<p>A partial answer: The lower the gravitational potential the slower the clocks, see <a href="http://en.wikipedia.org/wiki/Gravitational_time_dilation" rel="nofollow">Gravitational time dilation</a>. This means, as long as your flashlight is behind you in constant distance, it appears blue-shifted to you. With roles exchanged (light below you in constant distance) it appears red-shifted.
By the heterogeneity of the gravitational field the shift grows with the time, while falling in constant distance.</p>

<p>This is called the Einstein shift, which is not the same as the doppler effect. The doppler effect is caused by an objects velocity relative to the observer.</p>

CC BY-SA 3.0
2014-01-06T19:21:07.453Z
To clarify my understanding of your answer. There are two factors affecting the amount of redshift; the clock speed at the observer - a faster clock relative to the source means blueshift and vice versa, and the gravitational redshift. Due to the the steepness of the gravitation well growing exponentionally, I suspected we'd see even more shift then what is accounted for by gravitational time dilation alone?
frodeborli
CC BY-SA 3.0
2014-01-06T21:21:09.870Z
A faster clock (due to higher potential) of the observer relative to the source means redshift. The amount of shift depends of the difference of the gravitational potential. As this difference is higher when closer to the singularity for objects of fixed distance the shift grows while falling.
Gerald
CC BY-SA 3.0
2014-01-07T09:05:22.620Z
Thank you Gerald. And if both the flashlight and the observer were running at the same clock, while still at various distances from the black hole, there would be no shift in color? (For example if the flashlight itself was very heavy)
frodeborli
CC BY-SA 3.0
2014-01-07T15:03:16.543Z
If the flashlight is exactly that heavy to produce the same gravitational potential as at the location of the observer, there wouldn't be a net reshift, and the clocks would run synchronous.
Gerald
CC BY-SA 3.0
2014-01-07T18:57:38.363Z
But light itself is also affected by that clock rate? So being near a supermassive black hole, such as the one in the center of our galaxy, would make all light from sources further away look redshifted? Is "The Great Attractor" possibly such a source?
frodeborli
CC BY-SA 3.0
2014-01-07T21:06:11.430Z
Light from further away would look blue-shifted from the observer close to the black hole, because clocks - including oscillating light - run faster further away than closer to the black hole, more oscillations in the same time than the same kind of light source emits closer to the black hole. Clock is meant very general and includes light, actually it's the way time passes.
Gerald
CC BY-SA 3.0
2014-01-07T21:55:55.753Z
Thank you for your patience, Gerald. I'm going to accept your answer above, as I believe you've answered everything I wanted to know. You've properly explained the difference between Einstein shift and the doppler effect to me, i believe.
frodeborli
CC BY-SA 3.0
2014-01-06T18:51:48.937Z
2014-01-07T22:13:50.537Z
2014-01-07T22:13:50.537Z
Gerald
<p>If, hypothetically, me and my rocket powered flashlight were falling straight toward the center of a black hole. The flashlight is a few kilometers behind me in our travels toward the center of the black hole, but since it is rocket powered, it manages to maintain the exact distance to me for a while.</p>

<p>The point is; The distance between me and my flashlight is constant as long as I am observing it.</p>

<p>The photons coming from the flashlight would obviously not be rocket powered - and they would be affected by the black holes gravitation.</p>

<p>Would the light I see from the flashlight be shifted towards red or blue, even though the distance between me and my dear flashlight is maintained?</p>

<p>If so; switching the positions of me and my flashlight, would it change the color I'd observe?</p>

<p>If we turn off the rocket on the flashlight, I assume it would be redshifted regardless of which were closer to the singularity, and the magnitude of redshift would appear to accelerate?</p>

<gravity><redshift>
How does gravity affect the wavelength of light?
frodeborli
# How does gravity affect the wavelength of light?

(Asked by: username_0 on 2014-01-06)

If, hypothetically, me and my rocket powered flashlight were falling straight toward the center of a black hole. The flashlight is a few kilometers behind me in our travels toward the center of the black hole, but since it is rocket powered, it manages to maintain the exact distance to me for a while.

The point is; The distance between me and my flashlight is constant as long as I am observing it.

The photons coming from the flashlight would obviously not be rocket powered - and they would be affected by the black holes gravitation.

Would the light I see from the flashlight be shifted towards red or blue, even though the distance between me and my dear flashlight is maintained?

If so; switching the positions of me and my flashlight, would it change the color I'd observe?

If we turn off the rocket on the flashlight, I assume it would be redshifted regardless of which were closer to the singularity, and the magnitude of redshift would appear to accelerate?

**Tags:** gravity, redshift

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## 1 answers

### Answer by username_1 on 2014-01-06. Score: 3 | Accepted answer

A partial answer: The lower the gravitational potential the slower the clocks, see [Gravitational time dilation](http://en.wikipedia.org/wiki/Gravitational_time_dilation). This means, as long as your flashlight is behind you in constant distance, it appears blue-shifted to you. With roles exchanged (light below you in constant distance) it appears red-shifted.
By the heterogeneity of the gravitational field the shift grows with the time, while falling in constant distance.

This is called the Einstein shift, which is not the same as the doppler effect. The doppler effect is caused by an objects velocity relative to the observer.

> **Comments:**
> **username_0**: To clarify my understanding of your answer. There are two factors affecting the amount of redshift; the clock speed at the observer - a faster clock relative to the source means blueshift and vice versa, and the gravitational redshift. Due to the the steepness of the gravitation well growing exponentionally, I suspected we'd see even more shift then what is accounted for by gravitational time dilation alone?  
> **username_1**: A faster clock (due to higher potential) of the observer relative to the source means redshift. The amount of shift depends of the difference of the gravitational potential. As this difference is higher when closer to the singularity for objects of fixed distance the shift grows while falling.  
> **username_0**: Thank you Gerald. And if both the flashlight and the observer were running at the same clock, while still at various distances from the black hole, there would be no shift in color? (For example if the flashlight itself was very heavy)  
> **username_1**: If the flashlight is exactly that heavy to produce the same gravitational potential as at the location of the observer, there wouldn't be a net reshift, and the clocks would run synchronous.  
> **username_0**: But light itself is also affected by that clock rate? So being near a supermassive black hole, such as the one in the center of our galaxy, would make all light from sources further away look redshifted? Is "The Great Attractor" possibly such a source?  
> **username_1**: Light from further away would look blue-shifted from the observer close to the black hole, because clocks - including oscillating light - run faster further away than closer to the black hole, more oscillations in the same time than the same kind of light source emits closer to the black hole. Clock is meant very general and includes light, actually it's the way time passes.  
> **username_0**: Thank you for your patience, Gerald. I'm going to accept your answer above, as I believe you've answered everything I wanted to know. You've properly explained the difference between Einstein shift and the doppler effect to me, i believe.  

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