<?xml version="1.0" encoding="utf-8"?><feed xmlns="http://www.w3.org/2005/Atom" xml:lang="en-US"><generator uri="https://jekyllrb.com/" version="4.4.1">Jekyll</generator><link href="https://ulysseszh.github.io/feed/tags/imagination.xml" rel="self" type="application/atom+xml" /><link href="https://ulysseszh.github.io/" rel="alternate" type="text/html" hreflang="en-US" /><updated>2026-04-30T17:49:58-07:00</updated><id>https://ulysseszh.github.io/feed/tags/imagination.xml</id><title type="html"><![CDATA[Ulysses’ trip]]></title><subtitle>Here we are at the awesome (awful) blog written by UlyssesZhan!</subtitle><author><name>UlyssesZhan</name><email>ulysseszhan@gmail.com</email></author><entry><title type="html"><![CDATA[I want to invent a better way to read numbers]]></title><link href="https://ulysseszh.github.io/language/2023/01/11/english-numbers.html" rel="alternate" type="text/html" title="I want to invent a better way to read numbers" /><published>2023-01-11T22:51:41-08:00</published><updated>2023-01-11T22:51:41-08:00</updated><id>https://ulysseszh.github.io/language/2023/01/11/english-numbers</id><content type="html" xml:base="https://ulysseszh.github.io/language/2023/01/11/english-numbers.html"><![CDATA[<p>First, play a simple game. Read the number 7777 in English. Here is what you read:</p>
<blockquote>
Seven thousand seven hundred and seventy-seven.
</blockquote>
<p class="no-indent">
There are 14 syllables while the original number has only 4 digits. This makes it very tedious to read numbers in English. Thus, I want to invent a better way to read numbers!
</p>
<h2 data-label="0.1" id="single-digit-numbers">Single-digit numbers</h2>
<p>First, let’s improve the first few numbers in English. They are</p>
<table>
<thead>
<tr>
<th>Number</th>
<th>Original word</th>
<th>Pronunciation (BE)</th>
</tr>
</thead>
<tbody>
<tr>
<td>0</td>
<td>zero</td>
<td>/ˈzɪərəʊ/</td>
</tr>
<tr>
<td>1</td>
<td>one</td>
<td>/wʌn/</td>
</tr>
<tr>
<td>2</td>
<td>two</td>
<td>/tuː/</td>
</tr>
<tr>
<td>3</td>
<td>three</td>
<td>/θriː/</td>
</tr>
<tr>
<td>4</td>
<td>four</td>
<td>/fɔː/</td>
</tr>
<tr>
<td>5</td>
<td>five</td>
<td>/faɪv/</td>
</tr>
<tr>
<td>6</td>
<td>six</td>
<td>/sɪks/</td>
</tr>
<tr>
<td>7</td>
<td>seven</td>
<td>/ˈsevən/</td>
</tr>
<tr>
<td>8</td>
<td>eight</td>
<td>/eɪt/</td>
</tr>
<tr>
<td>9</td>
<td>nine</td>
<td>/naɪn/</td>
</tr>
<tr>
<td>10</td>
<td>ten</td>
<td>/ten/</td>
</tr>
<tr>
<td>11</td>
<td>eleven</td>
<td>/eˈlevən/</td>
</tr>
<tr>
<td>12</td>
<td>twelve</td>
<td>/twelv/</td>
</tr>
</tbody>
</table>
<p class="no-indent">
(I do not want to read 0 as /əʊ/ because it confuses with the letter O.) Here are several problems:
</p>
<ul>
<li>Some words have more than one syllable or end with a consonant, this makes it hard to pronounce them fast in continuous speech.</li>
<li>What do 10, 11, and 12 do here? They are not single-digit numbers and should not appear here.</li>
<li>Some of the words have strange spelling.</li>
<li><em>two</em> has the same pronunciation as <em>too</em>, and <em>four</em> has the same pronunciation as <em>for</em>, which may be confusing sometimes.</li>
</ul>
<p>I want to improve them as follows:</p>
<table>
<thead>
<tr>
<th>Number</th>
<th>Word</th>
<th>Pronunciation</th>
</tr>
</thead>
<tbody>
<tr>
<td>0</td>
<td>zoh</td>
<td>/zəʊ/</td>
</tr>
<tr>
<td>1</td>
<td>wah</td>
<td>/wɑː/</td>
</tr>
<tr>
<td>2</td>
<td>tue</td>
<td>/tjuː/</td>
</tr>
<tr>
<td>3</td>
<td>tee</td>
<td>/tiː/</td>
</tr>
<tr>
<td>4</td>
<td>foo</td>
<td>/fuː/</td>
</tr>
<tr>
<td>5</td>
<td>fah</td>
<td>/fɑː/</td>
</tr>
<tr>
<td>6</td>
<td>sou</td>
<td>/saʊ/</td>
</tr>
<tr>
<td>7</td>
<td>sah</td>
<td>/sɑː/</td>
</tr>
<tr>
<td>8</td>
<td>tay</td>
<td>/teɪ/</td>
</tr>
<tr>
<td>9</td>
<td>nye</td>
<td>/naɪ/</td>
</tr>
</tbody>
</table>
<p class="no-indent">
There are some advantages:
</p>
<ul>
<li>Most of the words have similar pronunciation as the original words, this makes it easy to remember them. The only exceptions are <em>sou</em> and <em>sah</em> because there too many single-syllable English words that start with /s/, and I have to avoid the existing words.</li>
<li>Each of the words contains only one simple syllable, which makes it easy to pronounce fast.</li>
<li>The spelling pretty much follows the pronunciation and avoids large discrepencies among accents.</li>
<li>The written length of the words are all the same (three letters), which makes it easier to sight-read.</li>
<li>The words are pretty distinguishable by their pronunciations so that they do not confuse when being said in a noisy environment.</li>
</ul>
<h2 data-label="0.2" id="positive-integers-smaller-than-1000">Positive integers smaller than 1000</h2>
<p>In English, the suffix <em>-ty</em> is used to get multiples of ten. This suffix will sound too similar to <em>tee</em> (3) in our improved words for digits. Therefore, I want to propose a new suffix <em>-ta</em>, pronounced as /tə/. The multiples of ten are then</p>
<table>
<thead>
<tr>
<th>Number</th>
<th>Word</th>
<th>Pronunciation</th>
</tr>
</thead>
<tbody>
<tr>
<td>10</td>
<td>wahta</td>
<td>/ˈwɑːtə/</td>
</tr>
<tr>
<td>20</td>
<td>tueta</td>
<td>/ˈtjuːtə/</td>
</tr>
<tr>
<td>30</td>
<td>teeta</td>
<td>/ˈtiːtə/</td>
</tr>
<tr>
<td>40</td>
<td>foota</td>
<td>/ˈfuːtə/</td>
</tr>
<tr>
<td>50</td>
<td>fahta</td>
<td>/ˈfɑːtə/</td>
</tr>
<tr>
<td>60</td>
<td>souta</td>
<td>/ˈsaʊtə/</td>
</tr>
<tr>
<td>70</td>
<td>sahta</td>
<td>/ˈsɑːtə/</td>
</tr>
<tr>
<td>80</td>
<td>tayta</td>
<td>/ˈteɪtə/</td>
</tr>
</tbody>
</table>
<p>For numbers smaller than 100 and not multiples of 10, they are just the sum of the corresponding multiples of 10 and single-digit numbers. The two parts hyphenizes to represent the number. The numbers 11 etc. are then</p>
<table>
<thead>
<tr>
<th>Number</th>
<th>Word</th>
<th>Pronunciation</th>
</tr>
</thead>
<tbody>
<tr>
<td>11</td>
<td>wahta-wah</td>
<td>/ˈwɑːtəˈwɑː/</td>
</tr>
<tr>
<td>12</td>
<td>wahta-tue</td>
<td>/ˈwɑːtəˈtjuː/</td>
</tr>
<tr>
<td>13</td>
<td>wahta-tee</td>
<td>/ˈwɑːtəˈtiː/</td>
</tr>
<tr>
<td>14</td>
<td>wahta-foo</td>
<td>/ˈwɑːtəˈfuː/</td>
</tr>
<tr>
<td>15</td>
<td>wahta-fah</td>
<td>/ˈwɑːtəˈfɑː/</td>
</tr>
</tbody>
</table>
<p>Now we can read all numbers smaller than 100.</p>
<p>For numbers starting from 100, English uses the word <em>hundred</em>, which is very long, and it is almost always followed by a useless word <em>and</em>. A better way to express numbers starting from 100 is just how we express those from 10. I want to propose a new suffix <em>-ha</em>, pronounced as /hə/, to represent multiples of hundred. The multiples of hundred are then</p>
<table>
<thead>
<tr>
<th>Number</th>
<th>Word</th>
<th>Pronunciation</th>
</tr>
</thead>
<tbody>
<tr>
<td>100</td>
<td>wahha</td>
<td>/ˈwɑːhə/</td>
</tr>
<tr>
<td>200</td>
<td>tueha</td>
<td>/ˈtjuːhə/</td>
</tr>
<tr>
<td>300</td>
<td>teeha</td>
<td>/ˈtiːhə/</td>
</tr>
<tr>
<td>400</td>
<td>fooha</td>
<td>/ˈfuːhə/</td>
</tr>
<tr>
<td>500</td>
<td>fahha</td>
<td>/ˈfɑːhə/</td>
</tr>
<tr>
<td>600</td>
<td>souha</td>
<td>/ˈsaʊhə/</td>
</tr>
<tr>
<td>700</td>
<td>sahha</td>
<td>/ˈsɑːhə/</td>
</tr>
<tr>
<td>800</td>
<td>tayha</td>
<td>/ˈteɪhə/</td>
</tr>
<tr>
<td>900</td>
<td>nyeha</td>
<td>/ˈnaɪhə/</td>
</tr>
</tbody>
</table>
<p class="no-indent">
Hyphenization gives other numbers below 1000:
</p>
<table>
<thead>
<tr>
<th>Number</th>
<th>Word</th>
</tr>
</thead>
<tbody>
<tr>
<td>101</td>
<td>wahha-wah</td>
</tr>
<tr>
<td>111</td>
<td>wahha-wahta-wah</td>
</tr>
</tbody>
</table>
<p>Sometimes we also benefits from just reading the digits. If the speaker know for sure that the audience will not confuse whether the number is a categorical data or a cardinal data, then the speaker can just read the digits. This makes the delivery of information about numbers really fast considering that now every digit only has one syllable.</p>
<table>
<thead>
<tr>
<th>Number</th>
<th>Word</th>
<th>Shortcut</th>
</tr>
</thead>
<tbody>
<tr>
<td>42</td>
<td>foota-tue</td>
<td>foo-tue</td>
</tr>
<tr>
<td>255</td>
<td>tueha-fahta-fah</td>
<td>tue-fah-fah</td>
</tr>
</tbody>
</table>
<h2 data-label="0.3" id="integers-starting-from-1000">Integers starting from 1000</h2>
<p>For numbers larger than 1000, just use how English deal with them, but use <em>thou</em>, <em>mill</em>, <em>bill</em>, etc. to replace the multi-syllable words <em>thousand</em>, <em>million</em>, <em>billion</em>, etc. Another way to do this is just reading digits.</p>
<table>
<thead>
<tr>
<th>Number</th>
<th>Word</th>
<th>Shortcut</th>
<th>Shorter-cut</th>
</tr>
</thead>
<tbody>
<tr>
<td>1,001</td>
<td>wah thou wah</td>
<td>wah thou wah</td>
<td>wah-zoh-zoh-wah</td>
</tr>
<tr>
<td>1,984</td>
<td>wah thou nyeha-tayta-foo</td>
<td>wah thou nye-tay-foo</td>
<td>wah-nye-tay-foo</td>
</tr>
<tr>
<td>65,535</td>
<td>souta-fah thou fahha-teeta-fah</td>
<td>sou-fah thou fah-tee-fah</td>
<td>sou-fah-fah-tee-fah</td>
</tr>
<tr>
<td>20,031,108</td>
<td>tueta mill teeta-wah thou wahha-tay</td>
<td>tue-zoh mill tee-wah thou wah-zoh-tay</td>
<td>tue-zoh-zoh-tee-wah-wah-zoh-tay</td>
</tr>
</tbody>
</table>
<p class="no-indent">
(Actually, I do not think using <em>thou</em>, <em>mill</em>, etc. is a good idea because they coincide with existing English words and are hard to remember (for foreigners), but I cannot come up with a better idea.)
</p>
<h2 data-label="0.4" id="decimals">Decimals</h2>
<p>For decimals, just use how English deal with them. Use <em>dot</em> instead of <em>point</em> to tell the decimal point because it is easier to pronounce. Also, I propose that we read <em>dot</em> as /dɒ/ instead of /dɒt/ to pronounce it faster.</p>
<table>
<thead>
<tr>
<th>Number</th>
<th>Word</th>
</tr>
</thead>
<tbody>
<tr>
<td>3.14</td>
<td>tee dot wah-foo</td>
</tr>
<tr>
<td>520.1314</td>
<td>fahha-tueta dot wah-tee-wah-foo</td>
</tr>
</tbody>
</table>
<h2 data-label="0.5" id="negative-numbers">Negative numbers</h2>
<p>For negative numbers, add <em>ne</em> (from the word <em>negative</em>), pronounced as /neɪ/, before the number.</p>
<table>
<thead>
<tr>
<th>Number</th>
<th>Word</th>
<th>Pronunciation</th>
</tr>
</thead>
<tbody>
<tr>
<td>-100</td>
<td>ne wahha</td>
<td>/ˈneɪˈwɑːhə/</td>
</tr>
</tbody>
</table>
<h2 data-label="0.6" id="fractions">Fractions</h2>
<p>English use ordinal numerals to represent fractions, which is very non-intuitive and sometimes ambiguous. There are various ways to represent fractions in English that do not involve ordinal numerals. For example, a half can be represented as</p>
<blockquote>
<ul>
<li>1 in 2,</li>
<li>1 (divided) by 2,</li>
<li>1 over 2,</li>
<li>1 to 2 <em>(only used for ratios)</em>,</li>
<li>1 slash 2,</li>
</ul>
</blockquote>
<p class="no-indent">
etc., but none of them is a good way because they may be ambiguous. We need to invent a new word. I propose using the word <em>ci</em>, from the second syllable in the word <em>reciprocal</em>, pronounced as /sɪ/.
</p>
<table>
<thead>
<tr>
<th>Number</th>
<th>Word</th>
</tr>
</thead>
<tbody>
<tr>
<td><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mfrac><mn>22</mn><mn>7</mn></mfrac></mrow><annotation encoding="application/x-tex">\frac{22}7</annotation></semantics></math></span></span></td>
<td>tueta-tue ci sah</td>
</tr>
<tr>
<td><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mfrac><mn>114</mn><mn>514</mn></mfrac></mrow><annotation encoding="application/x-tex">\frac{114}{514}</annotation></semantics></math></span></span></td>
<td>wahha-wahta-foo ci fahha-wahta-foo</td>
</tr>
<tr>
<td><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo>−</mo><mfrac><mn>1</mn><mn>5</mn></mfrac></mrow><annotation encoding="application/x-tex">-\frac15</annotation></semantics></math></span></span></td>
<td>ne wah ci fah</td>
</tr>
</tbody>
</table>
<h2 data-label="0.7" id="scientific-notation">Scientific notation</h2>
<p>How do you read <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>1.234</mn><mo>×</mo><mn>1</mn><msup><mn>0</mn><mrow><mo>−</mo><mn>8</mn></mrow></msup></mrow><annotation encoding="application/x-tex">1.234\times10^{-8}</annotation></semantics></math></span></span>?</p>
<blockquote>
One point two three four times ten to the power of negative eight.
</blockquote>
<p class="no-indent">
That is very long! I propose that we write the number as 1.234e-8, and just pronounce how it looks (pronounce the <em>e</em> here as /iː/).
</p>
<table>
<thead>
<tr>
<th>Number</th>
<th>Word</th>
</tr>
</thead>
<tbody>
<tr>
<td>1.234e-8</td>
<td>wah dot tue-tee-foo ee ne tay</td>
</tr>
</tbody>
</table>
<h2 data-label="0.8" id="ordinal-numerals">Ordinal numerals</h2>
<p>English uses the suffixes <em>-th</em>, <em>-st</em>, <em>-nd</em>, and <em>-rd</em> to represent ordinal numerals, which is actually a disaster…</p>
<ul>
<li><a href="https://english.stackexchange.com/q/36512/457522" target="_blank" rel="external">Do you like <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo stretchy="false">(</mo><mi>n</mi><mo>+</mo><mn>1</mn><mo stretchy="false">)</mo></mrow><annotation encoding="application/x-tex">(n+1)</annotation></semantics></math></span></span>st or <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo stretchy="false">(</mo><mi>n</mi><mo>+</mo><mn>1</mn><mo stretchy="false">)</mo></mrow><annotation encoding="application/x-tex">(n+1)</annotation></semantics></math></span></span>th?</a> (People surely have personal preferences over these two according to the answers, and there is no standard.)</li>
<li>You may know <a href="https://harrypotter.fandom.com/wiki/Platform_Nine_and_Three-Quarters" target="_blank" rel="external">Platform Nine and Three-Quarters</a> if you are a <cite>Harry Potter</cite> fan, but what is the ordinal of <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>9</mn><mfrac><mn>3</mn><mn>4</mn></mfrac></mrow><annotation encoding="application/x-tex">9\frac34</annotation></semantics></math></span></span>?</li>
<li>Because there are multiple suffixes, there is a disaster for non-native speakers to write ordinal numerals. You may <a href="https://www.google.com/search?q=21th" target="_blank" rel="external">Google “21th”</a> to see how many people have made the mistake.</li>
<li>How to ask a question w.r.t. an unknown ordinal numeral? <a href="https://english.stackexchange.com/q/21876/457522" target="_blank" rel="external">“The how many-th president is Barack Obama?”</a> (Most the answers in this link cannot generalize to other questions like “How many-th most beautiful person is Kat in the world?” or even weirder questions like “You love Kat the how many-th most in the world?”)</li>
</ul>
<p class="no-indent">
Therefore, we need to have a consistent way to turn any cardinal numeral into an ordinal numeral, and we need to invent a way to ask questions about ordinal numerals.
</p>
<p>I then want to use the word <em>ra</em> (abbreviation for <em>rank</em>, although this is an existing English meaning the hawk-headed sun god), pronounced as /rɑː/. Put the word <em>ra</em> before a number to represent the ordinal numeral.</p>
<table>
<thead>
<tr>
<th>Original word</th>
<th>Word</th>
</tr>
</thead>
<tbody>
<tr>
<td>first</td>
<td>ra wah</td>
</tr>
<tr>
<td>twentieth</td>
<td>ra tueta</td>
</tr>
<tr>
<td><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>n</mi></mrow><annotation encoding="application/x-tex">n</annotation></semantics></math></span></span>th</td>
<td>ra <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>n</mi></mrow><annotation encoding="application/x-tex">n</annotation></semantics></math></span></span></td>
</tr>
</tbody>
</table>
<p>To ask a question about an ordinal numeral, just convert the structure “ra + <em>num.</em> + <em>sth</em>” into “ra what + <em>sth</em>”, and form the rest of the sentence to get a question. If there is a definite article before <em>ra</em>, omit it in the question.</p>
<p>Example sentences:</p>
<blockquote>
<ul>
<li>The ra 21 century is the century of biology.</li>
<li>Kat wants a ra tue child.</li>
<li>The ra <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>n</mi></mrow><annotation encoding="application/x-tex">n</annotation></semantics></math></span></span> number in the Fibonacci sequence is <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msub><mi>F</mi><mi>n</mi></msub></mrow><annotation encoding="application/x-tex">F_n</annotation></semantics></math></span></span>.</li>
<li>Students board Hogwarts Express at the ra <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>9</mn><mfrac><mn>3</mn><mn>4</mn></mfrac></mrow><annotation encoding="application/x-tex">9\frac34</annotation></semantics></math></span></span> platform on the ra wah day in September.</li>
<li>The ra ne wah element in an array is defined to be its last element. Generally, the ra <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo>−</mo><mi>k</mi></mrow><annotation encoding="application/x-tex">-k</annotation></semantics></math></span></span> element in an array is defined to be its ra <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>n</mi><mo>−</mo><mi>k</mi></mrow><annotation encoding="application/x-tex">n-k</annotation></semantics></math></span></span> element, where <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>n</mi></mrow><annotation encoding="application/x-tex">n</annotation></semantics></math></span></span> is the length of the array.</li>
<li>In an arithmetic progression, the difference between the ra <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>n</mi><mo>+</mo><mn>1</mn></mrow><annotation encoding="application/x-tex">n+1</annotation></semantics></math></span></span> term and the ra <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>n</mi></mrow><annotation encoding="application/x-tex">n</annotation></semantics></math></span></span> term is the same for any <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>n</mi></mrow><annotation encoding="application/x-tex">n</annotation></semantics></math></span></span>.</li>
<li>Ra what president is Barack Obama?</li>
<li>Ra what most beautiful person is Kat in the world?</li>
<li>Ra what most do you love Kat in the world? / Ra what most in the world do you love Kat?</li>
</ul>
</blockquote>
<p>Although I invented a set of new rules for ordinal numerals, I do not delete the word <em>first</em> in English. The word <em>first</em> now means the frontmost thing in spatial or chronological order. The difference between being the first and being the ra wah appears when the things in order are uncountable or does not start from number 1. For example, we can say the first element in the interval <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo stretchy="false">[</mo><mn>0</mn><mo separator="true">,</mo><mn>1</mn><mo stretchy="false">]</mo></mrow><annotation encoding="application/x-tex">[0,1]</annotation></semantics></math></span></span> is <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>0</mn></mrow><annotation encoding="application/x-tex">0</annotation></semantics></math></span></span> if we order the elements from the small to large, but there is no <em>ra wah</em> element in it. For another example, because indices start from 0 in programming languages, the first element in an array is the <em>ra zoh</em> element instead of the <em>ra wah</em> element.</p>
<p>The word <em>first</em> can also be used in the following circumstances:</p>
<ul>
<li>In the structure <em>first… next… then… last…</em> or other similar structures;</li>
<li>In collocations like <em>in the first place</em>, <em>first of all</em>, etc.</li>
</ul>
<p>Sometimes, we may use <em>firstly</em> instead of <em>first</em> when it is used as an adverb (the same thing applys to other ordinal numerals). I would like to just replace <em>firstly</em>, <em>secondly</em>, etc. with <em>ra wah</em>, <em>ra tue</em>, etc.</p>
<h2 data-label="0.9" id="numerical-prefixes">Numerical prefixes</h2>
<p>Here is another mess with English: numerical prefixes. Just look at how many <a href="https://en.wikipedia.org/wiki/Numeral_prefix" target="_blank" rel="external">systems of numerical prefixes we have in English</a>.</p>
<p>However, I do not have motivations to fix the numerical prefixes… After all, <a href="https://english.stackexchange.com/a/28374/457522" target="_blank" rel="external">people will invent their own</a>. Also, there is an advantage of using numerical prefixes different from normal numerals: they avoid ambiguity in oral speech.</p>]]></content><author><name>UlyssesZhan</name><email>ulysseszhan@gmail.com</email></author><category term="language" /><category term="imagination" /><summary type="html"><![CDATA[Reading numbers in English is a pain. I want to invent a better way to read numbers!]]></summary><media:thumbnail xmlns:media="http://search.yahoo.com/mrss/" url="https://ulysseszh.github.io/assets/images/covers/2023-01-11-english-numbers.png" /><media:content medium="image" url="https://ulysseszh.github.io/assets/images/covers/2023-01-11-english-numbers.png" xmlns:media="http://search.yahoo.com/mrss/" /></entry><entry><title type="html"><![CDATA[How to construct mechanics in higher dimensions?]]></title><link href="https://ulysseszh.github.io/physics/2022/11/20/high-dim-mechanics.html" rel="alternate" type="text/html" title="How to construct mechanics in higher dimensions?" /><published>2022-11-20T17:56:52-08:00</published><updated>2022-11-20T17:56:52-08:00</updated><id>https://ulysseszh.github.io/physics/2022/11/20/high-dim-mechanics</id><content type="html" xml:base="https://ulysseszh.github.io/physics/2022/11/20/high-dim-mechanics.html"><![CDATA[<p>
  <em>This article is translated from a Chinese <a href="https://zhuanlan.zhihu.com/p/142546940" target="_blank" rel="external">article</a> on my Zhihu account. The original article was posted at 2020-05-28 21:24 +0800.</em>
</p>
<hr/>
<p>The spacetime of higher dimensions that we mention here refers to a Galileo universe with <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ι</mi></mrow><annotation encoding="application/x-tex">\iota</annotation></semantics></math></span></span> time dimensions and <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>χ</mi></mrow><annotation encoding="application/x-tex">\chi</annotation></semantics></math></span></span> space dimensions. It has affine structure, and we manually define a coordinate system on it. Galilean transformations include uniform-velocity motion (note that velocities are matrices (see below)), spacetime translation, and space rotation. The principle of Galilean relativity still holds.</p>
<p>The universe is <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo fence="true">(</mo><mi>ι</mi><mo>+</mo><mi>χ</mi><mo fence="true">)</mo></mrow><annotation encoding="application/x-tex">\left(\iota+\chi\right)</annotation></semantics></math></span></span>-dimensional, manually separated into an <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ι</mi></mrow><annotation encoding="application/x-tex">\iota</annotation></semantics></math></span></span>-dimensional subspace and a <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>χ</mi></mrow><annotation encoding="application/x-tex">\chi</annotation></semantics></math></span></span>-dimensional subspace, where the former is called the time space, and the latter is called the space space. World points (events) are described by the combination of time coordinates and space coordinates.</p>
<p>Note that, when we find the derivative of a function w.r.t. time, we need to distinguish the total partial derivative and the partial partial derivative. The former regards all variables as functions of time, while the latter one does not. We denote the total partial derivative of function <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>F</mi></mrow><annotation encoding="application/x-tex">F</annotation></semantics></math></span></span> w.r.t. the <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>j</mi></mrow><annotation encoding="application/x-tex">j</annotation></semantics></math></span></span>th time coordinate as <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><mi>F</mi></mrow><annotation encoding="application/x-tex">\partial_jF</annotation></semantics></math></span></span>, and the partial partial derivative as <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mfrac><mrow><mi mathvariant="normal">∂</mi><mi>F</mi></mrow><mrow><mi mathvariant="normal">∂</mi><msub><mi>t</mi><mi>j</mi></msub></mrow></mfrac></mrow><annotation encoding="application/x-tex">\frac{\partial F}{\partial t_j}</annotation></semantics></math></span></span>.</p>
<p>To avoid confusion, there is an example. For example, the total partial derivative of <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>F</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>q</mi><mo separator="true">,</mo><mi>t</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">F\!\left(q,t\right)</annotation></semantics></math></span></span> w.r.t. <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msub><mi>t</mi><mi>j</mi></msub></mrow><annotation encoding="application/x-tex">t_j</annotation></semantics></math></span></span> is <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><mi>F</mi><mo>=</mo><mfrac><mrow><mi mathvariant="normal">∂</mi><mi>F</mi></mrow><mrow><mi mathvariant="normal">∂</mi><mi>q</mi></mrow></mfrac><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><mi>q</mi><mo>+</mo><mfrac><mrow><mi mathvariant="normal">∂</mi><mi>F</mi></mrow><mrow><mi mathvariant="normal">∂</mi><msub><mi>t</mi><mi>j</mi></msub></mrow></mfrac></mrow><annotation encoding="application/x-tex">\partial_jF=\frac{\partial F}{\partial q}\partial_jq+\frac{\partial F}{\partial t_j}</annotation></semantics></math></span></span>. We can see that the total partial derivative has an extra term in addition to the partial partial derivative, which originates from the change of other independent variables of the function due to the change of time.</p>
<p>A system with <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>s</mi></mrow><annotation encoding="application/x-tex">s</annotation></semantics></math></span></span> DOF needs <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>s</mi></mrow><annotation encoding="application/x-tex">s</annotation></semantics></math></span></span> multivariable functions <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msub><mi>q</mi><mi>k</mi></msub><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>t</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">q_k\!\left(t\right)</annotation></semantics></math></span></span> to describe, where <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>k</mi></mrow><annotation encoding="application/x-tex">k</annotation></semantics></math></span></span> is the subscript, and <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>t</mi><mo>∈</mo><msup><mi mathvariant="double-struck">R</mi><mi>ι</mi></msup></mrow><annotation encoding="application/x-tex">t\in\mathbb R^\iota</annotation></semantics></math></span></span>. The numbers <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msub><mi>q</mi><mi>k</mi></msub></mrow><annotation encoding="application/x-tex">q_k</annotation></semantics></math></span></span> are called generalized coordinates. Generalized coordinates are mappings from the time space to the space space.</p>
<p>Then, the generalized velocities become matrices, whose each component represents each generalized coordinate changes w.r.t. each component of time coordinates. Written explicitly, it is <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>q</mi><mi>k</mi></msub></mrow><annotation encoding="application/x-tex">\partial_jq_k</annotation></semantics></math></span></span>. It can be regarded as the Jacobian matrix of generalized coordinates. We may think that the generalized velocities span a space called the velocity space.</p>
<p>Just like traditional universe with one time dimension, we define the action <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">S</mi></mrow><annotation encoding="application/x-tex">\mathcal S</annotation></semantics></math></span></span> as the volume integral of the function <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">L</mi></mrow><annotation encoding="application/x-tex">\mathcal L</annotation></semantics></math></span></span> in the time space, where the Lagrangian <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">L</mi></mrow><annotation encoding="application/x-tex">\mathcal L</annotation></semantics></math></span></span> is a scalar function defined on the space-velocity-time phase space. Written explicitly, it is <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">S</mi><mo><mi mathvariant="normal">≔</mi></mo><msub><mo>∫</mo><mi>M</mi></msub><mi mathvariant="script">L</mi><mspace linebreak="newline"/><mo separator="true">,</mo><mi mathvariant="normal">d</mi><msub><mi>V</mi><mi>t</mi></msub></mrow><annotation encoding="application/x-tex">\mathcal S\coloneqq\int_M\mathcal L\\,\mathrm dV_t</annotation></semantics></math></span></span>, where <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>M</mi><mo>⊆</mo><msup><mi mathvariant="double-struck">R</mi><mi>ι</mi></msup></mrow><annotation encoding="application/x-tex">M\subseteq\mathbb R^\iota</annotation></semantics></math></span></span> is a region in the time space, and <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="normal">d</mi><msub><mi>V</mi><mi>t</mi></msub></mrow><annotation encoding="application/x-tex">\mathrm dV_t</annotation></semantics></math></span></span> is the volume element in the time space.</p>
<p>Now, Hamilton’s principle still holds. What it says is that, if we regard <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">S</mi></mrow><annotation encoding="application/x-tex">\mathcal S</annotation></semantics></math></span></span> as a functional of the function <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>q</mi></mrow><annotation encoding="application/x-tex">q</annotation></semantics></math></span></span>, then the problem of finding the actual motion <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>q</mi></mrow><annotation encoding="application/x-tex">q</annotation></semantics></math></span></span> of the system is equivalent to solve the optimization problem: constraint the value of <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>q</mi></mrow><annotation encoding="application/x-tex">q</annotation></semantics></math></span></span> on <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="normal">∂</mi><mi>M</mi></mrow><annotation encoding="application/x-tex">\partial M</annotation></semantics></math></span></span>, and minimize <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">S</mi></mrow><annotation encoding="application/x-tex">\mathcal S</annotation></semantics></math></span></span>.</p>
<p>In this case, the Euler–Lagrange equation is (according to <a href="/math/2020/05/31/high-dim-euler-eq.html">a previous post</a>) <span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><munder><mo>∑</mo><mi>j</mi></munder><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><mfrac><mrow><mi mathvariant="normal">∂</mi><mi mathvariant="script">L</mi></mrow><mrow><mi mathvariant="normal">∂</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>q</mi><mi>k</mi></msub><mo fence="true">)</mo></mrow></mrow></mfrac><mo>=</mo><mfrac><mrow><mi mathvariant="normal">∂</mi><mi mathvariant="script">L</mi></mrow><mrow><mi mathvariant="normal">∂</mi><msub><mi>q</mi><mi>k</mi></msub></mrow></mfrac><mi mathvariant="normal">.</mi></mrow><annotation encoding="application/x-tex">\sum_j\partial_j\frac{\partial\mathcal L}{\partial\!\left(\partial_jq_k\right)}=\frac{\partial\mathcal L}{\partial q_k}.</annotation></semantics></math></span></span></span> Therefore, the momentum is defined to be the matrix
<span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><msub><mi>p</mi><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></msub><mo><mi mathvariant="normal">≔</mi></mo><mfrac><mrow><mi mathvariant="normal">∂</mi><mi mathvariant="script">L</mi></mrow><mrow><mi mathvariant="normal">∂</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>q</mi><mi>k</mi></msub><mo fence="true">)</mo></mrow></mrow></mfrac><mi mathvariant="normal">.</mi></mrow><annotation encoding="application/x-tex">p_{j,k}\coloneqq\frac{\partial\mathcal L}{\partial\!\left(\partial_jq_k\right)}.</annotation></semantics></math></span></span></span> Note that now the Euler–Lagrange equation is a set of second-order PDEs.</p>
<p>We try performing Legendre transformation on <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">S</mi></mrow><annotation encoding="application/x-tex">\mathcal S</annotation></semantics></math></span></span> and get Hamiltonian <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">H</mi></mrow><annotation encoding="application/x-tex">\mathcal H</annotation></semantics></math></span></span>.</p>
<p>As a function of space coordinates and velocities, the total derivative of <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">L</mi></mrow><annotation encoding="application/x-tex">\mathcal L</annotation></semantics></math></span></span> is <span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mi mathvariant="normal">d</mi><mi mathvariant="script">L</mi><mo>=</mo><munder><mo>∑</mo><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></munder><mfrac><mrow><mi mathvariant="normal">∂</mi><mi mathvariant="script">L</mi></mrow><mrow><mi mathvariant="normal">∂</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>q</mi><mi>k</mi></msub><mo fence="true">)</mo></mrow></mrow></mfrac><mtext> </mtext><mi mathvariant="normal">d</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>q</mi><mi>k</mi></msub><mo fence="true">)</mo></mrow><mo>+</mo><munder><mo>∑</mo><mi>k</mi></munder><mfrac><mrow><mi mathvariant="normal">∂</mi><mi mathvariant="script">L</mi></mrow><mrow><mi mathvariant="normal">∂</mi><msub><mi>q</mi><mi>k</mi></msub></mrow></mfrac><mtext> </mtext><mi mathvariant="normal">d</mi><msub><mi>q</mi><mi>k</mi></msub><mi mathvariant="normal">.</mi></mrow><annotation encoding="application/x-tex">\mathrm d\mathcal L=\sum_{j,k}\frac{\partial\mathcal L}{\partial\!\left(\partial_jq_k\right)}\,\mathrm d\!\left(\partial_jq_k\right)
+\sum_k\frac{\partial\mathcal L}{\partial q_k}\,\mathrm dq_k.</annotation></semantics></math></span></span></span> Substitute the Euler–Lagrange equation and the definition of momenta, and we have
<span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mi mathvariant="normal">d</mi><mi mathvariant="script">L</mi><mo>=</mo><munder><mo>∑</mo><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></munder><msub><mi>p</mi><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></msub><mtext> </mtext><mi mathvariant="normal">d</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>q</mi><mi>k</mi></msub><mo fence="true">)</mo></mrow><mo>+</mo><munder><mo>∑</mo><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></munder><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>p</mi><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></msub><mtext> </mtext><mi mathvariant="normal">d</mi><msub><mi>q</mi><mi>k</mi></msub><mi mathvariant="normal">.</mi></mrow><annotation encoding="application/x-tex">\mathrm d\mathcal L=\sum_{j,k}p_{j,k}\,\mathrm d\!\left(\partial_jq_k\right)+\sum_{j,k}\partial_jp_{j,k}\,\mathrm dq_k.</annotation></semantics></math></span></span></span> By the product rule, the first term in the formula above can be written as
<span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><msub><mi>p</mi><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></msub><mtext> </mtext><mi mathvariant="normal">d</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>q</mi><mi>k</mi></msub><mo fence="true">)</mo></mrow><mo>=</mo><mi mathvariant="normal">d</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><msub><mi>p</mi><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></msub><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>q</mi><mi>k</mi></msub><mo fence="true">)</mo></mrow><mo>−</mo><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>q</mi><mi>k</mi></msub><mtext> </mtext><mi mathvariant="normal">d</mi><msub><mi>p</mi><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></msub><mo separator="true">,</mo></mrow><annotation encoding="application/x-tex">p_{j,k}\,\mathrm d\!\left(\partial_jq_k\right)=\mathrm d\!\left(p_{j,k}\partial_jq_k\right)-\partial_jq_k\,\mathrm dp_{j,k},</annotation></semantics></math></span></span></span> and then we have <span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mi mathvariant="normal">d</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><munder><mo>∑</mo><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></munder><msub><mi>p</mi><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></msub><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>q</mi><mi>k</mi></msub><mo>−</mo><mi mathvariant="script">L</mi><mo fence="true">)</mo></mrow><mo>=</mo><mo>−</mo><munder><mo>∑</mo><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></munder><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>p</mi><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></msub><mtext> </mtext><mi mathvariant="normal">d</mi><msub><mi>q</mi><mi>k</mi></msub><mo>+</mo><munder><mo>∑</mo><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></munder><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>q</mi><mi>k</mi></msub><mtext> </mtext><mi mathvariant="normal">d</mi><msub><mi>p</mi><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></msub><mi mathvariant="normal">.</mi></mrow><annotation encoding="application/x-tex">\mathrm d\!\left(\sum_{j,k}p_{j,k}\partial_jq_k-\mathcal L\right)
=-\sum_{j,k}\partial_jp_{j,k}\,\mathrm dq_k+\sum_{j,k}\partial_jq_k\,\mathrm dp_{j,k}.</annotation></semantics></math></span></span></span> If we let
<span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">H</mi><mo><mi mathvariant="normal">≔</mi></mo><msub><mo>∑</mo><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></msub><msub><mi>p</mi><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></msub><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>q</mi><mi>k</mi></msub><mo>−</mo><mi mathvariant="script">L</mi></mrow><annotation encoding="application/x-tex">\mathcal H\coloneqq\sum_{j,k}p_{j,k}\partial_jq_k-\mathcal L</annotation></semantics></math></span></span>, then we have
<span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>q</mi><mi>k</mi></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">∂</mi><mi mathvariant="script">H</mi></mrow><mrow><mi mathvariant="normal">∂</mi><msub><mi>p</mi><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></msub></mrow></mfrac><mo separator="true">,</mo><mspace width="1em"/><munder><mo>∑</mo><mi>j</mi></munder><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>p</mi><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></msub><mo>=</mo><mo>−</mo><mfrac><mrow><mi mathvariant="normal">∂</mi><mi mathvariant="script">H</mi></mrow><mrow><mi mathvariant="normal">∂</mi><msub><mi>q</mi><mi>k</mi></msub></mrow></mfrac><mi mathvariant="normal">.</mi></mrow><annotation encoding="application/x-tex">\partial_jq_k=\frac{\partial\mathcal H}{\partial p_{j,k}},\quad\sum_j\partial_jp_{j,k}=-\frac{\partial\mathcal H}{\partial q_k}.</annotation></semantics></math></span></span></span> This is the new Hamiltonian equations, or canonical equations. We may find that it lacks the beauty of the form in one-dimensional time.</p>
<p>Problem 1: Prove that if we add the Lagrangian by the “total divergence” w.r.t. time of some function defined on space and time, the new Lagrangian describes the same mechanical system as the original. In other words, <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msup><mi mathvariant="script">L</mi><mo mathvariant="normal" lspace="0em" rspace="0em">′</mo></msup><mo><mi mathvariant="normal">≔</mi></mo><mi mathvariant="script">L</mi><mo>+</mo><msub><mo>∑</mo><mi>j</mi></msub><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><mi>f</mi></mrow><annotation encoding="application/x-tex">\mathcal L'\coloneqq\mathcal L+\sum_j\partial_jf</annotation></semantics></math></span></span> has the same equation of motion as <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">L</mi></mrow><annotation encoding="application/x-tex">\mathcal L</annotation></semantics></math></span></span>, where <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>f</mi></mrow><annotation encoding="application/x-tex">f</annotation></semantics></math></span></span> is an arbitrary function defined on the spacetime.</p>
<p>Problem 2: Prove by principle of Galilean relativity that the Lagrangian of a single free particle system is <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">L</mi><mo>=</mo><msub><mo>∑</mo><mrow><mi>j</mi><mo separator="true">,</mo><mi>k</mi></mrow></msub><mfrac><msub><mi>m</mi><mi>j</mi></msub><mn>2</mn></mfrac><msup><mrow><mo fence="true">(</mo><msub><mi mathvariant="normal">∂</mi><mi>j</mi></msub><msub><mi>q</mi><mi>k</mi></msub><mo fence="true">)</mo></mrow><mn>2</mn></msup></mrow><annotation encoding="application/x-tex">\mathcal L=\sum_{j,k}\frac{m_j}2\left(\partial_jq_k\right)^2</annotation></semantics></math></span></span>, where <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msub><mi>m</mi><mi>j</mi></msub></mrow><annotation encoding="application/x-tex">m_j</annotation></semantics></math></span></span> are constants (their physical meaning is mass, which means that mass is not scalar in time of higher dimensions), and <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msub><mi>q</mi><mi>k</mi></msub></mrow><annotation encoding="application/x-tex">q_k</annotation></semantics></math></span></span> are Cartesian coordinates. Find and solve its equation of motion, and hence derive the law of inertia.</p>
<p>Problem 3: Does the conservation of energy still hold?</p>
<p>A problem that I am too lazy to consider: Consider the Minkowski universe. Poincaré transformations are defined as those affine transformations that preserve the spacetime distance between events, and the spacetime distance is defined as the difference of the square of Euclidean distance in time space and the square of Euclidean distance in space space. The principle of special relativity guarantees that the equation of motion of closed systems is invariant under Poincaré transformations. Find the Lagrangian of a single free particle system.</p>]]></content><author><name>UlyssesZhan</name><email>ulysseszhan@gmail.com</email></author><category term="physics" /><category term="from zhihu" /><category term="classical mechanics" /><category term="imagination" /><summary type="html"><![CDATA[We can derive the equation of motion for mechanical systems in a Galileo universe with <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ι</mi></mrow><annotation encoding="application/x-tex">\iota</annotation></semantics></math></span></span> time dimensions and <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>χ</mi></mrow><annotation encoding="application/x-tex">\chi</annotation></semantics></math></span></span> space dimensions by generalizing the principle of relativity and Hamilton’s principle.]]></summary><media:thumbnail xmlns:media="http://search.yahoo.com/mrss/" url="https://ulysseszh.github.io/assets/images/covers/2022-11-20-high-dim-mechanics.png" /><media:content medium="image" url="https://ulysseszh.github.io/assets/images/covers/2022-11-20-high-dim-mechanics.png" xmlns:media="http://search.yahoo.com/mrss/" /></entry><entry><title type="html"><![CDATA[View of the world (physically rather than philosophically)]]></title><link href="https://ulysseszh.github.io/physics/2020/05/14/world-view.html" rel="alternate" type="text/html" title="View of the world (physically rather than philosophically)" /><published>2020-05-14T04:18:39-07:00</published><updated>2020-05-14T04:18:39-07:00</updated><id>https://ulysseszh.github.io/physics/2020/05/14/world-view</id><content type="html" xml:base="https://ulysseszh.github.io/physics/2020/05/14/world-view.html"><![CDATA[<h2 data-label="0.1" id="introduction">Introduction</h2>
<p>Have you ever thought of how can we describe the basic principles of our world (or universe), especially in a physical or mathematical way?</p>
<p>The thought itself seems like a philosophical problem (and is actually thought over by philosophers for thousands of years). However, maybe it can be interesting to think it over in another perspective.</p>
<p>Note that most of the definitions used below are different from the popular definitions!</p>
<h2 data-label="0.2" id="galilean-world">Galilean world</h2>
<p>Here is the basic principle of the Galilean world:</p>
<p class="no-indent">
<strong>Principle 1.</strong> The world is a Galilean structure with <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>3</mn></mrow><annotation encoding="application/x-tex">3</annotation></semantics></math></span></span>-dimensional space and <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>1</mn></mrow><annotation encoding="application/x-tex">1</annotation></semantics></math></span></span>-dimensional time.
</p>
<p>Here is the definition of a Galilean structure. A <em>Galilean structure</em> with <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>χ</mi></mrow><annotation encoding="application/x-tex">\chi</annotation></semantics></math></span></span>-dimensional space and <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ι</mi></mrow><annotation encoding="application/x-tex">\iota</annotation></semantics></math></span></span>-dimensional time is a <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>3</mn></mrow><annotation encoding="application/x-tex">3</annotation></semantics></math></span></span>-tuple <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo fence="true">(</mo><mi mathvariant="script">A</mi><mo separator="true">,</mo><mi>τ</mi><mo separator="true">,</mo><mi>ρ</mi><mo fence="true">)</mo></mrow><annotation encoding="application/x-tex">\left(\mathscr A,\tau,\rho\right)</annotation></semantics></math></span></span> with the following principles:</p>
<ol type="1">
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">\mathscr A</annotation></semantics></math></span></span> is a <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ν</mi></mrow><annotation encoding="application/x-tex">\nu</annotation></semantics></math></span></span>-dimensional affine space associated with the vector space <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msup><mi mathvariant="double-struck">R</mi><mi>ν</mi></msup></mrow><annotation encoding="application/x-tex">\mathbb R^\nu</annotation></semantics></math></span></span>, where <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ν</mi><mo><mi mathvariant="normal">≔</mi></mo><mi>χ</mi><mo>+</mo><mi>ι</mi></mrow><annotation encoding="application/x-tex">\nu\coloneqq\chi+\iota</annotation></semantics></math></span></span>;</li>
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>τ</mi><mo>:</mo><msup><mi mathvariant="double-struck">R</mi><mi>ν</mi></msup><mo>→</mo><msup><mi mathvariant="double-struck">R</mi><mi>ι</mi></msup></mrow><annotation encoding="application/x-tex">\tau:\mathbb R^\nu\rightarrow\mathbb R^\iota</annotation></semantics></math></span></span> is a linear mapping;</li>
<li>For <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo>∈</mo><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">a,b\in\mathscr A</annotation></semantics></math></span></span> such that <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>τ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>=</mo><mn>0</mn></mrow><annotation encoding="application/x-tex">\tau\!\left(a-b\right)=0</annotation></semantics></math></span></span>, the mapping <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ρ</mi></mrow><annotation encoding="application/x-tex">\rho</annotation></semantics></math></span></span> satisfies <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ρ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>=</mo><msqrt><msup><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mn>2</mn></msup></msqrt></mrow><annotation encoding="application/x-tex">\rho\!\left(a,b\right)=\sqrt{\left(a-b\right)^2}</annotation></semantics></math></span></span>.</li>
</ol>
<p class="no-indent">
To make the physical meanings of the above mathematical stuff clear, we
</p>
<ul>
<li>call <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">\mathscr A</annotation></semantics></math></span></span> the <em>universe</em> or a <em>Galilean space</em>,</li>
<li>call the points in the universe the <em>events</em>,</li>
<li>call <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>τ</mi></mrow><annotation encoding="application/x-tex">\tau</annotation></semantics></math></span></span> the <em>time</em>,</li>
<li>say two events <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>a</mi><mo separator="true">,</mo><mi>b</mi></mrow><annotation encoding="application/x-tex">a,b</annotation></semantics></math></span></span> are <em>simultaneous</em> iff <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>τ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>=</mo><mn>0</mn></mrow><annotation encoding="application/x-tex">\tau\!\left(a-b\right)=0</annotation></semantics></math></span></span>,</li>
<li>call <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ρ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">\rho\!\left(a,b\right)</annotation></semantics></math></span></span> the <em>distance between simultaneous events <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>a</mi><mo separator="true">,</mo><mi>b</mi></mrow><annotation encoding="application/x-tex">a,b</annotation></semantics></math></span></span></em>.</li>
</ul>
<hr/>
<p>Here is the second principle of the Galilean world:</p>
<p class="no-indent">
<strong>Principle 2</strong> (Galileo’s principle of relativity)<strong>.</strong> Laws of nature remain the same under Galilean transformation.
</p>
<p>Here is the definition of a Galilean transformation. An affine transformation <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>g</mi></mrow><annotation encoding="application/x-tex">g</annotation></semantics></math></span></span> over the Galilean space <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">\mathscr A</annotation></semantics></math></span></span> is called a <em>Galilean transformation</em> iff both of the following are satisfied:</p>
<ol type="1">
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="normal">∀</mi><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo>∈</mo><mi mathvariant="script">A</mi><mo>:</mo><mi>τ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>=</mo><mi>τ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>g</mi><mi>a</mi><mo>−</mo><mi>g</mi><mi>b</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">\forall a,b\in\mathscr A:
\tau\!\left(a-b\right)=\tau\!\left(ga-gb\right)</annotation></semantics></math></span></span> (preservation of intervals of time),</li>
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="normal">∀</mi><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo>∈</mo><mi mathvariant="script">A</mi><mo>:</mo><mi>τ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>=</mo><mn>0</mn><mo>⇒</mo><mi>ρ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>=</mo><mi>ρ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>g</mi><mi>a</mi><mo separator="true">,</mo><mi>g</mi><mi>b</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">\forall a,b\in\mathscr A:
\tau\!\left(a-b\right)=0\Rightarrow
\rho\!\left(a,b\right)=\rho\!\left(ga,gb\right)</annotation></semantics></math></span></span> (preservation of distance between simultaneous events).</li>
</ol>
<p>Galilean transformations form a group (why?) called the <em>Galilean group</em>, which is an <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo fence="true">(</mo><mi>ι</mi><mo>+</mo><mfrac><mrow><mi>χ</mi><mrow><mo fence="true">(</mo><mi>χ</mi><mo>+</mo><mn>3</mn><mo fence="true">)</mo></mrow></mrow><mn>2</mn></mfrac><mo fence="true">)</mo></mrow><annotation encoding="application/x-tex">\left(\iota+\frac{\chi\left(\chi+3\right)}2\right)</annotation></semantics></math></span></span>-dimensional Lie group (why?).</p>
<hr/>
<p><span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mtable rowspacing="0.25em" columnalign="right left" columnspacing="0em"><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><mi mathvariant="script">A</mi></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow/><mo><mi mathvariant="normal">≔</mi></mo><msup><mi mathvariant="double-struck">R</mi><mi>ν</mi></msup><mo separator="true">,</mo></mrow></mstyle></mtd></mtr><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><mi>τ</mi></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow/><mo><mi mathvariant="normal">≔</mi></mo><mrow><mo fence="true">(</mo><mi>t</mi><mo separator="true">,</mo><mi>x</mi><mo fence="true">)</mo></mrow><mo>↦</mo><mi>t</mi><mo separator="true">,</mo></mrow></mstyle></mtd></mtr><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><mi>ρ</mi></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow/><mo><mi mathvariant="normal">≔</mi></mo><mrow><mo fence="true">(</mo><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>↦</mo><msqrt><msup><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mn>2</mn></msup></msqrt></mrow></mstyle></mtd></mtr></mtable><annotation encoding="application/x-tex">
\begin{align*}
\mathscr A&amp;\coloneqq\mathbb R^\nu,\\
\tau&amp;\coloneqq\left(t,x\right)\mapsto t,\\
\rho&amp;\coloneqq\left(a,b\right)\mapsto\sqrt{\left(a-b\right)^2}
\end{align*}
</annotation></semantics></math></span></span></span> is a Galilean structure (why?). Here <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">\mathscr A</annotation></semantics></math></span></span> is called the <em>Galilean coordinate space</em>.</p>
<p>The following transformations on the Galilean coordinate space are Galilean transformations (why?):</p>
<ol type="1">
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mrow><mo fence="true">(</mo><mi>t</mi><mo separator="true">,</mo><mi>x</mi><mo fence="true">)</mo></mrow><mo>↦</mo><mrow><mo fence="true">(</mo><mi>t</mi><mo separator="true">,</mo><mi>x</mi><mo>+</mo><mi>v</mi><mi>t</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">\left(t,x\right)\mapsto\left(t,x+vt\right)</annotation></semantics></math></span></span>, where <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>v</mi><mo>∈</mo><msup><mi mathvariant="double-struck">R</mi><mi>χ</mi></msup></mrow><annotation encoding="application/x-tex">v\in\mathbb R^\chi</annotation></semantics></math></span></span> (<em>uniform motion</em>),</li>
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mrow><mo fence="true">(</mo><mi>t</mi><mo separator="true">,</mo><mi>x</mi><mo fence="true">)</mo></mrow><mo>↦</mo><mrow><mo fence="true">(</mo><mi>t</mi><mo>+</mo><mi>s</mi><mo separator="true">,</mo><mi>x</mi><mo>+</mo><mi>d</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">\left(t,x\right)\mapsto\left(t+s,x+d\right)</annotation></semantics></math></span></span>, where <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>s</mi><mo>∈</mo><msup><mi mathvariant="double-struck">R</mi><mi>ι</mi></msup></mrow><annotation encoding="application/x-tex">s\in\mathbb R^\iota</annotation></semantics></math></span></span> and <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>d</mi><mo>∈</mo><msup><mi mathvariant="double-struck">R</mi><mi>χ</mi></msup></mrow><annotation encoding="application/x-tex">d\in\mathbb R^\chi</annotation></semantics></math></span></span> (<em>translation</em>),</li>
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mrow><mo fence="true">(</mo><mi>t</mi><mo separator="true">,</mo><mi>x</mi><mo fence="true">)</mo></mrow><mo>↦</mo><mrow><mo fence="true">(</mo><mi>t</mi><mo separator="true">,</mo><mi>G</mi><mi>x</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">\left(t,x\right)\mapsto\left(t,Gx\right)</annotation></semantics></math></span></span>, where <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>G</mi><mo>∈</mo><mi mathvariant="normal">O</mi><mrow><mo fence="true">(</mo><mi>χ</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">G\in\mathrm O\left(\chi\right)</annotation></semantics></math></span></span> (<em>rotation</em>).</li>
</ol>
<p>Every Galilean transformation of the Galilean coordinate space can be represented uniquely as the composition of a rotation, a translation, and a uniform motion (why?).</p>
<p>All Galilean spaces with the same dimensions are isomorphic to each other (why?).</p>
<hr/>
<p>In fact, the two principles above are not enough to build up the whole classical mechanics. We need to define motion, velocity, and acceleration in our <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>3</mn><mo>+</mo><mn>1</mn></mrow><annotation encoding="application/x-tex">3+1</annotation></semantics></math></span></span> universe. Then, to describe how the motion is determined, we need the following principle:</p>
<p class="no-indent">
<strong>Principle 3</strong> (Newton’s principle of determinacy)<strong>.</strong> The motion is uniquely determined by initial positions and initial velocities.
</p>
<p>With this principle, we can conclude that the motion can be depicted by <em>Hamilton’s principle</em> <span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mi>δ</mi><mo>∫</mo><mi mathvariant="script">L</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>q</mi><mo separator="true">,</mo><mover accent="true"><mi>q</mi><mo>˙</mo></mover><mo separator="true">,</mo><mi>t</mi><mo fence="true">)</mo></mrow><mo>=</mo><mn>0</mn></mrow><annotation encoding="application/x-tex">
    \delta\int\mathcal L\!\left(q,\dot q,t\right)=0
</annotation></semantics></math></span></span></span> (why?), which leads to <em>Euler–Lagrange equation</em> <span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mfrac><mi mathvariant="normal">d</mi><mrow><mi mathvariant="normal">d</mi><mi>t</mi></mrow></mfrac><mfrac><mrow><mi mathvariant="normal">∂</mi><mi mathvariant="script">L</mi></mrow><mrow><mi mathvariant="normal">∂</mi><mover accent="true"><mi>q</mi><mo>˙</mo></mover></mrow></mfrac><mo>=</mo><mfrac><mrow><mi mathvariant="normal">∂</mi><mi mathvariant="script">L</mi></mrow><mrow><mi mathvariant="normal">∂</mi><mi>q</mi></mrow></mfrac></mrow><annotation encoding="application/x-tex">
    \frac{\mathrm d}{\mathrm dt}
    \frac{\partial\mathcal L}{\partial\dot q}=
    \frac{\partial\mathcal L}{\partial q}
</annotation></semantics></math></span></span></span> (why?).</p>
<p>According to Principle 2, for a closed system, its Euler–Lagrange equation should remain unchanged after a Galilean transformation (in a specific coordinate system, which in most cases is the Cartesian coordinate system utilized by Galilean coordinate space) acts on it, from which we can see that the universe is <em>time-homogeneous</em> (invariance under time translation), <em>space-homogeneous</em> (invariance under space translation), and <em>space-isotropic</em> (invariance under space rotation).</p>
<p>The rest (deriving the <em>Lagrangian</em> for some typical mechanical systems, and solving them) is just the normal classical mechanics, and is not related to the topic today.</p>
<h2 data-label="0.3" id="einsteinian-world">Einsteinian world</h2>
<p>We can build up the Einsteinian world similarly as we have done for the Galilean world.</p>
<p class="no-indent">
<strong>Principle 1.</strong> The world is an Einsteinian structure with <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>3</mn></mrow><annotation encoding="application/x-tex">3</annotation></semantics></math></span></span>-dimensional space and <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>1</mn></mrow><annotation encoding="application/x-tex">1</annotation></semantics></math></span></span>-dimensional time.
</p>
<p>Here is the definition of an Einsteinian structure. An <em>Einsteinian structure</em> with <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>χ</mi></mrow><annotation encoding="application/x-tex">\chi</annotation></semantics></math></span></span>-dimensional space and <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ι</mi></mrow><annotation encoding="application/x-tex">\iota</annotation></semantics></math></span></span>-dimensional time is a <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>4</mn></mrow><annotation encoding="application/x-tex">4</annotation></semantics></math></span></span>-tuple <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo fence="true">(</mo><mi mathvariant="script">A</mi><mo separator="true">,</mo><mi>τ</mi><mo separator="true">,</mo><mi>σ</mi><mo separator="true">,</mo><mi>ρ</mi><mo fence="true">)</mo></mrow><annotation encoding="application/x-tex">\left(\mathscr A,\tau,\sigma,\rho\right)</annotation></semantics></math></span></span> with the following principles:</p>
<ol type="1">
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">\mathscr A</annotation></semantics></math></span></span> is a <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ν</mi></mrow><annotation encoding="application/x-tex">\nu</annotation></semantics></math></span></span>-dimensional affine space associated with the vector space <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msup><mi mathvariant="double-struck">R</mi><mi>ν</mi></msup></mrow><annotation encoding="application/x-tex">\mathbb R^\nu</annotation></semantics></math></span></span>, where <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ν</mi><mo><mi mathvariant="normal">≔</mi></mo><mi>χ</mi><mo>+</mo><mi>ι</mi></mrow><annotation encoding="application/x-tex">\nu\coloneqq\chi+\iota</annotation></semantics></math></span></span>;</li>
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>τ</mi><mo>:</mo><msup><mi mathvariant="double-struck">R</mi><mi>ν</mi></msup><mo>→</mo><msup><mi mathvariant="double-struck">R</mi><mi>ι</mi></msup></mrow><annotation encoding="application/x-tex">\tau:\mathbb R^\nu\rightarrow\mathbb R^\iota</annotation></semantics></math></span></span> and <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>σ</mi><mo>:</mo><msup><mi mathvariant="double-struck">R</mi><mi>ν</mi></msup><mo>→</mo><msup><mi mathvariant="double-struck">R</mi><mi>χ</mi></msup></mrow><annotation encoding="application/x-tex">\sigma:\mathbb R^\nu\rightarrow\mathbb R^\chi</annotation></semantics></math></span></span> are linear mappings;</li>
<li>The linear mapping <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>a</mi><mo>↦</mo><mrow><mo fence="true">(</mo><mi>τ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo fence="true">)</mo></mrow><mo separator="true">,</mo><mi>σ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo fence="true">)</mo></mrow><mo fence="true">)</mo></mrow><mo>:</mo><msup><mi mathvariant="double-struck">R</mi><mi>ν</mi></msup><mo>→</mo><msup><mi mathvariant="double-struck">R</mi><mi>ν</mi></msup></mrow><annotation encoding="application/x-tex">a\mapsto\left(\tau\!\left(a\right),
\sigma\!\left(a\right)\right):
\mathbb R^\nu\rightarrow\mathbb R^\nu</annotation></semantics></math></span></span> has full rank;</li>
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="normal">∀</mi><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo>∈</mo><mi mathvariant="script">A</mi><mo>:</mo><mi>ρ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>=</mo><msqrt><mrow><mi>τ</mi><msup><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mn>2</mn></msup><mo>−</mo><mi>σ</mi><msup><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mn>2</mn></msup></mrow></msqrt></mrow><annotation encoding="application/x-tex">\forall a,b\in\mathscr A:\rho\!\left(a,b\right)=
\sqrt{\tau\left(a-b\right)^2-\sigma\left(a-b\right)^2}</annotation></semantics></math></span></span>.</li>
</ol>
<p class="no-indent">
To make the physical meanings of the above mathematical stuff clear, we
</p>
<ul>
<li>call <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">\mathscr A</annotation></semantics></math></span></span> the <em>universe</em> or an <em>Einsteinian space</em>,</li>
<li>call the points in the universe the <em>events</em>,</li>
<li>call <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>τ</mi></mrow><annotation encoding="application/x-tex">\tau</annotation></semantics></math></span></span> the <em>time</em>,</li>
<li>call <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>σ</mi></mrow><annotation encoding="application/x-tex">\sigma</annotation></semantics></math></span></span> the <em>space</em>,</li>
<li>call <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ρ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">\rho\!\left(a,b\right)</annotation></semantics></math></span></span> the <em>spacetime interval between events <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>a</mi><mo separator="true">,</mo><mi>b</mi></mrow><annotation encoding="application/x-tex">a,b</annotation></semantics></math></span></span></em>.</li>
</ul>
<hr/>
<p>Here is the second principle of the Einsteinian world:</p>
<p class="no-indent">
<strong>Principle 2</strong> (Einstein’s principle of relativity)<strong>.</strong> Laws of nature remain the same under extended Poincaré transformation.
</p>
<p>Here is the definition of a Poincaré transformation. An affine transformation <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>g</mi></mrow><annotation encoding="application/x-tex">g</annotation></semantics></math></span></span> over the Einsteinian space <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">\mathscr A</annotation></semantics></math></span></span> is called a <em>Poincaré transformation</em> iff <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="normal">∀</mi><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo>∈</mo><mi mathvariant="script">A</mi><mo>:</mo><mi>ρ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>=</mo><mi>ρ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>g</mi><mi>a</mi><mo separator="true">,</mo><mi>g</mi><mi>b</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">\forall a,b\in\mathscr A:
\rho\!\left(a,b\right)=\rho\!\left(ga,gb\right)</annotation></semantics></math></span></span>. Well, the definition is much simpler than that of Galilean transformation.</p>
<p>Poincaré transformations form a group (why?) called the <em>Poincaré group</em>, which is a <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mfrac><mrow><mi>ν</mi><mrow><mo fence="true">(</mo><mi>ν</mi><mo>+</mo><mn>1</mn><mo fence="true">)</mo></mrow></mrow><mn>2</mn></mfrac></mrow><annotation encoding="application/x-tex">\frac{\nu\left(\nu+1\right)}2</annotation></semantics></math></span></span>-dimensional Lie group (why?).</p>
<hr/>
<p><span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mtable rowspacing="0.25em" columnalign="right left" columnspacing="0em"><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><mi mathvariant="script">A</mi></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow/><mo><mi mathvariant="normal">≔</mi></mo><msup><mi mathvariant="double-struck">R</mi><mi>ν</mi></msup><mo separator="true">,</mo></mrow></mstyle></mtd></mtr><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><mi>τ</mi></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow/><mo><mi mathvariant="normal">≔</mi></mo><mrow><mo fence="true">(</mo><mi>c</mi><mi>t</mi><mo separator="true">,</mo><mi>x</mi><mo fence="true">)</mo></mrow><mo>↦</mo><mi>t</mi><mo separator="true">,</mo></mrow></mstyle></mtd></mtr><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><mi>σ</mi></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow/><mo><mi mathvariant="normal">≔</mi></mo><mrow><mo fence="true">(</mo><mi>c</mi><mi>t</mi><mo separator="true">,</mo><mi>x</mi><mo fence="true">)</mo></mrow><mo>↦</mo><mi>x</mi><mo separator="true">,</mo></mrow></mstyle></mtd></mtr><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><mi>ρ</mi></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow/><mo><mi mathvariant="normal">≔</mi></mo><mrow><mo fence="true">(</mo><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>↦</mo><msqrt><mrow><mi>τ</mi><mtext> ⁣</mtext><msup><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mn>2</mn></msup><mo>−</mo><mi>σ</mi><mtext> ⁣</mtext><msup><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mn>2</mn></msup></mrow></msqrt></mrow></mstyle></mtd></mtr></mtable><annotation encoding="application/x-tex">
\begin{align*}
    \mathscr A&amp;\coloneqq\mathbb R^\nu,\\
    \tau&amp;\coloneqq\left(ct,x\right)\mapsto t,\\
    \sigma&amp;\coloneqq\left(ct,x\right)\mapsto x,\\
    \rho&amp;\coloneqq\left(a,b\right)\mapsto
    \sqrt{\tau\!\left(a-b\right)^2-\sigma\!\left(a-b\right)^2}
\end{align*}
</annotation></semantics></math></span></span></span> is an Einsteinian structure (why?), where the constant <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>c</mi><mo>∈</mo><mi mathvariant="double-struck">R</mi></mrow><annotation encoding="application/x-tex">c\in\mathbb R</annotation></semantics></math></span></span> is called the <em>speed of light</em>. Here <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">\mathscr A</annotation></semantics></math></span></span> is called the <em>Minkowski space</em>.</p>
<p>The following transformations on the Minkowski space are Poincaré transformations (why?):</p>
<ol type="1">
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>a</mi><mo>↦</mo><mi>a</mi><mo>+</mo><mi>d</mi></mrow><annotation encoding="application/x-tex">a\mapsto a+d</annotation></semantics></math></span></span>, where <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>d</mi><mo>∈</mo><msup><mi mathvariant="double-struck">R</mi><mi>ν</mi></msup></mrow><annotation encoding="application/x-tex">d\in\mathbb R^\nu</annotation></semantics></math></span></span> (<em>translation</em>),</li>
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>a</mi><mo>↦</mo><mi>G</mi><mi>a</mi></mrow><annotation encoding="application/x-tex">a\mapsto Ga</annotation></semantics></math></span></span>, where <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>G</mi><mo>∈</mo><mi mathvariant="normal">O</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>ι</mi><mo separator="true">,</mo><mi>χ</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">G\in\mathrm O\!\left(\iota,\chi\right)</annotation></semantics></math></span></span> is an indefinite orthogonal matrix (<em>rotation</em>).</li>
</ol>
<p>Every Poincaré transformation of the Minkowski space can be represented uniquely as the composition of a translation and a rotation (why?).</p>
<p>All Einsteinian spaces with the same dimensions are isomorphic to each other (why?).</p>
<hr/>
<p>The rest is just the same as what we have done with Galilean world. You can find that the Einsteinian world is also space-homogeneous, time-homogeneous, and space-isotropic. Further more, it is time-isotropic.</p>
<h2 data-label="0.4" id="aristotelian-world-imagination">Aristotelian world (imagination)</h2>
<p>Although the Aristotelian world is not real, we can think of what it may look like.</p>
<p class="no-indent">
<strong>Principle 1.</strong> The world is an Aristotelian structure with <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>3</mn></mrow><annotation encoding="application/x-tex">3</annotation></semantics></math></span></span>-dimensional space and <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>1</mn></mrow><annotation encoding="application/x-tex">1</annotation></semantics></math></span></span>-dimensional time.
</p>
<p>According to Aristotle’s theory about the natural place, the world has something like a “center”, so the world cannot be space-homogeneous. However, he admits the invariance of natural laws over time, so the world is still time-homogeneous. It may be also reasonable to assume that the world is space-isotropic.</p>
<p>Thus, our definition of the Aristotelian structure should be non-affine, and the Aristotelian transformations should be composed of rotation and time translation.</p>
<p>However, although the space is non-affine, the time is affine. This makes it tricky to mix space and time together into an “<span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">\mathscr A</annotation></semantics></math></span></span>”. However, there is a workaround. We can define the universe still an affine space, while give it an origin. Since this origin only add limitations to space transformation instead of time transformation, we can make it an <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ι</mi></mrow><annotation encoding="application/x-tex">\iota</annotation></semantics></math></span></span>-dimensional affine subspace instead of a single point.</p>
<p>Here is the definition of an Aristotelian structure. An <em>Aristotelian structure</em> with <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>χ</mi></mrow><annotation encoding="application/x-tex">\chi</annotation></semantics></math></span></span>-dimensional space and <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ι</mi></mrow><annotation encoding="application/x-tex">\iota</annotation></semantics></math></span></span>-dimensional time is a <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>4</mn></mrow><annotation encoding="application/x-tex">4</annotation></semantics></math></span></span>-tuple <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo fence="true">(</mo><mi mathvariant="script">A</mi><mo separator="true">,</mo><mi>τ</mi><mo separator="true">,</mo><mi>o</mi><mo separator="true">,</mo><mi>ρ</mi><mo fence="true">)</mo></mrow><annotation encoding="application/x-tex">\left(\mathscr A,\tau,o,\rho\right)</annotation></semantics></math></span></span> with the following principles:</p>
<ol type="1">
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">\mathscr A</annotation></semantics></math></span></span> is a <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ν</mi></mrow><annotation encoding="application/x-tex">\nu</annotation></semantics></math></span></span>-dimensional affine space associated with the vector space <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msup><mi mathvariant="double-struck">R</mi><mi>ν</mi></msup></mrow><annotation encoding="application/x-tex">\mathbb R^\nu</annotation></semantics></math></span></span>, where <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ν</mi><mo><mi mathvariant="normal">≔</mi></mo><mi>χ</mi><mo>+</mo><mi>ι</mi></mrow><annotation encoding="application/x-tex">\nu\coloneqq\chi+\iota</annotation></semantics></math></span></span>;</li>
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>τ</mi><mo>:</mo><msup><mi mathvariant="double-struck">R</mi><mi>ν</mi></msup><mo>→</mo><msup><mi mathvariant="double-struck">R</mi><mi>ι</mi></msup></mrow><annotation encoding="application/x-tex">\tau:\mathbb R^\nu\rightarrow\mathbb R^\iota</annotation></semantics></math></span></span> is a linear mapping;</li>
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>o</mi></mrow><annotation encoding="application/x-tex">o</annotation></semantics></math></span></span> is an <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ι</mi></mrow><annotation encoding="application/x-tex">\iota</annotation></semantics></math></span></span>-dimensional affine subspace of <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">\mathscr A</annotation></semantics></math></span></span>;</li>
<li>For <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo>∈</mo><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">a,b\in\mathscr A</annotation></semantics></math></span></span> such that <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>τ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>=</mo><mn>0</mn></mrow><annotation encoding="application/x-tex">\tau\!\left(a-b\right)=0</annotation></semantics></math></span></span>, the mapping <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ρ</mi></mrow><annotation encoding="application/x-tex">\rho</annotation></semantics></math></span></span> satisfies <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ρ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>=</mo><msqrt><msup><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mn>2</mn></msup></msqrt></mrow><annotation encoding="application/x-tex">\rho\!\left(a,b\right)=
\sqrt{\left(a-b\right)^2}</annotation></semantics></math></span></span>.</li>
</ol>
<p>To make the physical meanings of the above mathematical stuff clear, we</p>
<ul>
<li>call <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">\mathscr A</annotation></semantics></math></span></span> the <em>universe</em> or an <em>Aristotelian space</em>,</li>
<li>call the points in the universe the <em>events</em>,</li>
<li>call <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>τ</mi></mrow><annotation encoding="application/x-tex">\tau</annotation></semantics></math></span></span> the <em>time</em>,</li>
<li>call <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>o</mi></mrow><annotation encoding="application/x-tex">o</annotation></semantics></math></span></span> the <em>center</em> of space,</li>
<li>say two events <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>a</mi><mo separator="true">,</mo><mi>b</mi></mrow><annotation encoding="application/x-tex">a,b</annotation></semantics></math></span></span> are <em>simultaneous</em> iff <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>τ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>=</mo><mn>0</mn></mrow><annotation encoding="application/x-tex">\tau\!\left(a-b\right)=0</annotation></semantics></math></span></span>,</li>
<li>call <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ρ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">\rho\!\left(a,b\right)</annotation></semantics></math></span></span> the <em>distance between simultaneous events <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>a</mi><mo separator="true">,</mo><mi>b</mi></mrow><annotation encoding="application/x-tex">a,b</annotation></semantics></math></span></span></em>.</li>
</ul>
<hr/>
<p>Then the task is to define the Aristotelian transformations.</p>
<p class="no-indent">
<strong>Principle 2.</strong> Laws of nature remain the same under Aristotelian transformation.
</p>
<p>Here is the definition of an Aristotelian transformation. An affine transformation <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>g</mi></mrow><annotation encoding="application/x-tex">g</annotation></semantics></math></span></span> over the Aristotelian space <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">\mathscr A</annotation></semantics></math></span></span> is called an <em>Aristotelian transformation</em> iff all of the following are satisfied:</p>
<ol type="1">
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="normal">∀</mi><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo>∈</mo><mi mathvariant="script">A</mi><mo>:</mo><mi>τ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>=</mo><mi>τ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>g</mi><mi>a</mi><mo>−</mo><mi>g</mi><mi>b</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">\forall a,b\in\mathscr A:
\tau\!\left(a-b\right)=\tau\!\left(ga-gb\right)</annotation></semantics></math></span></span>,</li>
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="normal">∀</mi><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo>∈</mo><mi mathvariant="script">A</mi><mo>:</mo><mi>τ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>=</mo><mn>0</mn><mo>⇒</mo><mi>ρ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>=</mo><mi>ρ</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>g</mi><mi>a</mi><mo separator="true">,</mo><mi>g</mi><mi>b</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">\forall a,b\in\mathscr A:
\tau\!\left(a-b\right)=0\Rightarrow
\rho\!\left(a,b\right)=\rho\!\left(ga,gb\right)</annotation></semantics></math></span></span>.</li>
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="normal">∀</mi><mi>a</mi><mo>∈</mo><mi>o</mi><mo>:</mo><mi>g</mi><mi>a</mi><mo>∈</mo><mi>o</mi></mrow><annotation encoding="application/x-tex">\forall a\in o:ga\in o</annotation></semantics></math></span></span>.</li>
</ol>
<p class="no-indent">
Notice the third condition, which makes it different from a Galilean transformation.
</p>
<p>Aristotelian transformations from a group (why?) called the <em>Aristotelian group</em>, which is an <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo fence="true">(</mo><mi>ι</mi><mo>+</mo><mfrac><mrow><mi>χ</mi><mrow><mo fence="true">(</mo><mi>χ</mi><mo>−</mo><mn>1</mn><mo fence="true">)</mo></mrow></mrow><mn>2</mn></mfrac><mo fence="true">)</mo></mrow><annotation encoding="application/x-tex">\left(\iota+\frac{\chi\left(\chi-1\right)}2\right)</annotation></semantics></math></span></span>-dimensional Lie group (why?).</p>
<hr/>
<p><span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mtable rowspacing="0.25em" columnalign="right left" columnspacing="0em"><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><mi mathvariant="script">A</mi></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow/><mo><mi mathvariant="normal">≔</mi></mo><msup><mi mathvariant="double-struck">R</mi><mi>ν</mi></msup><mo separator="true">,</mo></mrow></mstyle></mtd></mtr><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><mi>τ</mi></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow/><mo><mi mathvariant="normal">≔</mi></mo><mrow><mo fence="true">(</mo><mi>t</mi><mo separator="true">,</mo><mi>x</mi><mo fence="true">)</mo></mrow><mo>↦</mo><mi>t</mi><mo separator="true">,</mo></mrow></mstyle></mtd></mtr><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><mi>o</mi></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow/><mo><mi mathvariant="normal">≔</mi></mo><msup><mi mathvariant="double-struck">R</mi><mi>ι</mi></msup><mo>×</mo><mrow><mo fence="true">{</mo><mn>0</mn><mo fence="true">}</mo></mrow><mo separator="true">,</mo></mrow></mstyle></mtd></mtr><mtr><mtd><mstyle scriptlevel="0" displaystyle="true"><mi>ρ</mi></mstyle></mtd><mtd><mstyle scriptlevel="0" displaystyle="true"><mrow><mrow/><mo><mi mathvariant="normal">≔</mi></mo><mrow><mo fence="true">(</mo><mi>a</mi><mo separator="true">,</mo><mi>b</mi><mo fence="true">)</mo></mrow><mo>↦</mo><msqrt><msup><mrow><mo fence="true">(</mo><mi>a</mi><mo>−</mo><mi>b</mi><mo fence="true">)</mo></mrow><mn>2</mn></msup></msqrt></mrow></mstyle></mtd></mtr></mtable><annotation encoding="application/x-tex">
\begin{align*}
    \mathscr A&amp;\coloneqq\mathbb R^\nu,\\
    \tau&amp;\coloneqq\left(t,x\right)\mapsto t,\\
    o&amp;\coloneqq\mathbb R^\iota\times\left\{0\right\},\\
    \rho&amp;\coloneqq\left(a,b\right)\mapsto\sqrt{\left(a-b\right)^2}
\end{align*}
</annotation></semantics></math></span></span></span> is an Aristotelian structure (why?). Here <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi mathvariant="script">A</mi></mrow><annotation encoding="application/x-tex">\mathscr A</annotation></semantics></math></span></span> is called the <em>Aristotelian coordinate space</em>.</p>
<p>The following transformations on the Aristotelian coordinate space are Aristotelian transformations (why?):</p>
<ol type="1">
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mrow><mo fence="true">(</mo><mi>t</mi><mo separator="true">,</mo><mi>x</mi><mo fence="true">)</mo></mrow><mo>↦</mo><mrow><mo fence="true">(</mo><mi>t</mi><mo>+</mo><mi>s</mi><mo separator="true">,</mo><mi>x</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">\left(t,x\right)\mapsto\left(t+s,x\right)</annotation></semantics></math></span></span>, where <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>s</mi><mo>∈</mo><msup><mi mathvariant="double-struck">R</mi><mi>ι</mi></msup></mrow><annotation encoding="application/x-tex">s\in\mathbb R^\iota</annotation></semantics></math></span></span> (<em>time translation</em>),</li>
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mrow><mo fence="true">(</mo><mi>t</mi><mo separator="true">,</mo><mi>x</mi><mo fence="true">)</mo></mrow><mo>↦</mo><mrow><mo fence="true">(</mo><mi>t</mi><mo separator="true">,</mo><mi>G</mi><mi>x</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">\left(t,x\right)\mapsto\left(t,Gx\right)</annotation></semantics></math></span></span>, where <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>G</mi><mo>∈</mo><mi mathvariant="normal">O</mi><mtext> ⁣</mtext><mrow><mo fence="true">(</mo><mi>χ</mi><mo fence="true">)</mo></mrow></mrow><annotation encoding="application/x-tex">G\in\mathrm O\!\left(\chi\right)</annotation></semantics></math></span></span> (<em>rotation</em>).</li>
</ol>
<p>Every Aristotelian transformation of the Aristotelian coordinate space can be represented uniquely as the composition of a time translation and a rotation (why?).</p>
<p>All Aristotelian spaces with the same dimensions are isomorphic to each other (why?).</p>
<hr/>
<p>After building up the Aristotle world, how can we develop the mechanics here? Maybe it can be interesting.</p>
<h2 data-label="0.5" id="other-imaginations">Other imaginations</h2>
<p>Here are some other imaginations of a world:</p>
<ul>
<li>What about a space-anistropic universe?</li>
<li>What about defining the spacetime interval by multiplying space interval and time interval?</li>
<li>What about a time-heterogeneous universe?</li>
<li>What about making laws of nature unchanged under uniform acceleration?</li>
<li>What about…</li>
</ul>
<p>These can be materials for science fiction (novels or video games).</p>]]></content><author><name>UlyssesZhan</name><email>ulysseszhan@gmail.com</email></author><category term="physics" /><category term="imagination" /><category term="long paper" /><summary type="html"><![CDATA[The view of the world… Physically! In this article, I tried to use mathematical language to describe models of the physical world. A view of the world should include: a space (actually spacetime) with some mathematical structure on it (whose points are events in the world), a symmetry principle describing the symmetry of the world, and a motion law to describe the physics and dynamics of the world. This article proposed models for Galilean, Einsteinian, and even Aristotelian worlds. Can you come up with even other worlds?]]></summary><media:thumbnail xmlns:media="http://search.yahoo.com/mrss/" url="https://ulysseszh.github.io/assets/images/covers/2020-05-14-world-view.png" /><media:content medium="image" url="https://ulysseszh.github.io/assets/images/covers/2020-05-14-world-view.png" xmlns:media="http://search.yahoo.com/mrss/" /></entry></feed>