Author Archives: Andrei Derevianko

Why is the atomic many-body problem so difficult?

illustration of an atomOne could rightfully state that the atomic-structure problem has been around for a very long time. Yes, this is true - in fact quantum mechanics has been invented to explain atomic properties. Then why do we still struggle to solve it?

Should we be embarrassed by our inability to solve this basic problem? Sure we can solve it approximately, but solving it accurately is another story.

So what is holding us back? It is the very same entanglement and complexity of Hilbert space (that is where wave functions live) that makes quantum computing so powerful. To illustrate this enormous complexity, I'll take my favorite atom, cesium. It has 55 electrons. With 3 degrees of freedom per electron (x, y, and z), the Cs wave function depends on 55 \times 3 = 155 coordinates. As a result of the calculation I would need to store the wave function. If I  were to take a very poor grid of 10 points per coordinate, the storage would require $latex 10^{155}$ memory units.

$latex 10^{155}$ is of course an astronomically large number - in fact it exceed an estimated number of atoms in the Universe, 10^{80}. So even if we were able to compute the Cs wave function, there is no plausible way to store it in usable form.

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Stepping stones or the value of theory

photo of stepping stones across a lake

"Way In The Lake" by Evgeni Dinev

When invited to present colloquia talks, I usually talk about atomic clocks and I am sometimes asked what a theorist is doing in this highly experimental field. I was thinking about this question for a while and then I was on a plane browsing through an in-flight catalog (you know the one that advertises stuff that you may want but really do not need) and I realized how to answer this question.

There was an ad for a beautiful wristwatch accompanied by a story on the fine craft  of clockmaking. The story was comparing making a wristwatch to building a skyscraper out of matchsticks. So I thought, by that account, experimentalists working on atomic clocks are performing miracles. And if these  experimentalists are walking on water, my role as a theorist is to show them where the stepping stones are.

Related posts:

  1. When would an unanticipated “new physics” event be apparent to an unsuspecting experimentalist?
  2. The world's time

Pauli's letter or the importance of technical details

This is one of my favorite illustrations of the importance of technical details. Below I am posting a figure from a paper by Peter Woit, where the Pauli's letter is displayed.

In 1958, Wolfgang Pauli sent letters to some fellow physicists, making fun of Werner Heisenberg's exaggerated claims to have developed a successful unified
theory. A copy of his letter to J. Robert Oppenheimer is shown here.

A letter of Pauli to Oppenheimer

The letter says: Comment on Heisenberg Radio advertisement.
This is to show the world that I can paint like Titian. [Empty frame with jagged sides]. Only technical details are missing.

Here is  a link to a similar letter to Gamov.

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New group site

Welcome to our new site. Our ten-year-old webpage at UNR.edu has reached the end of its natural life cycle and we decided to move to a modern self-hosted WordPress platform. Since WordPress is not supported by the ever vigilant UNR system administrators, we opened a creatively-named domain dereviankogroup.com hosted by bluehost.com.

The site includes blogging section, where we plan on posting semi-regular updates on our research and various physics-related musings.

We would appreciate if you let us know how fast the site loads for you; this will help us identifying any performance issues with our webhost.

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