Philosophy / time travel.md.htmlon commit expand fields notes (6087721)
   1<!DOCTYPE html>
   2<html>
   3  <head>
   4      <meta charset="utf-8" />
   5      <title>time travel</title>
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 549  </head>
 550  <body class='markdown-preview'><h1>Time Travel and Quantum Mechanics</h1>
 551<h2>What is time?</h2>
 552<ul>
 553<li>
 554<p>time is what clocks measure</p>
 555<ul>
 556<li>measure time objectively, therefore does not help our subjective definition of time</li>
 557</ul>
 558</li>
 559<li>
 560<p><span class="math"><script type="math/tex">t</script></span> - just like another quantitative variable (period of time, not absolute)</p>
 561</li>
 562</ul>
 563<ul>
 564<li>
 565<p>fourth dimension</p>
 566</li>
 567<li>
 568<p>these describe properties of it, not its implications or how it works</p>
 569<ul>
 570<li>implications are generally determined through intuition</li>
 571</ul>
 572</li>
 573</ul>
 574<ul>
 575<li>
 576<p>sense of time is inherent to all humans - every human has memory and some idea that time is observable through changes in space</p>
 577<ul>
 578<li>computers have memory, do they have a concept of time?</li>
 579</ul>
 580</li>
 581</ul>
 582<p>Therefore, we have:</p>
 583<p><strong>1. Subjective time</strong></p>
 584<ul>
 585<li>presumably everyone experiences this notion of time</li>
 586<li>has the property of lapsing (time approaches <span class="math"><script type="math/tex">\infty</script></span>)</li>
 587</ul>
 588<p><strong>2. Physical time</strong></p>
 589<ul>
 590<li>represented by <span class="math"><script type="math/tex">t</script></span></li>
 591<li>objective</li>
 592<li>quantitative (period, not absolute)</li>
 593</ul>
 594<blockquote>
 595<p>All paradoxes arise from the fact that time lapses / flows - we are basing time on subjective time, not physical time</p>
 596</blockquote>
 597<h2>Concepts of time in physics</h2>
 598<h3>Thermodynamic time arrow</h3>
 599<ul>
 600<li>second law of thermodynamics:</li>
 601<li>total entropy cannot decrease over time (randomness of particles)</li>
 602<li>Hence, time has a specific direction (approaching infinite entropy)</li>
 603<li>This can be modelled as <span class="math"><script type="math/tex">t=e</script></span>. If <span class="math"><script type="math/tex">e</script></span> cannot be subtracted from, <span class="math"><script type="math/tex">t</script></span> can only go one way.</li>
 604<li>entropy is an irreversible process</li>
 605<li>therefore, time travel backwards is impossible</li>
 606</ul>
 607<h3>Block time</h3>
 608<ul>
 609<li>time is fourth dimension - all dimensions are related</li>
 610<li>universe exists as a whole, in four dimensions, just like a chair can exist in three dimensions (plus time)</li>
 611<li>you cannot isolate one value in one of the three dimensions and define it by itself - it would be ridiculous to say that a point (<span class="math"><script type="math/tex">x=2,y=1,z=3</script></span>) can exist if y and z are not defined</li>
 612<li>therefore, past and future must also exist as a relation of four values (<span class="math"><script type="math/tex">x,y,z,t</script></span>)</li>
 613</ul>
 614<ol start="2">
 615<li>
 616<p>Are the past and future real?<br>
 617By using the present as a proxy for the past or future, we can simulate them, effectively nesting times inside times (a simpler idea of a time machine - normal time machines lose the idea of the present).</p>
 618</li>
 619<li>
 620<p>Do events that occurred in the past / future exist?</p>
 621</li>
 622</ol>
 623<blockquote>
 624<p>What is the physical prompt to change to a different time?</p>
 625</blockquote>
 626<h2>Grandfather paradox</h2>
 627<ul>
 628<li>Grandfather is not necessary - could be the suicide paradox</li>
 629<li>Mathematical status - 1 bit (dead or alive):
 630<ul>
 631<li><span class="math"><script type="math/tex">x=</script></span> existence at 2 yrs, <span class="math"><script type="math/tex">y=</script></span> existence at 20 yrs</li>
 632<li><span class="math"><script type="math/tex">0=</script></span> dead, <span class="math"><script type="math/tex">1=</script></span> alive</li>
 633</ul>
 634</li>
 635<li><span class="math"><script type="math/tex">x=1</script></span> (alive - assumed because you must exist in the first instance)</li>
 636</ul>
 637<p><strong>(case 1)</strong> if <span class="math"><script type="math/tex">x=1</script></span>, then <span class="math"><script type="math/tex">y=1</script></span> (assuming you don’t die between) <span class="math"><script type="math/tex">\therefore x = y</script></span><br>
 638<strong>(case 2)</strong> if <span class="math"><script type="math/tex">y=1</script></span>, then <span class="math"><script type="math/tex">x=0</script></span> (because you killed yourself)<br>
 639This is not a contradiction - it simply states that <span class="math"><script type="math/tex">x=0 \lor 1</script></span> (one <em>or</em> zero)</p>
 640<p><strong><span class="math"><script type="math/tex">\therefore</script></span> both situations can exist (superposition) - suggesting multiple universe theory</strong></p>
 641<ul>
 642<li>Implies a link to quantum mechanics: dead or alive, or between (superposition):</li>
 643<li><span class="math"><script type="math/tex">1 \over 2</script></span> probability of killing grandfather/self (two parallel realities  - Murphy’s law)</li>
 644</ul>
 645<h3>Many worlds interpretation</h3>
 646<ul>
 647<li>different branches of superposition are different universes. Perception jumps between universes.</li>
 648<li>time can be defined only by the physical state of things at the time - this is the only way it can be derived. therefore, by changing the physical state, you are creating a new &quot;time (or timeline)</li>
 649</ul></body>
 650</html>