Testing Times for Nature’s constants
Atomic clocks, nuclear clocks, and space experiments are putting Nature’s fundamental laws to the test. Will they open the door to extra dimensions?
March 21, 2010
Light from quasars, such as this pair known as SDSS J1254+0846 about
4.6 billion light years away, could reveal Nature’s fickleness.
Credit: Chandra X-Ray Observatory
Numbers seem to rule our identities—telephone numbers, social security numbers, bank account numbers. The same is true for Nature, where the equations of physics rely on a set of precise digits, such as the speed of light or an electron’s charge. Nobody knows why Nature’s numbers, known as the fundamental constants, take the values that they do, but we do know that a slightly altered set of fundamental constants would dial up a vastly different universe.
Now some physicists are questioning whether these constants are, well, constant after all, and what the implications of their variation would be. Changing constants could signpost the existence of extra dimensions, or new fundamental forces, for instance.
In particular, physicists are scrutinizing the fine-structure constant
, or alpha
, which governs the strength of the electromagnetic force. If alpha were just four percent larger, carbon would not have been produced in stars; four percent smaller and we wouldn’t have oxygen. In other words, tinker too much with alpha and life as we know it could not exist.
Physicists’ faith in alpha’s constancy was first shaken in 1999, by a group of astronomers led by John Webb
of the University of New South Wales (UNSW), Australia. Webb’s team used the Keck telescope in Hawaii to study light from bright galaxies at the edge of the universe—called quasars—that had been absorbed by interstellar clouds. The clouds absorb certain wavelengths of light from the quasars and produce spectral lines; the spacing of these lines depends on the value of alpha. The team’s results
suggested that alpha’s value was one thousandth of a percent smaller 10 billion years ago.
Varying constants would
be a smoking gun for
- Doug Shaw
That change may sound tiny, but the implications were huge. "It was a paradigm shift," says Dmitry Budker
, of the University of California at Berkeley.
What’s at stake is our handbook for how the universe works: alpha is one of 26 numbers in the Standard Model of particle physics, which describes all known particles, the electromagnetic force, the weak force that governs radioactivity, and the strong force that determines how subatomic particles are bound together within the nucleus.
"Seeing varying constants would be a smoking gun for new physics," says physicist Doug Shaw at Queen Mary University in London, UK.
That "new physics" could take the form of the elusive theory of everything (TOE)
that physicists hope will link together the Standard Model and gravity. Some candidate TOEs, such as string theory
, require the existence of tiny curled-up extra dimensions. If these exist, then the constants we measure are just four-dimensional shadows of the true constants found in the higher dimensional fundamental theory. The constants would vary as the size and shape of the extra dimensions evolve, explains Shaw. "Varying constants would help eliminate competing theories to explain the origins of the universe," he says.
Sounds good, but in the decade since Webb’s team found that tantalizing evidence of a varying alpha, other astronomers have found contradictory quasar results
. The wrangling over alpha’s (in)constancy continues.
University of California, Berkeley
Credit: © Damon English
Budker hopes to end the controversy by searching for evidence for a varying alpha in the lab, backed by two grants
totaling $255,000, from the Foundational Questions Institute. His team have been measuring the frequency of radio waves absorbed by atoms of dysprosium—an element with energy levels that are exceptionally sensitive to changes in alpha—over a number of years. If that radio frequency changes over time, then they know that alpha has changed. The experiment is sensitive to changes of a few parts in ten million billion per year.
So far, Budker’s team has not found any significant variation in alpha. But they have set more stringent limits on its value, while others are joining the quest to pin down alpha in the lab.
The dysprosium atom essentially works as an atomic clock
, whose ’ticks’—set by different energy levels in the atom—are extremely sensitive to the value of alpha. Other experimenters are exploiting the fact that energy transitions in different atomic clocks have different dependencies on alpha. If two clocks, based on two different atoms are synchronized and left to run for a year, then any change in alpha will show up in a difference in the two clocks’ time-keeping.
If alpha varies, this will
be a huge leap forward
for our understanding of
- Dmitry Budker
Calculations by various physicists, including Shaw and independently Victor Flambaum at UNSW, also show that alpha could be affected by the Sun’s gravitational pull (see, for example, arxiv.org/abs/1002.4528
). Atomic clock experiments are already testing this idea by searching for seasonal variations in alpha due to the Earth’s changing distance to the Sun.
"The advances in the precision that they’ve achieved in lab tests over the past few years is remarkable," says Shaw. The accuracy is now better than one part in one hundred million billion—one in 1017
—and is still improving. If there is a change in alpha it’s smaller than this.
The lab experiments are now so precise that some people think they are closing in on the quasar data. "The most recent lab results have gone below the level at which you’d expect to see a variation if the quasar result is true," says Shaw.
Flambaum, a co-author of the original quasar publication, says that this does not necessarily mean that the quasar results suggesting that alpha varies were wrong. It may be that alpha varied in the past but is now constant."You cannot compare the cosmological measurements with the laboratory measurements directly, as it depends on the interpretive model," he says.
THE CLOCK IS TICKING FOR ALPHA
Atomic clocks, such as this caesium fountain clock, are testing alpha‘s
constancy with ever-increasing precision.
"Things have to be cleared up," agrees Budker. "There is serious evidence, and there is ongoing controversy."
While alpha seems to have passed this round of atomic tests, things are about to get tougher. The next generation of clocks—nuclear clocks—will make use of nuclear resonance frequencies to set the clock ticking at a faster rate than can be achieved in atomic clocks. As a result they could be up to a million times more sensitive to changes in alpha, says Flambaum.
Cosmologists are also scouring the relic radiation of the big bang—the cosmic microwave background (CMB)—for hints of a varying alpha in the early universe. Studies of the latest CMB data
from the Wilkinson Microwave Anisotropy Probe also suggest that alpha is boringly constant. However, new CMB data is being collected by the Planck satellite, and physicists still have a battery of other space tests for alpha.
So important is the task of uncovering whether the ’constants’ belie their name, it forms part of the European Space Agency’s Cosmic Vision for 2015-2025
. The planned STEP and MICROSCOPE satellite missions will take a less direct route for testing alpha along with another fundamental constant—the mass ratio of the electron to the proton. Tests on board will investigate what might be causing the constants to vary—that is, any changes in the size and shape of extra dimensions—by searching for new forces that evolving extra dimensions may induce. These forces may be small and would be masked on Earth by background vibrations, such as traffic on a nearby road. In space, however, they could cause test masses to move.
If alpha is found to vary, it will open up many more questions about why nature seems so particular about its numbers, says Budker. "If we discover that alpha varies, this will not be an answer, but it will take our understanding of the universe a huge leap forward."
Further reading: FQXi profile of Dmitry Budker (pdf)
Please read the important Introduction that governs your
participation in this community. Inappropriate language will not be tolerated and posts containing such language will be deleted. Otherwise, this is a free speech Forum and all are welcome!
AKINBO OJO wrote on May 17, 2014
"In many theories, the Sun perturbs the values of the constants by a factor roughly proportional to the Sun's Newtonian gravitational potential, which scales as the inverse of distance, r, between the Earth and the Sun. Since r fluctuates annually, reaching a minimum at perihelion in early January and a maximum at aphelion in July, the values of the constants, as measured here on Earth, should also oscillate in a similar seasonal manner." – D.J. Shaw and J.D. Barrow,
TIM GOLDEN BANDTECHNOLOGY.COM wrote on July 22, 2010
Isn't 10E-17 good enough precision?
Well, maybe 10E-23 is better, but the first is good enough to me.
In terms of where to seek change in alpha; I'd guess that it will be out of the planetary plane: one satellite did go there, but no anomalies were reported as far as I know. Go North!
DAN WINTER wrote on April 12, 2010
read all article comments
If time is defined by the period of rotating charge,
then synchronicity is defined by efficiency
of charge transfer.
Charge transfer efficiency can be shown
to be optimized by golden ratio perfected
fractality, in the form of phase conjugation.
My original Equation evidence at
Planck time - times golden ratio predicts: