The HUP in absence of observers

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Main Question or Discussion Point

I'm asking this after some previous thread about the HUP. If I understand well, the HUP is basically about measurements, that is, it imposes limitations to the quality of the information any observer may attempt to obtain about the universe.

My question is, how should we understand the HUP in the absence of any potential observers, say at the epochs when the universe was less than 8 or 10 billion years old, before (supposedly) any Life existed?

Does the HUP still play any role then and if so. how should it be interpreted or understood, when we can not talk of any attempts to make any measurements?

Thanks,

Answers and Replies

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PeterDonis
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2019 Award
If I understand well, the HUP is basically about measurements, that is, it imposes limitations to the quality of the information any observer may attempt to obtain about the universe.
Strictly speaking, this is true if you view quantum states as describing the probabilities of possible measurement results. But some interpretations of QM say that the quantum state is physically real, independent of whether any measurements are made or not. On these interpretations, the HUP is a restriction on the states that can be physically realized.

how should we understand the HUP in the absence of any potential observers
In these cases, you can still apply the HUP as a limitation on possible states: basically it limits how "close" a state can simultaneously get to an eigenstate of different non-commuting observables. What you end up doing with those states will depend on which interpretation of QM you are using. If the state just describes the probabilities of possible measurement results, then the only thing you can do with a quantum state of something 8 billion years ago is to evolve it forward in time to a time where there are some observers, in order to predict the probabilities of different possible measurements they could make. But if the state is physically real, you can talk about what states are possible 8 billion years ago independently of whether any measurement is ever going to be made.

If the state just describes the probabilities of possible measurement results, then the only thing you can do with a quantum state of something 8 billion years ago is to evolve it forward in time to a time where there are some observers, in order to predict the probabilities of different possible measurements they could make. But if the state is physically real, you can talk about what states are possible 8 billion years ago independently of whether any measurement is ever going to be made.
Wow, that sounds pretty heavy... so with different interpretations still being respectable, that means that scientists do not agree on what the universe was really like 8 billion years ago?

I seemed to have understood that "measurements" in absence of observers ought to be defined as just information records, information which gets imprinted in the physical configuration of the universe. And that such "information imprints" could be independent of any observers.

PeterDonis
Mentor
2019 Award
so with different interpretations still being respectable, that means that scientists do not agree on what the universe was really like 8 billion years ago?
The word "really" doesn't have a precise meaning, so this question doesn't really have a well-defined answer. However, if you are thinking that the first kind of interpretation I described, where all you can do with a quantum state referring to 8 billion years ago is to evolve it forward in time, implies that the universe 8 billion years ago was not "real", that's not the case. All such an interpretation is saying is that the quantum state we assign to 8 billion years ago is not a complete description of "reality" 8 billion years ago; it only captures the information needed to predict the probabilities of measurements we can make now (by evolving the state forward in time).

Also, the other kind of interpretation, where the quantum state 8 billion years ago is physically real, might not be as "ordinary" as you think it is, since the quantum state 8 billion years ago is going to describe amplitudes for different possibilities--which on one popular interpretation, means it's describing "many worlds". This is why I said, in my Insights article on quantum interpretations, that both kinds of interpretation require you to accept something unpalatable; the question is just which thing you find less unpalatable.

I seemed to have understood that "measurements" in absence of observers ought to be defined as just information records, information which gets imprinted in the physical configuration of the universe. And that such "information imprints" could be independent of any observers.
If you define "measurement" in such a way that it can occur in the absence of observers (where "observers" here has to mean something like "beings that are conscious the way we humans are"), then you can apply the HUP to these "measurements" to obtain limitations on their possible results. What the actual results mean depends on your interpretation of QM (but that's also true for "measurements" that do involve conscious observers).

atyy
Science Advisor