Your Brain is a Time Machine: The Neuroscience and Physics of Time

by Dean Buonomano

“Talk of the flow of time or the advance of consciousness is a dangerous metaphor that must not be taken literally.”

“The objective world simply is, it does not happen. Only to the gaze of my consciousness, crawling upward along the world line of my body, does a section of the world come to life as a fleeting image in space which continuously changes in time.”

Wow

the brain is a time machine: a machine that not only tells time and predicts the future, but one that allows us to mentally project ourselves forward in time. It is exceedingly easy to overlook the fact that without the ability to mentally travel into the future, our species would have never crafted an obsidian stone into a tool, or grasped that by planting seeds today we can ensure our future survival.

As Abraham Lincoln reportedly said, “The best way to predict the future is to create it,”

“Early expressions of future-oriented thought and planning consisted of learning to use, preserve, and then make fire, to make tools, and then to store and carry these with them. Furnishing the dead with grave goods; growing their own crops, fruits, and vegetables; domesticating animals as sources of food and clothing; . . . these all represent relatively recent developments in human evolution. Every single one is predicated on the awareness of the future.”

temporal contiguity

if cigarettes caused cancer a week after starting to smoke, as opposed to many decades later, the tobacco industry would never have become a worldwide trillion-dollar industry (chapter

These connections are implemented by the synapses, the interface between two neurons: a presynaptic neuron that is sending a signal out and a postsynaptic neuron that is receiving the signal. The inputs to any given neuron come from its presynaptic partners, each providing bioelectrical whispers. Excitatory synapses encourage the postsynaptic neuron to “fire”—that is, generate an output by sending an electrical signal to all its downstream neurons (its own postsynaptic partners). In contrast, inhibitory synapses attempt to persuade the postsynaptic neuron to keep quiet. With so many neurons the nervous system is the wiring diagram from hell. What determines which neurons are connected to which?

So our genes do not encode the strength of the synapses, but they determine the algorithms that govern the strength of the synapses.13 One learning rule in particular, spike-timing-dependent plasticity (STDP), beautifully illustrates how the temporal asymmetry

of cause and effect is built into our synapses. Consider the two neurons shown in Figure 2.2: neuron A is connected to B, and B in turn to A. Thus there are two synapses: A→B and B→A. We would say these neurons are recurrently connected: neuron A is the input to neuron B, and vice versa. Now let’s assume that each neuron is driven by distinct events in the outside world. Perhaps the owner of these two neurons is a baby named Zoe, and neuron A is driven by the sound of the letter z, and neuron B by the sound of the letter o; thus, whenever Mom and Dad say Zoe’s name, neuron A will fire right before neuron B and, for argument’s sake, let’s say that neuron A consistently fires 25 milliseconds before neuron B. The job of a synaptic learning rule is to strengthen or weaken synapses, according to the pattern of activity of the presynaptic and postsynaptic neurons. In this case STDP will preferentially strengthen the A→B synapse and weaken the B→A synapse. It took neuroscientists a surprisingly long time to hit upon this simple learning rule. It was only in the 1990s that STDP was conclusively demonstrated.14

the neurons and synapses within our brains manage to connect the dots between events separated by short and long intervals, allowing us to make sense of the events unfolding around us.

we can only detect such temporal patterns on the very narrow time scale of around a second. If you slow speech down too much, it becomes unintelligible, and if you speed a musical piece up too much, it ceases to be music (chapter 5).

So the brain is both an anticipation machine and a machine that tells time.

It quantifies the passage of time across a range of over twelve orders of magnitude—from the tiny difference in the time it takes a sound to arrive at the right ear versus the left ear, to the ability of some animals to anticipate the seasons.

If mice are kept in constant darkness, their circadian rhythm continues with a period of approximately 23.5 hours, resulting in a progressive leftward shift of the activity pattern.

Cyanobacteria are photosynthetic organisms, and photosynthesis is the ultimate daytime job.

one of the driving forces for the evolution of circadian clocks was the highly adaptive coordination of cellular functions with cycles of light and dark produced by the Earth’s rotation. The evolutionary

hamsters with circadian clock mutations can have free-running periods significantly below or above 24 hours. One such mutation results in a strain with an intrinsic circadian period of 22 hours. Compared to their wild-type counterparts these mutants have a shorter lifespan when living in a 24-hour world. Lesioning the suprachiasmatic nucleus of these animals actually prolonged their lives. This is a fascinating example of a situation in which part of the brain seems to be doing more harm than good.

the circadian clock does not have a minute hand, much less a second hand.

The biochemistry of translation/transcription feedback loops is simply too slow to be of any use when attempting to determine if the red light is about to change.

In natural environments the moon is the primary source of light at night, and a full moon enhances the ability of predators to see potential prey, so the most vulnerable phases of the life cycles of some animals occur out of phase with the full moon.

The sea worm’s internal circalunar clock can be demonstrated by the lunar equivalent of free-running circadian experiments. The circalunar clock must first be entrained, not by sunlight, but by moonlight (or in the laboratory by exposure to dim light for a few hours at night). If, after a period of entrainment, worms are kept in the laboratory under a constant day-night cycle, they nevertheless exhibit a 30-day reproductive rhythm. How do these worms keep track of this 30-day cycle? Do they use their circadian clock as a pendulum with a period of 1 day and count up to 30?

when sea worms were given a drug that altered their circadian rhythm, they still maintained a 30-day circalunar cycle.27 One more piece of evidence in support of our multiple clock principle.

we don’t feel the time of day like we feel the heat of midday sun. But we subjectively feel the passage of time, and are keenly aware of the duration of unfolding events. Clearly—as we will see next—the brain has other means to judge the passage of time. Means that transcend the passive measurement of time, and somehow generate a subjective sense of time’s passage.

Prospective timing is a true temporal task in that it relies on the brain’s timing circuits. In contrast, retrospective timing is in a sense not a timing task at all; it is rather an attempt to infer the passage of time by reconstructing events stored in memory.

Consciousness is a delayed account of not only what is happening in the external world, but of what is happening in the unconscious brain. For example, as we will see (chapter 12), by watching the neural activity within the brain it is possible to predict when people will voluntarily decide to move their finger up to 900 milliseconds before they actually do—hundreds of milliseconds before the subjects themselves seem to be aware of having “freely” decided to move their finger. So even if danger does kick the brain into an overclocking mode—resulting in sped-up actions—consciousness might be too slow to be guiding those actions. Thus the reports of “dominant mental quickness as demonstrated by the increased speed of thoughts”29 may just be another deception the unconscious brain imposes on the mind.

The active state of a network at a given time t is governed by the input to the neural network and its state (the active and the hidden state) at time t-1.

The “response” of both the pond and a neural network can be said to be state-dependent. Indeed,

eyeblink conditioning: by presenting an auditory tone followed by a puff of air to the cornea 250 ms later, over and over again, humans and other animals will learn to blink in response to the tone. But they do not blink as soon as the tone goes on: the blink is timed to precede the expected air puff. In other words, animals don’t only learn to blink, they learn when to blink.

The pattern is an emergent property: the whole is larger then the sum of the parts.

In the nineties, however, there was increasingly convincing evidence in rats and mice that new neurons were born in some parts of the brain—a process referred to as adult neurogenesis.

“the clock, not the steam engine, is the key-machine of the modern industrial age.”

Of all obstacles to a thoroughly penetrating account of existence, none looms up more dismayingly than “time.” Explain time? Not without explaining existence. Explain existence? Not without explaining time.

according to presentism only the present is real: all that exists exists in the perpetual present (in my use of the term, presentism does not imply that time is absolute). The past refers to a configuration of the universe that no longer exists, whereas the future represents a yet-to-be-determined configuration. Under eternalism, time has been spatialized into a full-blown dimension in which the past, present, and future are equally real. The universe becomes a four-dimensional “block” with one temporal and three spatial dimensions—the so-called block universe.3

physicist George Ellis, among others, advocates for a compromise: a four-dimensional block universe that only contains the past. Under this so-called evolving block universe theory, the present is the wave front that progressively freezes an undetermined future into an ever-growing and unchangeable past.4

Others believe that time is merely an abstraction, a very useful concept to help explain how the universe works, but unlike mass or energy, time would not be a fundamental ingredient of physics. To understand this view a bit better, recall that, in practice, clock time is always measured by change. No matter how accurate or inaccurate, clocks are always quantifying change of some physical phenomenon. The consequence of this fact is that it is always possible to express time as some other nontemporal physical measure. For example, quartz clocks and watches often mark the time with dials that revolve around a circular face, and when the minute hand goes from 12 to 6 we say 30 minutes have passed; but couldn’t we just as well say 180 degrees have passed? Or in the case of a pendulum clock with a base frequency of 1 Hz, instead of saying 30 minutes have passed, we could say 1,800 swings have elapsed. Indeed, the standard unit of time (the second) is not actually defined as some pure unit of time, but instead as 9,192,631,770 cycles of the radiation corresponding to the resonant frequency of cesium 133, which is approximately equivalent to how long it takes the earth to spin 1/240 of a degree around its axis.

The point is, clock time can be seen as a convention by which we standardize change.

The hodgepodge terms and theories about time—presentism, eternalism, tensed time, untensed time, the evolving block universe, relationalism, etc.—is a symptom of the fact that there is no consensus as to what time actually is. Nonetheless, to the extent that there is a favored theory in physics and philosophy, it is certainly eternalism. Eternalism, however, is not merely counterintuitive—it mocks one of the most universal features of human experience:

As explained by the contemporary philosopher Craig Callender: “The equations of physics do not tell us which events are occurring right now—they are like a map without the ‘you are here’ symbol. The present moment does not exist in them, and therefore neither does the flow of time.”7

So the second law of thermodynamics does not forbid balloons from unpopping, glasses from unbreaking, and ice cubes from unmelting, but it does the next best thing: it virtually ensures that they won’t.

measured, there is no going back. In fact, once the measurement is made, it is impossible to use Schrödinger’s equation to retrodict which slit the electron went through.12

quantum mechanics imposes an arrow of time upon the universe—

The fundamental equations of physics seem to imply that now is to time as here is to space, providing one reason many physicists and philosophers believe that we live in the block universe of eternalism.

If the ball leaves the hand before the game clock hits zero the shot counts.

General relativity offered an astonishing answer: gravity was not really a force per se, but the warping of spacetime.

“Matter is extended in space, but consciousness exists in time as surely as it proceeds from ‘I think’ to ‘I am.’”22

conscious awareness of external events can take hundreds of milliseconds to develop.

instantaneous consciousness, and whether the phenomenon of consciousness is compatible with the moments-within-a-moment solution to the block-universe/time-flow paradox.

relativity. So for now, even though the laws of physics seem to be most consistent with eternalism, we have no direct experimental evidence in support of eternalism, much less proof.