My talk with Daniel Rubin on Real Numbers and the Infinite in Analysis

A few days ago I had an online conversation with Dr Daniel Rubin who is a mathematician living in the US and who works in analysis, geometry and approximation theory. The topic was one close to my heart: Daniel wanted to hear of my objections to the status quo concerning the foundations of modern analysis: namely my rejection of “real number arithmetic” and why I don’t accept “completed infinite processes”. And naturally he wanted to do his best to rebut them.

Here is a link to our chat:

It is certainly encouraging to see that some analysts are willing to engage with the uncomfortable idea that their discipline might actually be in serious logical difficulties. Most of us are reluctant to accept that something we have been working on for years and years might actually be wrong. I applaud Daniel for the courage to engage with these important ideas, and to consider how they fit, or don’t fit, into his current view on analysis.

When we learn pure mathematics, there are many things that we at first don’t understand, perhaps because they are obscure, or perhaps because we are not smart enough — it is easy not to be sure which. Our usual reaction to that is: let me try to accept the things which are cloudy, and hopefully with further learning things will become clearer. This is a reasonable approach to tackling such a difficult subject. However it does require us to put aside our natural skepticism, and accept what the more established figures are telling us at critical points in the theoretical development, even if we imagine this is only temporary.

A good example is: “analysis is built from axiomatic set theory.” In other words the foundations of “infinite sets” and so the basic logical structure of the “arithmetic of real numbers” is a consequence of work of logicians, and can be taken for granted without much further inquiry. Or to put it less politely: it is not the job of an analyst to work out clearly the foundations of the subject; this is something that can be outsourced.

In this fashion dubious logical sleights of hand can creep into an area, transmitted from generation to generation and strengthened with each repeat. Young academics in pure mathematics are under a lot of pressure to publish to obtain a foothold in the academic ladder. This means they do not often have time to mull over those knotty foundational questions that might have been bugging them secretly at the backs of their minds. They probably don’t spend a lot of time on the history of these problems, many of which go back centuries, and in former times engaged the interest of many prominent mathematicians.

Later in their career, if our young PhD has been lucky enough to score an academic job, they might be in a position to go back over these core problems and think them through more carefully. But even then there is often not a lot of “academic reward” in doing so: their fellows are not particularly interested in endeavors that are critical of the orthodoxy — pure mathematics is quite different in this regard than science or even applied mathematics!

And journals are uniformly not keen on publishing papers on foundational issues, especially ones which challenge accepted beliefs. As pure mathematics rests on a premise of logical correctness, any questioning of that is seen as subversive to the entire discipline.

But maybe some serious consideration and debate of the underlying logical structure is just what the discipline really needs.

I certainly enjoyed our conversation and I think there are valuable points in it. I hope you enjoy it, and look forward to another public YouTube discussion with Daniel.

6 thoughts on “My talk with Daniel Rubin on Real Numbers and the Infinite in Analysis

  1. chwasungngen

    Totally agree from pure mathematics point of view
    On the other hand, analysis approach really push a great advanced in electronic etc. The analytic approach like small signal model is pure analysis, no algebra

  2. gentzen

    Still need to watch your debate. Since our last interaction, I learned that the famous physicist Nicolas Gisin has taken concept from ultrafinitism and intuitionistic mathematics, and started to run with it. Actually I would say he has achieved lift-off and is flying already. There is a recent Quanta story about it, but Gisin has published more papers on this subject since. Here is a quote from the Quanta story to get some feeling where he goes:

    It is a bit like the weather. Recall that we can’t precisely forecast the weather because we don’t know the initial conditions of every atom on Earth to infinite precision. But in Gisin’s indeterministic version of the story, those exact numbers never existed. Intuitionist math captures this: The digits that specify the weather’s state ever more precisely, and dictate its evolution ever further into the future, are chosen in real time as that future unfolds in a choice sequence.

    You might think that this is only intuitistic mathematics without ultrafinitism, but notice that the choice sequence can potentially be extended, but it is still possible that it will always stay shorter than some finite limit (even if the actual limit stays unknowable).

  3. Throwaway1

    When mathematicians talk about “completing infinite processes”, why isn’t the contradiction apparent to them? Infinite processes – by definition – cannot be completed. Maybe it helps to see it expanded: it is equivalent to saying “we will end the never-ending”. In how many ways does it need to be rephrased before this clicks? How much simpler can this simplest of contradictions be made apparent?

      1. Mark

        Well said both of you. It is drummed into us at a very early age, when we do not have the confidence or the vocabulary to properly disagree, that irrationals such as pi and the square root of two are constants, and that the infinite division process of 1 divided by 3 (in base 10) CAN complete and produce an infinite result with no remainder. But what I can’t fathom is why, after we have grown up, only a few of us appear to have the ability to fight against our indoctrination and see the crystal clear absurdities for what they truly are.

  4. Michael Jagdeo

    Consider the proposition: If pi is irrational, a circle has 0 lines of symmetry.
    Colloquially: irrational numbers like pi are not in fact, smooth.
    Practically: the de facto mantissa let’s us go on without going insane.


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