# Duality of time

1. Jun 26, 2010

### Yuripe

Time is integral part of spacetime.

If so, how would you explain the persistent unidirectional and significant flow of time and such a small dilation when time is influenced by spacetime manipulation?
Does time flow have to do with overall spacetime expansion?

2. Jun 26, 2010

### Eynstone

Time is not a 'part' of spacetime - the dichotomy between space & time is out of place in relativity.
Please explain what you mean by the 'flow' of time (in physicsl terms).

3. Jun 27, 2010

### Phrak

Welcome to Physics formum, Yuripe.

And I don't know why you you are in disagreement, Eynstone. Time-like R1 submanifolds of spacetime (world lines) are submanifolds of spacetime, aren't they?

Can you rephrase your question, Yuripe?

4. Jun 27, 2010

### Yuripe

My assumption that time is a part of spacetime is rather simple.
Mainly because it's space-time and because time is influenced along with space when spacetime is bend by gravity.

By flow of time I meant that there is an order of succession of things, there is an irreversible entropy and you can measure seconds "flowing" in your local spacetime. This rate of "flow" should be common across spacetime and is substantial compared to what gravity (bend of spacetme) needs to be to make some change to that rate of "flow".
Seconds success quite fast without any visible influence, but you need to make quite a big bend of spacetime if you want to make a very small change to that "flow".

Time won't stop in any location of an empty volume of spacetime, no matter how folded it is inside.
If you view it like this, then you could expect that there is some other cause of this "flow" then folded spacetime.

So I asked this question, if this "flow" of time could be the effect of the expansion of the whole spacetime in the universe. Because if so, this I think would nicely fit together "flow" of time with its part in spacetime.

5. Jun 27, 2010

### Phrak

You seem to have a cosmology question; are the rates of clocks dependent upon the expansion of the universe?

You might request to have one of the mentors move this thread to the Cosmology folder.

6. Jun 27, 2010

### Staff: Mentor

That is simply due to the fact that there is only one timelike dimension. If there were two or more timelike dimensions then you could have closed timelike curves in flat spacetime.

7. Jun 28, 2010

### Yuripe

How do you define timelike dimension?
What is different about it in accordance to plain dimension and how do you know there is only one?

8. Jun 28, 2010

### Yuripe

You are probably right, but its kind of mixed topic.
I'm trying to determine if that what we perceive as passing time can in reality be the effect of expanding spacetime and in accordance to that, if a gravity is just the local effect of mass on the rate of this expansion.

9. Jun 28, 2010

### Staff: Mentor

$$ds^2=-dt^2+dx^2+dy^2+dz^2$$

10. Jun 28, 2010

### Yuripe

Nice , but is this a definition of timelike dimension?
It looks to me as a description of spacetime and it doesn't say anything about why there is a minus sign before time component.

11. Jun 28, 2010

### my_wan

It can also be written:
$$ds^2=dt^2-dx^2-dy^2-dz^2$$
It really makes no physical difference.

I don't really get the problem with unidirectional time. You have processes that are more likely that others, making a reversal absurdly unlikely. But the physical progression states that we call a time flow are just as physical (classically) as a pool balls that, after bouncing around, end up back in the initial triangle pattern. It's a progression, not a direction.

The other issue is varying time rates, as in relativity. But if everything is defined by a classical state which evolves, how can it be presumed that time, which measures the changes, not vary under some circumstances, even at the most fundamental level?

Suppose time did stop for the next hour? But wait, how was it an hour if time was stopped? The mistake is to presume that time exist separately from the changes of states in the things we measure. If it's simply a change of state, then direction is simply an illusion of probabilities.

So my question to you is how is a change of state defined by a spacetime expansion fundamentally any different from a change of state defined by a glass breaking as it hits the floor? Are you implying that if the Universe was contracting, rather than expanding, that the glass would bounce up off the floor and unbreak? I think not. But if it was you wouldn't know it, because you'd simply be unreading my post and forgetting what yesterday will bring. And still wondering what the spacetime expansion that is unexpanding has to do with it, at least until you get too young and forget what you learned about expansion.

12. Jun 28, 2010

### Naty1

The expansion of the universe has varied since initial inflation ceased.....expansion was rapid and gradually slowed but did not stop and it seems to be accelerating right now....

Has the flow of time changed?? It doesn't appear to me that the expansion of the universe affects our local time to vary here in the Milky Way.

On the other hand as the universe expands, density decreases and hence gravity as well; so a distant observer could see our time apparently slower than at some time in the denser past.

13. Jun 28, 2010

### Staff: Mentor

When you get to a fundamental level you always find that all science is "a description" and never says "anything about why". If you want a "why" answer to a fundamental question then you need to see a philosopher or a priest, not a scientist. And such answers are not appropriate on this forum.

14. Jun 28, 2010

### Rasalhague

It might help to distingush between (1) the fact of spacetime having one timelike dimension and (2) the existence of a thermodynamic "arrow of time" (connected to the idea of entropy) which, at certain scales, gives a natural causal orientation to spacetime, a way to tell past from future.

http://xxx.lanl.gov/abs/gr-qc/0403121

15. Jun 28, 2010

### my_wan

Very good point. The timelike dimension is a mere coordinate choice and is no more physical, or have any more unique physical significance, than any coordinate choice.

16. Jun 28, 2010

### Rasalhague

At the risk of posting something inappropriate (albeit funny), here's one such answer in the 14th century English mystical treatse The Cloud of Unknowing [ http://www.lib.rochester.edu/camelot/teams/cloufrm.htm ] (lines 351-360). Why are events ordered one after another in time?

So that man schal have none excusacion agens God in the Dome and at the gevyng of acompte of dispendyng of tyme, seiing: "Thou gevest two tymes at ones, and I have bot o steryng at ones."

(So that man shall have no grounds for accusation against God at the Last Judgment when he must give account of how he has spent his time, saying, "You gave two times at once, and I have only one impulse at once.")

17. Jun 28, 2010

### petm1

I like thinking about this as a change in direction, space to time. Like the focal point within my eye where photons emitted at different times all come together in a pseudo-emission that I see as my now.

18. Jun 28, 2010

### matheinste

There is no differernce. It is just the fact that the sign of the time dimension is opposite to that of the spatial dimension that defines the metric, makes the interval not positive definite and so the spacetime geometry follows.

I don't think anyone would suggest that the sign of the time dimension in the expression for the interval gives time its direction. But of course it must the opposite sign to the spatial dimensions.

Matheinste.

19. Jun 28, 2010

### karkas

20. Jun 28, 2010

### Yuripe

According to SR time "flows" at different rates according to the speed of the object.
So time is relative to the speed, and speed is also relative to the observer.
If I start to move at certain speed and also take a role of the observer (of myself) than for me time will be passing at normal rate, same as I would be stationary.
If we now change to the perspective of the external observer who is slower than me or stationary, he would see that I'm living in "slow motion" in other words in time that runs slower.

I might suspect from the above, that there is a connection between the speed at which spacetime of the universe expands and perception of the rate at which time flows.

According to GR, local value of gravity has the same effect on time as speed in SR.
So higher gravity means slower time flow.

Let's say for now we are in point A in spacetime (A' in space) and I have two synchronized clocks.
I take one of them for a near light speed spin around the galaxy or into a high gravity location for a certain period of time.
Then I come back to compare readings on these clocks at point B in spacetime (same A' in space). What I see is that the clock I took is delayed to the clock I left.
I started this experiment at point A and ended in point B of my spacetime, what is different between these clocks is the spacetime distance between points A-B they traveled.

So how come the distance in spacetime from point A to B be different?
The time must've been flowing at different rates, and if the rates are different shouldn't there be a rate at which normally time flows when observer is stationary?