Time influences our social lives, our crops, and our circadian rhythms, among many other important life dimensions.
Timekeeping is so fundamental to us that our brains have their own time cells, which are responsible for inserting a time dimension into our memories.
The Hubble telescope’s image of the Crab Nebula, a supernova remnant and pulsar wind nebula in the constellation of Taurus. Pulsars provide a means of time measurement.
Along with all the fantastic scientific advancements and accumulated knowledge of humanity, our ways of measuring time have evolved over the centuries. Today, from atomic clocks to fast-rotating neutron stars, human ability to measure time has achieved incredible precision. If we were we to follow “true” time, we might end up abandoning our everyday time-tracking based on the Earth’s 24-hour rotation cycle. With further advances in our knowledge of the nature of time, we may soon add another chapter to our history of time measurement.
Until very recently, time was scientifically defined based on the Earth’s rotation.
The practice began with ancient civilizations, which would often produce their own calendars from observations of cycles of the natural world, using patterns of changes from day to night and season to season.
For a long time, we’ve understood the day based on the average time it takes for the Sun to reach its maximum height in the sky – which is a cycle of approximately 24 hours from one midday to the next. The most fundamental unit of time, the second, was mathematically derived by dividing down this approximation or, in other words, by reducing it to its lowest fraction. Given the irregularity of our planet’s rotation, though, in 1967 experts decided it was time for a change.
The word ‘second’ can be traced back to the sexagesimal (base 60) system used by the Babylonians in their astronomical calculations (Scientific American).
That year, at the 13th General Conference of the International Committee for Weights and Measures, researchers from around the world gathered to answer the question of how long a second actually is.
Global standardization of a precise measure for time was becoming increasingly necessary with technological advances in transportation, computerization, communication, and engineering. From that point forward, it was decided that the second should be based on a fixed numerical value instead of an astronomical measure. This fixed numerical value was based on the atomic properties of the element Cesium. According to a 1969 paper on the resolutions made during the Conference, by a scientist then working at the National Physical Laboratory in the UK, the main goal was increased precision.
As the paper stated, “An impressive step has been taken by discontinuing the use of astronomical events to define time in physics, for the change means that all physical laws in which time enters as a variable, and which have been tested experimentally by reference to periodic astronomical events, are assumed to be true if tested against periodic atomic events. The change in the definition of the SI unit of time represents a further step towards relating our basic units of measurement to atomic characteristics.”
As Dr. John Kitching explains, “An atom consists of negatively charged electrons orbiting a positively charged nucleus at a consistent frequency. The laws of quantum mechanics keep these electrons in place, but if you expose an atom to an electromagnetic field, such as light or radio waves, you can slightly disturb an electron’s orientation. And if you briefly tweak an electron at just the right frequency, you can create a vibration that resembles a ticking pendulum. Unlike regular pendulums that quickly lose energy, electrons can tick for centuries”.
The electron oscillations between energy levels in the winning element, Cesium-133, are so remarkably fast and measurable that the results formally defined one second as exactly 9,192,631,770 “ticks” of this Cesium isotope. The isotope contains 78 neutrons and 55 protons in its nucleus and gives unparalleled precision for time measurement in atomic clocks. Furthermore, as every atom of an elemental isotope is identical, scientists can independently produce perfectly consistent clocks, if they use the same Cesium-133 isotope and the same electromagnetic wave.
“Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) due to different numbers of neutrons in their nuclei. While all isotopes of a given element have almost the same chemical properties, they have different atomic masses and physical properties” (Wikipedia)
There are hundreds of atomic clocks in the world.
Due to variables like local gravitational effects, sometimes metrologists, who study measurement, need to iron out imperfections. Otherwise, our clocks would no longer register changes in light between day and night – and eventually maybe we’d watch sunsets early in the morning, or look up into the night sky while having a quick lunch between work shifts. To avoid this, we’ve created institutions, and the world’s timekeepers every so often add leap seconds to these atomic clocks, so we can maintain consistency in time and place.
Today, with many industries and technologies needing to measure time down to the nanosecond, we can find atomic clocks everywhere. They are in radio signal transmitters, GPS satellites, energy distribution, financial trading, and even the navigation technology in autonomous vehicles. But that does not mean scientists are done with improving our timekeeping methods.
A pulsar is a fast-rotating neutron star in which the symmetry axis of its magnetic field deviates from the axis of rotation, emitting synchrotron radiation along the dipole axis. Image: Elmi1966
Metrologists now contemplate possible new – and even more accurate – ways of defining the second.
A hertz is the measure of one event or cycle per second, and as Dr. Anne Curtis explains, “Optical frequencies oscillate much, much faster in the hundreds of terahertz. Hundreds of trillions of oscillations per second.” A terahertz equals a phenomenally large 1012 hertz, and the potential increase in precision from using light and optical technology makes experts believe a new and major change in the definition of the second is coming within the next decade or so.
Researchers like Dr. Liu Min, on another hand, make even bolder predictions. Based on an understanding of Albert Einstein’s theory of general relativity, he and his co-authors believe that as humans begin to colonize outer space, we should not restrict ourselves with a perspective of absolute timekeeping, or even hold the Earth as standard for time.
Increased accuracy in time measurement also provides greater precision for measuring distances in space.
As Einstein observed, space and time are connected as “spacetime”, a fluid-like fabric that each of us and everything around us occupies. Scientists have long been deeply invested in the task of measuring space, especially following astronomer Edwin Hubble’s 1929 observation that space is continually expanding. In 2012, using observations of pulsars from the Parkes Pulsar Timing Array (PPTA) project, Dr. George Hoggs and co-authors developed “the first pulsar-based timescale that has a precision comparable to the uncertainties in international atomic timescales”.
Pulsar schematic: neutron star in the middle, magnetic field curve lines, and emission zones represented by the protruding cones. Image: Roy Smits
Pulsars (from “pulsating radio sources”) are “highly magnetized rotating neutron stars that emit beams of electromagnetic radiation out of their magnetic poles.” Like a lighthouse, whose light can only be seen when pointed at the observer, the radiation from a pulsar can only be observed when a beam of emission is pointing toward the Earth.
Neutron stars are born when stars much more massive than our Sun explode in supernovas.
They are the dense, collapsed core left behind after the star’s outer layers are propelled into space. These stars are tiny, but extremely dense, and the closest one is still very far away from us: the neutron star named Calvera is 617 light-years away from Earth. The fastest spaceship humans have ever built would take about 11 million years to get there.
Victoria Kaspi Public Lecture: The Cosmic Gift of Neutron Stars
As Dr. Victoria Kaspi explains, pulsars are excellent clocks, comparable to the world’s best atomic clocks. “Having a pulsar, having this little cosmic superb clock in the sky, especially when you find it orbiting another star, it can allow you to make really sensitive measurements of dynamics, of how the star is moving. (…) When you have two starts in orbit around each other and you’re observing with a telescope, (and) when you have one of those stars being a pulsar, (…) by measuring that pulse period as the star moves away and toward you in this binary orbit, you can study the motion with exquisite precision”.
The time applications of pulsars are abundant.
Salvador Dali’s The Persistence of a Memory. “The orange clock at the bottom left of the painting is covered in ants. Dalí often used ants in his paintings as a symbol of decay”. Image: Wikipedia
As Dr. Cheng-Shi Zhao argues, pulsars “can be applied to many research fields, such as the establishment of the pulsar time standard, the detection of gravitational wave, the spacecraft navigation by using X-ray pulsars, [and] the pulsar time standard [could be used] to calibrate the atomic clock”.
So, why is timekeeping so fundamental and central to our lives?
Maybe it’s because, as societies grew and evolved, synchronization of our planet-wide exchange of information in time became increasingly important. Or maybe it’s because time is the one dimension around which we all base our comprehension of life, as if life is a voyage in time. As Albert Einstein famously stated, “Time and space are modes in which we think and not conditions in which we live”. Time, in this sense, might be a construct of the mind and not of physics.
But what if scientists proved that time is no more than some sort of illusion generated by the limitations of the way we perceive the universe?
As Andrew Zimmerman Jones shrewdly points out, “no individual frame of (a) movie changes or contains the passage of time, but it’s a property that comes out of how the pieces are strung together. The movement is real, yet also an illusion. Could the physics of time somehow be a similar illusion?”
