Question about Curvature caused by Mass

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    Curvature Mass
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Discussion Overview

The discussion revolves around the effects of mass on the curvature of spacetime and how this curvature influences the motion of a body in deep space. Participants explore concepts from general relativity, particularly the nature of acceleration and geodesics in spacetime, while also addressing hypothetical scenarios regarding the sudden appearance of mass.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why a body accelerates towards a planet when it suddenly appears, suggesting that it might be perceived as accelerating rather than actually increasing speed.
  • Another participant asserts that the body follows a geodesic in spacetime, which can result in a change in the angle to the time axis, indicating acceleration.
  • Concerns are raised about the conservation of energy and the implications of mass appearing suddenly, with some participants suggesting this violates principles of relativity.
  • There is a discussion about whether quantum mechanics permits the sudden appearance of mass, with differing opinions on the matter.
  • Participants express a desire for further clarification in layman terms, particularly regarding spacetime diagrams and the representation of objects at different speeds.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the implications of mass appearing suddenly or the compatibility of this scenario with conservation laws. There are competing views on the role of quantum mechanics in this context, and the discussion remains unresolved.

Contextual Notes

Participants note limitations related to the hypothetical nature of the scenario and its implications for established physical laws, particularly regarding energy conservation and general relativity.

Deepak K Kapur
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A body is moving in deep space substantially away from others influences of gravitational force ( just assume).

Suddenly, a big planet appears in It's vicinity. As per relativity it's not some force issuing forth from the planet that attracts the body towards itself. The body just keeps on moving as before, only the space is curved this time.So, it starts to move towards the planet. OK till now, I suppose.

My question is, 'why does the body accelerate/increase its speed.Why doesn't it keep moving at the same speed as before?

Or, do the people on the planet only SEE it to be accelerating towards the planet, as it is moving closer to the planet with time? Is the speed of the body same as before?

I hope there is nothing objectionable with this post
 
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Deepak K Kapur said:
Suddenly, a big planet appears in It's vicinity.
This part of the question violates the conservation of energy in a way that is incompatible with relativity. So the answer below will ignore that part.

Deepak K Kapur said:
why does the body accelerate/increase its speed.Why doesn't it keep moving at the same speed as before?
Remember that it is following a geodesic in spacetime, not just space. So a path that was originally parallel to the time axis can curve so that it is at an angle to the time axis. This is acceleration/increased speed.
 
Last edited:
Mass cannot just appear magically like that though.
 
Dale said:
This part of the question violates the conservation of energy in a way that is incompatible with relativity. So the answer below will ignore that part.

Remember that it is following a geodesic in spacetime, not just space. So a path that was originally parallel to the time axis can curve so that it is at an angle to the time axis. This is acceleration/increased speed.
I have kinda got the idea...

Can you please elaborate more and that too in layman terms.
 
rootone said:
Mass cannot just appear magically like that though.

I think quantum mechanics allows this, even if the probability is miniscule...
 
Deepak K Kapur said:
I have kinda got the idea...

Can you please elaborate more and that too in layman terms.
Are you familiar with spacetime diagrams? If so, how would you draw an object at rest, an object moving slowly, and an object moving fast?
 
Deepak K Kapur said:
I think quantum mechanics allows this, even if the probability is miniscule...
No it doesn't. Energy is conserved in QM. In any case, the question is about GR where this sort of thing is explicitly forbidden.
 
Dale said:
Are you familiar with spacetime diagrams? If so, how would you draw an object at rest, an object moving slowly, and an object moving fast?

Laymen are most certainly well versed in spacetime diagrams...
 
So, how would you draw an object at rest, an object moving slowly, and an object moving fast?
 

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