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

by Dean Buonomano

Place cells are neurons that fire, or “turn on,” when an animal is located in a specific place in a room—that is, a particular point in space.

Memory did not evolve to allow us to reminisce about the past. The sole evolutionary function of memory is to allow animals to predict what will happen, when it will happen, and how to best respond when it does happen.

Presentism, as the name hints, states that only the present is real. Under presentism, the past is a configuration of the universe that once existed, and the future refers to some yet-to-be-determined configuration.

Eternalism, in sharp contrast, states that the past and future are as equally real as the present.

natural time, clock time, and subjective time.

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.

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.

Which neurons are connected to which, and the strength of the synapse between them, is determined in part by synaptic algorithms—so called synaptic learning rules—programmed into our genes.

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

Maybe it is just as well if we face the fact that time is one of the things we probably cannot define. . . . What really matters anyways is not how we define time, but how we measure it.

Nocturnal animals generally have circadian clocks with a period shorter than 24 hours, while diurnal creatures, such as humans, tend to have circadian clocks with intrinsic periods slightly longer than 24 hours.

Circadian rhythms observed in the absence of any external signals are said to be free-running rhythms.

time had dilated, as his personal sense of time had slowed down compared to objective time.

chronostasis—the sensation that time is standing still. In

prospective timing: determining the passage of time starting from the present into the future.

retrospective timing: estimating the passage of time from some moment in the past up until the present.

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.

the more complex or challenging a task, the shorter the estimates of how much time was spent performing the task (53 versus 31 seconds). The opposite can happen with retrospective timing: the higher the cognitive load, the longer the task can seem (28 versus 33 seconds).

Consciousness is a delayed account of not only what is happening in the external world, but of what is happening in the unconscious brain.

Thus it appears that the perception of time is not the only mental faculty to be altered: the perception of space is altered as well.

There is even a name for the phenomenon of hearing multiple interpretations of a song: a mondegreen.

Time is to speech and music recognition as space is to visual object recognition.

Biological oscillators are generally used to time events of durations less than their period—infraperiod timing—whereas man-made clocks measure elapsed time for durations above their period—supraperiod timing.

The recurrent network has an interesting property of being able to remember what it was doing: even when bumped off its original trajectory it can “return” to and complete the task it was engaged in.

Elements are defined by the number of protons in their nucleus. The isotopes of an element refer to the variants that have different numbers of neutrons.

pulsilogium. It consisted of a weight on a string, which was attached to a horizontal ruler. The ruler allowed the length of the string to be shortened or lengthened. By changing the length of the string, the pulsilogium allowed the doctor to adjust the period of the swing of the pendulum to match the heartbeat of the patient. Thus the length of the string provided a fairly reproducible measure of heart rate.7

1905 the patent officer, Albert Einstein, published the paper On the Electrodynamics of Moving Bodies, which, in addition to abolishing the notion of absolute time, briefly describes a way to synchronize distant clocks.11

1967 an international consortium defined a second as: “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.”13 The

By measuring the time differences it takes a signal to arrive from different satellites, a GPS receiver can use a form of triangulation to figure out its latitude, longitude, and altitude.

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. To uncover the deep and hidden connection between time and existence . . . is a task for the future.

Among the many things the brain certainly did not evolve to understand was the brain itself. Another is the nature of time.

“It is utterly beyond our power to measure the changes of things by time. Quite the contrary, time is an abstraction, at which we arrive by means of the changes of things.”5

So the second law of thermodynamics only establishes an arrow of time provided that the universe started in a low-entropy state.

“Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.”

When we think we see motion at some instant, the underlying reality is that our brain at that instant contains data corresponding to several different positions of the object perceived to be in motion. My brain contains, at any one instant, several “snapshots” at once. The brain, through the way in which it presents data to consciousness, somehow “plays the movie” for me. . . . I see, coded in the neuronal patterns, six or seven snapshots of the kingfisher just as they occurred in the flight I thought I saw. This brain configuration, with its simultaneous coding of several snapshots, nevertheless belongs to just one.

Tongue-in-cheek,

The kappa effect, for example, establishes that when two events separated by the same temporal interval occur at larger distances from each other (reflecting higher speeds), people tend to judge those temporal intervals as longer.21

These and other observations are consistent with the notion that mentally traveling backward or forward in time relies in part on the same cognitive capacities we use to store and reconstruct autobiographical information about the past.

There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another which states that this has already happened. —DOUGLAS ADAMS

These rapid voluntary movements of the eyes are called saccades, and more rigorous experiments than trying to look at your own eyes in the mirror demonstrate that vision is partially suppressed during saccades.

“The reason we say that humans have free will is because we can’t predict what they will do.”15