Proton-electron mass ratio

Physicists speculate a lot about whether (or to what extent) the laws of nature are exactly the same in all parts of the visible universe. This question is sometimes known as the "fundamental constants" problem.

There are a variety of such fundamental constants. The best known is the speed of light. Many theories that are part of "alternative physics" are based on the idea that the speed of light (in a vacuum) is not a constant, and may have been different early in the history of the universe. There is very little, if any, solid evidence for this, however.

A closely related constant is the fine structure constant, fondly known as alpha. It is related to the speed of light by the equation α = e²(2ℎcε₀)^-1, where e is the charge of an electron, ℎ is the Planck constant, c is the speed of light, and ε₀ is the permittivity of free space.

There have been attempts to determine from astronomical data whether α may vary with time. This question can be investigated since α affects atomic spectra that are easily measured in a laboratory on Earth, and which can also be observed in distant astronomical objects. So far, no evidence of variability has been found. (See here, from June 2007.)

If α doesn't change with time, it is very unlikely that the speed of light does either (unless other constants also change in just the right way).

Another important fundamental constant is the ratio of the mass of a proton to the mass of an electron. (This ratio is sometimes denoted by μ, but that's confusing, as μ is used for other quantities in physics also.) In the standard model of particle physics, these masses, and their ratio, are free parameters that are not determined by the model itself. They are just there.

This ratio also affects atomic spectra, so it can also be investigated in astronomical studies. In April 2006, some evidence was reported for a difference between the laboratory value of the ratio and its value in distant quasars. The difference claimed was very small – 0.002% over a time span of 12 billion years. (See here, here.)

However, now there is more recent evidence from halfway across the universe, indicating that the ratio long ago is the same as it is now, although the time span is somewhat less (6 billion years):

Earth's laws still apply in distant Universe (6/19/08)

The laws of nature are the same in the distant Universe as they are here on Earth, according to new research conducted by an international team of astronomers, including Christian Henkel from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn. Their research, published today in Science, shows that one of the most important numbers in physics theory, the proton-electron mass ratio, is almost exactly the same in a galaxy 6 billion light years away as it is in Earth's laboratories - approximately 1836.15.

According to Michael Murphy, Swinburne astrophysicist and lead author of the study, it is an important finding, as many scientists debate whether the laws of nature may change at different times and in different places in the Universe. "We have been able to show that the laws of physics are the same in this galaxy half way across the visible Universe as they are here on Earth," he said.

The light actually comes from a quasar 7.5 billion light years away. But the spectral effect is due to ammonia molecules encountered when the light passes through a galaxy that is 6 billion light years distant.

Another account emphasizes that the latest result gives a constraint 10 times better than the 2006 result:

Changing physical constant may be constant after all (6/20/08)

There is good reason to trust the new result, Murphy says. The wavelengths at which the ammonia molecules absorb radiation depend more strongly on the proton-to-electron mass ratio than with other molecules, such as the molecular hydrogen that was used for the 2006 result. "Our constraint is 10 times better than those previously obtained," Murphy says.

Wim Ubachs, who led the 2006 analysis, agrees Murphy's result is "solid", but thinks there still might be a way to reconcile the two results. There remains the possibility that the constant varied between 6 billion and 12 billion years ago but has not varied since, he says.

Other accounts of this result:

• Experimenting in a galaxy far, far away (6/20/08)
  
• Far off in universe, the same laws found (6/21/08)
  
• Particles Retain Weight for Billions of Years (7/14/08)
  

Further reading:

Strong Limit on a Variable Proton-to-Electron Mass Ratio from Molecules in the Distant Universe – 6/20/08 research article in Science

Ammonia: Proton-electron mass ratio constant for 6 gigayears – 7/14/08 blog post on the research, and reasons why no variation in the ratio should be expected

Tags: fundamental constants