Saturday, May 4, 2024

Many-worlds does not Solve the Measurement Problem

Brian Greene's third interview is with Carlos Rovelli:
I'll now give you a brief 0:21 summary of where we have gotten so far all right in our first conversation with 0:26 philosopher Elise Crow we discussed the basics of quantum mechanics namely that 0:33 the theory has at its core the idea that the best you can ever do in our reality 0:39 is make probabilistic predictions ...

we also explored how through quantum 1:36 entanglement reality has what we call a nonlocal quality that is what you do 1:42 here can have an instantaneous Quantum impact on something way over there and 1:48 such non-locality we found can thread not just through space but also through time

all right in 1:56 our second conversation with physicist and author Sean Carol we explored a 2:01 number of proposed resolutions to the quantum measurement problem most notably 2:07 spontaneous collapse theories as well as the many worlds interpretation of 2:13 quantum mechanics in which every possible Quantum outcome actually 2:18 happens but each takes place in its own Quantum Universe

No. Nothing can have an instantaneous nonlocal impact, and many-worlds does not solve the measurement problem.

I am surprised Greene would say this, because the advocates of many-worlds theory do not even claim to solve the measurement problem.

They argue that there is a wave function of the universe, and that the function will explain everything we see. Any mystery might be answered by understanding this function better. But it does not tell us why we see one outcome.

Or so I thought. I asked ChatGPT:

The many-worlds interpretation (MWI) is a fascinating and controversial view in quantum mechanics. Let’s delve into it and explore how it relates to the quantum measurement problem.

The Quantum Measurement Problem

The measurement problem arises from the mysterious behavior of quantum systems during measurement or observation. Here’s a brief overview:

1. Wavefunction Collapse: Before a measurement, a quantum system exists in a superposition of all possible states described by its wave function. However, upon measurement, the wave function appears to “collapse” to a specific state, leading to the observed outcome. This collapse is puzzling because it seems to violate the unitary evolution of quantum mechanics.

2. Where Do Other Possibilities Go?: If all possible outcomes of a quantum measurement have real existence, why do we observe only one outcome? What happens to the other possibilities?

Many-Worlds Interpretation (MWI)

The MWI, proposed by physicist Hugh Everett, offers an intriguing solution to the measurement problem. Here are the key ideas:

1. Parallel Universes: According to the MWI, the universe doesn’t collapse into a single state during measurement. Instead, it branches into multiple parallel universes, each corresponding to a different measurement outcome. In other words, every possible outcome occurs in a separate universe.

2. Wave Function Never Collapses: In the MWI, there is no collapse of the wave function. Instead, the entire universe (including the observer) evolves as a superposition of all possible states. Each branch represents a different version of reality, with no need for collapse.

3. Replicas of You and Me: In these parallel universes, there exist replicas of you and me, leading other lives. Each replica experiences a different outcome of the measurement. These universes coexist but remain mutually isolated.

Controversy and Philosophical Implications

The MWI challenges our understanding of knowledge and reality:

Controversy: Debates about quantum interpretations can be passionate, and the MWI is no exception. Some physicists find it elegant, while others reject it due to its radical departure from classical intuition.

Epistemological Question: The MWI raises fundamental questions about what constitutes knowledge. It challenges us to consider what kind of theory we accept as a claim to understand the world.

In summary, the MWI provides an alternative perspective that avoids wavefunction collapse, but it remains a topic of ongoing debate. Whether it fully solves the measurement problem is still a matter of interpretation and personal preference12. Feel free to explore further or ask more questions! 😊

I cites this Quanta article:
Why the Many-Worlds Interpretation Has Many Problems ...

it seemed that the central problem for quantum mechanics was the mysterious rupture created by observation or measurement, which was packaged up into the rubric of “collapse of the wave function.” ...

But Bohr and colleagues didn’t bring wave function collapse into the picture just to make things difficult. They did it because that’s what seems to happen. When we make a measurement, we really do get just one result out of the many that quantum mechanics offers. Wave function collapse seemed to be demanded in order to connect quantum theory to reality.

So what Everett was saying was that it’s our concept of reality that’s at fault. We only think that there’s a single outcome of a measurement. But in fact all of them occur. We only see one of those realities, but the others have a separate physical existence too.

Okay, I guess the Everett do claim that they have solved somethiung here. As you can see, they have not.


  1. I think there's a subtlety that is worth considering about Bohr's position on collapse, which is different from Heisenberg's, in particular. See Don Howard's article "Who Invented the “Copenhagen Interpretation”? A Study in Mythology",
    What I take that to say is that Heisenberg and most others in the Copenhagen line took measurement to change the state, but Bohr took measurement to affect subsequent measurements. I think Bohr's take allows us to rethink the measurement problem.
    The discussion I suggest in JPhysA 2022, "The collapse of a quantum state as a joint probability construction", (DOI there), encodes Bohr's take as well as ideas about Koopman's Hilbert space formalism for classical mechanics and taking QFT to be a noisy signal analysis formalism.
    This gives us a no-collapse interpretation of state preparation formulated in the relatively abstract terms of the mathematics of joint and conditional probabilities that does not require Many Worlds. Whaddya think?

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