Unlike most of us, subatomic particles don't gain weight as they get older. The mass of these tiny bits of matter has remained constant over the last 6 billion years, recent astronomical observations indicate.
Believe it or not, but whether an electron was lighter or heftier in the past is a question of fundamental importance. Variations in particle masses and other so-called constants of nature, such as the speed of light, may help explain the mystery of dark energy and determine if hidden dimensions exist.
"Some theorists claim the physical constants should have varied over time," said Christian Henkel of the Max Planck Institute for Radio Astronomy in Bonn, Germany. "This is something that is part of the modeling of the universe."
Henkel and his colleagues have placed new limits on wavering constants. By observing the absorption of radio waves by molecules in the early universe, the researchers have shown that the mass ratio between two particles ? the proton and the electron ? has not changed from its present value by more than 2 parts in a million.
The results, reported in a recent issue of the journal Science, call into question previous measurements that claim to have seen variations in this mass ratio.
Sniffing out ammonia
Henkel came to study varying constants after he and some colleagues detected ammonia in a galaxy "halfway across the universe," he told SPACE.com. Such a faraway object offers a glimpse back in time, because the light we see from it now left the galaxy billions of years ago.
It was the first time this molecule had been seen so far away. The team identified the ammonia by its absorption of radio waves from a bright object called a quasar, located behind the galaxy. Only later did they realize that this absorption could provide information about the fundamental physics in the early universe.
"Ammonia is a molecule with a very special structure," Henkel explained. It looks like a pyramid (or tetrahedron) with three hydrogen atoms forming the base and one nitrogen atom on top.
Although most other molecules rotate faster when they absorb the energy from radio waves, ammonia actually flips inside out, with the nitrogen moving from above the hydrogens to below.
This flipping depends strongly on the ratio between the mass of the proton and the electron. Knowing this, Henkel's team compared their ammonia data to other molecules in the same galaxy and found the ammonia absorption had not significantly shifted from where it was expected to be.
The implication is that 6 billion years ago, protons weighed roughly 1,836 times more than electrons, just as they do now.
A changing world
"It is a puzzling result, but I am sure they did a thorough job," said Wim Ubachs of Vrije University in Amsterdam, The Netherlands.
Ubachs was one of the astronomers who wrote a 2006 research paper that claimed to see a change (20 parts in a million) in the proton-electron mass ratio. The experiment was similar to the current study, but in this case the molecule was not ammonia, but hydrogen.?
At face value, the two observations disagree, but Ubachs pointed out that his group's results correspond to a much earlier time, more than 11.5 billion years ago.
It is possible that the mass ratio changed in between the two measurements. Indeed, there are theories that the fundamental constants varied after the Big Bang but then stopped once dark energy (a hypothetical energy that produces a force that opposes gravity) began accelerating the expansion of the universe some 6 billion years ago. Some of these models claim that dark energy is somehow responsible for variations in the constants. Others, based on string theory, assume that extra spatial dimensions (beyond the three we can see) cause the constants to fluctuate in value.
To prove any of these wild ideas will take a lot more data; both Henkel and Ubachs have fresh observations that are in the process of being analyzed.
"We hope that the upcoming results will shed more light on the issue, which is very intriguing indeed," Ubachs said.
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