Simple relativity question [two ships]

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    Relativity Ships
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Discussion Overview

The discussion revolves around a scenario involving two identical ships traveling on parallel paths and whether they can determine their velocity by measuring the distance between them over time. The conversation touches on concepts of relativistic mass, gravitational attraction, and the implications of their relative motion.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions if the ships can determine their velocity by measuring the decreasing distance between them, suggesting that increased velocity might lead to increased gravitational attraction.
  • Another participant clarifies that since the ships are at rest relative to each other, they would measure each other's relativistic mass as identical to their rest mass.
  • There is a repeated emphasis on the need to consider relative motion, with one participant stating that if the ships are on parallel courses at the same speed, they are motionless relative to each other.
  • Some participants argue that increased momentum would lead to a decrease in gravitational attraction, suggesting that the time to collide would be increased.
  • One participant expresses a belief that the gravitational attraction, as perceived by others, is reduced and that the ships would attract more slowly.
  • Clarifications are made regarding the arrangement of the ships, confirming they are side by side.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between velocity, mass, and gravitational attraction, with no consensus reached on how these factors interact in the given scenario.

Contextual Notes

The discussion includes assumptions about the ships' isolation from external gravitational influences and the nature of their motion relative to each other, which may affect the interpretations of their gravitational interactions.

nearc
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two identical ships launch on parallel paths r distance apart. the are completely alone [nothing to affect them gravitationally [but themselves]]. once they reach some velocity v [they both reach the same velocity at the same time [everything is identical]] they turn off their engines and coast.

can someone on a ship determine the velocity v by measuring the decreasing distance between the two ships over time? [i.e. the faster they go, the more mass they have and the more they should be attracted to each other?]
 
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Since the two ships remain at rest relative to each other, each would measure the relativistic mass of the other to be identical to the rest mass.
 
Just a point of clarification: did you mean that the two ships are side by side or one in front of the other or some other arrangement?
 
nearc said:
two identical ships launch on parallel paths r distance apart. the are completely alone [nothing to affect them gravitationally [but themselves]]. once they reach some velocity v [they both reach the same velocity at the same time [everything is identical]] they turn off their engines and coast.

can someone on a ship determine the velocity v by measuring the decreasing distance between the two ships over time? [i.e. the faster they go, the more mass they have and the more they should be attracted to each other?]
You are not thinking in "relative" terms! The faster they go relative to what? If they are on parallel courses, at the same speed, they are each motionless relative to each other and each would measure the other to have its rest mass.
 
nearc said:
[i.e. the faster they go, the more mass they have and the more they should be attracted to each other?]
No, they do not have increased gravitational attraction.
 
With increased momentum another thread recently concluded the time to collide would be increased...meaning LESS gravitational attraction...!

I'll post that thread here if I can find it...
 
ghwellsjr said:
Just a point of clarification: did you mean that the two ships are side by side or one in front of the other or some other arrangement?

they are side by side
 
Naty1 said:
With increased momentum another thread recently concluded the time to collide would be increased...meaning LESS gravitational attraction...!

I'll post that thread here if I can find it...

i'd love to see that and another info an this thought.
 
I think the gravitational attraction as seen by everyone else is reduced, they would attract more slowly.
 
  • #10
nearc said:
ghwellsjr said:
Just a point of clarification: did you mean that the two ships are side by side or one in front of the other or some other arrangement?
they are side by side
In that case, what HallsofIvy said here is correct:
HallsofIvy said:
...
If they are on parallel courses, at the same speed, they are each motionless relative to each other...
Actually, what he said is also true if one was in front of the other after they reached their final velocity, but during their acceleration, they will be moving farther apart and end greater than the distance r between them.
 
Last edited:

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