To Saturn at (nearly) the speed of light

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Discussion Overview

The discussion revolves around the implications of sending a rocket to Saturn at nearly the speed of light, particularly focusing on the relativistic effects of time dilation and the perception of time for observers on Earth versus those on the rocket. Participants explore the complexities of how time and distance are perceived differently due to relativistic speeds.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the time it would take for an observer on Earth to see a rocket traveling to Saturn at near light speed, suggesting it could take a thousand years due to relativity.
  • Another participant counters that if the rocket travels at the speed of light, the time observed from Earth would be about an hour, based on the distance to Saturn.
  • Concerns are raised about the implications of time dilation, with one participant suggesting that if the rocket's clock shows less time has passed, it implies a misunderstanding of how time is perceived by different observers.
  • Several participants discuss the "twin paradox," indicating that the scenario presented is analogous to that thought experiment, where one twin travels at relativistic speeds and experiences less passage of time compared to the stationary twin.
  • There is confusion about whether the rocket could arrive at Saturn in less time than it takes light to reach there, with participants clarifying that while time and distance are perceived differently, the speed of light remains invariant across frames of reference.
  • One participant expresses a belief that time dilation is linked to mass increase and curvature of space, drawing parallels to black hole physics.

Areas of Agreement / Disagreement

Participants exhibit disagreement regarding the implications of time dilation and the perception of time for observers in different frames of reference. While some points are clarified, the discussion remains unresolved on several aspects of relativistic travel and its consequences.

Contextual Notes

Limitations include varying interpretations of relativistic effects, assumptions about observer perspectives, and the complexity of time dilation as it relates to speed and distance. The discussion does not resolve these nuances.

  • #61
.Scott said:
There may be parts of the universe that are either behind an event horizon or would move behind that horizon before you reached them.

Moving behind an event horizon doesn't make them unreachable to us; it just makes us unreachable to them. The accelerating expansion of the universe (assuming that our current model of that is correct) makes things unreachable both ways, so to speak.
 
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  • #62
PeterDonis said:
Moving behind an event horizon doesn't make them unreachable to us; it just makes us unreachable to them. The accelerating expansion of the universe (assuming that our current model of that is correct) makes things unreachable both ways, so to speak.
Right. It works both ways. Our event horizon (as seen from them) keeps us from getting to them and their event horizon (as seen by us) keeps them from getting to us.
 
  • #63
.Scott said:
Right. It works both ways. Our event horizon (as seen from them) keeps us from getting to them and their event horizon (as seen by us) keeps them from getting to us.

This amounts to saying that we and they are each inside a black hole with respect to the other. That's not correct. The cosmological horizon involved here is not the same as a black hole's event horizon. For one thing, its location in space changes with time. For another, its location in spacetime (i.e., which surface in spacetime it is) is different for different observers. Neither of those things are true for a black hole's event horizon.
 
  • #64
PeterDonis said:
This amounts to saying that we and they are each inside a black hole with respect to the other. That's not correct. The cosmological horizon involved here is not the same as a black hole's event horizon. For one thing, its location in space changes with time. For another, its location in spacetime (i.e., which surface in spacetime it is) is different for different observers. Neither of those things are true for a black hole's event horizon.
There are several ways to create an event horizon. One is with a black hole. Another is with simple continuous acceleration. Another is with expansion of space.
They all have the same characteristics - extreme time dilation, Hawking radiation.

I wasn't saying anything about black holes.
 
  • #65
I think that any null surface can be considered an event horizon. Once you cross it you cannot send signals back to the other side.
 

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