Fay Dowker: Past, Present and Future: The Science of Time

bonsoir good evening welcome to you all I'm massive a bow I'm the director of the philosophy department at the University of Geneva I'm very happy to extend a warm welcome to you for this conference this evening it will be a great pleasure for us to listen to feydau curve professor of theoretical physics at the Imperial College in London she will be speaking about the science of time this conference is dedicated to Stephen Hawking the physicist who passed away very recently and who directed fade-out kurz PhD thesis in Cambridge now it may seem strange for the philosophy department to invite physicists but when we wonder about fundamental notions such as time which is a a common object of research for both the philosophers and physicists it is very important for us to encourage such collaboration from the point of view of philosophy the fact of knowing whether time is belongs to the outside world researched by physics or whether it is part of the internal rules studied by a philosophy is something that shouldn't be taken for granted nonetheless of because if it's a colossal scale in the cosmos and because of its mysterious nature within us time is something that is fascinating for both these reasons it has therefore captivated Kant Lightning's says well Stephen Hawkins in the past it's a mysterious nature continues to nourish the work of physicists such as feydau Kerr as well as a contemporary philosophy and I hope that it will be a source of wonderment for us in the future with no further ado it is a pleasure for me to give the floor to the organizer of this evening of my colleague Christina mutation professor in science philosophy at the philosophy department and he will be introducing us to our speaker tonight fede Algar I will change to English in case you want to simultaneous translation now might be the moment to put that on Egbert Leo once said time is a great teacher unfortunately it kills all its students in spite of that philosophers and physicists have studied time and space for as long as they have been in business in fact the philosophy of space and time constitutes a central part of the philosophy of physics which in turn constitutes a central part of natural philosophy that is the philosophical consideration and reflection of the natural world that we find ourselves to inhabit natural philosophy in turn forms a central part of philosophy more general thus philosophy of space and time holds a rather important place in philosophy which is also justified by the fact that many philosophical accounts of our theories rely implicitly or explicitly on the nature and even the existence of space and time it is precisely the nature and indeed the very existence of space and time that is the central topic for the philosophy of space and time of course more specifically the philosophy of time is concerned for instance with questions such as does time pass it seems one of the most obvious features of the world as we experience it the time passes but it's a highly non-trivial and very deep philosophical question to consider whether that passing of time that flowing of time is part of the objective physic reality outside of us and independent of us or whether it rather only arises as for for us for human sentient beings like us to interact with that physical world also you might think one of the most important thing is that the present and to present things things presently existing are the only things that exist past things have existed but clearly no longer do and future things will exist but they do not yet but is that so is it only the case that the present is real or is the past and the future just as real perhaps it's time finite or infinite does it have a beginning or an end or both we also realize that many things many physical processes around us like the very simple things as melting of an ice cube in a glass of water seem to be directed they go in one direction only they are not the same as if I would show you a film of the things reversed you would immediately recognize which one is the true film and which one is the reversed film so there seems to be a directed of things in time but maybe it's time itself that has a direction these seemingly simple questions and obvious questions about most fundamental aspects of our world are relevant to anyone who seeks to an understanding of the cosmos and our place in it yet they are very hard to answer as it turns out these problems are all in one way or another I think aspects of the one central problem of the philosophy of time which I take to be a central problem of much of philosophy or at least of theoretical philosophy it concerns the reconciliation of what philosophers have come to call the manifest image the world as we human beings experience it the reconciliation with that the world as we experience it with the scientific image the world as it seems to be described and characterized by our best scientific theories how to reconcile the two is not always obvious and in particular in the philosophy of time and elsewhere it constitutes a very sensual philosophical problem just to make this a little bit more vivid the manifest image is teeming with activity objects are booming buzzing by changing their locations properties vivid perceptions are replaced by other ones we seem to be inexorably inexorably slippin into the future time or at least our experience in time seems a very busy and complicated sort of thing now contrast that to time as it appears in science and in particular in physical theories the little tea that we find in the fundamental dynamical equations of physics doesn't differentiate between past and future it doesn't speed up or slow down it doesn't pick out a time as the now it seems to be all there usually from minus infinity to plus infinity given to you at once and that sense no complicated structure at all so we seem to have when it comes to our concepts of time in the manifest image and the scientific image to eco another debate an explanatory gap in order to close this gap we urgently need the interdisciplinary conversations and indeed collaborations between physicists and philosophers so I hope you're no longer surprised to learn now that the philosophy department has been involved in the invitation of an eminent physicist to come here to Geneva today to talk about the science of time so with that I am very honored and very happy to introduce our speaker today professor Fay Talcher professor Fay Talcher she is a professor of theoretical physics at the Imperial College London as it's already been mentioned and she did their PhD under the supervision of professor Stephen Hawking at the universe city of Cambridge in 1990 after that she did postdoctoral work first in the astrophysics group at Fermi National Accelerator Laboratory in Batavia Illinois near Chicago then at the University of California at Santa Barbara and at the California Institute of Technology in Pasadena also in California of course after that she returned to her native UK and became a lecturer at Queen Mary University in London in 1996 and then joined Imperial College in 2003 she does research I'm sure she will be talking about that she will be she does research on the cause and set approach to quantum gravity which is the attempt to bring together aspects of relativity theory with aspects of quantum theory and she also works on the foundations of quantum mechanics to close saint augustine in his confessions once famously said what then is time if no one asks me I know what it is usually it's just you know check it's a quarter to seven yes so you know what time is but if no one sorry if I wish to explain it to him who asked me I do not know once you start to think to ask yourself about really what is the nature of time you realize that maybe this is far from evidence in the hope of obtaining some insights into the nature of time I would like to turn over to Professor Fei Dao who will be speaking today about past present and future the science of time please [Applause] [Music] [Applause] [Music] thank you very much Chris and good evening everyone so we're going to think about time tonight and just to get us in the mood here's a picture of a very beautiful and famous clock some of you may recognize it some of you have may may even have been to see it it's in the National Maritime Museum in Greenwich in the UK and its name is h4 it's about it it looks big on the picture but it's it's about this big H in h4 stands for Harrison John Harrison the designer and maker of this clock it took him about six years to make and it was completed in 1759 with this clock John Harrison solved the famous navigational problem called the longitude problem that's the problem of how the navigator on a ship can tell what the longitudinal position of the ship is if it's in the middle of the open sea after many days and weeks of sailing with no land in sight how does the navigator tell what the longitudinal position of the ship is so the way that Harrison and his Harrison's clock solves this problem is by being very accurate it's a very good clock it accurately measures the time and in particular is very stable and it accurately measures this time even when being carried upon a ship on a long voyage suffering many storms and rough seas before Harrison built h4 the clocks that were taken to see we're not very accurately used to lose and gain time in a random and unpredictable way so why is it that having an accurate clock one that's nice and stable and accurate even when carried on on the ship why does that solve the longitude problem well the navigator can do the following thing the Navigator takes the clock and before leaving port at some particular fixed known longitude the navigator will synchronize the clock say h4 with a clock that will stay behind at the port and the two clocks are both accurate and they synchronized at a particular time the navigator and the ship then leave and they go on their travels and at the moment when the navigator wants to tell what logic what the longitude or position of the ship is the Navigator looks at the clock h4 and sees what time that's showing and deduces that that must be the time in the at the port the port that the was the starting point of the journey the navigator then so the local time is known to the navigate the leaving point of the journey is known to the navigator the navigator will then find out the local time where the ship currently is just by an observation of the position of the Sun in the sky that will tell the navigator what time what the local time is at the ship and those two times the local time at the at the starting point of the journey the port and the local time where the ship currently is those two times together will determine the longitude because those two times will be different and the difference will tell the navigator how far east or west the ship has has sailed so for example if the difference in time if the the local time is six hours ahead of the time at the port then the ship will have sailed a quarter of the way around the globe and so to the east and so on so the difference in the times will tell the navigator where the ship is this was a solution a practical solution to the longitudinal navigation problem now there are at least two concepts of time involved in this story the first is the concept of local time so time at the port which is different from the time at the position of the ship now what that means is we're familiar with that that's just what we're familiar with with time zones and jetlag so that concept is basically an essentially a geometrical concept local time at a particular position on on the surface of the earth it's just a statement about what the position of the Sun is in the sky at that moment but there's another concept of time at play here if you think carefully about how the argument is made the about how to deduce the longitude the Navigator thinks to themselves well I have this clock and I see that it is showing let's say 10 o'clock I know therefore that at this very moment in at the port it's the local time is 10 o'clock so to make the argument the Navigator has to deduce from looking at the clock in their hand what the local time that at that moment at that very moment simultaneously with the navigator looking at the clock in the port far away many thousands of miles away the local time the Sun is at a particular position in the sky that is a necessary part of this argument for longitude there has to be this concept of far distant events being simultaneous being at the same time and I'm going to come back to this concept a little later in my talk so this story shows us how deeply embedded our understanding of time and the development of our understanding time is in the wider story of human history and the development of science and physics in particular and its relationship to our understanding of time was brilliantly expounded by professor Stephen Hawking in his famous book a brief history of time Stephen Hawking was my mentor and my teacher and I formally dedicate this lecture to his memory one of the concepts that Stephen only touched upon is the concept of the passage of time and this is something which chris mentioned in his introduction so the question of whether time really physically passes or whether it's simply a psychological aspect of it's a question of psychology the time doesn't really physically pass it's just that we experience it as passing this disagreement is a current and ongoing lack of consensus and I've chosen a couple of contemporary thinkers to quote to you to show you that this is an ongoing issue an ongoing debate so the first quotation I've chosen is from Paul Davies he's a theoretical physicist his expertise is in quantum field theory in curved space-time and he said the flow of time is an illusion and I don't know very many scientists and philosophers who had disagreed with that to be perfectly honest and presumably the explanation for this illusion has to do with something up here in your head and is connected with memory I guess laying down of memories and so on so it's a feeling we have but it's not a property of time itself time doesn't flow it's part of psychology and that is disagreed with by the philosopher of science philosophy of physics John Norton who wrote time really passes our sense of passages are largely passive experience of a fact about the way time really truly is objectively the fact of passage obtains independently of us time would continue to pass for the smoldering ruins were we and all sentient beings in the universe suddenly need to be snuffed out we have no good grounds for dismissing the passage of time as an illusion it has none of the marks of an illusion rather it has all the marks a novel of an objective process whose existence is independent of the existence of we humans now this debate about the nature of time is a very long-standing one as Chris said questions about the nature of time have been thought about discussed pondered since the time that we actually have records of human beings pondering questions and I've chosen another couple of quotations to illustrate that and I've chosen a couple of words to pin to the two positions of Davis and and Norton and those two words are being and becoming so on the side of Davis and of being we find Parmenides 250,000 years ago who said what is has no beginning and will never be destroyed it is whole still and without end it neither was nor will be it simply is now all together one continuous and on the other side of the dichotomy there is becoming so in support of John Norton view I've chosen a quotation from a book on Buddhist logic by chubowski & Sabarsky describes the buddhist doctrine of dependent origination as there is no matter no substance only separate elements momentary flashes of efficient energy perpetual becoming a flow of existential moments and I will return to Shabbat tsuki and this book on Buddhist logic at the end of my talk one of the joys of thinking about time the nature of time working on it is that it engages us with these tradition traditions of thought so one feels a connection with human beings throughout a long period of history have all been struggling with these with these questions so it's a current debate there is no consensus but from my own personal anecdotal experience in evidence I would say that Paul Davis is actually right that the majority of physicists and philosophers who think about such things agree with Parmenides and with him that in other words the position that the world simply is and that time does not flow is the majority view amongst physicists and philosophers that's a little unusual because probably the majority view amongst the general population is that of course time passes and so what's the reason for that why are why is the majority view amongst physicists and philosophers that time doesn't pass and that the universe simply is that past present and future events are all equally real they have the same physical status things that have happened in the past things that will happen in the future all they have the same physical state why is that well some claim I wouldn't disagree that the reason is that the current science our current best scientific theories supports that view they support the the position of being and of the so-called block universe so here is physicist Sean Carroll who says modern physics suggests that we can look at the entire history of the universe as a single four-dimensional thing that includes our own personal path through it which defines our world line this seemingly conflicts with our intuitive idea that we exist at a moment and move through time of course there's no real conflict just two different ways of looking at the same thing there is a four-dimensional universe that includes all of our world line from birth to death once and for all and each moment along that world line defines an instantaneous person with the perception that they are growing older advancing through time and it's going to be a major part of my job tonight to explicate this paragraph here by Sean Carroll the modern science the modern physics that he refers to is general relativity that's Einstein's theory of gravity and space-time it's our best current theory of space and time and my job part of my job is to explain to you why gen relativity leads us to this particular view this view of the universe as a block with past present and future invent events including my own death there already in what exists now to do that I'm going to take a step back back from general relativity to first give you the scientific view of space and time that pertained in science before gen relativity and that is the view of Newtonian physics I'm going to explain the picture the world picture that Newtonian physics gives us and I've called that Newtonian space so in Newtonian physics there is the concept of three-dimensional space three-dimensional space with objects in that space so objects like tables chairs flowers planets spaceships these are objects they particles they exist in three-dimensional space and they each have a position in three-dimensional space at a particular moment of time so we can choose a moment of time take a mental snapshot of three-dimensional space and in that snapshot everything that exists will have its position in three-dimensional space I'm going to call that that time that you choose that initial time at which we take the snapshot zero so that's that's the zero point the initial time and I'm going to invite you to envisage that snapshot just laid out there as what I've Illustrated here as a two-dimensional slice now here we come upon a perennial problem which is that we're limited in our ability to draw pictures of space-time because it's very difficult to draw anything other than one two or three dimensional things on a two-dimensional screen so really this snapshot of the world of the three-dimensional space with objects in it should be three dimensional we can't draw that so to stand in for three-dimensional space I'm just going to draw this two-dimensional space and these objects in the universe these are galaxies this is a picture that I've taken from a picture of the Hubble Deep Field of of distant galaxies so the galaxies are somewhere in this three-dimensional space but this is a two-dimensional picture remember of a three-dimensional space okay later on let's say an hour later we take another snapshot of three-dimensional space and the objects will have moved some of them and they'll take up new positions and I invite you to imagine taking that new snapshot and stacking it above the original one and labeling that snapshot with a number-one one stands for one hour and let's just keep doing that so another hour goes by we could take another snapshot of three-dimensional space the objects are whatever positions they're in now we take that snapshot and we lay it out above the other two snapshots at a position which I'll mark two and we keep going here's another snapshot an hour later at a position I mark three and so these snapshots are all stacked up one above the other in the order in which they are taken now I just chose one hour arbitrarily I could have chosen half an hour or every minute or every second and of course for each moment when we take the snapshot you can conceive of adding another one of these three-dimensional spaces so we stack them up like this and it is a postulate an axiom an assumption of Newtonian physics that for every moment of time continuously between zero and one there is a snapshot to be taken and there is three-dimensional space with objects in their positions at that time so these spaces can be stacked up and there are infinitely many of them between 0 and 1 infinitely continuously many of them so if you stack them up what you end up with is a four-dimensional block remember what I've drawn here of course is just a three dimensional block because we can't draw four dimensions these slices are really representing three-dimensional space if you stack up three-dimensional spaces one above the other then you'll get an extra fourth dimension and this will be a fourth four-dimensional block you can't draw that so this is just a three dimensional cartoon of what is really a four-dimensional space-time so in Newtonian physics time is spatialized as another dimension we can conceive of it as a fourth dimension we can also now study in Newtonian space-time physics the motion of these objects the planets for example and that was a very pertinent example for Newtonian physics because celestial mechanics is one of the greatest understanding celestial mechanics one of the greatest achievements of the Newtonian physics and what we can we can describe an object like a planet in terms of a world line in space-time so what's a world line well at the initial time the planet will have a particular position so it will be here say in space and in an hour later it will have moved and it will be here now in space and two hours later here in space three hours later over here in space and at every time in between it will have a position so it will there will be a continuous line of positions in space joining its initial position here at time zero to its initial final position here at at time three hours this world line is a full description of what this planet is doing and all the other planets will also and other things you mean the tables and chairs will also have their world lines in space Newtonian space-time so here's another one this is an object which starts off at the same position as our planet it goes off to a different position at times that after one hour it's at a different position at two hours and then it comes back to join the original planet at after three hours so these are two different world lines two different objects in space-time we can ask what is the time that elapses fall this red for the planet moving along red world line and the time that's easy to answer is three hours it takes three hours for this planet to do whatever it's doing and it also takes three hours for whatever is following this green world line to from this position here to this position here and that's true of anything that's moving in this space-time in if we want to know how long it takes we just take to do what it's doing we just take the initial let's say it starts at the initial on the initial slice in the initial space and it ends up on this final one it'll however it goes wherever it goes however it moves it will take three hours there is this universal concept of a global time it ticks away at the same rate for everything in this 3-dimensional space and it doesn't matter how you move or where you go so the notion of similar similar in a 'ti holds in this picture so at this in this three-dimensional space at this time one hour after the initial time the this there's a an event here which is the planet being here and this other thing the green thing being here and those two events are simultaneous it's the same time for those two things in this 3-dimensional space so that harks back to our understanding of how the longitude question was answered okay oh and I should mention that this picture is only a finite chunk of an infinite space-time in Newtonian physics so in Newtonian physics three-dimensional space is infinite it extends infinitely far in all the three directions and time is also infinite so as Chris said the really this is just a piece of an infinite space time and time really starts at minus infinity down here and goes up to plus infinity up there so we just draw a small piece of finite chunk of this infinite so the revolution of relativity changed this world picture to a new view a new world view came to pass it came into being with relativity and our new view of space-time is an Einsteinian view it shares and in this view space-time the world is a four-dimensional space-time it shares that characteristic with the Newtonian space-time is four-dimensional but there the difference that the similarities end I said that Newtonian space-time is made of space it's this stack of three dimensional spaces one above the other forming the four-dimensional space-time Einsteinian space-time is not made of space in fact in this new world view of Einstein of general relativity there is no such thing as three-dimensional space that concept doesn't appear at all anywhere in this theory so it's not made of stacks of space a stack of space what is it made of what is space-time made of in general relativity space-time is made of idealized events where you can think of an event as for example a firework a small firework that explodes that is an event it's something which is localized in space it's a small little thing and it lasts for only a short it has a short duration so it lasts for a short amount of time so it occupies a small piece of space-time now we can think of a smaller firework lasting for a shorter amount of time it's a faster burning firework that will be an event which occupies a smaller piece of space-time if you take the limit of an infinitely small firework lasting for an infinitely short time that is a point event and Einstein's space-time in general tivity is made of infinitely many continuously infinitely many such point events and they are arranged so that the collection of them all is four-dimensional there is no time coordinate no slicing of this four-dimensional space-time into spatial slight three-dimensional spatial slices there is however a notion of past and future so in the diagram the past is down here on the diagram this picture this world picture and the future is up here again this is just a finite chunk of space-time that that I've drawn you can conceive of this just being part of a larger space-time there are not-- there's no space but there are world lines still so here is a world line the world line is now not made of different positions of an object but it's made of different events along the world line of the object so for example the event of me having breakfast yesterday could be this event and me having lunch today and me having dinner later on this evening and me flying home to London tomorrow these could be events along my world line and it's an axiom of generality that in between all those events there are continuously many infinitely many other events so this is these are all the events along my world line for me ordered in the order that they occur to me and everything and everyone you also planets they all have their world lines in those time world lines made of the events that are associated with that object so here's another someone who was with me at breakfast yesterday and then goes off in on a different journey through space-time to different places and we meet at the airport tomorrow and in general activity in Einsteinian space-time the time that elapses for me along my world line is different from the time that elapses along the world line of someone else now that is true we don't notice in everyday life so it in my normal everyday life and your normal everyday life the difference is that elapse along our different world lines on the watches and clocks that we carry is not measurable because the differences are so tiny but if we were to be moving at speeds close to the speed of light with respect to each other or if we were going off to far-flung parts of the universe where there where there where there are black holes and hang around the horizon of the black hole for a long time then we would be able to detect the differences in the times that elapsed along different world lines and if you've seen the movie interstellar the whole premise of that film is that time elapses differently the different for people who go along different trajectories different world lines in space-time so time physical time in general tivity is not a global concept there is no such thing there's no such thing as simultaneous events in general tivity there are there is space-time there are world lines in space-time and time as we know it as we understand it time that actually passes and is physical passes along world lines only there is no space only space time and there is no similar to nante so here's another picture of the block this is I've gone down one dimension so before I had a pitch a piece of a picture a piece of space-time that was a three-dimensional depiction of a space since really four dimensional here I'm just sketched a cartoon in two dimensions of a four-dimensional space-time this is the beginning of the universe if it had a beginning if you didn't have a beginning then we have to stretch this bottom part of the picture all the way down to minus infinity here's the end of the universe if it has an end and in this block are all the events that will happen that are happening and that have happened and let me say that for example this star could represent the event which is me being born and this star could represent the event of me dying and these are all the events of my life strung out in this one-dimensional world line in space-time and here's another world line of some one these blue lines represent photons for example so this this could be the trajectory of a photon that's produced in some supernova somewhere it travels through space-time and intersects my world line here that means that I see the supernova so all of this exists this is the concept of the block universe in general tivity there is no space here no three-dimensional space just time passing time elapsing along world lines that block view does seem very very much at odds with my experience my subjective experience of course you can't experience what I'm experiencing so I just have to report to you that what it very strongly feels like to me is that I am somewhere right now on this world line so I'm here I born here I die I'm 52 so say you know I live an average life I'll probably be around here what it very strongly feels like to me is that I'm located somewhere on this line moving upwards on the line things are happening more and more events are happening and as they as they as they happen as these events happen the position along my world line is moving upwards so it's as if there's a very for me there's a very special illuminated spot luckily I have my laser pointer so I can actually illuminate it for you so so here we have this spot that my experience seems to correspond to coordinate with an illuminated spot that is constantly and inexorably unfortunately because you know I'm going to get there right that to my dad in extra be moving upwards and presumably I can't experience what you're experiencing because it's subjective but presumably you all feel the same right so if this was your world line you would feel that oh well that's my world line in generalities but I really strongly feel that I'm somewhere on that line right now moving upwards that that's corresponds to my experience my conscious experience that there is a special position on the line right now of course it's constantly changing it's moving but there is a special position a thing about general tivity is that there's no such thing in the theory as illuminated dots moving up world lines they just those dots don't exist and it's a good job they don't exist because it wouldn't make sense because if I've got a moving dot along my world line and the moving dot along your world line and all the other world lines in the universe as well with moving dots along them then there would be no way to coordinate where those dots should be so if my dot is here where's your dot there's no way to answer that question and so those dots just aren't there in the theory and general tivity there's no way to make this block universe view corresponds to our subjective experience I claim right however there's disagreement about that there's difficulty in communicating among scientists and philosophers and here's a conversation which I claim I've had many times with other physicists with and with philosophers that shows that there's just a mismatch in understanding of communication so thinker alien thing could be having a discussion about the block universe think a says well things happen in the block there is a there look there's a tree growing there's a supernova exploding there's a person experiencing time passing they're in the block think Abby says no no no the block is static it corresponds to events having happened not to them happening there in the block is the history of the growth of a tree there is a supernova having exploded there is a person having experienced time passing hey says no yeah the block does correspond to events happening Jesus no it doesn't yes it does right it's a debate that's going nowhere this I think cannot this disagreement this lack of understanding this lack of comprehension cannot be resolved within general tivity I believe I think that jet within General tivity it's as far as you can go to just take one of these positions and one cannot hope actually to persuade the other another person who has the contrary position I shall come clean here and say that I side with thinker be I side with John Norton and with the Buddhist philosophers that time does pass it's a physical process I believe and that general tivity does not do justice to it and at this point we will in my talk I'm going to leave the shores of known physics and if you like will now go forwards and speculate about the future so I've covered time in the past that time as we understand it in Newtonian physics time in the present that's time as we understand it in general ativy T and now I'm going to speculate about time as we might understand it in the physics of the future to do justice which I would like to to the temporal nature of our perception whilst maintaining the four-dimensional nature of the physical world in general tivity what seems to be required what seems to be called for is a growing block not a static block one that just exists once and for all laid out past present and future but a growing block and the process of becoming for space-time one in which the past is fixed and concrete but the future has yet to happen has yet to become and if this notion of a growing block is associated with the philosopher CD broad this idea has been a fruitful and useful heuristic in developing one approach to the problem of quantum gravity that approach is called causal set theory and Chris in his introduction mentioned the problem of quantum gravity generativity is superbly successful theory in its in its sphere of application however it has a problem and that problem is that indian relativity one has to assume that matter is deterministic that it follows deterministic laws of motion however we know that matter at its most fundamental level is not deterministic it doesn't behave in a deterministic way it's quantum mechanical and one of the features of quantum mechanical behavior is that it's unpredictable you can't know for sure what the outcome of the quantum experiments is going to be you can make probabilistic predictions but you don't know which of any particular a certain set of outcomes is going to occur so we do not know then how quantum matter gravitates we only have a theory of gravitation for matter that's deterministic we don't have a theory of how quantum not matter that's behaving truly quantum mechanically how it gravitates what's its gravitational field what how do we describe how quantum matter or the gravitational interactions of matter that's truly quantum we do not know with that's the problem of quantum gravity there are many different approaches and I work on one called causal set theory and in the remainder of my time tonight I'm going to briefly tell you a little about causal set theory so the basic idea behind causal set theory and remember this is speculation about a theory that might yet come to be in the future spacetime is not smooth and continuous but biti granular pixilated or atomic so just as we understood matter not to be smooth and continuous but actually made of atoms that for example a metal block if you try to divide that metal block in half and then in half again and then half again in half again you can't do that infinitely often eventually you reach the discrete atomic structure of the metal and there are atoms of the metal that can't be divided any more so space-time this four dimensional space-time of general relativity is also not infinitely divisible you take a piece of space-time you divide it in half and divide that in half of that eventually you get down to events which are fundamentally fundamental and indivisible and we call those space-time atoms so the idea is to propose but behind causal set theories to that space-time is comprises these atomic units of space-time space-time atoms but what binds these space-time atoms together what keeps them from just falling into a heap of dust what what gives them struck what gives the space-time formed of these atoms structure here we look to generativity for an answer spacetime in general tivity has a causal order the events of space-time in general ativy are causally ordered that causal order is fundamental to understanding generativity and what it means is that some events can cause li influence other events in space-time and some events can't so here I've drawn a pattern of causal relations that exist and the the rules for which events are causally related to other events is whether that event can send a signal to the other event so this event here which I've called D is to the causal past of this event a because it can send a photon along this blue line to a so this event D is in the it's causally related a it's in the causal past this event C is in the causal future of it this event D because D can send say a spacecraft or a Space Shuttle along this red world line to C but some events are causally unrelated and they have no there's no possibility of either of them influencing the other so C cannot influence a and the reason is that things can't travel faster than light so if C were to send something to a that something would have to travel faster than light and it can't and so C is causally disconnected from a and there's no sense in which C comes before a or a comes before C they're just unordered they have no causal relation to each other and this is true pair of events in space-time in general tivity they're either unrelated to each other and can't influence each other and there's no order between them or one is to the past of the other so we take that and we add it to our hypothesis that space-time is made of discrete space-time atoms and we end our those space-time atoms with an order so here are the space-time atoms described represented on my diagram by these dots and these arrows just represent the order the the the causal order that these atoms have so d3 seeds C D also precedes a which precedes B but C and a have no arrow between them and they are causally they are unordered there's no sense in which C comes before a or a comes before C they just have no order there's no ordering and this hypothesis about the structure of the underlying discrete structure of space-time was made independently by a talked by mere Heim and by bombelli lee Maya and Salkin and has been championed by Sorkin Raphael Sorkin particularly causal sets of the marriage of atomicity and causal order here's our causal set again made of these space-time atoms with the causal order between them it's a very small causal set if our hypothesis is right then the number of space-time atoms that you would need to describe a universe the observe our observable universe is rather larger than this it's 10 to the 240 roughly our hypothesis is that the continuum space-time of general relativity is an approximation to a discrete reality just like the fluid description of water is an approximation to the maleta true molecular structure of water and the hypothesis is that these space-time come into being in a continual random process of births so these each space-time Adam is born and the order in which they are born is their causal border so D is born before C D is born before a heir is born before C but there's no fact of the matter about whether C is born before a or a is the form of forth e they are unordered in their birth this order is not linear but it's a partial order this model in which the universe become this discrete universe comes into being has been called asynchronous becoming by Raphael sorting and in this model of the universe there are two types of physical thing one type is the space-time atoms and the order relations between them the other type of thing and that's the material the material of our four-dimensional world the other type is the process the birth process in which the elements come to be both these things are physically real and you need both and what corresponds to things happening is the process the atoms don't correspond to things happening it's the process in which they are born which correspond to the occurrence of events and to the passage of time that at least is the proposal that is being made in causal set theory so here's the conversation again think a and think could be they have the same disagreement this is just a repetition of what we had before a says well the block does correspond to events happening now B has something to hang their argument upon now these says well in causal set theory there are not just the space-time atoms that make up events but there's the birth process as well and it's the process of coming into being that corresponds to opening and this is what is missing in the block and this is what correlates with our subjective experience of the passage of time an event corresponds to a collection of space-time atoms with causal relations between them but the occurrence of the event corresponds to the birth of these atoms so I've presented to you if you like time in the past that's Newtonian time Newtonian space-time is about space and time in the present our very best understanding of space-time that's Einsteinian space-time and Einstein in space-time is about events and I've shown you a glimpse of vision perhaps of future physics in which time is not about space it's not about events it is actually about process so let me return now as I promised to this scherbotsky on dependent origination in Buddhist logic so again there's no matter no substance only separate elements momentary flashes of efficient energy perpetual becoming a flow of existential moments and he adds something must explain how the separate elements of the process of becoming are holding together to produce the illusion of a stable material world and the Buddhist philosophers proposed that what holds these events this becoming process together our causal laws and whether or not the Buddhist philosophers 2,500 years ago anticipated causal set theory we can debate but I think this illustrates that we should look to all of our intellectual heritage to get the best possible toolkit in order to make progress in fundamental physics and in philosophy we shouldn't limit ourselves to any particular tradition of thought but look most widely at all of human thought in order to give ourselves the best chance of making progress in the future and I will end there thank you very much for listening [Applause] [Music] thank you very much for this very very lovely table very insightful I hope now that we have at least remove pushed back some of the mysteries about the nature of time a little bit further now we have an opportunity to ask questions to the speakers and if you would like to ask a question please try to get my attention and I will then send somebody with a microphone to you so who would like to have a question here here's the first question good afternoon I'm not so familiar with the cause of set theory but I understand tell me if I'm wrong from what you said that causality as a fundamental nature in the theory if that's true by the setting is true my question would be what do you think of the thought experiment of John Wheeler regarding the delayed choice experiment which was confirmed by Allen a spell in the 85 in the 1985 I believe proving that along the decoherence process one could affect the duality between wave and particles after that process has occurred suggesting some people to talk about retro causality and even leading the great magician mathematician Alan Cohen in France talking about the past that is still moving that experiment showed that well suggest that some people say that the future is actually dictating the present I would like to have your views on this yes thank you for the question that's a brilliant question there are so many things to say about that one thing is to just say that all discussions of causality as in as far as quantum theory goes are hampered by our lack of consensus about what is really going on in a quantum system so on one view if one is doing a quantum experiment let's say in a box over here then one should not even talk about or think about what's going on inside the box all that one is doing is me is setting up some equipment outside the box in some way and then recording what that response of the equipment is so in fact there's no sense in which we can say what is happening inside the box as far as the quantum system is really concerned okay so that that's actually one that's a view that some people take we shouldn't discuss what's actually going on inside the box well we can't all we shouldn't so it's very difficult and there is no consensus on what people call the interpretation of quantum mechanics the answer to the question well what is what how how should we think of what picture should we have of the quantum world given that situation it's extraordinarily difficult to make conclusions about causation what is causing what to change when what is talking trying to talk about a quantum system in a situation in which there's no scientific consensus about how to talk about it whether we even should talk about it so that's a that's a sort of caveat however the the sorts of experiments and thinking that you refer to are indeed an incredibly important aspect of trying to make the physics of causal set theory quantum the dynamics that I alluded to in which the causal set elements are born in a partial order so far all we have is a classical model which is not quantum a classical model of how that can happen so we would like a quantum model and we would like it to respect relativistic causality that we would like that the question is can we do it in a way that is not in contradiction with experiments like the Bell experiments and the experiment you referred to and in such a way that there is a notion of relativistic causality but that begs the question what is that notion of relativistic causality in a quantum theory so part of our experiment part of our theoretical program is to try to formulate a condition of quantum relativistic causality which allows violations as a barrel inequalities and therefore is not into contradiction with experiment and yet is in some suitable sense yet to be yet to be discovered and formulated causal we're not nowhere near there yet but it's absolutely a crisis point that is a very fruitful place to look very carefully soon so it's very important for us yeah thank you there was a question over there thank you very much for this fascinating presentation I've really enjoyed you comments at beginning defining being and becoming and being could be compared to space and thus a physical space and becoming matter and matter is as you rightly said evolving constantly like complex adaptive systems so can we say that actually we are combination of two timelines two linearities one defining who we are in certain space but also who we are in the definition of meta evolution and maybe this will say combine to define time at a certain space matter time that would be difficult to realize with in general relativity because there's no such thing as three-dimensional space in general tivity so what you're proposing can't it it can't be compatible with general tivity whether or not future physics will somehow come back to this pre relativistic notion of there being three-dimensional space we do not know person myself I would strongly suspect not that so my my own view of the progress of science is that you just don't go backwards at least as far as I can tell in the history of science it's never the case that you go back to you know back to a previous point of view so so a general activity rules out such a such an idea but of course it's it is possible that in the future with three-dimensional space will will make a comeback and then perhaps something like what you're proposing could be could be developed we have time for one more question there's one here so many jokes a one of the big arguments of that the time is flowing in one direction is that some even doesn't seem to be a reversible and my question is based on what you said tonight is it because some even seems to be irreversible because the quantum atoms that you spoke about hour after the realization of the events dispersed in space-time I don't know if I'm clear [Music] good so they're in this process of becoming there is inherently a direction so in the sense that and it just distinguishes the past which is real those are the space-time atoms that have already been born and the future which is open and that doesn't exist yet so and the space-time atoms are born so space-time grows it accretes more and more space-time events and that's directional that's inherently directional it breaks there's no symmetry between past and future they're in in that sense what you refer to as however it could be the case that there is what people call another notion of time reversal symmetry is that what how Chris described it if you run a movie four words and run a movie backwards then you can tell which one's the right way right so eggs smashing the floor and break but they don't gather themselves up off the floor and reconstitute them so that's it might be that the space-time that grows you via this process of becoming once you have it is actually symmetrical so in the sense that once you have it the whole thing from the beginning of time to the end of time and you run it backwards that's also a perfectly good and possible space-time so it could even though the birth process has a direction it might be that there's also that there could be a sense in which the dynamics is time reversal symmetry so that's so the arrow of time and this fundamental fundamentally physical process of becoming which is which has it has a direction those are those are those are sort of there are they're logically distinct questions I don't think I understood your second question already thank you him I think because if there was a question here down in the center yes I the first time thank you so much for this presentation it's well very great second time I'm sorry you just about my question because I learned my first English lessons like five weeks ago and it's quite complicated for me to translate my perfection so you took in an example when you're talking about the two people coming together you remember this you know about the dimension and you talk about the event happenings and my question is what kind of events the events I'm sorry just even we see all the events happening and that you do not see if my question itself clear which are you asking about the events that comprise space-time space-time in general acidity I guess yes because you know you talking about the the event but even we see or we don't see well I mean general tivity is a gives us a world view of the entire universe including events that are yeah that we that we don't see directly or that are too distant from us so that their effects can't be can't be detected by us so yes the worldview of general relativity is is is a cosmological view so it's the space-time is is is the hot the entire physical world in ghent in general tivity so that that will include events that that we see and also events that so we don't see I'm not sure I answered answered your question very well so thank you very much we've come to the end of this talk please join me once again in thanking professor fade-out for her wonderful [Applause] [Music] [Applause] [Music]

Channel: Beyond Spacetime

Views: 26,869

Rating: 4.7824774 out of 5

Keywords: Beyond Spacetime, Geneva Symmetry Group, Fay Dowker, Philosophy of Time, Philosophy of Spacetime, Philosophy of Quantum Gravity

Id: LAFymPdAg5I

Length: 76min 38sec (4598 seconds)

Published: Fri May 25 2018