杏吧原创

One of the good things about being God is that there鈥檚 not much competition. From time immemorial, no one else has boasted the skills necessary to create a universe. Now that鈥檚 about to change. 鈥淧eople are becoming more powerful,鈥� says Andrei Linde, a cosmologist based at Stanford University in California. 鈥淢aybe it鈥檚 time we redefine God as something more sophisticated than just the creator of the universe.鈥�

Linde was prompted to make this wry observation by the news that a glittering prize is within physicists鈥� reach. For decades, particle accelerators have been racking up an impressive list of achievements, including creating antimatter and exotic particles never seen in nature. The next generation of these giant colliders will provide the hunting ground for the elusive Higgs boson, thought to be the source of all mass. These machines might even create mini black holes. Mighty as those discoveries and creations are, however, they pale into insignificance beside what Nobuyuki Sakai and his colleagues at Yamagata University in Japan have now put on the table. They have discovered how to use a particle accelerator to create a whole new universe.

The idea of creating another cosmos is not a new one; in fact it has a long history. 鈥淭he story really begins with the question of the origin of our own universe,鈥� says Eduardo Guendelman, a physicist at Ben Gurion University, in Beer Sheva, Israel. As physicists began to research what kick-started our universe, they realised that if it happened once, maybe it could happen again. The growing acceptance of the big bang model in particular, which suggested that space-time burst forth in an explosion of energy concentrated in a tiny space, opened up a new set of tantalising possibilities. 鈥淧eople immediately started to wonder what would happen if you put lots of energy in one space in the lab 鈥� shot lots of cannons together,鈥� Linde says. 鈥淐ould you concentrate enough energy to set off a mini big bang?鈥�

For theoretical physicists the initial answer was a resounding 鈥渘o鈥�. All of the particles that you would create in such a process would have their own gravity, pulling them together. So, instead of creating a baby universe in the lab, you would just create a black hole. The idea came back to life in 1981, however, when Alan Guth at the Massachusetts Institute of Technology proposed the theory of inflation, which suggests the universe went through a period of rapid expansion just after the big bang.

Guth鈥檚 idea solves many cosmological puzzles, but no one really knew how or why it happened. So he and his colleagues set about trying to find out what might have made our universe inflate 鈥� exactly the question that would help us create a new universe using our own tools.

Inflation theory, subsequently modified by Linde, relies on the fact that the 鈥渧acuum鈥� of empty space-time is not a boring, static place. Instead, it is subject to quantum fluctuations that cause strange bubbles to appear at random times. These bubbles of 鈥渇alse vacuum鈥� contain space-time with different 鈥� and very curious 鈥� properties.

According to our best understanding of physics, the universe鈥檚 energy is stored in different kinds of fields. One of these is the Higgs field, which plays a vital role inside bubbles of false vacuum: it keeps the energy density within the bubbles constant.

That has two effects. First, it creates a repulsive gravitation within the bubble, which causes the bubble鈥檚 space-time to swell. Though that might seem just what a budding creator requires, the effect doesn鈥檛 change the size of the bubble to an observer on the outside 鈥� the growth is only noticeable within the body of the false vacuum bubble. Nor are things improved by the second consequence of a constant energy density.

In 1987, Guth and Ed Farhi, also at MIT, demonstrated that bubbles which start out below a certain size collapse under the tension of the bubble walls before the space they enclose has a chance to expand. That happens because the pressure inside the bubble of false vacuum is always lower than the pressure of the true vacuum outside. In other words, if the bubble is too small, its walls will always have a tendency to collapse inwards.

Frustratingly, there鈥檚 even a problem with the larger bubbles. Although they could grow to cosmological scales in theory, they also need a kick-start before they will expand. In our early universe, that kick was provided by the big bang, a point with infinite energy density. Reproducing one of those in the lab isn鈥檛 exactly plain sailing.

In 1989, however, Willy Fischler, a cosmologist at the University of Texas in Austin, and his colleagues found a way out. They showed that the same random quantum processes that create a false vacuum bubble would also allow it to spontaneously turn into a larger one. Their calculations showed that once this larger bubble had been burped out, it could inflate into a new universe without a kick-start from infinite energy.

Only one problem remained for the wannabe creators of the universe: a crippling time delay. The spontaneous transition from small bubble to large bubble can take a long time to occur, and the initial bubble tends to collapse under the tension of its walls before it can transform itself into a larger bubble.

Guendelman and his colleague Jacov Portnoy, also at Ben Gurion University, took matters into their own hands, and came up with some ways to prop the bubble walls apart. They factored in fields of energy known as 鈥済auge fields鈥� to the small bubble鈥檚 surface to keep its skin taut, holding it at a constant radius. 鈥淲ith that in place, all you need to do is sit there, watch the small bubble and wait for the large bubble to appear,鈥� says Guendelman.

By this point, however, the home-made universe was starting to look rather cobbled together, to say the least. Though Guendelman and Portnoy鈥檚 trick was neat, no one knew how you might add these gauge fields into an experiment. Plenty of other obstacles remained, too. For a start, although Guth had calculated that the size of bubble needed was invitingly small 鈥� just 10^-26 centimetres across 鈥� there still wasn鈥檛 any realistic way to create the bubble in the first place. What鈥檚 more, one unfortunate consequence of quantum mechanics is that, even if you set up the right conditions for the process to occur and stop the bubble imploding, you still don鈥檛 know exactly when the inflation will happen. So even with a small bubble in place, and Guendelman and Portnoy鈥檚 theoretical fields doing their job, there was no guarantee it would turn into a baby universe any time soon. 鈥淵ou could be sitting and waiting from here to eternity,鈥� says Guendelman.

Blowing bubbles

For years, Guendelman and his colleagues sought to break through this impasse. What they needed was something that would create the bubble, then quickly blow it up into a universe.

And that is what Sakai has just found. The vital part of the new universe-creation tool kit is a magnetic monopole 鈥� a strange spherical particle that encapsulates an isolated north or south magnetic field. For would-be universe builders, the big attraction of a monopole is that it has a huge mass concentrated into a tiny volume, with an enormous energy density created by the Higgs field 鈥� just like false vacuum bubbles.

Sakai and his team realised that a seemingly stable monopole in our universe is always teetering on the brink of expansion 鈥� needing just a nudge to start it inflating. Hurling mass onto the monopole 鈥� increasing its energy density 鈥� will push it over the edge, leading to runaway inflation (). 鈥淥ur calculations show that, given enough energy, the monopole will inflate eternally,鈥� Sakai says.

This process could be triggered naturally. According to Sakai, if a monopole floating through space collided with another massive object it would gain the mass needed to trigger inflation. One candidate is a cosmic string 鈥� a kind of high-energy rip in space-time. Though we have yet to see one, cosmic strings may have been created as a by-product of the big bang.

But since we have no cosmic strings 鈥� and we鈥檇 like to remain in control of the project 鈥� Sakai suggests we might be able to trigger inflation by hurling particles onto a monopole in an accelerator. This would add mass, and thus energy, to the monopole, and make it blow up into an entirely new universe.

It would be an extraordinary achievement, of course, but what happens then? It鈥檚 one thing to create a universe, but quite another to know where to keep it. After all, an eternally inflating universe might be expected to take up quite a bit of space 鈥� the cupboard under the stairs simply won鈥檛 do.

Actually this wouldn鈥檛 be a problem, Sakai says. For a start, the process warps space-time enormously, so that it is no longer the Euclidean space we are familiar with. This highly distorted space doesn鈥檛 have the same geometry as normal space, so it鈥檚 not as if the universe would blow up and engulf us.

Also, the baby universe has its own space-time and, as this inflates, the pressure from the true vacuum outside its walls continues to constrain it. As these forces compete, the growing baby universe is forced to bubble out from our space-time until its only connection to us is through a narrow space-time tunnel called a wormhole (see Graphic).

鈥淪itting inside the monopole, you would see space expanding in every direction鈥�

In the end, space-time becomes so distorted that even this umbilical cord is severed. The baby universe鈥檚 space-time is left entirely divorced from our own. If you were sitting inside the monopole, you would see space expanding, rushing out in every direction 鈥� just as it did after the big bang in our universe. The view from our universe, outside the monopole looking in, would be rather less spectacular.

鈥淥nce disconnected, the baby universe will be locked inside a microscopic black hole鈥�

Sakai鈥檚 calculations show that, once disconnected, the baby universe will be locked inside a microscopic black hole which will not appear to grow in size. This mini black hole will emit Hawking radiation and quickly evaporate from our universe. It will continue to grow its own space-time, but will leave behind little trace of its presence in our universe. 鈥淲e would make this tiny little thing and before we know it, it has flown away 鈥� escaped from our grasp,鈥� Linde laments.

Vanishing trick

In fact, it will disappear so fast it may even be difficult to tell if we鈥檝e managed to create it at all, Sakai says. Plans are under way to detect mini black holes in the next generation of particle accelerators, such as the Large Hadron Collider at CERN, in Switzerland, so Sakai is hopeful that in the future we will be able to detect the fleeting birth cries of a baby universe (New 杏吧原创, 23 April 2005, p 38).

Everyone agrees it鈥檚 an enticing idea, though Fischler is not counting his baby universes before they鈥檙e hatched. 鈥淭he real question is whether it is doable,鈥� he says. There鈥檚 still a problem of raw materials, he points out. As yet, not even one monopole has been detected in nature.

Ironically, one of inflation theory鈥檚 greatest successes was to explain why we have had such difficulty finding these elusive monopoles, despite theoretical predictions that they should exist all around us. Inflation argues that our visible universe grew from a quantum fluctuation so small it contained only one monopole. That particle is out there somewhere, but the odds are against our bumping into it.

So if we aren鈥檛 likely to run into a natural monopole any time soon, just how will we get our hands on one? Maybe we could make one in the lab, Fischler concedes. Colliding an electron with a positron in a particle accelerator could, in principle, create a monopole-antimonopole pair. And, according to Sakai, we could then trigger inflation by crashing other particles onto our new monopole, adding more and more mass to it.

Generating a monopole-antimonopole pair in the lab is, however, incredibly problematic. Monopoles have huge masses compared to electrons and positrons. To make up this mass difference we would have to smash the initial particles together with far greater energies than are available in today鈥檚 accelerators. Even assuming that at some future time we would have the technology to reach these energies, the chances of success are still slim. 鈥淵ou would be colliding two ping-pong balls and hoping that two perfectly formed bowling bowls fall out,鈥� says Fischler. 鈥淚 would say that in practical terms this is even worse than hopeless.鈥�

OK, so it鈥檚 not exactly a school science project, but the budding universe creators are excited, nonetheless. 鈥淚 think our model is one of the most realistic for creating a universe in the lab because it uses materials that may well already be out there,鈥� Sakai says. Guendelman agrees. 鈥淥urs was just a theoretical idea, but they get a similar effect using something that is predicted to exist by well-known theories,鈥� he says. In addition, Sakai鈥檚 mechanism solves the waiting problem, Guendelman says.

The question is, would it be worth all the effort? Linde thinks so. 鈥淚 sat down and really thought about why we should even care about creating a universe in the laboratory,鈥� Linde says. 鈥淲e put energy into the baby universe to create it, but we can鈥檛 get any energy out of it 鈥� we can鈥檛 mine its resources.鈥� Once it鈥檚 formed, he adds, its space and time 鈥� though growing 鈥� is entirely divorced from our own. 鈥淲e can鈥檛 jump into this tiny thing and visit it,鈥� says Linde. 鈥淲e don鈥檛 seem to be able to communicate with it at all.鈥�

In the end, Linde realised he had overlooked the obvious motivation: good old-fashioned megalomania. 鈥淛ust imagine if it鈥檚 true and there鈥檚 even a small chance it really could work,鈥� he says. 鈥淚n this perspective, each of us can become a god.鈥�