Research News Highlight

Constant of nature not constant?

A team of scientists of the Laser Centre Vrije Universiteit (LCVU), Amsterdam, and of the European Southern Observatory in Chile have found indication for a small variation of one of the constants of nature: the ratio between the mass of the proton and the mass of the electron. They report this finding in the April 21 issue of Physical Review Letters, in a paper entitled:
Indication of a cosmological variation of the proton-electron mass ratio based on laboratory measurement and reanalysis of H₂ spectra
Click to download a reprint.

A variation of a constant of nature implies that the laws of nature are subject to change. The scientists at LCVU and at ESO (in Paranal Chile) discovered a variation by comparing the proton-electron mass ratio in molecular hydrogen as it is now and how it was 12 billion years ago. They performed extremely accurate measurements on spectral lines of hydrogen molecules in the laboratory and compared the results with the same lines observed in spectra of quasars. Quasars are astronomical objects that emitted light way back in the past (twelve Billion years ago). They are so far distant, that their light reaches the Earth only now. In the 'old light' a fingerprint of molecules of hydrogen or H₂, as they were then, is carried to Earthbound telescopes, in this case to the Very Large Telescope in Chile.

the VLT at Paranal, Chile, at 2.5 km altitude in the Andes
The laboratory measurements were performed with a special laser, developed in the Laser Centre VU Amsterdam, operating at the specific wavelengths absorbed by hydrogen molecules. Those wavelengths are in the extreme ultraviolet (XUV) between 90 en 110 nanometer. The beam of XUV-radiation is crossed with a beam of H₂ molecules in otherwise vacuum conditions (Click for picture of Vacuum chamber) Some essential parts of the laser setup are displayed below:

On the left the Pulsed-Dye Amplifier; on the right the ring dyelaser.
(Click on picture for higher pixel size)
In the expanding universe the quasars move away from us (at Earth). As a result of the cosmological redshift all spectral lines are shifted to the visible wavelength domain. Hence the light can penetrate through the Earth's atmosphere, and the spectra can be recorded accurately by means of a spectrometer connected to the telescope. For these observations use was made of the UVES spectrograph:

The "Ultraviolet and Visible Echelle Spectrograph".

The hydrogen molecule, built from two protons and two electrons, is the most abundant molecule in the universe. From a comparison of data obtained via laser spectroscopic measurements and from quasar observations, in combination with calculations of the structure of the H₂ molecule, it follows that the mass ratio of the proton and the electron may have changed in the mean time. We found indication that the mass ratio has become smaller by 0.002% in the past twelve Billion years. That seems just a little, but according to the laws of physics, this ratio should not vary at all.

The proton-electron mass ratio is an important fundamental constant of nature. This constant is dimensionless, so independent of any system of units. It is a number that can be measured and the current status is: M_p/m_e = 1836,1526726. See also the NIST website for the constants. Standard physics does not have an explanation as to why M_p/m_e has this value, nor can it provide an explanation as to why it would vary. Note that also the possibility of a variation in alpha, the fine structure constant, is a subject of current research. See: A Constant Worthy of the Name.

It is interesting to note in this respect that the German scientist F. Lenz, in the possibly the shortest paper ever to appear in Physical Review argued, that the value for Mp/me, as it was known then in 1951 (1836.12) exactly coincides with 6 times the number pi to the 5th power. The modern value of Mp/me no longer matches such an algebraic relation.

The laboratory measurements, the calculations on the hydrogen molecule, and the statistical analysis of the data were carried out by an LCVU-team lead by Prof. Wim Ubachs, further consisting of Dr. Elmar Reinhold (now associated with the European Space Agency ESA, in Noordwijk, the Netherlands), Dr. Urs Hollenstein (now at the ETH in Zürich, Switzerland) and Ruth Buning (Masters student at the Vrije Universiteit Amsterdam). The observations of quasar systems at ESO in Chile were carried out by a team headed by Prof. Patrick Petitjean (Institute d'Astrophysique de Paris, France) and Dr. Alexandre Ivanchik (Ioffe Institute St. Petersburg, Russia). The six scientists mentioned are all co-author of the paper describing the results.

Future

Further observations will provide definite evidence whether the here obtained indication of a variation of a constant of nature will hold. Possibly it can be determined more precisely than the currently obtained 3.5 sigma statistical uncertainty. At the moment accurate data exist only for 76 spectral lines in hydrogen, observed in two quasars; Q0405-443 and Q0347-383. It is important to identify additional bright quasar sources, that contain hydrogen spectral lines. In the laboratory further measurements will be carried out on the still missing spectral lines of H₂.

In the Laser Centre VU we will also adopt a different strategy. There is the possibility of searching for temporal variations of fundamental constants only based on laboratory measurements. Because the time intervals are then small (smaller than 5 years in general) the experimental precision must be dramatically increased. That is the goal of a new research programme (FOM-IPP-I11) of the group Atomic, Molecular and Laser Physics, that has recently been granted by the Netherlands funding organisation FOM (Fundamental Research of Matter). In this programme we will collaborate with the Optics Research Group at Delft Technical University, the Nederlands Meetinstituut, TNO Science and Industry and ASM Lithography. The programme focuses on precision measurements with ultrastable lasers in connection to various applications.

The ultraprecise measurements will be carried out with so-called "frequency comb lasers". For this revolutionairy principle, allowing to produce and measure optical frequencies at extremely high accuracy, the 2005 Nobel Prize was awarded to John Hall and Ted Hansch. In the mean time we have constructed such a laser system at LCVU, and already important scientific results have been obtained; these results have been published in Science.
In addition to frequency combs we also are engaged in the development of a molecular fountain that will be used for precision metrology.

The frequency comb laser setup at the Laser Centre VU

Press coverage of this research

This work was carried out with suppport from the Netherlands Institute for Space Research SRON.


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