Is time emergent from quantum entanglement?

Question

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?

• E. Moreva, G. Brida, M. Gramegna, V. Giovannetti, L. Maccone, and M. Genovese, "Time from quantum entanglement: An experimental illustration", Phys. Rev. A 89, 052122 (2014), arXiv:1310.4691.

Answer 1

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.

Answer 2

What's being rather plausibly suggested, in Moreva's experiment, is a physical apparition of the time dimension. In reality (as compared to what Physics Teacher has correctly described as her "toy model"), such an apparition could only occur (at least in the well-accepted model of time as having a thermodynamic basis) in an overall environment either extremely close to thermal equilibrium (as discussed by Aguirre and Carroll at https://arxiv.org/abs/1108.0417), or in one or more relatively small regions causally separated from the remainder of that environment's space: Of those two possibilities, the latter seems much the more probable, given the astronomical evidence for the existence of black holes.

However, although the inflationary cosmological models generally seem to accommodate variations in spatial scale, the only one I've found that implicitly requires such variations is Nikodem J. Poplawski's "cosmology with torsion" (detailed in many papers, written between 2010 and 2020, that are available free on Arvix), which is based on effects of gravitational collapse in the materialization and interactions of known types of subatomic particles, rather than having any basis in any hypothesized field of "inflaton" particles, which may remain extremely unlikely to be observable prior to their hypothetical decay into photons, electrons, etc.

Whereas the other inflationary cosmologies are (AFAIK) all based on General Relativity, the torsion-based one is derived from the more recent "Einstein-Cartan Theory" that was developed by Einstein through conversations with Elie Cartan, and is reportedly more complex mathematically, although its compatibility with the CMB data was reported to be complete in a 2015 analysis by Desai. Its basic difference is that it requires all fermions to have spatial extent, which was an assumption often incorporated into pop-sci texts prior to the many practical applications of quantum physics, that have resulted in the current wait for a theory of quantum gravity.

I'm bringing these facts to the attention of PSE's participants in the hope that some of those familiar with ECT might have the reputation (1,000) adequate to establish an "Einstein-Cartan" tag, which might facilitate comparisons between cosmologies by prospective students.

Update on this answer: Although Poplawski 1st described his model (in his 2010 "Cosmology with torsion") as "an alternative to cosmic inflation", it's generally described as a version or adjunct of inflation, with the bounce effects I've described being substituted for action of an inflaton field (which, unlike stars, doesn't rotate). In response to today's news about a successful and larger-scaled version of Moreva's experiment on "time from quantum entanglement", I'd like to point out the fact that the shape of Poplawski's model of local universes, each of which he describes as resembling a three-dimensional version of the surface of a ball, itself resembles the "two sheet" model of space locally appearing flat, but connected at many points by fermions whose curved surfaces form "bridges" between them, that was described by Einstein and Rosen in their 1935 paper at https://doi.org/10.1103/PhysRev.48.73 . To me, it's the intuited association between time and rotating or spinning objects (whether they're electrons or clock dials) that makes his model seem more consistent with the emergence of thermodynamic time than other relativistic models might be: Thermodynamics is the study of heat over time, and interaction or collisions between spinning objects is what would bring it most completely into the picture.