Sending Signal from Planet Near Gargantua: A Thought Experiment

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Discussion Overview

The discussion revolves around a thought experiment inspired by the film Interstellar, specifically focusing on the implications of sending a signal from a planet near the black hole Gargantua to a spacecraft where a character named Romilly is located. Participants explore the effects of time dilation and the nature of spacetime in the context of general relativity.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the signal would be delayed due to the stretching of spacetime, suggesting that light takes longer to reach Romilly as a result.
  • Another participant challenges the idea of spacetime stretching, asserting that spacetime is a fixed 4-dimensional geometry that does not change over time.
  • There is a discussion about the relevance of Kerr spacetime and its stationary nature, indicating that the conditions around a rotating black hole differ from those in an expanding universe.
  • Concerns are raised about the potential redshift of the signal, which could affect how the signal is perceived by Romilly.
  • Participants express uncertainty about the implications of proximity to the black hole and how it might influence the communication between the planet and the spacecraft.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the nature of spacetime and its implications for signal transmission. There are competing views regarding the concept of spacetime stretching and the effects of gravitational redshift.

Contextual Notes

Limitations include the lack of consensus on the interpretation of spacetime in the context of general relativity and the specific conditions around a rotating black hole. Some assumptions about signal transmission and time dilation remain unresolved.

kccrossen
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TL;DR
A discussion on the scenes of Interstellar, If a radio signal were to be sent from the planet near Gargantua to the space craft where Romilly was working on his theory what would happen.
The other day my friend asked me a really interesting question regarding the scene from interstellar where they go down to Miller's planet, where every hour on this planet is 7 years of Earth time. He asked me if they were to send a signal to the spaceship where Romilly was, what would happen.

My thought process was that any information travels at the speed of light, we know this, but would the signal be delayed, as spacetime is stretching apart? The speed of light itself doesn't change but the light is taking longer to get to him as space is being stretched between them. Is this a correct thought or am I just making up things?

Also wouldn't it depend on how close Romilly was to the planet? and how close the planet was to the black hole? If he were to get the signal, would the astronauts just talk extremely slowly, almost too slow to tell what they are saying since the time dilation is so large? I haven't taken general relativity, only special so I am just asking out of curiosity. Its a cool thought experiment. Let me know your thoughts.
 
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kccrossen said:
spacetime is stretching apart?

No, it isn't. First, spacetime doesn't change, it just is; it's a 4-dimensional geometry that already contains within it all the information about how things "change" with "time". It doesn't have to change at all.

Second, Kerr spacetime, which is the spacetime around a rotating black hole, is stationary, so even pop science descriptions that apply to, say, models of the expanding universe and talk about "space expanding" do not apply to Kerr spacetime. "Stationary" means that, from the standpoint of observers orbiting in the spacetime around Gargantua, nothing about the black hole itself or the space around it "changes" with "time".

kccrossen said:
The speed of light itself doesn't change but the light is taking longer to get to him as space is being stretched between them.

Why do you think "space is being stretched" in this scenario?
 
kccrossen said:
Summary:: A discussion on the scenes of Interstellar, If a radio signal were to be sent from the planet near Gargantua to the spacecraft where Romilly was working on his theory what would happen.
Any such signal would be also redshifted:
https://en.m.wikipedia.org/wiki/Gravitational_redshift
 

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