- #1
space-time
- 218
- 4
I have some questions. Let us assume for these questions that I am using the (- + + +) sign convention.
Firstly, we know that if you have a parameterized curve ξ(s), then you can find the proper time between two events at points s1 and s2 by using this formula (assuming that the curve is timelike):
dτ = ∫ sqrt( -gab(dxa/ds)(dxb/ds) ) ds
where the bounds of the integral are from s1 to s2
However, what if you do not have a parameterized curve, but you instead just have a simple vector, or you are simply given two events in a spacetime and then are told to find the proper time between the events?
For example, let's say we were working with Minkowski space and I pointed out two events to you. The first event would be
(t, x, y, z) = (1, 0, 0, 0).
The second event could be:
(t, x, y, z) = (5, 0, 0, 0).
Now I really don't think that the integral formula above would be applicable to this case here (mainly due to derivatives of constants being 0 and whatnot).
Instead, here is what I think that the proper steps to solve this problem are:
First, I would subtract the first event from the second event to get a vector [4, 0, 0, 0]. I believe that I would treat this vector as dxa when I applied the spacetime interval to it. In other words, dx0 = 4 and the other components equal 0.
Next, I should apply dτ2 = gabdxadxb
In the case of this example, dτ2 = -16
Finally, the proper time between the two events would just be sqrt( -dτ2 ), which would in this case just be 4.
Is that correct?
If so, I know that this was probably an easy and obvious example. It is just that the most recent textbook that I read from only focused on what to do to get the proper time between events on the ends of a parameterized curve, but it did not talk about instances where you do not have a parameterized curve, but just two events. This was bothering me.
Thanks
Firstly, we know that if you have a parameterized curve ξ(s), then you can find the proper time between two events at points s1 and s2 by using this formula (assuming that the curve is timelike):
dτ = ∫ sqrt( -gab(dxa/ds)(dxb/ds) ) ds
where the bounds of the integral are from s1 to s2
However, what if you do not have a parameterized curve, but you instead just have a simple vector, or you are simply given two events in a spacetime and then are told to find the proper time between the events?
For example, let's say we were working with Minkowski space and I pointed out two events to you. The first event would be
(t, x, y, z) = (1, 0, 0, 0).
The second event could be:
(t, x, y, z) = (5, 0, 0, 0).
Now I really don't think that the integral formula above would be applicable to this case here (mainly due to derivatives of constants being 0 and whatnot).
Instead, here is what I think that the proper steps to solve this problem are:
First, I would subtract the first event from the second event to get a vector [4, 0, 0, 0]. I believe that I would treat this vector as dxa when I applied the spacetime interval to it. In other words, dx0 = 4 and the other components equal 0.
Next, I should apply dτ2 = gabdxadxb
In the case of this example, dτ2 = -16
Finally, the proper time between the two events would just be sqrt( -dτ2 ), which would in this case just be 4.
Is that correct?
If so, I know that this was probably an easy and obvious example. It is just that the most recent textbook that I read from only focused on what to do to get the proper time between events on the ends of a parameterized curve, but it did not talk about instances where you do not have a parameterized curve, but just two events. This was bothering me.
Thanks