Image
© Tau'olunga, Wikimedia Commons
As the Earth orbits the sun, the strength of its gravity could vary because of interactions with an undiscovered force, nicknamed the "X-field".
Everyone has heard of Newton's apple. He watched it drop to the ground in the autumn of 1666, prompting him to ask a series of questions. "Why should that apple always descend perpendicularly to the ground?" Newton wondered. "Why should it not go sideways or upwards, but constantly to the Earth's centre?"

One question Newton didn't ask is whether apples or oranges fall differently. Or whether an apple would fall differently in the spring. They might seem peculiar concerns, but Alan Kostelecký, a physicist based at Indiana University in Bloomington, thinks they are important. He and his graduate student Jay Tasson have found that such flagrant violations of our best theory of gravity could easily have evaded detection for centuries.

What's more, in a paper published in Physical Review Letters (vol 102, p 10402), the pair have shown that investigating such unlikely-seeming possibilities could help us work out what makes the universe tick. "We have made a surprising and delightful discovery," Kostelecký says. "We might just catch a glimpse of the ultimate theory that underpins our universe."

This is the culmination of 20 years' work for Kostelecký. In 1989, he began to think about how to find a glitch in our best understanding of how the universe works, as provided by two great theories. The first is general relativity, Einstein's theory of how gravity works. The other is the standard model of particle physics, a quantum description of the matter around us and of all forces other than gravity.

At the moment, relativity and the standard model are incomplete. General relativity breaks down when gravity is very strong - when describing the big bang, for example, or the heart of a black hole. And the standard model has to be stretched to breaking point to account for the masses of the universe's fundamental particles. The two theories are also incompatible, having entirely different notions of time, for instance. This has made it impossible to unite the two in a single "theory of everything".

Image
© Unknown
The trouble is, despite their faults, relativity and the standard model are very good theories. Taken separately, they describe perfectly almost all physical phenomena known to science. If we want to know what the theory that unites them is going to look like, we have to find things that they cannot explain. "The challenge is to find those phenomena," says Kostelecký. This is what he and Tasson think they might now be able to do.

They have begun by launching an attack on an almost sacred premise of physics, known as Lorentz symmetry. This says that the laws of physics appear the same for anyone moving at uniform speed relative to you, whatever their orientation in space.

One consequence of Lorentz symmetry is that the universe should be isotropic: whichever way you look or travel, everything seems pretty much the same and behaves in the same way. There is no "up" or "down", and there is no direction in which light, people or planets can travel more easily.

So far, nothing in the universe has been shown to breach Lorentz symmetry. But that doesn't mean that Lorentz symmetry is inviolable. It just means that we may have been looking in the wrong place so far, or that experiments that have looked for symmetry violations have not been sensitive enough.

Kostelecký and Tasson have not picked on Lorentz symmetry at random. Various attempts to build a theory of everything have all suggested that it might break down. Among the most well known are approaches called string theory and loop quantum gravity.

Kostelecký hasn't pinned his hopes to a particular theory of everything, however. Instead he has taken a more open-ended approach that he hopes will give us an idea of where to look for Lorentz symmetry violations and inform future theories.

He and his colleagues use general relativity and the standard model as their starting point, then suggest ways to violate symmetry. They do this by positing that the universe is filled with as-yet unknown force fields that impose a "preferred" direction on space and therefore violate symmetry. The result is a theory Kostelecký calls the standard model extension, or SME.

By including all known forces and particles, and how they might interact with the new force fields, the SME exposes an assortment of hitherto ignored phenomena that might provide an observable violation of Lorentz symmetry. "Currently, experimentalists are working their way through the list," says Kostelecký.

So far they have drawn a blank. Researchers have looked at whether clocks tick faster in certain orientations in space, or whether the magnetic field of a material, which is created by the spin of electrons within it, changes with the orientation of the electrons' spin axis. So far, they have found nothing.

However, this doesn't mean we can assume that the force fields in the SME don't exist. Some fields may be invisible to photons, but visible to other particles like neutrinos. Or perhaps a field interacts strongly with gravity, but not with electromagnetism.

To see how the idea works, imagine one of Kostelecký's SME fields - let's call it the "X-field" - running through our solar system. The X-field, like a magnetic or electric field, has an orientation that can be pictured as a series of arrows. What happens when a particle such as a neutron or proton passes through it?

For starters, the field might impose a subtle effect on the particle's spin, or create a small phase shift in its trajectory. Or it could be that different types of particle respond differently to the field.

We have never noticed any such effects, so we have never detected any such field. But Kostelecký and Tasson point out that we may not have been looking in the right way. If the X-field and the sun's gravity affect each other, there may well be effects we haven't noticed.

Such interactions between the X-field and gravity could be a rich seam for scientists to mine. Kostelecký and Tasson's latest calculations show that these interactions could produce symmetry violations as much as 1030 times bigger than some of the ones that researchers have attempted to find so far.

That won't make the violations easy to detect, though: compared to the other fundamental forces, gravity is astonishingly weak, so variations caused by the X-field would still be very hard to measure.

One way we might find evidence of the X-field is to look for small differences in gravity's power at different times of the year. "Apples might fall at different rates in different seasons," says Kostelecký, though he can't predict when apples might falls faster. "It will be a cyclical effect."

This is because the gravitational pull of the sun could warp the X-field slightly. According to Kostelecký's calculations, gravity causes the X-field's arrows to tip towards the sun by an amount that depends on the strength of the gravitational field at that location. Suitably designed experiments might be able to detect how a particle's behaviour varies as the Earth circles the sun, due to this variation in the X-field at different locations in space.

Another possibility that Kostelecký and Tasson raise is that the X-field affects different particles in different ways. For example, each type of quark might "feel" the X-field to differing degrees. Or perhaps the number of electrons in an atom will determine how that atom couples to the field, and thus to gravity. It could even be that a combination of factors - for example the constituent particles of atoms and their position in space - will shape the finer details of how different objects couple to the X-field and gravity, producing unexpected effects. "Apples and oranges may fall at different rates," Kostelecký says.

The search begins

Though it might sound like a long shot, Kostelecký and Tasson's paper offers an exciting new insight, says physicist Mike Tobar at the University of Western Australia in Crawley. "This is an important development," he says. Ronald Walsworth of Harvard University agrees: "I expect several experimental groups will now search for the effects Kostelecký is proposing."

So where will they begin? As the effects will show up as anomalies in the way particles respond to gravity, Kostelecký and Tasson have proposed testing a modified version of Newton's universal law of gravitation. The idea is to see whether it is consistent when applied to varying combinations of particles - protons, neutrons and electrons - at different times and in different places. So far, only a tiny fraction of this new range of possible effects has been investigated.

One such investigation, by Eric Adelberger's group at the University of Washington in Seattle, searched for a difference in the ways titanium and beryllium respond to gravity.

"The Adelberger experiment does the analogue of comparing the fall of an apple and an orange at the same time," Kostelecký says.

No one is under any illusions. If there is a difference in gravity's pull for these different elements, it is going to be very small. This is why the Washington researchers got involved: they are experts at using excruciatingly sensitive torsion balances, which measure the gravitational pull between two masses, to probe this kind of situation.

In order to carry out the experiment, they also had to shield their balance from electric and magnetic fields and vibrations from nearby laboratories, while also compensating for the varying gravitational pull of the underground water table as it rose and fell at different times of year.

In the end, however, they found that there was no difference in the coupling of beryllium and titanium to gravity - to 1 part in 100 billion, at least.

Kostelecký is undaunted. Adelberger's experiment tested only one kind of interaction between the hypothetical field and gravity. Kostelecký believes that experiments carried out at different times of year might expose another aspect of the coupling. With the changing seasons, the relative orientation of the Earth's velocity and the X-field's arrows would change significantly.

If that fails, there are other options, including scope for antimatter to unravel the symmetry of the universe. "Apples and anti-apples may fall at different rates," Kostelecký says. This idea is even harder to explore: accumulating enough antimatter to make a body the mass of an apple, for instance, is beyond our current technological capabilities. Anti-hydrogen atoms have been made, though, and efforts are under way to see whether they fall differently to hydrogen atoms. "We may get results within the next decade," Kostelecký says.

Kostelecký is outlining other experiments that might reveal the fields postulated by the SME. Superconducting gravity sensors, lasers that probe the distance to the moon, atom interferometers and upcoming satellite-based gravity experiments such as microSCOPE and STEP; any or all of these might help find where that infuriating symmetry breaks down, and where that elusive ultimate theory of the universe has to kick in.

Well, that is the hope. Though Walsworth agrees that such experiments are important, he is not yet convinced they will reveal any symmetry violations. "It is not in any way certain that they exist, or that we humans will ever have the ability to find them," he says.

Adelberger is also cautious about the prospects, but thinks we should look anyway. He believes that the problem with reconciling relativity and quantum theory is so great that we cannot afford to leave any of our cherished principles untested. "It seems very likely that we are missing something huge in physics," Adelberger says. "I would be surprised if large Lorentz-violating effects are present, but it is definitely worth testing to see if nature respects my prejudices."

Bibliography

Marcus Chown's latest book is Quantum Theory Cannot Hurt You (Faber, 2008)