If the current is very weak, you are near a planet-stream, like spacetime around the Earth. In the same way, astronauts feel the gravitational pull of planets if they come too close. But if you swim to a different part of the lake, near one of the currents, you can start feeling the current dragging you with it. You cannot see or perceive their presence when you are very far away from these currents because the water around you remains still. Just imagine that there are strong streams flowing within certain parts of a lake. Spacetime close to these cosmic objects is no longer still and smooth but curved and whirly, just like strong streams or rivers. This scenario changes when we look at spacetime surrounding massive objects such as planets, stars, and black holes. This scenario is similar to astronauts floating freely in outer space, where they can easily move up and down and left and right. You can stay still, or you can swim in any direction easily. Objects and people can freely float in this lake. Spacetime far away from planets and stars is flat and smooth without any movement, just like a large, quiet lake. This analogy compares the structure of spacetime to the flow of water. Luckily, an analogy can come to our rescue as we try to understand spacetime. Thinking of all places and all times in the universe taken together is conceptually challenging-the universe is so vast, after all! How can we picture spacetime? Spacetime Flows Like a River For example, if you want to meet your friends after school, you need to agree on where you want to meet (the place) and when to meet (the time). Spacetime is our way of describing all events in the universe-where they happen and when they happen. The theory is based on Einstein’s insight that we should not think of space and time as separate but rather as connected elements that make up spacetime. Let us start with general relativity, which is Einstein’s theory that links space and time to gravity. This is why black holes are black and why we can only see the surrounding matter in this picture (photo credit: Event Horizon Telescope CC BY 4.0).Figure 1 - Black holes are regions in spacetime in which gravity is so strong that nothing can escape-not even light.This article uses a river analogy to explore why we cannot escape a black hole. Analogies are valuable learning tools in physics because they compare abstract science concepts to familiar concepts we know from everyday life. Although Einstein used advanced mathematics to develop his ideas about the universe, we can grasp basic elements of Einstein’s theories using pictures, analogies, and our imagination. Hardly anyone understood space and time better than Albert Einstein, the famous physicist with wild curly hair. To understand what is going on with black holes ( Figure 1), we first need to learn more about space and time. Therefore, black holes remain cosmic mysteries, and many scientists work hard to solve the mystery of black holes. Not even the laws of physics tell us what happens when something falls into a black hole-at least not yet. Because nothing can get out of black holes, physicists struggle understanding these objects. That is why we cannot see black holes-they are invisible to our eyes. Not even light can get out of these regions. Black holes are dark, dense regions in space where the pull of gravity is so strong that nothing can escape. What Are Black Holes?īlack holes are some of the universe’s most fascinating objects. The analogy offers a new way of thinking about space and time. In this article, we present an analogy that helps us make sense of these mysteries. In other words, black holes are huge cosmic mysteries. And more fantastic still: the laws of physics do not tell us what happens when something falls into a black hole and reaches its center. Why? Because once something has fallen into a black hole, it can never return. Physicists think that black holes are some of the universe’s most exciting objects to study. These objects are dark, dense regions in the universe, and their gravitational pull is so strong that nothing can escape them-not even light! This is why black holes are so black: without light, we cannot see them. Have you ever heard of black holes? Black holes sound like objects from a science fiction story.
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