Parallel transport of tangent vector....(geodesic)

In summary, the author states that any curve in 2-D cartesian coordinate system is a geodesic, but this is not correct. The same curve in 2-D polar coordinate (r,θ) ,at each λ the tangent vector points along the r(unit) and θ(unit) which differs along the curve ,that is the same curve is not a geodesic in 2-D polar coordinate system ,is it so?
  • #1
Apashanka
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https://www.google.com/url?sa=t&sou...Vaw3UvOQyTwkcG7c7yKkYbjSp&cshid=1551081845109
Here in page 55 it is written that geodesic is a curve whose tangent vector is parallely transported along the curve.
Now if there is a curve in 2-D which is determined by λ(length along the curve) .In 2-D cartesian coordinate system the tangent vector at every λ will point along the x(unit) and y(unit) direction that means they are parallely transported along the curve that means any curve in 2-D cartesian coordinate system is a geodesic.
But the same curve in 2-D polar coordinate (r,θ) ,at each λ the tangent vector points along the r(unit) and θ(unit) which differs along the curve ,that is the same curve is not a geodesic in 2-D polar coordinate system ,is it so??
 
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  • #2
Parallel transport means, loosely, that you move a vector along some path while keeping it pointing in the same direction. In Euclidean space, only straight lines have a tangent vector that, moved along the line and kept pointing in the same direction, remains a tangent vector. As a counter example, imagine a circular path. At some point the tangent points in the +x direction. Parallel transport it a quarter of the way round the circle. The vector still points in the +x direction, but the tangent vector here is in the +y direction.

Coordinates are irrelevant to this fact. The additional complexity in curved spacetime is in the definition of "pointing in the same direction". That's handled by the connection coefficients.
 
  • #3
Apashanka said:
In 2-D cartesian coordinate system the tangent vector at every λ will point along the x(unit) and y(unit) direction that means they are parallely transported along the curve that means any curve in 2-D cartesian coordinate system is a geodesic.
This is not correct. In flat space only straight lines parallel transport their tangent vector. I am not sure exactly why you would think that curved paths parallel transport their tangent vector. Could you explain your reasoning?
 
  • #4
Dale said:
This is not correct. In flat space only straight lines parallel transport their tangent vector. I am not sure exactly why you would think that curved paths parallel transport their tangent vector. Could you explain your reasoning?
IMG_20190225_142030.jpg

Here V1 and V2 are the tangent vectors at every λ which point along x(unit) and y(unit) throughout the curve which means they are parallel transported... that's why I am saying.
 

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  • #5
Apashanka said:
Here V1 and V2 are the tangent vectors at every λ which point along x(unit) and y(unit)

You are mistaken about what tangent vectors are. Tangent vectors point along the curve. They don't point along the coordinate axes.
 
  • #6
Apashanka said:
Here V1 and V2 are the tangent vectors
Oh, there is the mistake. Neither V1 nor V2 are tangent vectors to this curve. The tangent vectors would point diagonally along the curve.

A tangent vector can be thought of as being formed by taking two points on the curve, say ##(x(\lambda),y(\lambda))## and ##(x(\lambda+\Delta \lambda),y(\lambda+\Delta \lambda))##, drawing the line connecting those two points, and taking the limit as ##\Delta \lambda## goes to 0. V1 and V2 do not fit that description.
 
  • #7
Dale said:
Oh, there is the mistake. Neither V1 nor V2 are tangent vectors to this curve.
Yes sir I got it now they are the components...
 
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  • #8
PeterDonis said:
You are mistaken about what tangent vectors are. Tangent vectors point along the curve. They don't point along the coordinate axes.
Yes sir I got it now they are only the components...
 
  • #9
Apashanka said:
Yes sir I got it now they are the components...
For this case at any λ the tangent vector points in dx(λ)/dλ##\hat x##+dy(λ)/dλ##\hat y##
 
  • #10
Apashanka said:
For this case at any λ the tangent vector points in dx(λ)/dλ##\hat x##+dy(λ)/dλ##\hat y##
Yes, that is correct
 
  • #11
PeterDonis said:
You are mistaken about what tangent vectors are. Tangent vectors point along the curve. They don't point along the coordinate axes.
IMG_20190225_153523.jpg

Here T (tangent vector) is not pointing along the curve??
 

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  • #12
Apashanka said:
View attachment 239314
Here T (tangent vector) is not pointing along the curve??

No, T is pointing perpendicular to the curve. Here's an illustration of what is meant by parallel transport.

We have a curve drawn in blue. The tangent vectors at two points, A and B, are drawn in black. The vector drawn in gray is the parallel-transport of the tangent vector at A to the point B. Since the parallel-transport of the tangent vector at A is not equal to the tangent vector at B, that means the curve is not a geodesic.

parallel.jpg
 

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  • #13
stevendaryl said:
No, T is pointing perpendicular to the curve. Here's an illustration of what is meant by parallel transport.

We have a curve drawn in blue. The tangent vectors at two points, A and B, are drawn in black. The vector drawn in gray is the parallel-transport of the tangent vector at A to the point B. Since the parallel-transport of the tangent vector at A is not equal to the tangent vector at B, that means the curve is not a geodesic.

View attachment 239320
Yes sir got it now...thank you
 

What is parallel transport of a tangent vector along a geodesic?

Parallel transport is a mathematical concept used in differential geometry to describe the movement of a vector along a curved surface or manifold. It involves transporting a vector from one point to another while keeping it parallel to itself, without changing its magnitude or direction.

How is parallel transport related to geodesics?

Geodesics are the shortest curves on a surface or manifold, and they are used to define the concept of parallel transport. When a vector is transported along a geodesic, it remains parallel to itself, and the direction of the vector is determined by the curvature of the surface.

Why is parallel transport important in differential geometry?

Parallel transport is important because it allows us to define a notion of a straight line on a curved surface. This is crucial in understanding the geometry of curved spaces and is used in many applications, such as in general relativity and computer graphics.

What is the difference between parallel transport and normal transport?

In parallel transport, the vector being transported remains parallel to itself, while in normal transport, the vector may change its direction and magnitude. Normal transport is used to transport vectors along curves that are not geodesics, while parallel transport is used for geodesics.

How is parallel transport calculated?

The calculation of parallel transport involves using the connection or covariant derivative of the surface or manifold. This derivative takes into account the curvature of the surface and allows us to transport the vector while keeping it parallel to itself. The specific method of calculation may vary depending on the surface or manifold being considered.

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