Back to the Future
To hear them talk, you would think that time runs backward for the Aymara people, a South American Indian tribe living in Andean regions of Bolivia, Peru and Chile. In both words and gesture, they convey a sense that the past lies before them and the future behind. The same word in the Aymara language means both "front" and "past," while the word for "back" also connotes "future." The arrow of time has not been reversed, of course; the Aymara have merely reversed the spatial metaphors used in virtually every language to express the direction of time. It turns out the Aymara attach a great deal of importance to what they actually see, and this is reflected in their language. Since they can see the past rather than the future, their spatial metaphors for time place the past in their line of sight in front of them and the future behind them, where they can't see.
The Aymara's sense of moving backward into the future is startling only because the use of spatial references is so embedded in our understanding of time that we don't realize we are thinking metaphorically. In reality, of course, time does not move either forward or backward, at least not in the sense of an object moving through physical space. Time has no spatial direction or dimension whatsoever. If you strip away the metaphors, you are left with the thing itself, which remains an enigma.
Time can be measured with great precision, but it's not at all clear what is being measured. There would be no problem if units of time were regarded purely as an abstract measure, like ounces or kilometers. But they are not. We all have the palpable sense of time passing, even if we can't say what has passed. This experience of duration can be measured from one moment to the next -- and here's where the trouble starts. A single moment by definition has no duration, and once we start the clock running, our starting point in the present immediately vanishes into the past. We are beguiled by a sensation of passing time, but all we actually experience is a succession of vanishing moments that have no duration in themselves.
Our sense of time moving forward (or backward, as the case may be) depends entirely on our recollection of prior moments. Since the past no longer exists, we have only our memory to go on. Once memory goes, time ceases to have any duration. I experienced this firsthand with my mother as she descended deeper into dementia, and her life became a succession of vanishing moments with no anchorage in memory. While taking her around on errands, I soon realized she had no ability to gauge whether we had been driving for five minutes or five hours, because she couldn't remember when we got in the car.
My mother had virtually no short-term memory left in the last years of her life, and long stretches of her past were beyond her recall. Yet she still had some concept of time in the end. In a pattern common among dementia victims, her memory unwound in reverse. She still had childhood memories from 70 or 80 years ago, and she knew that she was now an old woman. Like Kurt Vonnegut's Billy Pilgrim in Slaughterhouse Five, she had come unstuck in time.
The medical term for the inability to form new memories is anterograde amnesia. Perhaps the best-documented case involves a patient identified as H.M., an otherwise intelligent adult male whose memory stopped following the partial removal of the hippocampus in his brain during an operation in the 1950s to control severe epileptic seizures. For H.M., Harry Truman would always be president, and the old man's face that stared back at him in the mirror would always come as a shock. Time had effectively reached a standstill.
Clive Wearing, a prominent British musician, suffered even more devastating memory loss when a herpes simplex virus attacked the hippocampus, as well as the frontal and temporal lobes of his brain. Wearing has only the sketchiest memories of his life before he fell ill in 1985 and nothing since. His life consists almost entirely of what can be held in his awareness from moment to moment, which he experiences as repeated instances of "waking up" throughout the day -- except that for him there is no repetition.
Does time cease to exist merely because we lack the cognitive ability to perceive it? The question bears superficial resemblance to the classic query about whether a tree falling in a forest makes a sound if there is no one to hear. In the case of a falling tree, however, acoustic equipment can still detect sound waves, whether or not there is anyone to hear. By contrast, no scientific equipment can detect time waves or particles in the absence of our subjective perception of time. We like to think a clock measures time, but it really measures nothing tangibly beyond its own ticking.
"Everything flows and nothing abides," wrote Heraclitus more than 2,500 years ago. "You cannot step twice into the same river, for other waters and yet others go flowing on." It is, of course, very easy to cast yourself headlong into the onrushing stream and imagine you are being carried along by powerful currents of time. It is far better to squat like a rock in the middle of the flow and allow life to swirl all around you. This is the secret of eternity.
Rafael E. Núñez and Eve Sweetser, "With the Future Behind Them: Convergent Evidence From Aymara Language and Gesture in the Crosslinguistic Comparison of Spatial Construals of Time," Cognitive Science, 2006.