Red shift / Blue Shift - Gravity Well Roundtrip

In summary, the conversation discusses the possible effects on light when passing through a gravitational well, specifically when shining a light towards a black hole and it returns back to the observer. The experts mention that the light will return to its original wavelength, as long as there is no change in distance between the observer and the black hole. They also mention that in more complex scenarios, such as cosmic expansion, the light may experience changes in wavelength due to the evolution of gravity wells. However, in the simplest scenario, energy conservation ensures that the light will return at its original wavelength.
  • #1
D.S.Beyer
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TL;DR Summary
Does wavelength change from trips through gravity wells?
If I shine a white light out into space from Earth, and it bends around a black hole and comes straight back to me so I can see it in my telescope, is the light red or blue shifted, or neither?

For sake of the thought experiment, let’s leave out cosmic expansion that would stretch it on it’s journey, and let’s say my frame of reference doesn’t move so we can leave out any doppler shift. I am purely interested if there is any effect on the light from going in and out of gravity wells. Does it return to its original wavelength (if viewed from the reference frame of its origination)?
 
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  • #2
D.S.Beyer said:
Does it return to its original wavelength
Yes. The photon gains just as much energy falling into a gravity well as it later loses climbing out of it. The picture changes with large structures under certain kinds of expansion, where gravity wells evolve during a photon's journey (the Sachs-Wolfe effect). But in the simplest scenario it's just energy conservation.
 
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  • #3
D.S.Beyer said:
I am purely interested if there is any effect on the light from going in and out of gravity wells. Does it return to its original wavelength (if viewed from the reference frame of its origination)?
The only thing that would have an effect is if the black hole (or mirror for that matter) is changing its distance from you, in which case there will be red/blue shift. The gravity doesn't have any effect at all from your perspective since you're measuring the light at the same potential as where it left you.

That said, if I shine a light at a (small, distant) black hole and it comes back to me, it will take longer to make the round trip (as measured by me) than light from a mirror placed at approximately the equivalent distance. But assuming no relative motion, both will come back at the original wavelength.
 

Related to Red shift / Blue Shift - Gravity Well Roundtrip

1. What is red shift and blue shift?

Red shift and blue shift are terms used to describe the change in the wavelength of light emitted by an object. Red shift occurs when the wavelength of light becomes longer, shifting towards the red end of the visible light spectrum. Blue shift occurs when the wavelength of light becomes shorter, shifting towards the blue end of the visible light spectrum.

2. What causes red shift and blue shift?

Red shift and blue shift are caused by the Doppler effect, which is the change in frequency and wavelength of a wave as the source or observer moves relative to each other. In the case of red shift, the source is moving away from the observer, causing the wavelength to appear longer. In the case of blue shift, the source is moving towards the observer, causing the wavelength to appear shorter.

3. How is red shift and blue shift used in astronomy?

Red shift and blue shift are used in astronomy to determine the motion of objects in space. By analyzing the shift in the wavelength of light emitted by an object, astronomers can calculate the velocity and direction of the object's movement. This is particularly useful in studying the expansion of the universe and the movement of galaxies.

4. What is a gravity well?

A gravity well is a concept in physics that describes the curvature of space and time caused by the presence of a massive object, such as a planet or star. The greater the mass of the object, the stronger the gravity well it creates. This means that objects with less mass will be pulled towards the object with greater mass due to the curvature of space-time.

5. How does a gravity well affect the red shift and blue shift of light?

In the case of red shift, the light emitted from an object near a gravity well will appear to have a longer wavelength due to the curvature of space-time. This is known as gravitational red shift. On the other hand, blue shift can occur when light is emitted from an object moving away from a gravity well, as the light has to travel against the curvature of space-time, causing the wavelength to appear shorter. This is known as gravitational blue shift.

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