# B Question about Curvature caused by Mass

1. Jul 7, 2017

### Deepak K Kapur

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

2. Jul 7, 2017

### Staff: Mentor

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.

Last edited: Jul 7, 2017
3. Jul 7, 2017

### rootone

Mass cannot just appear magically like that though.

4. Jul 7, 2017

### Deepak K Kapur

I have kinda got the idea...

Can you please elaborate more and that too in layman terms.

5. Jul 8, 2017

### Deepak K Kapur

I think quantum mechanics allows this, even if the probability is miniscule....

6. Jul 8, 2017

### Staff: Mentor

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?

7. Jul 8, 2017

### Staff: Mentor

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.

8. Jul 8, 2017

### Deepak K Kapur

Laymen are most certainly well versed in spacetime diagrams...

9. Jul 8, 2017

### Staff: Mentor

So, how would you draw an object at rest, an object moving slowly, and an object moving fast?

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