Public confidence in the "constants" of nature may be at an all-time low.
Recent research has found evidence that the value of certain fundamental parameters, such as the speed of light or the strength of the invisible glue that holds atomic nuclei together, may have been different in the past.
"There is absolutely no reason these constants should be constant," says astronomer Michael Murphy of the University of Cambridge. "These are famous numbers in physics, but we have no real reason for why they are what they are."
The observed differences are small — roughly a few parts in a million — but the implications are huge.
The laws of physics would have to be rewritten, and we might need to make room for six or seven more dimensions than the four — the three spatial ones, plus time — that we are used to.
Lines of evidence
The evidence for varying constants focuses primarily on studies of quasars.
Quasars are extremely bright, extremely distant interstellar objects powered by giant black holes. Some of them are so far away that their light was emitted 12 billion years ago, effectively making them windows into the early universe.
Astronomers study this ancient light to determine if the universe was different then than it is now. Specifically, they look at absorption spectra, which indicate the composition of invisible gas clouds that lie between us and the quasars.
The lines in each absorption spectrum reveal exactly what is in the gas clouds, since each atomic element has a "fingerprint" — a set of specific frequencies at which it absorbs light.
In 1999, Murphy and his colleagues found the first convincing evidence that these atomic fingerprints change with time.
Using data from the Keck Observatory in Hawaii, they detected a difference between billion-year-old quasar absorption spectra and the corresponding atomic spectra measured on Earth.
Some of these Earth-bound spectra were not well characterized, so Murphy and others recently performed careful lab experiments to confirm that there was indeed a shift.
A spectrum is basically light split into its component frequencies, much like when white light goes through a prism to produce a rainbow.
What's in a constant
Because the frequencies of absorption spectra depend on various parameters, the quasar observations are sometimes interpreted as indicating that light was faster in the past, or that the electron had a weaker charge.
But theorist Carlos Martins of the University of Cambridge tells LiveScience that this is not entirely correct.
"It doesn't make sense to talk about a varying speed of light or electron charge," he says.
This is because the values of these parameters include units that might change.
The speed of light, for instance, might be measured one day with a ruler and a clock.
If the next day the same measurement gave a different answer, no one would be able to tell just from the data if it was the speed of light, the ruler's length or the clock's rate of ticking that had changed.
To avoid this confusion, scientists use dimensionless constants — pure numbers that are ratios of measured quantities.
In the case of the shifts in Murphy's data, the relevant dimensionless constant is the fine structure constant (often designated by the Greek letter alpha), which characterizes the strength of the electromagnetic force.
The researchers found that alpha was smaller in the past. Other "famous numbers" would also not be immune to the vagaries of time.
"You would expect variation in all the fundamental constants," Murphy says.
It was therefore not entirely a surprise when — in April of this year — Patrick Petitjean of the Astrophysical Institute of Paris and his collaborators detected a change in the proton-electron mass ratio from molecular absorption lines in quasar spectra.
The mass variation can be interpreted as the strong nuclear force's coupling constant being larger in the early universe, Petitjean says.
A hole in the theory
Time-varying constants of nature violate Einstein's equivalence principle, which says that any experiment testing nuclear, gravitational or electromagnetic forces should give the same result no matter where or when it is performed.
If this principle is broken, then two objects dropped in a gravitational field should fall at slightly different rates.
Moreover, Einstein's gravitational theory — a key component of general relativity — would no longer be completely correct, Martins says.
A popular alternative to relativity, string theory — which is actually an untested hypothesis — predicts inconstant constants.
It assumes that sub-atomic particles are in fact one-dimensional vibrating strings and that the universe has 10 or more dimensions.
According to string theory, the extra dimensions are hidden from us, but the "true" constants of nature are defined in all dimensions.
Therefore, if the hidden dimensions expand or contract, we will notice this as a variation in our "local" three-dimensional constants.
Even if string theory is not correct, the current model of gravity will likely need to be revised to unite it with the other three fundamental forces, electromagnetism and the strong and weak nuclear forces.
"We have an incomplete theory, so you look for holes that will point to a new theory," Murphy says.
Varying constants may be just such a hole.
The other side
Not all quasar data is consistent with variations.
In 2004, a group of astronomers — including Petitjean — found no change in the fine structure constant using quasar spectra from the Very Large Telescope in Chile. No one has yet explained the discrepancy with the Keck telescope results.
"These measurements are so difficult, and at the extreme end of what can be achieved by the telescopes, that it is very difficult to answer this question," Petitjean says.
Other experiments outside astronomy have found no evidence for variation in the fine structure constant (alpha), although they do not examine the same very old period of time that quasars represent.
— Atomic clocks: By comparing extremely accurate clocks, researchers have shown that the current change per year in alpha is less than one part in a million billion.
— The Oklo mine: This uranium deposit in the African nation of Gabon was a natural nuclear reactor two billion years ago. An early study concluded that alpha has not changed more than 10 parts in a billion since the reactor ran. But a more recent analysis shows that this depends on certain assumptions.
— Anthropic arguments: For life to have arisen on Earth, many constants could not have been very different from what they currently are. For instance, if alpha changed by 4 percent, then carbon, the basic element of all life on Earth, could not have been made by the first generation of stars billions of years ago.
Forces of nature
The four fundamental forces can each be characterized by a dimensionless constant.
Strong nuclear force: Holds together the neutrons and protons in an atomic nucleus, overcoming the mutual repulsion of protons, which all bear the same positive electromagnetic charge. The most powerful of the four forces, it has an very small effective range.
Electromagnetism: Holds electrons around atoms; explains light; the basis of electricity and chemistry.
Weak nuclear force: Responsible for certain radioactive decays; only effective on subatomic particles. Current prevailing theory links it to electromagnetism.
Gravity: The attraction of all objects with mass for each other, it keeps planets, stars and galaxies from flying apart. The weakest of the four forces, its effective range is nearly infinite.
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