I found a paper: 'A New Method of Finding the Distribution of Prime Number', saying 



> 
> We stack discs and annuluses with certain rules then turn on the light to illuminate. The projection of annuluses corresponds prime number,and the projection of discs corresponds composite number. 
> 
> 
> 


See: <http://en.cnki.com.cn/Article_en/CJFDTOTAL-HNKX201201010.htm>


Are there other physics methods for number theory?


**Edit:** Answers and comments here and to the [corresponding meta question](https://meta.mathoverflow.net/questions/1749/why-has-the-question-optical-methods-for-number-theory-been-closed) have shown that the answer is yes, and much broader and more recent than the sieve of Eratosthenes. Also, a more informative link to the above paper is
<http://wenku.baidu.com/view/1d602350be23482fb4da4cc6.html?re=view>



Optical methods for number theory?
There's a way to use physics to calculate the digits of $\pi$. I quote from <https://math.stackexchange.com/questions/138289/intuitive-reasoning-behind-pis-appearance-in-bouncing-balls>


Let the mass of two balls be $M$ and $m$ respectively. Assume that $M=16\times100^nm$. Now, we will roll the ball with mass $M$ towards the lighter ball which is near a wall. How many times do the balls touch each other before the larger ball changes direction? (The large ball hits the small ball which bounces off the wall)


The solution is the first $n+1$ digits of $\pi$.


