杏吧原创

Editorial: We deny the inexplicable at our peril

It looks like physics works differently in different places. If so, everything we think we know about the cosmos may be wrong

IT鈥橲 not easy being . Sometimes, when he gives a talk about his work, he gets comments like, 鈥淚鈥檓 surprised you had the guts to say that.鈥�

Webb, who is an astronomer, doesn鈥檛 really understand what else he is supposed to do. 鈥淚鈥檓 just reporting what鈥檚 there in the data,鈥� he says. The trouble is, Webb鈥檚 data says we should rewrite the laws of physics.

Quite how, we don鈥檛 yet know. According to his team鈥檚 analysis of the light from distant galaxies, the way physics works may depend on the direction you鈥檙e facing. That might mean resurrecting the ether, the substance once thought to permeate all of space before Einstein abolished it over a century ago, or even invoking extra dimensions.

Either way, the implications of Webb鈥檚 result are huge. If his team is right, strikes at the heart of Einstein鈥檚 special theory of relativity and our understanding of the cosmos.

With so much at stake, it鈥檚 not surprising there is opposition. 鈥淭his would be sensational if it were real,鈥� says cosmologist Max Tegmark of the Massachusetts Institute of Technology, 鈥渂ut I鈥檓 still not completely convinced.鈥� Lennox Cowie, an astronomer at the University of Hawaii in Manoa, puts it more bluntly. 鈥淚 don鈥檛 believe it,鈥� he says.

Webb, who is at the University of New South Wales (UNSW) in Sydney, Australia, first raised physicists鈥� hackles about a decade ago, when he found some strange results after using the in Hawaii. He had been looking at quasars, which are extremely bright galaxies in the far reaches of the cosmos. As the quasar light passed through clouds of magnesium and iron atoms on its 12-billion-year journey to Earth, some of the light had been absorbed by the metal atoms.

Oddly, though, Webb鈥檚 analysis said the atoms had taken up the wrong kind of light. The wavelengths of light absorbed by magnesium and iron can be predicted using the equations of quantum electrodynamics, but the ones Webb recorded were different. Twelve billion years ago, it seems, iron and magnesium absorbed photons of different energies than the ones they absorb today.

Webb did have an explanation, though. The observations fitted perfectly if he changed one of the fundamental constants of nature, known as the fine-structure constant, or alpha. This is a central pillar in quantum electrodynamics and dictates, among many other things, which photons certain atoms will absorb.

罢辞诲补测鈥檚 is approximately 1/137. But Webb鈥檚 work showed that, billions of years ago, it must have been around one part in a million smaller.

Nobody believed the result was right, but neither could they find any flaws in Webb鈥檚 analysis (). The only other explanation was that something peculiar to the Keck telescope was to blame. So Webb turned to the (VLT) in Chile, and analysed the quasar light that it picked up.

Webb鈥檚 PhD student, Julian King, has just completed the analysis. 鈥淲hen I started I quietly hoped we鈥檇 find the same thing Keck found,鈥� says King. 鈥淭he worst case would have been no effect; then we would have had to start searching for the flaw at Keck.鈥�

King鈥檚 worries were unfounded. There was an effect: as with the Keck observations, the VLT found a slightly different alpha from the accepted value. But the big surprise was that this time the constant was bigger, not smaller ().

Webb thinks he knows why: alpha definitely varies, but in space not time. This startling conclusion comes from the fact that the Keck telescope and the VLT look out on different parts of the sky. Imagine drawing a straight line from the gas clouds used for the Keck observations to those that absorbed light on its way to the VLT (see diagram). According to King鈥檚 results, the value of alpha grows as you move along that line. 鈥淭he further 鈥榮outh鈥� you go, the more positive the change in alpha becomes,鈥� he says.

Broken symmetry

In other words, alpha seems to be small on one side of the universe and large on the other. Earth sits somewhere in the middle. 鈥淚f we didn鈥檛 violate the laws of physics in our previous results, we鈥檙e certainly violating them now,鈥� says Michael Murphy at Swinburne University of Technology in Hawthorn, Australia, who works with Webb.

Murphy is only half joking. Back in 2002, Alan Kostelecky, Ralph Lehnert and Malcolm Perry showed that any spatial variation in alpha would break Lorentz symmetry. This is a golden rule of Einstein鈥檚 special theory of relativity. It says that the laws of physics apply in the same way in all directions in the universe, and are not sensitive to the speed at which you are moving. So someone carrying out an experiment while travelling close to the speed of light from Earth to Alpha Centauri would get the same result as someone travelling the other way, and they would both agree with someone carrying out the same experiment in a lab back home ().

So it鈥檚 a real problem, even though the spatial variation in the laws of physics would be tiny from here to Alpha Centauri. 鈥淚t鈥檚 the principle of the thing,鈥� says Craig Hogan of Fermilab in Batavia, Illinois. 鈥淵ou can extrapolate to larger distances and expect physics to look entirely different far away.鈥�

That is unacceptable. For a start, our models of the universe are built on the assumption that everything works in the same way everywhere in space and time. If the laws of physics are a moveable feast, our carefully constructed history of the universe starts to fall apart. In its place all we have are, as the late great physicist John Wheeler put it, 鈥渃osmic by-laws鈥�. The only comfort is that this scenario might provide some support for string theory (see 鈥淚t came from another dimension鈥�, below).

Cosmic alignment

The violations of Lorentz symmetry have other implications, too. If the laws of physics depend on your orientation in the universe, it takes physics back more than a century 鈥� to the age of the ether.

The concept of the ether was developed by physicists in the 19th century because they thought there must be a medium through which light travelled, in the same way that sound requires air. They hypothesised that its presence could be detected by the movement of the Earth through it, which would create an 鈥渆ther wind鈥�. This would mean that experiments designed to measure the speed of light would find different results depending on which direction they faced.

However, a landmark experiment in 1887 by Albert Michelson and Edward Morley failed to find any evidence for such effects. Einstein concluded that the absence of the ether meant that the speed of light must be constant everywhere and he used this as the framework for special relativity.

That didn鈥檛 put an end to the search for evidence of Lorentz violation, however. We know that Einstein鈥檚 theories of relativity are not the final word, and Kostelecky and others have studied many things, such as the behaviour of neutrinos and the way neutrons spin, looking for where they break down. So far, they have drawn a blank. That鈥檚 what makes Webb鈥檚 result so intriguing.

So what should we do about it? One answer is to do nothing until we find another result that corroborates its claims.

Physicists are an extremely cautious breed. Even , an atomic physicist at the Free University in Amsterdam in the Netherlands, who has called Webb鈥檚 result the 鈥減hysics news of the year鈥�, urges care. 鈥淚t is exciting because it is a glimpse of new physics, something entirely different,鈥� he says. 鈥淏ut it has to be verified and reproduced.鈥�

One of the best ways to do that might lie in comparing Webb鈥檚 results with other heresies we have noticed. Just as magnetic north provides a 鈥渢his way up鈥� orientation for anyone on Earth, the line along which alpha grows can be considered as an 鈥渁xis鈥�, or dipole, that defines a direction through the cosmos. And there are other axes to consider. For example, analysis of the cosmic microwave background radiation has revealed what Jo盲o Magueijo and Kate Land of Imperial College London dubbed the 鈥�axis of evil鈥�. If the universe is the same whichever way you look at it, hot and cold spots of radiation will be randomly scattered across the sky. Yet the hot and cold spots seem to run in a particular direction through the cosmos, though this is very different to the axis of alpha.

The axis of alpha does line up with another cosmic anomaly, though. In 2008, Sasha Kashlinsky of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, noticed a huge phalanx of galaxies streaming across the universe. The galaxies are heading, for no obvious reason, towards a gap in the sky between the constellations of Centaurus and Vela (New 杏吧原创, 24 January 2009, p 50). Subsequent observations have confirmed the motion: they seem to be moving at around 1000 kilometres per second, perhaps under the gravitational influence of some gargantuan structure beyond the cosmic horizon, in a similar direction to the alpha axis. 鈥淚t looks as if our dipole is not very far away from theirs,鈥� Webb says.

A couple of other possibilities have arisen. There is a dipole in the abundance of deuterium in the early universe 鈥� and it runs parallel to the axis of alpha. Another dipole comes from the intensity of light emitted by supernovae, which King says is 鈥渧aguely near鈥� the axis of alpha. 鈥淚t鈥檚 far from conclusive proof, but people have run simulations of the statistical chance of all these dipoles aligning and it鈥檚 actually quite small,鈥� King says.

There are more options to be explored. Look at the way clouds of hydrogen molecules absorb quasar light, for example, and you鈥檒l find suggestive evidence of another axis.

Alignments aren鈥檛 the only source of corroboration. Further examinations of the amounts of helium and lithium in the early universe may reveal evidence of variation in other constants, such as those associated with the strong force that holds nuclei together.

In fact, evidence of variation in any of the dozens of constants of physics could help explain a varying alpha; it is generally agreed that if one constant varies, they all will. 鈥淚 couldn鈥檛 imagine a unified theory of the universe where alpha varies and other constants don鈥檛,鈥� says Victor Flambaum, one of Webb鈥檚 colleagues at UNSW.

Not all the evidence is to be found in space, though. Working with Julian Berengut, also at UNSW, Flambaum has drawn up a list of ways that alpha and other constants might be checked on Earth. A variation in alpha would, for instance, show up in atomic clocks as the Earth moves through space. Unfortunately, our best atomic clocks are not yet sensitive enough to detect a change of the level that Webb鈥檚 team find. However more sensitive clocks are in the pipeline ().

鈥淎 variation in alpha would show up in atomic clocks as we move through space鈥�

Meanwhile, Kostelecky suggests, the fact that varying alpha breaks Lorentz symmetry might provide another way to test it. 鈥淭his could lead to signals in lab experiments,鈥� he says.

Still, most physicists remain convinced that all this would be wasted effort. 鈥淭hese anomalies come and go,鈥� says Pedro Ferreira at the University of Oxford.

So what will it take for the world to believe in varying constants? The chance of Webb鈥檚 result not being due to chance is 99.9937 per cent. Though that sounds reliable, a scientific discovery traditionally has to be at 99.99994 per cent to count. 鈥淚 don鈥檛 believe that people will take it seriously even then,鈥� Ferreira says. It still wouldn鈥檛 be enough for anyone to consider rewriting the laws of physics, he reckons, not when the stakes are so high.

Webb is sanguine about this. 鈥淲hat we鈥檝e got here is the most precise measurement of physics that has ever been made over a large volume of the universe,鈥� he says. 鈥淣obody has said, 鈥榟ere is a problem with your analysis鈥�, or 鈥榟ere鈥檚 a systematic error that can explain your results鈥�. The only tool they鈥檝e got is to say, 鈥業 don鈥檛 believe it鈥�. People are clutching at straws.鈥�

鈥淣o one can point to a flaw in our analysis or an error explaining the results. They just say 鈥業 don鈥檛 believe it鈥欌��

There are plenty of precedents for anomalies seeding scientific revolutions. Unfortunately for Webb, the process usually takes decades for evidence to pile up, and vindication often comes after everyone involved in the original discovery has shuffled off this mortal coil. It might be hard being John Webb now, but perhaps he can draw comfort that his name might live on long after he is gone.