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--- courses:phy100:1 [2023/05/27 05:18] – [1. A tour in Space] asad
+++ courses:phy100:1 [2023/10/31 23:22] (current) – [3.2 Celestial sphere] asad
@@ Line 1: / Line 1: @@
 ====== 1. A Tour in Space and Time ======
 Can you see the connection between English 'star' and Bangla 'স্থির'? And why do English 'love' and Bangla 'লোভ' sound similar, and how do they relate to 'লুব্ধক', the brightest star in the sky called 'Sirius' in English?
+===== - Space-time and energy-matter =====
+In the year 2005, [[wp>Einstein]] discovered two surprising facts about the universe we live in when he was only 26 years old. The first was simply that [[wp>space]] ($S$) and [[wp>time]] ($T$) are the same thing:
+$$ \text{Space} = \text{Time}. $$
+The second is that [[wp>matter]] ($M$) and [[wp>energy]] ($E$) are two forms of the same thing. This was formalized in his famous equation $E=Mc^2$ which we will write in English as
+$$ \text{Energy} \equiv \text{Matter}. $$
+We measure space using the unit **meter** ('m' in short), time using **second** ('s'), energy using **joule** ('J') or **watt** ('W') and matter using **kilogram** ('kg').
+{{:courses:phy100:spacetime.png?nolink|}}
+The universe we observe from Earth can be thought of as a spherical ball (shown above) of spacetime filled with energy-matter. Here, space is confined to the surface and time to the radius, but they are connected. We are at the center of the sphere, and we can see a sphere at every radius which is basically the distance of the spherical surface from us. In 1915, Einstein found another surprising fact through his mathematical analysis:
+$$ \text{SpaceTime} \sim \text{EnergyMatter}. $$
+Space-Time (ST) and Energy-Matter (EM) are intimately related. EM tells ST how to bend, and ST tells EM how to move.
 ===== - A tour in Space =====
@@ Line 61: / Line 80: @@
 ===== - Maps of Space =====
+==== - Terrestrial sphere ====
 The curved surface of the spherical earth (the **terrestrial sphere**) is the space we will start to map first, as an orientation for this course. Since Aristotle described the round shadow of earth on the surface of the moon, no astronomer ever doubted that the earth is round. We have mapped the 2-dimensional surface of this sphere using a **geographic coordinate system** that has two references and two angles of measure: latitude and longitude. See the video first.
@@ Line 75: / Line 95: @@
 Can you see the similarity with the example of the marker? The two references here are like the two walls in the marker example. And the angles here are equivalent to those tile-blocks. Here we do not need the third dimension because we are not interested in measuring heights from the surface.
+[[https://astro.unl.edu/naap/motion1/tc_both.html|{{:courses:phy100:lat-long.webp?nolink|}}]]
+==== - Celestial sphere ====
 {{:courses:phy100:twospheres.png?nolink|}}
@@ Line 91: / Line 114: @@
 From 3:33 in the video, I remove the earth from the picture and you see both hemispheres of the sky, albeit not at one glance. You are seeing the interior surface of the celestial sphere. The gridlines clearly show the two poles NCP and SCP and the hazy Milky Way appears as a circular band; normally we see only half of this band at a time, the other half being blocked by the horizons.
-You also see the shapes of the **constellations**. A constellation is a group of stars that form a recognizable pattern when viewed from the earth. These patterns were used for navigation by our ancestors and gave rise to numerous myths throughout human history. The patterns are of course imaginary. Some of the stars in a constellation might be very close to us, while some are extremely distant.  As the surface of the earth is divided into many countries and oceans, the surface of the imaginary celestial sphere is divided into 88 constellations.
+[[https://astro.unl.edu/naap/motion1/cec_both.html|{{:courses:phy100:ra-dec.webp?nolink|}}]]
-We identify people using their countries. Similarly, we identify celestial objects using the constellations in which they belong. For example, the Great Orion Nebula belongs to the Orion constellation. And here is a map of the Orion Constellation, the country of Orion.
-{{https://upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Orion_IAU.svg/825px-Orion_IAU.svg.png?nolink&600}}
-You see the borders and also its neighboring constellations; this is how we would show the map of Bangladesh as well with India and Myanmar as neighbors. Three stars (Alnitak, Alnilam, Mintaka) constitute the belt of the Orion. Betelgeuse and Bellatrix are the hands, Saiph and Rigel are the legs. Our ancestors devised a way to remember this region of the sky using the shape of Orion, a mythic hunter.
-Use https://stellarium-web.org for exploring various constellations. And one of the most curious thing you can observe is the Ursa Minor clock:
-{{youtube>nOlw30viFu8?large}}
-\\
-The constellation Ursa Minor has 7 stars shaped like a bear as the ancients saw it. They look like a clock hand to me and if you rotate the sky for 24 hours in Stellarium, you see the hand move like the hour-hand of a 24-hour clock. This constellation was also called the Phoenician Bear. With the help of Ursa Minor, the mysterious Phoenicians navigated the Mediterranean sea and worked as the bridge from the Semitic aleph-ba-ta to Greek alpha-beta and finally to the world-wide alphabet.
 ===== - A tour in Time =====
@@ Line 164: / Line 175: @@
 | May  | Formation of the disk of our galaxy, the Milky Way. |
 | September  | Formation of our solar system. |
-| October | Plants create oxygen via photosynthesis. |
+| October | Algae create oxygen via photosynthesis. |
-| November | First life. |
 | 15 December | The most ancient fossils of life. |
 | 18 December | Animals with backbone arise. |
@@ Line 181: / Line 191: @@
 | 13.5 years | First stars and galaxies form. |
 | 4.5 years | Sun and the solar system form. |
-| 4 years | The first living organisms on earth appear. |
+| 3.7 years | The first living organisms on earth appear. |
-| 7 months | The first multicellular organisms appear. |
+| 3.5 years | The first multicellular organisms appear. |
 | 3 months | All continents of earth come together to form Pangea. |
 | 3 weeks | Dinosaurs go extinct due to a meteor, mammals fill the gap. |
-| 3 days | The first hominines appear in Africa. |
+| 2 days | The first hominines appear in Africa. |
-| 50 minutes | The first humans (//Homo sapiens//) evolve in Africa. |
+| 2 hours | The first humans (//Homo sapiens//) evolve in Africa. |
-| 6 minutes  | Humans are found all over the world. |
+| 25 minutes  | Humans migrate out of Africa to the whole world. |
 | 5 minutes | First agricultural villages are established. |
 | 3 minutes | First literate urban civilizations appear. |
 | 1 minute | The great civilizations of China, India, Persia and the Mediterranean flourish. |
-| 24 seconds | Turko-Mongols conquer India, Plague strikes Europe. |
+| 21 seconds | Muslims conquer northern India, Black Death strikes Europe. |
 | 15 seconds | A single capitalist world system emerges. |
-| 6 seconds | First industrial revolution. |
+| 7 seconds | First industrial revolution. |
-| 2 seconds | The 31-year war (first and second WW) begins. |
+| 3 seconds | The Thirty-one Years' War (first and second world wars) begins. |
-| Last second | Eight billion humans, a walk on the moon, electronic revolution. |
+| Last second | Seven billion humans, a walk on the moon, electronic revolution. |
+The 1-year calendar and the 14-year calendar aside, we have our own sense of time which seldom correspond to reality. The human psychological time cannot be represented in science, but only in poetry, as Shakespeare did in his famous series of 154 sonnets.
 <blockquote><poem>
@@ Line 217: / Line 229: @@
 <cite>William Shakespeare, Sonnet 19</cite>
 </blockquote>
-===== - Science and pseudoscience =====
-What is science? What is the difference between science (astronomy) and pseudoscience (astrology)?
-"Science is the **practice** that provides us with the most **reliable** (most warranted) statements that can be made, at the time being, on subject matter covered by the **community** of knowledge disciplines." -- Sven Ove Hansson.
+This sonnet in particular takes us back to one of the most ancient sense of cosmic time, the devouring Time who gives birth to children and then causes their death. We are all born in time, and it is Time who takes our life. The Greeks (and Romans) imagined that the god Chronos (Saturn) is Time who eats his own children except Zeus (Jupiter), who was saved by his mother. The march of the planets Saturn and Jupiter along the ecliptic was so significant for them. In Indian culture, কাল or মহাকাল is another name for the god Shiva who destroys all his creation periodically.
-{{ :courses:phy100:foursciences.png?nolink |}}
+===== - Science and pseudoscience =====
+If you get too depressed contemplating your own death at the hands of Time, think about the amazing achievements of Human Culture in the last one hundred thousand years, and specifically about its scientific achievements, at least for the sake of this course.
+This has been possible specially because a significant portion of the society decided to create something called 'science' and separate it from pseudoscience. The two were different but not always opposed to each other. For example, in the ancient and middle ages, astronomers depended on astrology for their livelihood. And the words 'astronomy' and 'astrology' were used interchangeably as you can see in the following poem.
 <blockquote><poem>
@@ Line 244: / Line 257: @@
 <cite>William Shakespeare, Sonnet 14</cite>
 </blockquote>
+Shakespeare does not care much about 'astrology' (astronomy for him) because he derives his knowledge not from the stars of the sky but from the eyes of his friend. The eyes are 'constant stars' (ধ্রুবতারা) for him.
+Eventually astronomy (science) separated itself from astrology (pseudoscience); too bad we lost the rights to use 'logy' as in 'biology' and, instead, had to vie for 'nomy.'  The modern advancements of science would not be possible without a rigorous 'scientific method' which must be followed irrespective of all social and cultural superstitions and prejudices. The method is simple.
+**Scientific method:** (1) Theorize a hypothesis consistent with the currently established theories in order to solve a new problem or describe a new natural phenomenon. (2) Devise realizable experiments that would either verify or falsify the hypothesis. (3) If the verification works, keep the hypothesis unchanged until new problems arise. (4) If the verification fails or falsification wins, go back to 1 and repeat the procedure with a new hypothesis.
+Sven Ove Hansson, a Swedish philosopher gave an useful definition of science: Science is the **practice** that provides us with the most **reliable** (most warranted) statements that can be made, at the time being, on subject matter covered by the **community** of knowledge disciplines. Can you differentiate astrology (a pseudoscience) from astronomy (a science) using this definition?
+{{ :courses:phy100:foursciences.png?nolink |}}
+Using this definition, we can unite all branches of science and think of them as branches of a single 'tree of knowledge' as shown above.