“TIME is a road without any bifurcations, intersections, exits, or turnarounds.” With that, neuroscientist Dean Buonomano sets up the meat of his new book, Your Brain is a Time Machine – and an intriguing difference between the way we animals navigate time as opposed to space.
Not that contrasting time and space makes the task of understanding time any easier, as Buonomano illustrates later: “The physicist’s talk on the nature of time ended on time, but it seemed to drag on for a long time.” This captures various notions of time: natural time, clock time and subjective time.
Natural time is what physicists fuss about. Is time real? Or is the passage of time an illusion, and do all moments in time exist in much the same way that all coordinates of space exist? Neuroscientists, on the other hand, fuss about clock time and subjective time.
To explain natural time, physicists and philosophers back eternalism, according to which the past, present and future are all equally real. “There is absolutely nothing particularly special about the present: under eternalism now is to time as here is to space,” writes Buonomano.
The other main explanation of natural time is presentism, according to which only the present moment is real – a view that tallies with our sense of subjective time. The past is gone, the future hasn’t happened yet. “Neuroscientists are implicitly presentists,” says Buonomano. “But despite its intuitive appeal, presentism is the underdog… in physics and philosophy.”
“Mental time travel is a human capacity. But to do it, biology first had to figure out how to keep time”
Buonomano decides it is time to simultaneously tackle the physics and the neuroscience of time. The title of the book is derived from the now well-regarded idea that our brains are prediction machines. Whenever we perceive something, theory says that what we perceive is not objective reality, but rather the brain’s best guess as to what’s causing the sensations impinging on the body. But popular accounts of the theory often ignore one dimension of the prediction machinery: time.
Buonomano points out that the brain is continuously making real-time predictions, not just of “what will happen next” but also of “when it will happen”. To do so, the brain needs complicated machinery for keeping time – to predict not just what will happen within microseconds, but what might happen in seconds, minutes, hours, even days, weeks, months and years.
This ability to predict the long-term future is reliant on memory. In fact, that’s really the main evolutionary use for memory, as a storehouse of the information needed to predict the future. With memory and cognition, our brains became time machines – we could travel back and forth in time. This mental time travel is a human capacity, distinguishing us from other animals, hence the book’s title. Scrub jays, oddly, seem to demonstrate similar abilities, but proof of mental time travel in animals is hard to come by as yet.
To indulge in mental time travel, biology first had to figure out how to keep time, not unlike how scientists in the 17th century invented the pendulum clock. Christiaan Huygens’s high-quality pendulum clocks were the first to keep time more accurately than clocks within the human brain.
Buonomano’s book is full of delicious details about the myriad ways in which cells – neurons and other types – tell the time. For example, there’s the complicated sounding suprachiasmatic nucleus, a cluster of neurons at the base of the hypothalamus that acts as a master circadian clock. Circadian clocks depend on pendulum-like oscillations of the levels of specific proteins. One of which is aptly named period.
But unlike our clocks, which can tell time over a vast range of intervals, the brain has no single clock. For example, lesions in the suprachiasmatic nucleus don’t alter the brain’s ability to discern temporal patterns at the scale of seconds: there are different clocks for that. If there’s one clear message about the neuroscience of timekeeping, it’s that neural circuits can wire themselves in response to regular external stimuli. In other words, they can keep time, all sorts of time.
Reading Buonomano’s book, it’s hard not to marvel at how time and timekeeping pervade our existence – whether in the form of the clocks and instruments we build or through the mechanisms inherent in our brains. Buonomano creates a sense of wonder about just how complex the temporal brain is and about what a spectacular job it does of timekeeping.
Buonomano writes lucidly, in an almost matter-of-fact fashion, choosing crystalline clarity over flowery prose. So the occasional writerly sentences stand out, for example, when he writes: “The duration of the beat of a hummingbird’s wing is as concealed to our sensory organs as is the drifting of the continents.”
Buonomano’s clear writing is most apparent when he writes about the physics of time. Given that his expertise is neuroscience, this is no small feat. His explanation for why Einstein’s special theory of relativity implies the existence a block universe – a 4D manifold of space-time in which here, there and everywhere exist alongside the past, present and future – makes a masterful case for eternalism.
Special relativity destroys the notion of simultaneity – the idea that two observers moving with respect to each other could agree on the timing of events. When speeds get close to the speed of light, the temporal order of events can be perceived differently by different observers.
Buonomano writes: “If we assume that all events that ave ever or will ever occur are permanently located at some point in the block universe… then the relativity of simultaneity becomes no more puzzling than the fact that two objects in space can appear to be aligned or not depending on where you are standing. Two telephone poles along a highway appear aligned if you are standing on the side f the road, but not if you are in the middle of the road – it is a question of perspective.” And so it is with time.
But eternalism clashes with our subjective experience of the flow of time: in other words, physics clashes with neuroscience. While it’s true that we feel the passing of time, and thus instinctively favour presentism, Buonomano points out that our notions of subjective time are intricately linked to our notions of space. He shows this with the metaphors we use to talk about time: “That was a refreshingly SHORT commercial. We have been studying time for a LONG time… I’m looking FORWARD to your reply; in HINDSIGHT that was a terrible idea.” For timekeeping, the brain co-opts the neural circuits that are used to represent space, thus treating time and space similarly, in a curious analogy to special relativity.
This leads to one of the most intriguing questions raised in the book: could our theories about physics be informed by the very architecture of our brain? “Now that we know that the brain itself spatializes time, it is also worth asking if the acceptance of eternalism has benefited from the fact that it resonates with the architecture of the organ responsible for choosing between eternalism and presentism,” writes Buonomano.
“Could our theories about physics be informed by the very architecture of our brain?”
The state of scientific knowledge about time is such that no straight answers are forthcoming. The book, a compelling read for the most part, somewhat peters out towards the end, with more questions raised than answers. Understandably so. “Our subjective sense of time sits at the center of a perfect storm of unsolved scientific mysteries: consciousness, free will, relativity, quantum mechanics, and the nature of time,” writes Buonomano.
Your Brain Is a Time Machine can be disquieting, as the implications settle in, for example, of inhabiting a universe in which all moments exist. But the book ultimately leads to an internal quieting, as one realises that all the profound scientific discoveries of the past century or so are struggling with a common enemy: time.
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