Light always wants to travel in a straight line

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Discussion Overview

The discussion centers on the behavior of light in curved spacetime and the implications for satellite orbits around Earth. Participants explore concepts related to the curvature of space caused by massive objects, the nature of satellite paths, and the conditions under which satellites might collide.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that light travels in straight lines but is bent by the curvature of space around massive objects like black holes.
  • One participant mentions that light from a star can reach us even when blocked by another body due to the curvature of space.
  • Another participant clarifies that Earth satellites travel in elliptical orbits that intersect, suggesting they can collide if at the same altitude.
  • There is a discussion about the minimal curvature of space around Earth, with one participant noting that light is only slightly deflected near Earth.
  • Questions arise regarding the definition of "parallel" orbits and whether satellites can collide if they are on such paths.
  • Some participants express confusion about the nature of elliptical orbits and their relationship to the concept of parallelism in this context.
  • A later reply emphasizes that light follows a geodesic in curved spacetime, not simply a straight line in space.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the nature of satellite paths and their potential for collision. There are competing views on the definition of parallel orbits and the implications of spacetime curvature on satellite movement.

Contextual Notes

There are unresolved questions regarding the definitions of parallel orbits, the extent of curvature around Earth, and the implications of general relativity on the paths of light and satellites.

UrbanXrisis
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this was posted before, but I was told to put it in this section...

Light always wants to travel in a straight line, however, the space is curved around massive objects such as black holes, so it would seem as if the light was being bent as it traveled around the black hole. The space around the Earth is curved.. so satellies traveling along parallel paths can actually collide with eath other right?
 
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i think your right, there's this thing i read where you can see a star which is actually behind another bodies which should block it, but because space curves, the light of the star still reaches us.
 
All Earth satellites travel in elliptic (usually close to circular) orbits with the Earth's center at a focus. Therefore these orbits will always lie in planes that intersect each other. The orbits will cross if they happen to be at the same distance from the center of the earth.
 
UrbanXrisis said:
this was posted before, but I was told to put it in this section...

Light always wants to travel in a straight line, however, the space is curved around massive objects such as black holes, so it would seem as if the light was being bent as it traveled around the black hole. The space around the Earth is curved.. so satellies traveling along parallel paths can actually collide with eath other right?
1] The space in Earth's vicinity is only veeeerrrry slightly curved; Earth is not very massive. A beam of light passing Earth will be deflected by an amount almost to small to detect with the finest of instruments.
2] Satellites have paths that form 'great circles' around the Earth; there are no satellites that travel parallel to each other. Yes, they will collide (eventually) *IF* they are at the same altitude. It's pretty hard to do though. Space - even the space around Earth - is pretty big.
 
yes, if they are at the same altitude, they will collide eventually. But can the collide if they are on parallel paths? Are you saying that the curvature is so small that they will not collide?
 
UrbanXrisis said:
yes, if they are at the same altitude, they will collide eventually. But can the collide if they are on parallel paths? Are you saying that the curvature is so small that they will not collide?
What does it mean for two elliptical orbits to be "parallel"? Also, are you talking about the paths of their orbits through space, or the path of their worldlines through spacetime? General relativity doesn't say objects take the shortest path through curved space, it says they take the path through curved spacetime with the greatest proper time (time as measured by a clock that follows that path through spacetime).
 
UrbanXrisis said:
But it's not possible for orbits to be parallel like that. If you draw the orbits as ellipses, each ellipse should have one focus as the center of the earth, while the parallel ellipses you drew don't share either focus.

edit: Actually, I'm confused by what you mean when you posted that picture. There are four sets of ellipses there, in two different planes, one ellipse in each plane being drawn with a solid line and a smaller ellipse drawn with a dotted line inside. I thought you meant "parallel" like ellipses in different planes are parallel, but then you said something about dotted vs. solid lines, so maybe you want me to ignore the extra plane and just consider a dotted ellipse inside a solid one? Either way, it would be better if you could find a picture without the extraneous stuff...
 
Last edited:
UrbanXrisis said:
Light always wants to travel in a straight line, ...
No. It wants to move on a geodesic. That's a worldline of extremal length. It is spacetime that is curved around a black hole, not simply space. The spatial portion of the worldline of light does bend.

..so satellies traveling along parallel paths can actually collide with eath other right?
No. They can't move on parallel paths if they are moving freely.

Pete
 

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