All you need is a wormhole, the Large Hadron Collider or a rocket that goes really, really fast

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'Through the wormhole, the scientist can see himself as he was one minute ago. But what if our scientist uses the wormhole to shoot his earlier self? He's now dead. So who fired the shot?'

Hello. My name is Stephen Hawking. Physicist, cosmologist and something of a dreamer. Although I cannot move and I have to speak through a computer, in my mind I am free. Free to explore the universe and ask the big questions, such as: is time travel possible? Can we open a portal to the past or find a shortcut to the future? Can we ultimately use the laws of nature to become masters of time itself?

Time travel was once considered scientific heresy. I used to avoid talking about it for fear of being labelled a crank. But these days I'm not so cautious. In fact, I'm more like the people who built Stonehenge. I'm obsessed by time. If I had a time machine I'd visit Marilyn Monroe in her prime or drop in on Galileo as he turned his telescope to the heavens. Perhaps I'd even travel to the end of the universe to find out how our whole cosmic story ends.

To see how this might be possible, we need to look at time as physicists do - at the fourth dimension. It's not as hard as it sounds. Every attentive schoolchild knows that all physical objects, even me in my chair, exist in three dimensions. Everything has a width and a height and a length.

But there is another kind of length, a length in time. While a human may survive for 80 years, the stones at Stonehenge, for instance, have stood around for thousands of years. And the solar system will last for billions of years. Everything has a length in time as well as space. Travelling in time means travelling through this fourth dimension.

To see what that means, let's imagine we're doing a bit of normal, everyday car travel. Drive in a straight line and you're travelling in one dimension. Turn right or left and you add the second dimension. Drive up or down a twisty mountain road and that adds height, so that's travelling in all three dimensions. But how on Earth do we travel in time? How do we find a path through the fourth dimension?

Let's indulge in a little science fiction for a moment. Time travel movies often feature a vast, energy-hungry machine. The machine creates a path through the fourth dimension, a tunnel through time. A time traveller, a brave, perhaps foolhardy individual, prepared for who knows what, steps into the time tunnel and emerges who knows when. The concept may be far-fetched, and the reality may be very different from this, but the idea itself is not so crazy.

Physicists have been thinking about tunnels in time too, but we come at it from a different angle. We wonder if portals to the past or the future could ever be possible within the laws of nature. As it turns out, we think they are. What's more, we've even given them a name: wormholes. The truth is that wormholes are all around us, only they're too small to see. Wormholes are very tiny. They occur in nooks and crannies in space and time. You might find it a tough concept, but stay with me. 

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A wormhole is a theoretical 'tunnel' or shortcut, predicted by Einstein's theory of relativity, that links two places in space-time - visualised above as the contours of a 3-D map, where negative energy pulls space and time into the mouth of a tunnel, emerging in another universe. They remain only hypothetical, as obviously nobody has ever seen one, but have been used in films as conduits for time travel - in Stargate (1994), for example, involving gated tunnels between universes, and in Time Bandits (1981), where their locations are shown on a celestial map

Nothing is flat or solid. If you look closely enough at anything you'll find holes and wrinkles in it. It's a basic physical principle, and it even applies to time. Even something as smooth as a pool ball has tiny crevices, wrinkles and voids. Now it's easy to show that this is true in the first three dimensions. But trust me, it's also true of the fourth dimension. There are tiny crevices, wrinkles and voids in time. Down at the smallest of scales, smaller even than molecules, smaller than atoms, we get to a place called the quantum foam. This is where wormholes exist. Tiny tunnels or shortcuts through space and time constantly form, disappear, and reform within this quantum world. And they actually link two separate places and two different times.

Unfortunately, these real-life time tunnels are just a billion-trillion-trillionths of a centimetre across. Way too small for a human to pass through - but here's where the notion of wormhole time machines is leading. Some scientists think it may be possible to capture a wormhole and enlarge it many trillions of times to make it big enough for a human or even a spaceship to enter.

Given enough power and advanced technology, perhaps a giant wormhole could even be constructed in space. I'm not saying it can be done, but if it could be, it would be a truly remarkable device. One end could be here near Earth, and the other far, far away, near some distant planet.

Theoretically, a time tunnel or wormhole could do even more than take us to other planets. If both ends were in the same place, and separated by time instead of distance, a ship could fly in and come out still near Earth, but in the distant past. Maybe dinosaurs would witness the ship coming in for a landing.

The fastest manned vehicle in history was Apollo 10. It reached 25,000mph. But to travel in time we'll have to go more than 2,000 times faster

Now, I realise that thinking in four dimensions is not easy, and that wormholes are a tricky concept to wrap your head around, but hang in there. I've thought up a simple experiment that could reveal if human time travel through a wormhole is possible now, or even in the future. I like simple experiments, and champagne.

So I've combined two of my favourite things to see if time travel from the future to the past is possible.

Let's imagine I'm throwing a party, a welcome reception for future time travellers. But there's a twist. I'm not letting anyone know about it until after the party has happened. I've drawn up an invitation giving the exact coordinates in time and space. I am hoping copies of it, in one form or another, will be around for many thousands of years. Maybe one day someone living in the future will find the information on the invitation and use a wormhole time machine to come back to my party, proving that time travel will, one day, be possible.

In the meantime, my time traveller guests should be arriving any moment now. Five, four, three, two, one. But as I say this, no one has arrived. What a shame. I was hoping at least a future Miss Universe was going to step through the door. So why didn't the experiment work? One of the reasons might be because of a well-known problem with time travel to the past, the problem of what we call paradoxes.

Paradoxes are fun to think about. The most famous one is usually called the Grandfather paradox. I have a new, simpler version I call the Mad Scientist paradox.

I don't like the way scientists in movies are often described as mad, but in this case, it's true. This chap is determined to create a paradox, even if it costs him his life. Imagine, somehow, he's built a wormhole, a time tunnel that stretches just one minute into the past.

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Hawking in a scene from Star Trek with dinner guests from the past, and future: (from left) Albert Einstein, Data and Isaac Newton

Through the wormhole, the scientist can see himself as he was one minute ago. But what if our scientist uses the wormhole to shoot his earlier self? He's now dead. So who fired the shot? It's a paradox. It just doesn't make sense. It's the sort of situation that gives cosmologists nightmares.

This kind of time machine would violate a fundamental rule that governs the entire universe - that causes happen before effects, and never the other way around. I believe things can't make themselves impossible. If they could then there'd be nothing to stop the whole universe from descending into chaos. So I think something will always happen that prevents the paradox. Somehow there must be a reason why our scientist will never find himself in a situation where he could shoot himself. And in this case, I'm sorry to say, the wormhole itself is the problem.

In the end, I think a wormhole like this one can't exist. And the reason for that is feedback. If you've ever been to a rock gig, you'll probably recognise this screeching noise. It's feedback. What causes it is simple. Sound enters the microphone. It's transmitted along the wires, made louder by the amplifier, and comes out at the speakers. But if too much of the sound from the speakers goes back into the mic it goes around and around in a loop getting louder each time. If no one stops it, feedback can destroy the sound system.

The same thing will happen with a wormhole, only with radiation instead of sound. As soon as the wormhole expands, natural radiation will enter it, and end up in a loop. The feedback will become so strong it destroys the wormhole. So although tiny wormholes do exist, and it may be possible to inflate one some day, it won't last long enough to be of use as a time machine. That's the real reason no one could come back in time to my party.

Any kind of time travel to the past through wormholes or any other method is probably impossible, otherwise paradoxes would occur. So sadly, it looks like time travel to the past is never going to happen. A disappointment for dinosaur hunters and a relief for historians.

But the story's not over yet. This doesn't make all time travel impossible. I do believe in time travel. Time travel to the future. Time flows like a river and it seems as if each of us is carried relentlessly along by time's current. But time is like a river in another way. It flows at different speeds in different places and that is the key to travelling into the future. This idea was first proposed by Albert Einstein over 100 years ago. He realised that there should be places where time slows down, and others where time speeds up. He was absolutely right. And the proof is right above our heads. Up in space.

This is the Global Positioning System, or GPS. A network of satellites is in orbit around Earth. The satellites make satellite navigation possible. But they also reveal that time runs faster in space than it does down on Earth. Inside each spacecraft is a very precise clock. But despite being so accurate, they all gain around a third of a billionth of a second every day. The system has to correct for the drift, otherwise that tiny difference would upset the whole system, causing every GPS device on Earth to go out by about six miles a day. You can just imagine the mayhem that that would cause.

The problem doesn't lie with the clocks. They run fast because time itself runs faster in space than it does down below. And the reason for this extraordinary effect is the mass of the Earth. Einstein realised that matter drags on time and slows it down like the slow part of a river. The heavier the object, the more it drags on time. And this startling reality is what opens the door to the possibility of time travel to the future.

Right in the centre of the Milky Way, 26,000 light years from us, lies the heaviest object in the galaxy. It is a supermassive black hole containing the mass of four million suns crushed down into a single point by its own gravity. The closer you get to the black hole, the stronger the gravity. Get really close and not even light can escape. A black hole like this one has a dramatic effect on time, slowing it down far more than anything else in the galaxy. That makes it a natural time machine.

I like to imagine how a spaceship might be able to take advantage of this phenomenon, by orbiting it. If a space agency were controlling the mission from Earth they'd observe that each full orbit took 16 minutes. But for the brave people on board, close to this massive object, time would be slowed down. And here the effect would be far more extreme than the gravitational pull of Earth. The crew's time would be slowed down by half. For every 16-minute orbit, they'd only experience eight minutes of time.

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Inside the Large Hadron Collider

Around and around they'd go, experiencing just half the time of everyone far away from the black hole. The ship and its crew would be travelling through time. Imagine they circled the black hole for five of their years. Ten years would pass elsewhere. When they got home, everyone on Earth would have aged five years more than they had.

So a supermassive black hole is a time machine. But of course, it's not exactly practical. It has advantages over wormholes in that it doesn't provoke paradoxes. Plus it won't destroy itself in a flash of feedback. But it's pretty dangerous. It's a long way away and it doesn't even take us very far into the future. Fortunately there is another way to travel in time. And this represents our last and best hope of building a real time machine.

You just have to travel very, very fast. Much faster even than the speed required to avoid being sucked into a black hole. This is due to another strange fact about the universe. There's a cosmic speed limit, 186,000 miles per second, also known as the speed of light. Nothing can exceed that speed. It's one of the best established principles in science. Believe it or not, travelling at near the speed of light transports you to the future.

To explain why, let's dream up a science-fiction transportation system. Imagine a track that goes right around Earth, a track for a superfast train. We're going to use this imaginary train to get as close as possible to the speed of light and see how it becomes a time machine. On board are passengers with a one-way ticket to the future. The train begins to accelerate, faster and faster. Soon it's circling the Earth over and over again.

To approach the speed of light means circling the Earth pretty fast. Seven times a second. But no matter how much power the train has, it can never quite reach the speed of light, since the laws of physics forbid it. Instead, let's say it gets close, just shy of that ultimate speed. Now something extraordinary happens. Time starts flowing slowly on board relative to the rest of the world, just like near the black hole, only more so. Everything on the train is in slow motion.

This happens to protect the speed limit, and it's not hard to see why. Imagine a child running forwards up the train. Her forward speed is added to the speed of the train, so couldn't she break the speed limit simply by accident? The answer is no. The laws of nature prevent the possibility by slowing down time onboard.

Now she can't run fast enough to break the limit. Time will always slow down just enough to protect the speed limit. And from that fact comes the possibility of travelling many years into the future.

Imagine that the train left the station on January 1, 2050. It circles Earth over and over again for 100 years before finally coming to a halt on New Year's Day, 2150. The passengers will have only lived one week because time is slowed down that much inside the train. When they got out they'd find a very different world from the one they'd left. In one week they'd have travelled 100 years into the future. Of course, building a train that could reach such a speed is quite impossible. But we have built something very like the train at the world's largest particle accelerator at CERN in Geneva, Switzerland.

Deep underground, in a circular tunnel 16 miles long, is a stream of trillions of tiny particles. When the power is turned on they accelerate from zero to 60,000mph in a fraction of a second. Increase the power and the particles go faster and faster, until they're whizzing around the tunnel 11,000 times a second, which is almost the speed of light. But just like the train, they never quite reach that ultimate speed. They can only get to 99.99 per cent of the limit. When that happens, they too start to travel in time. We know this because of some extremely short-lived particles, called pi-mesons. Ordinarily, they disintegrate after just 25 billionths of a second. But when they are accelerated to near-light speed they last 30 times longer.

It really is that simple. If we want to travel into the future, we just need to go fast. Really fast. And I think the only way we're ever likely to do that is by going into space. The fastest manned vehicle in history was Apollo 10. It reached 25,000mph. But to travel in time we'll have to go more than 2,000 times faster. And to do that we'd need a much bigger ship, a truly enormous machine. The ship would have to be big enough to carry a huge amount of fuel, enough to accelerate it to nearly the speed of light. Getting to just beneath the cosmic speed limit would require six whole years at full power.

The initial acceleration would be gentle because the ship would be so big and heavy. But gradually it would pick up speed and soon would be covering massive distances. In one week it would have reached the outer planets. After two years it would reach half-light speed and be far outside our solar system. Two years later it would be travelling at 90 per cent of the speed of light. Around 30 trillion miles away from Earth, and four years after launch, the ship would begin to travel in time. For every hour of time on the ship, two would pass on Earth. A similar situation to the spaceship that orbited the massive black hole.

After another two years of full thrust the ship would reach its top speed, 99 per cent of the speed of light. At this speed, a single day on board is a whole year of Earth time. Our ship would be truly flying into the future.

The slowing of time has another benefit. It means we could, in theory, travel extraordinary distances within one lifetime. A trip to the edge of the galaxy would take just 80 years. But the real wonder of our journey is that it reveals just how strange the universe is. It's a universe where time runs at different rates in different places. Where tiny wormholes exist all around us. And where, ultimately, we might use our understanding of physics to become true voyagers through the fourth dimension. 

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霍金解读爱因斯坦的“虫洞”理论

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  虫洞是根据爱因斯坦相对论预测的连接时空中两个不同地点的假想“隧道”或捷径,上面的三维图轮廓集中呈现了这一点:负能量将时间和空间拖入一条隧道入口,并在另一个宇宙出现。虫洞至今仍是一种假设,因为从没有人见过,但在一些电影中被描述成时间旅行的通道,比如《星际奇兵》(1994年)和《时光大盗》(1981年),前者将虫洞描述成宇宙之间有门的隧道,后者则是天体图中展现了虫洞的位置

  继警告人类勿主动与外星人接触以后,英国著名物理学家斯蒂芬-霍金又抛出一个惊人言论。在一篇探讨如何建造“时间机器”的文章中,霍金详细分析了人类如何利用自然规律实现“时间旅行”的伟大梦想。尽管这一概念看起来有些荒诞,但他仍认为人类终有一天会实现这一梦想。以下是文章主要内容。

  寻找穿越第四维的通道

  大家好,我是斯蒂芬-霍金,是物理学家、宇宙学家及梦想家,尽管身体不能活动,只能通过电脑与大家交流,但从内心中我是自由的,自由地探索宇宙,思考以下重大问题:时间旅行是否可行?能否打开一个回到过去的通道,或找到通向未来的捷径?我们最终能否利用自然规律成为掌控时间的主人?

  在科学界,时间旅行一度被认为是歪理学说。过去因为担心有人会把怪人的标签贴在自己身上,我对这个问题常常避而不谈。但现在,我不再那么谨小慎微了。事实上,我更像是建造了巨石阵的那些人。我对时间痴迷已久,如果有一台时间机器,我会去拜访风华正茂的玛丽莲-梦露,或是造访将望远镜转向宇宙的伽利略。或许,我还会走到宇宙的尽头,破解整个宇宙湮灭之谜。

  为了让这一切从虚幻变成现实,我们应以物理学家的角度来重新审视时间——即第四维。这个问题没有听上去那么晦涩难懂。每个好学的孩子都知道,任何物体都以三维形式存在,包括坐在轮椅上的我。一切物体都有宽度、高度和长度。此外,还有一种长度——时间的长度。例如,虽然一个人可能活了80岁,但巨石阵的石头却数千年屹立不倒。太阳系的运行将持续数十亿年。

  一切物体都有时间以及空间的长度。时间旅行意味着我们要经过第四维。要想搞明白这一点,我们可以想象正在从事一种日常活动,比如开车。开车沿直线行驶,是在一维中旅行。向左转或是向右转,则是二维旅行。驱车上下山路意味着又多增加了高度,所以是在三维空间内。那么我们怎样才能实现时间旅行?怎样才能发现穿越第四维的通道呢?

  无处不在的“虫洞”

  让我们暂时从科幻电影中寻找答案吧。在此类电影中,通常会有一台巨大而高能耗的时间机器,这台机器产生通往第四维的通道——“时光隧道”。时光旅行者——勇敢但可能有些鲁莽的人,做好我们大家所知道的准备,然后走进时光隧道,来到一个他们想要到达的时间里。这一概念可能有些牵强,事实可能与之存在着天壤之别,但该想法本身不是那么的疯狂。

  物理学家们也在思考时光隧道,但我们的角度不同。我们想搞清过去或未来的通道是否存在于自然规律中?事实证明,我们认为确实是这样的。而且,我们还给它们起了一个名字:虫洞。其实,虫洞无处不在,只是因为太小,我们肉眼看不到罢了。虫洞非常小,存在于时空的隐蔽处和缝隙里。你或许认为这是一个难以理解的概念,请耐心听我继续解释吧。

  任何物质都不是平整无暇和实心的,如果仔细观察,会发现它们上面都存在小孔和裂缝,这是一个基本的物理原理,同样适用于时间。即便是像台球一样的东西,上面也有裂缝、褶皱或空洞。现在容易说明这种情况也存在于第一个三维中。相信我,这一原理同样适用于第四维。时间也存在许多微小的裂缝、褶皱和空洞。在最小的刻度下——比分子甚至原子都小,我们来到一个称为量子泡沫(quantum foam)的地方,这是虫洞存在之处。

  时空中的微小隧道或捷径不停地在这个量子世界中形成、消失和重新形成。它们可以连接两个隔离的空间以及两个不同的时间。不幸的是,现实生活中这种时光隧道非常狭小,即使发现了它们,我们也不能从这个缝隙穿过——可这正是“虫洞时间机器”概念的前进方向。部分科学家认为,或许有一天捕捉到一个虫洞,将它放大数万亿倍,令其足够的大,能让人甚至飞船进入。

  如果我们拥有足够的能量和先进的技术,将来或许甚至能在太空中建造一个巨型虫洞。我并不是说一定可以做到,但如果真的有这种装置,那么确实很了不起。一端在地球的附近,另一端则在遥远的星球附近。从理论上讲,虫洞或时光隧道不仅仅能把我们带到别的星球。如果两端在同一个地方,且由时间而非距离分离,在遥远的过去,飞船就能在地球附近自由出入。或许恐龙会看到飞船登陆的场景。

  “疯狂科学家”悖论

  如今,我意识到以四维方式思考并不容易,虫洞是一个令你绞尽脑汁的概念。我一直想做一个简单的实验,揭示人类通过虫洞的时间旅行是否可行,或是现在,或是未来,我喜欢简单的实验和成功后的香槟酒。所以,我将自己最喜欢的两件事情结合起来,探讨时间旅行是否可行。让我们设想一下这样的场景,我参加一个为未来旅行者举办的欢迎宴会。

  由于出现意外,我没有让别人知道,直至欢迎宴会结束以后。我写好了邀请函,注明了准确的时间和空间坐标。我希望它能以这样或那样的形式存在数千年。或许,未来一天有人会发现邀请函上的信息,利用虫洞时间机器回到我的宴会,证明时间旅行将来是可行的。

  与此同时,时间旅行贵宾应该随时会降临,五个或一个。但就在我说话的工夫,仍没有人到来,真是惭愧。我希望至少未来的“环球小姐”能踏进这扇门。这项实验为何不奏效?一个原因可能是回到过去的时间旅行所存在的问题——我们称之为悖论,一个众所周知的问题。探讨悖论是一件很有趣的事情,最著名的悖论通常被称为“祖父悖论”。

  我有一个新的简化版本——“疯狂科学家”悖论。我不喜欢一些电影中科学家被描述成疯狂的群体,但在这种情况下,确实如此。这个家伙决心建立一个悖论,即便付出生命代价在所不惜。可以想见,他是在建造虫洞——仅需一分钟就来到过去的时光隧道。通过虫洞,这位科学家可以看到他一分钟以前的自我。

  如果这位科学家利用虫洞向以前的自我开枪,会发生什么事情?他现在已经一命呜呼。那又是谁开的枪呢?这便是一个悖论,听上去毫无意义。但这却是那种让宇宙学家做噩梦的状况。这种时间机器会违反整个宇宙所遵循的基本规则。我认为一切皆有可能。如果真是如此,那么就没有任何办法阻止整个宇宙陷入混乱。所以,我认为有些事情总会发生以阻止这种悖论。

  探索通向未来的“钥匙”

  在某种程度上,这或许是科学家永远不会发现他面临向自己开枪境地的原因。在这种情况下,我只能遗憾地告诉大家,虫洞本身就是一个问题。最后,我认为像这样的虫洞不能存在,原因就是反馈。如果你有到摇滚演唱会现场观看演出的经历,你可能会辨别出这种尖利的噪音。这就是反馈,引起反馈的原因很简单。声音进入麦克风,通过电线传播,经由扩音器令声音放大,在一个环状物内绕来绕去,每次令声音比上一次更大。如果没人阻止,反馈能够破坏音响系统。

  虫洞也会遇到这种问题,只不过声音换成了辐射。一旦虫洞变大,大自然的辐射物会进入,最终形成一个环路。反馈变得如此强劲,最终摧毁虫洞。虽然微型虫洞确实存在,也有可能在某一天不断膨胀,但持续时间不会太长久,所以不能当作时间机器使用。这是没人能及时回到我晚会的真正原因。任何通过虫洞和其他方式回到过去的时间旅行或许都是不可能的,否则,悖论就会出现。

  因此,遗憾的是,回到过去的时间旅行应该永远不会上演。对于寻找恐龙的人来说,这会令他们大失所望,但对于历史学家而言,他们可以彻底解脱了。故事到此并未结束。这并没有使所有的时间旅行不可行。我确实对时间旅行深信不疑,对通向未来的时间旅行更是如此。时间就像河流,我们每一个人仿佛被时光的流动无情地卷走,只不过时光是另一种形式的河流——以不同速度、在不同地点流动,这是通向未来的“钥匙”。

  一百多年前,爱因斯坦最早提出了这一概念。他认为,世上应该存在让时间慢下来的地方,以及让时间加速的地方。他绝对是正确的,证据恰恰就在我们的头顶。这便是全球定位系统,简称GPS。一个卫星网络正在地球周围轨道运行,它们使得卫星导航成为可能,同时还表明时间在太空的运行速度快于在地球上。每一艘太空飞船内部都是一台运行精确的钟表。虽然如此精确,但每天仍会快十亿分之一秒左右。

  卫星导航系统必须为此做出矫正,否则,微小的差异就会扰乱整个系统,令地球上所有的全球定位系统每天都会出现大约6英里(约合9.7公里)的误差。你可以想象由此造成的后果。钟表并不存在这样的问题,走得快是因为时间在太空比在地球运行快。造成这种非同寻常影响的原因是地球的质量。爱因斯坦发现,物质会减缓时间运行速度,就像是河的下游一样。物体越重,对时间的阻力越大。这种惊人的事实为通向未来的时间旅行开启了大门。

  引力无穷的超大质量黑洞

  恰恰在银河系中心,距离地球2.6万光年远的地方,拥有银河系中最重的天体——一个质量相当于四百万个太阳的超大质量黑洞,在自身引力作用下,它被压缩为一个点。距离这个超大质量黑洞越近,遭遇的引力就越强。一旦距离其过近,连光线都无法逃脱,会被吞噬。这样的超大质量黑洞对时间具有显著的影响,令其减缓的速度远远超过银河系中的任何物体。这使得它是台“天生的时间机器”。

  我喜欢想象宇宙飞船如何能充分利用这种现象。如果某个航天机构正在控制从地球发射的探测器,他们会发现绕轨道运行一圈的时间为16分钟。对于飞船上的勇敢者来说,靠近这个超大质量物体,时间就会慢下来。在这里,引力影响远比地球引力极端。机组人员的时间将会减慢一半。对于原本每圈要耗费的16分钟,他们其实仅经历了8分钟。

  想象一下,当飞船及机组人员绕这个黑洞运行五年时,别的地方已经过去了十年。当他们回到家乡,地球上的人比他们老了五岁。所以,超大质量黑洞就是一台时间机器,当然,这还不是非常的实用。超大质量黑洞之所以比虫洞更有优势,是因为不会激发悖论。此外,它不会因反馈走上自我毁灭之路。

  然而,通向未来之旅并非一路坦途。地球距离未来世界漫长无边,让我们距离未来非常遥远。幸运的是,我们还有另一种时间旅行方式,这也是我们建造货真价实的时间机器最后、也是最大的希望。旅行速度必须超级快,甚至比避免被吸进黑洞所需要的速度还快。这是因另一个涉及宇宙的奇怪事实所致。宇宙中存在着速度限制,即每秒钟18.6万英里(约合30万公里),亦称光速。

  任何物体不能超越这一速度。这也是科学界最成熟的理论原则之一。无论是否相信,以接近于光速的速度旅行可以将你送达未来世界。要想理解这一点,我们可以想象一个具有科幻色彩的交通系统——一条遍布地球周围的轨道,为超高速火车准备的轨道。我们将利用这列想象出来的火车,尽可能地接近于光速,看它如何变成一台时间机器。列车上的乘客购买了通向未来的单程车票。火车开始加速,越来越快,不久开始绕地球一圈圈运行。

  如何突破速度限制

  达到光速意味着绕地球运行速度要飞快,比如每秒钟绕7圈。不过,无论这列火车的动力有多强劲,它永远也无法到达光速,因为物理学原理令其做不到这一点。假设它接近光速,距离这一终极速度还有一点距离。现在非同寻常的事情发生了。列车上的时间相对于地球开始减缓,就如同靠近超大质量黑洞一样,而且有过之而无不及。列车上一切物体的活动都变慢。这是为了保护速度限制,原因并不难理解。

  想象一个孩子跑向迎面而来的火车。他前冲的速度增加至列车的速度上,所以,他难道不能在意外中突破速度限制吗?答案是否定的。自然规律会令列车上的时间减缓,使得这一幕永远不会发生。这个孩子跑得再快,也不能打破速度限制。时间总是会减慢,足以“保护”速度限制。这一事实源于耗费多年踏上未来之路的可能性。

  想象一下这样的场景:2050年1月1日,一列火车离开车站,绕地球轨道一圈又一圈,直至100年以后,最终在2150年新年夜停下来。此时,乘客们在世上的时间也只剩下一周,因为身在火车上,时间过得非常慢。当他们离开火车,会发现一个完全不同于车上环境的世界。在一周内,他们已经在通向未来的道路上前进了100年。

  当然,建造一列能达到这种速度的超高速火车的可能性微乎其微。不过,我们已经在位于瑞士日内瓦的世界最大的粒子加速器——大型强子对撞机——建造了这样的装置。大型强子对撞机位于瑞士和法国交界地下100米深处一条总长16英里(约合25.75公里)的环形隧道内,一旦开足马力,这台对撞机能在瞬间从零加速至每小时6万英里(约合每小时9.7万公里)。

  令动力和粒子的速度变得越来越快,直至它们能以每秒1.1万圈的速度绕隧道运行,这时,速度将接近于光速。但是,就像是上面描述的那列火车一样,它们永远无法到达这一终极速度,最快只能达到光速的99.99%。发生这种情况的时候,进行时间旅行是不切合实际的。由于一种称为兀介子的“短命”粒子,使我们了解了这一点。通常情况下,兀介子会在250亿分之一秒内分解。当它们被加速至接近光速时,寿命是以前的30倍。

  “天上一日,地下一年”

  情况的确就那么简单,如果我们想踏上未来之旅,那么速度必须快。我认为实现这一目标的唯一途径是进入太空。在人类历史上,速度最快的载人飞船“阿波罗”10号,速度为每小时2.5万英里(约合每小时4万公里),但要实现在时间中旅行,我们的速度必须是“阿波罗”10号速度的2000倍。按照这种思路,我们应该先制造一个巨大的飞船,里面可以装载着巨量燃料,令其加速至接近于光速,在全负荷动力运行下,实现这一目标仍需要六年时间。

  由于飞船如此的庞大和沉重,最初的加速度相对平缓。随着时间的推移,飞船速度非常快,不久即覆盖广大区域。一年以后,我们到达了系外行星。两年后,它的速度将达到光速的一半,距离太阳系越来越遥远,再过两年它可能会已经达到光速的90%。在发射四年后,飞船距离地球30万亿英里(约合4.8万亿公里)远,这意味着飞船将开始时间旅行,届时,它的速度接近于光速,在船上呆一天,相当于在地球上呆两天。

  再经过另外两年全负荷动力飞行,飞船将到达其最高速度——相当于光速的99%。在这一速度下,在船上呆一天,那就意味着在地球上度过一年的时间。飞船确实“飞进”了未来。时间变慢还有另一个优势,从理论上讲,这意味着我们一生当中可以跨越无尽的距离。探索银河系边缘之旅将耗费80年的时间。

  然而,未来之旅真正惊奇之处在于,揭示整个宇宙有多么的奇特。在宇宙中,不同的地点,时间运行的速度不同;微小的虫洞存在于我们周围每一个角落;最终,我们将利用掌握的物理学知识,成为穿越第四维的真正宇宙旅行者。