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### Author Topic: What is a tensor?  (Read 2534 times)

#### Arthur Geddes

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##### What is a tensor?
« on: 25/04/2016 19:42:03 »
Could someone provide a practical description of tensors of first & second order, etc.?  Trying to get this relativity thing (A.R., Absolute Relativity is looking good) & i'm hitting a roadblock with my limited understanding of differential linear algebra.

#### PmbPhy

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##### Re: What is a tensor?
« Reply #1 on: 25/04/2016 23:40:37 »
Could someone provide a practical description of tensors of first & second order, etc.?  Trying to get this relativity thing (A.R., Absolute Relativity is looking good) & i'm hitting a roadblock with my limited understanding of differential linear algebra.

A tensor is typically defined in two equivalent ways. One way is what I refer to as the geometric definition. The second way is what I refer to as the analytic definition. I created two webpages to explain these in detail for each viewpoint.

Geometric definition: A tensor is defined as a multi-linear map from vectors and 1-forms to the set of real numbers. See:
www.newenglandphysics.org/physics_world/math_phy/05_tensors_via_geometric/tensor_via_geometric.htm

Analytic definition: An equivalent way of defining a tensor, one commonly used in relativity, is a set of numbers which transform from one system of coordinates to another by a certain transformation rule. See:
http://www.newenglandphysics.org/physics_world/math_phy/04_tensors_via_analytic/tensors_via_analytic.htm

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#### Atomic-S

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##### Re: What is a tensor?
« Reply #2 on: 26/04/2016 07:46:40 »
A point worth keeping in mind is that a distinction is to be made between a mathematical value and a function whose domain is a set of such mathematical values and whose range is another set of such (or perhaps a different type of) mathematical values, the function expressing the mapping between one and the other. We may thus speak of the velocity of an object as a vector, and we may loosely speak of the windspeed as a vector, but to be semantically precise, the latter should be spoken of as a vector function of position because it does not necessarily have a single value, but a whole set of values varying from place to place. In some equations, this distinction is assumed and may not be immediately obvious to the uninitiated. The letter V could be used for either, but the significance is not the same. To be more precise, the expression for the windspeed case would be V(x, y, z) , or you could equivalently write V(r) with r being position expressed vectorially with respect to some reference point.

#### Atomic-S

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##### Re: What is a tensor?
« Reply #3 on: 26/04/2016 08:31:58 »

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#### PmbPhy

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##### Re: What is a tensor?
« Reply #4 on: 26/04/2016 08:34:02 »
Quote from: Atomic-S
A point worth keeping in mind is that a distinction is to be made between a mathematical value and a function whose domain is a set of such mathematical values and whose range is another set of such (or perhaps a different type of) mathematical values, the function expressing the mapping between one and the other.
What does this have to do with the definition of tensors? This appears to be off topic to me. I

Suggestion: If you had any point that was on topic then you should have mentioned that vectors are examples of tensors, i.e. a vector is a tensor of rank 1. It'd be a good idea to keep this in mind for the future. Otherwise people might take this as an opportunity to drag the subject way off topic.

Quote from: Atomic-S
We may thus speak of the velocity of an object as a vector, and we may loosely speak of the windspeed as a vector, but to be semantically precise, the latter should be spoken of as a vector function of position because it does not necessarily have a single value, but a whole set of values varying from place to place.
The velocity of a particle is a vector whose value depends on the position of the particle along its trajectory where the position is in turn a function of time. I.e. V = V(r(t)). In this case the velocity is a function of time whereas wind-speed is a vector field which depends on both position and time, i.e. V = V(r, t).

Note: A vector is a tensor of rank 1 and a scalar is a tensor of rank 0. The term scalar as its used in physics and tensor analysis is a real valued function, possibly of position, whose value remains unchanged upon a change in coordinates.

More on vectors since they're examples of tensors: There are two kinds of vectors (1) free vectors and (2) bound vectors. The former refers to vectors with fixed initial and terminal point whereas the later only the magnitude and direction completely determine the vector. For details please see: https://en.wikipedia.org/wiki/Euclidean_vector

An example of a second rank tensor is the Newtonian tidal force tensor. See: http://www.newenglandphysics.org/physics_world/mech/07_tidal_force_tensor/tidal_force_tensor.htm

This tensor determines the forces on an object in a non-uniform gravitational field.
« Last Edit: 26/04/2016 08:38:49 by PmbPhy »

#### Arthur Geddes

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##### Re: What is a tensor?
« Reply #5 on: 26/04/2016 16:07:16 »
Quote from: Atomic-S
A point worth keeping in mind is that a distinction is to be made between a mathematical value and a function whose domain is a set of such mathematical values and whose range is another set of such (or perhaps a different type of) mathematical values, the function expressing the mapping between one and the other.
What does this have to do with the definition of tensors?

The point of the thread is to further understanding of gravity as understood by G.R..  Atomic-S' reply is inline with my train of thought; i was gonna mention.  Gravity is a 'acceleration field/spacetime shape resolution' issue; i think we're gonna get to "metric" & variance issues soon, if this goes well.

I'm eyeing an interpretation of me on a non-spinning planet in the middle of nowhere; in spherical coordinates & considering also the G field from a massless point in the vicinity.

Anyone wanna jump into that?

I have two questions wrt equivalence: if i'm accelerating at 9.8 m/s2, why am i not going the speed of light?  I'm old enough, to be sure.  And; if i'm experiencing a force but *not* accelerating; am i traveling in a circle in some sense?

& remember, i'm on a non-spinning (apparently earth-sized) planet in free space.

#### Arthur Geddes

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##### Re: What is a tensor?
« Reply #6 on: 26/04/2016 18:57:39 »
An example of a second rank tensor is the Newtonian tidal force tensor. See: http://www.newenglandphysics.org/physics_world/mech/07_tidal_force_tensor/tidal_force_tensor.htm

This tensor determines the forces on an object in a non-uniform gravitational field.
So, the tidal force tensor describes the local spacetime (cuz it's about the 'metric?' or just space?) relative to a massless test point in free fall?  .. Kinda by definition cuz relativity just says this is transposable to other reference frames (i.e. a tensor?)

How does this relate to geodesics?  & if i have rocket boots holding me stationary in that potential field, what's happening to that acceleration?

#### Arthur Geddes

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##### Re: What is a tensor?
« Reply #7 on: 26/04/2016 19:01:09 »
if i have rocket boots holding me stationary in that potential field, what's happening to that acceleration?
Maintaining my potential energy...  Hmmm.. potential energy release is entropic relief....  The more intense the field, the quicker the release... & time goes slower...  is there a constant in there somewhere?

#### jeffreyH

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##### Re: What is a tensor?
« Reply #8 on: 26/04/2016 19:29:22 »
To use your rocket boots you are simply countering the instantaneous velocity at your altitude with an input of constant energy. Thus you are creating an artificially engineered 'inertial' frame of reference. However, this is still an accelerating frame of reference.

#### Arthur Geddes

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##### Re: What is a tensor?
« Reply #9 on: 27/04/2016 03:36:28 »
To use your rocket boots you are simply countering the instantaneous velocity at your altitude with an input of constant energy. Thus you are creating an artificially engineered 'inertial' frame of reference. However, this is still an accelerating frame of reference.
Yeah; i'm asking how that presents mathematically.  If free fall is a straight line on a geodesic(?); stationary in a potential gradient must be a non-straight line in some respect... mustn't it?

#### Atomic-S

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##### Re: What is a tensor?
« Reply #10 on: 27/04/2016 06:51:55 »
Quote
Yeah; i'm asking how that presents mathematically.  If free fall is a straight line on a geodesic(?); stationary in a potential gradient must be a non-straight line in some respect... mustn't it?
I believe you are correct about that. The worldline of someone standing "stationary" on the Earth's surface is not straight in spacetime, because spacetime is bent.

#### Atomic-S

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##### Re: What is a tensor?
« Reply #11 on: 27/04/2016 06:57:14 »
Quote
I have two questions wrt equivalence: if i'm accelerating at 9.8 m/s2, why am i not going the speed of light?  I'm old enough, to be sure.  And; if i'm experiencing a force but *not* accelerating; am i traveling in a circle in some sense?
I believe why you are still where you are rather than going at the speed of light is because while  you are accelerating at 9.8 m/s2 away from the Earth's center (note that the force you feel is equivalent to that generated by an outward, not inward, acceleration), spacetime is accelerating (as defined by the geometry of its distortion as it manifests itself from one time to the next) inward and a counterbalancing rate.

#### Atomic-S

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##### Re: What is a tensor?
« Reply #12 on: 27/04/2016 07:12:04 »
Quote
How does this relate to geodesics?  & if i have rocket boots holding me stationary in that potential field, what's happening to that acceleration?
It is being eaten up by spacetime bending in the other direction.

#### evan_au

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##### Re: What is a tensor?
« Reply #13 on: 27/04/2016 11:30:56 »
Quote from: Arthur Geddes
if i'm accelerating at 9.8 m/s2, why am i not going the speed of light?  I'm old enough, to be sure.
If we apply some physics from early high school, you would expect your velocity to be:
v = at    where:
v: velocity at time t
a: acceleration (9.8m/s2 in this example)
t: elapsed time

According to this formula, to reach the speed of light v=c=3x108 m/s would take t=3x108/9.8 = 3x107 seconds = 347 days, or just under a year.

However (and it's big however!), Einstein showed that you can never accelerate a massive object to reach the speed of light - no matter how hard you accelerate, or for how long.

These concepts normally come up at the end of high school or early university physics.

We don't notice it with our daily lives here on Earth, but when you accelerate objects close to the speed of light, strange things occur: time slows down, and mass increases, so it gets harder and harder to get closer to the speed of light. Oddly, this is not visible to the person being accelerated.

Such peculiarities are very familiar to physicists at the LHC, who try to get particles traveling as close as possible to the speed of light so they can do experiments at very high energies.

See: https://en.wikipedia.org/wiki/Speed_of_light#Upper_limit_on_speeds

So you may feel old, but you are not nearly old enough to reach the speed of light, when accelerating at 9.8m/s2!

#### Arthur Geddes

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##### Re: What is a tensor?
« Reply #14 on: 27/04/2016 17:16:43 »
It is being eaten up by spacetime bending in the other direction.
This is what i intuit but; this is the first i've heard it from someone else.

Spacetime *is* the aether &, deviations from flat *is* acceleration field.  But, no circle &, what about time?  I intuit eating space & spewing time to conserve 4D volume; just cuz.

Woo hoo!

#### Arthur Geddes

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##### Re: What is a tensor?
« Reply #15 on: 27/04/2016 17:23:40 »
However (and it's big however!), Einstein showed that you can never accelerate a massive object to reach the speed of light - no matter how hard you accelerate, or for how long.
I don't believe this is a relativistic case.

You're suggesting i'm radiating & less than a year old?

#### PmbPhy

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##### Re: What is a tensor?
« Reply #16 on: 01/05/2016 20:56:18 »
Quote from: Arthur Geddes
The point of the thread is to further understanding of gravity as understood by G.R..
Oh! Okay. Thanks for mentioning that. Had you not then I'd never have known since nobody mentioned gravity until I did in the last post. I'm curious. Why didn't you explain that in your opening post? How were we to know that the point of this thread is as you just said? Please understand that I'm not trying to be snide whatsoever. I'm merely trying to understand if I missed something that was obvious to others and not to me.

Quote from: Arthur Geddes
Atomic-S' reply is inline with my train of thought; i was gonna mention.
I'm confused here my dear sir. Please explain how so? I only ask because I can't see how its related since its almost completely unrelated to tensor analysis. I asked Atomic-S What does this have to do with the definition of tensors? but he chose not to answer my question. Perhaps if you describe the train of thought that you mentioned above then I'll understand. Thanks.

Quote from: Arthur Geddes
Gravity is a 'acceleration field/spacetime shape resolution' issue; ..
Gravity is the reason why objects, free of all forces other than gravity, fall at a rate which is independent of the mass of the object. That is precisely what Einstein meant when he spoke of gravity. What you referred to, i.e. spacetime shape, are actually tidal gradients in the gravitational field. In the language of general relativity, spacetime curvature.

#### PmbPhy

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##### Re: What is a tensor?
« Reply #17 on: 01/05/2016 21:06:40 »
Quote from: Arthur Geddes
Yeah; i'm asking how that presents mathematically.  If free fall is a straight line on a geodesic(?); stationary in a potential gradient must be a non-straight line in some respect... mustn't it?
What do you mean by a straight line on a geodesic? A geodesic is defined here: https://en.wikipedia.org/wiki/Geodesic

There are several equal ways to define geodesics. They are
1) The straightest possible line in a curved space.
2) A path of extremal length.
3) The path, which is identical to zero acceleration of a point.

I provide a concise definition of those in my webpage on geodesics. See:
http://www.newenglandphysics.org/physics_world/math_phy/03_geodesics/geodesics.htm

However if you're in a gravitational field then the spatial trajectory is not a straight line in space.

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#### Arthur Geddes

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##### Re: What is a tensor?
« Reply #18 on: 02/05/2016 03:22:53 »
I'm merely trying to understand if I missed something that was obvious to others and not to me.

From the original post: "Trying to get this relativity thing..."

Quote

Quote from: Arthur Geddes
Atomic-S' reply is inline with my train of thought; i was gonna mention.
I'm confused here my dear sir. Please explain how so? I only ask because I can't see how its related since its almost completely unrelated to tensor analysis.

It's complicated; don't worry about it.

Quote
I asked Atomic-S What does this have to do with the definition of tensors? but he chose not to answer my question. Perhaps if you describe the train of thought that you mentioned above then I'll understand. Thanks.

It has to do with a particular type of tensor in a particular application to describe the nature of "relatively straight lines."

Quote

Quote from: Arthur Geddes
Gravity is a 'acceleration field/spacetime shape resolution' issue; ..
...actually tidal gradients in the gravitational field. In the language of general relativity, spacetime curvature.
Right; different languages; one including relative shapes.

#### jeffreyH

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##### Re: What is a tensor?
« Reply #19 on: 02/05/2016 09:23:41 »
Quote from: Arthur Geddes
if i'm accelerating at 9.8 m/s2, why am i not going the speed of light?  I'm old enough, to be sure.
If we apply some physics from early high school, you would expect your velocity to be:
v = at    where:
v: velocity at time t
a: acceleration (9.8m/s2 in this example)
t: elapsed time

According to this formula, to reach the speed of light v=c=3x108 m/s would take t=3x108/9.8 = 3x107 seconds = 347 days, or just under a year.

However (and it's big however!), Einstein showed that you can never accelerate a massive object to reach the speed of light - no matter how hard you accelerate, or for how long.

These concepts normally come up at the end of high school or early university physics.

We don't notice it with our daily lives here on Earth, but when you accelerate objects close to the speed of light, strange things occur: time slows down, and mass increases, so it gets harder and harder to get closer to the speed of light. Oddly, this is not visible to the person being accelerated.

Such peculiarities are very familiar to physicists at the LHC, who try to get particles traveling as close as possible to the speed of light so they can do experiments at very high energies.

See: https://en.wikipedia.org/wiki/Speed_of_light#Upper_limit_on_speeds

So you may feel old, but you are not nearly old enough to reach the speed of light, when accelerating at 9.8m/s2!

The equivalence of an accelerating frame of reference and a gravitational field is highlighted by Evans post. In less than one year of acceleration we reach a point equivalent to approaching the event horizon of a black hole where the object is approaching light speed. If we cannot reach light speed that begs the question can gravity force an object to reach light speed at the event horizon of a black hole.

#### PmbPhy

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##### Re: What is a tensor?
« Reply #20 on: 03/05/2016 22:51:02 »
Quote from: Arthur Geddes
From the original post: "Trying to get this relativity thing..."
Quote
Ahhh! I understand now. For some reason I thought you only had special relativity (SR) in mind because people almost always study SR before they study general relativity (GR). Thanks for making that clear. :)

#### evan_au

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##### Re: What is a tensor?
« Reply #21 on: 04/05/2016 12:46:28 »
Quote from: JeffreyH
can gravity force an object to reach light speed at the event horizon of a black hole?
For a "distant observer", an object can reach just over c/3 before it disappears inside the event horizon.

If you scroll down to the graphs and equations on the following web page, you will see that the answer is "yes and no", all depending on your frame of reference.
http://physics.stackexchange.com/questions/170502/will-an-object-always-fall-at-an-infinite-speed-in-a-black-hole

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#### jeffreyH

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##### Re: What is a tensor?
« Reply #22 on: 04/05/2016 18:03:26 »
Quote from: JeffreyH
can gravity force an object to reach light speed at the event horizon of a black hole?
For a "distant observer", an object can reach just over c/3 before it disappears inside the event horizon.

If you scroll down to the graphs and equations on the following web page, you will see that the answer is "yes and no", all depending on your frame of reference.
http://physics.stackexchange.com/questions/170502/will-an-object-always-fall-at-an-infinite-speed-in-a-black-hole

Quite a while back now I did produce a set of graphs much like those of John Rennie in the linked thread. I was never convinced by some of my own conclusions. I may revisit this with a new perspective.

#### Arthur Geddes

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##### Re: What is a tensor?
« Reply #23 on: 04/05/2016 18:52:14 »
"Apparent horizon" compelled me to look up "Photon sphere:" https://en.wikipedia.org/wiki/Photon_sphere & i wonder if these are essentially the same thing to a falling observer...?

Note "ds=o (a light-like interval)", i.e. relative to the photon.

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##### Re: What is a tensor?
« Reply #23 on: 04/05/2016 18:52:14 »