# Is gravity a force or a property of space?

## Main Question or Discussion Point

I have a question. I understand that one of the main problems with a theory of everything is the unification of gravity with the other three forces. Could this be due to the fact gravity may not be a force at all but a property of space? If Einstein essentially defined gravity as such, then why is it still treated like a force even after successful confirmations of GR?

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Charge is a property of electrons, it still exerts a force; I'm not sure why something can't be both a property and a force (or actually how any force could not result from a property of something)

Ken G
Gold Member
If Einstein essentially defined gravity as such, then why is it still treated like a force even after successful confirmations of GR?
It's because GR is not consistent with quantum mechanics when you go to the Planck scale. So one or the other, or both, is wrong at that scale. I really don't know why so many people expect gravity to act more like a quantum mechanical force at those scales, instead of just breaking down and becoming something completely different, but I think it's the simple case of looking for your keys under the streetlight before you look for them in the shadows.

A.T.
I have a question. I understand that one of the main problems with a theory of everything is the unification of gravity with the other three forces. Could this be due to the fact gravity may not be a force at all but a property of space? If Einstein essentially defined gravity as such, then why is it still treated like a force even after successful confirmations of GR?
In Newtons model gravity is an interaction force. In Einsteins model gravity is an inertial force. The more general concept of non-Euclidean spactime geometry that also causes other effects than mass attraction is called gravitation.

We still use Newton's model, because it is simpler mathematically and yields almost the same results as Einstein's for most applications.

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I have a question. I understand that one of the main problems with a theory of everything is the unification of gravity with the other three forces. Could this be due to the fact gravity may not be a force at all but a property of space? If Einstein essentially defined gravity as such, then why is it still treated like a force even after successful confirmations of GR?
Theories of everything are not scientific, let us better to ignore them...

Effectively one of the difficulties on unifying the known interactions with gravity is because gravitation in GR is geometrical instead of a true force as electromagnetism. However, it is perfectly possible to consider gravity as a force in the modern field theory of gravity (FTG), and thus allowing a future unification with the rest of interactions.

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Could it be that depending on point of reference :) , it either property or the force. Because it sounds like gravity is similar to temperature which can be hot or cold. May be for the source of temperature it's a property but for recipient (observer) it's a force?

It's because GR is not consistent with quantum mechanics when you go to the Planck scale. So one or the other, or both, is wrong at that scale. I really don't know why so many people expect gravity to act more like a quantum mechanical force at those scales, instead of just breaking down and becoming something completely different, but I think it's the simple case of looking for your keys under the streetlight before you look for them in the shadows.
I've heard this repeated a lot, but why is it the case they don't work together at such small scales? What is the proper QM model at larger scales then?

why is it still treated like a force even after successful confirmations of GR?
Things are not so simple.

THREE ROADS TO QUANTUM GRAVITY,Lee Smolin, Chapter 12:

We...looked at three different approaches to quantum gravity....black hole thermodynamics, loop quantum gravityu and string theory. While each takes a different starting point, they all agree that when veiwed on the Planck scale, space and time cannot be continuous....On the Planck scale, space appears to be composedof fundamental discrete units. ...It is possible that the different approaches represent different windows onto the same quantum world...and if this is so there must be a way iof unifying them all into a single theory.
For other alternative theories of gravity, try here:
Alternative Theories,
http://en.wikipedia.org/wiki/Gravity#Alternative_theories

I understand that one of the main problems with a theory of everything is the unification of gravity with the other three forces. Could this be due to the fact gravity may not be a force at all but a property of space?
Gravity is a force and according to GR, it appears to be a property of space.