Every year, we travel through time.
In autumn, we travel forward in time by one hour, and in the spring, we travel back in time by one hour. Every four years we gain 24 hours in February, and every three or so years an extra second is added to a minute.
Time appears, and then – *poof* – disappears again. But, wait a minute (whatever a minute is). Time cannot spring in and out of existence, can it? Time loans the universe a second, an hour, or possibly a day until the deadline whereby the universe must pay time back, right? But where has time been all this time?
Time is hard to define. We measure time in years (the time it takes Earth to orbit the Sun), days (one rotation of the Earth) and lunar months (the time it takes the moon to wax and wane). Time – hours, minutes, seconds, milliseconds, nanoseconds – are all manmade constructs. We made them up.
And time is a concept that doesn’t necessarily apply to the universe.
Time has always been inextricably linked with the Sun. The ancient Egyptians and Babylonians used sundials that roughly divided daylight into 12 equal segments. 60-minute hours and 60-second minutes are the product of the ancient Mesopotamian sexigesimal (base 60) numbering system. The French attempted to use the decimal system (base 10 rather than 12) for time-keeping, but that never caught on. The Greeks improved the sundial by marking gradations on sundials to indicate the divisions of time during the day.
And then the Scientific Revolution (1550-1700) came along. According to Vincenzo Viviani, Galileo’s first biographer, 20-year-old Galileo got bored during prayers at the Cathedral of Pisa in 1583. As he daydreamed, something caught his eye: a swinging altar lamp. Curiosity got the better of him and he swung the lamp to find out how long it took to swing back and forth. He used his pulse to time large and small swings.
Galileo discovered something remarkable that nobody else had: the period of each swing was exactly the same. Then, the pendulum clock was born – the most accurate way of timekeeping at the time. After that, other clocks had developed such as the H4 (1759), an accurate chronometer for the determination of longitude, and the Swiss (1944). It was only a matter of time until wristwatches became a fashion statement; everyone had to tell the time.
In 1928, all other clocks were rendered redundant as a new clock was in town, one with no moving parts: the quartz crystal clock. Then, in 1955 physicists Louis Essen and Jack Parry developed, at the National Physical Laboratory, a clock unlike any other: Caesium I, the world first atomic clock. The atomic clock, now on display in the Science Museum, outperformed existing pendulum and quartz clocks. The clock kept time by counting the vibration of caesium atoms, it was so accurate that it would only gain or lose one second every 300 years.
And like that, time changed. No longer was a second defined as 1/86,400th (24 hours x 60 minutes x 60 seconds) of a day. In 1967, the 13th General Conference of Weights and Measured defined a second as, “9,192,631,770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom”.
The official atomic second is slightly shorter than Coordinated Universal Time (UTC). So every three years, a “leap second” is normally added to the year to make up the difference (before the first leap second was added in 1972, UTC was 10 seconds behind atomic time).
Since then, scientists have honed in on the model with incredible precision. The latest modification to the strontium lattice atomic clock is accurate to one second in some 15 billion years – that’s a bit longer than our universe has existed.
Now that the history lesson is over, what is time? To understand time you will have to do one little thing: give up all your intuitions about how time works.
If you flick through a book, you’ll notice that the book doesn’t have infinite pages. It has a beginning, middle and end. Humans possess a thing called a narrative bias; we make sense of the world around us through stories.
All stories have a beginning, middle and end. To process the overwhelming information around us, our brain compresses information in narratively, dropping facts that don’t fit the story. We are the protagonist of our narratives, of course; complex characters with fears, hopes and mild insanities.
Everything beyond our skin is a prop, a two-dimensional character made to fit snugly into our story (“that hipster-looking dude in Pret with a MacBook”, “that phone-obsessed girl on the Tube”, “that saint who helped me carry my shopping bags”, etc.)
This isn’t something you do on purpose; narratives are so important to your survival that it’s the very last thing you give up before you’re just a sack of skin. Without a narrative you wouldn’t have a conscious and without a conscious, there would be nothing but white noise ringing in your ears.
If you’re confronted with the thought that the beginning and the end don’t exist – they’re just a manmade invention used to perceive the space around us – your brain will fall into a state of disarray (cognitive dissonance). The cure is to either accept the new belief and ignore/disregard the old belief, or vice versa.
So it’s high time you threw your thoughts on time out of the window.
Time is inextricably related to space, so to understand time you must understand spacetime. Why? Because the speed of light in a vacuum is independent of the motions of all observers (particles count as observers). This is Albert Einstein’s theory of special relativity (presented in 1905).
To understand special relativity you must first familiarise yourself with the speed equation: speed (s) = distance (d)/time (t).
The speed of light is the same for all observers, and this is known as “c” for constant (as in c in E=mc2, where is E is energy and M is the mass of an object). So the speed of light for any observer is constant regardless of the speed the observer is going. For the speed of light to remain constant, something in the speed equation has to give way, that something is time. It turns out time slows down when you travel faster and faster, nearing the speed of light. What could be 10 minutes for the object could be 20 minutes for the observer. So what’s the future for the fast-moving object could be the past for the slow-moving observer, and vice versa.
This relativistic effect is called time dilation. So the faster an object travels, the slower the time passes. That’s why moving clocks are slower than stationary ones. For example, observer A is on a slow-moving train called “train A”. She measures time using her wristwatch. Observer B, passing train A at high speed, has the exact copy of A’s wristwatch. Yet, from the point of view of A, B’s wristwatch runs more slowly than her own. And this is why Einstein said “time is an illusion”.
In addition, fast-moving objects cover a greater distance during a fixed time interval than slower-moving objects. So back to the speed equation:
Special relativity also applies to the concept of simultaneity. If observer A judges two events to be simultaneous, observer B, moving relative to A, might not agree. Discrepancies in the order of events can occur as a result of the invariance of the speed of light. This YouTube video explains this concept visually.
According to Merrian-Webster’s definition, spacetime is “a system of one temporal and three spatial coordinates by which any physical object or event can be located”. So if spacetime isn’t constant, then what is? The spacetime interval. The spacetime interval is the spacetime difference between two separate events. The equation:
(Spacetime interval)2 = (the distance between two events)2 – (speed of light)2 x (the time between two events)2
Since the equation involves subtraction, the spacetime interval can be positive, negative or zero. When the spacetime interval is positive, two events are said to be seperated by “space-like interval”. In a space-like interval, nothing can get from one event to another, and there are always observers who disagree about which of the two events happened first. In a YouTube video, physicist Professor Brian Cox of Manchester University gives an example: “The Sun is eight light-minutes away – it takes light eight minutes to get from the Sun to the Earth. If the Sun exploded now, it will take me eight minutes to notice. So let’s say the Sun explodes, and from my point of view, four minutes elapsed. I still don’t know it’s exploded, so there’s nothing the Sun can do to cause something to happen on the Earth – it’s completely disconnected from it for eight minutes”.
In a space-like interval, the order of events can be swapped. Person X moving at near-light speeds past the Solar System could see person Y talking about the Sun exploding eight minutes after the Sun has exploded, and person Z could see person Y talking the Sun exploding eight minutes before the Sun has exploded.
When the spacetime interval is negative or zero, two events are said to be separated by “time-like interval” and “light-like interval”, respectively. In both cases, signals or things can get from one event to the other, and everyone agrees on its sequence. In a YouTube video, Professor Cox gives an example: “If I throw a ball at you and I knock you off the wall. The ball caused you to fall off the wall. There’s no way in Einstein’s theory that could cause you to swap the order of events around. . . you couldn’t fall off the wall and then I throw the ball”.
So although we can’t agree about past, present, future, time or distance, we all agree about causality. This might seem weird because we think time is responsible for causality, but it’s actually the other way around. Agreeing about temporal anything is because of causality. You could say causality is the only thing that’s “real”. So what does causality have to do with spacetime? Everything.
Shortly after Einstein’s theory of relativity was proposed, Hermann Minkowski, Einstein’s former mathematics professor, noticed the spacetime interval resembled a weird version of a “distance” formula called the “non-Euclidean geometry”. So he proposed something pretty epic: maybe reality – the universe we’re living in – isn’t a 3D space that evolves in time, maybe it’s a 4D “non-Euclidean” space that’s just there. Ie, what if spacetime has just always existed? No evolution. No time. Just there.
This 4D space would be spacetime, where its points correspond to all events, ever:
Image: YouTube screengrab
To support Minkowski’s proposal, a 2014 paper published in journal Physics Letters B, says the universe had no beginning and will have no end (a “block universe”), according to a new model that applies quantum correction terms to complement Einstein’s theory general relativity. The model also helps solve the problem of dark matter and dark energy.
What some cosmologists now believe is that rather than matter collapsing, causing a “Big Bang”, the matter bounced (“the Big Bounce”). Ie, they believe the universe has energy levels and goes through a cycle of collapses and bounces.
“[The Big Bounce] is actually in doubt because we now know that our universe is not going to re-collapse. It is actually going to expand forever because of dark energy. 70 per cent of our universe is made of an unknown type of force or energy that we call “dark energy”, whose main impact on the universe is to make the expansion of the universe accelerate with time – so not only is the universe growing with time, it’s growing at an ever accelerating speed,” says astrophysicist Roberto Trotta of Imperial College London to the science radio talk show The Naked Scientists, based at the University of Cambridge.
“The Big Crunch will end the universe and the end will be a state of darkness where all the matter will have been sucked into black holes . . . but it’s about 200bn years in the future so we’ve got nothing to worry about,” he adds.
Our perception of time and space is arbitrary and inherently meaningless. It’s like the xy-grid we used in school. It’s useful for talking about the information dotted on it, but on its own is meaningless – it’s just there. The information dotted on the grid would always be, regardless of the xy-grid.
Are you real? Sort of. If you are the sequence of events at which you were present then you are a non-Euclidian geometric object in 4D space. Ie, you are your own spacetime interval – you are a line segment that joins the events from your birth to your death. Do you move around that line segment? No. You are it. There is no motion in spacetime – it’s tenseless. The manmade concept of past/future/present tense is ultimately meaningless (remember the xy-grid analogy). So your future isn’t predetermined, it already exists.
An analogy to understand spacetime: imagine we’re all reading The Lord of the Rings. We agree on the events of the story, but we don’t agree on where they happened on the page, how many pages there are between events, and the order of some of those events, and yet, we’re all reading the same book. But only there are no pages…no numbers…and there is no book. All of it is a figment of our imagination in order to perceive whatever it is.
The illusion of space and time is explained visually in a YouTube video by research scientist Gabriel Perez-Giz of New York University.
If we throw Einstein’s theory of general relativity (published in 1915) into the mix, the idea of space and time becomes even more complex. There could be multiple spacetimes (universes called “multiverse”) with different dimensions, making it hard to ascertain which one this – the one we’re living in – is.
“[Some researchers] postulate the existence of parallel universes, so our universe is made of [a 4D universe], but what if there were additional dimensions – that we cannot actually penetrate ourselves,” Trotta says on The Naked Scientists radio show.
To conclude, why do we perceive spacetime as, well, space and time? No one knows. Not yet, at least.