Why does time only have one direction?

Why does time only have one direction?
Greetings, johnzxcv. I think what you mean to say is that time is '1dimensional'.
Time of course has 2 directions  past & future  but we experience only the flow from past to future. I would say that is the way nature is  and in the Old Testament even God Himself is constrained by time since He sends Noah's flood upon earth due to human misbehaviour, which previously He did not know in advance would happen otherwise He would not have been angry enough to do it. Christians however often claim that "God is outside time."
I suspect however that you are looking for another type of answer  which I cannot give.
TFOLZO
The question is a very good one, and at the same time it is not one that modern physics can answer with any amount of certainty. What comes closest to addressing this is a fairly recent hypothesis called "Causal Dynamical Triangulations"  the basic idea is that smooth and classical spacetime is an emergent phenomenon from the arrangement of fundamental building blocks called "pentachorons" ( or 4simplexes ). These fundamental building blocks can be joined together only in very specific ways, just like Lego blocks can be joined only in a specific way ( raised ends into corresponding holes, but not raised end to raised end or hole to hole )  in the case of pentachorons, you can consistently join them only in such a way that the timelike edges of these 4dimensional entities are aligned with one another. If you join very many of such 4simplexes, you get a manifold that looks macroscopically smooth and is timeoriented, i.e. that has a preferred direction in time ( the future ). So the basic proposal of CDT is that spacetime is smooth only on macroscopic scales, but discrete on microscopic scales, and that the preferred time direction of our world arises from the way these discrete and fundamental building blocks are arranged.Thanks a lot.
What I mean is why we can experience only the flow from past to future?
Perhaps that question will never be answerd!
Take careful note that I called this a hypothesis  it is an idea that shows some promise, but it is not yet at a stage where it has reached the status of a falsifiable theory or model, and it is not taught in standard textbooks yet. While the basic idea is simple, the mathematical details of this hypothesis are devilishly difficult, and much work is still to be done on this.
Those are two different questions. Time being 1dimensional is unrelated to the question regarding why objects which are located at points along its worldline but only proceeding in one dimension with increasing time values.
Even though time has two directions objects can't move in two directions, only one direction, i.e. forward in time, i.e. along increasing values of proper time along its world line.
And anyone of them who says that is quite wrong.
Yes it can. Time is a cumulative measure of motion or change. That goes back to ancient Greece. There is no direction other than that inferred from the fact that if motion from A to B doesn't happen, motion from B to C can't happen. There's a sequence to events, and we assign an abstract direction to this, but there is no actual direction. Try pointing towards the future with your right hand, and to the past with your left. Try hopping towards the future or the past. No can do.
I don't think it does, Markus. Nor do I think that Smolin, who helped "popularize" it, and who wrote a whole book about time, understands time at all. Methinks somebody is lost in maths.
You experience change and motion. The idea that there's some kind of "flow" from the past to the future is just a convention. Something you were told about when you were a kid, and never really thought about. Time doesn't really flow, and you don't really travel to the future at one second per second. Have a look at the OP of time travel is a fantasy for more about that.Originally Posted by johnzxvc
Hmm, there are noncrank theories out there that time needs to have two dimensions in order to be able fully reconcile the standard model with GR and QM. Try googling Itzhak Bars.
Then you can start by reading this Scientists zero in on why time flows in one direction by Sean Carroll and Jennifer Chen
Scientists zero in on why time flows in one direction
Well, Shawn Carroll is an excellent physicist, and his theory sounds very intriguing. I think his approach is a reasonable one, per the summary.
In my view, if at all points space is expanding away from a Big Bang event, then if one assigns a 4space vector that points back to the bang (from any spacetime location), the direction of time is in the opposite direction (which would be the direction of ct per Minkowski). The big question in my mind, is why we cannot see various moments in time (locations upon a ct axis), as we simultaneously hold the inch locations on a ruler? Both those questions should be answered in unision IMO, to have a better validation the theory is correct. (1) why does time seem to have a direction, and (2) why do we live in only the ever changing instant (time's flow) when relativity theory requires all moments in time to exist on an equal footing.
Light travels at the rate of c. We measure time and space from light's propagation. Yet, as a Minkowski figure shows us, the rate of light is related only to the geometry of the static worldlines in 4space, the relative angular orientation between worldlines of a body and the photon within the 4d continuum. None of this explains why one's own lineofsimultaneity advances at (seemingly steady) rate, let alone it's direction of advancement along the worldline (time's arrow).
IMO, physicists should consider more unresolved matters "collectively in unison" as they build their models of reality. Not to imply that Sean Carroll and Jennifer Chen are not doing that, as I have not read their theory. However, it's often enough the case that physicists approach unresolved matters without considering all the bases in collective, one reason so many different competing theories exist. You then end up with wrong theories, or at best "partially correct theories", but no correct theory. And, that's just an opinion, and nothing more On the other hand, these are very difficult questions, and it's sometimes the case that the correct model eventually arises from the prior mistakes or partially correct theories of giants.
Thanx for that reference.
Thank You,
SinceYouAsked
Last edited by SinceYouAsked; 08092014 at 11:14 PM.
An ideal gas. A system of noninteracting particles (or particles that interact through contact forces only where any potential can be ignored for all practical purposes) such as that found in a low pressure gas, etc.
Please note that when a physicists speaks of noninteracting particles he's disregarding extremely small contributions such as gravitational forces so small as to be virtually undetectable. In an ideal gas the Van der Waal forces are taken to be zero.
In reply to Johnzxcv, re: time.
There is no direction of time...there is "NOW" and the continuation of "NOW", from one instant to another.
Cheerio!
To use Farsight's favorite kind of argument,
Hermann Minkowski: Space and Time (Wikisource)The concepts about time and space, which I would like to develop before you today, have grown on experimental physical grounds. Herein lies their strength. Their tendency is radical. Henceforth, space for itself, and time for itself shall completely reduce to a mere shadow, and only some sort of union of the two shall preserve independence.
Raum und Zeit (Minkowski) – WikisourceDie Anschauungen über Raum und Zeit, die ich Ihnen entwickeln möchte, sind auf experimentellphysikalischem Boden erwachsen. Darin liegt ihre Stärke. Ihre Tendenz ist eine radikale. Von Stund′ an sollen Raum für sich und Zeit für sich völlig zu Schatten herabsinken und nur noch eine Art Union der beiden soll Selbständigkeit bewahren.
Figure 2 is interesting. It's likely the world's first published lightcone diagram.
O (origin point)
diesseits von O = this side of O (past timelike region)
jenseits von O = beyond O (future timelike region)
zwischen Hyperbel = intermediate hyperbola
Nachkegel = aftercone (future null cone)
Vorkegel = forecone (past null cone)
raumartiger Vektor = spacelike vector
zeitartiger Vektor = timelike vector
To understand it, consider how one defines the distance between two points. In 3space, it is given by Pythagoras's theorem, but Minkowski showed that that theorem can be extended to spacetime by including time as an additional space dimension, but one that contributes in the opposite direction. Thus, space contributions and time contributions will partially or completely cancel, and a nonzero result can have either sign.
Mathematically, in special relativity, spacetime has a metric g, a symmetric real constant 2tensor. A 4vector x has a length invariant composed with g: X = x.g.x This is rather obviously a generalization of Pythagoras's theorem. By some mixing and rescaling of coordinates, g can be turned into a diagonal matrix of +1's and 1's. There are two possible sign conventions for g, time + space , and time  space +, and one sees both of them in the literature. I'll use the timelike convention here: time +, space .
So a vector can fall into three regions:
 X > 0  timelike
 X = 0  null
 X < 0  spacelike
Massive objects have timelike trajectories, while electromagnetic and gravitational waves have null trajectories because of their masslessness. Thus, the speed of light in a vacuum is a result of the geometry of spacetime.
In reply to Ipetrich, re: your #13.
I like your post...still. I believe it is a "false value" to ascribe physicality to time functions. To measure is one thing...to imply the measure gives substancequality to a thing can be a mistake.
To enumerate a thing does not give it "substance", even though it implies it...unfortunately, it's about all we have to describe an intangible aspect of "reality".
.....
If electromagnetic and gravitational waves have "null" velocities...it implies there must be a "continuum of self", which in turn means that there is a "preexistence of potential".
I see no way around this, other than "magic particles" that wink in and out of existence as per need. The only other possibility is an aethercontinuum!
(Thanks for reading!)
Continuing further, we can classify metric spaces, spaces with distance measures.
If the metric has all the same sign of signature, then the space is called a Euclidean one, after that famous geometer.
If one sign is opposite the others, then the space is called a Minkowskian one, after the first one to recognize such a metric.
I don't know of any general name for two or more signs being opposite two or more signs.
Itzhak Bars seems to propose a spacetime metric with signature ++... two timelike coordinates and two spacelike ones.
There's an interesting oddity for two or more times. In general, a velocity u = dx/dτ, where x is the coordinate vector and τ is the proper time. For timelike motion, u.g.u = 1. Let's now see about that oddity, a possible trajectory in time:
The times:
t1 = (1/ω)*sin(ω*τ)
t2 =  (1/ω)*cos(ω*τ)
The velocities:
u1 = cos(ω*τ)
u2 = sin(ω*τ)
u.g.u = 1 is easy to verify.
So an object can go around and around and around in time, unlike for a single time, where an object can only go in one direction in it.
Let's now consider how a 4velocity's components are related in a Minkowskian space. I will divide its components up into a time component, u_{t}, and a space one, the 3vector u_{s} with length u_{s}. One finds
u_{t}^{2}  u_{s}^{2} = 1
or
u_{t} = + sqrt(u_{s}^{2} + 1)
Thus, the u_{t} solution is split into two branches, a positive one >= 1 and a negative one <= 1, with a gap in between. That may impose a direction on time, because a particle would have to jump from forward motion in time to backward motion in time, and vice versa.
If u is a null vector, then
u_{t} = + u_{s}
also with two branches, but separated by the zero vector. Thus, one has this classification of directions in spacetime:
But for a Euclidean space, there is only one comparable category, one that includes all directions.
 Forward timelike
 Backward timelike
 Forward null
 Backward null
 Spacelike
It's also interesting to consider coordinate transformations that leave the distance measure unchanged. These are given by
x' = R.x + D
going from x to x' with matrix R and vector D. The R's satisfy R^{T}.g.R = g . For a Euclidean, allsamesign metric, the group of {R,D}'s is called the Euclidean group, and the group of R's the orthogonal group. For spacetime, the group of {R,D}'s is called the Poincaré group and the group of R's the Lorentz group.
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