"[ computerized voice ] hello." "My name is stephen hawking " "Physicist, cosmologist..." "And something of a dreamer." "Although I cannot move..." "Narrator: ..." "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?" "Check it out." "Time travel was once considered scientific heresy." "I used to avoid talking about it" "For fear of being labeled 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 " "As the fourth dimension." "It's not as hard as it sounds." "All physical objects, even me and my chair," "Exist in three dimensions." "[ beeping ]" "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," "These stones will last much longer " "For thousands of years." "And the solar system will last for billions of years." "Everything has a length in time, as well as space." "Traveling in time" "Means traveling through this fourth dimension." "To see what that means," "Let's do a bit of normal everyday traveling" "Just to get a feel for it." "A fast car makes it a bit more fun." "[ tires screeching ]" "Drive in a straight line," "And you're traveling 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 traveling in all three dimensions." "[ tires screech ] [ engine revs ]" "But how on earth do we travel in time?" "How do we find a path through the fourth dimension?" "[ tires screech ]" "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." "[ electricity crackling ]" "A time traveler," "A brave, perhaps foolhardy individual" "Prepared for who knows what," "Steps into the time tunnel..." "And emerges who knows when." "[ whooshing ]" "[ beeping ]" "The concept may be far-fetched," "And the reality may be very different than this," "But the idea itself is not so crazy." "Physicists have been thinking about tunnels in time, too," "But we covered 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." "[ indistinct conversations ]" "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." "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." "Take this pool table." "The surface looks flat and smooth," "But up close, it's actually anything but." "It's full of gaps and holes." "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, as well." "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 centimeter 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 one 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 the earth," "And the other far, far away, near some distant planet." "Theoretically, a wormhole could do even more." "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 the earth," "But in the distant past." "Maybe dinosaurs would witness" "The ship coming in for a landing." "[ dinosaurs bellowing ]" "Now, I realize 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." "Narrator:" "I like simple experiments and champagne." "So I've combined two of my favorite things" "To see if time travel from the future to the past" "Is possible." "I'm throwing a party A welcome reception for future time travelers." "But there's a twist." "I am not letting anyone know about it" "Until after the party has happened." "Here is the invitation," "Giving the exact coordinates in time and space." "I am hoping a piece of it, in one form or another," "Will survive for many thousands of years." "Maybe one day someone living in the future" "Will find the information" "And use a wormhole time machine to come back to my party," "Proving that time travel will one day be possible." "My time-traveler guests" "Could be arriving any moment now." "5... 4... 3... 2...1." "[ clock ticking ]" "What a shame." "I was hoping a future miss universe" "Was going to step through the door." "So, why didn't the experiment work?" "I think one of the reasons might be because" "Of a well-known problem with time travel to the past " "The problem of 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." "[ gun cocking ]" "Imagine somehow he's built a wormhole..." "A time tunnel that stretches" "Just one minute into the past." "It may not sound like much," "But even one minute of time travel" "Can cause real trouble." "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?" "[ silenced gunshot ] ugh!" "He's now dead..." "Killed before he'd even finished assembling the pistol." "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 the 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." "[ heavy-metal rock music plays ]" "If you've ever been to a rock gig," "You'll probably recognize this screeching noise." "[ microphone feedback ]" "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 mike..." "It goes 'round and 'round in a loop," "Getting louder each time." "If no one stops it," "Feedback can destroy the sound system." "[ electricity crackles ]" "I think 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 someday," "It won't last long enough to be of use as a time machine." "That's the real reason no one came to the party." "In fact, I believe 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." "Narrator:" "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 traveling into the future." "The idea was proposed by albert einstein Over 100 years ago." "He realized" "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 31 satellites in orbit around the earth." "[ beeping ]" "The satellites make satellite navigation possible." "But they also reveal" "That time runs faster up here..." "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 here" "Than it does down below," "And the reason for this extraordinary effect" "Is the mass of the earth." "Einstein realized that matter drags on time," "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." "I admit this is a difficult concept to understand," "So let's take a simple example." "This is the great pyramid of giza." "It weighs over 40 million tons," "And, like all heavy things, it's actually slowing down time." "The effect is small," "Billions of times smaller than that of the earth." "But if we exaggerate it drastically..." "You can see the principle at work." "Close to the pyramid, everything is slowed down," "Again, like the sluggish part of a river." "Here time itself is passing slower" "Compared to how it's passing further away." "But what if people near the pyramid" "Look outwards?" "They must see the opposite effect." "Because they are slowed down," "They must see time in the distance" "As running fast." "It's a simple result of the mass of the pyramid." "This distortion opens the door" "To the possibility of time travel." "So what we need to really travel in time" "Is something much more massive than a pyramid..." "And I know just the thing." "Right in the center of the milky way, 26,000 light-years from us," "Lies the heaviest object in the entire galaxy" "Hidden by a vast cloud of gas and stars." "Narrator:" "It's a supermassive black hole" "Containing the mass of 4 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." "So it's wrapped in a sphere of darkness 15 million miles in diameter." "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 someday be able to take advantage" "Of this spectacular phenomenon." "Of course it would first have to avoid being sucked in." "The trick, I think," "Would be to aim just off to the side" "So they'd miss it." "They'd have to be on exactly the right trajectory and speed," "Or they'd never escape." "Get it right, and the ship would be pulled into orbit," "A giant circle 30 million miles in diameter." "Here it would be safe." "Its speed would be enough" "To keep it from falling any further in." "If a space agency were controlling the mission" "From earth or anywhere else far away from the black hole," "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 near the pyramid or planet earth." "The crew's time would be slowed down by half." "For every 16-minute orbit," "They'd only experience 8 minutes of time." "'round and 'round they'd go, experiencing just half the time" "Of everyone far away from the black hole." "The ship and its crew would be traveling through time." "Imagine they circle the black hole" "For 5 of their years." "10 years would pass elsewhere." "When they got home," "Everyone on earth would have aged 5 years more" "Than they had." "The crew of the spacecraft would return to a future earth." "They would have made a journey not only in space," "But in time." "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." "Traveling through the fourth dimension" "Will never be a walk in the park." "But it turns out" "There is a surprisingly straightforward way to do it." "You just have to travel very, very fast..." "Narrator: ..." "Much faster than even the high speed" "Required to keep out of the supermassive black hole." "This is because of 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." "I realize this sounds weird, but trust me." "It's one of the best established principles in science." "Believe it or not," "Traveling 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 the 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 on board." "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 traveling large distances into the future." "Imagine the train left the station" "On January 1, 2050." "It circles the 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 than the one they'd left." "In one week," "They'd have traveled 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 60,000 miles per hour" "In a fraction of a second." "Increase the power," "And the particles go faster and faster" "Until they're whizzing around the tunnel" "At 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%" "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." "These particles are real-life time travelers." "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." "Narrator:" "The fastest manned vehicle in history" "Was apollo 10." "It reached 25,000 miles per hour." "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." "[ radio chatter ]" "The ship would have to be big 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." "Man:" "T-minus 10 seconds..." "Man: 4, 3... 2, 1..." "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 just one week," "It would have reached the outer planets," "Gas giants like neptune." "After two years, it would reach half light speed" "And be far outside our solar system." "Two years later, it would be traveling" "At 90% of the speed of light speed" "And passing our closet star system," "Alpha centauri." "Around 30 trillion miles away from earth..." "And 4 years since launch..." "The ship begins to travel in time." "For every hour of time on the ship," "Two hours pass on earth," "A similar situation to the spaceship" "That orbited the massive black hole." "But there's more to come." "After another two years of full thrust," "The ship would reach its top speed -- 99% 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 human 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." "Captions paid for by discovery communications"