I Is time really moving backwards?

[jbriggs444]

Okay, so I suppose we could imagine a really twisted space-time where a body is rotated by shear forces. But does that not still imply an energy transfer? I mean, you've completely given up the concept of conservation of energy and the conservation of momentum, fundamental physicals principals, in all your arguments. I don't understand why that is

I would say any non-accelerating observer to keep it simple, one in free fall in the GR sense, free from external forces.

That is wrong. A frame of reference is not an observer. There is more to it than that. Details. We often use an "observer" as a metaphor for a frame of reference -- the coordinate system in which that observer is at rest. In flat space-time, identifying an inertial observer is adequate to identify a unique coordinate system (up to rotation) in which that observer is at rest at the origin.

However, there is no rule that a frame of reference needs to have a physical observer at rest at its origin.

 

[valenumr]

As for the twisted space-time, nope. I just consider initial conditions for a thought experiment separate from the running of the thought experiment.

Energy transfer? From what to what? You've not yet closed your system.

Conservation of energy and conservation of momentum? Those are intact. As long as we are dealing with closed systems [and space-times that are friendly].

Can you slow down and explain what you think a frame of reference is and why we cannot use more than one to describe the same physical system?

What do you mean from what to what? A gravitational field is generated by a mass, the energy is extracted from that field, thus reducing the energy of said mass.

 

[Dale]

I'm pretty confident that there are observations that can be made to contradict this claim.

I am pretty confident that there are not.

I mean, you've completely given up the concept of conservation of energy and the conservation of momentum, fundamental physicals principals, in all your arguments. I don't understand why that is

We have not. But what you still fail to recognize is that forces transfer momentum not energy. Power transfers energy. It is possible to have a non-zero force and a zero power.

 

[valenumr]

OK, fair enough.
That is wrong. A frame of reference is not an observer. There is more to it than that. Details. We often use an "observer" as a metaphor for a frame of reference -- the coordinate system in which that observer is at rest.

Sorry, "observer". I'm pretty sure when I said an observer uninfluenced by outside force we were in agreement.

 

[jbriggs444]

What do you mean from what to what? A gravitational field is generated by a mass, the energy is extracted from that field, thus reducing the energy of said mass.

You spoke of an energy transfer. I asked for clarification -- transfer from what to what.

I am having trouble deciphering what you mean by having the gravitational field subtract from the energy of the object whose mass is responsible for that field. This is supposed to have something to do with maintaining a satellite in orbit about that mass?

 

[jbriggs444]

Sorry, "observer". I'm pretty sure when I said an observer uninfluenced by outside force we were in agreement.

I was extending the benefit of the doubt. We agree that an observer on a geodesic path can define a coordinate system that is approximately inertial over some local region of space-time.

If you want to go from that to talk about this object now being in orbit under the effects of gravity then the local region is necessarily too large for "approximately inertial" to be even remotely correct.

 

[valenumr]

You spoke of an energy transfer. I asked for clarification -- transfer from what to what.

I am having trouble deciphering what you mean by having the gravitational field subtract from the energy of the object whose mass is responsible for that field. This is supposed to have something to do with maintaining a satellite in orbit about that mass?

Satellites in orbit do in fact extract energy from the planet's rotational velocity. But I don't understand your misunderstanding.

 

[jbriggs444]

Satellites in orbit do in fact extract energy from the planet's rotational velocity. But I don't understand your misunderstanding.

They can, yes, through tidal effects. They can also inject energy into the planet's rotation in the same way. They can radiate energy gravitationally. These are minor effects and not what I thought we were discussing.

 

[valenumr]

They can, yes, through tidal effects. They can also radiate energy gravitationally. These are minor effects and not what I thought we were discussing.

I'll just go back to my original point. A physical body will only deviate from the geodesic if energy is applied. That's all I said. It's not a difficult concept.

 

[jbriggs444]

I'll just go back to my original point. A physical body will only deviate from the geodesic if energy is applied. That's all I said. It's not a difficult concept.

But it is still wrong.

 

[valenumr]

I'll just go back to my original point. A physical body will only deviate from the geodesic if energy is applied. That's all I said. It's not a difficult concept.

To be fair, I may have claimed that it should be observable as to which body had energy applied to it due to the deviation, but the two are intertwined.

 

[jbriggs444]

To be fair, I may have claimed that it should be observable as to which body had energy applied to it due to the deviation, but the two are intertwined.

Bullet and gun. Pebble and blob of putty. The deviation from the geodesic may be either [kinetic] energy positive or [kinetic] energy negative.

 

[valenumr]

But it is still wrong.

Aaaaand were back to square one. If you want to throw out GR and fundamental laws of physics that transcend even that... Well..

 

[jbriggs444]

Aaaaand were back to square one. If you want to throw out GR and fundamental laws of physics that transcend even that... Well..

We could step back to Newtonian mechanics. Things are simpler there.

 

[Dale]

A physical body will only deviate from the geodesic if energy is applied. That's all I said. It's not a difficult concept.

It is not difficult but it is wrong. Uniform circular motion remains an obvious counterexample

 

[valenumr]

We could step back to Newtonian mechanics. Things are simpler there.

We could step back to Newtonian mechanics. Things are simpler there.

You do realize that "every action has an equal and opposite reaction" is still a fundamental tenet of physics, right?

 

[Dale]

Bullet and gun. Pebble and blob of putty. The deviation from the geodesic may be either [kinetic] energy positive or [kinetic] energy negative.

Or neutral.

 

[valenumr]

It is not difficult but it is wrong. Uniform circular motion remains an obvious counterexample

I'm really going to need a better explanation, because uniform is a non-deviating path along the geodesic (only there is no such thing, but elliptical orbits).

 

[jbriggs444]

It is not difficult but it is wrong. Uniform circular motion remains an obvious counterexample

To flesh this out a bit, suppose that we have two billiard balls connected by a thread spinning around each other. They are in orbit about the Earth. Their center of mass traces out an approximate geodesic. The two balls individually are each subject to a force. They each deviate significantly from a geodesic path.

Ordinarily, we would not say that much energy transfer is taking place. In particular, in a frame of reference anchored to the center of mass, the energy transfer rate is exceedingly near zero.

 

[Dale]

Aaaaand were back to square one. If you want to throw out GR and fundamental laws of physics that transcend even that... Well..

Nobody is throwing out any laws of physics, we are simply addressing a misunderstanding on your part.

Do you have a bit of calculus background? Can I write simple differential equations and you will be able to follow?

 

[valenumr]

To flesh this out a bit, we have two billiard balls connected by a thread spinning around each other. They are in orbit about the Earth. Their center of mass traces out an approximate geodesic. The two balls individually are each subject to a force. They each deviate significantly from a geodesic path.

Nobody is throwing out any laws of physics, we are simply addressing a misunderstanding on your part.

Do you have a bit of calculus background? Can I write simple differential equations and you will be able to follow?

Yes. It's been a while since I've exercised those skills, but I've had career experience in mathematical modelling, so it's been applied.

 

[valenumr]

Or neutral.

So you both claim that a fundamental principal of GR, an "observer" in "free fall" along a geodesic can deviate from that path without any external force applied? Ok.

 

[weirdoguy]

You do realize that "every action has an equal and opposite reaction" is still a fundamental tenet of physics, right?

No, it is not fundamental, because it's not always true, especially in relativity. Consider two positive charges moving along perpendicular straigt lines. Consider magnetic forces that they act on each other. Those "action and reaction" forces are perpendicular, hence not opposite.

 

[jbriggs444]

So you both claim that a fundamental principal of GR, an "observer" in "free fall" along a geodesic can deviate from that path without any external force applied? Ok.

No. Our claim is different. It is that an object may deviate from a geodesic path without a change in energy.

The proviso is that the frame of reference used to evaluate the energy change must not be one in which the object is initially at rest.

That then leads to the discussion of "what is a reference frame". One could also be drawn into a discussion of "what is energy in the context of General Relativity and is it conserved"?

 

[valenumr]

No, it is not fundamental, because it's not always true, especially in relativity. Consider two positive charges moving along perpendicular straigt lines. Consider magnetic forces that they act on each other. Those "action and reaction" forces are perpendicular, hence not opposite.

It is commentary on conservation of momentum and energy. I'm pretty most folks would agree that those are fundamental principles rooted in Newtons laws. Even qft still respects those laws of conservation.

 

[Dale]

Ok, so first some definitions $\vec p$ is momentum and $\vec F$ is force. Force and momentum are related by $\vec F = \frac{d}{dt}\vec p$ so a force is a change in momentum. Note in particular that both force and momentum are vectors.

Now, energy is a scalar $E$ so it is impossible that $\vec F = \frac{d}{dt}E$ because the left hand side is a vector and the right hand side is a scalar. So we will define a new scalar $P$ called power such that $P= \frac{d}{dt}E$.

What we would like to do is to determine $P$ in terms of $\vec F$. Then $\vec F$ (a vector) will give us the change in $\vec p$ (a vector), and $P$ (a scalar) will give us the change in $E$ (a scalar).

The proof can be found elsewhere, here I will simply assert the result $P=\vec F \cdot \vec v$ where $\vec v$ is the velocity. This is a scalar, like it should be so $\frac{d}{dt}E=\vec F \cdot \vec v$

Now, the part that specifically contradicts your assertion is that it is possible for $\vec F \cdot \vec v =0$ even if $\vec F \ne 0$. This happens if $\vec v$ is perpendicular to $\vec F$ (as in the case of uniform circular motion) or if $\vec v=0$. Hence a force can change momentum without changing energy.

 

[valenumr]

No. Our claim is different. It is that an object may deviate from a geodesic path without a change in energy.

The proviso is that the frame of reference used to evaluate the energy change must not be one in which the object is initially at rest.

That then leads to the discussion of "what is a reference frame".

I've said previously that energy and momentum are observables dependent upon the observer wrt to the observed. But the fact of the matter is deviations from the geodesic can be distinguished by experiment, by all observers. This is acceleration.

 

[jbriggs444]

I've said previously that energy and momentum are observables dependent upon the observer wrt to the observed. But the fact of the matter is deviations from the geodesic can be distinguished by experiment, by all observers. This is acceleration.

Proper acceleration is an invariant. Kinetic energy is not.

 

[valenumr]

OK, fair enough.

Proper acceleration is an invariant. Kinetic energy is not.

See, now we are agreeing. I have said that energy and momentum can be disagreed among observers, but mass is invariant. But. BUT. Acceleration (deviation from the geodesic) is observable and measurable, thus it is possible to tell where energy has been applied. Do you want an illustrative example?

 

[jbriggs444]

See, now we are agreeing. I have said that energy and momentum can be disagreed among observers, but mass is invariant. But. BUT. Acceleration (deviation from the geodesic) is observable and measurable, thus it is possible to tell where energy has been applied. Do you want an illustrative example?

Your claim is not correct in general. In particular, a claim that invariant acceleration means invariant energy transfer is suspect.

Note that there are certainly scenarios where one can find an energy invariant in spite of energy being frame-relative in general. For instance, the energy lost to friction when a bag of cement slides across the bed of a pick-up truck. Despite the kinetic energy of both bag and pick-up truck being frame-relative, the energy lost to friction is invariant. [As I recall, @Dale and I discussed this years ago and he had decided that this scenario led to something invariant even in the context of special relativity]

An illustrative example would be helpful to work out our disagreement, yes.