Time - Measurement And Meaning

[Music] night took up night [Music] the emergence of modern man has been closely associated with the development of the Sciences of the many physical phenomena that men have tried to understand few have held the fascination or proven so elusive as time for at least 5,000 years men have noted the seasons recorded the solstices regularly divided the year in short have made calendars [Music] measuring time with calendars is really a way of associating the Earth's rotations on its axis with its yearly Revolution about the Sun as the Earth revolves around the sun we make a sort of clock out of it the sun is the center the Earth is the clock hand and the 12 hours are January February March April but as the Earth revolves around the Sun it also rotates on its axis producing days and nights the axial rotation of the planet and its revolution around the Sun are unrelated the Earth turns about 365 and A4 times for every Revolution but not exactly men have tried for thousands of years to find some system which easily fits days into years the search for a simple calendar has failed because the rotation and revolution are not simply related today with a system of leap years and oddly divided months we have a workable way of handling the odd relationship between the Earth's rotation and its annual Journey around the Sun what precisely is a day ancient Mediterranean peoples defined a day as the period between sunrises with the invention of water clocks like this one they were able to divide the day into smaller [Music] units when Galileo invented a pendulum operation clock it was possible to prove that A Day defined by successive transits of a distant star was more constant than the day based on the [Music] sun historically hours minutes and seconds have been arbitrary divisions of a day now however short periods of time like the second can be defined independently of celestial events [Music] [Music] [Music] n mechanisms which tell time are usually based on some periodic motion something that repeats itself over and over again electric clocks in America are driven by alternating current with the frequency of 60 cycles per second that period of a 60th of a second is controlled by the motion of atoms in atomic clocks thus the turnings of the planet no longer determine hours and seconds the first attempt to define the second scientifically was to take the average solar day and divide it into 86,400 equal parts but the Earth's rotation proved too erratic to serve as a useful standard in the 1950s scientists studying ancient Chinese records of eclipses discovered that the lengths of days were shorter then than now thus men learned that gradually as the years pass the Earth's rotation is slowing down ultimately the Earth may slow down to a single rotation per year so that it always keeps its face towards the sun just as the moon now rotates to keep the same face toward the Earth if we continue continued to define the second by the rotation of the earth seconds would grow gradually longer with every passing day so the next refinement in the definition of the second was to measure precisely how many seconds there had been in the year 1900 and to use that as the standard second the third and most recent refinement has been to define the second as the duration of a little over 9 billion periods of the light waves that are emitted by an atom of cesium a clock based on this system ticks off seconds with very little error the principle of our cesium atomic clock is really quite simple when cesium metal is heated the individual atoms begin to emit light at a frequency of about 9 billion cycles per second and those atoms are emitting a very precise frequency more unburying than almost anything else in the universe we send the stream of vibrating atoms into a little chamber that's resonating at the same frequency about 9 billion times a second the resonating chamber interacts with the atoms inside it it keeps the atoms vibrating the atoms keep the chamber resonating and the Chamber controls a crystal of quartz rock which is sending out a radio signal in other words the atoms control a radio signal and the signal controls a clock [Music] for instance here is a demonstration of how short a time interval is easily measurable today we'll send an electrical pulse through about 1 and2 M of wire and see if our Computing counter can measure how long it takes the pulse to travel from one end of the wire to the other the measurement of the intervals is actually very rapid the computer takes a moment to average them out it takes the pulse of electricity let's see that's about 6,300 millions of a second to Go Through the Wire people ask us why we would ever need to measure times this short but actually when we're considering nuclear events 100 million of a second is such a long period of time that we would hardly ever talk about it we are more concerned with time intervals in the neighborhood of a billion of a second or shorter [Music] a billionth of a second is called a nanc in the time it takes a firecracker to explode a million NCS go by [Music] in a nanc a Jet Plane will travel about a 100,000 of an inch Which is far enough for it to plow through several thousand air molecules inside of those air molecules a great deal is happening in a billionth of a second some subatomic particles will live out their entire history protons in the atom's nucleus will oscillate thousands of billions of times in a nanc for a long time men considered atoms to be the smallest parts of nature recently even smaller constituents of matter have been discovered the So-Cal Elementary particles over 100 of them have been classified th far with names such as electrons neutrons kons neutrinos and anti neutr no the lifetime or the Decay process of these particles is extremely short Su to the order of 10us 24 seconds which is one trillionth over a trillionth of a second in machines like this cyclotron at the University of California atomic particles are accelerated to high velocities and studied physicists have recorded events taking place in quadrillions of seconds and shorter some physicists believe that the evidence of course when we talk about backwards time we're usually speaking of a rather specialized interaction among electrons positrons and gamma rays according to one model an electron May sometimes reverse its charge in direction of spin thus becoming a positron in the process it also reverses its direction in time and begins traveling toward the past this backward journey in time May last for Nan seconds or centuries until the particle collides with an electron and is annihilated it's not at all like the dramatic reversals of time found in science fiction here we see a typical photograph in an elementary particle experiments the white lines that you see here are tracks left by the particles entering a bubble chamber the time intervals of particle physics are so brief they can only be measured indirectly by such methods as measuring tracks particles leave in liquid hydrogen it takes light a trillionth of a trillionth of a second to cross the nucleus of a hydrogen atom and that is also the shortest Decay time of any atomic particles we believe that mass and energy travel in discrete bundles or Quant and some theoretical physicists have suggested that time also passes in discrete units each unit is about 10 to Theus 24 seconds if we start with this as a unit of time and lay out a th000 trillion of them end to end we still have only a billionth of a second we have an unexpected clue as to the direction of the arrow of time it is in the concept of entropy the persistent increase in entropy means that things tend toward a diffuse chaotic and disordered State and with every passing moment the things of the UN Universe are becoming more disordered the continuing increase of entropy means that an orderly little drop of ink will diffuse into a billion Parts at the first opportunity increasing entropy means that the air in your room will tend toward the most disordered State possible it will never gather together under your bed and leave you to suffocate it means that if left alone every Rock and every planet will slowly erode away everything is slowly slipping into chaos As Time Marches On entropy Marches On indeed they may be more related than we imagine notice what happens when we make our movie violate the law of increasing entropy when the cards go to a more ordered state it seems that we have reversed time the cards jumping up violate the law of gravity but they also violate the law of increasing entropy we can always tell when a movie is going backwards if it violates the law of increasing entropy what we conceive of as time may be simply the Relentless increase of entropy of decay of Randomness but now we are pushing the limits of what we can say of the nature of time we simply don't know what time is such speculations belong properly in metaphysics and in Psychology the psychology of subjective [Music] time extremely brief events are as inconceivable to us as the eons that pass on stars and [Music] planets we are talking now of subjective time the time that we sense [Music] our view of the universe is peculiarly fashioned by our life spans for a man whose life is less than a century a billionth of a second is inconceivably brief but if our lives happen to be as short as that of a particle called a k mson a second would seem a vastly long period of time we have talked about man's subjective view of time spans and we have talked about the measurable intervals of physics but now we aren't talking about anything or rather what we're talking about now is now what is now how long is now is now a psychological idea is it a notion in physics when it's now on Earth is it now throughout the Galaxy the idea of now assumes simultaneous moments throughout the Universe but in physics events which are simultaneous for one Observer may not be simultaneous for another in other words now is not a meaningful term in physics psychologically there is a continually moving point of in time which we call now this now separates the past from the future it is always now time is a long string of Nows which stops abruptly at the [Music] present looking back on them we see these Nows as instants in time and as points in our movement through space [Music] so far we have suggested that time as defined in physics moves forward uniformly unchanging throughout the Universe yet we know that our personal sensation of the duration of time intervals is affected by circumstances presumably to an insect whose life is brief and whose movements are rapid our activities are slow and lumbering [Music] relative size affects our subjective perception of [Music] time but we are now going to suggest that there are conditions under which the measurable rate of time changes [Music] physicists have now proven that the duration of an interval is related to the velocity at which a thing is moving through space for example a clock ticking off seconds on a very distant star moving very rapidly away from the earth would seem to us to take hours or even months between ticks this notion of time slowing down or time delation was first suggested by Albert Einstein his ideas radically changed our view of fundamental physics [Music] many Concepts in physics are first posed as thought [Music] problems a famous thought problem is the twin paradox it involves two brothers one is going to a distant star and will return [Music] [Music] as the astronaut is accelerating away from the earth that is his velocity is continually increasing he notices that clocks on Earth are going faster than his clock however as he approaches 99% at the speed of light he stops accelerating he is going at an unchanging velocity but what is this clocks on Earth are no longer going faster than his clock in fact they're going [Music] slower the spacecraft slows down to go around the planet since his speed is again changing he observes that clocks on Earth are again going faster than his clock now he's returning home he has had a dazzling trip time seeming to slip back and forth what will the earth be like when he gets back he is 12 years older his brother however has grown into an old man the world is no longer interested in his little experim [Music] experiment here at the 2m long Stanford University accelerator experiments have been performed which conclusively prove that under certain circumstances time can be caused to slow down we have a problem in this stainless steel tube directly above me we're sending Pine mesons produced by our accelerator into the bubble chamber these P mesons are very very short-lived particles in fact they only live about 26 billionths of a second in that amount of time you can only expect them to go about 27 ft before decaying even if they were going at the speed of light on the other hand our bubble chamber is located 250 ft Downstream of where I'm standing the question then is how do the particles live long enough to go into the chamber the answer is relativity we're standing on the 82in bubble chamber at the Stanford linear accelerator Center and right now my meons are going into the chamber at the rate of 30 per second 15 each time you hear the noise in the background now the P mesons are traveling so fast in fact at over half a billion miles hour that in their SpaceTime frame length appears to be contracted and to us time appears to be dilated now what do I mean by that well the 400t distance through which the peson have to travel in order to get to the chamber looks to a peson as if it is only about 4 ft long which is quite a difference also in terms of length of time dilation effect time is slowed down as we observe it by a factor of over a 100 so that instead of living only 26 billions of a second the peson lives long enough to travel 2700 ft before It ultimately decays this is quite a bit longer of a distance than the 400 ft that we've been talking about thus in actual controlled EXP experiments physicists have shown that time is variable with regard to Velocity the same equations that showed how Mass may be transformed into energy in a nuclear reaction showed that objects traveling at different velocities exist in different time [Music] frames the great shifting mutable Universe which Einstein revealed necessitated redefining our most basic Notions energy Mass space gravity and [Music] time gradually we learn to predict physical events to describe operations mathematically but our definitions are strictly operational as with length and mass the other basic physical Concepts which we use operationally but can't Define time can also be used and measured but finally we find it impossible to answer that most compelling question what is [Music] time there's no use trying Alice said one can't believe impossible things I dare say you haven't had much practice said the queen when I was your age I always did it for half an hour a day why some times I've believed as many as six impossible things before breakfast [Music]