Is gravity both a force and not a force?

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Because of gravity, you need to define inertial frame by zero acceleration when there is no force, not by zero accelerometer readings,
If you cannot detect a universal force as the one above, how do you know that there is "zero acceleration" then?
 
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jbriggs444
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If you cannot detect a universal force as the one above, how do you know that there is "zero acceleration" then?
In science, if you can never detect it, you can safely ignore it and let Occam's razor cut it away.
 
  • #28
ISamson
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Okay, I know there are many other discussions regarding this exact topic, but I might (probably not) have found an easier way to think of gravity being a force or not a force.
Just like light can be a particle or a wave from how you measure it, to my understanding so can gravity. As I am told, NASA look at gravity as a force (newtonian gravity), for their calculations, because GR equations take too long and make a tiny difference. However, when calculating gravity in much larger masses (maybe like galaxies), general relativity, where gravity is a distortion of spacetime, is needed as it becomes far more accurate.
This means depending on how you need it, gravity can be used as a force (for planets and asteroids etc) and with it not being a force (for larger masses possibly stars and galaxies etc.)

I don't know if this is correct. I have just been reading so much about if gravity is a force or not and I am trying to get some answers).

Could this work as an over simplified answer?
Newtonian gravity makes an exact description of small things in orbit, like satellites or planets, General Relativity is more accurate in calculating and predicting the behaviour of very large bodies, like black holes and galaxies. So, you're right.
 
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PeterDonis
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Newtonian gravity makes an exact description of small things in orbit, like satellites or planets, General Relativity is more accurate in calculating and predicting the behaviour of very large bodies, like black holes and galaxies.
This is not correct. Newtonian gravity is not an "exact" description for anything; it is an approximation in all cases. It is a reasonably good approximation for cases where gravity is very weak and all objects are moving relative to each other much slower than light. That includes many problems involving large things like galaxies as well as small things like satellites and planets. However, our measurements are accurate enough that we can detect deviations from Newtonian gravity even for satellites orbiting the Earth (Google for "Gravity Probe B"), as well as for planets orbiting the Sun (the extra precession of Mercury's perihelion is one of the classic tests of GR).
 
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Before dealing with gravity and general relativity, one should ask and answer an analogous question in Newtonian mechanics: Is inertial force a force or not a force? After answering that one, the gravity case is conceptually much easier.
 
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In science, if you can never detect it, you can safely ignore it and let Occam's razor cut it away.
I was trying to point out a conceptual inconsistency in Newtonian mechanics. Had Einstein ignored it, we would not have general relativity today.

Before dealing with gravity and general relativity, one should ask and answer an analogous question in Newtonian mechanics: Is inertial force a force or not a force? After answering that one, the gravity case is conceptually much easier.
Yes, that's what I meant with 'forces' causing 'relative acceleration'.
 

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