# Does time exist?

1. Aug 1, 2014

### EEngineer91

I recognize the analytical usefulness of having time, a parameter. However, does time really exist? If so, is there a universal time? Can we measure time without a clock, since clocks are just spatial units we can compare with. I notice how in all texts we always have another observer's clock shifted....but what if the clock shifted due to length contraction etc. How can there be a definite measurement of time in a certain frame when there is no way to check its accuracy?

2. Aug 1, 2014

### phinds

I suggest a forum search. This topic is beaten to death here several times a year.

HINT: at the bottom of every thread, including this one, there is a list of related threads. That's a good place to start.

3. Aug 1, 2014

Yes.

4. Aug 1, 2014

### EEngineer91

So how can you check you time measurement without using another clock? I think you're dismissing this. If I measure time in a certain frame, what is the time on the Universal frame? How can I check these without using a clock that can be manipulated by the relative velocity of such a frame?

5. Aug 1, 2014

### A.T.

LOL, there are 3 other threads with exactly the same title.

6. Aug 1, 2014

### ChrisVer

The time is not a parameter- it's a dynamical variable in the SR and GR framework, and all theories which try to incorporate them (for example QFT in contrast to QM).
In any case time exists, as space does. The same questions about clocks can be asked for the spatial measurements with a rule let's say. So in exactly the same way, someone would ask "does space exist?"
The main thing is that you can correlate everything with the appropriate transformation (at least in SR this works, I am not sure about GR at the moment, but I think you can-that's why the covariant derivatives exist)

7. Aug 1, 2014

### EEngineer91

Again, no one is denying the analytical use of time. My problem is, there is no absolute time that can be measured objectively, so can it be "real"?

8. Aug 1, 2014

### ChrisVer

what do you mean by "real"? Of course it's not imaginary (hehehe-mathematical joke?)...
There is not absolute time as there is no absolute space. This doesn't mean that space or time doesn't exist... However you can correlate the clocks by Lorentz transformations in SR.... The absolute quantity in both SR and GR is the proper time/length (or in general , invariant quantities that are the same for all the observers)....

9. Aug 1, 2014

### Staff: Mentor

The term "real" is generally a philosophical term (from ontology), not a scientific one. If you wish to discuss it here we must begin with a scientific definition of the term.

10. Aug 1, 2014

### EEngineer91

How can you define proper time since there is no non-inertial reference frame. For there to be proper time there must be a "proper" reference point, which does not exist....but all of space is traveling in different directions at different speeds. Relativists just define proper time as the time seen by one referenced observer, and compare all other time measurements to the original one....this does not mean the proper time is absolute

11. Aug 1, 2014

### ChrisVer

I don't know, but I think this is a game of words?
By proper time/length I meant the quantity:
$ds^{2}= n_{\mu \nu} dx^{\mu} dx^{\nu}$
Or in GR you make the change: $n_{\mu \nu} \rightarrow g_{\mu \nu}(x)$. In the differential geometry framework this is also called the 1st fundamental form, and it's invariant. That means that once you measure it to be A in one reference frame, then all other RF will measure it to be A.
This is because in SR it's invariant under Lorentz Transformations of your space-time variables.

In GR it's reparametrization invariant (it's the same for every coordinate system under a coordinate transformation :$x^{\mu} \rightarrow \bar{x} ^{\mu}(x^{\rho})$). So either for the coordinate system $x$ or $\bar{x}$ this length will be the same.

12. Aug 1, 2014

### EEngineer91

"That means that once you measure it to be A in one reference frame, then all other RF will measure it to be A."

So what is the proper time in the twin paradox? The time measured from earth or the ship? They measure two different apparent travel times

13. Aug 1, 2014

### ChrisVer

The proper time is the length of the worldline they scan...either you take the earthling or the traveler's rest frame, the mover worldline's length will be the same for both RF. The same for any other RF you can choose...
The time you are talking about is the reference dependent time/the observers clock's time (or in my notation $x^{0}$ )

14. Aug 1, 2014

### Staff: Mentor

And each one's measured travel time is the proper time *for that observer*. Proper time is a property of worldlines and observers; there is no one proper time that every observer observes, or that every observer agrees is "the" proper time. Each observer has their own.

15. Aug 1, 2014

### EEngineer91

Exactly! No absolute notion of time, this is my main point

16. Aug 1, 2014

### Staff: Mentor

But there's also no absolute notion of space. Do you think space is "real"? If so, time is just as "real" as space is. If not, then why are you so concerned about time as opposed to space?

(Also, the word "absolute" can have more than one meaning. The proper time experienced by a given observer traveling along a given worldline in spacetime is absolute in the sense that every observer will agree on its numerical value, even if only that particular observer directly observes it. This is in contrast, for example, to two spatially separated events happening "at the same time", which is *not* absolute even in this sense: different observers in relative motion can disagree about whether two spatially separated events happened at the same time.)

17. Aug 1, 2014

### ChrisVer

And in this point everyone agrees... Nobody said that time is absolute. Absolute are the invariant quantities and time -alone- is not one of them... what I disagree on is that you say that time is not "real"...

18. Aug 1, 2014

### ChrisVer

PeterDonis are posts #13 and #14 contradicting each other or not? Because at first glance I think we said opposite things... maybe you can correct me? or is it my misconception of what you wrote?

19. Aug 1, 2014

### Staff: Mentor

They're not; they're just focusing on different aspects. You're focusing on the fact that every observer agrees on the proper time along a particular worldline; I'm focusing on the fact that different observers travel on different worldlines.

20. Aug 1, 2014

### EEngineer91

"Real" means different things to different people. My contention is both space and time are creations of the mind to analyze the world. Space and time do in a sense exist, but cannot be measured absolutely.