Potential energy and GR, something doesn't add up

In summary: The mass at 1Km above the Earth begins to fall because it has an increased gravitational pull. The energy that is added to the system is the kinetic energy of the falling mass.
  • #1
tbitz
9
0
Here is a thought experiment that doesn't seem to add up.

Suppose you have a unit of mass at rest on the surface of Earth that spontaneously transforms to photos all moving radially outward into space. At some distance (say 1Km) the photons revert back to the original mass but are now 1Km above the surface of the Earth at rest. During this process no external energy has been added.

Now the mass at 1Km above the Earth begins to fall. When it passes it's original location it now has the same mass as when it started but has this additional kinetic energy. Where did this additional energy come from?

Tony
 
Physics news on Phys.org
  • #2
sorry, my error
 
Last edited:
  • #3
This is nothing to do with GR. When the mass spontaneously becomes photons, they would have to radiate in ALL directions in order to conserve momentum. That includes up and down. This should solve your problem.
 
  • #4
Not really, the isotropic radiation could be reflected by an external parabolic miror to the point 1 km high. The back reaction on the mirror would explain the change in momentum.

The answer to the problem is the photons at 1 km would be gravitationally red shifted (GR) and therefore of less total energy than originally. When converted back into mass it would be less than the original mass by the loss of potential energy/c2. When that fell back to Earth it would regain that loss of mass/energy as kinetic energy and theoretically could be brought to a halt and that KE changed into extra mass so it ends up the same mass as it began, (perfect efficiency assumed of course!)

I hope this helps.

Garth
 
  • #5
Garth said:
Not really, the isotropic radiation could be reflected by an external parabolic miror to the point 1 km high. The back reaction on the mirror would explain the change in momentum.

The answer to the problem is the photons at 1 km would be gravitationally red shifted (GR) and therefore of less total energy than originally. When converted back into mass it would be less than the original mass by the loss of potential energy/c2. When that fell back to Earth it would regain that loss of mass/energy as kinetic energy and theoretically could be brought to a halt and that KE changed into extra mass so it ends up the same mass as it began, (perfect efficiency assumed of course!)

I hope this helps.

Garth

Thats a good explanation. Thanks. (I'll have to assume the math works).

On the bit about momentum, what if there is enough mass to create one photon at a given wavelength. It would travel out radially so no mirrors needed. The question now is the momentum was zero before the transformation, but non-zero after the single photon was created. How can that be?

Is the answer simply this cannot happen because it would violate conservation of momentum?

Tony
 
  • #6
No process can create a photon in a vacuum - there has to be two photons. A single photon can be created near a massive particle, since the massive particle can recoil.
 
  • #7
tbitz said:
Here is a thought experiment that doesn't seem to add up.

Suppose you have a unit of mass at rest on the surface of Earth that spontaneously transforms to photos all moving radially outward into space. At some distance (say 1Km) the photons revert back to the original mass but are now 1Km above the surface of the Earth at rest. During this process no external energy has been added.

Now the mass at 1Km above the Earth begins to fall. When it passes it's original location it now has the same mass as when it started but has this additional kinetic energy. Where did this additional energy come from?

Tony
In the given frame of reference the spacetime is static (guv = constant in time) then the 4-momentum of the original particle will separate into two photons, each with an energy (i.e. E = P0) is a constant of motion. So the total energy of the two photons remains constant. Therefore when they return to the same location and once again form a single particle the energy and the mass will be the same as when the process started.

Pete
 

1. What is potential energy?

Potential energy is the energy that an object possesses due to its position or configuration in a system. It is often referred to as stored energy because it has the potential to do work when released or converted into other forms of energy.

2. How does potential energy relate to general relativity (GR)?

In general relativity, potential energy is considered as a form of energy that contributes to the overall curvature of spacetime. This curvature affects the motion of objects and determines how they interact with each other in the presence of gravity.

3. What is the difference between potential energy and kinetic energy?

Potential energy is the energy that an object has due to its position or configuration, while kinetic energy is the energy that an object possesses due to its motion. The two forms of energy are interchangeable, and an object can convert from potential energy to kinetic energy and vice versa.

4. Why do we use potential energy in the context of general relativity?

In general relativity, potential energy is used to describe the curvature of spacetime and how it affects the motion of objects. This is because potential energy is a fundamental concept in physics and plays a crucial role in understanding the behavior of particles and systems in the universe.

5. What does it mean when something "doesn't add up" in the context of potential energy and general relativity?

When something doesn't add up in the context of potential energy and general relativity, it means that there is a discrepancy or inconsistency in the calculations or theories. This could indicate a gap in our understanding of the universe and may require further research and experimentation to resolve.

Similar threads

  • Special and General Relativity
Replies
5
Views
976
  • Special and General Relativity
Replies
5
Views
821
  • Special and General Relativity
Replies
6
Views
1K
Replies
1
Views
92
  • Special and General Relativity
2
Replies
62
Views
4K
  • Special and General Relativity
Replies
6
Views
2K
  • Special and General Relativity
Replies
9
Views
1K
  • Special and General Relativity
Replies
5
Views
958
  • Special and General Relativity
Replies
16
Views
1K
  • Special and General Relativity
2
Replies
37
Views
3K
Back
Top