[Ras72]

I believe GR breaks down at the atomic scale and for black holes.
(And its predictions appear to be slightly off as in the fly-by and the Pioneer anomalies and, more radically, in the recent and unconfirmed INFN neutrino experiment.)
Could some one please expand on the small scale limit of applicability? At what scale does it start to fail and how?
Thank you

[swansont]

Ras72, on 25 October 2011 - 03:57 PM, said:

I believe GR breaks down at the atomic scale and for black holes.
(And its predictions appear to be slightly off as in the fly-by and the Pioneer anomalies and, more radically, in the recent and unconfirmed INFN neutrino experiment.)
Could some one please expand on the small scale limit of applicability? At what scale does it start to fail and how?
Thank you

For GR "small scale" for distance is the Planck length, which is much smaller than an atomic nucleus. Scales on the order of Planck units are where you need a quantum theory of gravity.

http://en.wikipedia....ki/Planck_units

[Ras72]

swansont, on 25 October 2011 - 04:15 PM, said:

For GR "small scale" for distance is the Planck length, which is much smaller than an atomic nucleus. Scales on the order of Planck units are where you need a quantum theory of gravity.

http://en.wikipedia....ki/Planck_units

Are GR's predictions in perfect agreement at the nucleus scale?How does the failure manifests in probing down from the 100 billion billions planck length units of the nucleus diameter?Is it a growing discrepancy with experimental mass results? Or does it appear more suddenly?Is it found to be regular for different particles?

[DrRocket]

Ras72, on 25 October 2011 - 03:57 PM, said:

I believe GR breaks down at the atomic scale and for black holes.
(And its predictions appear to be slightly off as in the fly-by and the Pioneer anomalies and, more radically, in the recent and unconfirmed INFN neutrino experiment.)
Could some one please expand on the small scale limit of applicability? At what scale does it start to fail and how?
Thank you

No one knows the limits. Many share your thoughts.

I think the Pioneer anomaly has been explained conventionally. The neutrino expereiment is still up in the air, but most people suspect that there is an error in the experiment.

It is known that general relativity and quantum theory are not compatible. Most, but not all (e,g. Roger Penrose), think general relativity will give way to a theory of quantum gravity.

To really determine the limits we will need a new and better theory. There is a lot of ongoing research to develop such a theory, but it does not yet exist.

GR may break down at the Planck scale, but so far that is just speculation. You can find all sorts of rank speculation stated as fact in popularizations. No one really has a clue what goes on at that scale.

[Ras72]

DrRocket, on 25 October 2011 - 09:39 PM, said:

No one knows the limits. Many share your thoughts.

I think the Pioneer anomaly has been explained conventionally. The neutrino expereiment is still up in the air, but most people suspect that there is an error in the experiment.

It is known that general relativity and quantum theory are not compatible. Most, but not all (e,g. Roger Penrose), think general relativity will give way to a theory of quantum gravity.

To really determine the limits we will need a new and better theory. There is a lot of ongoing research to develop such a theory, but it does not yet exist.

GR may break down at the Planck scale, but so far that is just speculation. You can find all sorts of rank speculation stated as fact in popularizations. No one really has a clue what goes on at that scale.

Thank you Dr Rocket. Your answer is right on point. The need to postulate the existence of dark matter and dark energy is also potentially indicative of an imperfect understanding of gravity.
Wikipedia relates however of growing consensus in favor of dark matter and energy.

[URAIN]

Ras72, on 28 October 2011 - 08:40 PM, said:

Thank you Dr Rocket. Your answer is right on point. The need to postulate the existence of dark matter and dark energy is also potentially indicative of an imperfect understanding of gravity.
Wikipedia relates however of growing consensus in favor of dark matter and energy.

Actually what is the Dark matter and dark energy?

where it is exist?

Do we able to know existence of this ?

I questioned this but no one answering acurately.

[DrRocket]

URAIN, on 9 November 2011 - 10:28 PM, said:

Actually what is the Dark matter and dark energy?

where it is exist?

Do we able to know existence of this ?

I questioned this but no one answering acurately.

Nobody knows for sure.

[URAIN]

DrRocket, on 10 November 2011 - 04:41 AM, said:

Nobody knows for sure.

Why these are considering in the science, the medium which has much trusted in the world than anything. Even GOD. What is use of considering such things which are unknown or has not any existence?

[imatfaal]

URAIN, on 11 November 2011 - 03:54 PM, said:

Why these are considering in the science, the medium which has much trusted in the world than anything. Even GOD. What is use of considering such things which are unknown or has not any existence?

We have experimental results and observations which do not fit with our theories and understanding. We therefore postulate ideas that might help explain the anomalous results - some ideas are good others are bad - those that are good are tested and retested and eventually are accepted as true.

We have two problems - one is that galaxies do not rotate and behave the way we expect them to, we thus postulate an unseen "dark matter" which interacts gravitationally and will bring the observed results of galactic behaviour (which we know are correct) into line with theory. The second is that the expansion of the universe is accelerating - and we believe that an acceleration must be driven - dark energy is the "placeholder" for all the ideas that try and guess what that driver of acceleration is. Neither dark matter (which is fairly well developed ) nor dark energy (whiich is just a name for an unknown phenomenon) are accepted as correct yet - dark matter is getting close but we need to actually start detecting it.

[URAIN]

imatfaal, on 11 November 2011 - 04:25 PM, said:

We have two problems - one is that galaxies do not rotate and behave the way we expect them to,

Please clear it (I am not understanding). As per theory, Does galaxies must have to rotate?

This post has been edited by URAIN: 12 November 2011 - 01:34 PM

[insane_alien]

URAIN, on 12 November 2011 - 01:32 PM, said:

Please clear it (I am not understanding). As per theory, Does galaxies must have to rotate?

If they did not rotate then they would collapse in only a few billion years.

[IM Egdall]

URAIN - I think you misunderstood what has been said. Galaxies do rotate. So from that we can see how fast stars at the edges of galaxies are moving.

But the outer stars are orbiting so fast they should be flying off into outer space -- that is if nothing held them but the gravitational pull of the visible stars. So, physicists conclude, there must be more matter, thus more gravity, out there than we can see. No one nows what this additional unseen matter is -- it is called "dark matter".

Dark matter is also used to explain the motions of groups of galaxies, and other astronomical phenomena. For more, see link:

http://en.wikipedia....iki/Dark_matter

This post has been edited by IM Egdall: 12 November 2011 - 07:36 PM