This paper shows that 2 photons in a toy universe (isolated box) are seen as static to an outside observer unless interacted with/entangled with. It states that it shows that time emerges from entanglement, I don't know much about physics but wouldn't that imply that without entanglement, time wouldn't exist? But time isn't a physical tangible thing so how can it emerge from something?
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$\begingroup$ How do I read the full paper on there? $\endgroup$ – Time4Tea Aug 20 '18 at 17:50
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$\begingroup$ Click the download pdf. $\endgroup$ – Oisin Spain Aug 20 '18 at 17:58
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1$\begingroup$ FYI this idea was introduced by Page and Wootters in 1983. Credit where credit's due... $\endgroup$ – Mark Mitchison Aug 21 '18 at 14:44
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An interesting approach.
To answer your question,
time isn't a physical tangible thing so how can it emerge from something?
The idea is to think of time in two ways.
In one sense, "time" is the time that is in the equations of physics. That's the t in the equations of the paper, it's the parameter that describes how the states of all systems in the universe change.
However, actual measurements from within the universe cannot measure "t". All they can do is look at the correlation between the state of one thing - say, the hands of a clock - and the state of another thing - say, the conditions of a chemical reaction. So when we actually measure time, what we're measuring is these correlations.
The paper investigates a toy model, showing that measurements of such correlations from within the system - within the universe - will reveal that things correlate in this way, so that it will appear to things within it that time passes, that clocks measure time and there are processes and states that change in time. At the same time, the overall state of the system does not change in the parametric time t, and if it were possible to make measurements on the whole system from the outside the measurer would see that nothing changes, his measurement results appear all static and the state appears all static.
In this way time "emerges" from the physics: it is the observed time from within the system that emerges, out of how correlations are measured from within.
wouldn't that imply that without entanglement, time wouldn't exist
Yes, if that model captures the way time emerges in the real world - measured time would not exist (but parametric time would!) without entanglement.
That this is actually true is far from certain, even for the writers. They're just offering it as a motivation to explore that idea.
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$\begingroup$ When you say measured time would not, but parametric time would exist without entanglement, what do you mean? What's the difference between measured and parametric time? $\endgroup$ – Oisin Spain Aug 21 '18 at 16:03
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$\begingroup$ Parametric time is the parameter t in the equations, it is what moves every system equally through "(parametric) time". Measured time is the correlation between events as seen from within the universe. It's the fact that we see the clock hand moving 12 hours as the day becomes night, it's the time we actually measure from clocks. In the model WITHOUT entanglement when parametric time moves forward measured time would not. $\endgroup$ – PhysicsTeacher Aug 21 '18 at 17:06
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$\begingroup$ So, time would still exist though without entanglement, like planets still move and all, right? Just not the measured time? $\endgroup$ – Oisin Spain Aug 21 '18 at 17:35
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$\begingroup$ Planets moving are a kind of clock. You can look up in the sky, see that one star (planet) is in a different position from where it was before, and thus deduce that time has passed. Without entanglement, according to the model, measurements from within the universe will reveal no such clocks. Time (including your subjective time, as it's one part of your body measuring another part of you) would stand still. $\endgroup$ – PhysicsTeacher Aug 21 '18 at 17:39
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$\begingroup$ So, does it change anything though? Since we have entanglement, whether or not time emerges from it, we still have time? $\endgroup$ – Oisin Spain Aug 21 '18 at 18:04
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Time is a physical property which cannot exist without Space and vice versa. There is no Time or Space by themselves in the universe. There is only SpaceTime. SpaceTime is curved. Observation is entanglement. Observation is Thought. Thought is a physical property when within the physical universe. Thought is never static and curves with SpaceTime. Though SpaceTime is never static. Yet absent Thought, it appears so, but this is an illusion in the universe; Merely a reflection of the divergence of Space and Time in the linear: Infinity.
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$\begingroup$ Aside from not answering the question, this answer is incoherent. $\endgroup$ – quant Feb 28 at 14:09
