I Is time really moving backwards?

[jbriggs444]

I didn't mean to imply global, I just meat it could be treated as a local rest frame, as it would be in fee fall.

OK, fair enough. Let me try to flesh out this intuition a bit so that I can follow along.

We have this rest frame (inertial coordinate system) that is anchored on an object moving along a geodesic. We have to keep the frame small to try to keep curvature effects to a minimum. So it is something of a "world tube" around the object.

Of course, this will only be an approximately inertial frame. If one moves away from the center of the tube, one sees that small particles deflect from the inertial trajectories that we would naively predict.

Yes, those deflections are from "gravity". And yes, we could explain those deflections as "fictitious forces".

So yes, we are in agreement.

 

[valenumr]

That is false, as I described previously. That is exactly what I was objecting to.

So you've lost me... How is it possible for an object in free fall to do anything other than continue upon its path without a force applied? This is supposed to be true even in Newtonian physics.

 

[Dale]

So you've lost me... How is it possible for an object in free fall to do anything other than continue upon its path without a force applied? This is supposed to be true even in Newtonian physics.

A force is applied. That is not the same as energy. Forces transfer momentum, not energy. Energy is transferred by power, not force.

 

[valenumr]

A force is applied. That is not the same as energy. Forces transfer momentum, not energy. Energy is transferred by power, not force.

It's really not possible to apply a force to an object without applying energy... This is a fundamental principal of physics, is it not?

 

[valenumr]

It's really not possible to apply a force to an object without applying energy... This is a fundamental principal of physics, is it not?

Well, to be fair... Transferring energy, which is consevered.

 

[jbriggs444]

It's really not possible to apply a force to an object without applying energy... This is a fundamental principal of physics, is it not?

The same force can provide energy, can provide no energy or can extract energy from a moving object. It all depends on the reference frame you choose. There is no invariant fact of the matter.

Take a sack of concrete in the back of pick-up truck that is accelerating westward after the light turns green. Does the force of friction provide energy to the sack (because the truck is moving west) or subtract energy (because the road is rotating east)?

 

[jbriggs444]

Well, to be fair... Transferring energy, which is consevered.

Conserved but not invariant.

 

[valenumr]

OK, fair enough.
The same force can provide energy, can provide no energy or can extract energy from a moving object. It all depends on the reference frame you choose. There is no invariant fact of the matter.

Take a sack of concrete in the back of pick-up truck that is accelerating westward after the light turns green. Does the force of friction provide energy to the sack (because the truck is moving west) or subtract energy (because the road is rotating east)?

Let's not be pedantic here. You know what I mean. Without any impetus to the body, it will continue on its path. If it deviates, it will increase in "velocity", "momentum", and "energy", the latter two are conserved, so they must come from somewhere.

 

[jbriggs444]

Let's not be pedantic here. You know what I mean. Without any impetus to the body, it will continue on its path. If it deviates, it will increase in "velocity", "momentum", and "energy", the latter two are conserved, so they must come from somewhere.

Let us please be pedantic here.

This is general relativity. This is complicated stuff. Tiny implicit assumptions can trip you up. Energy is not necessarily conserved or even well defined. If we are in a space-time where energy is conserved, deflection from a path due to a real force can provide kinetic energy, remove kinetic energy or be energy-neutral.

 

[valenumr]

Let us please be pedantic here.

This is general relativity. This is complicated stuff. Tiny implicit assumptions can trip you up.

I still don't see your argument. The energy of said system is still conserved in GR, is it not? If you can explain how a body in free fall could accelerate WRT it's local geodesic with no energy transferred to said body, I would be very willing to digest that.

 

[jbriggs444]

I still don't see your argument. The energy of said system is still conserved in GR, is it not? If you can explain how a body in free fall could accelerate WRT it's local geodesic with no energy transferred to said body, I would be very willing to digest that.

Pebble strikes lump of clay. Kinetic energy decreases.

If you are going to conserve energy, you need to close your system. The lump of clay is in the system.

There is another prejudice that you are imposing on the system -- you are trying to account for the energy of your object using a frame of reference that is tied to the object before the force (from God's hand) was applied. It would be equally valid to use a frame of reference that is tied to the object after the force was applied. Then one could argue that forces (from God's hand) always reduce kinetic energy.

 

[valenumr]

A force is applied. That is not the same as energy. Forces transfer momentum, not energy. Energy is transferred by power, not force.

Dale, would you agree that a change in momentum also results in an increase in energy, and that both are conserved? One cannot come without the other.

 

[valenumr]

Dale, would you agree that a change in momentum also results in an increase in energy, and that both are conserved? One cannot come without the other.

Edit: sorry, change in energy, not increase.

 

[A.T.]

would you agree that a change in momentum also results in an increase in energy,..

Not in uniform circular motion, for example.

 

[Ibix]

Dale, would you agree that a change in momentum also results in an increase in energy, and that both are conserved? One cannot come without the other.

No he wouldn't. Circular motion is an (already stated) example with an applied force that causes acceleration but no energy change.

 

[jbriggs444]

Dale, would you agree that a change in momentum also results in an increase in energy, and that both are conserved? One cannot come without the other.

It is well known that momentum can change without an accompanying change in energy.

For point-like particles with no internal degrees of freedom, energy indeed cannot change without there also being an accompanying change in momentum.

I think, however, that you are angling for the special case of a [point-like] particle at rest at the origin and wanting confirmation that there can then be no change in either energy or momentum without an accompanying change in the other.

Yes, if the system is closed then both energy and momentum are conserved.

 

[Dale]

would you agree that a change in momentum also results in an increase in energy

No, I already gave a counter example: uniform circular motion.

both are conserved?

Yes, both are conserved locally

 

[valenumr]

No he wouldn't. Circular motion is an (already stated) example with an applied force that causes acceleration but no energy chan

No he wouldn't. Circular motion is an (already stated) example with an applied force that causes acceleration but no energy change.

In what sense? Angular momentum? If we are talking gravitational circular motion, than there is no "force" per se. If you are spinning a sling around, it definitely has increased energy.

 

[Dale]

If you are spinning a sling around, it definitely has increased energy.

Not if you are spinning it in uniform circular motion.

 

[jbriggs444]

In what sense? Angular momentum? If we are talking gravitational circular motion, than there is no "force" per se.

With Newtonian gravity there is a force.

With gravity in General Relativity, the trajectory is only circular in space, not in space-time. But yes, under this model, there is no force.

If you are spinning a sling around, it definitely has increased energy.

An ideal (no air resistance) sling spinning at a constant rate has constant energy despite the presence of a continuing force.

 

[valenumr]

With Newtonian gravity there is a force.

With gravity in General Relativity, the trajectory is only circular in space, not in space-time. But yes, under this model, there is no force.

An ideal (no air resistance) sling spinning at a constant rate has constant energy.

Ok, I'm understanding two disconnects here. First, energy is more of a scalar and momentum is more of a vector.

But second, you are missing my point that an obect in free fall can do nothing other than continue upon its path without an application of energy.

The sling doesn't start spinning at a constant velocity from zero without energy applied, and and planet orbiting a start will continue to do so indefinitely without external forces.

And energy and momentum are measures only relative to observers. The only thing any external observers can agree upon is mass.

 

[jbriggs444]

Ok, I'm understanding two disconnects here. First, energy is more of a scalar and momentum is more of a vector.

Yes. Nicely noted.

But second, you are missing my point that an obect in free fall can do nothing other than continue upon its path without an application of energy.

But this is wrong. Repeating does not make it less so. A moving object in free fall can be deflected by a sideways force without a change in speed.

The sling doesn't start spinning at a constant velocity from zero without energy applied

As I suspected, you want to consider the special case of an object initially at rest that is then subject to a force from God's hand.

 

[valenumr]

Yes. Nicely noted.

But this is wrong. Repeating does not make it less so. A moving object in free fall can be deflected by a sideways force.

As I suspected, you want to consider the special case of an object initially at rest that is then subject to a force from God's hand.

And I will question how you get to the second point without any energy.

 

[Dale]

you are missing my point that an obect in free fall can do nothing other than continue upon its path without an application of energy

We are not missing the point. We are explicitly stating that it is wrong.

The sling doesn't start spinning at a constant velocity from zero without energy applied

It can start spinning from a non-zero inertial velocity without energy applied. And there is always an infinite number of frames where it starts with non-zero velocity.

 

[jbriggs444]

And I will question how you get to the second point without any energy.

With the flick of a pencil. Reference frames are arbitrary. As is energy. Remember it is not invariant.

Edit: From the usual perspective, it is improper to question how the initial conditions of a thought experiment were created. We take them as given. The important part is what happens while the thought experiment is being carried out.

 

[valenumr]

We are not missing the point. We are explicitly stating that it is wrong.

It can start spinning from a non-zero inertial velocity without energy applied. And there is always an infinite number of frames where it starts with non-zero velocity.

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

 

[jbriggs444]

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

You are going to experimentally observe the non-existence of an infinite number of reference frames even though "frames of reference" are abstract mental constructs with no physical existence?

Perhaps we should step back and consider the question: What is a frame of reference?

Me, I usually settle for "coordinate system". @Dale, probably goes for something like "tetrad field".

 

[valenumr]

You are going to experimentally observe the non-existence of an infinite number of reference frames even though "frames of reference" are mental constructs with no physical existence?

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.

 

[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.

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?

 

[valenumr]

You are going to experimentally observe the non-existence of an infinite number of reference frames even though "frames of reference" are mental constructs with no physical existence?

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

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?

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