Why two mass attracts each other?

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Handsa, as i have already said, gravity is always attractive because acceleration is indistinguishable from gravity.
As acceleration is indistinguisable,we can say that acceleration is same as gravitational field,or field that is attractive. Note that you cannot show acceleration to be repulsive and thats how it answers your question.
 
eram said:
I've heard this before. "The clock does not march, but time marches on", right?

YES. TRUE in the case of Black-Hole.


Einstein asked this question, "Does the Inertia of a Body Depend upon its Energy-Content?" :)
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YES. So?

Does it mean "energy has inertia"?


ash64449 said:
look clocks don't simply dialate. It is due to the true nature of space-time itself.
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This is in other words, "Time dilation due to gravity". Here the "Time as indicated by a local clock" dilates.
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well,i think by 'real' time,he is talking about proper time. And indeed proper time dosen't change as it is the part of time-like invariant interval between two events.
 
Farsight said:
Tricky. I have a memory like an elephant, but no way could I pour out The Foundation of the General Theory of Relativity. I could do SR fairly easily from scratch, but not GR. Sure I could retell post #158 with a stick in the sand. And I could write out $$R_{\mu \nu} - {1 \over 2}g_{\mu \nu}\,R + g_{\mu \nu} \Lambda = {8 \pi G \over c^4} T_{\mu \nu}$$. Given enough time I could dredge up more, but I'd be struggling.
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Well I think that settles it! Professionals (and I'm being presumptuous enough to speak for scientists in general) have a very different notion of what it means to understand a theory of physics than you do.

In my mind, when you really understand something you don't really need to remember much! For instance, for my sins I have to give a GR class from time to time, and I would never recommend the students simply try to learn, verbatim, the lecture notes - I would simply ask that they understand them. Any student that managed to do that (say a mid 2:1 student) would certainly be able to reproduce a lot of basic GR (geodesic equation, variation if Einstein-Hilbert action, derive Schwarzchild solution, get physical conservations laws via Killing vectors....). I would also guess that they wouldn't proclaim their understanding of the subject is better than professional researchers!
 
ash64449 said:
Handsa, as i have already said, gravity is always attractive because acceleration is indistinguishable from gravity.
As acceleration is indistinguisable,we can say that acceleration is same as gravitational field,or field that is attractive. Note that you cannot show acceleration to be repulsive and thats how it answers your question.
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That's not true. Gravity can be repulsive when the 'pressure term' in the cosmological metric is dominant. Essentially that's how our universe began as an inflating soliton.
 
Guest254 said:
Well I think that settles it! Professionals (and I'm being presumptuous enough to speak for scientists in general) have a very different notion of what it means to understand a theory of physics than you do.

In my mind, when you really understand something you don't really need to remember much! For instance, for my sins I have to give a GR class from time to time, and I would never recommend the students simply try to learn, verbatim, the lecture notes - I would simply ask that they understand them. Any student that managed to do that (say a mid 2:1 student) would certainly be able to reproduce a lot of basic GR (geodesic equation, variation if Einstein-Hilbert action, derive Schwarzchild solution, get physical conservations laws via Killing vectors....). I would also guess that they wouldn't proclaim their understanding of the subject is better than professional researchers!
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What kind of a sin could that possibly be? Seems like it would be an honor expounding on such a theoretical model. I'm impressed.
 
brucep said:
What kind of a sin could that possibly be? Seems like it would be an honor expounding on such a theoretical model. I'm impressed.
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The "for my sins" comment probably needs a qualifier. I don't mind teaching in general (I actually quite like teaching some students), but for the most part I'd rather be working on my research. :)
 
Guest254 said:
The "for my sins" comment probably needs a qualifier. I don't mind teaching in general (I actually quite like teaching some students), but for the most part I'd rather be working on my research. :)
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I figured as much. Tell me something about your field of research. So I can do some reading. Good luck in your future.
 
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