Brian Greene - The Science Of Time Near A Black Hole

How does the concept of time change near a black hole? Does time have a direction? How did Einstein describe the distinction between past, present and future? These are some of the questions that we will explore in this video as we embark on a journey through the mysterious and mind-bending world of black holes and the concept of time. Join us as we dive into the incredible power of strangeness of these cosmic behemoths and discover the implications of traveling to a black hole, from the extreme physical effects on the body to the fundamental changes in our understanding of space, time and gravity. In principle, if you stay outside the event horizon of a black hole, right, if the black hole has a mass m, then there's a beautiful little formula that you can work out for how big the black hole is. You take two, multiply it by Newton's constant g, multiply it by the mass of the black hole and divide through by c squared. And that number tells you how close you can safely get to a black hole without being pulled in. And if you stay outside that number, 2dm over c squared, then in principle you can go to the black hole's edge, turn around and come back. And if you do that, when you come back, you will note that you come back to the future. If you spend time near the edge of a black hole, time elapses there more slowly. So clocks back on earth will be ticking fast while your clock is ticking off time slow. So if you spend like a year hanging out near the edge of a black hole and then you come back to earth, it might be 10 years in the future or a thousand years in the future, a billion years into the future. So the journey that you're talking about may seem to be a journey through space. You go from earth to the black hole and then you come back, but it's actually also a journey through time because when you go to the black hole and you come back, it will not be the amount of time that you think should have elapsed. It'll be much more on planet earth. Einstein's theory of relativity revolutionized our understanding of space and time. According to this theory, time is not absolute and universal, but instead it's relative and depends on the observer's motion and gravitational field. The passage of time is described as a fourth dimension along with the three dimensions of space. However, the concept of time having direction is still a topic of debate among physicists. Some believe that time moves only in one direction, from the past to the future, known as the arrow of time. Others suggest that time is symmetric and the arrow of time arises from the initial conditions of the universe. Einstein's relativity gives a very different way of thinking about time, in which we imagine that every moment is as real as every other moment. It's not just that the present is real and the past is gone, the future is yet to be. Every moment is as real as every other. Past still exists, future exists, present exists. And this is what gave rise to Einstein's own poetic description, where he said, the difference, the distinction between past, present and future is only an illusion, however persistent. And we are so familiar with the fact that when you look out at physical happenings in the world, they seem to always unfold in one temporal orientation. And in fact, there's an intuitive sense that time has a direction. Time has what we call an arrow, right? The canonical examples that we'd like to use, you know, if you drop an egg, it splatters, but it never unsplatters. You know, if someone dives off a diving board, they go into the water, but you never see somebody jumping out of the water, untumbling and landing on the diving board. And that's an important fact, because when you see reverse run films, the reason you laugh is because it's so ludicrous relative to your understanding, your intuition about the nature of time. But here's the amazing fact. When you look at the laws of physics as currently configured, all laws of physics, when properly interpreted, so I won't go into the nuances of that, but all laws of physics have the quality that they treat the future and the past symmetrically, identically on equal footing. Which means when we physicists study the laws of motion, any sequence of events that can unfurl in one direction, the reverse sequence is allowed by the laws of physics. And in fact, there's an algorithm for ensuring that if you have, for instance, any physical unfolding, if you want to reverse it, all you need to do is reverse the motions of the particles that are involved. The implications of time's relativity become particularly striking near black holes, where the extreme gravitational pull slows down time. As you approach a black hole, the gravitational force becomes stronger and stronger, causing time to slow down. If you are able to survive these extreme conditions, you could potentially experience some of the most extreme phenomena in the universe. Thus, the understanding of the nature of time is crucial to understanding the behavior of black holes and the effects they have on their surroundings. Near the vicinity of a black hole, and if you don't understand this, it doesn't matter. We can put down a space-time diagram. If you remember from high school, this is where we have time on this vertical axis and you set off a beam of light that fills out a cone, so-called light cone. And what happens is the geometry of space and time is so distorted by a black hole that beyond what's called the event horizon, the direction of time and space is so twisted that as light propagates, it cannot get out of the edge. It cannot go beyond the event horizon of the black hole, and that's why no light can get out. That's why the black hole is black. The idea of traveling to a black hole and coming back is something that we often see in science fiction, but is not merely a fictional concept. It is theoretically possible, but the extreme gravitational forces would have serious physical effects on your body. The difference in gravitational forces between your head and feet could stretch your body to the point of being torn apart. If you are near a waterfall and you are, say, paddling your kayak, if you can paddle faster than the rate at which the water is flowing toward the waterfall, you can get away. But if you can't paddle faster than the water is flowing, then you can't get away and you're doomed to fall down the waterfall. Here's the idea. The analogy is, space itself falls over the edge of a black hole, sort of like a waterfall of space, and the speed at which space travels over the edge of a black hole is equal to the speed of light. Nothing can go faster than the speed of light, so near a black hole you're doomed, so you might as well just paddle right toward the black hole and go on a joy ride down the throat of the black hole itself. So that's another way of thinking about it. Edge of a space in some sense is flowing over the edge. It's flowing over the edge at a speed equal to the speed of light. Since nothing can go faster than the speed of light, you can't paddle upstream. And if you can't paddle upstream, you can't get away from the black hole. You're doomed and you will fall into the black hole. The implications of traveling to a black hole are not just physical. If you are able to survive these extreme conditions, you could potentially experience some of the most extreme phenomena in the universe. As you approach a black hole, its gravitational pull becomes stronger and stronger, causing time to slow down. Then the time warp associated with being nearer to the black hole versus farther from the black hole could result in a significant time warp, time difference between clocks that are ticking at those different locations. And of course there have been examples in popular culture that have made use of that scientific fact. Interstellar, right? So in interstellar, if you recall, there's a black hole and some of the astronauts go relatively close to that black hole, and that affects their passage of time. And just to remind you, Kip Thorne, who was the science advisor and had the initial idea, I believe, for this film, made sure that everything was as scientifically accurate as it could be. So this is an example where just a few hours, according to, you know, Matthew McConaughey, translates into 23 years up on the mothership, because time is elapsing so slow in the powerful gravitational pull of a black hole relative to the rate at which time is elapsing elsewhere. Of course, all this is purely theoretical at the moment, but the fact that it's been possible to contemplate such a journey is a testament to the incredible power and strangeness of black holes. Despite their mysterious and often perplexing properties, black holes continue to fascinate and inspire scientists and the public alike. While the idea of traveling to a black hole may still be the stuff of science fiction, the advances in our understanding of black holes and the nature of time are paving the way for new discoveries and insights into the workings of the universe. Who knows what other mind-bending discoveries await us in the future?