"Does the universe have a boundary in space?”"Does the universe have a boundary in space? Let's have a dialogue on this issue. I ask, you answer. Of course, I will ponder and write down your possible answers.
Q: Let us first assume that the universe is infinite, that it has no boundary. Please imagine what this boundless universe without boundaries looks like, and then tell me.
A. It has no boundary on all sides.
Q: You're just repeating my question. What I want to ask is exactly what it means to have no boundaries.
A: I first thought of our Earth, the Solar System, and the Milky Way. Then I thought of other galaxies beyond the Milky Way, and other galaxies beyond other galaxies, and so on until infinity.
Q: Right, it is impossible for us to directly imagine something without a boundary. In order to imagine something without a boundary, we first imagine a part of it. This part has a boundary, and then imagine a part adjacent to it. This way, we can gradually expand and synthesize it. However, no matter how many parts you imagine and combine them, the result you get is still a limited thing with boundaries. What you said about pushing it to infinity is just empty talk. You can't really do it in your imagination.
A: I admit that I can't do it. When my imagination tried to push infinitely, it stopped, and I had to help it with words, saying to myself, Just keep pushing forward like this…
Q: Exactly. This means that we cannot imagine a universe without boundaries. Now, let's assume that the universe has a boundary. Can you imagine what lies beyond its boundary?
A: There should be nothing, otherwise it wouldn't be a border.
Q: You are right. If there is still something, we must locate its boundary outside those things until there is nothing. That was to say, there was only emptiness outside its boundary. Now you have a similar problem: You have to imagine the emptiness beyond the boundaries of the universe. This emptiness has no boundaries.
A: I can't imagine it.
Q: From this, it can be seen that it is inconceivable whether the universe has a boundary or not.
In the conversation above, we basically repeated Kang Sui's discussion. In fact, when he argued that the universe could not have no boundary, he was relying on the same reason: We cannot imagine infinite space, whether it is empty or filled with objects. If I had to choose, I would rather believe that the universe has no boundary, because imagining the infinite with content can still start from its limited parts, but imagining the infinite with emptiness cannot even find such a starting point.
Modern Cosmologists proposed a hypothesis based on Einstein's general theory of relativity: Our universe is limited in space but has no boundaries. How can there be no boundaries? Because its space is a curved and closed gravitational field, this space is neither connected to the void nor to other objects. As for whether there are other universes outside our universe, we will never know, so there is no need to consider. Unfortunately, philosophy often ignored the advice of science and focused on the unknowable. We couldn't suppress our curiosity: If there was neither a void nor another universe outside our limited universe, what could it be?”
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