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?