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Since I don't want to abuse to the ASK QUESTION form, and since those questions are all about time, I decided to write them all here. Hope it's ok.

First Question

Could it be possible (how?) to create some regions, in the Universe, in which the times seems like to stop (for a little while)? I mean: if we could be able to hide some event from the light, would be able the same to hide it from the time?

Second Question

Since there is a deep connection among observers light and the passage of time, wold the time still exist if there weren't any observer? Why?

Third Question

Isaac Newton introduced the idea of a $t$ variabile, which stands for time, to describe how do objects move. But Quantum Mechanics does consider the time in a very different way with respect upon the Classical view of Newton. Nay, according to the Planck scale units the time variable is not applicable. So shall we come back to Newton's period (and earlier) and try to eliminate the $t$ variable?

Namely: could we reformulate the entire Quantum Mechanics theory/formalism without using the time variable? I read that Carlo Rovelli did something about.. but no clue where to find some material!

Thank you all for the attention. If someone thinks I should split the question in three different ones, just tell.

Les Adieux
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    Time is that which the clock shows. As long as you can find a clock, you got time. – CuriousOne Mar 02 '16 at 00:21
  • @CuriousOne I completely disagree. – Les Adieux Mar 02 '16 at 00:22
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    That is your right in a free country, but protesting against a simple definition is kind of a waste of time, intellectually speaking. – CuriousOne Mar 02 '16 at 00:23
  • @CuriousOne I think we may arguing for days and decades about what time really is (or isn't).. I mean: are you sure that it's something that arises just because you have a watch? – Les Adieux Mar 02 '16 at 00:24
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    Time, in physics, is that which the clock shows. There is no other useful definition of time. We can argue about which kinds of systems constitute suitable clocks... that is a potentially fruitful argument, but pretty much everything else belongs into the philosophy department where they have been unable to come up with a common definition of the word "is" for the last 2500 years. – CuriousOne Mar 02 '16 at 00:27
  • @CuriousOne What is time, for a particle? What is time for quantum mechanics? Not simply something related to a clock, I guess.. But I may be wrong (anyway thank you for arguing >.<) – Les Adieux Mar 02 '16 at 00:31
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    A "particle" is the choice of a physicist to approximate the motion of an extended body with the dynamics of its center of mass. I don't think that such choices have a need for time, we usually don't write down the time when a physicist decides to go for a simplifying assumption. – CuriousOne Mar 02 '16 at 00:34
  • @CuriousOne It's all interesting.. but now we are discussing about what time is, and not about my questions which are slightly different! :D – Les Adieux Mar 02 '16 at 00:36
  • I was simply trying to help you to think about time in a rational way. Unless you understand what physicists mean by "time", none of the answers you may get will be any useful. – CuriousOne Mar 02 '16 at 00:38
  • You might be interested to read What is time, does it flow, and if so what defines its direction?. While it doesn't directly address the points you raise it explains how (all but one) physicists view time. – John Rennie Mar 02 '16 at 06:56

2 Answers2

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Question 1:

If a region of space contained no energy (and thus no matter or information), it would not be possible to measure the passage of time in that region. It's important to mention however, that such regions do not exist as there will always be quantum fluctuations.

Question 2:

According to basically all accepted theories, the answer is yes. If everything on earth that can "observe" were to disappear, only to magically reappear 100 years later, we could figure out that it had been 100 years using any number of different methods.

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Question1:

If you and your clock are sufficiently close together and in free fall, you are in a local Lorentz frame and there is no way for you to locally tell (such as by seeing your clock slow down) what the mass distribution (ie: different regions of space) is around you.

However, if you put the clock in a region of space that is different than yours, then it is possible to see that clock speed up, slow down, or even approach stopping wrt your clock. For, example put the clock near a black hole and you observe the clock from far away. To you, the clock will appear to slow down or even stop as it approaches the Schwarzschild radius. Conversely, if you were near the black hole and the clock was far away, you would see the clock going faster than your own clock.

Question2:

Since x,y,z,t are part of a four vector, you question about t existing is a question about x',y',z', and t' existing in a boosted frame. So your question is really does all space-time exist, not just t, if there is no observer. Hmmm...kind of philosophy.

Question 3:

I don't think you can eliminate t as a parameter for evolving physical systems because it is unique in always advancing to positive values for observers in all frames as the system evolves.

You could imagine parameterizing the change in all physical systems by the angle of your clock hand in its clockwise advance. However, another observer in a frame rotating a little faster than the clock hand would see the clock hand rotating backward toward negative angle. This exemplifies t as unique (and indispensable in parameterizing the change in systems) in that it only changes toward the positive direction. There is no frame we can transform to in which t reverses direction. You can't get away with just using the angles, boosts, or spatial positions of an object to parameterize evolution.

Gary Godfrey
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