Direction of time

[Razee01]

I don't know how it sounds, what is the direction of time? Is it towards the center of the universe, the big bang or opposite?

Thanks.

[swansont]

Unlike motion in space, where you can move in the + or - direction on a set of axes, your motion in the time dimension is only in the + direction.

[michel123456]

1 hour ago, Razee01 said:

I don't know how it sounds, what is the direction of time? Is it towards the center of the universe, the big bang or opposite?

Thanks.

Interesting question. Under normal circumstances you will get answers that say time has no direction (unlike space where you have directions).

However:

When you look at the night sky & far away at a star, you see the star as it was a long time  ago. The entire night sky is an image of objects as they once were. In other words you are looking at the past of the Universe. The farther away, the more in the past. The closest in space the closest in time.

Thus one should conclude that since Time goes from the past to the present & to the future, Time goes from the outside to the inside.

That counts for each observer: each observer see time going from out to in, where out is the past. The present touches exactly your skin and your eyes. The future is inside you (but that gets somehow philosophical).

[Strange]

59 minutes ago, michel123456 said:

Under normal circumstances you will get answers that say time has no direction (unlike space where you have directions).

I would say that time has one direction, but spatial dimensions have two directions.

[beecee]

9 hours ago, Razee01 said:

I don't know how it sounds, what is the direction of time? Is it towards the center of the universe, the big bang or opposite?

Thanks.

I first came across the terminology of the "arrow of time" in Stephen Hawking's book, "A Brief History of Time" 

In it he describes the thermodynamic arrow of time, that is entropy increasing with time, and the asymmetrical nature of time, the cosmological arrow of time, or the direction of the expansion of the universe, and the Psychological arrow of time, or the flow of time from the past to the future.

From memory [long time since I read the book] he speaks of the possibility of the universe recollapsing [or the reversal of the cosmological arrow] yet the flow from past to future continues in that one direction. In essence, while time is variable, it is also asymmetric or one directional.

[studiot]

Neither the original question, nor any of the 'answers'  are compatible with time being a coordinate axis for a physical dimension.

The all refer to some function or property being measured or plotted against this axis.

As with all such axes, direction is just a convention. 

It is gratifying that the aussies have the same convention for which way is up that we do. 

Further I think this is an issue of the philosophy of Science rather than the subject of relativity.

[michel123456]

10 hours ago, studiot said:

Neither the original question, nor any of the 'answers'  are compatible with time being a coordinate axis for a physical dimension.

The all refer to some function or property being measured or plotted against this axis.

? Please expand.

[studiot]

4 hours ago, michel123456 said:

? Please expand.

Sure thing.

 

It is often said that time is different from distance/length/position because we can proceed in either direction.

But let us examine the truth of this proposition with the help of a couple of diagrams.

 

 

In diagram (1) we have a sketch of my house placed against a distance scale or axis.

We can indeed wlak out of the front door and answer the question "How far is it from the corner of my door to the corner of the house" either way

Or we can look at the diagram and measure either way on the diagram.

Diagram (2) show shows the pressure dropping along a water pipe.
I have included a non retern valve partway along to preclude flow in one direction.
So water, like time, cannot flow upstream.

So here is a realworld example of something, plotted along an axis, that can only flow one way.

Yet we can look at the plot and measure in either direction.

In diagram (3) we return to time.
We can calculate, backwards or forwards, how long it was from the nearby last battle on English soil to when I bought my present house in diagram (1)

Of course we cannot travel in time back to either of these events, just as water cannot flow upstream in our pipe.
But we can traverse the distance from the front door to the corner of the house.
So what is different?

Nothing really. All are plots/graphs/diagrams drawn against some definining axes.

But, in general, that which is plotted is not necessarily the same as the axes themselves.

So when we are using a time axis, we are talking about events, states, positions or other properties plotted against time.
We are not directly taking about time itself.

So of course, like any other such plot, we can view the entire plot (or at least a section of it) together formeasurement and comparison purposes.

Edited March 13, 2019 by studiot

[michel123456]

17 minutes ago, studiot said:

Sure thing.

 

It is often said that time is different from distance/length/position because we can proceed in either direction.

But let us examine the truth of this proposition with the help of a couple of diagrams.

 

 

In diagram (1) we have a sketch of my house placed against a distance scale or axis.

We can indeed wlak out of the front door and answer the question "How far is it from the corner of my door to the corner of the house" either way

Or we can look at the diagram and measure either way on the diagram.

Diagram (2) show shows the pressure dropping along a water pipe.
I have included a non retern valve partway along to preclude flow in one direction.
So water, like time, cannot flow upstream.

So here is a realworld example of something, plotted along an axis, that can only flow one way.

Yet we can look at the plot and measure in either direction.

In diagram (3) we return to time.
We can calculate, backwards or forwards, how long it was from the nearby last battle on English soil to when I bought my present house in diagram (1)

Of course we cannot travel in time back to either of these events, just as water cannot flow upstream in our pipe.
But we can traverse the distance from the front door to the corner of the house.
So what is different?

Nothing really. All are plots/graphs/diagrams drawn against some definining axes.

But, in general, that which is plotted is not necessarily the same as the axes themselves.

So when we are using a time axis, we are talking about events, states, positions or other properties plotted against time.
We are not directly taking about time itself.

So of course, like any other such plot, we can view the entire plot (or at least a section of it) together formeasurement and comparison purposes.

I am not sure what you are speaking about.

Do you know the Minard map? Interestingly time flows in both directions. https://en.wikipedia.org/wiki/File:Minard.png  It describes the campaign of Napoleon in Russia & its losses.

 

In this map, time & displacement are essentially the same.

[studiot]

28 minutes ago, michel123456 said:

Do you know the Minard map? Interestingly time flows in both directions. https://en.wikipedia.org/wiki/File:Minard.png  It describes the campaign of Napoleon in Russia & its losses.

No I have never heard of this map before.

But it is an interesting example of graphic display of vainglorious loss of life.
I could not determine from the text, but I assume that black going and light buff is returning ?

It is interesting also because the map is dated Paris 1869, but France (but not Russia) converted to the metric system in 1790 and the Celsius scale whilst Russia remained on the Reaumur till much later. Millimetres were used in the distance scale I see.

 

It does however show that direction along an axis is a convention that we can choose.

[michel123456]

52 minutes ago, studiot said:

No I have never heard of this map before.

But it is an interesting example of graphic display of vainglorious loss of life.
I could not determine from the text, but I assume that black going and light buff is returning ?

It is interesting also because the map is dated Paris 1869, but France (but not Russia) converted to the metric system in 1790 and the Celsius scale whilst Russia remained on the Reaumur till much later. Millimetres were used in the distance scale I see.

 

It does however show that direction along an axis is a convention that we can choose.

The width is the number of soldiers, each mm for 10,000

The large light brown is the invading army crossing  the Niemen river (422.000 troops). The black is the return trip from Moscow(only 10,000).

The full legend can be found on the link to the map.

 

English: The map's French caption reads:

Figurative Map of the successive losses in men of the French Army in the Russian campaign 1812–1813.
Drawn up by M. Minard, Inspector General of Bridges and Roads in retirement. Paris, 20 November 1869.

The numbers of men present are represented by the widths of the colored zones at a rate of one millimeter for every ten-thousand men; they are further written across the zones. The red [now brown] designates the men who enter into Russia, the black those who leave it. —— The information which has served to draw up the map has been extracted from the works of M. M. Thiers, of Segur, of Fezensac, of Chambray, and the unpublished diary of Jacob, pharmacist of the army since October 28th. In order to better judge with the eye the diminution of the army, I have assumed that the troops of prince Jerome and of Marshal Davoush who had been detached at Minsk and Moghilev and have rejoined around Orcha and Vitebsk, had always marched with the army.

The scale is shown on the center-right, in "lieues communes de France" (common French league) which is 4444 m (2.75 miles).

The lower portion of the graph is to be read from right to left. It shows the temperature on the army's return from Russia, in degrees below freezing on the Réaumur scale. (Multiply Réaumur temperatures by 1¼ to get Celsius, e.g. −30 °R = −37.5 °C) At Smolensk, the temperature was −21° Réaumur on 14 November.

Edited March 13, 2019 by michel123456

[studiot]

17 minutes ago, michel123456 said:

The red [now brown] designates the men who enter into Russia, the black those who leave it.

 

Thank you for that correction.

(and the translation) +1

I thought that the numbers were losses, not survivors since I had understood that most of the losses came towards the end of the return trip as the weather became colder.

But either way, the losses were truly staggering.

[md65536]

On 3/13/2019 at 5:45 AM, studiot said:

Of course we cannot travel in time back to either of these events, just as water cannot flow upstream in our pipe.
But we can traverse the distance from the front door to the corner of the house.
So what is different?

Nothing really. All are plots/graphs/diagrams drawn against some definining axes.

Time is like distance. (They're both measures of lengths between events)

You can walk from the door to the corner of the house and then back, and say, "I'm decreasing the distance that I walked," but you're not really. You're decreasing the displacement between you and the door. The total distance you've walked can only increase. Time as measured by clocks is like that, it's the total elapsed.

The problem isn't that there's some hidden physical difference by what must be the "true physical meaning of time" and "true meaning of space", it is merely a difference between the measurements we're choosing to consider, and of choosing the wrong analogies to compare them.

A temporal analogy to displacement might be the minimum communication time between two locations, which would be the length of a light-like path between them. This can be increased or decreased, but it's not a measure we call 'time'.

[FreeWill]

On 3/12/2019 at 11:18 AM, Razee01 said:

I don't know how it sounds, what is the direction of time? Is it towards the center of the universe, the big bang or opposite?

Thanks.

Time is positively forward pointing since the first moment from the center of the universe

Center of the universe: 1D point (empty space-time, a plank unit) at the first moment of reality

On 3/12/2019 at 11:45 AM, swansont said:

Unlike motion in space, where you can move in the + or - direction on a set of axes, your motion in the time dimension is only in the + direction.

Does this mean that Time is a linear, forward pointing vector? 

Edited May 30, 2019 by FreeWill

[Strange]

8 minutes ago, FreeWill said:

+ Forward from the center of the universe (1D point (empty space-time, a plank unit?) at the first moment of reality)

There is no centre of the universe.

And your answer seems to be tautological: "what is the direction of time?" "forward in time" (although I don't think a better answer is possible).

[FreeWill]

8 minutes ago, Strange said:

There is no centre of the universe.

Yet we can not say this as a fact.

I would expect a symmetric development though if the start is 1D.

8 minutes ago, Strange said:

And your answer seems to be tautological: "what is the direction of time?" "forward in time" (although I don't think a better answer is possible).

True. Swansont expressed it already.

Do you think time is linear?

Edited May 30, 2019 by FreeWill

[Strange]

2 minutes ago, FreeWill said:

Yet we can not say this as a fact.

Of course not, because it is science. 

However, the universe having a centre (or a boundary, which is probably the same thing) would be inconsistent with our current models.

3 minutes ago, FreeWill said:

I would expect a symmetric development though if the start is 1D.

There is no reason evidence of a "start" and certainly no evidence that it started as 1D (a line? really?)

[FreeWill]

35 minutes ago, Strange said:

However, the universe having a centre (or a boundary, which is probably the same thing) would be inconsistent with our current models

What are the signs of inconsistencies with the current models, of a system which has center and boundaries? I think actually everything has a physical center, if we can recognize and determine its boundaries. 

35 minutes ago, Strange said:

There is no reason evidence of a "start" and certainly no evidence that it started as 1D (a line? really?)

My bad, thanks for the correction!

What is the dimension of a point?

 

Edited May 30, 2019 by FreeWill

[Strange]

35 minutes ago, FreeWill said:

What are the signs of inconsistencies with the current models, of a system which has center and boundaries?

As I understand it, the mathematics of GR requires the manifold to be differentiable (smooth and continuous everywhere). A boundary, implied by a centre, could conflict with this.

35 minutes ago, FreeWill said:

What is the dimension of a point?

Zero. So a point has no physical existence. (Neither does an ideal, 1D, line.) It is just a mathematical abstraction.

[Phi for All]

5 hours ago, FreeWill said:

Center of the universe: 1D point (empty space-time, a plank unit) at the first moment of reality

The BB was NOT an explosion from a center point. 

Also, we don't know that "reality" began at that time. Reality isn't a good word to use in this context, since science is our observations and measurements of the natural world. Reality sort of implies something beyond that, a realness outside what is observed.

We can't even say time started with the BB (that this was the "first moment"). It might have started again from a prior iteration.

[FreeWill]

 

 

 

5 hours ago, Phi for All said:

The BB was NOT an explosion from a center point. 

 

Difficult to determine. What is against it? What was the BB then?

Not an explosion, rather a singular event from a point. (evolution of empty space-time with a certain rate for example).

Note please, that calling the event the Big Bang suggest an explosion (radical radial expansion). 

 

[beecee]

Simply put, the BB was the evolution of space and time [as we know them] from a hot dense state at t+10-43 seconds. All of space and time [spacetime] that we observe was packed within the volume of an atomic nucleus, and as such it can be realized that the BB happened everywhere at the same instant...

[Strange]

8 minutes ago, FreeWill said:

Difficult to determine. What is against it? What was the BB then?

 Not an explosion, rather a singular event from a point. (evolution of empty space-time with a certain rate for example).

Note please, that calling the event the Big Bang suggest an explosion (radical radial expansion). 

The Big Bang was a snappy name, invented by Fred Hoyle, to compare with the (equally catchy) Steady State theory. (He always denied it was intended to be derogatory.)

It is a model that describes the ongoing expansion of the universe from an early hot, dense state. The universe was then, and is still, uniformly full of matter. (Hence not an explosion.) As the universe expanded it cooled (like when you release the pressure from an aerosol can) which allowed structures like stars and galaxies to form.

 

[FreeWill]

5 hours ago, Phi for All said:

Also, we don't know that "reality" began at that time.

I am 999.99..9% sure that reality did not begin with the time we know. I expect that the Universe is way bigger than we can yet observe or imagine. Energy and matter are from somewhere. I think they are a consequence of a process which I expect to be the expansion of the initial inertial frame: SpaceTime. (Could we say that the resistance of spacetime is 0?) 

3 minutes ago, Strange said:

The universe was then, and is still, uniformly full of matter.

Currently, the Universe is not uniformly full of matter. Interstellar/intergalactic space vs Back whole/center of a galaxy.

[Strange]

6 minutes ago, FreeWill said:

I expect that the Universe is way bigger than we can yet observe or imagine.

It is certainly much bigger than we ca observe. The observable universe is about 90 billion light years in diameter. The whole universe is probably several orders of magnitude larger, if not infinite. I don't know if you can imagine that or not.

6 minutes ago, FreeWill said:

Energy and matter are from somewhere. I think they are a consequence of a process which I expect to be the expansion of the initial inertial frame: SpaceTime. (Could we say that the resistance of spacetime is 0?) 

Don't bring your speculative ideas up in someone else's thread.

6 minutes ago, FreeWill said:

Currently, the Universe is not uniformly full of matter. Interstellar/intergalactic space vs Back whole/center of a galaxy.

On large scales it is.  The current large scale structures probably evolved from quantum scale variations in the early universe that expanded.