Quantum mechanics and General Relativity overlap

[daveian]

I am not an expert on cosmology, merely an interested layman, so I hope my question is not either too stupid or obvious.
As I understand it from reading books (currently Brian Greene's "Fabric of the Cosmos"), general relativity (GR) is used for analysing large massive objects whereas quantum mechanics (QM) works for small light objects and that the 2 theories have not yet been successfully unified. But this poses a question, What about the middle ground? At what point in the spectrum from large/ massive to small/ light does GR not function and vice versa for QM. Is it sudden or gradual fall off in effectivness for either theory.
Greene refers to the centre of a black hole (massive and tiny) where both GR and QM are needed but can't be used because they don't fit together.

 

[bcrowell]

Welcome to PF!

Yes, there are situations where you need both quantum mechanics and gravity. We have no experiments or observations that probe those situations. If we had empirical information about such situations, it would be a lot easier to construct a theory of quantum gravity.

 

[mathman]

When both quantum theory and general relativity are both needed (for exasmple inside a black hole) it ends up with mathematical nonsense.

 

[Macch]

I was wondering, is there any overlap between Quantum Information Theory and General Relativity? The black hole information paradox could be solved by devoloping knowledge about quantum information in strong gravitational field?

If someone wants to study this overlap between these 2 differentes areas of physics, what would be better: having a PhD in Quantum Information Theory or in Cosmology?

thanks for the help

 

[sardar]

Your understanding is correct that general relativity and quantum mechanics are two separate theories that have not yet been successfully unified. However, it is important to note that they are not necessarily limited to analyzing only large or small objects, respectively.

General relativity is a theory of gravity and space-time, and it can be applied to any object with mass, regardless of its size. Similarly, quantum mechanics can be applied to any system, regardless of its mass or size.

The issue arises when trying to apply these theories to extreme conditions, such as the center of a black hole. In these extreme conditions, both theories break down and we need a new, unified theory to accurately describe what is happening.

To answer your question about the middle ground, there is no specific point where one theory stops working and the other takes over. It is a gradual transition, with both theories becoming less accurate as we approach extreme conditions.

In terms of the black hole example, we currently do not have a complete understanding of what happens at the singularity (the center of a black hole). Both general relativity and quantum mechanics give us conflicting predictions in this scenario, showing the need for a unified theory.

In summary, general relativity and quantum mechanics are both valid theories that can be applied to a wide range of objects and systems. However, they break down in extreme conditions and a unified theory is needed to accurately describe these situations.