A spaceship in a science-fiction novel returns to Earth after traveling at near light speed. The crew lands only to find that, while they have experienced a few short months in space, their loved ones have aged decades. This well-worn plot device — based on Albert Einstein’s Theory of Special Relativity — has been around for 100 years yet still manages to perplex us.
Einstein showed that the speed of light is a constant. Under Special Relativity, the famous physicist also defined time as that thing you measure using a clock. The freaky time dilation that happens as you approach the speed of light is a consequence of these two facts. Defining time in this way also inextricably connects it to space. Clocks can count seconds using a turning gear, a photon bouncing between two mirrors, or a vibrating atom. But there is no way to measure time without having an object move through space. Interestingly, this link between time and space is present in just about every known human language, deeply embedded in the way we talk about the passage of days, months, and years. Which means that in a way Einstein stumbled upon and schematized something we all inherently know — that time remains subtly beyond our grasp.
Before we go into that, we need to talk about how Special Relativity works. Physics textbooks often introduce the theory using a simple time-measuring device: a photon bouncing between two mirrors. The mirrors are set up in just the right way so that it takes the photon one second to travel between them. You imagine building a clock like this and leaving it on the Earth; while simultaneously placing a second clock on a really fast spaceship. At low speeds, the spaceship-riding clock won’t show much of a difference compared to the Earth-bound clock. But as the ship gets closer and closer to the speed of light, the clock’s photon will travel through an appreciable extra bit of distance to reach the mirror on the other side, not just up and down but also in the direction of travel (check out the image for more explanation).
Measuring a second on the spaceship now takes longer compared to a stationary clock on Earth — that’s the ‘relative’ part of Relativity. Time, according to the spaceship clock, has slowed down. A few years may pass for space-based crew, but those back at home might experience hundreds. This isn’t just theoretical mumbo-jumbo. The 1971 Hafele-Keating experiment placed extremely precise atomic clocks — which measure time using the vibrations of cesium atoms — on airplanes and flew them in opposite directions on the Earth. When the clocks were returned to the same place and compared, their readings were off from one another in exactly the way that Special Relatively predicted.
Almost a century after Einstein’s revolution in physics, linguist George Lakoff and philosopher Mark Johnson published their groundbreaking book Metaphors We Live By, which systematically shows that metaphor underlies a large part of everyday language. Metaphors are inescapable and all-pervasive, interwoven so fully into our thought processes that we barely notice them. It’s only when you start to pay attention that you realize books don’t literarily break ground; shovels do. And its threads that are interwoven in cloth, not abstract concepts into other abstract concepts.
Some of our most basic temporal expressions in English are rooted in spatial metaphors. Consider the correspondence between the sentences “Traveling from New York to L.A.” and “Working from Monday to Friday.” We can similarly discuss riding before an army in order to arrive before a battle begins. And you might as easily ask your friend to meet you around the campfire as around lunchtime. But lunchtime doesn’t have any physical sides, so how can anything be ‘around’ it? Linguistically speaking, we are taking our everyday experiences with the placement of objects in space and analogizing them to the order of events in time. This is so ingrained that it can be hard to remember that time doesn’t literally pass in the same way as a train, nor can it go forward or backwards any more than it can go diagonal.
Languages often place the future in front of us: We look forward to an event and are relieved when a stressful time is behind us. But occasionally, it can be below us. We might be excited about an upcoming holiday or remember the stories that have passed down through the generations. Chinese has similar expressions, calling last month shànyuè (up.month) and next month xiàyuè (down.month). A rarer case is found the indigenous language Aymara, spoken in Peru, where the past is rendered as nayra timpu (‘the time before my eyes’) and tomorrow as q’ipi uru (‘the day at my back’). Some linguistic constructions conceive of the immediate past and future as nearby, with times earlier and later as farther away. A French person calls a great-grandfather their arrière-grand-père (the person behind their grandfather) and a great-grandson their arrière-petit-fils (the person behind their grandson).
Einstein’s native German has events happening an diesem Tag (on this day) or in dem Moment (at the moment), two phrases that use obvious spatial metaphors. It’s possible that while creating Relativity, the famous physicist was subtly influenced by his conversational experiences. Using the language of science, he placed central importance on the link we always seem to make between time and space; showing that you can’t measure one without using the other.
“Time and space are intimately interconnected,” says physicist Thomas O’Brian, chief of the National Institute of Standards and Technology’s time and frequency division. “In our normal human lives we experience time and space as different things. But this is an illusion based on the combination of living our lives in an immediate environment where the time/space connections are not so apparent and our limited human perceptions.”
Our eyes see light and our ears hear sounds. Yet our brains do not, to the best of our understanding, measure perfect time. Perhaps one day neuroscience will clarify how the architecture of our brains senses it, but for now “the neural bases of time perception remain shrouded in mystery.” Both physics and linguistics provide ways of grasping at time — using the moving parts of a clock and spatial metaphor, respectively. But as to what time really is, we cannot yet say.
This story was originally published on the defunct website now.space on 5/23/2016