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?

  • $\begingroup$ How do I read the full paper on there? $\endgroup$
    – Time4Tea
    Aug 20, 2018 at 17:50
  • $\begingroup$ Click the download pdf. $\endgroup$ Aug 20, 2018 at 17:58
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    $\begingroup$ FYI this idea was introduced by Page and Wootters in 1983. Credit where credit's due... $\endgroup$ Aug 21, 2018 at 14:44
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    $\begingroup$ Credit has been given, see reference [9] of Moreva et al. $\endgroup$
    – jim
    May 11, 2020 at 17:28
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    $\begingroup$ Time is a perfectly physical thing. Proof: You can measure time. In face there are dynamical equations that determine how time behaves (the Einstein field equations) just like there are dynamical equations that determine how electric fields behave (the Maxwell equations). $\endgroup$
    – user87745
    May 14, 2020 at 20:07

2 Answers 2


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.

  • $\begingroup$ 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? $\endgroup$ Aug 21, 2018 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. $\endgroup$ Aug 21, 2018 at 17:06
  • $\begingroup$ So, time would still exist though without entanglement, like planets still move and all, right? Just not the measured time? $\endgroup$ Aug 21, 2018 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. $\endgroup$ Aug 21, 2018 at 17:39
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    $\begingroup$ Also, is parametric time as you state like coordinate time? And is measured time like proper time? $\endgroup$ Aug 22, 2018 at 13:09

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.

  • $\begingroup$ Poplawski's cosmology allows a formation that is (per Guth's "The Inflationary Universe") not practicable in models of field-based inflation: By the addition of mass to its large star, a species faced with the extinction imposed by that star's life cycle might, if that star would've happened to be of a size only marginally inadequate for its collapse into a black hole, have added mass allowing it to collapse into a black hole rather than collapsing into a neutron star, thereby initiating the formation of a local universe eventually containing stars liable to collapse into smaller black holes. $\endgroup$
    – Edouard
    May 21, 2020 at 20:14
  • $\begingroup$ (I'm posting my previous comment as a "suggestion for improvement", as my own credentials in physics are inadequate to sustain such a possibility through theoretical verification, although I'd gladly accept whatever edit to my answer, from someone with credentials appreciably better than my own, might lend credibility to the possibility of such verification. The motivation of those adding mass to their star would, of course, be the eventual appearance of a down-scaled replica of their observable region.) $\endgroup$
    – Edouard
    May 21, 2020 at 20:28
  • $\begingroup$ My "update" is an attempted meld of the world as perceived by biological creatures (like ourselves) with the results of the recent experiment described in the last of my comments on Physics Teacher's answer to the OP's question. To visual learners (who may all be biological), I'd say those results leave Poplawski's torsion-based inflation, set in the interiors of black holes, preferable to inflation based on scalar fields that don't rotate and never have a value of zero. $\endgroup$
    – Edouard
    Dec 24, 2020 at 11:56

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