# Time emergent from entanglement

• B
• TheQuestionGuy14
In summary, the article and the thread discussing it state that time emerges from quantum entanglement. If you assign a quantum state to the universe as a whole, it does not change with respect to the time parameter that gets imposed by non-relativistic QM.f

#### TheQuestionGuy14

[Moderator's note: Spin off from a previous thread since this topic belongs in the QM forum.]

Again: we cannot discuss out of context quotes from an unknown source. Either give a specific link to the actual discussion you're asking about or this thread will be closed.

https://physics.stackexchange.com/questions/423835/is-time-emergent-from-quantum-entanglement

Last edited by a moderator:
Coordinate time is a coordinate label that you put on events in spacetime (more precisely, one of four coordinate labels that you put on each event, assuming you are using appropriate coordinates).

Proper time is the time elapsed on a clock between two events on the clock's worldline.

Then what is the person who replied talking about? The experiment wants to show that time emerges from quantum entanglement, he states only measured time is, and parametric time isn't, what is he talking about?

Proper time is the "distance" along a timelike worldline. If you arrange a set of parallel inertial (i.e. straight) timelike worldlines, this is one direction of a grid. If you agree a zero on all of the lines (preferably so that the zeros form a line orthogonal to each timelike line) then you have coordinate time.

It's exactly like the distinction between the length of any old line and the length along a set of parallel straight lines (which you'd call a y-coordinate) in Euclidean geometry.

So what is this guy I quoted in OP explaining? Is he correct or incorrect?

So what is this guy I quoted in OP explaining? Is he correct or incorrect?
My reading is that he's showing that clock measurements emerge from quite simple quantum systems. Einstein said time is what clocks measure - the paper is a (partial?) explanation of why that is so.

I may not be reading it right. I've only read the abstract and the stack exchange post.

bhobba
Not coordinate time or proper time. So if you were hoping to get information about what those are, that StackExchange thread is not the way to do it--nor is the paper linked to there.

In quantum mechanics (more precisely, non-relativistic QM), the time ##t## is a parameter; the state of the overall quantum system is a function of this parameter, but the parameter is just imposed by the theory, it has nothing to do with anything that's measured or anything physical.

"Measured time" in the paper is the change in the correlations between the subsystems of the overall quantum system as measurements are made on them, interpreted as evidence of "time flowing", as part of a proposal made by the authors of the paper on how we, as individual subsystems in the universe, could perceive time to be flowing (or, to put it another way, things to be changing), when, if you try to apply non-relativistic QM to the whole universe and assign the whole universe a quantum state, the Hamiltonian you get out of it says the state of the whole universe never changes at all as a function of the parameter ##t##.

As above, none of this has anything to do with either coordinate time or proper time as those concepts are used in relativity. A discussion of the paper and the StackExchange thread really belongs in the QM forum, not here, if that's what you want to ask about. But such a discussion has nothing to do with coordinate time or proper time.

OK, so, speaking of the experiment, is it correct? Does time emerge from entanglement? And if so, how could the universe be static to an external observer if planets, stars, people are clearly moving?

Does time emerge from entanglement?
Clock measurements can emerge from entanglement. Clock measurements are not time.

(See disclaimer in my previous post)

Clock measurements can emerge from entanglement. Clock measurements are not time.

(See disclaimer in my previous post)
But the experiment states that from an external observer, the universe would be static, as the observer is not entangled with the photons, only when an internal observer entangles with the photon pair, time can be measured. Thus time itself emerges from entanglement.

how could the universe be static to an external observer

There is no "external observer" for the universe as a whole. That is one key reason why I am skeptical of the argument given in the paper.

TheQuestionGuy14
the experiment states that from an external observer, the universe would be static

That's not actually what the math says. The math just says that if you assign a quantum state to the universe as a whole, it does not change with respect to the time parameter that gets imposed by non-relativistic QM.

As I said in my previous post, the idea of an "external observer" for the universe as a whole doesn't make sense, so any such interpretation of what the math says does not seem correct or useful to me.

Also, as I've noted, the math in question is non-relativistic QM, and a non-relativistic model does not seem correct or useful for the universe as a whole. At the very least, a valid model for something like this would need to be formulated in quantum field theory; but we don't have a good quantum field theory of gravity, so we don't have a good way of formulating models like this for the universe as a whole.

bhobba, Ibix and TheQuestionGuy14