1# Time Travel and Quantum Mechanics 2 3## What is time? 4- time is what clocks measure 5 - measure time objectively, therefore does not help our subjective definition of time 6 7- $t$ - just like another quantitative variable (period of time, not absolute) 8 9 10- fourth dimension 11 12- these describe properties of it, not its implications or how it works 13 - implications are generally determined through intuition 14 15 16- sense of time is inherent to all humans - every human has memory and some idea that time is observable through changes in space 17 18 - computers have memory, do they have a concept of time? 19 20Therefore, we have: 21 22**1. Subjective time** 23- presumably everyone experiences this notion of time 24- has the property of lapsing (time approaches $\infty$) 25 26**2. Physical time** 27- represented by $t$ 28- objective 29- quantitative (period, not absolute) 30 31> All paradoxes arise from the fact that time lapses / flows - we are basing time on subjective time, not physical time 32 33## Concepts of time in physics 34 35### Thermodynamic time arrow 36- second law of thermodynamics: 37- total entropy cannot decrease over time (randomness of particles) 38- Hence, time has a specific direction (approaching infinite entropy) 39- This can be modelled as $t=e$. If $e$ cannot be subtracted from, $t$ can only go one way. 40- entropy is an irreversible process 41- therefore, time travel backwards is impossible 42 43### Block time 44- time is fourth dimension - all dimensions are related 45- universe exists as a whole, in four dimensions, just like a chair can exist in three dimensions (plus time) 46- 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 ($x=2,y=1,z=3$) can exist if y and z are not defined 47- therefore, past and future must also exist as a relation of four values ($x,y,z,t$) 48 49 502. Are the past and future real? 51By 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). 52 533. Do events that occurred in the past / future exist? 54 55> What is the physical prompt to change to a different time? 56 57## Grandfather paradox 58 59- Grandfather is not necessary - could be the suicide paradox 60- Mathematical status - 1 bit (dead or alive): 61 - $x=$ existence at 2 yrs, $y=$ existence at 20 yrs 62 - $0=$ dead, $1=$ alive 63- $x=1$ (alive - assumed because you must exist in the first instance) 64 65 66**(case 1)** if $x=1$, then $y=1$ (assuming you don't die between) $\therefore x = y$ 67**(case 2)** if $y=1$, then $x=0$ (because you killed yourself) 68This is not a contradiction - it simply states that $x=0 \lor 1$ (one *or* zero) 69 70**$\therefore$ both situations can exist (superposition) - suggesting multiple universe theory** 71 72- Implies a link to quantum mechanics: dead or alive, or between (superposition): 73- $1 \over 2$ probability of killing grandfather/self (two parallel realities - Murphy's law) 74 75 76### Many worlds interpretation 77 78- different branches of superposition are different universes. Perception jumps between universes. 79- 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)