The Science of Time Explained by Brian Greene

we all feel its existence it makes sense of our world  it's elusive fluid and dynamic time it has baffled thinkers for millennia  yet no one knows what it actually is   time is the most familiar quality of  experience because there's nothing that   takes place that doesn't take place within an  interval of time and yet at the same time it is   perhaps the most mysterious quality of the world  so it's a wonderful confluence of the familiar and   the deeply mysterious all in one little package  if you were to ask me what is time i don't really   know i don't think anybody does i can say what  time gives us it allows us the language for   talking about change it allows us to envision the  events of the universe being spread out in this   temporal timeline and in that way allows us to  see the patterns that unfold within time allows   us the structure and the organization to think  about things in that kind of a progression but   what actually is it i don't really know and that's  so strange because we can measure it there are   laboratories in the world that measure this thing  called time to spectacular precision but you know   if you go up to the folks and say what is it that  you're actually measuring i don't know that they   can really articulate the kind of answer that  you would expect from those who are engineering   a device that can measure something called time  to that level of precision so it's a very curious   combination as a general definition time is the  continued sequence of existence and events that   occur in an apparently irreversible succession  from the past through the present into the future time has long been an important subject  of study in science but defining it in   a manner applicable to all fields without  circularity has consistently eluded scientists   in general relativity the physical nature of time  is addressed with respect to events in space time   for example the collusion of two  particles the explosion of a supernova or the arrival of a rocket ship and time is one  of our most precious commodities perhaps the most   precious commodity i mean we enter the world we're  given a certain allotment of time we don't know   how much time we're given now we have spent  countless hours decades centuries pondering the   nature of time and there is a lot we understand  about time but if we even reflect on the starting   point where you would imagine that we begin  our exploration of time the definition of time   we still are hard pressed to give that definition  right time certainly has to do with change that's   how we recognize that time has elapsed but trying  to really get at the heart of what time actually   is proves extraordinarily difficult john wheeler  one of the great physicists of our age he famously   said that time is nature's way of ensuring  that everything doesn't happen all at once and   certainly that is a quality of time but trying to  get at the heart of what it is is difficult what   we have done to try to get a handle on time over  the course of many centuries is devise ever better   ways of measuring the passage of time we try to  find processes out there in the natural world   that are cyclical that are repetitive and we have  improved upon it by making use of atomic clocks   in which for instance this is an example where a  cesium atom can be excited by bombarding it with   laser pulses and then it itself will pulse and  in fact we now define the second by counting the   number of vibrations of the radiation that comes  out of this cesium 133 atomic species so this is   a a wonderful progression in the history of trying  to measure this thing called time because atomic   clocks they are so accurate they're so reliable  in their repetitive process their cyclical process   that some of these devices will lose less  than a second in a million or 10 million years   in ancient greece some philosophers thought  that time is not a reality but a concept   even some theoretical physicists today  think that time is nothing but an illusion   until einstein's reinterpretation of the  physical concepts associated with time and space   time was considered to be the same everywhere in  the universe with all observers measuring the same   time interval for any event but nothing could  be further from the truth theories were devised   for the possibility of absolute time but the  physicists working on these problems found that   the idea was even more counter-intuitive than  einstein's relativity equations one of the most   frequent questions people wonder when thinking  about time is whether it's physically possible to   go back in time [Music] but a central problem with  time travel the past is the violation of causality   i do think that at the macroscopic level there  is a fundamental notion of causality that does   emerge from a starting point that may not have  causality built in so i certainly would allow   that at the deepest description of reality when  we finally have that on the table we may not see   causality directly at that fundamental level but i  do believe that we will understand how to go from   that fundamental level to a world where at the  macroscopic level there is this notion of a causes   b a notion that einstein deeply embraced in his  special theory of relativity where he showed that   time has qualities that we wouldn't expect based  on experience you and i if we move relative to   each other our clocks tick off time at different  rate and our clocks is just a means of measuring   this thing called time so this is really time that  we're talking about time for you and time for me   are different if we're in relative motion he then  shows in the general theory of relativity that   if we're experiencing different gravity to more  precisely different gravitational potentials time   will elapse for us at different rates these  are things that are astoundingly strange   that give rise to a scientific notion of time  travel this is how far einstein took us in wiping   away the old understanding of time and injecting  a new understanding of its qualities so there's   so much about time that's counterintuitive but i  do not think that we're ever going to wipe away   causality if we universally agree on cause and  effect einstein showed in his thought experiments   the people travel at different speeds though  these effects are typically minuscule in the   human experience they become much more pronounced  for objects moving at relativistic speeds   approaching the speed of light the predictions  of the theory of relativity for time dilation   have been repeatedly confirmed by experiment so  if you keep wondering if time travel into the   future is possible the answer is absolutely  yes an individual can follow an einsteinian   strategy and propel themselves into the future  in some sense more quickly so if if i wanted to   see what's happening on planet earth 1 million  years from now einstein tells me how to get   1 million years from now build a ship that can  go out into the universe near the speed of light   turn around and come back let's say it's a  six month journey out a six month journey back   and einstein tells me how fast i need to travel  how close to the speed of light i need to go so   that when i step out of my ship it will now be one  million years into the future on planet earth and   this is not something where there's differences of  opinion in the scientific community any scientist   who knows anything about what einstein taught us  agrees with what i just said and so that kind of   travel to the future is absolutely allowed by the  laws of physics there are engineering challenges   there are even biological challenges right they're  g-forces that you're going to experience you know   so there's all sorts of stuff embedded in this but  those i will call the details and those details   notwithstanding the universe allows  this kind of travel to the future [Music] as for going back in time we've mentioned before  that it is a direct violation of causality   giving rise to the possibility of a temporal  paradox some interpretations of time travel   resolve this by accepting the possibility of  travel between branch points parallel realities   or universes but we will discuss time travel  into the past more extensively in another video thanks for watching did you like this video   then show your support by subscribing and  ringing the bell to never miss videos like this

Channel: Science Time

Views: 253,136

Keywords: science, brian greene, the science of time explained by brian greene, brian greene time travel, is time travel possible, what is time, is time an illusion, brian greene science time, science time, space, cosmos, einstein, general relativity, physics, wormhole, warp speed, go back in time, time machine, future time travel, time, spacetime

Id: dNbrI18BOAI

Length: 10min 1sec (601 seconds)

Published: Sat Jul 09 2022