Can Distance Alone Prove Gravitational Binding?

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Discussion Overview

The discussion centers on whether the distance between two bodies can be used to prove they are gravitationally bound, particularly in the context of celestial bodies such as the Earth and the Moon, and the Milky Way's motion towards the Great Attractor. The scope includes theoretical considerations and implications for gravitational binding.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant suggests that distance alone is insufficient to prove gravitational binding, emphasizing the need for additional parameters such as velocity and mass.
  • Another participant agrees that velocity is crucial, stating that without it, one cannot determine the orbital status of a body, citing the example of the Earth and the Moon.
  • A third participant questions whether gravitational binding could be proven in the context of the Milky Way's motion towards the Great Attractor, implying a lack of clarity on the matter.
  • A later reply asserts that it is possible to estimate the masses of the bodies and measure velocities through redshift, thus providing the necessary parameters to assess gravitational binding.

Areas of Agreement / Disagreement

Participants generally agree that distance alone is not sufficient to prove gravitational binding, but there is disagreement on the implications for specific cases, such as the Milky Way's motion. The discussion remains unresolved regarding the ability to definitively prove gravitational binding in those contexts.

Contextual Notes

Limitations include the dependence on accurate measurements of mass and velocity, as well as the assumptions made about the stability of orbits based on these measurements.

wolram
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Can the distance between two bodies be calculated to prove they are gravitationally bound?

Use two bodies with known mass.
 
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You need to know velocity as well as distance in order to know this. If you took a snapshot of the Earth and the Moon you wouldn't know whether the Moon was in orbit, moving away from the Earth at sufficient velocity to escape or indeed plunging straight towards it!

Edit: you would also need to know that masses, since that will tell you what velocities would indicate stable orbits. So that's three things, position, velocity and mass.
 
Thankyou Wallace.

From that i guess there is no way the milky way and its motion towards the great attractor could be proven by calculation to be cause by gravity?
 
No, since we can get an estimate of the masses of the bodies involved and measure velocities via redshift, hence we have (to some level of precision) the three things I mentioned.
 

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