[On a simple sidewalk in Barzakh]

Socrates: One, two, three, four, five, six, but where is the seventh one, Ishtar? Yesterday I was a guest of you seven, in return today you were supposed to be my guests.

Ishtar: Juno is late as usual.

Socrates: If we don’t have all seven, our discussion won’t be as good.

Rabi: I heard Juno went to a Buddhist monastery between Lake Manasarovar and Rakshastal in the morning and hasn’t returned yet. How about we leave Barzakh and go down to Earth? You can host us there.

Socrates: Not a bad idea. And the discussion I want to host you with will be best suited by the shores of Lake Manasarovar. Let’s go then.

[Between Lake Manasarovar and Rakshastal]

Socrates: What’s up, Juno? We missed you so much that we came all the way from the sky to Earth.

Juno: I was going to Barzakh, but seeing the shadow of clouds in the lake, I forgot about you all. You know how rare clouds are here.

Socrates: It’s better if I don’t look at the clouds too much. I’ve already heard a lot of Aristophanes’ scolding without seeing the clouds.

Juno: Okay, no need to look at the clouds. Let’s go to the roof of that monastery, everyone come along. From the roof, you can see Rakshastal, Lake Manasarovar, and Mount Kailash together.

Socrates: With so many things to see, will you still need my gift of words?

Juno: That’s the test, Socrates. Yesterday we gave you a huge discussion on the constitution of an ideal state, in return today you have to tell us something so good that our eyes won’t turn anywhere else.

[On the roof of a Buddhist monastery by the shores of Lake Manasarovar.]

Socrates: I will talk about the fourteen billion years of history of the universe. Our European successors coined the word ‘universe’ by combining ‘uni’ and ‘verse’. ‘Uni’ means one, ‘verse’ means to transform, ‘universe’ means ‘transformed into one thing’. Yesterday you were talking about the nation, the institution created to form that nation is named ‘university’ because the meaning of ‘universe’ is found within it. Just as a university unites all students and teachers to create a national identity, the word universe brings everything in the cosmos into one single entity. I have many parts like hands, feet, nose, mouth, ears, heart, but all together I am one single person. Similarly, within the cosmic web, there are many galaxies, gases, stars, planets, satellites, asteroids, but all together the universe is one single entity.

The fourteen billion years of history of this universe can be divided into seven ages: Particle, Galactic, Stellar, Planetary, Chemical, Biological, and Cultural ages. The first three hundred thousand years are the Particle age, and the last three hundred thousand years are the Cultural age.

Rabi: The Brahmaputra river, born from many glaciers near Lake Manasarovar, also has seven stages.

Socrates: Exactly. And there is a metaphorical relationship with time as well. What do you think?

Rhea: Yes, like a river, our time only flows in one direction.

Socrates: Then we can elevate the comparison of the universe’s time with the Brahmaputra river from a metaphor to an allegory. The seven stages of the Brahmaputra (Angsi, Tsangpo, Siang, Brahmaputra, Jamuna, Padma, Meghna) can be compared to the seven ages of the universe. Shakespeare spoke of the seven ages of human life from childhood to old age. The universe has grown quite large after passing through seven ages since its birth. The Brahmaputra falls into the Bay of Bengal through the Meghna. Do you see any relationship between the sea and the time of the universe?

Rhea: If all the rivers from Angsi to Meghna are compared to the past and present, then the sea is undoubtedly a metaphor for the future.

Socrates: Exactly. Just as the river is narrow like the past, the sea is vast like the future. In the past, only one set of events happened in my life, like a river bound by two banks. But in the future, many things can happen, there are endless possibilities like the sea.

Rhea: Wonderful, Socrates, your introduction is excellent.

Socrates: Thank you. But remember Rabindranath’s words: ‘When I open my eyes to this light, your gaze will be fulfilled’. If you do not open the eyes of your mind, I will not be able to see. You must first take the responsibility of showing the relationship between each age of the universe and each stage of the Brahmaputra. Then I will talk in detail about each age over the next seven days.

Rhea: We agree. Who will talk about which?

Socrates: You can understand. Your names coincidentally match the seven days of the week or the seven planets of our time. Rabi with Sunday and the Sun, Shashi with Monday and the Moon, Mars with Tuesday; Hermes being another name for Mercury matches with Wednesday, Juno as Jupiter’s consort matches with Thursday, Ishtar as Venus’s predecessor matches with Friday, and Rhea as Saturn’s consort matches with Saturday. Rabi will start with the comparison of the Particle age with Angsi, then everyone will follow one by one, and Rhea will finish with the comparison of the Cultural age with Meghna. Rabi, start.

Rabi: If the Particle age is the first three hundred thousand years of the universe’s history, then it is very easy to find a similarity with the Angsi river. The Particle age is a time that humans have not yet been able to observe directly. Just as it is difficult to find the source of Angsi, it is equally difficult to find the source of the universe. The Angsi river also clarifies the conflict between metaphor and science. The first stage of the Brahmaputra is not just Angsi; many rivers combine into one stream to form the Tsangpo at one stage. You may have chosen Angsi to keep the total number at seven, or because the name sounds short and beautiful. Just as brevity, beauty, metaphor, simile, and allegory work in poetry, trying to do the same in science will lead to many complications.

Socrates: You are absolutely right, Rabi. We will use metaphors the way Shakespeare did. When he calls Juliet the sun, we must always remember that Juliet does not actually become a burning ball of gas. Shashi, it’s your turn now.

Shashi: The comparison of the Tsangpo with the Galactic age can be made more naturally. The feeling of vastness in galaxies can be felt by looking at the whiteness of Tibet on both banks of the Tsangpo. Galaxies and their clusters are the largest structures in the universe, and the Tsangpo is the longest part of the Brahmaputra. If we try to make more comparisons, people will start calling us fundamentalists.

Socrates: That’s enough. Comparisons with the moon also don’t sound good for long. Mars, start your battle now.

Mars: The Siang in Arunachal is indeed a huge battlefield, as violent as the birth of planets and stars from the massive clouds of gas and dust in the Stellar age. The Yarlung Tsangpo Canyon is the deepest gorge on Earth, six kilometers deep. The Siang river has carved this gorge through the Himalayas over millions of years in love with the Bay of Bengal. The roar of this deep gorge cannot be explained to someone who hasn’t heard it, just as the violent and terrifyingly beautiful scene of the birth of a solar system from a rotating gas cloud cannot be seen by any living person.

Socrates: Perfect. Hermes, what are you thinking, looking at Manas?

Hermes: I was thinking about the sudden change in the character of the river when the Siang descends from the mountains into the plains of Assam. This change can easily be compared to the first one or two billion years of the Planetary age. After its birth, the Earth was as hot as fire, bombarded by thousands of rocks from space. After billions of years of this war, when the oceans emerged on the Earth’s surface, a home of love was created inside the sea, meaning a factory for creating life. If the Siang in Arunachal is the battlefield, then the Brahmaputra in Assam is the garden of peace.

Socrates: Such a beautiful comparison never occurred to me before. Juno, it’s your turn now.

Juno: After entering Bangladesh from Assam, the Brahmaputra becomes the Jamuna. This Jamuna has no relation to India’s Yamuna. Still, since the word Jamuna brings to mind the love of Radha-Krishna and the Taj Mahal on the banks of the Yamuna in Agra, the comparison of the Jamuna river with the Chemical age is very natural. If the hydrothermal vents created under the sea in the Planetary age are the home of love, then in the Chemical age, the first life forms were born in this home, capable of creating a lineage through pairing. This union can be beautifully compared to the confluence of the Jamuna and Padma (Ganges) near Dhaka in Rajbari.

Socrates: This union is indeed interesting. Think about it, the Ganges and the Brahmaputra are born almost from the same place near Lake Manasarovar, the Brahmaputra from the northern slope of the Himalayas, the Ganges from the southern slope. After traveling a long way, they meet in a place called Rajbari under the names Padma and Jamuna. If the Jamuna is a symbol of chemistry, the Ganges can be a symbol of culture, as one of the oldest state cultures in the world has grown on the banks of the Ganges. Well, Ishtar seems impatient, start.

Ishtar: It is very symbolic that the Biological age begins in a place where the two largest rivers of South Asia meet. What flows from Rajbari to Chandpur under the name Padma is actually the result of the confluence of two large rivers (the Brahmaputra as Jamuna, the Ganges as Padma). Since the Padma is one of the most revolutionary rivers in the world in terms of flow, its comparison with the Biological age is fitting. Because in this age, the universe’s history saw the greatest revolution with the filling of a planet with billions of species of plants, animals, fungi, and bacteria.

Socrates: Undoubtedly the greatest. Rhea, it’s your responsibility to finish now.

Rhea: I think the revolution of culture was greater than that of life, and the comparison of the Cultural age with the Meghna is the most natural. In the beginning, the universe was like a single continuous gas, this gas gradually fragmented into many different things, but all these creations always wanted to unite with other creations. Culture is the best way to unite different things. The Meghna river does the same. Many rivers in Bangladesh fall into the Meghna, and the Meghna rushes towards the Bay of Bengal with everyone, just as culture wants to rush towards the future with many people, carrying many dreams in its heart.

SOCRATES: Excellent conclusion, Rhea. Since the existence of time has come, I think we should think about it a little more.

Rhea: Yes, and we should get out of the metaphor and focus on a little real science.

Socrates: Why not? There are some mathematical laws of the universe whose ultimate source is unknown to humans. Moreover, even we the dead do not know it yet. But we know the objects on which these rules operate. Everything in the universe that obeys rules can be put into four categories: space, time, energy, matter (abbreviated as ‘STEM’). Einstein worked on these four things in the early 20th century. According to his special relativity, space and time are the same thing, and energy and matter are the same thing. General relativity (GR) describes the fundamental relationship between spacetime and energy-matter. I do not understand the mathematical basis of this superstructure, although understanding it is a major task in my future eternity.

Ravi: But if you don’t understand this mathematical basis, is there any point in discussing it? What I’ve learned from studying and researching relativity until I died is that people get it ninety percent wrong if they don’t know the math.

SOCRATES: Why ninety, I would say ninety-nine percent of the mistakes I make myself. My only knowledge is that I know nothing.

Ravi: Then the gift of your words will do us more harm than good.

SOCRATES: Oh, because my words harmed the youth, I had to drink hemlock and die. So now I am not afraid of harming anyone. But if you really want no one to be harmed, then please tell us yourself what is the best way to think of spacetime in the universe.

Ravi: Whatever I say in Bengali or English is bound to be wrong, only math can say it right. Since you can’t understand the math of spacetime, listen to the ninety-nine percent wrong version. After Einstein’s discovery of GR in 1915, he thought that the universe’s spacetime would be highly curved due to the large amount of energy-matter. The more matter there is, the more space bends. He also made a mathematical model of this four-dimensional curvature. Let alone humans, it is impossible for us to visualize the four-dimensional box of this model. But if we reduce the three dimensions of space to two dimensions, a three-dimensional model of space-time becomes very clear to the eye. You will understand if I draw on the tablet. Look at this:

The surface of this sphere, known as Einstein’s curveball, is two-dimensional space, and time is along the radius. In this model all the energy-matter of the universe is at a given time only on the surface of a sphere of a given radius. Radius increases with time, and the size of the universe increases with surface area. Einstein didn’t think of increasing the radius at first, he believed in a stable universe like Aristotle. He also added a constant called the cosmological constant to his equation so that the radius does not change. But later, around 1930, after receiving proof of the expansion of the universe, the constant was removed. In the 21st century, people brought this constant back not to stop the radius, but to control the rate at which the radius increases.

At present we know that the Universe is the same in all directions and has the same average density everywhere. This is also understood by the sphere of this three-dimensional spacetime. Think of the surface of the sphere as the surface of the Earth, and imagine that you are floating on a raft in the Pacific where everywhere you look is just water and water, the same in all directions, and the density of water is the same everywhere you go.

SOCRATES: But, Ravi, if I look up, I will not see the same.

Ravi: Hey, Socrates, you’re still as stubborn as ever. If we reduce the three dimensions of space to two dimensions in the beginning, then can there be anything called space other than the surface of the Pacific?

SOCRATES: No.

Ravi: And if there is no space without this surface, how can you look up? Looking up will require three dimensions of space which we have already reduced to two dimensions for the sake of this model.

SOCRATES: Well, I see.

Ravi: The fact that the universe looks the same in all directions is called the cosmological principle. Two more interesting phenomena of our universe can be explained using this three-dimensional model. The first is like this. Hubble found that all distant galaxies are moving away from us, and the farther away they are, the faster they are moving away. Actually galaxies are not moving, spacetime is expanding. If we think of the above sphere as a balloon, and if each galaxy is a spot on this balloon, then as the balloon gets bigger the radius or time will increase, the surface area of ​​the balloon will also increase and the spots (galaxies) will move away from each other. Each speck will feel that all the other specks are moving away from it, each will feel that it is at the center of the universe, since everything is moving away from it. But actually no one is the center. The surface of a sphere has no center, but if you stand anywhere on the surface and look around, you feel at the center of everything.

SOCRATES: Excellent. And what is the second?

Ravi: The second one is like this. Wherever we point our telescope from Earth, we can see up to the same distance in all directions. Since the speed of light is constant according to Einstein’s special relativity, our horizon is limited in both space and time. An example will make it clear. Let’s say we have three telescopes, named X, Y, Z, and let’s say all galaxies have the same absolute or true brightness. So the more sensitive the telescope, the more distant galaxies it can see. Now suppose X can see galaxies up to 1 billion lightyears away, Y can see up to 2 billion lightyears away, and Z up to 3 billion lightyears away. So the universe observed by X is a balloon with a radius of one billion lightyears, the balloon radius of Y is two billion lightyears, and the universe observed by Z is 3 billion lightyears in radius.

SOCRATES: But our three-dimensional spacetime balloon doesn’t match that. In the picture balloon the radius was time, in the observable universe of your three telescopes the radius is space, because its unit is light-years.

Ravi: This is where you have to think. Although the light-year is a unit of distance, it is related to time. Looking at a galaxy a billion light years away means seeing what it looked like a billion years ago, because it took a billion years for light to travel from there. The distance that light travels in one year at the speed of 300,000 km per second is one light year, or about 10 trillion km. So even in the balloon of the observable universe radius is actually time. X’s balloon has a radius of one billion years, because it can see a billion years into the past.

SOCRATES: I have two observations here. First of all, the picture of Einstein’s curveball, is it not a metaphor?

Ravi: Good point. It’s true that Curveball isn’t a real picture of the universe. Our world is four-dimensional, reducing it to three dimensions is mathematically trivial but psychologically illusory. But the curveball is still not a metaphor, its place is above metaphor, maybe analogy.

Socrates: I agree. Another observation is this. Looking at your map of the observable universe, I feel like I never see myself, never see the present, only see others, only see the past. If you can’t see yourself, is it possible to know yourself? I doubt my words now.

Ravi: If we can know about the present by looking at the past, can’t we know ourselves by looking at others?

SOCRATES: We have a better understanding of the universe’s present because we have a timeline of the past. In the case of society, it is not possible to understand the present without the past. However, I have doubts about whether it is possible to know oneself by looking at other people. But anyway, I thought I’d take this opportunity to show you the timeline I made.

This timeline has seven events from each era of the universe. So there are 49 total events. There is one slide for each event, and below the slides is a navigation that can be zoomed in and out and panned left and right. It will be useful for our meditation. Browsing the timeline can be a meditation on walking a tightrope into the history of the universe. I think it would be good for all of us if everyone, from Ravi onwards, looked at the seven events of the era and made some comments. Ravi?

Ravi: Yes, of course. At the outset, I must say again that studying history in this way without maths is not honorable for me. Still reading. The first event of the Particle Age is of course the Big Bang, which gave birth to our Universe approximately fourteen billion years ago. Ever since the universe was born, it has been expanding, sometimes fast, sometimes slowly. Along with the Big Bang, the Universe suddenly expanded in a phenomenon called inflation. After that all forces were born within one trillionth of a second. All the elementary particles, quarks, electrons, protons, etc. arrived within the first second. Within the first 15 minutes, multiple protons gathered to form the nucleus of an element like helium. At first energy was more dense than matter. Fifty thousand years after the Big Bang, matter defeated energy and increased in density. And after 300,000 years, electrons combine with proton nuclei to form atoms, and photons are freed from the tyranny of free electrons. We can still observe these free photons as microwaves. Through them we created the first image of the universe. This is where the particle age ends. Shashi?

Shashi: I saw the seven events of the galactic era on the timeline. The story goes like this. At the end of the particle age, the universe was essentially a single gas, with a density almost equal everywhere, but with some variation. Where the density is a little higher, in the dark age those areas tend to become denser under the influence of dark matter. One hundred million years after the Big Bang, the Dark Ages ended, two hundred million years later, galaxies and stars began to form from the accumulation of gas in areas of overdensity; Its name is Cosmic Dawn. In the first six hundred million years, the intergalactic medium formed, and in eight hundred million years, our galaxy, the Milky Way, was born. A billion years after the Big Bang, the Universe was filled with galaxies, and within two and a half billion years the quasars, the most massive and active galaxies, were born. The Galactic Era ended when clusters of galaxies formed three billion years after the Big Bang. Mars?

Mars: The start of the Stellar Age is estimated to be 10.4 billion years ago, three and a half billion years after the Big Bang. At that time, most stars were forming inside the galaxies of the Universe. Ten billion years ago, Population One stars were born, which are young, metal-rich, and reside in the galaxy’s disk. Nine billion years ago many clusters of stars formed and our galaxy got its thin disk. After a billion years, many voids were formed in the universe due to the formation of superclusters of galaxies. Six billion years ago, the repulsive dark energy won out over the attractive gravity, causing the expansion of the universe to accelerate. Five and a half billion years ago the Milky Way took on a spiral shape, and 4.6 billion years ago the Stellar Age ended with the birth of our Solar System. Hermes?

Hermes: The first important event in the planetary era was the birth of the Inner (closest to the Sun) planets 4.55 billion years ago. After 50 million years, the Sun attains the status of a main sequence, i.e. full-fledged star. About 4.4 billion years ago the hot Earth cooled and the oceans were born. But after that, for about 300 million years, there was a huge bombardment of rocks from space on the earth. At the end of this disaster, the continental crust was formed which is still moving, still changing the shape of the continents. 3.6 billion years ago, all the continents of the world joined together to form the Valbara supercontinent. This era ends here. Juno?

Juno: Socrates, you began the Chemical Age 3.6 billion years ago, when organic life probably first evolved from inert matter in hydrothermal vents on the ocean floor. The first fossils of organisms are found three and a half billion years ago. The first successful organisms, bacteria, left the sea and came to land about three billion years ago. A hundred million years later, large amounts of cyanobacteria in the oceans began releasing oxygen into the Earth’s atmosphere through photosynthesis. The first eukaryote to form a nucleus inside the cell appeared 2.7 billion years ago. The amount of oxygen in the atmosphere increased greatly two and a half billion years ago, three hundred million years later the ozone layer was formed. The chemical age ended with ozone. Ishtar?

Ishtar: Biodiversity began to increase during the Biological Era. The first complex cells were formed one and a half billion years ago. Many cells came together to bring about the multicellular revolution 600 million years ago. Two hundred million years later, animals emerged from water on land. Two hundred million years ago, warm-blooded animals appeared, and 65 million years ago, the asteroid impact ended with the extinction of the dinosaurs. Rhea?

Rhea: The Cultural Age begins with the birth of the first hominines approximately 7 million years ago. Animals of the genus Australopithecus and Homo appeared on earth between 4 and 1 million years ago. And the first modern humans walked the earth probably three hundred thousand years ago in Africa. Humans began to spread across the globe from Africa over a hundred thousand years ago. Fifty thousand years ago humans started a great revolution in religion, music and art. Agriculture began 10,000 years ago, followed by the first states 5,000 years later. Globalization began five hundred years ago, with its second wave beginning two hundred years ago with the Industrial Revolution. Here is the end of cultural age.

SOCRATES: It was really necessary to listen to everyone. You understand, I am not talking about the history of all places in the universe, I am talking about the history of the humans. The Universe came here as part of human history. This is why the Galactic Age focuses on the Milky Way, the Stellar Age focuses mainly on the stars of the Milky Way, the Planetary Age focuses on the Solar System, the Chemical and Biological Age focuses on Earth’s biosphere, and the Cultural Age focuses solely on human culture. It is our history, our meditation.