# Most basic description of what time actually is

1. Nov 28, 2003

### mikelus

I thought about the most basic description of what time acually is and came up with this.
Time is the distace between two points in space.
im not a person who reads alot of physic books so I don't know alot about the subject. Do you agree or disagree?

2. Nov 28, 2003

### mathman

Length is the distance between two points on space. Time is how long it takes to get from one point to the other.

Sometimes the obvious happens to be true.

3. Nov 28, 2003

### Sniper__1

how did you come up with this? i disagree i believe time is not real but a construct of the human mind because in your theory different speeds of travel wuld cause different times so it cant be true.

4. Nov 29, 2003

### mikelus

let me rephrase it
time is the space between two points in space.

5. Nov 29, 2003

### jcsd

You can find the distance ($ds$) between two points in n-dimensionsal (Euclidian) space from:

$$ds^2 = {x_1}^2 + {x_2}^2 + {x_3}^2 .... + {x_{n-1}}^2 + {x_n}^2$$

which in our three dimensional world is:

$$ds^2 = x^2 + y^2 + z^2$$

notice no need for any time dimension to find the distance between two objects in space.

6. Dec 3, 2003

### mikelus

time

Is there an exact measurment for time?

7. Dec 4, 2003

8. Dec 4, 2003

### Sariaht

The fourth coordinate is ict. This might be important in a thread called time.

9. Dec 4, 2003

### jcsd

No, we're talking about seperation in space not spacetime, space tiome has the metric (depending on the formalism you prefer):

$$ds^2 = (x^1)^2 + (x^2)^2 + (x^3)^2 - (x^4)^2$$

Where $x^4 = ct$.

10. Dec 4, 2003

### jcsd

Unless of course your talking about Euclidian quantum gravity which does form a four-dimensional locally Euclidan space with one dimension equivalent to -ict.

11. Dec 7, 2003

### mikelus

what is the basic concept of spacetime?

does it relate in that time does not exist without space and space with out time.

with out two points in time does space exist?

12. Dec 7, 2003

### jcsd

mikelus:

Above I gave the equation for finding the distance between two points in (Euclidian, i.e. flat) space (if we take one of the points as the origin):

$$ds^2 = x ^2 + y^2 + z^2$$

In relativity there are a group of equations which show us how differnt observers travelling at different speeds view time and space:

$$x' = \gamma(x - ut)$$
$$y' = y$$
$$z' = z$$
$$t' = \gamma(t - \frac{ux}{c^2})$$

Where the primed dimensions (e.g. $x'$) is how an observer travelling along the x axis with speed $u$ views these dimesions compared to how an observer in this rest frame views them. $\gamma$ is equal to $(1 - u^2/c^2)^{1/2}$

If we apply these transformations to the first equation, we find:

$$d(s')^2 = \gamma^2(x - ut)^2 + y^2 + z^2$$

this means that $ds^2 = d(s')^2$ if and only if $u = 0$. So observers travelling at different speeds will not agree on the distance between two objects in space.

If we look at the metric for (flat) spacetime given in my first post in a simlair way we see it is of the form:

$$ds^2 = x^2 + y^2 + z^2 - c^2t^2$$

And we as in the metric for space we perform the same opertaion for an obnserver travelling at a different speed (i.e. perform a Lorentz tarnformation) we get the equation:

$$d(s')^2 = \gamma^2(x - ut)^2 + y^2 + z^2 - c^2\gamma^2(t - \frac{ux}{c^2})^2$$

if we expand this out we get:

$$d(s')^2 = \frac{x^2 - 2utx + u^2t^2}{1 - \frac{u^2}{c^2}} + y^2 + z^2 - \frac{c^2t^2 - 2utx + \frac{u^2x^2}{c^2}}{1 - \frac{u^2}{c^2}}$$

if we factorise and divide we find:

$$d(s')^2 = x^2 + y^2 + z^2 - c^2t^2 = ds^2$$

So we should say that two observers will always agree on the distance between two points in (flat) spacetime, even if they are travelling at different speeds. This is why the concept of spacetime is so useful as we use it to find equations valid for all observers not for just one particular observer.

You may of also noticed that the Lorentz transfomration is very simalir to rotating something in space, in this way we can think of velocity as a rotation in spacetime.

Last edited: Dec 7, 2003
13. Dec 9, 2003

### HeavensWarFire

You should all read my post.

Time doesnt exist, in the way you think a Chair exists. Time is a an abstract concept that relates to change.

14. Dec 9, 2003

### HeavensWarFire

You failed to prove yur explanation of absolute time

You seem to confuse absolute spead measure, with absolute time measure. The question is not about the actual distance between 2 points, but rather, what is in fact the smallest unit of time measurable.

Heres why i think yur formula doesnt work: you fail to take into consideration the idea of infinite divisability. You take a foot, and you dived that in half. Take this half, and divide that in half. Repeat this process of diving each, and every successive half, and you will never find a terminating point. Hence, your math formula for distance is only contextual, and it does not answer any of the paradoxes of Zenos.

To define an absolute unit of time, you need to first define an absolute unit of distance that you can not mathetimatically subdivided, and you need an absolute unit of speed. Once you answer these issues, then maybe you can come up with an actual unit of time that can not be further subdivided into smaller units.

15. Dec 9, 2003

### HeavensWarFire

Hello

Knock, knock:
Space is simply the absence of "matter." without matter, you could never have an atomic notion of a void.

Secondly, time seems to only come into consciousness when you start to think about distance, and speed.

But over, and beyond that, theres no such thing as time. Mathetimatically, you an infinitely add, or subdivide, hence theres not absolute measure of any kind in the truest sense.

16. Dec 9, 2003

### jcsd

Re: You failed to prove yur explanation of absolute time

They're not my formulas they're Pythagoras's, Lorentz's and Minowski's, they lie at the heart of special relativity.

I'd also like to tell you taht ds, x, y, z and ct do not take discrete values (this should be rather obvious as there are only a few cases where they can all take on non-zero but discrete values).

I'm not confused, all I was doing was explaining the well-known concept of space-time, It appears YOU are confused.

17. Dec 9, 2003

### HeavensWarFire

Try reading first

How nice of you to ignore the main crux of my thoughts.

Ok,wise guy, tell me, how much time has there been since the creation of all life? And how much time will there be there once all is killed off in what is called entropy? Surely you know, for you have many theories in your head.

Perhaps you do not understand the idea of "infinite divisability"? Tell, what is the smallest unit of measure that you can think of? Surely you know? Did you not know that mathematically speaking, many things are endless? A foriegn concept to you is it? And perhaps you are equally confused with what is called a Paradox?

x=length
y=width
z=depth

This is just the standard variables for the dimensions of the space, object, continuum, it doesnt tell me anything about what is the smallest unit imaginable of anything, whether we are speaking of distance, time, or speed.

18. Dec 10, 2003

### jcsd

No, you don't understand, where did I say that the temporal and spatial dimensions weren't continuous? Infact if you look at the formula:

$$ds^2 = x^2 + y^2 + z^2 - c^2t^2$$

You find there are very few values you can put in that would allow ds, x, y, z and ct to be discrete, off the top of my head I can only think of 1 set of values (and therefore mutiples of these values) that allow them all to be natural numbers: ds = 3, ct = 3 and where x y and z take the values 1, 1 and 4 (that's of course not to say there aren't more), the point is you can't make the formula discrete without excluding a whole set of solutions.

Now stop blathering.

19. Dec 10, 2003

### HeavensWarFire

lol

Ahahahahahahahahah...........

Alright man, i will speak at your level: a formula for how you may go about in solving a certain equation is not the same as the demonstration.

Your formula is cute, but, you still havent furnished me with an actual answer to the question of whether or not, mathematically you can actually arrive at the smallest unit possible. And until you do that, your above means nothing to me. Theory, and practise are not of the same realm. So tell me in what way your formula is of any significance? What can you solve with your theory exactly?

20. Dec 10, 2003

### jcsd

1) this is not my thewory and I never mad such a claim, it was a brief introduction to the concept of spacetime a key part of Einstein's theory of relativity. Relativyt is a key part of modern physics it's applications are num,erous and it has been tested many times.

2) I did not make any comment on whether time is discrete or continuous, infact relativity does tactitly assume that time is continuous, so why do you keep on going on about the infinite divisbilty of time?

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