快猫短视频

SUPPOSE you had a little gadget that could explain all the greatest mysteries of the Universe. Like the scientific equivalent of some ancient oracle, you could consult it to learn how the laws of physics emerged in the fireball of the big bang. Press the right buttons and it would tell you why the cosmos is ballooning outwards ever faster, and why matter, rather than its nemesis antimatter, dominates the Universe.

The gadget has already been invented. It doesn鈥檛 look like much: a blob of helium chilled almost to absolute zero. But according to Grigori Volovik of Helsinki University of Technology and the Landau Institute for Theoretical Physics in Moscow, this refrigerated droplet can help explain some of the Universe鈥檚 most elusive secrets. It can lay bare the nature of gravity and the intimate workings of black holes, for example. It exposes the newborn Universe to scrutiny and tracks down the origin of physical laws and elementary particles. And the helium oracle has already pronounced on the major quest in physics, the search for a quantum theory of gravity. 鈥淚t doesn鈥檛 exist,鈥� Volovik says.

His ideas are controversial, of course, and intensely complex. But they are winning some prominent supporters, and Volovik鈥檚 following could be about to grow. In a book due to be published next year, Volovik has spelled out exactly why helium is the oracle every cosmologist should be consulting.

His conviction arises from three decades of work with superfluid helium-3, a weird liquid that defies common sense. When helium is cooled to near absolute zero, it stays liquid but starts to follow quantum rather than classical rules. Effectively, the atoms lose their individuality and begin to influence each others鈥� movements. Helium-3 can then perform tricks like flowing along pipes without friction and defying gravity by climbing the walls of a beaker.

We have long known that superfluid helium has profound mathematical links with the cosmos. For a start, as with every other moving fluid, the way sound moves through superfluid helium is mathematically similar to the way light moves in a gravitational field. But, with helium, the parallels run much deeper.

The first indication of helium鈥檚 peculiar aptitude for cosmology came with its mimicry of 鈥渃osmic strings鈥�. Most scientists believe that in the very first moments of the Universe, there was only one force. Within a split second, this force decayed into the four very different forces we know today: gravity, the strong and weak nuclear forces, and the electromagnetic force. In the 1970s, Tom Kibble of Imperial College, London, suggested this process would thread the cosmos with stringy flaws or 鈥渄efects鈥�: long, thin cracks 鈥� or cosmic strings 鈥� in the fabric of the expanding Universe.

The idea was appealing because matter could have clustered around the strings to form galaxies, explaining why the galaxies we see today lie along filaments separated by giant voids.

It was hard to see how this idea could be tested until, 15 years later, Wojciech Zurek of the Los Alamos National Laboratory in New Mexico noticed that the maths underpinning cosmic strings is the same as the maths describing similar kinds of defects in superfluid helium. As liquid helium cools into the superfluid state, it undergoes a phase transition, akin to water turning into ice. This can leave behind little flaws. When different parts of the fluid choose to move in different directions, lines of rotating vortices are created along the border where these regions meet.

Zurek made clear predictions about what scientists could expect to see in experiments that generate vortices in superfluid helium by spinning it in a rotating cryostat (快猫短视频, 21 September 1996, p 46). If Kibble鈥檚 idea about cosmic strings was right, there should be an obvious relationship between the smallest vortices that can expand and be detected, and the chamber鈥檚 rotation speed. Volovik and his colleagues at Helsinki University鈥檚 Low Temperature Laboratory decided to put the theory to the test in 1995. And sure enough, the numbers matched up. 鈥淲e checked that Kibble was right 鈥� his scenario does work,鈥� says Volovik.

In the past couple of years, measurements of the cosmic microwave background 鈥� radiation left over from the big bang 鈥� have indicated that cosmic strings may not have played such an important role in shaping the distribution of galaxies. Nevertheless, confirming that such defects may have formed was a major triumph for helium cosmology, proving its predictive power. So Volovik has now set his sights on bigger things.

Indeed, he鈥檚 looking for the holy grail of physics. The phenomena that excite physicists most occur at high energies and on the tiniest scales. These are the conditions of the Universe鈥檚 birth. Yet our theories simply don鈥檛 work at these scales, and our particle accelerators aren鈥檛 powerful enough to achieve the highest energies. But while the real Universe guards its secrets, the helium universe tells all.

Volovik鈥檚 oracle is a superfluid state of the helium-3 isotope called 3He-A. Superfluid helium-3 can exist in two phases, A and B, depending on its temperature and pressure. Volovik claims the A phase accurately mimics almost all the properties of space-time, which is the four-dimensional fabric of the Universe.

Far from being nothing, the vacuum of empty space is a frothing sea of virtual particles that pop in and out of existence by briefly borrowing energy from the vacuum. 3He-A is just as complicated. It will never solidify, even if you could chill it to the unattainable zero kelvin. And at a whisker above absolute zero, 鈥渜uasiparticles鈥�, packets of sound energy, start to appear. The slight sloshing of atoms at anything above zero kelvin generates little sound waves and 鈥� like all kinds of waves 鈥� these consist of packets of energy of various sizes.

The quasiparticles can be detected and, what鈥檚 more, their behaviour follows some of the same mathematical laws that govern the behaviour of elementary particles in a vacuum, such as electrons, neutrinos and quarks. For instance, some quasiparticles interact with each other in the same way that gravitons, the hypothetical carriers of the gravitational force, interact with matter particles such as the electron, according to Volovik. In fact, he says, 3He-A, like space-time, can reveal all the physics of both gravitational and electromagnetic fields.

His analysis of 3He-A has led him to some startling conclusions. First of all, he says, helium indicates that gravity is not a fundamental property of the Universe. In other words, it wasn鈥檛 there from the beginning. As the helium liquid cools from a high-energy, chaotic state into the low-energy, superfluid state, order begins to take shape. First the quantised state emerges. Only then do quasiparticles showing specific properties 鈥� the analogues of gravity and electromagnetism, for instance 鈥� start to appear. He believes this shows the newborn cosmos was born in a lawless state.

In the real Universe, everything was superhot and full of energy at first. Then, as the big bang fireball cooled, a single force that briefly ruled the Universe emerged after about 10^鈭�43 seconds, according to Volovik. Sometime after that the forces we know today 鈥� gravity, the strong nuclear force and so on 鈥� budded off. 鈥淥ut of the chaos Einstein鈥檚 theories surfaced, and forces like the strong and weak nuclear interactions emerged,鈥� he says.

He joins a growing clan of physicists, including Nobel prizewinner Robert Laughlin of Stanford University in California, who argue that the laws that we hold to be fundamental and the basis of reality are in fact emergent properties, spawned by the random chaos of the vacuum. It鈥檚 a provocative statement but, in Volovik鈥檚 view, there鈥檚 nothing really radical about it. Think of an 鈥渆mpty鈥� jar, which actually contains billions of air molecules jigging around and randomly bumping into each other. It鈥檚 chaotic, but from this mess the properties we call temperature and pressure emerge. Couldn鈥檛 the laws of space-time emerge in the same way?

If Volovik is right, trying to find a theory of quantum gravity is a waste of time. He knows that鈥檚 another message few people will want to hear: huge numbers of physicists spend their days trying to marry general relativity 鈥� Einstein鈥檚 theory of gravity 鈥� with quantum mechanics. They hope such a theory will enable them to describe what happens when space-time, forces and matter are confined to tiny scales and endowed with enormous amounts of energy 鈥� as at the very beginning of the Universe. Without it, they say, we can never understand the birth of the Universe or the weird conditions at the hearts of black holes.

But so far, no one has found a convincing theory of quantum gravity. 鈥淎fter 70 years of research, in spite of numerous achievements, quantum gravity is still far from realisation,鈥� says Volovik. He sees this as confirmation that it鈥檚 simply not possible to 鈥渜uantise鈥� general relativity. In the highest energy conditions, the quantum gravitational field would become so swamped by the chaos of other particles and fields that it would be impossible to isolate anything that looked like gravity.

He does have some support on this point. 鈥淚t鈥檚 speculative, but I think there is a very good chance that he is correct,鈥� says Matt Visser of Victoria University in Wellington. 鈥淚t鈥檚 possible that gravity is simply a low-energy approximation to something that is radically different at short distances and high energies. If this is the case, 鈥榪uantum gravity鈥� makes no sense at all.鈥�

If so, scientists will need to change tack. The whole point of the exercise would be different, Visser says. The aim would be to find some quantum theory, no matter how bizarre, from which general relativity emerges at low energies. 鈥淭his is very different from trying to quantise general relativity itself,鈥� he says.

And the powers of Volovik鈥檚 oracle don鈥檛 stop there. There are plenty of other questions helium can address, he says. 快猫短视频s would love to test a key prediction about black holes, for example. Theory suggests that black holes trap matter and even light inside a region called the event horizon. Nothing can escape. But in the 1970s, the British scientist Stephen Hawking showed that tiny amounts of energy leak from black holes as a result of pairs of particles popping up at the horizon. One half of the pair could fall in, while the other could escape. The escaping particle would steal some of the energy 鈥� effectively the mass 鈥� from the black hole. Eventually, the black hole might evaporate through this 鈥淗awking radiation鈥�.

The trouble is that this radiation is extremely weak. So weak, in fact, that a typical black hole in space would take far longer than the current age of the Universe to evaporate completely. Being realistic, astronomers have no hope of ever detecting Hawking radiation from black hole candidates in space.

But some researchers now believe that carefully tuned flows of superfluids can mimic the physics of black holes (快猫短视频, 18 March 2000, p 22). A flow of superfluid helium, for example, creates an 鈥渆vent horizon鈥� wherever the flow of the superfluid is faster than the maximum speed of the quasiparticles. Like the photons of light that can鈥檛 climb out of a black hole, the quasiparticles can鈥檛 escape the superfluid鈥檚 current.

Volovik and his colleagues are planning to create a helium 鈥渂lack hole鈥� using two superfluids sliding over each other. They showed earlier this year that this set-up can give rise to an event horizon for quasiparticles on the boundary between the two flows. Instead of a particle stealing energy from a black hole as Hawking radiation, a few quasiparticles can leak out by stealing kinetic energy from the fluid鈥檚 motion (see ). Although this quasiparticle analogue of Hawking radiation may turn out to be too small to measure, it could provide the first chance scientists have to see some detailed black hole physics in action.

Visser is inspired by this possibility. 鈥淭he single most exciting aspect of all these analogue models is that they hold out the hope 鈥� not quite a guarantee 鈥� that we may be able to do laboratory experiments on analogue black holes in the not-too-distant future,鈥� he says. 鈥淒irect experimental evidence for analogue Hawking radiation would have a truly stunning impact.鈥�

Another problem Volovik is consulting his helium oracle about concerns the accelerating expansion of the Universe. In 1998, scientists announced results from observations of supernova explosions that suggest the expansion of the Universe is speeding up. It seems as though space exerts a repulsive force, sometimes known as the cosmological constant, through some kind of 鈥渄ark energy鈥�.

What鈥檚 the source of the dark energy? No one really knows. A good guess might be that it stems from the energy of the seething vacuum. But if you use the equations of quantum mechanics to tot up just how much vacuum energy there should be in the Universe, adding the contributions from virtual particles and random fluctuations of fields, you get a huge figure. The strength of its repulsion should be enormous 鈥� about 120 orders of magnitude too big to cause the observed acceleration.

Flummoxed, many scientists blame the discrepancy on the highest energy fluctuations, which are a hidden factor because theorists haven鈥檛 yet managed to calculate them. Some of these contributions would be negative. Could these contributions to the vacuum energy almost cancel out the others, leaving a very small cosmological constant? Yes, says Volovik 鈥� because helium says so. He doesn鈥檛 even need to do an experiment to prove it.

Imagine, he says, that people lived in superfluid 3He-A. 鈥淭he people are made of elementary quasiparticles, just like we are made from elementary particles,鈥� he suggests. In superfluid helium the quasiparticles do not exert any kind of force on the atoms in the liquid. Because there鈥檚 no interaction between the quasiparticle people and the helium atoms, the people would think they were living in empty space. If the quasiparticle people tried to compute the energy of their vacuum, by adding up the energies of fluctuations in the 鈥渆mpty space鈥� around them, they鈥檇 get a huge answer. 鈥淲hen they compared their estimate with reality, they would also find that the estimate is many orders of magnitude too big,鈥� says Volovik. 鈥淪o for them that cosmological constant problem is as puzzling as it is for us.鈥�

But, from outside the helium cosmos, we can see all the components of the atomic structure 鈥� unlike in our Universe, nothing is hidden. And so we can calculate its various effects, Volovik says. The effect of the helium atoms, which mimic the effects of the highest energy physics in the Universe, is to counter the vacuum energy. When you add everything up, the total is a small value 鈥� and crucially, it鈥檚 not zero, matching the small observed acceleration in the Universe鈥檚 expansion.

Another puzzle helium could help solve is this: why is the Universe dominated by matter, rather than antimatter, when theory says the big bang should have created equal amounts of both? Today, the antimatter is nowhere to be seen. Presumably, it clashed with the matter and vanished in a puff of radiation. But for any matter to be left behind to build stars and galaxies, there must have been slightly more matter than antimatter around.

One leading theory is that, as the Universe evolved, the vacuum itself spawned a tiny excess of quarks over antiquarks. This was predicted back in the 1960s and encapsulated in something known as the Bell-Jackiw-Adler anomaly. If this bias really exists, it should show its hand in superfluid helium by creating a tiny, extra force on a spinning vortex. Volovik and his colleagues have confirmed in experiments that this force does exist. Once again, helium has confirmed that an equation which may describe events in the early Universe is correct.

Volovik says helium can explain a host of other things. Why space appears to be completely flat, rather than curved as Einstein鈥檚 equations say it easily could be, for example. Why galaxies are threaded with tangled magnetic fields. And which of the 鈥渇undamental鈥� constants of the Universe, such as the speed of light, are genuinely fundamental and unchanging.

So far, though, he hasn鈥檛 gathered many disciples. He puts that down to an unhealthy divide between the physicists who play with general relativity and those who scrutinise the harsh realities of solid, real-world experiments. 鈥淭he dirty solid body is in great contrast to the beautiful mathematical world of general relativity,鈥� he says. 鈥淔or many people, the idea that general relativity is not fundamental and can be violated at high energy is unbearable.鈥�

Ulf Leonhardt of St Andrews University is also trying to make artificial black holes. He thinks Volovik鈥檚 only problem is his own cleverness. Most scientists just don鈥檛 have a broad enough grasp of the issues involved to recognise the parallels between cosmological and superfluid theories. Leonhardt thinks Volovik鈥檚 work is as sound and rigorous as it is provocative.

Even those who are familiar with both condensed matter and general relativity have some reservations, however. According to Visser, fluids such as liquid helium serve as a fine model for about half of general relativity. The other half is less certain. He鈥檚 unsure whether superfluid helium can really explain the cosmological constant, for instance.

鈥淭here鈥檚 room for debate there,鈥� says Visser. 鈥淭here are several technical issues that seem to stand in the way of getting a clean version of the Einstein equations out of these analogue models. I think it鈥檚 possible, maybe even plausible, but I do not see a rigorous proof.鈥�

But such proofs may soon emerge. The European Science Foundation has created a programme called COSLAB (Cosmology in the Laboratory) to explore the links between condensed matter, particle physics and cosmology. 快猫短视频s from 10 European nations are now tinkering with condensed matter such as superfluid helium to see what 鈥� if anything 鈥� it might tell us about black holes, the dark energy that drives the expansion of the cosmos, and defects in space-time.

Of course, there鈥檚 no guarantee that helium will expose the Universe鈥檚 deepest secrets. But according to Visser, we should at least try to test some of the notions that nature has stubbornly kept out of our reach. The helium oracle might turn out to be one of the best ideas in physics, he says. 鈥淚t is often quite remarkable how much really deep and fundamental physics can be found hiding in unexpected places.鈥�