[ T i m e T r a v e l ]
[ the science of gettin' back to the future... ]
We've all seen in the movies where our protagonist hero steps into the invisible hole in the air and gets transported a million years forward or back in time. Or when they drive that really cool DeLorean up to 88 miles an hour until it disappears and reappears in some other period when the road in front of it is actually finished. We bet you're wondering, is that stuff possible? Can I go back and forward in time? Maybe, get the winning lottery numbers of tomorrow, come back to today and strike it rich? Or is that just Hollywood's interpretation of science? You may be surprised.
[ black holes. the quicker picker upper of the universe. ]
Well, it's true that it's not already possible to do the time traveling thing yet. There are some reasons why. First of all, there's this thing called space-time.. that all things exist on. It's the "fabric" of space... it's what actually holds space.. and time... together. You can't see it, but it's there. Think of it as... say.. a blanket. Now, when you put a bowling ball on a blanket, what happens? It curves slightly inwards, toward the ball you put on it, right? Yup. In physics, we call this the curvature of space-time. The object on the "blanket" of space-time has mass... or in other words, it's made of stuff. And all objects with mass exert gravity. No, that doesn't mean you and your friends will go flying through the air and get stuck on Uncle Bob the next time he comes over... as all masses exert some kind of gravity, but if they're anything smaller than a planet it's not really noticeable.
So we got this space-time, and when you put stuff on it that has mass, which exerts gravity, it curves in towards the object. And that means that stuff coming towards a really massive object will slightly bend toward it; it can only follow the space-time it's riding on. And yes, that does mean that time gets a little bit distorted.
What's all this nonsense got to do with time travel anyway, you ask? Simple. Since time will be distorted near objects of extremely high mass (and that mass will have to be VERY great, meaning a planet or star as large as our sun or even more), physics theory tells us that time travel IS possible.
How can you actually travel through time then? There are two possibilities, theoretically. And by theoretically, we mean they're not far away from the imagination of Hollywood, only that they're from the imagination of physicists that use mathematical equations about 5 miles long to prove their theory.
The first theoretical way to do it is to pass by an EXTREMELY massive black hole. Now, what is a black hole, exactly? Think back to the space-time mumbo jumbo I explained above. Now think of a star. A star has a life cycle... it can't just live forever. It ignites, burns bright for billions and billions of years, expands, burns out, and dies. But sometimes, just sometimes, it doesn't just "die".
[ stars that collapse inward, and not the Hollywood kind ]
Sometimes a star reaches the end of its life cycle, expands to become a big red ball twice its original size, and then something really weird happens. Remember when I said all objects in space-time have mass? And mass has gravity, right? Would you believe me if I said - if a star of a mass so great reaches the end of its life cycle, it can actually collapse in on itself? Yeah, you heard me right. The intense gravity of a dying star of about... say, a few hundred solar masses (by the way, a "solar mass" is the geek term we use to define the mass equal to one of our Sun) can pull in on itself. After that, it shrinks. It doesn't lose mass, and it's still shrinking. Let's say this star has... about 500 solar masses. That's 500 times as massive as our sun. (We'll need to check it into a weight loss program of some sort) It's still shrinking, because its gravity is so strong that it can't stop pulling in on itself. Still 500 masses.
[ the star that lost 500 solar masses in appearance... again, not Hollywood ]
Yup, it's 500 solar masses, and right about now it's the size of a golf ball. It's been shrinking without anything to stop it... that gravity is getting stronger the smaller it gets. Because the more mass packed into a smaller space equals more gravity. This baby's 500 times our sun at the size of a golf ball, a marble, make that a fruit seed. The gravity is now so strong at this point that the space-time (yeah, remember that?) is starting to curve - all the way around this object. Why? If the gravity's so strong, not even space-time can lay flat. And since light travels through space-time, light's gonna curve all around this thing too.
[ the 2 week star weight loss plan... complete! ]
The star is now crushed itself and its 500 solar masses into a single, one dimensional point - meaning it's become the next step up from well, nothing. The gravity is now infinite. This means, yeah, you guessed it... it's a black hole. Space-time would now be totally curved around this thing. Light? Don't even think about it.... light coming from the star gets pulled right back in. Meaning you can't see it. Try to shine a light on it... you'll see nothing. What do we call this point that our dying star just shrunk itself to? We call it, a singularity.
[ event horizon... no, it's not just a movie ]
So we got this singularity which is a point of 500 solar masses compressed into almost nothing which has so much gravity that space-time is curved all around it, all over it like a bad date in a movie theater. Believe me, I know. But anyway, so light can't come out of this thing. And it can't be reflected back to you, since it sucked all the light that went into it. What about the light that is too far from the black hole to get sucked in.... but not far enough to get away?
Well, they're stuck. These light particles... called photons, which are, well, particles of light, are trapped all around the black hole in a sphere called the Schwarzchild Radius. It can't get out, and is too fast to get back in. We non-geeks call it... the event horizon. We can't see what exists inside of the event horizon, and what's inside can't see us. Not that they'd be alive to see us, anyway. And light that whizzes by the event horizon (but NOT into it) can pass without getting sucked in, but will be bent severely. Meaning, the black hole's gravity could pull it enough to give it a sharp right or left turn. Some scientists believe that this effect.. the Doppler effect can sometimes gives us the wrong idea on where stars are in the universe, simply because the light from these stars are gettin' curved with the massive objects that swerve light around 'em.
[ what's this got to do with time travel already?!?! ]
Well, space-time that is curved around a black hole is curved infinitely. Time will be curved around it infinitely also, since time exists as a part of space-time. (It's got the word 'time' in it, doesn't it? Hehe..) This is where the first theory for time travel comes in. Usage of a black hole as transport.
[ warp speed, captain ]
We can't actually go in a black hole. The tidal forces of the gravity inside a black hole would literally rip a human apart, along with the strongest spaceship. There is another alternative.
Time is infinitely curved around a black hole, right? What about outside the event horizon? It's curved severely, but not all the way. And the gravity here isn't strong enough to kill you, if you moved fast enough. The theory is to fly past a black hole, possibly even orbit around it, outside the event horizon where space-time is curved. Time here moves at a different rate than it does elsewhere. It moves slower. But a human or a clock would not be able to tell the difference in its own behavior (well of course, a clock can't think, but anyway), because a human's brain would be slowed down relative to the time it's in. Observers at a distance (smart guys who wanna see the action but not get killed) would notice that the guy's movin' a lot slower than he usually does. The non-thinking clock he has with him... also going slower. This isn't really traveling through time, it's just delaying it for the astronaut floating around the black hole. Captain, we have time warp.
Also, there is the possibility of wormholes, tunnels that are theoretically inside or near the singularities of black holes, that can connect to a whole different part of space-time, since space-time is curved. This idea is really kinda shaky though, since this idea is extremely theoretical and even if they did exist, they'd be far too unstable for people to go through. It is time travel though, and could lead to the future, or even the past.
[ the 2nd theory of time travel transportation ]
The other theory that the geeks... er, scientists have thought up, is this. As you read in the light speed section, time slows down for people and things that travel at or near the speed of light. Of course, as of now, and according to theory, nothing can travel at the speed of light because of the infinite amount of energy it would take to move something that fast. But if someone could travel close to that speed, then their time would slow down relative to people back on Earth. Here's an example. An astronaut dude goes in a spaceship with a clock in front of him. His clock reads... August 15th, 1999 AD. He travels to 99.999% the speed of light, as fast as he can theoretically go. He notices nothing changed besides a strong urge to vomit. Otherwise, he's fine. His clock seems to be going at the same rate as when he left, to him. He's chillin' in his spaceship... goes to visit some stars. He turns around and heads back... 5 years later, according to his clock. He gets to Earth. He lands, steps out, reads a clock on a billboard, and gets smacked in the head by a flying car. He wakes up and realizes, the date he saw on that billboard has changed. It says August 15th, 1,500,233,133 AD, welcome to Earth, home planet of the Galactic Federation. What's happened here??
Traveling at light speed for what he thought was 5 years has caused time dilation, or in other words, it slowed him down immensely according to Earth's clock. According to the law of time dilation, the faster an object in space-time travels... the more the space-time around him curves itself, and when he closely approaches light speed, it curves almost infinitely. This allows him to go through time at a much slower rate than everyone else. When he left a billion years ago, Earth saw the astronaut dude's clock slow down to almost a standstill. To him, nothing has changed. His brain adjusted. But a billion years went by back on Earth while it took him only 5 years. Way to time travel into the future, eh?