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'We can build a real time machine'
Travelling in time might sound like a flight of fancy, but some physicists think it might really be possible. BBC Horizon looked at some of the most promising ideas for turning this staple of science fiction into reality.
Ron Mallett has a dream: He wants to travel in time.
This isn't mere fantasy - Mallett is a respected professor of physics.
"I think of myself as being an ordinary person with a passion, and my passion is the possibility of time travel," he says.
Prof Mallett has wanted to build a time machine for most of his life. His passion, he explains, can be traced to a tragic event early in his life.
Ron's father, a heavy smoker, died of a heart attack at the age of 33 - when Prof Mallett was just 10 years of age. Ron was devastated and withdrew into his books.
"A year after that when I was 11, I came across the book that changed everything for me. That was The Time Machine, by HG Wells," the University of Connecticut physicist told the BBC's Horizon programme.
"The cover caught my attention, but it was when I read the inside, and it said: 'Scientific people know very well that time is just a kind of space and that we can move forward and backwards in time, just as we can… in space'.
"When I read that I said: 'This is wonderful!'."
Prof Mallett explains: "If I could build a time machine, then I could go back into the past and see my father again and maybe save his life and change everything."
Time travel may sound far-fetched, but scientists are already exploring several mysteries of nature that could one day see Ron's dream fulfilled.
Albert Einstein thought the three dimensions of space were linked to time - which serves as a fourth dimension. He called this system space-time, and it's the model of the Universe that we use today.
But Einstein also thought it was possible to fold space-time, creating a shortcut between two distant locations. This phenomenon is called a wormhole, and it can be visualised as a tunnel with two openings, each emerging at different points in space-time.
Wormholes might exist naturally in the cosmos; indeed, scientists in Russia are trying to use radio telescopes to detect them. But using wormholes for time travel won't be straightforward.
The nearest ones could be many light-years away. And even if you could get to them and then survive the journey through them, there's no guarantee where you'd end up.
But some physicists have speculated that we might be able to conjure up bespoke wormholes at some point in the future - though we currently have no idea how.
Physics also predicts that wormholes would have a habit of collapsing, crushing whatever's inside them. If a time machine is ever to exploit them, we'd have to find a way to stop this inconvenient feature.
The mysterious phenomenon of dark energy might provide a solution. In the 1990s, astronomers found that the expansion of the Universe was speeding up, rather than slowing down as might have been expected.
"Something out there is having an 'anti-gravity' effect - it's pushing rather than pulling. We don't know what that is, but it makes up most of the Universe. We call it dark energy," says Prof Tamara Davis, a cosmologist at the University of Queensland in Australia.
A wormhole will only work for time travel if its "mouth" can be held open for long enough that it allows something to travel through it. That requires something called negative energy, which doesn't really exist in the everyday world.
But the dark energy that permeates the cosmos fits the bill - if we can figure out what it is, we might be able to prop open a wormhole long enough to go in one end and out the other.
"We don't know whether we are able to make a wormhole, whether that's technically within our capabilities… But who knows what a future human civilization is going to be able to do," says Prof Davis.
"Technology has advanced so rapidly that maybe space and time themselves are something that can come under our control."
Wormholes exist at the more speculative end of physics, offering one approach to travelling in time. But Ron Mallett has another.
He has drawn up plans for an actual time machine, and his concept was inspired by a book he read at age 12 about Albert Einstein's equations.
Prof Mallett has built a table-top device that illustrates principles he thinks could be used to build a real, working time machine. First, lasers are used to generate a circulating beam of light. The space inside this "ring laser" should become twisted, "like stirring a cup of coffee", the University of Connecticut professor explains.
Because space and time are intimately connected, warping space should also warp time. Prof Mallett's theoretical work has shown that, given enough laser intensity in a small enough space, it should be possible to alter the normally linear timeline we all inhabit.
"If space is being twisted strongly enough, this linear timeline is going to be twisted into a loop. If time all of a sudden is twisted into a loop that allows us the possibility of travelling into the past," says Ron Mallett.
However, in order to make it work, the concept would require vast amounts of power and a way of shrinking everything to a microscopic scale.
But once we have a time machine, using it successfully will require a detailed understanding of time itself.
The generally accepted view is that the Universe is an unchanging "block" of space-time; this idea arises directly from Einstein's equations.
"What's important about the model is the idea that the past, present and future are all equally real. So you can think of everything that ever did exist, does exist or will exist as all somehow being out there in space-time," says Dr Kristie Miller, director of the Centre for Time at the University of Sydney, Australia.
"The dinosaurs are all out there somewhere in the past doing dinosaur stuff, we're all here now and all of the future is out there somewhere in space-time too."
One way to visualise the block model is to think of other places in time as being like other places in space: "We are here in Sydney, but there are other people located in Singapore and London. Those places are perfectly real, it's just that we aren't at them," says Dr Miller.
This is good news for the budding time traveller, because it suggests there is nothing to stop us from swapping where we are now for some other place and time.
But, importantly, it also implies that the past, present and the future are already written, so that if we were to travel back in time, we wouldn't be able to alter it. To take an oft-quoted example, we shouldn't be able to kill someone's grandparent so that their descendant will cease to exist in the future.
The block model treats our everyday concept of time as an illusion, a way that humans rationalise reality. But Prof Lee Smolin, from the Perimeter Institute in Waterloo, Canada, disagrees. He believes that the passage of time is a real and fundamental phenomenon.
"Time travel is probably impossible," he says. "If what's real is the present moment and the past is only real in the sense that there are memories and records of it in the present, and the future is still to exist… there's nowhere to go."
His colleague Prof Neil Turok, director of the Perimeter Institute, thinks the weird world of quantum physics could be crucial to answering this question.
This area of physics emerges at very small scales, where the rules of classical physics we learnt about in our school textbooks break down. For example, in the quantum world, it might be possible for a particle to be in many places at once.
"I think it's clear to me that there is some probability of us going backwards in time," he says. "In quantum physics, nothing is impossible - particles travel through walls!"
Prof Turok explains that time travel remains a distant hope because "no one really has any plausible idea of how to go backwards in time right now". But he adds: "One should never say never, because some clever person will come along and tell you how to break the rule."
Horizon: How To Build a Time Machine was broadcast on BBC Two on Tuesday 10 July and is now available on the BBC iPlayer.
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