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- </head>
- <body class='markdown-preview'><h1>Time Travel and Quantum Mechanics</h1>
-<h2>What is time?</h2>
-<ul>
-<li>
-<p>time is what clocks measure</p>
-<ul>
-<li>measure time objectively, therefore does not help our subjective definition of time</li>
-</ul>
-</li>
-<li>
-<p><span class="math"><script type="math/tex">t</script></span> - just like another quantitative variable (period of time, not absolute)</p>
-</li>
-</ul>
-<ul>
-<li>
-<p>fourth dimension</p>
-</li>
-<li>
-<p>these describe properties of it, not its implications or how it works</p>
-<ul>
-<li>implications are generally determined through intuition</li>
-</ul>
-</li>
-</ul>
-<ul>
-<li>
-<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>
-<ul>
-<li>computers have memory, do they have a concept of time?</li>
-</ul>
-</li>
-</ul>
-<p>Therefore, we have:</p>
-<p><strong>1. Subjective time</strong></p>
-<ul>
-<li>presumably everyone experiences this notion of time</li>
-<li>has the property of lapsing (time approaches <span class="math"><script type="math/tex">\infty</script></span>)</li>
-</ul>
-<p><strong>2. Physical time</strong></p>
-<ul>
-<li>represented by <span class="math"><script type="math/tex">t</script></span></li>
-<li>objective</li>
-<li>quantitative (period, not absolute)</li>
-</ul>
-<blockquote>
-<p>All paradoxes arise from the fact that time lapses / flows - we are basing time on subjective time, not physical time</p>
-</blockquote>
-<h2>Concepts of time in physics</h2>
-<h3>Thermodynamic time arrow</h3>
-<ul>
-<li>second law of thermodynamics:</li>
-<li>total entropy cannot decrease over time (randomness of particles)</li>
-<li>Hence, time has a specific direction (approaching infinite entropy)</li>
-<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>
-<li>entropy is an irreversible process</li>
-<li>therefore, time travel backwards is impossible</li>
-</ul>
-<h3>Block time</h3>
-<ul>
-<li>time is fourth dimension - all dimensions are related</li>
-<li>universe exists as a whole, in four dimensions, just like a chair can exist in three dimensions (plus time)</li>
-<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>
-<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>
-</ul>
-<ol start="2">
-<li>
-<p>Are the past and future real?<br>
-By 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>
-</li>
-<li>
-<p>Do events that occurred in the past / future exist?</p>
-</li>
-</ol>
-<blockquote>
-<p>What is the physical prompt to change to a different time?</p>
-</blockquote>
-<h2>Grandfather paradox</h2>
-<ul>
-<li>Grandfather is not necessary - could be the suicide paradox</li>
-<li>Mathematical status - 1 bit (dead or alive):
-<ul>
-<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>
-<li><span class="math"><script type="math/tex">0=</script></span> dead, <span class="math"><script type="math/tex">1=</script></span> alive</li>
-</ul>
-</li>
-<li><span class="math"><script type="math/tex">x=1</script></span> (alive - assumed because you must exist in the first instance)</li>
-</ul>
-<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>
-<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>
-This 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>
-<p><strong><span class="math"><script type="math/tex">\therefore</script></span> both situations can exist (superposition) - suggesting multiple universe theory</strong></p>
-<ul>
-<li>Implies a link to quantum mechanics: dead or alive, or between (superposition):</li>
-<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>
-</ul>
-<h3>Many worlds interpretation</h3>
-<ul>
-<li>different branches of superposition are different universes. Perception jumps between universes.</li>
-<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>
-</ul></body>
-</html>