# Is the probability zero?

by aquaregia
Tags: probability
 Mentor P: 13,654 Infinity is not a number. Yet another thing every mathematician worth his or her salt knows.
P: 121
 Quote by D H Infinity is not a number. Yet another thing every mathematician worth his or her salt knows.
DH,

I am suprised at you. Whether or not infinity is a number is a matter of definition, from the viewpoint of the field of mathematics.

There are different definitions. It is a matter of preference, or perhaps of convenience.

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When my oldest daughter Charlotte was six, she was scheduled to take an entrance exam to get into second grade in a private school.

I had stong suspicions that she would be asked what was the largest number she knew.

With mischief in my heart, I taught her to answer "infinity."

The time for the test came.

The tester reported to us that when she asked Charlotte what was the biggest number she knew, Charlotte smiled a big confident smile and answered,

"Infinity! And, infinity is equal to ten, because ten is the biggest number I know!"

:)

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"A number is an answer to the question 'how many elements are in that set?' "

DJ
P: 121
 Quote by D H One way to define the real numbers is via infinite series (they are called Cauchy sequences; google that phrase). It is non-mathematicians, not mathematicians, who have a problem with saying $$\sum_{n=1}^{\infty} \frac 1 {2^n} \equiv \lim_{N\to\infty}\sum_{n=1}^N \frac 1 {2^n} = 1$$ Non-mathematicians (and high school math teachers, boo!) are the ones who say $0.999\cdots \ne 1$. The limit of a sequence, if it exists, is a specific number. The limit does not differ from this number by some infinitesimal amount. It is the number. What makes some mathematicians cringe is lack of rigor. One place where this occurs is the non-rigorous use of infinitesimals by engineers and physicists. Mathematicians switched from Newton's infinitesimals to the epsilon-delta notation formalism developed by Weierstrass because this formalism is mathematically sound. Physicists clung to the shorthand infinitesimals because they work (mostly). Mathematicians finally made the concept of infinitesimals rigorous in the 1960s by means of the non-standard analysis (another phrase to google). Anything that is true in standard real and complex analysis is true in the non-standard analyses. In particular $$\left(\sum_{n=1}^{\infty}\frac 1 {2^n}\right)-1=0$$ whether one is using standard or non-standard analysis.
DH, That is a beautiful explanation. I'm really impressed.
P: 121
 Quote by D H While philosophers might be vexed by Zeno's paradox, mathematicians most definitely are not because any mathematician worth their salt knows that $$\frac 1 2 + \frac 1 4 + \cdots = \sum_{n=1}^{\infty}\frac 1{2^n} = 1$$
Right on, DH!

Yeah, Zeno's paradox is difficult for modern mathematicians to understand. I will try to explain the paradox and try to explain while it is difficult for us moderns to understand at the same time. I will do this by giving a modern formulation of the paradox that shows where Zeno thought differently from the way we do.

Suppose a hare is chasing a rabbit and the hare goes twice as fast as the rabit.

The hare starts out 120 miles behind the rabbit and the hare runs at 60 miles an hour. The rabbit runs at 30 miles an hour. The race is track is four miles long.

After one hour, the hare is one half mile behind the rabbit. After another half hour the hare is 1/4 mile behind the rabbit.

The nth time period has length one hour / 2^n.

As each time period passes, the hare halves his distance to the rabbit.

To actually catch the rabbit, the hare would have to run for as long as it takes to transverse an infinite number of these intervals.

But, in the real world, infinity does not exist.

Hence the hare never catches the rabbit.

But we all know that the hare passes the rabbit when they have both run for one hour, and after two hours, the hare crosses the finish line.

This material is quasi-original with me, so, it may well stand improvement. Please let me know if it does.

There are too many dirrerent ways to explain this in modern terms to make a start on it. But, I think what I have written catches the essential idea and illustrates the different viewpoints of the relationship between mathematics and the real world.

Actually, the viewpoint of mathematicians even two centuries ago was closer to the Greeks.

DJ

"By quasi-original," I mean,

"I claim no credit for being the original proporter of these ideas in case they should happen to be right, but, I accept full responsibility for them if they are wrong."
Mentor
P: 13,654
 Quote by DeaconJohn DH, I am suprised at you. Whether or not infinity is a number is a matter of definition, from the viewpoint of the field of mathematics. There are different definitions. It is a matter of preference, or perhaps of convenience.
You're right; I should have said infinity is not a real number. There are other definitions where infinity is a number -- the extended real number line, for example.

 But, in all seriouness, how about this for a "universal" definition of a number ---- "A number is an answer to the question 'how many elements are in that set?' "
These are the cardinal numbers. What set has half of an element? pi elements? 1-i elements?
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Emeritus
P: 38,428
 Quote by DeaconJohn DH, I am suprised at you. Whether or not infinity is a number is a matter of definition, from the viewpoint of the field of mathematics.
What D H meant to say is that infinity is not a member of the set of a real numbers (or complex numbers for that matter). That is what is normally understood by "numbers" with no other adjectives. As far as the real or complex numbers are concerned, whether or not infinity is a member is NOT a matter of preference or of convenience.

 There are different definitions. It is a matter of preference, or perhaps of convenience. ---------------------------------------------------------------- When my oldest daughter Charlotte was six, she was scheduled to take an entrance exam to get into second grade in a private school. I had stong suspicions that she would be asked what was the largest number she knew. With mischief in my heart, I taught her to answer "infinity." The time for the test came. The tester reported to us that when she asked Charlotte what was the biggest number she knew, Charlotte smiled a big confident smile and answered, "Infinity! And, infinity is equal to ten, because ten is the biggest number I know!" :) ------------------------------------------------------------------------- But, in all seriouness, how about this for a "universal" definition of a number ---- "A number is an answer to the question 'how many elements are in that set?' " DJ
P: 121
 Quote by D H You're right; I should have said infinity is not a real number. There are other definitions where infinity is a number -- the extended real number line, for example.

English is not a "context-free" language, and I mis-interperted the the context in which you were making your statment. In other words, it was "my bad," not "your lack," that resulted in my suprise.

 Quote by D H These are the cardinal numbers. What set has half of an element? pi elements? 1-i elements?
Right-on, DH. My suggestion that this be a "universal" definition changed the "context" that required me to specify that I was only talking about "cardinal numbers."

My last paragraph should have read:

"A cardinal number is an answer to the question, 'How many elements are in a set?' "

I really would like to know your opinion on this proposal. For example, this proposal would exclude the cardnalities of classes from the collection of cardinal numbers.

[A Note for Beginners: If I remember correctly, Bertrand Russel introduced "classes" into Zermelo-Frankel (ZF) set theory for the purpose of getting his paradox (Russell's paradox) out of set theory. If I understand correctly, one can consider the "class of all sets" in Russel's extension of ZF. In other words, a "class" is like a "really big set." Nobody cared - (correction - no mathematicians cared) - except those working in mathematical logic - until recent years when category theory began to take over.]

[Another Note for Beginners: The ubiquity of category theory in modern mathematics is a movement kicked off by Alexander Grothendieck's application of category theory to derive his generalization of the Hirzebuch-Riemann-Roch theorem in the 1950's. Category theory was developed (mostly by Grothendieck) in the thousands of pages of tomes called EGA and SGA that stand for the French equivalents of "Exposition of" - and "Seminar in" - "Algebraic Geometry." There were other giants - such as Grothendieck's advisor Jean Dieudonne, the mysterious Jean Paul Serre, and the incredible Pierre Deligne - whose mathematical work played major roles in bringing category theory to the attention of the mathematical community.]

Another example of the kind of thing I am asking about is whether or not the infinities in "non-standard analysis" match up with the infinite cardinal numbers in the usual set theory that mathematicians use. In other words, did Robinson include - or try to include - the cardinalities of classes in his "non-standard analysis."

(I have the impression that there are logical problems that are best avoided by excluding the sizes of classes from the collection of cardinal numbers. I don't remember what they are, except, i have the impression that they are related to Russel's paradox.)

(I'm guessing, here, that the collection of cardinal numbers form a class and not a set. I would like to know if that is correct.)
P: 121
 Quote by DeaconJohn Right on, DH! Yeah, Zeno's paradox is difficult for modern mathematicians to understand. I will try to explain the paradox and try to explain while it is difficult for us moderns to understand at the same time. I will do this by giving a modern formulation of the paradox that shows where Zeno thought differently from the way we do. The paradox is this. .......... This material is quasi-original with me, so, it may well stand improvement. Please let me know if it does. There are too many dirrerent ways to explain this in modern terms to make a start on it. But, I think what I have written catches the essential idea and illustrates the different viewpoints of the relationship between mathematics and the real world. Actually, the viewpoint of mathematicians even two centuries ago was closer to the Greeks. DJ "By quasi-original," I mean, "I claim no credit for being the original proporter of these ideas in case they should happen to be right, but, I accept full responsibility for them if they are wrong."
Sorry, guys, looks like I'm bringing up a subject that has already been wrung dry. And, been "wrung dry" by you guys in the not too distant past. Ah, one of the dangers of being a newbee.

However, I think that the viewpoint that I propose has a different spin than the viewpoints expressed in your previous posts. I am proposing that the crux of Zeno's reasoning (expressed in modern language) is that he didn't believe that an infinite quantities of things really existed in the real world.

Assuming that is what he was thinking, he might have been right of course, in which case one explanation of his paradox is that you can't keep dividing time in half. When you get down to a certain granularity, you take an instantaneous jump accross the atomic length of time.

If I remember correctly, the Greeks did believe that matter was not infinitely divisible, that's what they (e.g., Lucretius?) meant by by "atom," the smallest, indivisible, unit of matter. Granted (a correction to my previous post) they did think that time had no beginning, so, my statement that they did not believe that infinity existed in the real world was a little hasty. But, it does not seem unreasonable to me that they thought that an infinity of intervals of time did not exist.

This seems more reasonable to me than the supposition that they thought that the sum of the geometric series diverged. More accurately, that they thought that it was impossible for an infinite number of terms to have a finite sum. Even though that is "literaly" what Zeno said, I suspect he took his "paradox" as a demonstration that infinite quantities do not exist i nature.

What do you all think? I know that at least some of yon haave thought about it.

Here is a pointer to one of your posts that suggests a different intrepertation than the one that I am purporting.

Yours,

DJ

P.S.

It's important to consider what the ancients thought. They were excellent mathematicians (Archimedes is credited with discovering the essence of the calculus) and they had a different "worldview" than we do. So, from an objective viewpoint (like for example from the viewpoint of Bayesian probability theory), there is a non-zero probabiity that they wer more right than we are in their conception of realiity.
Mentor
P: 13,654
 Quote by DeaconJohn This seems more reasonable to me than the supposition that they thought that the sum of the geometric series diverged. More accurately, that they thought that it was impossible for an infinite number of terms to have a finite sum. Even though that is "literaly" what Zeno said, I suspect he took his "paradox" as a demonstration that infinite quantities do not exist i nature. What do you all think? I know that at least some of yon haave thought about it.
My first thought is that we are getting way off topic here.

My second thought is that Zeno simply thought any series with an infinite number of terms had to diverge, and yet it obviously doesn't. Paradox. Not surprising since he predates the concept of a convergent infinite series by a mere 2100 years or so.
P: 121
 Quote by D H My first thought is that we are getting way off topic here. My second thought is that Zeno simply thought any series with an infinite number of terms had to diverge, and yet it obviously doesn't. Paradox. Not surprising since he predates the concept of a convergent infinite series by a mere 2100 years or so.
Thanks, DH!

I guess you're right about getting a little "off topic." Time to move on!

DJ
 P: 328 The next person to say "1 / infinity = 0" gets a complimentary punch in the face. You might as well say "1 / applesauce = 0" Because applesauce is just as much a number as infinity.
P: 272
 Quote by Archosaur The next person to say "1 / infinity = 0" gets a complimentary punch in the face.
*cough cough*

 Quote by Archosaur You might as well say "1 / applesauce = 0" Because applesauce is just as much a number as infinity.
True, if by "applesauce" you mean "some element of the extended/projective/hyper-real numbers that has greater magnitude than any finite real number."
 P: 328 So here is a similar problem, given an continuous uniform distribution over the interval [0,1], what is the probability that any number picked at random is rational?
 HW Helper Sci Advisor P: 3,682 0, naturally.
 PF Patron Sci Advisor Thanks Emeritus P: 38,428 The point being that 0 probability on a continuous probability distribution does NOT mean "impossible" nor does probability 1 mean "certain to happen".
P: 328
 Quote by quadraphonics True, if by "applesauce" you mean "some element of the extended/projective/hyper-real numbers that has greater magnitude than any finite real number."
A greater magnitude? Infinity doesn't have a magnitude! I do not mean your proposed jargon!

I mean applesauce!!!
P: 328
And don't cough at me.

If 1/infinity = 0 then 1 = 0 * infinity.
If 1/infinity = 0 then 2*1/infinity = 2*0, also known as 2/infinity = 0
If 2/infinity = 0 then 2 = 0 * infinity

If 1 = 0*infinity AND 2 = 0*infinity
Then 1=2

P: 272
 Quote by Archosaur A greater magnitude? Infinity doesn't have a magnitude!
The extended (and projective) real lines both contain elements with a larger magnitude than any finite number, called infinity.

You might also have heard of Transfinite numbers.

 Quote by Archosaur I do not mean your proposed jargon!
It's not my proposal. The extended and projective reals have been around since long before I was born.

 Quote by Archosaur I mean applesauce!!!
Unlike infinity, I have never encountered a number system that includes an element "applesauce."

 Quote by Archosaur And don't cough at me.
No, the expression $0*\infty$ is undefined in both the extended and projective real numbers, although $1/\infty=0$ does indeed hold in both of them.