Does Time Expand in All Directions in the Universe?

[write4u]

Thank you for your patience and your encouraging responses (except the last one

A clock at the corner where you start will tell you that 4 minutes have passed, but the number of minutes that you will have aged is (different)

Is this due to the direction or the speed of travel relative to the starting point?

1. An object moving in space traces out a curve in spacetime.

TY, this is basically what I was trying to express. I believe it is called a "world line", e.g. a chronology of its own existence within spacetime?

Earlier you asked me to clarify the term "set" of world lines which would afford a probability calculation. I believe those sets are called "world braids".

 

[Fredrik]

Is this due to the direction or the speed of travel relative to the starting point?

It's just the speed.

http://en.wikipedia.org/wiki/Time_dilation

 

[Whitefire]

Because a point doesn't determine a direction. You need something like a point and a timelike curve through that point, or a point and a spacelike hypersurface through that point.

Well, you will not get any argument from me here. Agreed 100%. However, can't you determine a direction from several points? You don't need to slice the Earth's surface into square yards to determine which way is up-- 3 points are enough. And if you substitute points for observers... why the slices?

@write4u: If there is any direction for time, 'future' is like 'forward' when driving a car. On the other hand, considering the fact that each point/observer moves into its 'future' with the 100% speed, I would say that 'future' (like 'forward') is a relative term and therefore a relative direction. When you compare many relative directions you can get a larger picture: that they are not necessarily the same; my relative 'future' and galaxy X relative 'future' are not the same futures/directions. Like with going 'west' by car. I can move 'west' by going south-west, west or north-west. I am always going 100% forward, but it is better to understand my north-west movement as what it is in relation to the larger frame of reference, not as 'less efficient movement west' (a.k.a: relative slower progress of time). In the case of time, we need the largest frame of reference possible--the entire universe, or at least what we see of it.

you can't visualize it

please ... a space-like *surface* ...

 

[WannabeNewton]

You are stuck thinking about vectors in the elementary sense from Euclidean space. First of all, we are trying to define a time-like basis vector at every point of space-time so obviously we need a vector field, we can't just take two points and subtract them to get a vector like in elementary physics. And if we want a notion of space at every instant of time then we need space-like hypersurfaces that fit together nicely under a continuous parameter - which we call "time". You can't visualize these hypersurfaces (the word surface here doesn't mean surface embedded in $\mathbb{R}^{3}$, which are the only kind of surfaces you can visualize).

 

[write4u]

Well, you will not get any argument from me here. Agreed 100%. However, can't you determine a direction from several points? You don't need to slice the Earth's surface into square yards to determine which way is up-- 3 points are enough. And if you substitute points for observers... why the slices?

@write4u: If there is any direction for time, 'future' is like 'forward' when driving a car. On the other hand, considering the fact that each point/observer moves into its 'future' with the 100% speed, I would say that 'future' (like 'forward') is a relative term and therefore a relative direction. When you compare many relative directions you can get a larger picture: that they are not necessarily the same; my relative 'future' and galaxy X relative 'future' are not the same futures/directions. Like with going 'west' by car. I can move 'west' by going south-west, west or north-west. I am always going 100% forward, but it is better to understand my north-west movement as what it is in relation to the larger frame of reference, not as 'less efficient movement west' (a.k.a: relative slower progress of time). In the case of time, we need the largest frame of reference possible--the entire universe, or at least what we see of it.

As I understand it, the time frame of the car where everything associated with the car all travel in the same direction at the same speed is called a 'world braid'. A set of world lines moving in the same coordinated direction through spacetime. Can we say that technically a person is a world braid, and if I was born @ 4:00 pm, May 24, 1953, my world braid as a person today spans 60 years, locally.

But what happens when the car breaks down and stops moving? The ensemble is no longer moving in any direction in space. Yet the car, parts, occupants all continue to go forward in time. But for the individual parts time has no specific direction other than forward and depending on the properties of the individual parts.

One might say that the world braid of coordinated movement of traveling in a certain direction has paused, but the ensemble itself of course continues on in spacetime, slowly decaying until the car is no longer a world braid but a random collection of individual world lines. A wheel falls off the car, I decide to start walking, I have a heart attack from the stress, etc.

Can a universal spacetime coordinated be established at all? Do we have a 'theoretical' map of every spacetime coordinate?
If east, west, north, south are 'local' directions, can a direction be identified within spacetime other than as another spacetime coordinate? How does one express; I am 'here at this time' but I am on my way "there when I get there", except by reference to local coordinates?

Aside from the accepted science of the properties of spacetime, when we are dealing with the creation of an individual world line or world braids (chronologies of individual events), I see the time part of an individual world line as a by-product created from the chronology of the various durations of these physical events (changes). I don't think this is in conflict with current spacetime science, is it?

 

[Whitefire]

You are stuck thinking about vectors in the elementary sense from Euclidean space

Yes, but it is mainly because I am convinced that 'now', even this 'now' we can't really see or experience because it is space-like, can be treated as euclidean space, and we can imagine and visualise it; in fact, we do it all the time. Given enough data, you could reconstruct such space-like hypersurface from, say, 8 minutes ago, all the way to the Sun, and as I understand, this reconstruction would be an euclidean space.

@write4u: I am sorry but I cannot just accept the idea that if you don't see the changes, this must mean that time doesn't flow. If you put a stone and a clock next to each other, do you really think that time doesn't flow for the stone, only because it doesn't show it? I do sometimes wonder whether absolute zero = time stop, but it seems like mixing symptoms with the cause of the sickness.

 

[Fredrik]

Yes, but it is mainly because I am convinced that 'now', even this 'now' we can't really see or experience because it is space-like, can be treated as euclidean space, and we can imagine and visualise it; in fact, we do it all the time. Given enough data, you could reconstruct such space-like hypersurface from, say, 8 minutes ago, all the way to the Sun, and as I understand, this reconstruction would be an euclidean space.

This works when we're dealing with inertial coordinate systems in SR, but it doesn't work in GR, or even when we're dealing with non-inertial coordinate systems in SR.

 

[write4u]

Yes, but it is mainly because I am convinced that 'now', even this 'now' we can't really see or experience because it is space-like, can be treated as euclidean space, and we can imagine and visualise it; in fact, we do it all the time. Given enough data, you could reconstruct such space-like hypersurface from, say, 8 minutes ago, all the way to the Sun, and as I understand, this reconstruction would be an euclidean space.

@write4u: I am sorry but I cannot just accept the idea that if you don't see the changes, this must mean that time doesn't flow. If you put a stone and a clock next to each other, do you really think that time doesn't flow for the stone, only because it doesn't show it? I do sometimes wonder whether absolute zero = time stop, but it seems like mixing symptoms with the cause of the sickness.

I may not have expressed it with clarity. I agree with your example of the stone. And of course, if we wait long enough we can indeed see the stone 'age' (I used the 'wheel falling off' the car').

 

[Ogr8bearded1]

As one layman to another, perhaps it is the view that is off and causing problems. Maybe time is outside of what you are trying to say. I'll give an example. I'm in a car traveling North so my direction to all I can see is North. However the Earth is rotating, so to an observer not on Earth it would appear I'm traveling very slowly North and quickly East IF he were mapping your progress as a space point. Think of East as the forward of time, regardless of your true direction you head East. Even if the movement is OUTSIDE of your ability to observe. So I went North and time went forward. If I turn around and head South, I'm still really moving East and time is going forward. Even if I go East, my view of how fast I'm going East is still slower than my actual movement East and I'm still going forward in time. So yes, time has a direction, but it is one outside of perception. You notice its effects (the Sun moves higher in the sky) but not its affects on your direction of physical travel.

Also, I think its important to not confuse time with the measurement of time. Some say time is movement and without movement there is no time. To me, they are talking about the measurement of time (and that rock, its atoms are moving even if it is still.) Smolin says time may be fundamental. If you haven't read the post and seen the talks I'd say you should give it a look here. https://www.physicsforums.com/showthread.php?t=683198

One of the best thoughts on time I have heard was when someone stated "Do we stand still and time moves, or do we move and time stands still?" If we can't really answer this question then figuring out 'why' it always, to our perception, moves forward may be impossible.

 

[write4u]

@write4u: I am sorry but I cannot just accept the idea that if you don't see the changes, this must mean that time doesn't flow. If you put a stone and a clock next to each other, do you really think that time doesn't flow for the stone, only because it doesn't show it? I do sometimes wonder whether absolute zero = time stop, but it seems like mixing symptoms with the cause of the sickness.

After a little more reading I discovered I was talking about a 'world knot', where a world line ceases.

 

[Fredrik]

I'm closing this thread because the original question has been answered, and the discussion has deteriorated into something that doesn't resemble physics. I should have closed it long ago. Physics Forums is a resource for those who want to learn what the best theories of physics say. It's not a place for philosophical speculation that has very little to do with physics.

The question about whether time has a direction (in special and general relativity) is answered e.g. by some of my posts in this thread:

The only thing that can define a direction of time is the kind of stuff that WannabeNewton is talking about. First you slice up spacetime into 3-dimensional hypersurfaces labeled by a real parameter t, so that each event belongs to exactly one of these hypersurfaces. Now if we want to find the direction of time at an event p, we would look at the hypersurface that p belongs to. There are two directions that are orthogonal to this hypersurface at p. In one of these directions, t is increasing, and in the other direction, t is decreasing. The direction that's orthogonal to the hypersurface and such that t is increasing, can then be considered the direction of time at p.

Note that this direction depends on our choice of how to do the "slicing".

...a point doesn't determine a direction. You need something like a point and a timelike curve through that point, or a point and a spacelike hypersurface through that point.

In relativity, there are lots of directions that are labeled "timelike" (by a precise mathematical definition). Together they identify a region of spacetime that's sometimes called "the chronological future". The union of that set and its boundary is then called "the causal future". If what we mean by "the future" is one of these sets, then no, the future is not a direction. It's a set that identifies lots of different directions, not just one.

This is why we need something other than just an event in spacetime to single out which one of them to call the direction of time.