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Time Travel and Buddhism

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One of the chapters in my upcoming book(s) Physics and Buddhism This chapter explores a few themes: 1. Delayed Choice quantum eraser making the notion of causality not as straightforward to analyze, 2. Listing the many time machines in general relativity 3. Listing the laws of time travels and analyzing them. 4. Buddhism's comments on time travel with the help of philosophy of time.
  Ng Xin ZhaoNgxinzhaomonk@gmail.com12,300 words.Physics and Buddhismby Ng Xin Zhao  Physics and Buddhism Book 1 CHAPTER 1 Time Travel and Causality To end off the analysis with regards to time, I go to the most speculative topic of all. Time travel. Specifically, time travel to the past. Because seriously, we are alltravelling forward in time, at different rates according to our velocity as determined by special relativity and by gravity as determined by general relativity. So it is relatively straightforward to go to near the event horizon of black holes, or go near the speed of light, where time runs slow in order to fast-forward ourselves into the future, like whatthe devas do. Backwards time travel is not that straightforward or easy or may not even be possible. Once in the future, if backwards time travel is not permitted, there is nocoming back. We would be stuck in the future. For those few who get confused about time travel to the future, thinking that thefuture is uncertain, how can we travel to uncertain future? Just think of time travel tothe future is like you going to hibernation, suspended animation or a deep long sleeplike Sleeping Beauty. Time goes on without your participation and once you got out, youfind that many years had passed, the future had become your present moment and thereis no turning back. So for the rest of the chapter, time travel means time travel to thepast.  Well, there are plenty of time machines discovered in Physics, so far none of them works in our universe. Which might be a good thing. Time travel presents the oneof the ultimate philosophical conundrum of untangling the web of the chains of causeand effect. Deep within our intuition is that causes precedes effect and that the pastcannot be changed.  With time travel, it seems our grasp on reality slips from us. If we can go back intime and convince our grandfather to become a monk before he marries and have a kid,then would we be born? If we are not born, then we cannot convince our grandfather to become celibate, thus we would be born to convince him to become a monk. What would actually happen? Would reality implode due to this time paradox? Would it loopon indefinitely like a broken clock, spoiling time everywhere? The principle which is broken here is that the past is changed, in which weconvinced our grandfather to become a monk. However, more importantly, that changehas ripples of causes and effects which stops the change in the first place, so the effectsis going back in time to before the cause. Yet there would be no problem had the effectnot able to influence the cause. However, we should not be too hasty to lay blame on what went wrong here,there is this thing called the delayed choice quantum eraser experiment which messesup our intuition of how cause and effect should work in time as well. Delayed choice quantum eraser is a delayed version of the quantum eraser. The  Physics and Buddhism Book 2  quantum eraser 1  is a simple experiment. Prepare a laser, pass it through a beam splitter.In the picture of the photon, individual quanta of light, the beam splitter randomly allows the laser to either pass straight through, or to be reflected at 90 degreesdownward. Put a mirror at both paths to reconnect the paths to one point, at thatpoint, either put a beam splitter back in to recombine the laser paths or do not. Havetwo detectors after that point to detect which paths did the photon go. Instead of naming the paths A and B, I use Arahant Path and Bodhisatta Path. If there is a beam splitter, we lose the information of which paths did the photonsgo. Light from both paths will come together to go to only one detector. If we take outthe beam splitter, we get the information of which path did the photon went, if detector1 clicks, we know it went by the Bodhisatta path, if detector 2 clicks, we know it went by the Arahant path. So far nothing seems to be puzzling. Yet, let us look deeper, is light behaving asparticle of a single photon or as waves which travels both paths simultaneously? If lightis behaving like a single photon, then the addition of the beam splitter at the end shouldfurther make it randomly either go through or reflected, thus both detectors should havethe chance to click. Yet what is observed is that when the second beam splitter isinserted, only one of the detectors click. Light is behaving like waves, so that both pathsmatters and interference happens at the second beam splitter to make the path convergeagain and lose the information of which paths did the light took. Take out the beamsplitter at the end, then we can see which path light took, detector 1 or 2 will randomly click, thus it behaves like a photon to us. So how light behaves depends on our action of whether to put in the beamsplitter or not. Actually the more important thing is it depends on whether or not weknow which path did the light took or was it erased. More complicated experiment 2 shown below adds a polarisation rotator (90 o ) at one of the paths and two polarisersafter the end beam splitter shows that even through the polarisation rotator can allow us to tell which path did the photon took, the two polarisers (45 o ) after the beamsplitters can erase that information, making light behave like waves and only trigger oneof the detectors. If we try by any means to peek at or find out which path light took tothe end, light would end up behaving like particles and trigger both detectors.  Physics and Buddhism Book 3 Note that the second experimental set up did not actually erase the information but rather just scrambles it. The information can be there, but as long as no one canknow it, light can behave like waves. It is potential information which can be known thatmatters. So if we have an omniscient person like the Buddha, even he could not know  which path the photon took if the information is erased and inference happens so thatonly one detector clicks. If he tries to find out and found out which path the light took even by some supernatural psychic powers or special powers of a Buddha, then he would have changed the nature of light to particles and make two detector clicksrandomly. Here is a bit more terminology to make you more familiar with the experiment before we go on further. Light behaves coherently with wave phenomenon of interference so that only one detector is triggered when information of which path ittook is not available, or erased. Light behaves like a particle, or photon, or decohered, orits wave function collapses to choose a path, randomly triggering either of the detectors,interference does not appear when information about which path it took becomesavailable, or not erased, even in principle. So now onto the delayed choice quantum eraser. It is the experimental set upsuch that the light has already passed through the beam splitter at the start then wedecide if we want to know its path or erase that information. In the first experimentabove, just decide to insert or not insert the end beam splitter after the laser light haspassed through the start beam splitter and are on the way to the end beam splitter. Thepaths can be made super long, but of the same length to make them indistinguishable,and the decision to insert the end beam splitter or not can be linked to a quantumrandom number generator so that it is really last split second decision and at random.Our normal intuition tells us that light has to decide if it is going to be a particle or waveat the starting beam splitter. However, it turns out that the decision can be made evenafter that, while it is on its way along both paths as a wave or along one of them as aparticle!Other more complicated set ups 3  involves splitting the light into entangledphotons and letting one half of the split photons to be detected first, then apply thedecision to either erase the information or not to onto the second half of the photons, which by virtue of its entanglement would affect whether interference pattern appearsat the detected first half of the photons. Note that this cannot be used to send messages back in time, or receive messages from the future because we need information from the
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