Main Question or Discussion Point
I read somewhere that gravity is not a force. Is this true? What does it mean?
There is a class of forces in physics known as fictitious forces or inertial forces. Whether they act as real force or not depends on your choice of coordinate system. If you chose surface of the Earth as your reference point, for example, you have to account for the force of gravity for all of your equations to make sense. But if you happen to be inside an elevator during a free-fall, you will not experience gravity. For all intents and purposes, gravity just goes away.I read somewhere that gravity is not a force. Is this true? What does it mean?
In general relativity, they don't count gravity as a force, but its effects are taken into account in the equations of the spacetime curvature. In the limit of low speed and weak curvature, we can say that gravity acts approximately like a force (which is the physics that was around before Einstein's relativity).I read somewhere that gravity is not a force. Is this true? What does it mean?
No, you can't. Newtonian POV: We can feel every real force but gravity. There is no way to directly sense the gravitational force. However, note that we don't "feel" fictitious forces.For all practical purposes, we can say that Gravity 'produces' a force - because we can all feel it.
Exactly. Worrying about what different theories call things is just a semantics game. What really matters is how well physics predicts experimental outcomes. In those realms where Newtonian mechanics is (approximately) valid, Newtonian mechanics and relativity will agree on experimental outcomes, sensor readings, etc.I think physicists don't get too worked up about what exactly constitutes a force.
This is the sort of thing laymen hear on Carl Sagan programs. Actually, I suspect that most professional physicists would get it wrong.according to the theory of General Relativity, there is an equivalence to a body falling in free fall to an identical body out in free space traveling at a constant velocity..
The is clearly incorrect. If you are standing here on Earth you are experiencing gravity, and as you yourself said, gravity invoves spacetime not being Euclidian. So we are not in "our Euclidian space".what a big mass (like a planet) does is warp or curve space in such a way so that objects flying about freely in this curved space appears to us in our Euclidian space...
I agree with that - the "force of gravity" acts on a spring scale when you stand on it. The "different explanation" is to say that that force is not due to gravitation but due to acceleration. And as others said, whatever you call it or how you interpret it, everyone agrees that the spring is compressed by a real force.For all practical purposes, we can say that Gravity 'produces' a force - because we can all feel it. Modern Physics takes matters further than that, of course, but the same 'force' effects are observable even when they are 'explained' slightly differently.
I'm very much a 'purist' and I disagree with what you call the 'purist' view for reasons similar to the ones you mention.The 'purist' view is strongly against allowing gravity to be a force. [..].
If these two travellers stop walking, do they experience anything? Is there any Force acting on them to make them continue getting closer to the Pole? I do understand this is just an analogy but I feel it is too far away from the point about the 'reality' or otherwise of a gravitational force. You would need to say what the equivalent of a force is for these two travellers. It certainly couldn't be the same as the 'force' that pulls two masses together because the analogy is not 1:1.Imagine two travellers standing at different points along the equator. Now both of them start walking north, towards the north pole. What happens ? The further north they walk, the closer they get to one another ! They approach each other not because of any force acting between them, but because of the geometry of earth's surface. Gravity works the same - as time passes, two bits of matter will gravitate towards each other, not because of any force, but because of the geometry of space-time.
You are right, it isn't at 1:1 analogy. However ( and I failed to mentioned that ), once one understands that the northward direction corresponds to time, and the distance between the travellers corresponds to spatial separation, then the situation is clear : there is a tendency for two bits of matter to approach one another over time, purely based on the geometry of space-time. This is, I believe, quantified via the Raychaudhuri equation.If these two travellers stop walking, do they experience anything? Is there any Force acting on them to make them continue getting closer to the Pole? I do understand this is just an analogy but I feel it is too far away from the point about the 'reality' or otherwise of a gravitational force. You would need to say what the equivalent of a force is for these two travellers. It certainly couldn't be the same as the 'force' that pulls two masses together because the analogy is not 1:1.
Thanks for the precision!OK, my "opinion" is that a Force will produce an acceleration or change of shape. (That's Newtonian - based). How one measures these changes is, to my mind, irrelevant.
For a definition of Field in this context, I'd say that the presence of a Field will produce a Force on an object with a particular property - e.g. mass / charge / current. So F=mG and F=qE for instance. So the Field is force per unit of some property.
So we could continue the discussion with those definitions - or with others, perhaps(?).
However if we read the Equivalence Principle correctly (as I see it) then it is not that "gravity is just geometry" but rather that we only see dynamic evolution of test particles and so the boundary between gravity and geometry is indistinguishable. We can vary our choice of space-time geometry and introduce a "physical" force of gravity and not see any difference in predictions. I think this means that it is the geometry which is "not physical" rather than the gravitational force.
Gravitational forces (except tidal effects) don't exist in GR because forces cannot be 'transformed away' by going into free fall. This was Einstein's great insight.
Yet Einstein's GR does not depend on traditional 'fields'....such fields are a different model than his final geometric interpretation of spacetime curvature.Einstein called in his 1916 GR paper the gravitational field a "field of force".
I cited him in the context of his explanation of fields in GR. Do you know by chance how with that geometric interpretation of GR the words "force" and "field" were redefined, so that those words have different meanings than the same words of 1916?[..] such fields are a different model than his final geometric interpretation of spacetime curvature.
Quote from whom, I'm not sure but it doesn't matter.Gravitational forces (except tidal effects) don't exist in GR because forces cannot be 'transformed away' by going into free fall. This was Einstein's great insight.