WHAT makes gravity so special? In our everyday lives, it is the most familiar of the four fundamental forces of nature: drop an object, and it falls. Yet many features of gravity remain puzzling. Compared with the other forces, gravity is unimaginably weak. And while physicists have succeeded in formulating quantum theories of the strong force, electromagnetism and the weak force, gravity stubbornly remains the odd one out: decades of effort to marry gravity and quantum mechanics are less convincing.
According to quantum theory, all forces are transmitted by particles – the photon for the electromagnetic force, the W and Z bosons for the weak force and gluons for the strong force. These force carriers are more than theoretical flights of fancy, experiments have detected them. Yet no one has ever seen the particle thought to convey gravity. So where is it? There is a partial explanation for this no-show: gravity’s weakness reflects the fact that its particle – the graviton – should hardly ever interact with matter.
Four years ago, Tony Rothman, a physicist at Princeton University was chatting with fellow physicist Freeman Dyson about the elusiveness of gravitons. In fact, gravitons are thought to be so elusive that Dyson wondered whether it was possible to detect one at all. And if gravitons are undetectable, do they really exist?
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The question prompted Rothman to join forces with Stephen Boughn of Haverford College in Pennsylvania, with whom Dyson had also discussed the puzzle. “My first reaction was that Freeman’s question should be easy to answer,” says Rothman. Yet as he and Boughn investigated further, they realised that the resolution was far from obvious. In fact, their study leaves physicists questioning the reality of gravitons and wondering if it is time to re-think their approach to quantum gravity.
The first problem Rothman and Boughn had to tackle was where to look for gravitons. Despite gravity being so all-pervasive, that isn’t as easy as it might sound. When something falls under gravity, its atoms must exchange gravitons – assuming they exist – with atoms in the Earth. These exchange gravitons would be conjured up from the quantum vacuum, but they would exist for such a short time that they would be impossible to detect. What Rothman and Boughn needed was a source that forges gravitons and spits them out, just as a light source spews out photons.
After casting around for possible sources, they concluded that black holes were an enticing prospect. Black holes are not quite the self-contained bodies they are sometimes portrayed to be: they can lose mass through what is known as Hawking radiation, which calculations indicate should be made up of a mix of photons, neutrinos and gravitons.
Out of the black hole
However, calculations show that only about 1 per cent of the radiation from a black hole is in the form of gravitons. What’s more, only small black holes produce a significant amount of Hawking radiation; the bigger one like those formed by collapsing stars emit very little.
The black holes most likely to be a source of gravitons are therefore mini-black holes of the kind thought to have been created in the extreme and violent conditions of the big bang. However, calculations show that mini-black holes with a mass of less than about a billion tonnes emit Hawking radiation so strongly that they will have evaporated during the lifetime of the universe, while those heavier than 10 million billion tonnes will produce gravitons with too little energy to detect. “There is only a limited range of black hole masses that can provide a detectable source of gravitons,” Rothman says.
There is, however, a second possible source of gravitons that may be more promising than black holes. In stars, the seething temperatures rip electrons from their atoms and these electrons are able to roam free. As they fly through the electric fields of nearby charged nuclei they are accelerated and, like all accelerated charges, spit out photons in a process called bremsstrahlung. Accelerating electrons should also emit gravitons, though calculations predict far fewer of them than bremsstrahlung photons. “We are talking about a ten-billion-billionth of the luminosity of the sun being emitted in the form of gravitons,” Rothman says. “Nevertheless, in a typical star there are a lot of electrons and nuclei, which compensates for this.” Based on previous work by Robert Gould at the University of California in San Diego, Rothman and Boughn calculate that the most promising sources of gravitons are super-dense white dwarfs and neutron stars.
Having identified where gravitons are most likely to be produced, the next step is to work out how best to detect them. After examining several possibilities, Rothman and Boughn zeroed in on the gravitational analogue of the photoelectric effect. When a photon of light with enough energy strikes an atom of certain materials it kicks out an electron, generating a small electric current that signals the arrival of the photon: this is the normal photoelectric effect. There should, in theory, also be a “gravitoelectric effect”, although no one has ever seen it: a graviton with enough energy to kick out an electron ought to produce an electric current that would reveal its presence.
So the pair knew where to look and what they might look for. But there was still a snag. Compared with the electromagnetic force, gravity is extremely feeble. To get an idea of just how feeble, consider a fridge magnet. Despite the whole of the Earth pulling on the magnet, its magnetic field holds it firmly to the fridge’s steel body. The gravitational force between a proton and an electron in a hydrogen atom is about 1040 times weaker than the electromagnetic force between them. This weakness reflects the extreme rarity with which gravitons interact with particles of matter, and for graviton hunters this spells trouble. “It is this incredibly weak interaction that makes directly detecting a single graviton phenomenally difficult,” Rothman says.
If anyone knows the ins and outs of how to detect weakly interacting particles it is those physicists who study neutrinos, so it was to this group that Rothman and Boughn turned for inspiration. Neutrinos interact with matter solely through the weak nuclear force. Although about 100 billion neutrinos from the sun are passing through every square centimetre of your skin every second, not one of them is likely to be stopped by the atoms in your body. To maximise their chances of bagging one of these elusive particles, neutrino hunters build huge detectors containing a large amount of matter. Even though a neutrino is extremely unlikely to be stopped by a single particle, it has a reasonable chance of being stopped by one of the trillions upon trillions of particles that make up the detector. “This would also be the strategy for detecting a graviton,” Rothman says.
The biggest neutrino detector, now being built at the South Pole, will use a cubic kilometre of Antarctic ice to search for neutrinos. “The graviton, however, is 1021 times less likely to interact with matter than even a neutrino,” Rothman says. “Because of its phenomenally weak interaction with matter, we’re talking about a detector that utterly dwarfs a neutrino detector.”
He’s talking about something very large indeed. In fact, according to Rothman and Boughn’s calculations it would have to be the biggest detector conceivable, something similar in mass to Jupiter. “Much bigger and the detector would shrink under its own gravity and become a brown dwarf,” says Rothman. Drifting through interplanetary space, the detector’s vast surface would be a web of glistening electronics. Building such a behemoth would be a colossal technical challenge and enormously expensive – clearly way beyond anything conceivable today.
“The detector would have to be similar in mass to Jupiter”
But let’s suppose it will one day be possible to build one. Would such a detector be capable of bagging a graviton? Rothman and Boughn calculate that during the lifetime of the universe, a detector placed as far from the sun as Earth is now would detect about 1000 gravitons. Placing the detector the same distance from a super-dense white dwarf or neutron star would collect up to a billion gravitons. That’s one every decade or so.
Even supposing that the detector works perfectly, and each graviton hit produces an electrical pulse, the problems go on. Millions of other particles would rain down on such a vast detector every second, and many of them would produce the same electrical signals as gravitons. The most troublesome particles are expected to be neutrinos, but the good news is that although they rarely interact with matter they are positively sociable compared with gravitons and so, in principle, could be shielded. Yet there is a problem: that you would need an impossible amount of shielding material. “Neutrinos can penetrate light years of lead,” says Rothman. “That much shielding would collapse into a black hole.”
And there is another possible pitfall. It took physicists more than 10 years to accept that the photoelectric effect proved the existence of photons, as Einstein had postulated in 1905. The clinching evidence came in 1916 when Robert Millikan of the California Institute of Technology in Pasadena plotted the number of electrons detected against the frequency of the light he used. “In other words, we might have to detect large numbers of gravitons in order to persuade physicists of their existence,” says Rothman. Given the monumental practical difficulties, it comes as no surprise to learn that Rothman and Boughn see no realistic chance of ever detecting a graviton. “I’d bet my house that nobody in this universe will ever detect one,” says Rothman.
Does this mean anything for physicists seeking a quantum theory of gravity? “Gravitons might not be physical entities but metaphysical entities,” Rothman says. “In which case quantising gravity may not be achievable, or even sensible.”
Other physicists disagree. They say that even if Rothman and Boughn are right, and gravitons are in practice undetectable, this has no implications for our quest for a quantum theory of gravity. “I don’t think detectability of the single particle state is necessary,” says Ted Jacobson of the University of Maryland. “It is the field in quantum theory that is the fundamental thing, not the particle.” James Hartle, a physicist at the University of California, Santa Barbara, takes a similar view. “As far as we know, everything else is quantum, so it is reasonable to pursue the quantisation of gravity,” he says.
“The question of whether or not we can detect a single graviton is quite irrelevant to the issue of constructing a fundamental theory of gravitation,” agrees Nobel prizewinner Steven Weinberg of the University of Texas, Austin. “There are plenty of things that we cannot observe – quarks, galaxies beyond the cosmological horizon, and so on – but that appear as respectable ingredients of our theories.” He sees no reason to require every ingredient to be observable. “The only reasonable demand is that enough of the consequences of the theory should be observable to give us a chance to verify or refute it.”
However, Lee Smolin at the Perimeter Institute in Ontario, Canada, believes that the lack of gravitons could force theorists to rethink their ideas about a quantum theory of gravity. “We still need a single theory that underlies both quantum theory and general relativity,” he says. “If there is no reason for it to be based on gravitons, this only widens the options for that theory.”
Some physicists do not rule out the possibility of detecting gravitons, despite Rothman and Boughn’s calculations. Ian Moss of the University of Newcastle upon Tyne in the UK suggests that they could be produced by smashing electrons together in a particle accelerator, though this approach faces practical obstacles almost as daunting as the mega-detector. It would require a particle accelerator capable of bringing electrons up to the so-called Planck energy, where gravity becomes comparable in strength to nature’s other fundamental forces. This is thought to be 10 million billion times the energy achievable in the Large Hadron Collider, due to switch on next year. Moss points out, however, that if, as string theory suggests, the universe has more dimensions than the four we experience, then they may conspire to reduce the Planck energy, allowing gravitons to be seen at lower energies.
Rothman can’t be drawn to a final conclusion. “All we did was a calculation,” he says. “We are not at all sure that the concept of the graviton is dead.”
