摘要：Making a wormhole that a human could theoretically travel through would require an upside-down universe and negative energy
IN YOU go and out you pop – in a galaxy far, far away. Such is the incredible promise of the wormhole, a hypothetical portal in which space-time funnels into a narrow passage only to open up somewhere else, possibly on the other side of the universe.
It sounds fantastical, but 50 years ago many said the same about black holes, which also involve highly warped space-time. “We study wormholes partly for fun and partly, more seriously, to see what is physically allowable for space-time,” says theorist Toby Wiseman at Imperial College London. “And of course – who knows – perhaps one day, in the very far future, this could be an actual technology.”
Despite their mythic reputation, there is nothing especially outlandish about wormholes. They are predictions of Albert Einstein’s general theory of relativity, which says that mass creates gravity by warping the fabric of the universe. General relativity has allowed for an ever-enlarging universe, the big bang and black holes. In that context, wormholes seem no more far-fetched. In fact, Einstein himself was one of the first to provide a mathematical description of them, in the mid-1930s.
“The great thing about general relativity is that you can write down any space-time you want, plug it into the Einstein equations and translate it into what matter you would need to support it,” says Wiseman. So if you want space-time to look like a wormhole, you need a certain sort of matter.
What sort? The trick is to find something that can prop open a wormhole without it collapsing straight away. In 2020, Juan Maldacena at the Institute for Advanced Study (IAS) in New Jersey and Alexey Milekhin, then at Princeton University, predicted that matter that interacts via gravity alone could produce a wormhole big enough for humans to fit through. However, we are yet to identify matter that interacts via gravity alone, such as the dark matter thought to keep galaxies from flying apart, with the characteristics required. “As far as we know, this construction can’t actually be embedded in our universe,” says Wiseman.
Then again, ordinary matter could generate traversable wormholes, according to Ping Gao at the Massachusetts Institute of Technology, Daniel Jafferis at Harvard University and Aron Wall at the IAS – albeit in a type of upside-down model universe in which gravity emerges as a “holographic” property.
Their trick was to employ the quantum property of entanglement, by which measurements on one particle can instantaneously affect the properties of another, an arbitrary distance away. Performing measurements on pairs of particles entangled over a wormhole would stop the wormhole collapsing, they say.
The simulated wormhole that wasn’t
Proof of this concept seemed to arrive last year when Jafferis and others claimed to have simulated a wormhole on Google’s Sycamore quantum computer, with the transport of a particle from one quantum chip to another.
But Norman Yao at the University of California, Berkeley, and his team disputed the claim, saying that whatever happened didn’t bear the hallmarks of holographic gravity and therefore wasn’t a wormhole proper. Hrant Gharibyan at the California Institute of Technology says it was more like conventional quantum teleportation: “The hardware needed [to simulate this type of wormhole] might be another 10 years away.”
Still, we don’t live in the upside-down universe in which these wormholes could exist. Besides, any passage through a wormhole would demand negative energy to keep it open during transit. Quantum physics permits some of this, but only so much. The result, says Wiseman, is that passage though wormholes, if they exist, is almost certainly forbidden to be quicker than any conventional route. “If this is true, there’s no shortcut,” he says.