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--- courses:ast100:0 [2025/02/05 00:37] – [2. Seven Ages, Seven Rivers] asad
+++ courses:ast100:0 [2025/02/05 00:40] (current) – [4. Timeline] asad
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 **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.
 ===== - Space-Time-Energy-Matter =====
+**Socrates**: Excellent conclusion, Rhea. Since the existence of time has come up, I think we should think a bit more about it.
-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 focus a bit on real science, stepping out of metaphors.
-Rhea: Yes, and we should get out of the metaphor and focus on a little real science.
+**Socrates**: Why not? The universe has some mathematical rules whose ultimate source humans do not know. Besides, we who are dead do not know them yet either. But we are familiar with the things these rules work on. Everything that follows rules in the universe can be categorized into four categories: space, time, energy, and matter (abbreviated as 'STEM'). Einstein worked with these four things at the beginning of the twentieth century. His special relativity says that space and time are the same thing, and energy and matter are the same thing. General relativity (GR) tells us 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 for my future eternal life.
-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.
+**Rabi**: But is there any benefit in discussing this if we do not understand the mathematical basis? What I understood from studying and researching relativity until my death is that without knowing math, people understand it ninety percent wrong.
-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 I myself understand it ninety-nine percent wrong. My only knowledge is that I know nothing.
-SOCRATES: Why ninety, I would say ninety-nine percent of the mistakes I make myself. My only knowledge is that I know nothing.
+**Rabi**: Then your gift of words will harm us more than benefit us.
-Ravi: Then the gift of your words will do us more harm than good.
+**Socrates**: Oh, it was because my words harmed the youth that 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 about the universe's spacetime.
-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.
+**Rabi**: Whatever I say in Bengali or English will inevitably be wrong; only math can tell the truth. Since you are incapable of understanding the math of spacetime, listen to the ninety-nine percent wrong version. After discovering GR in 1915, Einstein thought that due to the vast amount of energy-matter, the universe's spacetime would be very curved. The more matter there is, the more space curves. He also created a mathematical model of this four-dimensional curvature. But it is impossible for humans or even us to visualize this four-dimensional box model. But if we reduce the three dimensions of space to two dimensions, a three-dimensional model of spacetime becomes very clear before our eyes. You will understand if I draw it on a tablet. Look at this:
-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:
 {{:courses:phy100:spacetime.png?nolink|}}
-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.
+The surface of this sphere known as Einstein's curveball is two-dimensional space, and along the radius is time. In this model, all energy-matter in the universe exists only on the surface of a sphere with a specific radius at a specific time. The radius increases with time, and so does the size of the universe with the surface area. Initially, Einstein did not think about increasing the radius; he believed in a static universe like Aristotle. To keep the radius from changing, he added a constant called the cosmological constant to his equation. But later in 1930, after finding evidence of the expansion of the universe, he removed this constant. In the twenty-first century, people have brought back this constant not to stop the radius but to control its expansion speed.
-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.
+Currently, we know that the universe looks the same in all directions and its density is roughly equal everywhere. This can also be understood through this three-dimensional spacetime sphere. Think of the surface of this sphere as Earth's surface and imagine you are floating on a raft in such a place in the Pacific where wherever you look you see only water and water; everything looks the same in all directions and wherever you go, water density remains equal.
-SOCRATES: But, Ravi, if I look up, I will not see the same.
+**Socrates**: But Rabi, if I look up, won't it look different?
-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?
+**Rabi**: Oh Socrates, you are still as stubborn as ever. If we reduce three dimensions of space to two dimensions from the start, can there be anything called space other than the surface of the Pacific?
-SOCRATES: No.
+**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.
+**Rabi**: And if there is no space other than this surface, how will you look up? To look up you need three-dimensional space which we have already reduced to two dimensions for this model.
-SOCRATES: Well, I see.
+**Socrates**: Okay, understood.
-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.
+**Rabi**: The fact that everything looks similar in all directions in the universe is called the cosmological principle. Using this three-dimensional model we can explain two more interesting phenomena about our universe. The first one is like this. Hubble observed that all distant galaxies are moving further away from us and those further away are moving faster. Actually, galaxies are not moving; spacetime is expanding. If we think of the above sphere as a balloon and each galaxy as a dot on this balloon's surface, then as the balloon gets bigger with increasing radius or time, its surface area increases and dots (galaxies) move away from each other. Each dot will think that all other dots are moving away from it; each will feel like it is at the center of everything since everything else is moving away from it. But actually no one is at center; there is no center on a sphere's surface but standing anywhere on its surface looking around makes one feel like they are at center.
-SOCRATES: Excellent. And what is the second?
+**Socrates**: Wonderful! And what about second one?
 {{https://upload.wikimedia.org/wikipedia/commons/thumb/9/98/Observable_Universe_with_Measurements_01.png/768px-Observable_Universe_with_Measurements_01.png?nolink&500}}
-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.
+**Rabi**: The second one goes like this. No matter which direction we point our telescope from Earth we will see up to an equal distance in all directions. According to Einstein's special relativity since light speed is constant our horizon is limited both in space and time directions. An example will make it clear; suppose we have three telescopes named X Y Z and assume all galaxies have same absolute brightness then more sensitive telescope sees further galaxies now suppose X sees maximum 1 billion light years away Y sees up to 2 billion light years away Z sees up to 3 billion light years away then X's observed universe would be balloon with radius 1 billion light years Y's balloon radius would be 2 billion light years Z's observed universe radius would be 3 billion light years.
-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.
+**Socrates**: But this doesn't match our three-dimensional spacetime balloon; in picture balloon radius was time but your observable universe radius for three telescopes is space because its unit is light year.
-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.
+**Rabi**: Here lies thinking point though light year unit distance it's related with time seeing galaxy 1 billion light years away means seeing how it was 1 billion years ago because light took 1 billion years to reach here light travels at speed 300000 km per second distance covered by light in one year called one light year or nearly 10 trillion km so observable universe balloon radius actually time X's balloon radius 1 billion years because it sees up to 1 billion years past.
-SOCRATES: I have two observations here. First of all, the picture of Einstein's curveball, is it not a metaphor?
+**Socrates**: Here I have two observations first isn't Einstein's curveball picture also kind 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.
+**Rabi**: Good point true curveball isn't real picture our world four-dimensional reducing it three dimensions mathematically trivial but mentally imaginary yet curveball can't be called metaphor it's above metaphor maybe analogy fits better.
-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.
+**Socrates**: Okay agreed second observation goes like this your observable universe map makes me feel self never seen present never seen only others seen only past seen if self can't be seen can self known? I'm doubting my own words now.
-Ravi: If we can know about the present by looking at the past, can't we know ourselves by looking at others?
+**Rabi**: If by seeing past we learn about present can't by seeing others through seeing learn about self?
 ===== - Timeline =====
+**Socrates**: We have understood the present of the universe well because there is a timeline of the past. It is also impossible to understand the present of society without the past. I have doubts about whether it is possible to know oneself by seeing others in the case of humans. However, be that as it may, I think I can take this opportunity to show you the timeline I created.
-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.
 <html><iframe src='https://cdn.knightlab.com/libs/timeline3/latest/embed/index.html?source=1j2ZU6TEkcz8jwc3p_Yi1kVtfULD2aARnt66QtzBGAAs&font=Default&lang=en&initial_zoom=2&height=750' width='100%' height='750' webkitallowfullscreen mozallowfullscreen allowfullscreen frameborder='0'></iframe></html>
-[[https://cdn.knightlab.com/libs/timeline3/latest/embed/index.html?source=1j2ZU6TEkcz8jwc3p_Yi1kVtfULD2aARnt66QtzBGAAs&font=Default&lang=en&initial_zoom=2|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?
+[[https://cdn.knightlab.com/libs/timeline3/latest/embed/index.html?source=1j2ZU6TEkcz8jwc3p_Yi1kVtfULD2aARnt66QtzBGAAs&font=Default&lang=en&initial_zoom=2|In this timeline]] seven events from each age of the universe have been included. So there are a total of 49 events. Each event has a slide, and below the slide is a navigation that can be zoomed in and out and panned left and right. This will be useful for our meditation. Browsing the timeline can be a meditation of walking towards the history of the universe on a thin rope. I think it would be good for all of us if everyone, starting from Rabi, looks at the seven events of their respective ages and comments on them. Rabi?
-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?
+**Rabi**: Yes, of course. First, I must say again, studying history in this way without math is not honorable for me. Still, I am reading. The first event of the Particle age is definitely the Big Bang, through which our universe was born approximately fourteen billion years ago. Since its birth, the universe has been expanding, sometimes fast, sometimes slow. Immediately after the Big Bang, an event called inflation caused the universe to expand suddenly. Within one trillionth of a second, all forces were born. Within the first second, all elementary particles, quarks, electrons, protons, etc., appeared. Within the first 15 minutes, multiple protons gathered to form the nucleus of elements like helium. Initially, the density of energy was higher than that of matter. Fifty thousand years after the Big Bang, the density of matter increased, surpassing energy. And after three hundred thousand years, electrons combined with the nucleus of protons to form atoms, and photons were freed from the oppression of free electrons. These free photons can still be observed as microwaves. Through them, we have created the first picture of the universe. This is where the Particle age ends. Shashi?
-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?
+**Shashi**: I saw the seven events of the Galactic age on the timeline. The story goes like this. At the end of the Particle age, the universe meant a single gas, with almost equal density everywhere, but with some variation. Where the density was a bit higher, those places became denser under the influence of dark matter during the Dark Age. The Dark Age ended one hundred million years after the Big Bang, and two hundred million years later, galaxies and stars began to form from gas accumulations in over-dense areas; this is called the Cosmic Dawn. Within the first six hundred million years, the intergalactic medium formed, and within eight hundred million years, our galaxy, the Milky Way, was born. One billion years after the Big Bang, the universe was filled with galaxies, and within two and a half billion years, quasars, the most massive and active galaxies, were born. The Galactic age ends three billion years after the Big Bang when galaxies form clusters. Mars?
-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?
+**Mars**: The Stellar age is considered to have started 10.4 billion years ago, meaning three and a half billion years after the Big Bang. At that time, the most stars were being formed within the galaxies of the universe. Ten billion years ago, Population I stars were born, which are young, metal-rich, and reside in the disk of galaxies. Nine billion years ago, many clusters of stars formed, and our galaxy got its thin disk. One billion years later, the formation of superclusters of galaxies created many voids in the universe. Six billion years ago, repulsive dark energy triumphed over attractive gravity, causing the expansion speed of the universe to increase. Five and a half billion years ago, the Milky Way got its spiral shape, and 4.6 billion years ago, the Stellar age ended with the birth of our solar system. Hermes?
-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?
+**Hermes**: The first important event of the Planetary age is the birth of the inner (close to the sun) planets 4.55 billion years ago. Five million years later, the sun reached the main sequence, meaning it achieved the status of a full-fledged star. Approximately 4.4 billion years ago, the hot Earth cooled down, and the oceans were born. But for the next three hundred million years, the Earth was bombarded by huge rocks from space. At the end of this disaster, the continental crust formed, which is still moving and changing the shape of continents. 3.6 billion years ago, all the continents of the Earth joined together to form the Vaalbara supercontinent. This is where this age ends. Juno?
-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?
+**Juno**: Socrates, you started the Chemical age 3.6 billion years ago when possibly the first organic life was created from inorganic matter in hydrothermal vents under the sea. The first fossil of life is found from three and a half billion years ago. The first successful life, bacteria, came to land from the sea about three billion years ago. One hundred million years later, a large amount of cyanobacteria in the sea began to release oxygen into the Earth's atmosphere through photosynthesis. The first eukaryote with a nucleus in its cell appeared 2.7 billion years ago. The amount of oxygen in the atmosphere increased significantly two and a half billion years ago, and three hundred million years later, the ozone layer formed. The Chemical age ends with the ozone layer. Ishtar?
-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.
+**Ishtar**: In the Biological age, the diversity of life began to increase. One and a half billion years ago, the first complex cells formed. Six hundred million years ago, many cells came together to bring about the revolution of multicellular life. Two hundred million years later, animals came out of the water onto land. Two hundred million years ago, warm-blooded animals were born, and sixty-five million years ago, the Biological age ended with the extinction of dinosaurs due to an asteroid impact. Rhea?
-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.
+**Rhea**: Approximately seven million years ago, the Cultural age began with the birth of the first hominins. The creatures of Australopithecus and Homo genus appeared on Earth four to one million years ago. And the first modern humans probably walked the Earth three hundred thousand years ago in Africa. Humans began to spread from Africa to the rest of the world one hundred thousand years ago. Fifty thousand years ago, humans started a huge revolution in religion, music, and art. Agriculture began ten thousand years ago, and five thousand years later, the first states appeared. Globalization began five hundred years ago, and its second wave started two hundred years ago with the Industrial Revolution. This is where the Cultural age ends.
+**Socrates**: Hearing from everyone was really necessary. You can understand, I am not telling the history of every part of the universe, but actually the history of humans. The universe has come here as part of human history. This is why the Milky Way is mentioned in the Galactic age, the focus is mainly on the stars of the Milky Way in the Stellar age, the main subject of the Planetary age is the solar system, and the subjects of the Chemical and Biological ages are the Earth's biosphere, and the subject of the Cultural age is only human culture. This is our history, the subject of our meditation.