**Science News**

###### Curated by RSF Research Staff

# Entanglement of quantum clocks by gravity

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### Gerard ’t Hooft on the Future of Quantum Mechanics

### Weighing a Star with Light

### New measurements exceed Heisenberg uncertainty limit; is this experimental evidence for non-orthodox quantum theories?

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Time forms the basis of our every experience, yet it remains a challenging factor to define in a consistent scientific model. Most advanced models of unified physics considers time as an emergent property, not an intrinsic attribute of the physical world. This partly emerges from the treatment of time in general relativity, where time becomes inextricably merged with space and becomes relative to ones frame of reference.

In general relativity, it is commonly imagined that any given reference frame uses a latticework of clocks to record events, where each location in space has a corresponding idealized clock. The clocks can then be used to locate events in spacetime. In this clock latticework picture, clocks are considered as external objects that do not interact with the rest of the universe. This raises some considerations, as within the framework of the connected universe we understand that "everything affects everything else", so how accurate is it to consider our clocks as behaving independently from the surrounding system?

Considering the clocks as occupying a special role where they are unaffected by the state of the surrounding system places them on an unequal footing with the rest of the physical system, and must therefore be artificial. In the words of Einstein:

Now a new study by physicists at the Vienna Center for Quantum Science and Technology evaluates the general relativistic notion of clocks from a quantum perspective. What the research team discovered is that the higher the precision of a given clock, and hence the better ability to locate events in spacetime from that clock -- the greater its total energy. Because of the mass-energy equivalence, this means that it will have a greater gravitational field than a less precise clock. The stronger gravitational field will induce relativistic time dilation, so that all nearby clocks (considering again our clock latticework of relativistic theory) will necessarily get entangled.

The entanglement of clocks through relativistic time dilation introduces limitations to the measurability of time as recorded by the clocks, since they are not the independent, absolute entities of classical theory, but instead are interacting, co-dependent systems. This shows how the notion of time may begin to breakdown at the quantum level. When systems are considered at the macroscopic level, the entanglement of the myriad clocks at each point in spacetime becomes less evident, and the classical notion of time is recovered.

Furthermore, the research team describes how the general relativistic notion of time dilation emerges from the average mass-energy of a gravitating quantum system. The study is an interesting treatment of unified physics and should hold insights into quantum gravity research.

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