If space is not continuous, then is calculus wrong?

[jessjolt2]

good evening jess,

Do you understand the difference between 'continuous' and infinitely divisible?

no?
doesnt infinitely divisible mean continuous?

 

[Studiot]

Good evening jesse

no?
doesnt infinitely divisible mean continuous?

Consider the following rather strange function which consists of all the numbers between 0 and 1, none of the numbers between 1 and 2, all the numbers between 2 and 3, none of the numbers between 3 and 4 ... and so on.

Is is infinite? Yes

Is it continuous? No

Is it infinitely divisible? Yes

This function is, of course, all the tops or bottoms of a perfect square wave.

 

[jessjolt2]

Good evening jesse
Consider the following rather strange function which consists of all the numbers between 0 and 1, none of the numbers between 1 and 2, all the numbers between 2 and 3, none of the numbers between 3 and 4 ... and so on.

Is is infinite? Yes

Is it continuous? No

Is it infinitely divisible? Yes

This function is, of course, all the tops or bottoms of a perfect square wave.

i kind of see your point, but i do not see how this relates to my question?

and based on current mathematics, this function is infinitely divisible on the interval (0,1), (2,3), etc, but it is not divisible at all on (1,2), etc...

my question is, what if space is not infinitely divisible on any interval?...and in this case we would need to use mathematics which takes this discreteness of space/time into account. I notice many people here are saying how good calculus is as a model of reality. but that is not my point. i do not want to know what MODELS reality, i want to know what IS reality...basically i want to know what math mirrors and PERFECTLY describes reality.

 

[sophiecentaur]

What seems to be missing from Zeno's paradox is the fact that the successively smaller and smaller distances require successively shorter and shorter times to travel over. If the speed is constant then the time taken is the same whether you divide the total distance by the speed or do it the hard way by summing thsee smaller and smaller times. So it isn't the maths that disagrees with reality. What's wrong is the way that people interpret what the maths is telling them.

 

[sophiecentaur]

i do not want to know what MODELS reality, i want to know what IS reality...basically i want to know what math mirrors and PERFECTLY describes reality.

That is asking too much, I think. All we can expect is to produce models that are closer and closer to 'reality'. By closer to reality, I mean to be able to predict things with better and better accuracy.
I could be wrong. One day I could wake up, having recently died, and find some geezer in a long white beard telling me the exact answer to everything - but I won't hold my breath.

There are other views about the purpose and meaning of Science, of course but they haven't yet been proven, any more than my view.

 

[SteveL27]

That is asking too much, I think. All we can expect is to produce models that are closer and closer to 'reality'. By closer to reality, I mean to be able to predict things with better and better accuracy.
I could be wrong. One day I could wake up, having recently died, and find some geezer in a long white beard telling me the exact answer to everything - but I won't hold my breath.

There are other views about the purpose and meaning of Science, of course but they haven't yet been proven, any more than my view.

If science is measurement; and if all measurement is approximate; then science must always be approximate.

The question of whether there even is anything that counts as "ultimate reality" is an unknowable mystery.

I often think that if we discovered an equation that would fit on a t-shirt that explains everything there is to know about the working of the universe ... that would tell us more about ourselves than it does about the universe.

The universe is not an equation.

Ok enough philosophy for one day. I'm off to San Francisco. But first I have to go halfway there ...

 

[sophiecentaur]

Yes. Ultimate Reality is a naive goal because it needs, yet, to be defined.

 

[Studiot]

That is asking too much, I think

Exactly

You have to study something as it is not as you want it to be.

 

[Dembadon]

Without getting too philosophical, I don't accept the premise that mathematics has the ability to be "wrong." It can be used/applied incorrectly, but to suggest that it can be wrong is analogous to assigning blame to a tool for being used improperly, rather than the person who used the tool.

 

[Fredrik]

The axioms that define the real numbers were inspired by human intuition about positions along a straight line. However, in modern mathematics, an "axiom" isn't "something that's so obvious that it doesn't need to be proved". (This is how my high school teacher defined the word "axiom", but it's completely incorrect). It's just a statement that's a part of a definition. A definition simply associates an English word or a phrase with a set that does satisfy the axioms. So once we have defined the real numbers, it's impossible for theorems about real numbers to be objectively wrong. The theorems will hold for what the definition calls "real numbers". (This would be the members of a set that satisfies the axioms that define "the set of real numbers").

i do not want to know what MODELS reality, i want to know what IS reality...basically i want to know what math mirrors and PERFECTLY describes reality.

We all do, but we will never know this. There's no method we can use to obtain that information, and even if we already had it, it would be impossible to prove that what we have is a perfect description of reality.

 

[Hootenanny]

Without getting too philosophical, I don't accept the premise that mathematics has the ability to be "wrong." It can be used/applied incorrectly, but to suggest that it can be wrong is analogous to assigning blame to a tool for being used improperly, rather than the person who used the tool.

I disagree here. It is possible for mathematics to be inconsistent, in which case, I would consider it to be "wrong".

 

[D H]

Without getting too philosophical, I don't accept the premise that mathematics has the ability to be "wrong." It can be used/applied incorrectly, but to suggest that it can be wrong is analogous to assigning blame to a tool for being used improperly, rather than the person who used the tool.

I disagree here. It is possible for mathematics to be inconsistent, in which case, I would consider it to be "wrong".

What's worse, we don't know if the continuum (the reals) are inconsistent or incomplete. Gödel's incompleteness theorems kinda get in the way.

 

[Studiot]

we don't know if the continuum (the reals) are inconsistent or incomplete

This sounds interesting, you are presumably talking about something other than Cauchy sequences here.

Would you care to elaborate?

 

[D H]

What's worse, we don't know if the continuum (the reals) are inconsistent or incomplete.

This sounds interesting, you are presumably talking about something other than Cauchy sequences here.

Would you care to elaborate?

I already did. You left out the elaboration, which was the very next sentence in the post you quoted: "Gödel's incompleteness theorems kinda get in the way."

 

[pwsnafu]

I already did. You left out the elaboration, which was the very next sentence in the post you quoted: "Gödel's incompleteness theorems kinda get in the way."

There's nothing stopping you moving outside the reals to determine if the reals are inconsistent.

 

[Studiot]

I already did

I know you mentioned Goedel.

However I thought that Cauchy showed the real line to be complete so I would be very interested to see G's theorem used to refute this.

 

[Dembadon]

I disagree here. It is possible for mathematics to be inconsistent, in which case, I would consider it to be "wrong".

I probably lack the mathematical maturity it would take to understand an example, but just in case I'm able to figure it out, could you indulge me and give me an example anyways?

The reason I ask is because, logically, it is possible for an argument to be valid even though the premise(s) is/are inconsistent.

 

[bp_psy]

I know you mentioned Goedel.

However I thought that Cauchy showed the real line to be complete so I would be very interested to see G's theorem used to refute this.

Those are two completely different concepts one deals with the logical foundation of mathematics the other with the existence of gaps in the reals.

 

[HallsofIvy]

Saying that "the reals may be inconsistent" is misleading. What is meant, is not the real line but the axioms defining the real number system. It is a set of axioms, not a set of points or numbers, that is "consistent" or "inconsistent". What Goedel showed was that any set of axioms, large enough to encompass the natural numbers (so this includes any set of axioms large enough to encompass the real numbers, which include the natural numbers) is either inconsistent or incomplete.

"Inconsistent" means that it would be possible to prove both a theorem and its negation. Which would, of course, make a "proof" of anything meaningless.

"Incomplete" means there exist some theorem that can be stated in terms of the axioms but cannot be either proved or disproved in those axioms. (We could, of course, add that theorem as an axiom, but then there would be some other theorem that could neither be proved nor disproved.)

Yes, it it were shown that the axioms for the real numbers (or even just the natural numbers) were inconsistent, that would be a disaster for mathematics. However, that has never been proven and most mathematicians (I would think anyone who thought doing mathematics was worthwhile!) believes that the axioms are consistent and so must be incomplete- which is no big deal. It only means that there will always be more mathematics to discover.

 

[Studiot]

Thank you for that clarification, HOI.

So, if I understand you correctly, the comment about the reals is about axioms, not the real line itself.

In this thread (I think) we are discussing the connection between the real number line itself and spatial reality not the axioms or definitions of either so if you are able to further clarify the relevence that would be grand.

 

[Fredrik]

I probably lack the mathematical maturity it would take to understand an example, but just in case I'm able to figure it out, could you indulge me and give me an example anyways?

The reason I ask is because, logically, it is possible for an argument to be valid even though the premise(s) is/are inconsistent.

I think anyone who has enough mathematical maturity to have heard the words "mathematical maturity" will understand this example:

Definition: A set X is said to be stupid if
(1) X={0,1}.
(2) X={1}.

The mathematics of stupid sets can be said to be "wrong", because the set of axioms that defines the term (the statements (1) and (2)) is inconsistent (i.e. the axioms contradict each other). Let's prove a theorem in the theory of stupid sets:

Theorem: Every stupid set contains the number 5.

Proof: Let X be an arbitrary stupid set. We will prove that 5\in X by deriving a contradiction from the assumption that 5\notin X. So suppose that 5\notin X. Since X is stupid, \{1\}=X=\{0,1\}, but this contradicts \{1\}\neq\{0,1\}.

Since we didn't have to use the assumption 5\notin X, the method we used to prove this can be used to prove any statement. Let P be an arbitrary statement.

Theorem: P

Proof: Suppose that P is false. Let X be an arbitrary stupid set. Then \{1\}=X=\{0,1\}. Since this contradicts \{1\}\neq\{0,1\}, P must be true.

So in the theory of stupid sets (and in all other inconsistent theories), every statement is true.

 

[D H]

I probably lack the mathematical maturity it would take to understand an example, but just in case I'm able to figure it out, could you indulge me and give me an example anyways?

The reason I ask is because, logically, it is possible for an argument to be valid even though the premise(s) is/are inconsistent.

So in the theory of stupid sets (and in all other inconsistent theories), every statement is true.

As is every statement's contradiction.

Dembadon, there are some mathematicians who intentionally play with inconsistent mathematics (e.g., http://plato.stanford.edu/entries/mathematics-inconsistent/, http://books.google.com/books?id=pipfBn8mCTwC). The only way forward with such systems is to throw out reductio ad absurdum, or proof by contradiction. This is an extremely powerful tool that most mathematicians are loath to surrender. That is why Halls in an earlier post said that most mathematicians deal with Gödel's incompleteness theorems by believing "that the axioms [of the reals] are consistent and so must be incomplete".

 

[Dembadon]

Thank you for the example, Fredrik, and the explanation, D H.

 

[lugita15]

What's worse, we don't know if the continuum (the reals) are inconsistent or incomplete. Gödel's incompleteness theorems kinda get in the way.

Actually the first order theory of real closed fields is complete and provably consistent, as shown by Tarski, because it is too weak for Godel's Theorem to apply. You can only talk about the natural numbers, and thus make the system susceptible to Godel, if you use the second order theory of real numbers.

 

[Studiot]

Actually the first order theory of real closed fields is complete and provably consistent, as shown by Tarski, because it is too weak for Godel's Theorem to apply. You can only talk about the natural numbers, and thus make the system susceptible to Godel, if you use the second order theory of real numbers.

Please explain further.

This philosophy of maths / very pure maths is beyond my normal field, but interesting.

 

[lugita15]

Please explain further.

This philosophy of maths / very pure maths is beyond my normal field, but interesting.

OK, a first order theory is a theory that only allows quantification over individuals, not quantification over sets. So in the first order theory of real numbers, you can say "for all real numbers x, ...", but you can't say "for all sets F of real numbers, ..." (for that you'd need the second-order theory).

Now the standard first-order theory of natural numbers is called Peano Arithmetic (PA), and Godel's theorem applies to any theory which can prove all the facts about natural numbers that PA can prove. But it turns out that in the theory of real closed fields (RCF), which is the standard first-order theory of real numbers, we can't even talk about natural numbers! The axioms of RCF are all the axioms for fields, plus one additional axiom which is stated http://en.wikipedia.org/wiki/Real_closed_field#Definitions". You may be thinking, if the field axioms define both the multiplicative identity (1) and addition, that would be enough to talk about natural numbers; you could just say that natural numbers are 1, 1+1, 1+1+1, and so on. The problem is, what does "and so on" mean? It's not precise enough for a formal theory.

Here's a proper definition of natural numbers. A set F of real numbers is called hereditary if it is closed under the successor operation; in other words, x+1 \in F whenever x \in F. Then we can say that a natural number is a real number which belongs to all hereditary sets containing 1. If you stare at that long enough, you'll find that it works: 1 is a natural number because it belongs to all sets containing 1, so in particular it belongs to all hereditary sets containing 1. 2 is a natural number because it is 1+1, so any hereditary set that contains 1 automatically contains 2. Etc. Since this definition used the phrase "ALL hereditary sets", any statement about natural numbers is necessarily second-order in the theory of real numbers.

Because of all this, Godel's theorem doesn't apply to RCL, so Tarski was able to show that RCL is complete, consistent, and even decidable (meaning you can write an algorithm which decides whether any given first-order statement about real numbers is true or false, something you can never dream of doing with the natural numbers).

 

[SteveL27]

Because of all this, Godel's theorem doesn't apply to RCL, so Tarski was able to show that RCL is complete, consistent, and even decidable (meaning you can write an algorithm which decides whether any given first-order statement about real numbers is true or false, something you can never dream of doing with the natural numbers).

Thank you for that explanation. Curious about one point: You are saying that first order statements are decidable. Are second-order statements decidable too? Or not? For example the least upper bound property, which quantifies over sets (for all nonempty sets bounded above etc.) is a second order property. Does this mean there are undecidable second-order statements about real closed fields?

 

[D H]

Here's a proper definition of natural numbers. A set F of real numbers is called hereditary if it is closed under the successor operation; in other words, x+1 \in F whenever x \in F. Then we can say that a natural number is a real number which belongs to all hereditary sets containing 1.

Hmmm. The set of all hereditary set that contain 1 looks to me to be more like \mathbb Z rather than \mathbb N. Isn't \mathbb N (or a set polymorphic to it) the hereditary set that contains 1 but contains no element whose successor is 1?

Regarding the reals, isn't the concept of a supremum second order?

 

[lugita15]

Thank you for that explanation. Curious about one point: You are saying that first order statements are decidable. Are second-order statements decidable too? Or not? For example the least upper bound property, which quantifies over sets (for all nonempty sets bounded above etc.) is a second order property. Does this mean there are undecidable second-order statements about real closed fields?

Yes, any second-order theory of real numbers have undecidable statements. The axioms of the standard second-order theory of real numbers are the axioms for fields along with the least-upper-bound axiom, which states that every nonempty set which has an upper bound has a least upper bound (as you mentioned, this is a second-order statement). Then you can define the natural numbers using a second-order statement about real numbers, and in fact you can prove (it's a pretty simple exercise) all 5 of Peano's axioms, including the full second-order principle of mathematical induction, which begins with "for all of natural numbers ...".

So the second-order theory of real numbers contains within it all of second-order Peano arithmetic, which means that Godel's theorem definitely applies.

 

[lugita15]

Hmmm. The set of all hereditary set that contain 1 looks to me to be more like \mathbb Z rather than \mathbb N. Isn't \mathbb N (or a set polymorphic to it) the hereditary set that contains 1 but contains no element whose successor is 1?

Regarding the reals, isn't the concept of a supremum second order?

\mathbb Z is definitely not the intersection of all the hereditary sets containing 1; \mathbb N is a hereditary set containing 1, but it doesn't contain the rest of the integers. And the interval \left[1,\infty\right) is a hereditary set containing 1, so \mathbb N is not the only hereditary set containing 1.

Yes, the definition of supremum is second-order, which is why we can't use the least-upper-bound axiom for the first-order theory. Instead, we have to use a more complicated axiom which yields the same consequences as LUB, at least for first-order statements. For instance, we can take the intermediate value theorem for all polynomials as our axiom. See http://en.wikipedia.org/wiki/Real_closed_field#Definitions" for other equivalent axioms.

(The fact that you're not actually using LUB comes with a cost: even the algebraic numbers satisfy the first-order theory. This means that somewhere deep in the proofs that pi and e are transcendental, we are implicitly using a second-order consequence of the least upper-bound property. I wonder what that is ...)