Gravitational Redshift really verification of GR?

In summary, the conversation discusses how the gravitational redshift can be derived using classical gravity and the equations E=mc^2 and E=hv. This prediction is the same for classical and general relativity, leading to the question of how redshift can be used as verification of GR. However, it is pointed out that the gravitational redshift factor corresponds to the change in gravitational time dilation and the Pound and Rebka experiment proves gravitational redshift, but not the validity of the Schwarzschild solution.
  • #1
thehangedman
69
2
If we assume:

[itex]E = mc^{2}[/itex]

and for photons:

[itex]E = hv[/itex]

Then we can derive an effective mass:

[itex]m = \frac{hv}{c^{2}}[/itex]

And using simple classical gravity obtain:

[itex]hv - \frac{GMm}{r} = hv - \frac{GMhv}{c^{2}r} = Constant[/itex]

You can derive the constant by evaluating the equation above at the limit as r goes to infinity. This then gives you the gravitational red shift, all without using anything from GR.

So, since this prediction is the same for classical AND GR, how can red shift be used as verification of GR? I'm not questioning GR, just wondering why this is still listed as verification when it's clearly not.
 
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  • #2
thehangedman said:
If we assume:

[itex]E = mc^{2}[/itex]

and for photons:

[itex]E = hv[/itex]

Then we can derive an effective mass:

[itex]m = \frac{hv}{c^{2}}[/itex]

And using simple classical gravity obtain:

[itex]hv - \frac{GMm}{r} = hv - \frac{GMhv}{c^{2}r} = Constant[/itex]

You can derive the constant by evaluating the equation above at the limit as r goes to infinity. This then gives you the gravitational red shift, all without using anything from GR.

So, since this prediction is the same for classical AND GR, how can red shift be used as verification of GR? I'm not questioning GR, just wondering why this is still listed as verification when it's clearly not.

That's not correct. The gravitational redshift factor corresponds the to change in gravitational time dilation [itex]\frac{d\tau}{dt}-1=\frac{1}{\sqrt{1-\frac{2GM}{rc^2}}}-1[/itex].
 
  • #3
If you are referring to the Pound and Rebka experiment, it proves gravitational redshift but it does not prove the validity of the Schwarzschild solution (even when we ignore rotation) as the experiment is not accurate enough to do that.
 

1. What is gravitational redshift?

Gravitational redshift is a phenomenon in which light waves from an object are stretched and appear to have a longer wavelength when observed from a region with a stronger gravitational field.

2. How does gravitational redshift relate to Einstein's theory of General Relativity?

Einstein's theory of General Relativity predicts that the presence of a strong gravitational field can cause light waves to lose energy and therefore have a longer wavelength, resulting in gravitational redshift.

3. How is gravitational redshift verified as a prediction of General Relativity?

Gravitational redshift has been observed and verified through experiments, such as the Pound-Rebka experiment, which measured the shift in the wavelength of light emitted from a source at different distances from a strong gravitational field.

4. Can gravitational redshift be explained by other theories besides General Relativity?

While there are other theories that attempt to explain gravitational redshift, such as the Newtonian theory of gravity, they do not fully account for the observations and are not as widely accepted as General Relativity.

5. How does gravitational redshift affect our understanding of the universe?

Gravitational redshift is an important concept in understanding the effects of gravity on light and the behavior of objects in strong gravitational fields, such as black holes. It also has implications for measuring the expansion of the universe and the distribution of matter and energy in the universe.

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