Inertial and non inertial frames

[Jeronimus]

Maybe someone could define acceleration. To me, acceleration is the context of SR, is that which is responsible for changing which inertial frame of reference an object becomes at rest in.

I could not possibly define it to be the change in velocity over time (seen from the observer's point of view who is supposed to be accelerating), since in SR an observer considers himself at rest.

So here i am, waiting for how you define acceleration in the context of SR seen from an accelerating observer's point of view.

 

[Mentz114]

Maybe someone could define acceleration. To me, acceleration is the context of SR, is that which is responsible for changing which inertial frame of reference an object becomes at rest in.

I could not possibly define it to be the change in velocity over time (seen from the observer's point of view who is supposed to be accelerating), since in SR an observer considers himself at rest. So here i am, waiting for how you define acceleration in the context of SR seen from an accelerating observer's point of view.

Try the Wiki article. There's a definition there.

http://en.wikipedia.org/wiki/Four-vector

Imagine you're sitting in a small boat on a calm lake so the boat is at rest wrt the water. If you then throw a brick towards aft so it leaves the boat, what happens ? The brick was accelerated by your arm and the reaction ( Newton's third) pushes the boat and you. You are still at rest in the boat even if it accelerates, as long as you go with it.

 

[Jeronimus]

Imagine you're sitting in a small boat on a calm lake so the boat is at rest wrt the water. If you then throw a brick towards aft so it leaves the boat, what happens ? The brick was accelerated by your arm and the reaction ( Newton's third) pushes the boat and you. You are still at rest in the boat even if it accelerates, as long as you go with it.

Nice example, except that you do not feel acceleration when you push the brick away of you. What you feel/sense is different parts of your body being accelerated differently, and the resulting change of your body structure. This is what you sense, and not acceleration itself.
At the point your fingers touch the brick, your fingers get compressed because the brick resists being accelerated (why it resists and why inertial mass is connected to gravitational mass i do not know yet exactly). This compression moves through your body, and changes the structure which then can be sensed by you. But that is not acceleration you sense.

 

[Mentz114]

Nice example, except that you do not feel acceleration when you push the brick away of you. What you feel/sense is different parts of your body being accelerated differently, and the resulting change of your body structure. This is what you sense, and not acceleration itself.
At the point your fingers touch the brick, your fingers get compressed because the brick resists being accelerated (why it resists and why inertial mass is connected to gravitational mass i do not know yet exactly). This compression moves through your body, and changes the structure which then can be sensed by you. But that is not acceleration you sense.

Suppose you had a friend on the boat and she threw the brick ? Would you feel anything ?
Whether you did or not, an accelerometer would register the push on the boat.

You can make an accelerometer by standing a brick an its small face on a skateboard. Give the skateboard a good push and the brick falls over !

 

[Jeronimus]

Suppose you had a friend on the boat and she threw the brick ? Would you feel anything ?
Whether you did or not, an accelerometer would register the push on the boat.

You can make an accelerometer by standing a brick an its small face on a skateboard. Give the skateboard a good push and the brick falls over !

Of course it falls over, since you accelerated the bottom part of the brick touching the skateboard first. You accelerated different parts of the brick differently with the resulting change in position you measured. But you did not measure acceleration itself.

 

[PAllen]

Maybe someone could define acceleration. To me, acceleration is the context of SR, is that which is responsible for changing which inertial frame of reference an object becomes at rest in.

Where did you get this idea from? Acceleration of a body in an inertial frame in SR is taken as derivative of 4-velocity by proper time.

I could not possibly define it to be the change in velocity over time (seen from the observer's point of view who is supposed to be accelerating), since in SR an observer considers himself at rest.

Again, where do you get this from. SR says only inertial observers consider themselves at rest. The standard SR equations only apply to inertial frames; observers at rest in these frames detect no acceleration (e.g. using an accelerometer).

So here i am, waiting for how you define acceleration in the context of SR seen from an accelerating observer's point of view.

SR typically does not deal with accelerated observers; certainly Einstein never defined accelerated observers for SR. It readily deals with accelerated motion in inertial frames.

It is possible to define coordinates representing an accelerated observer, in SR. If you do, acceleration is now absolute derivative of 4-velocity by proper time. This type of derivative involves what are called connection coefficients (which are zero in an inertial frame in SR). In these coordinates, the coordinate 3-velocity of the observer will be zero (its 4-velocity will not be zero - it will have a time component). However, because of the nature of the absolute derivative, the 4-acceleration will be nonzero, and the proper acceleration (given by the norm of the 4-acceleration) will be what is measured by an accelerometer.

 

[Jeronimus]

Where did you get this idea from?

It's a pretty simple idea. You accelerate(instantaneous to keep it simple) and as a result, someone who observes you accelerating will conclude that you are no longer at rest in your former inertial frame of reference, but you are at rest in another frame.

Switching the frame an object is at rest in, is a direct result of acceleration. (seen by an observer in another inertial frame of reference)

I could continue with what acceleration is a result of, but i'd rather not.

Again, where do you get this from. SR says only inertial observers consider themselves at rest. The standard SR equations only apply to inertial frames; observers at rest in these frames detect no acceleration (e.g. using an accelerometer).

But isn't accelerating an object, seen from an observer in another IFR at rest, a constant switch of IFRs the object is at rest in? I think it is.

 

[TrickyDicky]

For the record, if the worldline of a particle is parametrised by proper time as $x^\alpha(\tau)$, in any coordinate system in GR or SR, inertial or non-inertial, then the 4-velocity U and the 4-acceleration A are defined by
$$\begin{align} U^\alpha &= \frac{dx^\alpha}{d\tau} \\ A^\alpha &= \frac{dU^\alpha}{d\tau} + \Gamma^\alpha_{\beta \gamma} U^\beta U^\gamma \end{align}$$
Both U and A are 4-vectors, i.e. rank-1 tensors.

All of the above is true in both SR and GR, in inertial or non-inertial coordinates. Of course in Minkowski coordinates in SR, all the $\Gamma^\alpha_{\beta \gamma}$s are zero so the expression for 4-acceleration can be simplified.

I thought this was well-known stuff covered in every GR textbook but apparently not from the evidence of this thread.

Nope, you are wrong, I hoped someone would come up to correct it but apparently, and incredibly, no one did.
Have you tried reading Carroll notes for instance? Then you should realize that in GR the expression with christoffel corrections transforms as a tensor, not as a contravariant vector.

 

[Dale]

Maybe someone could define acceleration.

Again, your question is ambiguous in this context. You need to specify if you want a definition for coordinate acceleration or proper acceleration. Both are easy to define. Coordinate acceleration is the second time derivative of an object's position. Proper acceleration is the first covariant derivative of the object's tangent vector.

So here i am, waiting for how you define acceleration in the context of SR seen from an accelerating observer's point of view.

The above definitions hold for any point of view, inertial or not.

 

[TrickyDicky]

I'm sorry, but I that answer still isn't helping me make any sense of your post #85. In what way is the GR definition of proper acceleration different than the SR definition, so that "mixing the 4-acceleration in SR with its definition in GR" will lead to problems/errors?

See my answer to DrGreg.

 

[TrickyDicky]

When the rank isn't specified then it should be understood that the word "tensor" refers to a tensor of an arbitrary unspecified rank, including rank 0 and rank 1.

Then I guess for you a book entitled "Tensor calculus" should deal with real functions differential calculus and with vector calculus, right?

 

[stevendaryl]

Nope, you are wrong, I hoped someone would come up to correct it but apparently, and incredibly, no one did.
Have you tried reading Carroll notes for instance? Then you should realize that in GR the expression with christoffel corrections transforms as a tensor, not as a contravariant vector.

What are you talking about? Are you saying that Dr. Greg wrote down the wrong expressions for 4-velocity and 4-acceleration? Are you saying that he's wrong to call them 4-vectors? Are you saying that he's wrong to say that a 4-vector is a rank-1 tensor? Are you saying that he's wrong that this is "well-known stuff"? What do you mean wrong?

 

[PAllen]

It's a pretty simple idea. You accelerate(instantaneous to keep it simple) and as a result, someone who observes you accelerating will conclude that you are no longer at rest in your former inertial frame of reference, but you are at rest in another frame.

Switching the frame an object is at rest in, is a direct result of acceleration. (seen by an observer in another inertial frame of reference)

I could continue with what acceleration is a result of, but i'd rather not.

You made a statement about SR. As has been clarified multiple times, SR distinguishes inertial and non-inertial frames, and, as formulated by Einstein, dealt only with inertial frames. Also, FYI, pre-relativistic physics made the same distinction. Newton in Principia had extensive discussions of the absolute character of rotation and acceleration. SR did not change this (and GR, not really either).

But isn't accelerating an object, seen from an observer in another IFR at rest, a constant switch of IFRs the object is at rest in? I think it is.

There is a limited sense in which you can treat it this way. However, a derivative is has a value at a single point. If the comoving inertial frame changes, there is accelerated motion in each and every co-moving inertial frame involved.

 

[TrickyDicky]

What are you talking about? Are you saying that Dr. Greg wrote down the wrong expressions for 4-velocity and 4-acceleration? Are you saying that he's wrong to call them 4-vectors? Are you saying that he's wrong to say that a 4-vector is a rank-1 tensor? Are you saying that he's wrong that this is "well-known stuff"? What do you mean wrong?

Try reading what you quote.

 

[stevendaryl]

Try reading what you quote.

What you said made no sense, which is why I asked for clarification. Everything that Dr. Greg wrote was correct, so I have no idea what you are talking about when you call it wrong.

 

[Nugatory]

Nope, you are wrong, I hoped someone would come up to correct it but apparently, and incredibly, no one did.
Have you tried reading Carroll notes for instance? Then you should realize that in GR the expression with christoffel corrections transforms as a tensor, not as a contravariant vector.

I'm still having trouble understanding what you mean when you say this... How is "transforming as a tensor" different than "transforming as a contravariant vector"?

 

[stevendaryl]

Try reading what you quote.

Specifically, you wrote:

...in GR the expression with christoffel corrections transforms as a tensor, not as a contravariant vector.

That makes no sense, for several reasons. First, there is no such thing as the "GR expression". The expression is the same whether you are in GR or SR. The difference between the two is that the metric tensor is fixed in SR, but not in GR. But in both cases, you can define the connection coefficients and define 4-velocity and 4-acceleration, and the expressions are the same. As Dr. Greg points out, in SR it's possible to choose coordinates so that the connection coefficients are everywhere zero, but you don't have to use those coordinates, they're just simpler to work with.

The second reason what you wrote made no sense is that you said "the expression transforms as a tensor, not as a contravariant vector". That makes no sense because a vector is a special case of a tensor. If you want to say that a vector can't be a tensor, then 4-velocity is not a tensor, by that definition.

Don't tell me to reread your original post. If what you had written made sense to me, then I wouldn't have asked for clarification.

 

[stevendaryl]

I'm still having trouble understanding what you mean when you say this... How is "transforming as a tensor" different than "transforming as a contravariant vector"?

The answer is: there is no difference, TrickyDicky is confused on this point.

 

[stevendaryl]

The answer is: there is no difference, TrickyDicky is confused on this point.

There is a difference of opinion about what a vector or tensor, etc. is. Some people define these things in terms of matrices that transform in such and such a way under coordinate transformations. I don't like that definition. I prefer to know what a vector "is", rather than how it transforms. You can derive how it's components transform under coordinate changes if you know what a vector is.

The geometric way of defining things is this:

1. A parametrized path is a (continuous, differentiable, blah, blah) function $P(s)$ mapping real numbers to points in space (or spacetime).

2. A real scalar field is the reverse: a function $\Phi(p)$ mapping points in space (or spacetime) to real numbers.

3. A tangent vector is a linear approximation to a parametrized path. It characterizes the path locally. Technically, if $P(s)$ is a real scalar field, then we can identify the tangent vector $d/ds P(s)$ with the operator $v$ that acts on scalar fields as follows: $v(\Phi) = d/ds (\Phi(P(s))$

4. A cotangent vector is a linear approximation to a real scalar field. It characterizes the field locally. Technically, if $\Phi(X)$ is a real scalar field, then we can define a cotangent vector $\nabla \Phi$ to be that operator $w$ that acts on tangent vectors as follows: $w(v) = v(\Phi)$

5. Higher-level tensors are multilinear functions of tangent and cotangent vectors.

This way of defining things doesn't even mention coordinates or coordinate transformations, but is sufficient to deduce how vectors and tensors transform.

 

[harrylin]

[..] So here i am, waiting for how you define acceleration in the context of SR seen from an accelerating observer's point of view.

Hi Jeronimus,
In the context of SR, acceleration is defined relative to a to-be-specified inertial reference system (or, with "proper" acceleration: relative to a class of instantaneously co-moving inertial reference systems).
In equation: a=dv/dt. If an object accelerates relative to one inertial system, then it accelerates relative to all inertial systems.

But isn't accelerating an object, seen from an observer in another IFR at rest, a constant switch of IFRs the object is at rest in? I think it is.

Yes - however that is a complicated way of looking at it.

 

[stevendaryl]

Nope, you are wrong, I hoped someone would come up to correct it but apparently, and incredibly, no one did.
Have you tried reading Carroll notes for instance? Then you should realize that in GR the expression with christoffel corrections transforms as a tensor, not as a contravariant vector.

Here's what Sean Carroll says:

A straightforward generalization of vectors and dual vectors is the notion of a tensor...
From this point of view, a scalar is a type (0, 0) tensor, a vector is a type (1, 0) tensor, and
a dual vector is a type (0, 1) tensor.

 

[Dale]

Then I guess for you a book entitled "Tensor calculus" should deal with real functions differential calculus and with vector calculus, right?

Do you have an example of a book entitled "tensor calculus" that does not deal with rank-0 tensors (scalars) and rank-1 tensors (vectors)?

 

[TrickyDicky]

Do you have an example of a book entitled "tensor calculus" that does not deal with rank-0 tensors (scalars) and rank-1 tensors (vectors)?

Don't change what I said, I talked about calculus. They always use vectors(that always depend on a basis and therefore vector equations are depedent on the coordinate system used) in the first pages to build the concept of tensors (with the property that they are not basis-dependent and therefore tensor equations are valid on any coordinates, do you see now the difference?)
The first one I found in google books, by Synge and Child is a good example.

 

[TrickyDicky]

Here's what Sean Carroll says:

This is not about terminology, I have no problem with vectors being 1-tensors, this is a conceptual issue, instead of getting mad you could step back and try understanding what I'm saying.
Since you have Carroll notes, go to the part 3 about curvature, and see how he gets a (1,1) tensor from the covariant derivative of a contravariant vector: eq. 3.1 and following.

 

[PAllen]

Don't change what I said, I talked about calculus. They always use vectors(that always depend on a basis and therefore vector equations are depedent on the coordinate system used) in the first pages to build the concept of tensors (with the property that they are not basis-dependent and therefore tensor equations are valid on any coordinates, do you see now the difference?)
The first one I found in google books, by Synge and Child is a good example.

I have this book (the first one I used, long ago). They certainly say vectors and scalars are rank 1 and zero tensors. They use the same letter T for all cases:

T no super scripts - scalar; rank 0 tensor
T one superscript - vector (contravariant); rank 1 tensor
T more sub/superscripts - higher rank tensor.

It is instructive that they don't use, say V for vector and T for tensor.

But this is all silly. Everyone knows tensor is the general term, vector the special case, and there should be no argument unless people go looking for one.

 

[stevendaryl]

Don't change what I said, I talked about calculus. They always use vectors(that always depend on a basis and therefore vector equations are depedent on the coordinate system used) in the first pages to build the concept of tensors (with the property that they are not basis-dependent and therefore tensor equations are valid on any coordinates, do you see now the difference?)
The first one I found in google books, by Synge and Child is a good example.

I think you are confused. A vector is a special case of a tensor. A vector equation is "coordinate free" in exactly the same sense that a tensor equation is (since it is) a tensor equation.

An important distinction is between a vector and the n-tuple of components of the vector. The former is coordinate-independent, while the latter is coordinate-dependent.

A 4-vector $V$ can be written as a linear combination of basis vectors:

$V = \sum V^\alpha e_\alpha$

where $e_\alpha$ is the set of basis vectors.

The components $V^\alpha$ change when you change coordinates, and the basis vectors $e_\alpha$ change when you change coordinates, but the combination $V = \sum V^\alpha e_\alpha$ is the same in any coordinate system.

 

[stevendaryl]

This is not about terminology, I have no problem with vectors being 1-tensors, this is a conceptual issue, instead of getting mad you could step back and try understanding what I'm saying.

I need help from you doing that, because what you wrote makes no sense to me.

Since you have Carroll notes, go to the part 3 about curvature, and see how he gets a (1,1) tensor from the covariant derivative of a contravariant vector: eq. 3.1 and following.

I have equation 3.1 in front of me, and it doesn't help at all to clarify what you're talking about. He writes:

$\nabla_\mu V^\nu = \partial_\mu V^\nu + \Gamma^\nu_{\mu \lambda} V^\lambda$

What does that have to do with your statement

Then you should realize that in GR the expression with christoffel corrections transforms as a tensor, not as a contravariant vector.

We're actually talking about two different, though related, things. If $V$ is the velocity 4-vector, then you can define an operation producing another 4-vector, $A$ via:

$A^\nu = d/d\tau V^\nu + \Gamma^\nu_{\mu \lambda} V^\mu V^\lambda$

That produces a 4-vector. In contrast, if $V$ is a vector field then you can define an operation producing a [1,1] tensor via:

$\nabla_\mu V^\nu = \partial_\mu V^\nu + \Gamma^\nu_{\mu \lambda} V^\lambda$

Those are different, but related operations.

 

[Dale]

Don't change what I said

Take your own advice. Your tailspins like this are rather amusing to watch.

I will look at the book you mentioned, but I bet that it uses rank-1 and -0 tensors and probably even calls them vectors and scalars.

 

[stevendaryl]

$A^\nu = d/d\tau V^\nu + \Gamma^\nu_{\mu \lambda} V^\mu V^\lambda$

That produces a 4-vector. In contrast, if $V$ is a vector field then you can define an operation producing a [1,1] tensor via:

$\nabla_\mu V^\nu = \partial_\mu V^\nu + \Gamma^\nu_{\mu \lambda} V^\lambda$

Those are different, but related operations.

The relation is kind of subtle. If you have a 4-vector $V$ and you extend it to a vector field, then the relationship between the two is

$D_\tau V^\nu = (\nabla_\mu V^\nu) V^\mu$

where $D_\tau V^\nu = d/d\tau V^\nu + \Gamma^\nu_{\mu \lambda} V^\mu V^\lambda$

I don't know what the official term is for the operator $D_\tau$.

 

[PAllen]

The relation is kind of subtle. If you have a 4-vector $V$ and you extend it to a vector field, then the relationship between the two is

$D_\tau V^\nu = (\nabla_\mu V^\nu) V^\mu$

where $D_\tau V^\nu = d/d\tau V^\nu + \Gamma^\nu_{\mu \lambda} V^\mu V^\lambda$

I don't know what the official term is for the operator $D_\tau$.

Absolute derivative is what I've used.

 

[TrickyDicky]

But this is all silly. Everyone knows tensor is the general term, vector the special case, and there should be no argument unless people go looking for one.

Couldn't agree more, this is the silliest discussion, even more knowing I've never argued against this and has nothing to do with the real discussion involving DrGreg's post, stevendaryl and me.
But I have it well deserved for following certain person's favourite sport.

 

[TrickyDicky]

We're actually talking about two different, though related, things. If $V$ is the velocity 4-vector, then you can define an operation producing another 4-vector, $A$ via:

$A^\nu = d/d\tau V^\nu + \Gamma^\nu_{\mu \lambda} V^\mu V^\lambda$

That produces a 4-vector. In contrast, if $V$ is a vector field then you can define an operation producing a [1,1] tensor via:

$\nabla_\mu V^\nu = \partial_\mu V^\nu + \Gamma^\nu_{\mu \lambda} V^\lambda$

Those are different, but related operations.

Man, finally it dawns on you I was referring to a vector field, what else would you take the covariant derivative of in curved space (like GR's)?, that is the difference with SR, in flat spacetime the covariant derivative reduces to a simple directional derivative and you get another 4-vector.
As you see this has nothing to do with calling vectors 1-tensors as some have wanted to pollute the discussion with.
I just can't make out how so many people can misunderstand this when I was specifically speaking abou the difference SR/GR, and specifically talking about the "covariant derivative" of a vector field, we weren't at any moment talking about the tangent space of vector uniquely because at that space it makes no sense to talk about the difference SR/GR, and besides as I said covariant derivatives are always of vector fields, with the only caveat that in the flat special case it happens to be equivalent to a directional derivative (the Christoffel symbols vanish), and the result is another vector. But in the general manifold case the result is a order 2 tensor.

 

[Muphrid]

A directional covariant derivative of a vector field is still a vector field in GR, just as the directional derivative of a vector field is a vector field in SR.

What is a directional derivative in SR? Pick a vector $a^\mu$. The directional derivative of the vector field $V^\nu$ is $a^\mu \partial_\mu V^\nu$. Change $\partial$ to $\nabla$, and you have the correct analogue in GR. Both are still vector fields.

If instead you don't wish to consider directional derivatives, you can just look at $\partial_\mu V^\nu$ (in SR) and $\nabla_\mu V^\nu$ (in GR). Both are 2-index objects that marry the divergence and curl.

 

[PAllen]

Man, finally it dawns on you I was referring to a vector field, what else would you take the covariant derivative of in curved space (like GR's)?, that is the difference with SR, in flat spacetime the covariant derivative reduces to a simple directional derivative and you get another 4-vector.
As you see this has nothing to do with calling vectors 1-tensors as some have wanted to pollute the discussion with.
I just can't make out how so many people can misunderstand this when I was specifically speaking abou the difference SR/GR, and specifically talking about the "covariant derivative" of a vector field, we weren't at any moment talking about the tangent space of vector uniquely because at that space it makes no sense to talk about the difference SR/GR, and besides as I said covariant derivatives are always of vector fields, with the only caveat that in the flat special case it happens to be equivalent to a directional derivative (the Christoffel symbols vanish), and the result is another vector. But in the general manifold case the result is a order 2 tensor.

No, there are two separate operations:

1) absolute derivative along a curve; this is equally valid in curved and flat spacetime; in flat the connection components vanish. Absolute derivative of a contravariant vector along a curve is a contravariant vector (along the same curve). As I see it, this is defined and motivated without respect to (2) - or can be.

2) Covariant derivative. This makes a tensor of one higher rank than its operand. Covariant derivative of contravariant vector field produces rank two mixed tensor.

3) A derived fact is that if you extend a vector along a path to be a field (doesn't matter how), apply (2), then contract by the original vector field and restrict to the curve, you get the same result as (1).

I am of the school to treat (3) as an interesting consequence, but understand (1) as a separate operation, with no need to go through (2) and (3) to get there.

 

[TrickyDicky]

A directional covariant derivative of a vector field is still a vector field in GR, just as the directional derivative of a vector field is a vector field in SR.

What is a directional derivative in SR? Pick a vector $a^\mu$. The directional derivative of the vector field $V^\nu$ is $a^\mu \partial_\mu V^\nu$. Change $\partial$ to $\nabla$, and you have the correct analogue in GR. Both are still vector fields.

If instead you don't wish to consider directional derivatives, you can just look at $\partial_\mu V^\nu$ (in SR) and $\nabla_\mu V^\nu$ (in GR). Both are 2-index objects that marry the divergence and curl.

Directional covariant derivative?? That's a first, I only know the covariant derivative of a vector field, and in general manifolds it is a 2-index tensor. When the Christoffel coefficients can be made to vanish like in the flat case the covariant derivative reduces to directional derivative and gives another vector field.