# I How much time do we occupy?

1. Aug 15, 2015

### hlustisa

Hello

If we have this 4D strucuture (being time one of the dimensions) is it right to think that, similarly to the fact the matter ocuppies a portion of three spatial dimensions, it also has a length in time?

If this question makes any sense, would be possible to know how much time we occupy? It would be just a fraction of second, is it more? I mean, the fact we are moving in time torwards what we call the future, doesn't mean that we can't occupy a finite portion of the time dimension at any given "instant".

Another thing I was never able to understand: If matter is able to bend space-time around itself, does it mean that not only space is bent, but also time, right? The time would bent over the space, and as we move in time we also move in space towards the object that causes time to bent. In this case, the same way earth's gravity acts all around it (in space) , it alsos acts in time? Does the earth from this exact instant in time generate a gravitational field that can be felt back in time, or even in the future? Could we measure it? Does the gravitational field we experience now is the result from the sum of gravity generated by many earths on different time instants?

Forgive me if it is all BS, but those are honest questions from a non physicist.

2. Aug 15, 2015

### Orodruin

Staff Emeritus
The question does not make sense. A moment is by definition not extended in time. An object will define a four-dimensional region of space-time (world volume), but this is due to the object existing at different moments. As a similar question, consider the following:

What is the extent in the x-direction of the circle x^2 + y^2 < 1 at x = 0.5? Since you are asking about x = 0.5, the object you are going to look at has no extension in the x-direction. The circle itself is extended in the x-direction, but this is more related to an object actually defining a world volume than having an extension in time at a fixed moment (which, as stated, does not make sense).

3. Aug 15, 2015

### hlustisa

Ok. I got it. It depends on how I define the object. I could define it as having 0 length in time, which means is a "different" object at any given instant, or as a 4D object considering it exists in different instants.

We tend to look at objects and measure their volume. Take a look at a ball for instance, we perceive how much space it occupies because It has a boundary (it is arbitrarily defined, but its a boundary), The particles that make up the ball are connected or related to each other in such manner that we think about it as a unity. In this context, my question was about perceiving this same unity in time. During a finite amout time, that ball is a unity, and then it is not tight together anymore, or not bounded to its previous version so strongly that it becomes another object entirely.

Thank you for your answer.

Do you have any coments about the last question?

4. Aug 15, 2015

### Staff: Mentor

Hi hlustisa, welcome to PF,

If we are lucky we occupy 70 or 80 light years of time. In spacetime we are much longer in the time dimension than in any of the spatial dimensions.

5. Aug 15, 2015

### hlustisa

hahahaha =]

This is actually the point. If I ask you where your body ends and the environment around you begins, you can easily answer me.
But in time, do you really think you are exact the same "object" from cradle to grave? Is there a "natural" range in time in which you get so decoupled from your previous versions that you become something else?

I am asking this because of my other question, related to effect of the gravity in time. I wonder if there is a time range in which we fell the effect of future's earth gravity or something like that.

Thank you for the reply!

6. Aug 15, 2015

### Orodruin

Staff Emeritus
This is a more philosophical than physical question. You can draw the boundaries of a system under study as you wish as long as you specify how that system interacts with its surroundings.

This sounds like something taken from reading popular science and trying to think like it. It does not have any physical coherence and is not very well defined. Also be advised that the discussion of personal theories is not allowed at Physics Forums.

7. Aug 15, 2015

### Vespa71

A very short time at bigbang time may be long here,, Wich is good, I suppose.I didn`t understand the question.

8. Aug 15, 2015

### hlustisa

I am sorry. I did not realize this could be qualified like that. I don't mean to go against the rules of the community. I could not find a way to delete the thread.

9. Aug 15, 2015

### Staff: Mentor

Uhh, that wasn't a joke. I was answering the question. In spacetime an object is as long in time as its "lifetime" times c. So for a human that is 70-80 light years long.

That is a philosophical question, but I don't think that the answer is important. Whatever events you choose as defining the beginning and the end of the "object", the corresponding length in the time direction in spacetime is that duration times c. I would choose birth and death for a human, with a corresponding 70-80 light year length, but if you wanted to choose different events then you could certainly do so. The science part is clear, only the philosophy is vague.

10. Aug 15, 2015

### Staff: Mentor

It's ok, that was just a friendly reminder. We can close the thread on that note, though.

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