Let's imagine a line where cuts have been made to mark all rational points (I don't believe this is possible, but let's go with it for now). I believe you cannot mark any more points on this line. If you throw a dart in between the rational points then you will hit an indivisible line segment. That is as discrete as it gets, and even then the line is securely continuous. — keystone
The cuts are 0-dimensional so they are illusions of convenience. If you throw a dart at the line you will always hit the line, never the cut. The cuts have measure 0 after all. — keystone
The problem here is that the real number line is the mathematical object that was in question, surely? So as a construction, it hosts both the rational and the irrational numbers as the points of its line.
Now,
like Peirce and Brouwer, you might want to make more ontic sense of this by employing the notion of intervals.
And so the claim becomes that reality has a fundamental length – the unit one interval. There is a primal atom of 1D-ness or continuity. There is an object or entity that begins everything by already being both extended and truncated. And in being both these things as a primal symmetry state – possessing a canonical oneness both as a contradiction and an equality – it can then become the fundamental length that then gets either endlessly truncated, or endlessly extended.
The equality of 1 can be broken by a move in either direction. And each move creates a ratio - a rational number - that speaks to the number of steps taken away from home base. If you can count upwards to a googol/1, then you can divide downwards to 1/googol. The reciprocal relation between extension and truncation is right there explicitly in the symmetry breaking of the unit 1 interval.
Now as reciprocal directions of arithmetic operations, each would seem to extend infinitely. Or at least, they are unbounded operations. The higher you can count with this system, the smaller you can divide its parts. But could you reach actual zero, or actual infinity? Well the obvious problem is that this would mean squeezing your origin point - the unit 1 - out of actual existence. The whole way of thinking would lose its anchoring conception of the truncated interval that set the whole game of approaching its dichotomous limits going.
So yes. If you just think of numbers as rationals – ratios that embody reciprocal actions on a fundamental length – then actual completed infinities and actual 0D points or cuts become anathema to the metaphysical intuition.
What then happens when you add the irrationals? Does that change anything?
The usual way of picturing it is that the number line becomes so infinitely crowded by markable points that it is effectively a continuum. Infinite extension and infinite truncation become the same thing. A new state of symmetry. A new unit 1 state. The continuum is that which is neither truncated nor extended. The concept of a finite length anchoring things is dissolved and becomes vague.
In hierarchy theory terms – Stan Salthe's "basic triadic process" – this is a familiar state of affairs. The small has become so small that it just fuses into a steady blur. The large has by the same token become so large that it has completely filled the field of view.
You wind up in a world where there is a global bound that arises because continuity has been extended so much that its truncated ends have crossed the event horizon (as cosmology would call it). And likewise, the local bound has become so shrunk in scale that it is a fused blur of parts (a wavefunction as quantum physics would call it). And then that leaves us, as the observing subject, surrounded by a bunch of medium sized dry goods – objects that embody both truncation and extension in terms of looking like composite wholes made of divided parts.
So we can make sense of the rationals as intervals on a continuum – the symmetry of the unit 1 being equally broken in both its complementary directions. And then when this asymmetry is maximised, you wind up in this hierarchical situation where you live in a world of truncated lengths, but then are semiotically closed in by a global bound of holographic continuity (synechism or constraint, in Peirce speak) and a local bound of quantum discreteness or fluctuation (tychism or spontaneity, in Peirce speak).
But that is connecting with the physics. A richer notion of mathematics that includes time and energy along with space. I'll get back to the issue of the irrationals.
Now for me, it seems clear enough that the familiar irrational constants – pi, phi, e, delta – are again unit 1 ratios, but ratios generated by growth processes. So e for example scales the compounding growth of a unit 1 square, not a unit 1 interval. That is why it is incommensurate. It is a unit 1 dropping in on the numberline from a higher dimensionality.
Surds have the same story. That then leaves the unbounded cloud of numbers with random decimal expansions. Some like pi, phi, e, delta have their generators in a higher dimension as I argue. But that isn't even a drop in the ocean of all the infinitesimals that seem to exist and so make the numberline infinitely dense with dimensionless points or cuts.
One view that appeals is that all these meaningless numbers with random decimal expansions mark nothing in particular. They are in fact the tychic spontaneity of Peircean semiotics – the inability of nature to suppress or constrain all its surprises. Down at the ground level of truncation, it is just fluctuation – the seething instability of the quantum vacuum, filled with its virtual particles and zero point energy (to use some much abused terms).
But maybe also, all these random fluctuations are actually ratios of some kind – visitors from another dimension like the growth constants. Maybe they all have a generator that makes each of them a unit 1 story in a bigger picture, one with infinite dimensionality. Or unbounded dimension.
So the infinitesimals becomes a blur of unit 1-ness that results from a numberline living in infinite relational dimensionality, just as Cantorian infinity establishes its own unit 1-ness by becoming the point where counting goes "supra-luminal" – crosses the event horizon in physical terms.
Something is certainly going on here with the irrationals. Some definitely have their higher dimensional generators like the unit square, the unit rectangle, the unit circle, the unit Pythagorean triangle. Crisp and necessary mathematical structure arises out of the swamp of algebraic symmetry breaking.
Do the rest of the irrationals have a similar story of geometric necessity behind them? Or are they just a blur of surprises and accidents. A blur of differences that don't make a differences and hence that which serves as the blank and continuous backdrop to the constants of nature which in turn have the most supreme importance.
I think Peirce and Brouwer argue towards a world where the number line starts off from the conception of truncated intervals – the infinity of truncation~extension operations that can act on the unit 1 length. That gives us the rationals that are completely at home in the world so defined.
Then you get the intrusions from higher mathematical dimensions – ratios or symmetry breakings from a larger universe of rational shapes. These pop on the line in ways that don't fit so exactly.
This then leads to a view of the number line that is a continuum of fluctuations. But now we are counting all digits with random decimal expansions as a something rather than a nothing.
It is information theory all over again where both meaning and nonsense are assigned the same bit-hood status. Signal and noise are in the eye of the beholder. What matters in the new counting system is there is the truncated interval – the ensemble of microstates. Information theory becomes about counting all actualised differences, not the differences that make a difference (even if that dichotomy can then be recovered with other relational measures like the notion of mutual information).
Anyway, the picture I have in mind is a continuum which is composed of rational intervals – truncated lengths that appear over all possible length scales. Eventually these lengths become either too large or too small and so exceed our pragmatic grasp. We live in a world where energy and time matter, along with (3D) space. And so it is simply a fact of living in that world which imposes a cosmic event horizon and a Planck scale cut-off. As an intuitionist would say, we haven't got the time or energy to pursue either the promise of unbounded interval expansion or unbounded interval truncation.
But into this rational continuum creeps some irrational numbers that really matter – the rare visitors from higher-D ratios.
And also then, we have the noisy background crackle that is the randomness of all the other unbounded decimal expansions we can imagine as truncation operations. If they have generators, then in the spirit of Kolmogorov complexity, there is no actually simpler algorithm available but to print out every entropic digit. They represent the limit of the generatable. Another way of saying they are merely the meaningless noise that can't be squeezed out of any system. The quantum uncertainty that one knows one finds at the Planck cut-off of any system made of actual material stuff.