Ross–Littlewood paradox
Encyclopedia
The Ross–Littlewood paradox (also known as the balls and vase problem or the ping pong ball problem) is a hypothetical problem in abstract mathematics and logic
designed to illustrate the seemingly paradox
ical, or at least non-intuitive
, nature of infinity
. More specifically, like the Thomson's lamp
paradox, the Ross–Littlewood paradox tries to illustrate the conceptual difficulties with the notion of a supertask
, in which an infinite number of tasks are completed sequentially.
The problem starts with an empty vase and an infinite supply of balls. An infinite number of steps are then performed, such that at each step balls are added as well as removed from the vase. The question is then posed: How many balls are in the vase when the task is finished?
To complete an infinite number of steps, it is assumed that the vase is empty at two minutes before noon, and that the following steps are performed:
This guarantees that a countably infinite
number of steps is performed by noon. Since each subsequent step takes half as much time as the previous step, an infinite number of steps is performed by the time two minutes has passed.
At each step, ten balls are added to the vase, and one ball is removed from the vase. The question is then: How many balls are in the vase at noon?
The following procedure outlines exactly how to get a chosen n number of balls remaining in the vase:
Let i denote the number of the operation currently taking place.
Let n denote the desired final number of balls in the vase.
Then:
Clearly, the first n odd balls are not removed, while all balls greater than or equal to 2n are. Therefore, exactly n balls remain in the vase.
.
, if infinitely many operations have to take place (sequentially) before noon, then noon is a point in time that can never be reached. On the other hand, to ask how many balls will be left at noon is to assume that noon will be reached. Hence there is a contradiction implicit in the very statement of the problem, and this contradiction is the assumption that one can somehow 'complete' an infinite number of steps. This is the solution favored by mathematician and philosopher Jean Paul Van Bendegem.
. A sampling of some of these discussions:
Mathematical logic
Mathematical logic is a subfield of mathematics with close connections to foundations of mathematics, theoretical computer science and philosophical logic. The field includes both the mathematical study of logic and the applications of formal logic to other areas of mathematics...
designed to illustrate the seemingly paradox
Paradox
Similar to Circular reasoning, A paradox is a seemingly true statement or group of statements that lead to a contradiction or a situation which seems to defy logic or intuition...
ical, or at least non-intuitive
Intuition (knowledge)
Intuition is the ability to acquire knowledge without inference or the use of reason. "The word 'intuition' comes from the Latin word 'intueri', which is often roughly translated as meaning 'to look inside'’ or 'to contemplate'." Intuition provides us with beliefs that we cannot necessarily justify...
, nature of infinity
Infinity
Infinity is a concept in many fields, most predominantly mathematics and physics, that refers to a quantity without bound or end. People have developed various ideas throughout history about the nature of infinity...
. More specifically, like the Thomson's lamp
Thomson's lamp
Thomson's lamp is a puzzle that is a variation on Zeno's paradoxes. It was devised by philosopher James F. Thomson, who also coined the term supertask....
paradox, the Ross–Littlewood paradox tries to illustrate the conceptual difficulties with the notion of a supertask
Supertask
In philosophy, a supertask is a quantifiably infinite number of operations that occur sequentially within a finite interval of time. Supertasks are called "hypertasks" when the number of operations becomes innumerably infinite. The term supertask was coined by the philosopher James F...
, in which an infinite number of tasks are completed sequentially.
The problem starts with an empty vase and an infinite supply of balls. An infinite number of steps are then performed, such that at each step balls are added as well as removed from the vase. The question is then posed: How many balls are in the vase when the task is finished?
To complete an infinite number of steps, it is assumed that the vase is empty at two minutes before noon, and that the following steps are performed:
- The first step is performed at one minute before noon.
- The second step is performed at 30 seconds before noon.
- The third step is performed at 15 seconds before noon.
- Each subsequent step n is performed at 2 minutes before noon.
This guarantees that a countably infinite
Countable set
In mathematics, a countable set is a set with the same cardinality as some subset of the set of natural numbers. A set that is not countable is called uncountable. The term was originated by Georg Cantor...
number of steps is performed by noon. Since each subsequent step takes half as much time as the previous step, an infinite number of steps is performed by the time two minutes has passed.
At each step, ten balls are added to the vase, and one ball is removed from the vase. The question is then: How many balls are in the vase at noon?
Vase contains infinitely many balls
The most intuitive answer seems to be that the vase contains an infinite number of balls by noon, since at every step along the way more balls are being added than removed. This is the solution favored by mathematician John Byl.Vase is empty
Suppose that the balls of the infinite supply of balls were numbered, and that at step 1 balls 1 through 10 are inserted into the vase, and ball number 1 is then removed. At step 2, balls 11 through 20 are inserted, and ball 2 is then removed. This means that by noon, every ball labeled n that is inserted into the vase is eventually removed in a subsequent step (namely, at step n). Hence, the vase is empty at noon. This is the solution favored by mathematicians Allis and Koetsier. It is the juxtaposition of this argument that the vase is empty at noon, together with the more intuitive answer that the vase should have infinitely many balls, that has warranted this problem to be named the Ross–Littlewood paradox.Depends on the conditions
The number of balls that one ends up with depends on the order in which the balls are removed from the vase. As stated previously, the balls can be added and removed in such a way that no balls will be left in the vase at noon. However, if ball number 10 were removed from the vase at step 1, ball number 20 at step 2, and so forth, then it is clear that there will be an infinite number of balls left in the vase at noon. In fact, depending on which ball is removed at the various steps, any chosen number of balls can be placed in the vase by noon, as the procedure below demonstrates. This is the solution favored by philosopher logician Tom Tymoczko and mathematician logician Jim Henle.The following procedure outlines exactly how to get a chosen n number of balls remaining in the vase:
Let i denote the number of the operation currently taking place.
Let n denote the desired final number of balls in the vase.
Then:
- for i = 1 to infinity,
- if i ≤ n, then remove the ball with number 2i
- if i > n, then remove the ball numbered n+i
Clearly, the first n odd balls are not removed, while all balls greater than or equal to 2n are. Therefore, exactly n balls remain in the vase.
Problem is underspecified
Although the state of the balls and the vase is well-defined at every moment in time prior to noon, no conclusion can be made about any moment in time at or after noon. Thus, for all we know, at noon, the vase just magically disappears, or something else happens to it. But we don't know, as the problem statement says nothing about this. Hence, like the previous solution, this solution states that the problem is underspecified, but in a different way than the previous solution. This solution is favored by philosopher of mathematics Paul BenacerrafPaul Benacerraf
Paul Joseph Salomon Benacerraf is an American philosopher working in the field of the philosophy of mathematics who has been teaching at Princeton University since he joined the faculty in 1960. He was appointed Stuart Professor of Philosophy in 1974, and recently retired as the James S....
.
Problem is ill-formed
The problem is ill-posed. To be precise, according to the problem statement, an infinite number of operations will be performed before noon, and then asks about the state of affairs at noon. But, as in Zeno's paradoxesZeno's paradoxes
Zeno's paradoxes are a set of problems generally thought to have been devised by Greek philosopher Zeno of Elea to support Parmenides's doctrine that "all is one" and that, contrary to the evidence of our senses, the belief in plurality and change is mistaken, and in particular that motion is...
, if infinitely many operations have to take place (sequentially) before noon, then noon is a point in time that can never be reached. On the other hand, to ask how many balls will be left at noon is to assume that noon will be reached. Hence there is a contradiction implicit in the very statement of the problem, and this contradiction is the assumption that one can somehow 'complete' an infinite number of steps. This is the solution favored by mathematician and philosopher Jean Paul Van Bendegem.
External links
This problem and its variations appear frequently on the sci.math newsgroupNewsgroup
A usenet newsgroup is a repository usually within the Usenet system, for messages posted from many users in different locations. The term may be confusing to some, because it is usually a discussion group. Newsgroups are technically distinct from, but functionally similar to, discussion forums on...
. A sampling of some of these discussions: