The big bang

FizixFreak

why is it that before big bang TIME could not exist?

Chalnoth

Well, that's more a statement about certain very specific models of the big bang, not necessarily a statement about reality.

Basically, in some models, such as in Stephen Hawkings' no boundary proposal, there simply isn't any time before the big bang. Asking "what came before the big bang" is analogous to asking "what lies north of the north pole." This is because in his no boundary proposal, the space-time manifold doesn't actually have any sort of edge, just like there is no end to the surface of the Earth (in the sense of people who thought the Earth was flat thought of an edge). It is, however, finite, wrapping back on itself in a very specific way. Thus what we see of as "time" has a beginning of sorts, but there is nothing "before" it (just as the Earth has a point that is furthest north, but with nothing north of that point).

FizixFreak

so time was ''created'' after the big bang .
cant i say that the big bang actually triggered the creation of the ''things'' that could experience time rather than saying that big bang caused the creation of time (as before it there was nothing or none that could measure or evaluate time)????????????

Chalnoth

It depends upon the model. We don't yet know which model is an accurate description of reality.

FizixFreak

if we say that time was ''created'' after big bang wouldn't that imply that time only has existence when there is some one or some thing that can feel it but relativity gives a different picture of time to us?

Chalnoth

No, not at all. You don't need an observer to experience time. But space and time themselves exist on what is called a manifold. Without a manifold, you have no space, no time.

FizixFreak

so before the big bang that manifold existed the bang just expanded it right????? (or that is what i understood when i did some research on string theory).

Chalnoth

No. Time is a direction within the manifold. There is no "before" or "after" outside of it.

FizixFreak

time is a direction??????
i didn't quite understood that.

Chalnoth

Yes. In the same way that up/down, east/west, and north/south are directions.

FizixFreak

but if you call time as direction it means it existed before the big bang?
and does the time represents all known direction or just one specific direction.

Chalnoth

No, because the direction only has meaning within the manifold. It doesn't have meaning outside of the manifold.

Chronos

Science tries to avoid the 'God' hypothesis. Not because scientists hate 'God', but, because they wish to explain as much as possible about the universe without resorting to 'miracles' [which history has proven to be a bad idea]. I have no problem accomodating a 'God' in my universe, just understanding the role s/he plays.

Passionflower

To address a common misunderstanding: time is not an actual dimension on the manifold.

The confusion arises because often a coordinate chart is used where an observer's x₀ (or sometimes denoted as t) is identical to his proper time. For instance a rest frame in Minkowski spacetime using Cartesian coordinates or Fermi normal coordinates in curved spacetimes.

Curved spacetime is a four dimensional manifold but no single dimension is explicitly time.

So what is time? Well for any timelike observer time is the metric distance between two events on his worldline.

In GR worldlines can simply end (at a singularity), by time symmetry (and GR is time symmetric) that implies that worldlines can simply begin as well. Hence according to GR it is possible that for a given observer time can have a begin and an end.

Chalnoth

Well, obviously there's ambiguity as to which sort of direction on the manifold we can identify with time. There is no definitive direction that is associated with time, and different observers will see time as being different directions on the manifold. But then, this is the same with all other directions as well, so time isn't exactly special in regard to this ambiguity.

FizixFreak

so may i say that time is a direction but the answer of WHICH DIRECTION changes with observer?????
or may be that time exists within the three dimensions of space occupying some part of all three dimensions?????

Chalnoth

No, time is definitely separate from the three dimensions of space. Which direction in space-time you see as time depends upon things like your velocity and acceleration.

Mathematically, time is exactly the same as the other dimensions, except that the sign of a metric component associated with time is opposite from the spatial dimensions. For example, if the spatial dimensions have positive metric components, then time has a negative metric component. With this convention, if you find the metric distance between two different times for a particular observer, you get a negative number (a timelike distance...this is the actual time that the observer sees on their clock). If you find the metric distance between two simultaneous events, by contrast, you get a positive number (a spacelike distance: this is the distance you would measure between these events in a reference frame where they occur simultaneously).

Finally, if you find the metric distance between two points in the travel of a light ray (for example, from when a light ray is emitted to when it is absorbed), you always get zero. So light rays themselves act as a boundary between timelike distances and spacelike distances.