AskUs: Explain to me the theory that states black holes can be used for time travel

So one of our reader asked us this question the other day: Explain to me the theory that states black holes can be used for time travel

ANSWER

Get ready for a crash course in advance astrophysics.

First you have to know that space isn’t a non-existent thing but a thing in itself. We call it space-time. So when something rests on space-time it distorts. Imagine a bed sheet. If i stretch it out and put a bowling ball on it the bed sheet warps under the weight of the ball. This is how space-time works, but it isn’t like a bed sheet, its existence is in more than 3 dimensions so the human mind can’t conceive of the true shape of space-time. So the objects will distort space and cause it to have bumps and grooves all over the place. The more massive the object the more massive the bump or depression. A black hole is an object that is so heavy that the bottom of the dip is infinitely deep, so it takes the relative description of a hole.

Now lets talk about light. Light travels at the universal constant. That constant is how fast two points can “talk” to each other. Light wont slow down or speed up from that constant, it always travels that distance at the same rate. Other things that travel that fast would be gravitational waves, and any other mass-less object. Light also travels along space, so it moves with the bumps and hills in space-time.

Now gravity! So when an object comes close to another object in space. The object approaches it travels in a straight line until it reaches the next object’s space-time distortion. So imagine rolling a marble past the bowling ball. The marble always travels straight. When it hits the depression of the bed sheet the marble “turns.” That turning comes from the distortion, the marble is still traveling straight but the bed sheet is turning underneath it causing the marble to turn. The marble will continue to orbit until it leaves the “gravity well” or until it hits the bowling ball.

Now for time travel. Imagine to photons of light traveling towards a massive object. One will pass the gravity well on the side and the other will enter. The light has enough speed to go in and go out with out too much problem, however it will still travel the same distance as the other light beam. So when the light beam leaves the gravity well it will still be in line with the other light beam. But light doesn’t speed up! So in the gravity well time slowed down in order to allow the light to cover the same distance, because the universal constant must be preserved. What is being preserved is causality between the two edges of the gravity well.

So imagine you’re running a race. Person A gets to run straight to the finish line. Person B must go down and up a ravine to get to the same point. You both run the same speed and you both get to the same finish line at the same time. We observe this with light all the time, so the only way both light beams get to the same place at the same time, time must travel slower in the ravine so that light can cover more distance with the same speed.

Black holes only magnify this effect because of the massive amount of distortion it puts onto space, so the amount of time dilation is noticeable to humans. Oh and speed messes with time as well. The closer to the speed of light you get, the slower time travels in order to make sure you don’t pass the universal constant. So if you travel really fast along the outer edge of an event horizon time will dilate so much that thousands of years will go by in no time from your perspective.