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High-Z Helium: Is QED Failing?

by John G. Cramer

Alternate View Column AV-167
Keywords: two-electron, atoms, high-Z, helium, QED, quantum, electrodynamics
Published in the May-2013 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 12/4/2012 and is copyrighted ©2012 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

 

Physics is an experimental science.  The intricate dance between physics theory and experiment proceeds with well defined dance steps: a set of experimental observations suggests a theory, which is usually formulated mathematically; the theory makes predictions for the results of new experiments that have not yet been performed; experimental tests are made and the results compared with the theoretical predictions; the theory is either supported by the experimental tests or falsified and shown to be in need of modification; the falsified theory is either modified and improved to agree with the observations or replaced with a better theory; new experimental tests are made; and the dance goes on ...  This, in a somewhat oversimplified form, is the "scientific method",  the way that science progresses.

In general, the method works very well, but scientific progress can be inhibited in certain areas because the theories are too good.  The Standard Model of particles and fields, which includes quantum electrodynamics (QED) to describe electromagnetic phenomena at the quantum level and quantum chromodynamics (QCD) to describe strong-interaction phenomena at the quantum level, works altogether too well.  The Standard Model is in good agreement with essentially all of the data collected by physics experiments during the past several decades.  The recent discovery of the Higgs boson (see my column in the December-2012 issue of Analog) represents a great triumph for the Standard Model, because it had predicted the existence of the Higgs decades earlier.

Yet, in another way, the Standard Model is an unsatisfactory and temporary theory.  It fails to provide any deep understanding of the inner workings of the universe.  Instead, it depends on the ad hoc values of about two dozen arbitrary parameters, for example the masses of quarks, leptons, and bosons, the strengths of the fundamental forces, and the strengths of interconnections between particles. Some of these parameters are set to zero, while others have definite values that must be derived from measurement. We have no very good ideas about where these parameters come from, how they were set, or how they are related to each other.  We are confident that there must be some better, more fundamental theory that lies behind the Standard Model's façade. Up to now, however, we have been unable to make the Standard Model "break", to find places where its predictions conspicuously fail, to find a crack in the solid and unblemished wall of theory that might provide some inkling of what lies behind it.  Until such a break occurs, we are stuck with the Standard Model and its imperfections.


This situation may have just changed, and changed in an unexpected regime.  Quantum electrodynamics, which in many ways is the most trusted part of the Standard Model, may be showing signs of a crack.  Experimentalists at the National Institutes of Science and Technology in Gaithersburg , Maryland have been pursuing an ongoing series of experiments to test QED using their electron-beam ion trapping (EBIT) facility.  EBIT technology was first developed at the Lawrence Livermore Laboratory in about 1988 and has been duplicated and improved at several other laboratories including NIST.  It provides a way of simultaneously trapping some selected highly ionized atoms in an electromagnetic field in a very high vacuum while bombarding them with a beam of accelerated electrons.  The trap consists of a 3 tesla magnetic field limiting the trapped ion’s transverse motion and a set of electrically charged cylindrical tubes limiting the trapped ion’s longitudinal motion.  Highly charged ionized atoms finding themselves in this combination of crossed fields have nowhere to escape and are repeatedly struck by a beam of fast electrons, producing interesting and otherwise unreachable states of atomic excitation.  The trap can be set to trap ionized atoms having a particular charge-to-mass ratio, while rejecting all other atoms, so that specific atomic systems can be studied.

To understand the significance of the EBIT technology as applied to the recent measurements, let's start by reviewing a bit of atomic physics.  A neutral atom of atomic number Z consists of a heavy nucleus with a positive electric charge +Ze orbited by Z light-weight electrons, each with one unit -e of negative electric charge.  It is rather like a tiny electric solar system with a massive central star orbited by many light-weight planets.

The simplest atom in the periodic table of elements is hydrogen, with a nucleus (a proton) of charge +e orbited by a single electron of charge -e.  If this hydrogen atom is "kicked" to an excited state, for example by collision with a fast electron, its orbiting electron will find itself in a more loosely bound orbit that is farther away from the nucleus.  It will then undergo one or more "quantum jumps" to smaller and more tightly bound orbits, releasing excess energy along the way by emitting one photon of light with each jump, until it reaches the atom's "ground state", the lowest allowed orbit around the hydrogen nucleus.  It remains in that state until another collision occurs.

Helium is the second simplest atom in the periodic table of elements.  It has a nuclear charge of +2e, and is orbited by two electrons.  That sounds like only a modest change from hydrogen, but in the early days of atomic physics it was a show-stopper.  The first fairly realistic atomic model, developed by Neils Bohr, could accurately predict the observed "spectrum" (the wavelengths of emitted photons) observed when hydrogen was excited, but it utterly failed to predict the spectrum for helium.  The helium spectrum has a number of visible-light photon "lines" spanning the red to violet, including a bright yellow one, with a few electron-volts of energy that are produced by transitions among the higher excited states, along with some higher energy ultraviolet photons at around 39 electron volts that are produced by transitions to the ground state of helium.  Accurately predicting the energies of these photons proved impossible with the Bohr model, and had to wait for the full-blown development of multi-body quantum mechanics and QED.  The electromagnetic interactions between the orbiting electrons, as well as their electromagnetic interactions with the nucleus, had to be accurately described.  It was necessary to solve the quantum three-body problem accurately in order to account for all of the interactions between the helium nucleus and its two orbiting electrons.

The EBIT technology offers a new way of studying the spectra of modified helium atoms, as a test of three-body QED predictions.  In particular, one can change the nuclear charge Z from 2 to some larger value and see that happens.  What is expected to happen is that the observed spectral lines of helium should systematically be shifted to smaller wavelength values by a factor of 4/Z2.  Changing the Z value is done by starting with some heavy atom (neon, argon, titanium, iron, barium, uranium, …), removing all but two of its electrons, selectively trapping it, and bombarding it with electrons while observing the photons it produces during quantum jumps from a "high" orbit to one closer to the nucleus.  The recent work at NIST has focused on high-precision measurements on helium-like titanium, a two-electron atom with a nuclear charge of Z=22.   These results, with rather small uncertainties, were combined with previous data, most of it with somewhat larger uncertainties, on the photons from high-Z helium-like atoms with Z=16, 18, 19, 21, 23, 24, 26, 32, and 36 and compared with the predictions of 3-body QED.

The most energetic photons from electron transitions in helium have energies of around 39 electron volts.  The photon energy scales as Z2, so analogous photons observed in the helium-like atoms with  a nuclear charge of Z=22, should have energies that are (22/2)2=121 times higher.  The most energetic  photons from the helium-like atoms, studied with high precision bent-crystal spectroscopy, have energies around 4,750 electron volts, which is in the soft x-ray region.  The energy vs. Z of the most energetic photons from these studies of helium-like atoms were compared with the predictions of quantum electrodynamics, a part of the Standard Model that, up to now, has had an essentially unblemished record in predicting the results of experimental measurements.  It was found that the data are systematically larger in energy than the 3-body QED predictions by about 0.1 to 0.6 electron-volts, depending on the value of Z.  Further, the deviations in the heavier high-Z helium-like atoms appear to grow as Z3.  The reported discrepancy with QED has a statistical significance of about 5 standard deviations.  Thus, QED, a central and highly trusted  component of the Standard Model, seems to be failing in a very fundamental and consistent way.

Does this mean that new physics is lurking just offstage, and that the Standard Model is wrong in some fundamental way?  Or is this only an indication that the theorists who are applying QED to this system have missed some subtle effect that only appears in high-Z 3-body systems or that is some unexpected artifact of the trapping technique?

It's too early to say, but I hope that this represents a real crack in the Standard Model that will lead us to understand in a deeper way just how the universe works.  QED has been considered the least likely place for the Standard Model to fail, and many of the QED techniques and assumptions are carried over to QCD.  Perhaps we are doing both QED and QCD wrong, and the consequences are just now showing up as tiny energy shifts in the excitations of high-Z atoms.  This could be the breakthrough, the crack in the wall, that physics has been needing for decades in order to progress to the next level of understanding.


References:

EBIT Technology:

"Overview of the Electron Beam Ion Trap Program at NIST", J. D. Gillaspy, et al, Physica Scripta T59, 392-395 (1995).

Test of QED:

"Testing Three-Body Quantum Electrodynamics with Trapped Ti20+ Ions: Evidence for a Z-dependent Divergence Between Experiment and Calculation", C. T. Chantler, et al, Physical Review Letters 109, 153001 (2012).


SF Novels by John Cramer:  my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .

AV Columns Online: Electronic reprints of about 170 "The Alternate View" columns by John G. Cramer, previously published in Analog , are available online at: http://www.npl.washington.edu/av.


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