The physics of time travel, by Dr Pieter Kok

hi I'm Peter Koch from the Department of physics and astronomy at the University of Sheffield I'm a lecturer in quantum information theory and in my spare time I like to watch movies in particular time-travel movies some of my favorites of course are Terminator Back to the Future and my all-time favorite is primer the problem with time-travel movies is that sometimes they don't get the physics right and as a physicist this bothers me an example of this is when Marty McFly is playing the guitar and Back to the Future 1 he he has a photograph on the neck of his guitar and his brother and sister are slowly fading out of the photograph as his parents are drifting apart this there was no physical mechanism for this and so I'd like to study time-travel in a little bit more detail a bit more scientifically and oh I have to go to my lecture ah sorry ah now I've got to go change yeah this happens all the time right so time travel gives rise to all sorts of paradoxes for example you can go back in time and to the point where your grandfather has not had any children yet kill him that means he won't have any children one of your parents will not have been born and so you could not have been born and so you can't go back in time - clearly grandfather in which point he lives has children and you will be born so you can go in back in time and it goes around and around and around so you see that there's a paradox there and this is the sort of thing that as a theoretical physicist I love and I use mathematics to try and solve this paradox I also like to simplify this paradox a little bit because we don't have to involve grandpa in this scenario we can just do it all by ourselves for example a diagram here so this is our time machine time runs in the vertical direction so lower is earlier and later is higher up on the board and here is our time machine it consists of two parts you the bit that you go in and then you pop out earlier so lower out of this part of the time machine and so now we have the situation where I'm going into the time machine pop out earlier and encounter my younger self I killed myself and that means that that person cannot go into the time machine any more and so we have the paradox again and mathematically speaking we can just say dead or alive that's all we need so that's just one bit of information so let's say zero is dead and one is alive then we can say that my younger self has a bit value zero one my older self has a bit value of zero and one we call it X and y and then when my older self kills my younger self that is a controlled bit flip because I go from one alive to zero dead but at all happens if I am already alive so if I'm one so that's why it's controlled and then when we work through the maths we find that X plus y equals y which means that X must be equal to zero now that means that I wasn't born in the first place and so of course if I'm not born at all then there is no paradox now in quantum mechanics you can solve the time travel paradox in a much more elegant way because in the classical situation we have we determined that you are dead right from the start in quantum mechanics you can be dead or alive or most importantly you can be in a superposition of being dead in a life and you may already be aware that this is possible because of course Schrodinger's cat is dead or alive and in a superposition so when you allow superpositions of 0 and 1 for these bit values in quantum mechanics it turns out you can be any state of dead and alive when you go in and there is a resolution for the time travel paradox so the die machine does not impose any restrictions on the worlds before it came into existence that is a very important point and how does quantum mechanics do this how does quantum mechanics solve this time travel paradox well for that you could go to the many-worlds interpretation in which different branches of the superposition are actually different universes and what happens when you go back in time and you kill your older self you actually jump from one universe to another this is how quantum mechanics solves time travel oh I need to go to my lecture sorry I'm sorry

Channel: The University of Sheffield

Views: 296,382

Keywords: Time, travel, sheffield university, Sheffield Uni, Uni of Sheffield, Univerisity of Sheffield, Physics, Time travel, Dr Pieter Kok, Student, 2019, universe, quantum mechanics

Id: uz9eLjO2BrA

Length: 5min 43sec (343 seconds)

Published: Tue Feb 05 2013