borisburke: A crude analogy:
I'm 6ft away from my door when I hear a knock. I move towards the door following this simple rule: Each step takes one second and covers half of the remaining distance. EG: After one second, I'm 3ft from the door. After two seconds, I'm 1.5ft from the door etc. How long will it take for me to reach the door?
Of course, I will never reach the door. Not after a trillion years. Never. No matter how many steps I take, there will always be half of the last step remaining.
In practice, one step takes not one second, but rather a number of seconds proportional to the distance moved. For example, maybe you move at one foot per second.
Then, the number of seconds it takes to take n steps will not increase without bound, and after 6 seconds you'll have taken the infinite amount of steps required to reach the door.
borisburke: 0.9 is 0.1 less than 1.
0.99 is 0.01 less than 1.
And so on. Use n number of nines, the difference is always n-1 zeros followed by a 1.
Let n be infinite. Then, after n (infinite) nines, the difference is an infinite number of zeros followed by a 1. Thing is, if we have a number like this, then every approximation of the number, no matter how precise, is 0, as you're never going to see the 1 at the end; therefore, that 1 doesn't come up. It's smaller than every positive number, so it has to be zero or negative (and negative can be ruled out).
By the way, if you *really* want that 1 to mean something, there are ways to describe that in mathematics:
* Transfinite ordinals. ω represents ordinal infinity, so ω + 1 represents the ordinality of an infinite sequence followed by 1 more element. (Note that this is different from transfinite cardinals, which are different.) In other words, this would like "follow this infinite path to the end, and once you reach it, turn left.
* For dealing with infinitessimals as actual mathematical objects, there's nonstandard analysis.
borisburke: In practical terms, 0.9r is as close to 1 as makes no difference. But in absolute logical terms, where 1 = true, and not 1 = false. Then 1/3*3=false.
Give me a positive real number that is smaller than the absolute difference between .9 repeating and 1.