In String Theory, one of the most crucial calculations involves summing all the numbers from 1 to infinity, which obviously should be infinite. But not in ST, where the Rieman Zeta Function is used, and gives a value of -1/12??? Yes, if you add all the numbers from 1 to a million, you get a big number; if you add all the numbers from 1 to a trillion, you get an even bigger sum. But should you add all of them to infinity, not only do you get a fraction, but a negative one? How do physicists get it so wrong? Well, they did get it wrong on quantum entanglement (click on, Quantum Entanglement Demystified)so why not on this one!
Here's a short version of a proof (attributed to Euler):
Euler started with this:
(1) 1 + x + x2 + x3 + · · · =1/(1 − x)
He differentiated both sides:
(2) 1 + 2x + 3x2 + · · · =1/(1 − x)2
He set x = −1 and got this:
(3) 1 − 2 + 3 − 4 + · · · =1/4
Then Euler considered this function (now known as the Riemann Zeta function):
(4) ζ(s) = 1−s + 2−s + 3−s + 4−s + · · ·
He multiplied by 2−s:
(5) 2−s ζ(s) = 2−s + 4−s + 6−s + 8−s + · · ·
Then he subtracted twice the second equation from the first:
(6) (1 − 2·2−s) ζ(s) = 1−s − 2−s + 3−s − 4−s + ···
and setting s = −1, he got:
(7) −3(1 + 2 + 3 + 4 + · · · ) = 1 − 2 + 3 − 4 + · · ·
Since he already knew the right-hand side equals 1/4, he
concluded:
(8) 1 + 2 + 3 + 4 + · · · = −1/12
So what is wrong in this deduction? Take equation (3).
(3) 1/4 = 1 − 2 + 3 − 4 + 5 - 6 + 7 - 8 · · ·
We can regroup it as (take the first two, then the next two, etc.):
(3a) 1/4 = − 1 − 1 − 1 − 1 · · ·
Or we can regroup (3) as (take the first and third, second and fourth, etc.):
(3b) 1/4 = 4 − 6 + 8 − 10 + 12 − 14 · · ·
Now regroup (3b) as:
(3c) 1/4 = − 2 − 2 − 2 − 2· · ·
Comparing (3a) and (3c), we get that:
(9) 1/2 = 1/4 ???
We can conclude that Euler's deduction was plain wrong.
In all the texbooks, you will find that equation (1) is true only if |x| < 1 or -1 < x < 1, so by setting s equal to -1 in equation (3), we have made equation (1) divergent.
Now in all String Theory textbooks, we are told that ζ(-1)= -1/12 can be derived vigorously. I'm looking into that and will report on it in a later post.
Stay tuned.
Monday, January 03, 2011
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