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Humans typically measure time by the change in material things. The movement of hands on a clock. The oscillations of a cesium atom.

But can time also be measured by just changes in energy, and in the absence of matter?

According to evolutionary universe theories there was a point in the evolution where there was no matter - just energy. Was the concept of time nonsensical at this point?

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  • $\begingroup$ Are you talking about the radiation-dominated era? $\endgroup$
    – HDE 226868
    Commented Nov 16, 2014 at 17:20
  • $\begingroup$ @ HDE 226868 perhaps - if that was a stage in the evolution where matter was non-existent. Do physicists measure time through that part of the evolution without the existence of matter? $\endgroup$
    – docscience
    Commented Nov 16, 2014 at 17:32
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    $\begingroup$ The universe was filled almost entirely by radiation, but it wasn't perfectly matter-free. And yes, time still existed. $\endgroup$
    – HDE 226868
    Commented Nov 16, 2014 at 17:33
  • $\begingroup$ Related: physics.stackexchange.com/q/15371/2451 , physics.stackexchange.com/q/17056/2451 , physics.stackexchange.com/q/39141/2451 and links therein. $\endgroup$
    – Qmechanic
    Commented Nov 16, 2014 at 17:34
  • $\begingroup$ @Qmechanic those links - yes related, but it does not answer my question. One point in that discussion is that time is entropy related. Entropy can be measured in the absence of matter, I believe. So then can time? $\endgroup$
    – docscience
    Commented Nov 16, 2014 at 17:41

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First, the concept of matter and energy are entagled since Einstein. Most people have an intuition on the word energy as some untouchable fuild with destructive power,mainly due to hollywood. But energy and mass have a much more specific, mathematical meaning in physics. So instead, let me rephrase the question as "does it makes sense to talk about time in the abscence of "sutff"?"

That´s certainly a good question. We do physics by specifying positions of stuff (like in classical mechanics) in space and time, or value of a "stuff" called fields in space and time (like electromagnetism). Mostly all known physics work by assuming a background and describing how things (like tensor fields, spinor fields,etc) evolve in this background. There may even be a relationship between the background and stuff that lives in it (like in general relativity, where geometry is determined by the stress energy tensor). But ultimately the concepts of stuff and background are independent.

It makes sense to talk about the geometry of empty space-time in general relativity, and quantum field theory even tells us that there is no empty space-time at all.

So according to our current most accepted theories, space and time have a meaning even in the abscence of any "material" thing such as clocks, atoms, fields, etc. And we´ve got to this point, where we invented the concepts of space and time to explain the motion of things, and now they "have a life by themselves".

But could we recast physics in such way that time gets meaning only upon the motion of stuff? Ernst Mach used to think that

It is utterly beyond our power to measure the changes of things by time. Quite the contrary, time is an abstraction at which we arrive by means of the changes of things.

This kind of thought has inspired people like Einstein to develop general relativity. But as we can see he hasn´t completely succedded in implementing Mach´s ideas, as it still makes sense to talk about empty space in relativity. There is however some very interesting steps in this direction, mainly due to Julian Barbour and collaborators where time is an emergent concept from "change". In the causal dynamical triangulation approache to quantum gravity, the whole 4d space-time arena is recovered from "something else". It may be that theories like this end up giving completely new testable physics.

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  • $\begingroup$ "... space and time have a meaning even in the absence of any "material" thing". How this? You forget the person who thinks about it - a material thing. Einstein view was that without matter there is no spacetime $g_{\mu\nu}=0$. $\endgroup$
    – JanG
    Commented May 23 at 8:04
  • $\begingroup$ Here the corresponding link physics.stackexchange.com/a/698092/281096 $\endgroup$
    – JanG
    Commented May 23 at 8:10
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But can time also be measured by just changes in energy, and in the absence of matter?

If there are no massive particles present, then general relativity has a symmetry called conformal invariance. In the presence of this symmetry, there is no way to measure time.

According to evolutionary universe theories there was a point in the evolution where there was no matter - just energy.

Not true. According to standard cosmological models, there has never been a time when the universe contained no massive particles.

Related: Feedback on the paper, 'CCC-predicted low-variance circles in CMB sky and LCDM' by V. G. Gurzadyan and R. Penrose

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Only if matter and energy were not intertwined. How can you exhibit and measure energy without the matter it works upon?

Energy is just a property used to explain matter. See summary http://profmattstrassler.com/articles-and-posts/particle-physics-basics/mass-energy-matter-etc/matter-and-energy-a-false-dichotomy/

Thinking outside the box: Does the idea of time stop in a complete vacuum of matter and energy? How sticky is this vacuum? Does it require an entire universe void of matter and energy or just a small pocket? Would the existence of any matter or energy act explosively against that void?

Ultimately, is time just a property of matter and energy? If so, then time is non existent in a universe without matter and its properties. Going back to your universe origins statement, what properties does matter completely unbound to energy and energy completely unbound to matter exhibit and what is the interaction when they combine?

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