Sep 8, 2025 03:27 AM
SABINE HOSSENFELDER
https://youtu.be/wh5NAM0oNaQ
VIDEO EXCERPTS (many missing details): Some ideas in physics sound so strange you’d think they can’t possibly be true. Dualities are one of them. They say that two entirely different theories can describe the exact same physical system. Not just approximately. Exactly.
You can calculate something using one theory, or using the other — and you’ll get the same result. This raises the interesting question of why we use only one description of reality and not the other. Or, if you are more philosophically minded: what is real in the first place.
Let me start with an example you might already be familiar with...
[...] And these are the two sides of the duality. You can describe the world with quantum waves in space, by position, or you can describe them by their frequency decomposition in momentum space. Physicists switch between the two all the time...
[...] We don’t think about the world in terms of momentum space. Why is that? Well, that’s where things get interesting. I think it’s because of our own constitution. It’s because we are always somewhere, in space. This is how we are used to making sense of the world, by way of location and differences between locations. It’s real to us. Momentum space on the other hand is a mathematical tool that’s sometimes convenient, but that doesn’t correspond to our experience.
Still, on a fundamental level you might ask, isn’t both equally real, or equally made up? If this makes your head spin, try thinking about it in momentum space — it’s even worse.
Then there’s the best-known duality in modern theoretical physics, with a rather intimidating name, the AdS/CFT correspondence. It says that a theory of gravity like general relativity in a space with a negative cosmological constant – that’s the AdS space -- is equivalent to a quantum theory without gravity in a space with one dimension less. This is very perplexing if you think abou it. It's like there is this one dimension that you don’t actually need....
[...] And again that brings up the question, well, in which sense is one description more real than the other?
Here, too, I think the reason is not that one is objectively better or more meaningful. It’s that one of them fits our experience. You see we are used to describing what happens in terms of things that have locations and that do things with other things.
But with these dualities, it’s always only one side that has such a description. The other side is highly non-local with nothing resembling particles.
[...] So I think the lesson here is that, for one thing, our personal experience of the world has a major influence on the theories that we develop. And second, it makes no sense to claim that a certain description of our observation is real, as there can be multiple different ones.
This is also why I just get confused if people ask whether particles are real. I don’t know what this even means. We can use certain mathematical descriptions that we call particles to describe our observations. Or we can use something else entirely. So are particles real? I don’t know. I also don’t need to know. It’s not my job. I just need to know how to describe observations, one way or another.
[...] I find this fascinating because dualities show us that the way we describe the universe isn’t unique. We pick the description that makes the most sense to us. But there are entirely different ways to see the world. And sometimes I wonder if an alien species that has entirely different experiences would use a completely different way to explain the same physics...
There’s Another Way to See Reality. It’s Just as True. ... https://youtu.be/wh5NAM0oNaQ
https://www.youtube-nocookie.com/embed/wh5NAM0oNaQ
https://youtu.be/wh5NAM0oNaQ
VIDEO EXCERPTS (many missing details): Some ideas in physics sound so strange you’d think they can’t possibly be true. Dualities are one of them. They say that two entirely different theories can describe the exact same physical system. Not just approximately. Exactly.
You can calculate something using one theory, or using the other — and you’ll get the same result. This raises the interesting question of why we use only one description of reality and not the other. Or, if you are more philosophically minded: what is real in the first place.
Let me start with an example you might already be familiar with...
[...] And these are the two sides of the duality. You can describe the world with quantum waves in space, by position, or you can describe them by their frequency decomposition in momentum space. Physicists switch between the two all the time...
[...] We don’t think about the world in terms of momentum space. Why is that? Well, that’s where things get interesting. I think it’s because of our own constitution. It’s because we are always somewhere, in space. This is how we are used to making sense of the world, by way of location and differences between locations. It’s real to us. Momentum space on the other hand is a mathematical tool that’s sometimes convenient, but that doesn’t correspond to our experience.
Still, on a fundamental level you might ask, isn’t both equally real, or equally made up? If this makes your head spin, try thinking about it in momentum space — it’s even worse.
Then there’s the best-known duality in modern theoretical physics, with a rather intimidating name, the AdS/CFT correspondence. It says that a theory of gravity like general relativity in a space with a negative cosmological constant – that’s the AdS space -- is equivalent to a quantum theory without gravity in a space with one dimension less. This is very perplexing if you think abou it. It's like there is this one dimension that you don’t actually need....
[...] And again that brings up the question, well, in which sense is one description more real than the other?
Here, too, I think the reason is not that one is objectively better or more meaningful. It’s that one of them fits our experience. You see we are used to describing what happens in terms of things that have locations and that do things with other things.
But with these dualities, it’s always only one side that has such a description. The other side is highly non-local with nothing resembling particles.
[...] So I think the lesson here is that, for one thing, our personal experience of the world has a major influence on the theories that we develop. And second, it makes no sense to claim that a certain description of our observation is real, as there can be multiple different ones.
This is also why I just get confused if people ask whether particles are real. I don’t know what this even means. We can use certain mathematical descriptions that we call particles to describe our observations. Or we can use something else entirely. So are particles real? I don’t know. I also don’t need to know. It’s not my job. I just need to know how to describe observations, one way or another.
[...] I find this fascinating because dualities show us that the way we describe the universe isn’t unique. We pick the description that makes the most sense to us. But there are entirely different ways to see the world. And sometimes I wonder if an alien species that has entirely different experiences would use a completely different way to explain the same physics...
There’s Another Way to See Reality. It’s Just as True. ... https://youtu.be/wh5NAM0oNaQ
