muon neutrino, tau neutrino. The first three have a charge of -1, and the three neutrinos have a charge of 0, making them neutral. The heaviest quark is top, and the lightest is up. The heaviest lepton is the tau, and the lightest is the electron, our most beloved object.

RABI: Great description, not explanation, description. All of this can only be explained by math, which of course we are excluding. In this model, you can think of particles of matter (fermions) as bricks, and particles of force (bosons) as cement. Just as a wall can be made by joining bricks with cement, so anything in the universe can be made by joining fermions with bosons.

RHEA: But you told Socrates in the beginning that you would ‘describe’ the birth of matter from energy. I understand that bosons are all created by symmetry breaking from particles of the TOE force. But how exactly did fermions form after the Big Bang?

RABI: Good question. Since energy and matter are equivalent, one was created from the other after the Big Bang all the time. This phenomenon is called ‘pair production’ which I will try to explain with this figure. A particle of matter and an antiparticle can be formed from two particles of energy or force. This does not violate any conservation laws, because the total amount of product before and after the reaction is the same.

RHEA: So can matter and antimatter be created from energy even now?

RABI: No. Because it takes a lot of energy to create matter through pair production, it requires temperatures much higher than today which does not exist even in the center of stars, but the Universe was hot enough just after the Big Bang. Antiparticle of electron is called positron. The temperature required to form an electron-positron pair is 10 billion Kelvin, the temperature required to form a proton-antiproton pair is 10 trillion Kelvin, and the various quark-antiquark pairs require about 1 quadrillion Kelvin. Since the temperature of the universe was one quadrillion kelvin at an age of 1 picosecond, quarks were first born between 1 picosecond and 1 nanosecond. Then at the age of 1 microsecond, protons are created from energy, each of which contains two up quarks and one down quark. And the electrons were born when the universe was about 1 second old.

RHEA: Interesting. But you only talked about particles of matter. In each of these cases, don’t we also get antiparticles of antimatter?

RABI: Yes, antimatter has always been created along with matter. But the interesting thing is that from the very beginning, for some unknown reason, there was a little more matter than antimatter in the universe. Now you see in the figure that just as matter-antimatter can be formed from energy, so also matter-antimatter can completely annihilate each other and return to energy. This destruction, known as annihilation, was also underway. But since there was a little more matter than antimatter, some matter remained even after all the antimatter was destroyed. After annihilating all the antiquarks, many quarks survived, after annihilating all the antiprotons, many protons survived, and after annihilating all the positrons, many electrons survived.

RHEA: Where did the energy created from all these annihilations go? For example, where are the photons produced in all electron-positron annihilations?

RABI: Excellent question. These photons are called the Cosmic Microwave Background or CMB. It was with these photons that we were able to take the first pictures of the Universe, when it was three hundred thousand years old, and had a temperature of 3,000 kelvin.

RHEA: But you jumped from the victory of the electrons in a battle with the positrons at the age of 1 second of the universe to the age of 300,000 years. What happened in the middle?

RABI: Well let me go back a bit. In fact, sitting on this ice has frozen my head. Can’t we take a little walk on the ice-molten water of the Angsi River? Jesus also walked on water.

HERMES: Come on, everyone. Like Aristotle’s Peripatetics we can talk while walking on water.

RABI: The touch of ice-cold water on my feet opened my mind. The biggest event that happened when the universe was beteen 1 second and 10 minutes old was nucleosynthesis, the building of the heavy nucleus of an atom by proton-neutron pairs. It is part of our previous building project, like joining bricks with cement. The simplest of the 92 elements in the periodic table is the hydrogen atom. An atom’s identity is determined by the number of protons in its nucleus. A lone proton is called hydrogen, even if it does not have an electron attached to it. But to make the next element helium, two protons and two neutrons are needed. The process of creating helium from hydrogen (called nuclear fusion) is shown in this figure, which happened during the first ten minutes of the universe.

RHEA: But why within the first ten minutes?

RABI: The reason is again temperature. To create helium through fusion, the temperature must be at least 1 billion kelvin. By the age of ten minutes, the temperature of the expanding universe had dropped to 1 billion degrees, so this process only happened in the first ten minutes. At first, the chemical element in the universe was only hydrogen (meaning single protons), in the first ten minutes 24% of the hydrogen was converted into helium, and the remaining 76% remained hydrogen. The chemical composition of the universe is still the same. Astronomers’ observations have also shown that the surface of the oldest stars in our galaxy is 24 percent helium, the rest almost entirely hydrogen.

RHEA: I understand how the nucleus, the center, of hydrogen and helium was formed. But why could not electrons combine with them to make neutral atoms? Is that because of the temperature?

RABI: Yes. The higher the temperature, the greater the kinetic energy (speed) of the particles of matter. Potential energy between electrons and protons loses the battle with kinetic energy. That is why for the first three hundred thousand years electrons were free, protons did not love them. But photons were in love with electrons. If the electrons are free, the photons cannot travel along their normal straight lines, repeatedly colliding with the electrons, getting scattered. Wherever the temperature is high, electrons are free, and photons are scattered. Such is the current state inside the Sun. For this reason the first 300,000 years of the universe can be compared to a star like the Sun. The temperature at the surface of the Sun is a few thousand kelvin, and as you move towards the center the temperature increases to several million kelvin. Similarly, at the age of 300,000 years, the temperature of the universe was a few thousand, from there, the temperature will increase as you go towards the Big Bang.

RHEA: And if we come toward the present, we see that the temperature and density are decreasing day by day due to the expansion of the universe.

RABI: Yes. And the density of energy decreased more rapidly with temperature than the density of matter. This is why when the universe was 50,000 years old, the density of energy was less than that of matter. The first fifty thousand years were the reign of energy, after which the reign of matter began.

RHEA: And exactly what happened at the age of three hundred thousand years?

RABI: Then the temperature dropped to about 3000 kelvin and the electron could no longer remain free. The proton binds the electron with its potential energy, creating a neutral hydrogen atom. After that the helium atom was also born. And since there are no more free electrons, the photon starts its free journey in a straight path. The event didn’t happen exactly at three hundred thousand years, it happened over a period of three hundred thousand to one million years, but I’m using three hundred thousand for poetic reasons.

RHEA: Well, I understand what the CMB of your figure is. It’s understandable why it’s called a background, because it’s the first picture we’ve taken of the universe, meaning the very back, the background of everything. The figure shows a colorbar from -300 to +300 microkelvin. White-green means 0, the more you go towards the blue, the more negative the temperature, and the more towards the red, the more positive. What does this mean?

RABI: Here zero means the average temperature of the universe at the age of 300,000 years. The temperature difference of the entire universe with respect to this average is shown in this map. It is a map of the entire universe. The universe was then a single gas of hydrogen-helium, with temperatures almost the same everywhere, but with slight variations. The variation from one place to another is a few hundred microkelvins at most. Gas in blue areas is slightly cooler than average, and in red areas slightly warmer.

RHEA: Ok I understand. But what does this ‘microwave’ of the CMB mean? Why are these photons called the microwave background?

RABI: When the CMB radiation occurred, the temperature of the universe was 3000 degrees, so the wavelength of the CMB was in the infrared range, a few micrometers, about the size of a needle tip. But with the expansion of the universe, this length has grown to a few millimeters today, about the thickness of our fingernails, which falls in the microwave range of the electromagnetic (EM) spectrum. So we had to take this CMB image with a microwave telescope. The lower part of the EM spectrum figure you see in blue shows how easily light can pass through Earth’s atmosphere and reach us. The blue color here symbolizes opacity, which ranges from 0 to 100, the higher the opacity, the lower the transparency.

RHEA: Yeah, so it turns out that microwave light from space doesn’t reach the ground very well, Earth’s atmosphere destroys it. So how was the CMB image taken?

RABI: That picture was taken by a space-telescope called ‘Planck’ made by the European Space Agency.

SOCRATES: I think we should take this opportunity to discuss light. What is light? What is the wavelength and frequency of light?

RABI: Light is electromagnetic (EM) energy. Light is made of photons. Light can also be thought of as a wave. We are now walking over the Angsi River. If I kick the water with my foot, a wave will be created on the surface of the water. Light is also a wave that is created only within the EM field. I can make waves in the water with my feet. But the EM field requires charged particles, such as electrons, to create waves. When an electron is accelerated, waves are created by the interaction of the electric and magnetic fields. Just as water waves have wavelengths and frequencies, so do EM waves or light. Rhea can explain this very well with the example of a sea beach.

RHEA: Socrates, you yourself know everything very well, but you pretend to know nothing.

SOCRATES: My daemon does not think so. Anyway, tell me if you’re going to make waves or not.

RHEA: Ok, instead of making waves in the air with words, I am directly making waves in the water. We are very close to the Tsangpo River. Look there, an empty boat tied to the shore. I will make waves in the middle of the river with this boat, you all stand on the bank with your feet in the water so that you can actually feel the waves. Now I’m pedaling too hard. Socrates, put aside your irony, tell me how the waves seem.

SOCRATES: It seems to come more frequently, meaning that the distance from one crest of the wave to the next is shorter.

RHEA: You have understood both wavelength and frequency. The distance between two adjacent peaks is the wavelength. And frequency is how often one wave (peak) comes after another. I changed pedal speed many times. Did you get a change in the wave?

SOCRATES: Yes. The faster the velocity changes, the shorter the wavelength of the wave, and the higher the frequency. Shorter the wavelength the higher the frequency, lower the frequency the longer the wavelength.

RABI: Let’s all get on the boat and finish the rest.

[All eight in a boat, on the Tsangpo River.]

RHEA: I have another question about the figure of the EM spectrum. In order of wavelength, radio light is the longest, followed by microwaves, infrared, visible, ultraviolet, and X-rays, and gamma rays the shortest. The shorter the wavelength, the higher the frequency, the higher the energy. When did people discover this vast range of light?

RABI: Until the 19th century, people did not know any light except visible light. Two German scientists were the first to discover light other than visible light. Heinrich Hertz discovered radio light in 1888, and Wilhelm Röntgen discovered X-rays in 1895. Then all light from radio to gamma-rays gradually became available to humans. The name of the device that captures light is telescope. Since Mars was the only astronomer among us, he should lead the telescope discussion.

MARS: I have a small portable smart telescope with me. The behavior of all telescopes at all frequencies can be explained with this small telescope. It is a visible light telescope, operating at a wavelength of 400 to 700 nanometers, i.e. at a frequency of approximately 400 to 700 terahertz. The unit of frequency Hertz is named after Heinrich Hertz. We see light of different frequencies as different colors. If the wavelength of light is 450 nanometers, its color is blue, if it is 550 nanometers, it is green, and if it is 700 nanometers, it is red. There are actually not seven colors between red and blue, but an infinite number.

RABI: So why did people think of exactly seven colors in the rainbow?

MARS: Isaac Newton attributed seven colors to the rainbow for the same reason that we divide the history of the universe or the river Brahmaputra into seven parts, i.e. for poetic reasons. Many colors are also visible in the images of this telescope. We can use frequency and color as synonyms.

RABI: Since the night is here, give us a demonstration of this telescope.

MARS: The main parts of any modern telescope are three: collector, detector, processor. The figure shows all three that you can match with the telescope. The vertical slab mounted on the tripod is an altitude-azimuth mount, with an optical tube attached. At the lower end of the tube is the collector, in this case an 11-cm diameter mirror, and at the upper end is the detector, in this case a CCD sensor. The sensor is placed where all the light falling on the curved mirror of parabolic shape meet after reflection. If you follow the arrow you will understand. The sensor converts all photons of light into electrons and sends them through wires to the processor, which is housed inside the vertical mount. In this case, the processor is a Raspberry Pi mini-computer with 64 GB of storage, and a WiFi modem. The computer power switch is on the surface of the vertical mount. And here’s where the power comes from—the battery housed inside the horizontal part of the mount, just above the tripod. When switched on, the telescope itself will create a WiFi network to which we can connect with the phone’s ‘Unistellar’ app. The telescope has to be controlled with this app.

RABI: What is the meaning of the specifications of the telescope in the picture?

MARS: The efficiency of any telescope is determined mainly by two parameters: resolution and sensitivity. A telescope’s resolution is the size of the smallest object it can see, specified as angular resolution. The lower the value, the better the telescope, because it can see smaller objects. Its unit here is arcsecond. Remember that a circle is divided into 360 parts, each part being one degree. An arcminute is one 60th of a degree, and an arcsecond is one 60th of an arcminute. Our telescope has a resolution of 1.72 arcseconds, not bad for an amateur. Angular resolution depends on the size of the mirror, and pixel resolution depends on the sensor, hence sensor resolution. Pixel resolution refers to the total number of pixels in an image, just like a camera. Our sensor has a resolution of 4.9 megapixels, which means that the image it produces contains 4.9 million pixels. The area of ​​the sky covered in an image is given as the field-of-view, which in our case is about 40 arcminutes. The Moon is about the size of a finger, about 30 arcminutes. So taking a picture of the moon with this telescope will cover almost the entire screen, because almost the entire field-of-view will be occupied by the moon alone.

RABI: And what is sensitivity?

MARS: Sensitivity refers to how dim objects a telescope can see. In this case, the sensitivity is defined by the limiting magnitude, which here has a value of 18. I am explaining. Following the ancient Greeks, astronomers developed a logarithmic system to express the apparent brightness of any celestial object. In this system, the higher the brightness of the object, the lower its magnitude. The Sun has an apparent magnitude of -26, the Moon -13, bright stars 0, and humans can see a maximum of magnitude 6 with the naked eye. Our telescope’s limiting magnitude is 18, meaning it cannot see objects above magnitude 18. From 6 to 18, we can take pictures of many objects with it which can never be seen with the naked eye.

RABI: So we end the particle age discussion by taking a picture of an object in the sky?

MARS: We cannot take a very good picture from the boat. Today, let us see the sky with our naked eyes. Going further along the Tsangpo River, we will take pictures from the shore another day.