Photons can't accelerate, can they?

[questionposter]

In physics there is the problem in unifying gravity with EM and Quantum mechanics. Perhaps this is the result of our unsuccessful attempt to unify gravity with them, that we are unable to answer such a question satisfactorily?

 

Well if people don't know then people like Swan should just say "we can't answer that with our current knowledge" instead of sending me on this wild goose-chase for an answer. I'm not going to all-of-a sudden think Swan is stupid or doesn't have any credibility just because he can't answer a question that we don't have the technology to answer right now.

Edited September 10, 2011 by questionposter

[davey2222]

I think there may be some theory like a broken-symmetry something somewhere in this?

 

I am curious if you had an answer to this problem. Mind sharing with us?

Edited September 10, 2011 by davey2222

[csmyth3025]

I think the issue questionposter brought up was about conservation of energy. If the photon gets blue-shifted due to gravity, where did the energy to blue-shift the photon come from?

The old textbooks used to refer to gravitational potential energy as the energy of position. In this case that description is helpful. A photon (or a baseball) that's very far away from the Earth will have a maximum amount of potential gravitational energy due to it's position far away from the Earth. If the baseball has no net velocity relative to Earth when it starts to "fall" from this very large distance, the greatest speed it will attain (ignoring atmospheric drag) is about 11.2 km/s before it hits the ground.

(ref. http://en.wikipedia....cape_velocities )

 

If the baseball is made of titanium metal it would weigh about 1 kg (~2.2 lb). It's kinetic energy would be E_k = 1/2mv² = 0.5 kg*(1.12*10⁴ m/s)² = ~62.72 MJ (megajoules) This is the amount of energy produced by the combustion of about half a gallon of gasoline.

 

If you were to shoot this same baseball back up into space, it would take the same amount of energy to get it back to the same point it started.

 

When the baseball is very far away from Earth, its gravitational potential energy relative to the surface of the Earth is 62.72 MJ and its kinetic energy relative to the surface of the Earth is zero. When the ball lands on the ground its gravitational potential energy relative to the surface of the Earth is zero and its kinetic energy is 62.72 MJ.

 

The ball doesn't "take away" any of Earth's gravity. It simply converts one form of the energy it already possesses (its gravitational potential energy relative to the surface of the Earth) to another form of energy (its kinetic energy when it reaches the surface of the Earth).

 

The blue-shifting of the wavelength of a photon that travels directly to Earth from some arbitrarily large distance works in a similar fashion, but involves equations that I can't yet understand or manipulate.

 

Chris

 

PS I think Swansont is very capable of providing you with an even more thorough explanation than I've offered. I think he's just having trouble trying to simplify a general relativistic solution in terms that you'll have some hope of understanding. There is no mystery here.