# 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

By the minus sign in the metric:
$$ds^2=-dt^2+dx^2+dy^2+dz^2$$

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?

21. Jun 28, 2010

### Naty1

not at all...I see that stated here from time to time and disagree completely....

Ask most any qualified physicsts if they seek both "how" and "why" and I'd hope they answer "of course"....That's one reason we have physical interpretation discussions here...not only "what is the math" but also "what does it mean?"

It's true, for example, no one really knows why fundamental particles differ from one another, but you can be sure most physicsts would like to know why. We just haven't figured that out yet.

Once we figure the origin of anything.... energy,time, mass, or particles, for example, ....we should get important insights into everything....

22. Jun 28, 2010

### Staff: Mentor

This is actually a perfect example to make my point. In current theory there is no answer to that question. The only way to answer that question would be in terms of a new theory. That new theory would then be the fundamental theory and again, you could not get a "why" answer to a fundamental question about this new theory.

Admittedly, my argument is somewhat tautological. You cannot get a scientific answer to a fundamental question and fundamental questions are the ones that current theories cannot answer. But the point is that there are such questions, they have no answer other than "it fits the data".

23. Jun 28, 2010

I agree DaleSpam, science cannot and should never be expected to answer a fundamental 'why'. We can reduce the number of our unknowns by relating them to each other, but there is no experiment or model that can provide an answer to the last, fundamental 'why'.

24. Jun 28, 2010

### Staff: Mentor

The use of the word "flows" is rather confusing and obscures your meaning.

Everything that SR says is encapsulated in the expression I posted above which is called the (Minkowski) metric. The ds term is the time measured by a clock and the other terms are the time and distance measured in an inertial coordinate system.

From the metric you can see that in SR time is more similar to distance (Pythagorean theorem or arc length formula) than it is to anything to do with flow. A geometric analogy is much more appropriate than a material analogy.

Last edited: Jun 28, 2010
25. Jun 28, 2010

### Rasalhague

In some of their uses, "why" and "how" overlap. In fact, the OP actually used the expression "how would you explain". If someone asks "Why does the amount of time between two events depend on the speed of the spacetime coordinate system you chart them in?" they might mean exactly the same as someone who asks "How does...?" (They may be looking for an explanation of the way this happens, how such an unintuitive notion can be self-consistent, what the theory actually says, what the jargon means, what it predicts, what makes physicists are convinced this makes a good model for the universe we live in.) All perfectly reasonable questions.

When a distinction is made between why and how, why can connote: "I want more insight." Someone may feel they understand a method; they can plug numbers into a formula and get the right answer on a test, but they aren't satisfied with this level of understanding; they want to know more: where does this formula come from, how can it be derived, what structures of knowledge does it relate to, is it a special case of something, are there equivalent ways of getting the same result, does this algebra have a geometric interpretation, is this case analogous to other aspects of nature. Again, good questions.

Or the why may come from someone who knows as much as anyone does about the theory, in which case they may be looking for a further insight that they feel is missing from current models. Not necessarily a bad question, depending on who's asking!

Or it can be the why that DaleSpam refers us to philosophers and priests for, the teleological why that Richard Dawkins warns against, the kind that asks for an answer such as that medieval author gave: "why do events succeed each other in time?" meaning "for what purpose?", "to what end?", where the only satisfying answer can be one that personifies nature or its governing preinciples as something with a rational purpose.