We started our exploration of time by discussing the deep connection between astronomy and our earliest thoughts on time

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Allen Carson
5 years ago
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time and finished with this hypnotic animation, which

time and space spinning, orbiting, and tagging along as

1 We started our exploration of time by discussing the deep connection between astronomy and our earliest thoughts on time and finished with this hypnotic animation, which nicely emphasizes the motion of the Earth through both time and space spinning, orbiting, and tagging along as the Solar System moves through the Milky Way and the wider universe. 1

We ll return to this connection between space and time later when we explore general relativity for now, I would

For many scientists, philosophers, and artists our constant motion into ever-new locations in the universe made

Many bold new ideas emerged in the 300 years following Galileo s work, including modern views on geology and

2 We ll return to this connection between space and time later when we explore general relativity for now, I would note what this change in our view of the Earth s motion did to our view of time. For many scientists, philosophers, and artists our constant motion into ever-new locations in the universe made it clear that old ideas of Earth s constancy and the soothing cycle of time were false. Many bold new ideas emerged in the 300 years following Galileo s work, including modern views on geology and biology, but also the general conceit and wide-spread acceptance of progressivism the idea that we can shape the future in a positive way and avoid repeating events of the past. 2

Far more than ever in human history, we had adopted the directional arrow as our dominant metaphor for time a historical development that persists to this day.

The chandelier in the Cathedral of Pisa that set Galileo thinking.

3 Far more than ever in human history, we had adopted the directional arrow as our dominant metaphor for time a historical development that persists to this day. While retro chic is always fun*, it s what s new that really drives our modern cultures! *Okay always fun at Halloween parties! The chandelier in the Cathedral of Pisa that set Galileo thinking. However, while this move towards modernism and its focus on the novelty of moments in time was developing, physics was quietly building a strong case against it. Our friend Galileo arguably started the assault, with observations of a swinging chandelier while he was a college student in Italy in the 1580s. 3

His work would eventually lead to the development of the pendulum clock, a truly revolutionary device at the time and one that we ll return to very soon.

4 His work would eventually lead to the development of the pendulum clock, a truly revolutionary device at the time and one that we ll return to very soon. But much more significantly, Galileo s analysis of the pendulum s movement along with many other experiments he conducted related to motion arguably launched modern physics itself. The basic pendulum is the first well studied harmonic oscillator, a key concept in modern atomic physics! Within less than a century of his death, Galileo s legacy had been picked up by giants such as Pascal, LaPlace, and Newton and the fundamental laws of motion still taught in the first quarter of every physics student s coursework were solidly in place. 4

By the late 1800 s, these laws of motion were being extended to not only deep space and the

atoms and molecules that made up matter around us.

around us was and still is staggering, and the explosion of technology they produced has

But upon closer inspection, these laws of motion had a remarkable feature about them they are

5 By the late 1800 s, these laws of motion were being extended to not only deep space and the motions of the planets, but also to the smallest particles physicists imagined at the time the atoms and molecules that made up matter around us. The success of these physical laws in predicting and explaining the behavior of the world around us was and still is staggering, and the explosion of technology they produced has changed almost every aspect of life. But upon closer inspection, these laws of motion had a remarkable feature about them they are completely reversible in time. In other words, the standard laws of motion that describe the behavior of the physical world around us have no directional arrow in time. Forward, backwards it s all the same to physics. 5

This apparent timelessness in the behavior of the universe is encapsulated beautifully in the idea of Poincaré s Return, from Henri Poincaré s work in the late 1800 s on the flow of particles in a

6 This apparent timelessness in the behavior of the universe is encapsulated beautifully in the idea of Poincaré s Return, from Henri Poincaré s work in the late 1800 s on the flow of particles in a closed system. No matter what the original state or size of the system, nor how many particles are involved, the particles are guaranteed to eventually return arbitrarily close to their initial state the past, in other words, will always return! 6

And I ll note this problem has not gone away since Poincaré s time in fact, as we ll see when we discuss cosmology, it s a central theme in modern views of the long-term past and future of our entire

7 And I ll note this problem has not gone away since Poincaré s time in fact, as we ll see when we discuss cosmology, it s a central theme in modern views of the long-term past and future of our entire universe. But does Poincaré s Return really mean that all of time is truly cyclic, and that the arrow of time is truly an illusion? This crisis in our understanding has spurred scientists to explore time in ever-increasing levels of detail, and across scales that run from the age of the universe (over 13 billion years) to the smallest theoretically possible length of measurable time (the socalled Planck time just over seconds!). To understand fully the results of this work and the limits of our knowledge, we need to understand how such measurements are made. In other words, how do we answer that age-old question: What time is it? 7

These days, you d most likely answer that by looking at your phone but your phone doesn t keep track of the time itself.

Those machines broadcast the time continuously, and help keep our complex world in sync but even they don t actually measure the time either.

measure the vibrations of fundamental particles of matter, and astronomical clocks and satellites that carefully track the rotation of the

8 These days, you d most likely answer that by looking at your phone but your phone doesn t keep track of the time itself. Like any other networkconnected device, it uses a Network Time Protocol to communicate with a time server somewhere on the internet. Those machines broadcast the time continuously, and help keep our complex world in sync but even they don t actually measure the time either. Time servers are kept accurate by synchronizing them with our most robust measurements of the repeating cycles of time atomic clocks that measure the vibrations of fundamental particles of matter, and astronomical clocks and satellites that carefully track the rotation of the Earth. These two widely disparate types of clocks beautifully tie together the most advanced nuclear physics and the simple stellar observations of our time-keeping ancestors. 8

9 Time Server Time Lord We have already discussed the very earliest of true timemeasuring instruments, the sundial (you can even visit two of them right here on campus!). They are simple and effective but because they rely on sunlight, they re also unreliable because of weather, and basically useless at night! 9

10 (though they can even be made to work on other planets!) A somewhat more reliable timekeeping device was the water clock generally a simple jar with a hole in it that would allow water to flow out at a steady rate. As the water flowed out, markings on the jar indicated how much time had elapsed. 10

Simple as they were, these devices were much more reliable than sundials they could be used

in use throughout the ancient Mediterranean, Africa, and Asia.

the container emptied, for example, or as the temperature of the water rose or fell could

11 Simple as they were, these devices were much more reliable than sundials they could be used indoors, at night, and in poor weather and by at least 350 BC, such water clocks were commonly in use throughout the ancient Mediterranean, Africa, and Asia. However, water clocks were far less accurate than sundials changes in the rate of water flow as the container emptied, for example, or as the temperature of the water rose or fell could significantly alter the length of time it took for the clock to fill or empty, and required clock builders to get creative in their designs. 11

12 Some of these clocks became remarkably complex, with geared faces, moving statues, elaborate art work, and even noisemakers and bells to chime the hours our first alarm clocks. Reproduction of the Jayrun Water Clock in the Nationaal Beiaard- en Natuurmuseum Asten (Netherlands) 12

Water clocks like these were widely used for over a thousand years, but beginning in the late 1600 s they were gradually replaced by the newly-invented pendulum clock.

version. Schematic of Huygen s first pendulum clock. These devices were much more accurate and reliable than water clocks keeping time to within 10 seconds per day and a lot less wet!

13 Water clocks like these were widely used for over a thousand years, but beginning in the late 1600 s they were gradually replaced by the newly-invented pendulum clock. Galileo s exploration of the physics of a pendulum s motion had led him to suggest it could be used as a timekeeping device, and in 1656 Christiaan Huygens produced the first successful version. Schematic of Huygen s first pendulum clock. These devices were much more accurate and reliable than water clocks keeping time to within 10 seconds per day and a lot less wet! They could easily be made small, so that reliable timekeeping devices began to appear for the first time in the homes of the growing middle class of merchants and skilled tradesmen. 18 th -century pendulum clock for the home. 13

Right around the same time, in 1675, Huygens collaborated with the

It used the tension of a coiled piece of metal to control the rotation of

14 Right around the same time, in 1675, Huygens collaborated with the English physicist Robert Hooke to produce the first springdriven clock. It used the tension of a coiled piece of metal to control the rotation of a flywheel, and converted that rotation into the movement of hands on a clock face. Further, it could be made small enough to hold in one hand the first pocket watch! A 17 th century pocket watch manufactured by Thomas Tompion. 14

th and 19 th century they revolutionized economies,

15 It s hard to overstate the impact that readily available, accurate watches and clocks had on human cultures of the 18 th and 19 th century they revolutionized economies, transformed militaries, and forever changed societies all over the world. 15

We saw last time how the development of accurate clocks in the 18 th and 19 th centuries transformed human cultures over the world.

We saw last time how the development of accurate clocks in the 18 th and 19 th centuries transformed human cultures over the world. They also allowed for the precise physical measurements of time needed