In 1655 the English mathematician John Wallis published a book in which he derived a formula for pi as the product of an infinite series of ratios. Now researchers from the University of Rochester, in a surprise discovery, have found the same formula in quantum mechanical calculations of the energy levels of a hydrogen atom.

"We weren't looking for the Wallis formula for pi. It just fell into our laps," said Carl Hagen, a particle physicist at the University of Rochester. Having noticed an intriguing trend in the solutions to a problem set he had developed for students in a class on quantum mechanics, Hagen recruited mathematician Tamar Friedmann and they realized this trend was in fact a manifestation of the Wallis formula for pi.

"It was a complete surprise - I jumped up and down when we got the Wallis formula out of equations for the hydrogen atom," said Friedmann. "The special thing is that it brings out a beautiful connection between physics and math. I find it fascinating that a purely mathematical formula from the 17th century characterizes a physical system that was discovered 300 years later."

The researchers report their findings in the *Journal of Mathematical Physics*.

In quantum mechanics, a technique called the variational approach can be used to approximate the energy states of quantum systems, like molecules, that can't be solved exactly. Hagen was teaching the technique to his students when he decided to apply it to a real-world object: the hydrogen atom. The hydrogen atom is actually one of the rare quantum mechanical systems whose energy levels can be solved exactly, but by applying the variational approach and then comparing the result to the exact solution, students could calculate the error in the approximation.

When Hagen started solving the problem himself, he immediately noticed a trend. The error of the variational approach was about 15 percent for the ground state of hydrogen, 10 percent for the first excited state, and kept getting smaller as the excited states grew larger. This was unusual, since the variational approach normally only gives good approximations for the lowest energy levels.

Hagen recruited Friedmann to take a look at what would happen with increasing energy. They found that the limit of the variational solution approaches the model of hydrogen developed by physicist Niels Bohr in the early 20th century, which depicts the orbits of the electron as perfectly circular. This would be expected from Bohr's correspondence principle, which states that for large radius orbits, the behavior of quantum systems can be described by classical physics.

"At the lower energy orbits, the path of the electron is fuzzy and spread out," Hagen explained. "At more excited states, the orbits become more sharply defined and the uncertainty in the radius decreases."

From the formula for the limit of the variational solution as the energy increased, Hagen and Friedmann were able to pull out the Wallis formula for pi.

The theory of quantum mechanics dates back to the early 20th century and the Wallis formula has been around for hundreds of years, but the connection between the two had remained hidden until now.

"Nature had kept this secret for the last 80 years," Friedmann said. "I'm glad we revealed it."

**Explore further:**
Good quantum states and bad quantum states

**More information:**
"Quantum mechanical derivation of the Wallis formula for pi," by Tamar Friedmann and C.R. Hagen, *Journal of Mathematical Physics* , November 10, 2015. DOI: 10.1063/1.4930800 , http://scitation.aip.org/content/aip/journal/jmp/56/11/10.1063/1.4930800

## OdinsAcolyte

For all the talk about godlessness all humans personify the mysterious.

See?

## Jayded

## Jim4321

## Osiris1

## ECat

Nov 10, 2015## enaskanenas

## betterexists

Dinosaurs never heard of pie. The Great Missing Recipe!

If you don't believe, just Do Google IMAGE Search for Apple Pie.

e was The Secret between Pi & Pie!

## someone11235813

## animah

Atoms are not spherical in mathematical terms, they are more like clouds or swarms. Because electron trajectories and positions cannot be computed for any given point in time, they can't be said to have clear boundaries (except at absolute zero).

That said they are spherical in layman's terms (because superimposition of electron trajectories over time average out to a round shape), so this mathematical relationship is very intuitive. A nice change for quantum physics :-)

## Steve 200mph Cruiz

A discovery does not mean you create something, it means you notice something that was already there

Ecat,

No it is not spherical, this is what the electron shell looks like around a hydrogen atom:

upload.wikimedia.org/wikipedia/commons/c/cf/HAtomOrbitals.png

The shape of the electron bubble changes dramatically depending on the state and how much energy the electron contains.

Just imagine how complicated it gets in atoms with more electrons which are repelling each other while doing all this as well, and then in molecules where electrons and even entire atoms start getting shared between other atoms and nuclei.

## Job001

This neurological response is called the Hindsight Bias: https://en.wikipe...ght_bias

Very good gentlemen, this advances our modeling of atoms by directly showing the transition from quantum to classical in one small atom. Statistically the Heisenberg uncertainty becomes smaller the more massive or energetic the numbers are. Lovely!

## Jim4321

## del2

## Bulbuzor

Now I thought it was a case closed that electrons do not orbit the atom's nucleus; they just pops here and there in a "bubble" of probability. Is he referring to an "orbit" because he mentioned Bohr or am I missing something? By orbit does he refer to the probability bubble being more "sharply defined" or he says 20th century physics are right?

## Jim4321

Bulbuzor: For large n, the velocity and radius both increase. Hence, the uncertainty principal allows for more and more localized wave packets. With the right combination of wave functions, one can find the Bohr atom electron as a limiting case. Both quantum and classical theory are correct in the this limit.

## Hyperfuzzy

## vlaaing peerd

It's a secret if something is kept intentionally hidden, which it wasn't. So it was rather just unknown.

## big_hairy_jimbo

Final Result =

3.1415926143129

Where as the accepted value is

3.1415926535897

Apparently if you use every second ratio, then you get the square root of 2!!!!

Fascinating!!!!

## animah

Indeed: http://math.stack...equal-pi

:-)

## adam_russell_9615

More specifically, I think Id say it was a connection between physics and geometry.

## FredJose

I always thought geometry was a branch of maths?

Quite clearly, our mathematical discoveries are constraint by the world we live in. No matter what it is that some mathematician discovers in his/her wildest imagineerings, it always comes down to something connected to the whole set of things we are allowed to discover.

It's as if there's a prescribed path of discovery to which we are constrained, no matter what we do or claim to do as creative and original thought..

## jljenkins

If so, it illustrates the effects of population growth. One mind doing the work of 50 is better because the 50 can't directly correlate their insights. Intelligence is becoming diluted. Which would explain many of the legends in their own mind on this site.

## big_hairy_jimbo

That series of square roots of two's looks like it might generate pi more accurately with fewer iterations. I'm going to have to compare the two now :-)))))

## Yves_Moreau

A fun fact? Maybe. But the title "New derivation of pi links quantum physics and pure math" is totally overblown.