1<!DOCTYPE html>
2<html>
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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 "time (or timeline)</li>
649</ul></body>
650</html>