# Clock in the universal sense?

1. May 6, 2015

### Atlas3

Does Universe Spacetime have clock of it's own? A period?

2. May 6, 2015

### Staff: Mentor

I'm not sure I understand the question. What would it mean for spacetime to "have a clock of its own"? What sort of observation would you make to test whether it did or not?

3. May 6, 2015

### Chalnoth

What spacetime are you talking about?

In general, I'd have to say the answer is no: clocks are built from matter. It would certainly be possible to use gravity as part of your clock design (in a way, the Earth's orbital period is a sort of clock). But you still need matter involved to make it work (if there's no Earth and no Sun, the "orbital period of the Earth" doesn't make any sense).

4. May 6, 2015

### bahamagreen

Are you asking if the block (universe) has a clock (spacetime subject to change of configuration)?

I believe the block universe idea suggests that for the entire configuration of spacetime the "history" is fixed, but if the block universe can change configuration it would seem that the entire history would become different but still consistent (as we know it). We would not know of the change because it includes all of the history, including all the consistent memories. To track a change would require being "outside" of the changing spacetime with some way to compare the previous configuration to a subsequent configuration... but there is not accounting for "outside" and any measures from within a changing spacetime would always be consistent.

The closest thing I can think of is perhaps the Many Worlds interpretation - but instead of the many worlds you might have "many configurations" of spacetime, each generation of another world really being the expression of a change in spacetime configuration (and each of those changes presenting a consistent self history)... the Many Worlds is liked for its power and consistency but it also requires "thinking outside the block"... :)

5. May 6, 2015

### Atlas3

I will need to familiarize myself with what you have described. I have considered many parallel universes of infinite size experiencing periodicity of life time. Big Crunch ending like a bang beginning. But other parallel universes experiencing their own time period independently. A massive universal period. A function of time normalized 0 to 1 period of expansion / contraction of each whole. Please keep in mind I am going to familiarize myself with the suggestions above as someone may be on this train of thought. I thank you

6. May 6, 2015

### Chalnoth

That's not a viable clock model, as there's no way to measure across different big bang/crunch events. Also, if there are many big bang/crunch events, chances are the period of time associated with each varies dramatically.

7. May 6, 2015

### Atlas3

Time exists as a period of many things. Solar time for example is one. Earth rotation provides one for instance. Expansion of the universe has a period as well. We don't know the period of he universe.
Can you give a little explanation how you suppose the period would vary? I think it would as well but cannot put a few words on it. I have the same idea because it would vary depending upon the maturation of gravity for an event, and that would vary.

Last edited: May 6, 2015
8. May 6, 2015

### Chalnoth

Looks like you were cut off.

It's possible to use the expansion of the universe to set a specific meaning for "now" (when the CMB has the same average temperature at every point). But it's still not really possible to use the expansion as a clock of any sort. Clocks typically depend upon having predictable, repeating behavior, and the expansion of our universe doesn't do that.

That is, you can use the expansion to sync up clocks that are far away, but you can't use it as a clock in and of itself.

9. May 6, 2015

### Atlas3

I
We would not exist to measure time across these events, as we would cease to exist as would our solar system, but time would continue in the absence of our solar timekeeping.
The overall process of a repeating reconfiguration could be considered a pulse. Periodic but varying periods does not make a regulated clock but it does make a wave which could not be measured by us. But could be time proceeding. It can't be measured by us. We would not exist any longer nor would our reference frame for time. A reconfiguration would reset the maturation period which would vary. Also consider more than one universe experiencing this life.

10. May 6, 2015

### Staff: Mentor

All of these proposals (which are speculative) are still cases of matter changing configuration. None of them are examples of spacetime having a "clock of its own". So it looks like the answer to the question you posed in your OP is "no".

11. May 6, 2015

### Atlas3

Can the fabric of space time be considered to exist in isolation of matter?

12. May 6, 2015

### Staff: Mentor

Since there is matter in the universe, we have no way of testing whether spacetime could exist in a universe with no matter at all.

13. May 6, 2015

### Atlas3

Can spacetime be constructed without matter? Mathematically.

14. May 6, 2015

### Staff: Mentor

Yes, flat Minkowski spacetime is a mathematical solution to the Einstein Field Equation with zero stress-energy (and zero cosmological constant). But again, because there is matter in our actual universe, this mathematical solution does not describe it.

15. May 6, 2015

### Atlas3

Can curved spacetime be described within Minkowski spacetime as a system?

16. May 6, 2015

### Atlas3

I agree it could not be used as a clock in the time keeping sense. But does the possibiltity of periodicity in expansion allow negative expansion?

17. May 6, 2015

### Atlas3

Thank you for your answers to my questions. It astonishing to me that I had the physical thought to think was a possibility. I came up with this without being taught or research. I had a moment of pure physics one night. Actually cosmology. I only reached a point in education recently to formulate questions to what i imagined in my mind one night. I have been pursuing this in college for a few years since. Now I am finding out things like this are possibilities in fact. Its amazing and thank you again.

18. May 6, 2015

### Staff: Mentor

If by "negative expansion" you mean "contraction", then yes, there are mathematical solutions that describe contracting universes.

19. May 6, 2015

### Atlas3

can the contraction and expansion be a cycle without a crunch or bang mathematically? I was told it occurs like a step function, however, after big bang in theory.

20. May 6, 2015

### Staff: Mentor

No; at least, not with classical GR. There are speculations that quantum effects might allow a "bounce" scenario where contraction turns around and becomes expansion before a crunch/bang occurs; but those are just speculations at this point.