Physics is the business of figuring out the structure of the world. So are our
brains. But sometimes physics comes to conclusions that are in direct conflict
with concepts fundamental to our minds, such as the realness of space and time.
How do we tell who’s correct? Are time and space objective realities or human-invented concepts?
There’s been quite a few surprises in the history of the development of physics. And now we may be
at one of those special junctions again—many physicists suspect that the next step will
pull the rug out from under us by showing that space and time are not quite real.
How do we prepare for such a massive frameshift? To be fair, our ideas about space and time have
not always been the same. In a previous episode we talked about the two radically opposing ideas:
In the “Absolute” space and time view, championed by Newton, space exists on its own, with no
regard to any objects or entities, and time also exists, its passage governed by a cosmic clock.
At the time, this was a hefty proposition. Many philosophers and scientists, such as
Leibnitz and Descarte considered space and time as “relational”,
as a network of distances between objects or succession of events.
Ultimately, we saw that Newton’s pure vision of absolute space and time couldn’t be right.
But if the dimensions don’t ultimately “look like” our impression of them, what are they? Does
our mental experience of space and time resemble the world external to our subjective experience?
Newton clearly thought that there must be a very close correspondence. Leibnitz on the other hand
thought that we build our experience of space through distilling positional relations that
are inherent to the connections between objects, rather than space being a standalone container
for those objects. Another prodigious thinker who thought similarly was Immanual Kant. He
initially took Newton’s side on the reality of space and time, but after what he calls
a Copernican revolution in his thinking, he came to believe that space and time are not physically
real but are constructs of the mind--inborn principles by which we organize the world.
Last time we turned to Einstein as the ultimate tie breaker. In his essay about “the problem of
space”, Einstein wrote that “concepts of space and time are free creations of the human intelligence,
tools of thought, which are to serve the purpose of bringing experiences into
relation with each other.” Uh. OK, so we don’t just believe Einstein because he’s Einstein.
But we sure don’t dismiss something Albert says without thinking very hard about it.
So, are space and time, absolute and fundamental, or relational and conceptual?
There might be new clues in the mechanisms by which brains manage space and time, so today we’re
going to do something unusual. We’re going to do some neuroscience to try to understand how these
dimensions are dealt with in the brain and what that implies about their existence in the world..
Ok. To be spatially capable organisms, we need our brain to do two things. Tell us 1)
Where things are in relation to us, and 2) Where everything is in relation to each other in space.
The latter, the allocentric map of space, is more relevant to our discussion so we’ll focus on that.
To navigate the environment, brains seem to generate a mental representation of
the surrounding environment. This is often called a ‘cognitive map’.
In 1971, neuroscientists John O'Keefe and Jonothon Dostrovsky found the first
evidence for this map in the brain. They were monitoring neural activity in rats
as they wandered around their environments. They noticed that a neuron in a section of a
rat’s hippocampus would fire rapidly every time the rat entered a particular region.
And when the rat moved a bit further,
a different neuron would fire, and so on. They’d discovered place cells,
and some have argued that they represent our internal map. In a given environment,
specific place cells correspond to specific locations, and those neurons remain fixed on
those locations until you go to a new environment. Then, the place cells remap to the new space.
But, how do these cells know where the rat is?
In 2005 Edvard and May-britt Moser discovered another group of neurons that behaved a little
like the place cells. These are in an area next to the hippocampus—the entorhinal cortex.
They also fire when an experimental rat passes through a particular location in
their physical environment. But that same neuron will also fire when the
rat enters any number of other locations at equal distances from each other—locations
that together form a hexagonal grid spanning the current environment.
These are the grid cells, and they appear to tile whatever space we’re
currently in into grids of multiple different orientations and scales.
Although each new space is tiled differently, individual grid cells
represent a fixed scale--they are like a rigid ruler providing something like
metric information with different resolutions. Imagine how this plays out in your own head.
You’re walking across a large room and a given neuron in your entorhinal cortex clicks every
3 meters. A different neuron fires more often - approximately every meter - while another
is clocking your progress every 10 meters, and so on. Any location in the space elicits
a unique pattern of “firing grid cells” which in turn leads to the firing of a unique place
cell. One view is that the brain is doing an inverse fourier transform, through which
a combination of signals from multiple different grid cells stack to excite localized place cells.
Now there are a lot of unknowns here,
and different interpretations about what all these cells are doing, which we’ll come back to.
Regardless, the combination of grid cells and place cells seem to be a key part of the machinery
behind your sense of your surrounding space. And it has some very telling properties. It seems like
it includes a coordinate grid. In that sense, it’s a distinctly Newtonian picture of space,
with coordinates fixed to the current environment and independent of your location within it,
and also independent of things in that environment. This makes it feel absolute
rather than relational. Perhaps you remember that in the relational view, space is thought
of as a network of distance relationships between objects, and that’s not what grid cells represent.
But this is only the tip of the iceberg of our spatial processing
machinery. We talked about our allocentric modeling of space,
which appears absolute. But we also perform egocentric processing—understanding the
surrounding space in context of ourselves, and this is by definition relational.
We use depth perception to help construct the grid, and we use our internal sense of
our velocity and direction of motion to update our position on the grid.
These relational distance measures help us build and update a sense of space that
is absolute and not centered on ourselves or anything else.
Our hippocampus and entorhinal systems seem hard-wired to represent space the way they do,
so perhaps it’s understandable why Newton—and now many of us—have developed
this intuition of space as an absolute entity that’s independent of its contents.
We haven’t yet got to the big question of whether the space in our heads is just in our heads,
although we already have some useful clues. Before we get to the really hard question,
we need to spend a little time on … time.
So, Newton thought that time was as absolute as space, ticking away everywhere at the same
rate even absent anything to experience it. Einstein showed that the rate of clocks depends on motion
and gravity, but also hinted that he didn’t buy the idea of time as independent of things
that experience time. He said that “Time is what clocks measure.” suggesting that time isn’t an
absolute thing in itself, but rather something that emerges from the behavior of matter.
We’ve talked about the origin of time in this sense before. Any thing that undergoes internal
change can be thought of as a clock. For a good clock, that change is regular. Perhaps periodic
like the swinging of a pendulum or measured by accumulated change, like the sand in an hourglass.
Now, we have no “time receptors,” in our brains, as we do for other features of perception, like color,
and pitch. But we know that our brains can tell time. We wake and sleep on a regular cycle,
we can guess when a certain amount of time has passed with varying accuracy,
we can sync up our limbs for coordinated movement, and we can keep a beat—or at
least some of us can. We must have an internal clock of some sort. Although
actually it’s no longer generally believed that we have a single master clock. Rather,
different brain regions probably find their own ways to model or track the passage of time.
One top candidate is the rhythmic activity created by populations of neurons firing in sync. These
brain waves repeat on a regular timescale, from 0.02 to 600 cycles per second (Hz),
and some of them might be creating something like a ticking clock that other neurons can
organize themselves around. We also track time on different scales through our circadian rhythm,
through the accumulation and ordering of our memories, and through other neural methods.
All of these inner clocks track experienced time, and are only loosely correlated with external
time. On very short timescales our timing is rather good and consistent—we coordinate our
limbs and our senses, and we’re good at guessing how much time has passed up to several seconds.
But our guesses get worse for longer time periods, and our sense of time is warped by
whatever’s going on—time flies when you’re having fun, but a watched pot never boils.
In general, our time perception does not seem absolute. It seems distinctly relational and
very malleable—we remember temporal ordering of events and estimate intervals between events.
But we don’t have an inbuilt proxy for Newton’s singular cosmic clock.
This may be why we wear a watch but don’t carry personal yardsticks!
OK, so we’ve talked about how the brain models space and time. Things get really
interesting when we bring these together. In fact the brain’s distinction between
space and time is not so cut and dry. For example, researchers have seen that under
certain conditions hippocampal cells seem to track progression of time rather than place.
They fire sequentially as time passes for a rat running in place in a wheel.
In fact it now seems that place cells can fire with new locations OR with the passage of time.
They may even have a much more general function, and that’s tracking sequences.
I already talked about brain rhythms as internal coordinating clocks. There is one
such brain wave in the hippocampus: the theta cycle, periodic neuronal pulsing at 4-10 Hz.
On every pulse of the theta cycle, a chain of place cells will fire in rapid succession—in
the middle is the neuron representing the current location or time. But first,
a group of neurons representing the recent past will fire in sequence,
and then a sequence representing the upcoming places in the trajectory. On each theta wave, you
live partially in your immediate past and future. This suggests that place cells may ultimately
reflect your executed and planned trajectory rather than specifically space and time.
This trajectory may be in actual space, or in an abstract space of thought and intention,
or even a logical chain of reasoning. Evidence in favor of this is that we know that the
hippocampus is critical for laying down memories. It indexes past experiences in a way that enables
us to recall them in sequence—almost as though it was applying a coordinate system to them.
It seems that the machinery that may initially have evolved for enabling navigation through
space has been co-opted into a much bigger role. This is also true of the grid cells,
which seem to play a role in mapping 2-dimensional abstract spaces. For example, they’re active
when we build mental models that involve the relationships between pairs of related variables.
We seem to use grid cells and place cells to help us organize the world in very general ways,
and navigation in real space and in mental space could be using the same algorithm.
OK, let’s regroup. We started all of this by asking if the space and time of our
minds corresponds to physically real entities. Some pretty smart people, including Einstein,
thought that perception of space and time are mental constructs. Now let me be very clear;
they were not saying that the external world isn’t real. They believed that there’s something
out there that has an independent existence to us. That something exhibits regularities
that our brains partition into space and time. Leibnitz, Kant, and Einstein felt
that those regularities only take on our familiar experience of space and time within our minds.
So does the neuroscience we learned agree with them? We can’t answer that directly but we can
try to say whether our brains are capable of such a feat. And the answer to that looks to be a yes.
Many researchers believe that our mechanisms for tracking time and space are really general
purpose algorithms for tracking sequences of events and mapping the relationships between
continuous variables. And with navigation being such an essential function for survival,
it seems likely that these systems evolved with the original purpose of doing this job for space
and time. John O’keefe, discoverer of place cells put it this way: let’s assume that the
world is an n-dimensional energy soup. Animals on all levels of the evolutionary scale develop
systems sensitive to various aspects of this soup; these become their version of reality.
One evolutionary development led to a set of systems which divided the soup sharply into
discrete objects and provided a spatial framework for containing these objects.
In other words, arranging the world into what and where and when is our brain’s most efficient and
meaningful way of carving nature at its joints. The fundamentality and primacy of space and time
may stem from the fact that we have no alternative way of partitioning our experience. Many
scientists are accepting the demise of spacetime as a fundamental entity. In future episodes in
this series we’ll get back to the implications of this in physics.What fundamental structures and
processes give rise to external regularities that our brain represents as spacetime?