Today, time rules our lives. We live each day with the moments broken up into hours, minutes and seconds. We never seem to have enough time. But can you imagine not being able to tell time at all, where the movements of the Sun and the stars was the only way to know what time it was? Let’s learn about the history of time, methods of telling time, and Einstein’s historic discovery that time isn’t as fixed as we thought it was.
- History of telling time
- Ancient ways of telling time
- Stonehenge and other ancient calendars — About.com
- Sumerian Calendar
- Sundial history
- Early clocks — About.com
- More early clocks
- Build a water clock
- History of mechanical pendulum clocks — About.com
- John Harrison and the Longitude problem — Royal Observatory
- Book: Longitude: The true story of a lone genius who solved the greatest scientific mystery of his time, by Dava Sobel
- Leif Erikson
- Chronometers — Wiki
- Quartz oscillators — Wiki
- How Atomic Clocks work — HowStuffWorks
- Atomic clocks are not radioactive! –HowStuffWorks
- More on atomic clocks
- Cesium — Wiki
- The NIST Cesium Fountain Atomic Clock
- Muons and Time Dilation — GSU
- Time Must Be Relative — Caltech
- Speed of light and time — Argonne National Lab
- Twin Paradox — University of New South Wales
Download the transcript
Fraser Cain: Back in the United States Pamela?
Dr. Pamela Gay: Yes, yes I am finally.
Fraser: I think we need to apologize to the listeners that we’ve been a little irregular over the last couple of weeks. A lot of it was just it was a lot more complex to get podcasts done from England and Germany and so on.
Pamela: The 9-hour time difference kind of did us in.
Fraser: Yeah and you were pretty fried from all of your meetings. So I think when we started the new questions show we mentioned that there would be times when we wouldn’t be able to get them done and this was one of those times.
You’re back, I’m good, we’ve got an episode today and we’ll be getting back on schedule with the question shows so everything is moving on. Thank you very much everyone for your patience and understanding and I’m sure it will happen again. But we’re going to keep the regular shows coming out as regular as possible.
So today: Time. Time rules our lives. We live each day with the moments broken up into hours, minutes and seconds. We never seem to have enough time. Can you imagine not being able to tell time at all? Where the movements of the Sun and the Stars was the only way to know when it was?
Let’s learn the history of time, the methods of telling time and Einstein’s historic discovery that time isn’t as fixed as we thought it was. Well so in a world without technology, how did people tell time?
Pamela: Luckily we always have our Celestial timekeepers. There’s the Sun that allows us to know when it passes from Noon to Noon. We can tell when the Sun hits its highest point in the Sky measure when it happens the next day and we know that 24-hours have gone by.
We can measure the passage of the years using alignments of the Sun. When is it at its most southern point on the horizon; when is it at its most northern part on the horizon? When is it exactly in-between those two points at sunrise or sunset?
Fraser: Right and a day being one complete rotation of the Earth on its axis.
Pamela: And a year is one complete orbit around the Sun.
Fraser: Right and I guess this was a pattern that ancient peoples figured out [Laughter] pretty quickly.
Pamela: Well it kinda helps with harvest and things like that.
Fraser: Yeah, and you know evolution clearly with sleeping, waking and all that. So we’ve known about it I guess billions of years.
Pamela: And it mostly works for us being able to simply say day versus night and winter versus summer; kind of a convenient system.
Fraser: How things get more sophisticated from there?
Pamela: There’s always that person who wants to write down everything. Who wants to detail when everything occurred and they want to be as precise as possible. We all know these people. These people set out to start planning ways to delineate our days.
It also helps when you’re trying to set up meetings and when you’re trying to plan visits and all the sorts of things that humans as social creatures like to do. To meet all these different needs, people looked for ways to delineate time. Some of the earliest means just used shadows. The shadow of a stick will be shortest at Noon. It will be longest and pointed one way in the morning and longest and pointed another way in the evening. You can divide up the path of the shadow into various segments and measure the passing of the day.
But that doesn’t work so well at night and human beings do things at night too. So we’re also always looking for other mathematically predictable ways to break up time. Beyond us measuring days and years we also like to try and define months. Unfortunately, our Lunar cycle, our Moon’s passage around and around the Earth doesn’t divide evenly into a year.
So there have been whole systems of trying to work out calendars that are based on this cycle that basically resonates between eleven cycles, twelve cycles, thirteen cycles, varying numbers of years that you count which way and the other way trying to figure out how to develop a Lunar calendar.
Fraser: Right this is the problem that the Moon takes about 29 days to go around the Earth and then if you try and divide those in, you don’t get 12 Lunar months in a calendar year, you get like 12 1/2, right?
So if you’re saying on every third Moon is when we plant crops, after awhile you get completely out of sync with the Sun and with the true seasons that the Earth is experiencing as it is going around the Sun.
Pamela: This is where you end up coming up with crazy leap year systems. It just doesn’t quite work. But this mostly kinda sorta twelve did eventually lead us to our 12-month year. It gets you somewhere. It just doesn’t get you all the way there.
Between combining the Moon to celebrate many religious holidays; using the Sun to track the passing of the seasons to delineate one year accurately, you can develop calendars that allow us to mark the passing of time for many year cycles of how the Moon and the Sun beat against one another.
Fraser: Right and ancient peoples developed all kinds of technologies to track that calendar. Like Stonehenge and Pyramids and to know when the Solstices were and when the Equinoxes are to know what good times to plant or harvest are.
Pamela: Eventually we found ways to make it all work but we had to settle down for a 365-day calendar that is based purely on the Sun but has this crazy leap day every few years and on certain centuries.
Building calendars is one of the most complicated things that humanity has figured out how to do. But that’s just bookkeeping. Trying to measure time – the passage of moments, seconds, hours – that starts to become more of a technological problem. It’s in its own way very hard to solve as well.
Fraser: Okay so what were the first methods of actually breaking up the day into – because I mean like days and Lunar months and calendar year – those are all natural occurrences, right? Those are based on the way the movement of the objects in the Solar System so beyond then it’s completely artificial, right?
Pamela: Right and so somehow in ancient times it fell on to this “we’re going to have a sixty-second minute; we’re going to have 60-minute hour” this all comes down in some way to 360 degrees in a circle.
It was the Sumerian civilization that came up with this crazy sexagesimal system that we’re using. In devising Sun Dials we look for okay let’s make the day be 12 hours; let’s make the night be 12 hours. There are actually systems that changed the length of an hour so that as the days got shorter and longer with the passage of the seasons the hours actually changed during daylight and night time to reflect this change the length of an hour would need to be to keep both the day and the night 12.
Eventually we decided no, we’re just going to make everything the same length. This started to make it easier to build mechanical apparatuses or I guess a candle you would say is a chemical apparatus. One of the early ways of keeping time was to use a water clock. You fill a vessel with water, let the water come out of a set diameter opening, fill set volumes and you can measure the passage of time in how water transfers from one vessel to another.
You can also burn candles. If the candles are all made of similar composition of the same diameter and height, they will take the same amount of time to burn. So between water clocks and precisely made candles, we found ways to measure equal passages of time in both the day and night even when the Sun wasn’t to be seen behind the clouds.
Fraser: Right a Sun Dial is a wonderful technology for measuring the passage of time. You could even build them yourself. If you want a project to do around the house, build a Sun Dial. Get the kids involved and help them mark out the hours and so on. But as you said, a Sun Dial doesn’t work when the Sun goes down.
Pamela: This started the quest for “how do we precisely measure time”? Candles are fairly precise but you always have to worry about them blowing out. Maybe the wick is a slightly different composition. Difference affects them. Temperatures SERIOUSLY affect water clocks. If it gets too cold your water clock just plain freezes and well time doesn’t stop at the passage of time according to your clock does stop.
This got people looking for increased ways to make more complicated systems that work more accurately. With water clocks you have to worry about as the height of the water inside the vessel drops the water flows out of it at different rates. They played with the geometries of the vessels.
People wanted to do things like add alarms. One ingenious way that Plato came up with for building an alarm clock was you float a container that when it gets too high dumps little metal BBs out that makes a terrible racket and wakes everyone up. You want to start to be able to do all sorts of complicated things like ring the passing of the hour, ring the passing of the quarter hour.
It was actually in the Western Hemisphere, it was Monks who needed to know when to pray that developed some of the first mechanical clocks that were based on pendulum technologies. One of the nice things that comes out of Physics is if you pull back anything that’s hanging and just let it swing freely, in the absence of any external forces – in the absence of drag, in the absence of friction – Gravity alone will allow this pendulum to continually swing at the same rate.
The reality is that real pendulums are dealing with air and friction and so they do slow down. But by adding things like weights and winding clocks, you can get this pendulum system more and more precise. They were actually able to develop systems hundreds of years ago that could keep time as accurately only losing one minute per day which is pretty amazing. I think I have some watches that haven’t worked that accurately.
Fraser: Right so they had an accurate clock before a lot of people really needed them yet – except for them [the Monks]. They needed to pray on set times of the day.
But for most people… like it just blows my mind to think about just living your day. You get up, do work and eat dinner and go to bed and you don’t really think about what time it is because we spend so much of our day concerned about what time it is.
Try it sometime. Spend a whole day and don’t look at a clock. You’ll go crazy.
Pamela: Yes [Laughter] so we started off with these early mechanical clocks. Then we built pendulum clocks. The next real thing that we had to figure out how to overcome was “how do we keep time on a tilting swaying bumping up and down ship out at Sea”?
Fraser: Why is that important?
Pamela: Well when you look up you can see the passage of the night by how the Stars rotate through the Sky. If you know what day of the year it is you know which Stars should be where in the Sky at what time.
Since the location of the Stars is a function of where you are and what time it is, you can figure out your position on the Planet if you know the time and you measure the position of the Stars.
Fraser: Right and I guess when you’re out at Sea, the position of where you are on the Planet is VERY important. If you’re 50 kilometers off where you think you are then you might be crashing into a reef.
Pamela: And that would definitely be a bad thing. So by developing clocks that worked out at Sea… there is actually, the British government issued a longitude prize to the person who could build the first clock that worked out at Sea.
This award eventually went to John Harrison in 1759. By being able to know what time it is and measure positions of the Stars, they were able to very accurately build maps that noted sand banks, bad places to go and that allowed you to find the safe harbors.
This made sailing the Ocean much, much safer and allowed us to start having our global economy as ships sailed around the entire globe.
Fraser: Right before that a lot of sailors would only sail at day and they would only sail in view of land, knowing precisely where they were. They couldn’t just sort of go right out into the middle of the Ocean and know that they were going to be on course to where they were going.
Pamela: And there were exceptions who did make it. Leif Erickson made it to North America. There were exceptions but it wasn’t a safe way to conduct business. The clock allowed global transportation to become safer.
Fraser: Right. That’s great. Okay I remember seeing an A&E movie and the final outcome for the longitude prize was this clock that looked sorta like a big pocket watch, [Laugher] right?
Very accurate, totally invulnerable to the bouncing of the waves and was very accurate and the sailors just loved it. Alright so technology marches on. What comes after that?
Pamela: Well, since then we’ve been working to refine and shrink our ability to tell time. The two don’t always go hand-in-hand. Our most accurate clocks are still room-size devices.
In the passage of the building of clocks we had Chronometers, we had Quartz Oscillators – which we still use, and everyone still can buy Quartz watches down at the local wherever you go to do your shopping.
The real breakthrough came with Atomic clocks. Atomic clocks have allowed us to test Relativity. They’ve allowed us to very accurately map the gravitation of our Planet. They’ve allowed us to basically answer all the questions about where and when things have occurred because you need to tie both of them together when you’re making accurate measurements.
Fraser: So HOW does an Atomic clock work?
Pamela: First I’m going to tell you how it DOESN’T work. Lots and lots of people out there think that Atomic clocks are based off of radioactive decay and they’re not. The way the most accurate Atomic clocks that we use work is you take Cesium and you get it resonating in a particular transition.
The way Atoms work is there’s a bunch of different allowed energy levels for Electrons. An Electron bouncing between 2 allowed energy levels will give off a Photon of light that corresponds to that transition. So if you excite an Electron it will bounce to a higher energy level.
Then it will spontaneously decay because no one likes to stay excited for too long. It will spontaneously decay back down to a lower energy level. In different devices like lasers and masers you can set up a resonance so that they keep giving off light. They keep oscillating between these 2 allowed energy levels.
So with masers it happens to work out with this particular way that we set up Cesium that it is nine billion one hundred and ninety-two million, six hundred and thirty-one thousand seven hundred and seventy cycles of this up and down transitioning that leads to the passage of one second.
Fraser: So you could then I guess break up a second into 9 billion parts if you wanted to, right? Each one of these jumps is one 9 billionth of a second.
Pamela: This is defined by Quantum Mechanics. At the end of the day the passage of time is defined by the Physics of our Universe and that’s just kinda cool.
But, it started off as something where we counted heartbeats, where we counted the passage of the Sun. We looked at the passage of the seasons.
At the end of the day it all comes down to Quantum Mechanics and the transitions of energy. We even can measure the changes in the passage of time by looking at relative velocities and who’s accelerating and who isn’t.
Fraser: This is one of the trends that Science is always trying to do. They’re trying to turn all these measurements – meters, kilograms and so on – into properties of the Universe and time is a great one.
So now you can send instructions to Space Aliens and say “here’s how you measure time”. They could build an Atomic clock and measure time the exact same way.
We could sync up our clocks and everything would be fine. So this is great. We’ve got Atomic clocks. Time is perfect. Time is being measured down to the 9 billionth of a second but time is relative.
Pamela: And that’s one of the cool things about the passage of time is we don’t even perceive it as constant. We all know that when you’re having one of those really boring days, the minutes never seem to pass by and when you’re having a really exciting day it seems like everything’s gone in a single heartbeat.
Well time itself isn’t constant. One great example of this is the lifetime of a Muon. It is an unstable sub-atomic particle. If you just create a Muon in a laboratory and allow it to hang out on the counter, it’s only going to hang out for 2.2 times 10 to the negative 6th of a second which really isn’t that much time.
Now, when these Muons are created in our upper Atmosphere by Cosmic rays hitting the Atmosphere, they’re plowing through Space at .998 times the speed of light. If time wasn’t affected by velocity as they tried to make it through the Atmosphere, they’d only make it about 660 meters – .66 kilometers.
But because of the dilation of time, because time is measured by how you perceive light to be moving – and everyone perceives light to be moving at the exact same velocity – this Muon that is moving so fast appears to someone standing on the surface of the Earth to now suddenly live fifteen times longer basically – 34.8 times 10 to the negative 6 seconds. It’s now able to go 10.5 kilometers through our Atmosphere such that it can be detected.
Fraser: So, from the Muon’s point of view, it’s lasting a certain amount of time. But because it’s moving so fast from our point of view it lasts longer.
Pamela: And to the Muon its life is always the same brief moment, the same 2.2 times 10 to the negative 6 seconds. It’s the non-moving person that sees the life of the Muon stretch out, to see it age slower.
Fraser: Right, now we don’t want to completely redo our shows on Relativity but this is the discovery or the suggestion [Laughter] I guess and then the evidence from Einstein that it’s the speed of light that’s constant.
It’s time that then has to change; it has to give for people depending on where they are.
Pamela: And one of the hard parts about this is people are always like “but how do you know who’s moving?” There’s this idea of the Twin Paradox where you take 2 individuals – 2 twins – and you stick one of them on a rocket ship and send that one out into Space on a high speed adventure, reverse their direction and bring them back down to Earth.
After they’ve gone on this high speed adventure, when they come back to the Planet years and years later, they’ll have hardly aged compared to their twin who just stayed on the Planet.
If frame is changeable, if each of us carries our coordinate system around with us why is it that the one that did all the moving didn’t perceive himself as standing still while the one the planet Earth was the one that was perceived to be doing the moving? In this case is comes down to well “who accelerated”?
If you have 2 individuals that start off standing side-by-side one of them has to accelerate. There are all sorts of Physics that tie into doing work, to using energy that ties into doing the acceleration. It’s whoever does the acceleration that experiences the change in the passage of time.
The way to envision why they see time is different is you can imagine that you have 2 mirrors and you measure the passage of time by how long it takes for a pulse of light, a packet of light to bounce from one mirror down to the other.
If you’re holding these 2 mirrors one meter apart for you, the passage of time goes top mirror, bottom mirror, and top mirror. How long it takes the light to go 2 meters distance. Fine, not a big deal to figure out, you just measure. To the person who sees you moving, the light has to go a lot more than 2 meters because you’re moving sideways while the light is moving.
So it has to go not just the one meter from top to bottom but it also has to go the distance you’ve traveled. It’s actually cutting across some large diagonal as it goes from bottom mirror to top mirror and back again.
Fraser: Right, you would see it as going in a zigzag pattern. As the person is zipping past you, you would see that – if you could follow the trace of the light as it went up and down – you would see not just a line up and down, you would see a zigzag because the person is moving sideways.
Pamela: And in order for both of you to see the light take the same amount of time to travel between those 2 points, the person who sees it going the shorter distance has to perceive time moving at a slower rate. That’s the only way they can both see the light having the same velocity.
Fraser: From, I guess intellectual exercise to reality these wonderful Atomic clocks have demonstrated this.
Pamela: Yeah, we stick them on Spaceships and send them around the Planet. We stick them on airplanes and send them across the Oceans. We can actually measure this change in how much time is passed from one Atomic clock compared to another Atomic clock.
It’s one of the great things about having this technology that breaks down the seconds into over 9 billion intervals is we can start to accurately measure how time passes for one observer compared to another.
Fraser: So now we’re going to get into the questions my [Laughter] 4-year old would ask. Is time a fundamental part of the Universe or is it something that we as human beings perceive? Do you know what I mean?
Pamela: Yeah, I do and as near as we can tell, time is actually a fundamental aspect of the fabric of Space and well we say Space and Time. It is part of the probabilities of I have a lump of radioactive materials and in some passage of time half of that material will have decayed.
It’s part of what is the duration of time that a Muon is going to live? All of these different Quantum Mechanics effects have built in to them the passage of time. So it seems to be part of the actual everything that we experience.
Fraser: And so it is part of the Universe. This is one of those questions like ‘what’s outside the Universe”? Part of the problem is that time is part of the Universe. If you’re experiencing time then you’re in the Universe.
If you try to get outside of the Universe then you wouldn’t experience time and so it would be very hard to see things and so on. Now, I’ve heard that time could be reversible in many mathematical formulas, right?
Pamela: Right, this is where we talk about reversible reactions. This plays a large part in Thermal Dynamics but the actual passage of time; it’s always going in one direction. We’re stuck with that.
Fraser: Do we know why? According to all these formulas, couldn’t you just turn it around and it would still work? The time would work both ways but it seems to be …
Pamela: But the thing is not all reactions are reversible. You can’t un-decay a Plutonium Atom. You can’t un-decay a Muon Particle. All of these non-reversible reactions force our Universe to march in one direction.
Fraser: But, we talked about some of the important numbers in the Universe. We talked about how the force of Gravity is this; Alpha constant is that and time moving forward at the rate that it does. Is that a fundamental constant?
Pamela: I don’t think it’s even so much a constant as it’s just part of the framework that everything sits on top of. Time is part of the X Y Z and C of Space.
We look at the Universe in terms of 3-dimensions and in the passage of time which is at least in part dictated by the speed of light. So you can say that the speed of light is a constant but the direction of time is not so much a constant as it just is.
Fraser: Right and for some things like Photons, they don’t experience time at all.
Pamela: Right and that’s just one of those weird philosophical questions to deal with.
Fraser: Well, I think we’ve sort of broken our brains [Laughter] and I think we’ve run out of time. I had so many puns planned but I had to really control it. Thanks Pamela and it’s great to have you back. We’ll talk again.