Relational quantum mechanics

Relational quantum mechanics (RQM) is an interpretation of quantum mechanics that changes how we think about tiny particles. In RQM, the state of a particle depends on the relationship between an observer and the particle. This idea was first explained by Carlo Rovelli in 1994.

RQM is inspired by special relativity, which shows that what we see depends on where we are. It also uses ideas from Wheeler about information theory. In RQM, different people might describe the same particle in different ways. For example, one person might see a particle in one state, while another person sees it in many states at once, called a superposition. RQM says both descriptions can be right because the state depends on who is looking.

This approach helps solve some tricky questions in quantum mechanics, like the measurement problem and ideas about whether things happen in a definite way everywhere at once, called local realism. RQM shows that reality is about relationships between things, not one single description of everything.

History and development

Relational quantum mechanics started when Carlo Rovelli noticed that some problems in quantum mechanics were similar to old problems with Lorentz transformations before special relativity was discovered. He thought that, just like old ideas about time were incorrect, assuming that a system's state looks the same to every observer can cause confusion.

Later, scientists like Lee Smolin and Louis Crane used these ideas to explore the beginning of the universe. In 2020, Rovelli wrote a book called Helgoland to help explain these concepts. In 2023, he and Emily Adlam updated the theory with new rules about how different viewpoints relate to each other.

The problem of the observer and the observed

When we study quantum mechanics, we think about what happens when someone looks at a tiny particle or system. This is a big question in a theory called Relational Quantum Mechanics.

Imagine one scientist, named O, looking at a tiny particle called S. Scientist O can describe what happens when they look at S. But there is another scientist, named O′, who wants to know what happens to both O and S together. Even though both scientists are talking about the same event, they might tell the story in different ways. This shows us that in quantum mechanics, what we observe can depend on who is doing the observing. This idea connects to famous thought experiments like Wigner's Friend, which helps us understand different ways people can think about quantum theory.

Central principles

Relational quantum mechanics says that the state of a quantum system depends on who is looking at it. This idea comes from the thought that, just like in special relativity, where things look different depending on your viewpoint, quantum systems might seem different to different observers.

The theory has two main ideas: first, that all systems—big or small—are quantum systems, and second, that quantum mechanics is complete as it is, without any hidden rules. Because of this, the description of a system is always tied to a specific observer. For example, one observer might see a system in one state, while another observer sees something different.

Consequences and implications

In relational quantum mechanics, the state of a quantum system changes depending on who is observing it. This means that two people might describe the same system in different ways because of their own points of view.

One important idea is that an observer can check if their description of a system is right, but they cannot always know the exact state of the system without interacting with it. This can lead to situations where two observers seem to get different results, but these differences happen because each observer’s description depends on their own interactions with the system.

The theory also suggests that systems can only interact if they are close enough in space and time, following the rules of special relativity. This shows how a system's location and its interactions depend on each other.

Relationship with other interpretations

Relational quantum mechanics (RQM) works in a different way from some other ideas about quantum mechanics. It does not match well with hidden variables theories, which think there are secret factors that explain quantum actions. RQM believes quantum mechanics tells the whole story and does not need extra ideas.

RQM is a little like the Copenhagen interpretation, but it says that any interaction — not just big ones with machines — changes how we understand quantum systems. This is not the same as the Copenhagen idea that only big interactions make changes.

RQM also shares some points with the many-worlds idea, but it does not think there is one big description of the whole universe. Instead, it says there are many smaller, linked descriptions.

RQM works well with the consistent histories approach, which studies chains of events instead of single moments. RQM helps explain how these chains can still make sense even when they depend on what the observer sees.

Main articles: Copenhagen interpretation, Bohm's interpretation, many-worlds, consistent histories

EPR and quantum non-locality

RQM provides a special way to understand the EPR paradox. It shows that nothing travels faster than light in a Bell test experiment. The idea of locality works for everyone.

In the EPR experiment, something makes two electrons spin in linked ways. These electrons move far apart to two observers, Alice and Bob, who check the spins. The spins match perfectly: if Alice finds her electron spinning up, Bob finds his spinning down, and the other way around. This matching seems to happen right away, even over big distances, which might look like faster-than-light communication.

In RQM, someone needs to touch a system to see its features clearly. Since Alice and Bob are too far apart to check both electrons together, they can't know each other's answers right away. Only later, when they meet and talk, do they see the matching between their answers. This way, RQM explains the matching without breaking the rules about faster-than-light travel or locality. Everyone still sees answers that fit the normal ideas of quantum mechanics.

Derivation

Relational quantum mechanics (RQM) says that the state of a quantum system depends on how the observer and the system relate to each other. This idea was first introduced by Carlo Rovelli in 1994. RQM is inspired by special relativity, which shows that observations can change depending on where the observer stands.

RQM can be built from a few basic ideas, or postulates, from experiments. These postulates help explain how we get information about quantum systems. One key idea is that there is a limit to how much information we can get from a quantum system. Another idea is that we can always learn new things about a system. These simple starting points help us understand quantum mechanics better.

Problems and discussion

Relational quantum mechanics says that what we know about something changes based on how we look at it. It is like how things appear different depending on where you stand. This idea focuses on how a system acts in different situations, instead of on things that never change, like an electron's mass. Some people think that even things like mass can only be known when we interact with the object.

Main article: phase space
Main article: state space