The quantum state of the Universe

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Discussion Overview

The discussion revolves around the quantum state of the universe, particularly focusing on the nature of entanglement and the implications of measurements on quantum states. Participants explore theoretical concepts related to quantum mechanics, including the behavior of distant particles and the definition of entangled states.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants propose that the dimension of the space of quantum states grows exponentially with the number of particles, leading to expectations of finding entangled states more frequently.
  • Others question the assumption that the universe is not in an entangled state, asking for definitions of what constitutes a "weird" quantum state.
  • A participant defines "weird" as a state that cannot be expressed as a product of individual particle states, suggesting that distant particles should evolve towards such states if not interacting.
  • Concerns are raised about the implications of distant particles not being in a product state, questioning the existence of long-distance correlations that are not observed.
  • Some participants clarify that entangled states are those that cannot be expressed as products of individual states, and there is uncertainty about the validity of this term for physically distant particles.
  • Another viewpoint suggests that measurements play a crucial role in defining the universe, with interpretations like many worlds and decoherence being mentioned as potential resolutions to the discussion.
  • A later reply emphasizes that the state of large parts of the universe is complex and not simply a product of single particle states, referencing the cluster property of quantum mechanics.

Areas of Agreement / Disagreement

Participants express differing views on whether the universe is in an entangled state, with no consensus reached on the definitions or implications of "weird" quantum states. The discussion remains unresolved regarding the nature of distant particles and their quantum states.

Contextual Notes

Participants note limitations in definitions and assumptions regarding entanglement and the behavior of quantum states, particularly in relation to measurements and the implications for the universe as a whole.

accdd
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The dimension of the space of quantum states of multiple particles grows exponentially as the number of particles increases. I would have expected to more likely find the quantum state of many particles in a strange state (such as an entangled one) but it is not so, why? Why isn't the universe entangled? Just because macroscopic objects are continually hit by particles, forcing measurements? If so, then do single particles far away in vacuum (not subject to continuous measurements) evolve towards entangled states? Why isn't the universe in a weird quantum state?
Sorry for the mistakes, i am using a translator.
 
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Why do you think the universe is not in an entangled state? How do you know the universe is not in a "weird quantum state"? Perhaps it is. What is the definition of "weird" in this context?
 
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By weird quantum state I mean a state that cannot be expressed as a product of individual particles.
I know the universe is not in a weird quantum state because I have read that if the particles are distant and their respective wave functions do not overlap, we can approximate the wave function of both as the product of the individual wave functions, and this is not a weird quantum state.
If we have distant particles that are not interacting with anything their quantum state should evolve towards a state that is not the product of the individual particles. Wouldn't this create long-distance correlations that are not observed? What is the evidence that the universe is in a "weird" quantum state?
 
accdd said:
By weird quantum state I mean a state that cannot be expressed as a product of individual particles.
That makes no sense to me. Are you using weird to mean entangled?
accdd said:
What is the evidence that the universe is in a "weird" quantum state?
I don't have a working definition of "weird".
 
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If entagnled is any state that cannot be expressed as the product of the states of the individual particles that compose it, then yes. I used the word "weird" because I don't know if either the word entangled is valid for describing states of physically distant particles.
 
There are quantum states and there is the Universe. Through measurements, quantum states become the Universe. Obviously, measurements are ubiquitous otherwise you'd have no Universe. Throw in observers or many worlds and the mystery is solved(btw those 2 are the most popular interpretations along with decoherence which also employs many worlds)
 
accdd said:
The dimension of the space of quantum states of multiple particles grows exponentially as the number of particles increases.
The dimension is countably infinite already for a single particle, and this does not change for multiple particle states. Only in quantum field theories one may have Hilbert spaces with an uncountable dimension.
accdd said:
I would have expected to more likely find the quantum state of many particles in a strange state (such as an entangled one) but it is not so, why? Why isn't the universe entangled? Just because macroscopic objects are continually hit by particles, forcing measurements? If so, then do single particles far away in vacuum (not subject to continuous measurements) evolve towards entangled states? Why isn't the universe in a weird quantum state?
Sorry for the mistakes, i am using a translator.
The state of any large piece of the universe is not even approximately the product of single particle states. This is consistent with the cluster property that asserts that particles far away from each another are asymptotically independent. This makes the 2-particle state approximately the product of two single particle states. But any open, convex part of the universe containing an observer and two far away particles is nevertheless in a very complex multiparticle state.
 
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