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?
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.
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..
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. [..].
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.
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.
Einstein called in his 1916 GR paper the gravitational field a "field of force".
[..] 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.
Thanks for the clarification! You seem to say that "Gravity is not a force" really means "Gravity is not a four-force". That is a concept that may need more elaboration, in particular for non-falling observers. And what happened to Einstein's gravitational field?It's precisely because we identify the four-force as something that all freely-falling obsrevers agree on. An object that is freely falling experiences zero four-force, and hence, because the four-force is more useful than ordinary force, we tend to use that versus ordinary force. [..]
No, you can't. At least not correctly. The centrifugal force explanation of the tides is fundamentally flawed; it happens to get the right result thanks to a lucky cancellation of errors. That circular motion explanation utterly fails for a Moon-sized object falling straight toward the Earth. When that object is at a distance of 384,400 km from the center of the Earth, the tidal forces exerted by it on the Earth would be exactly the same as those exerted by the Moon itself when it is 384,400 km from the center of the Earth.You can explain the tides in terms of motion in a circle, with all the 'forces' we're familiar with.
The spacecraft example above leads to how tidal forces can be addressed in GR. Suppose the distance between the spacecraft's center of mass and the accelerometer is many orders of magnitude less than the distance between the spacecraft and planet. This means we can linearize the tensor mathematics that describes the curvature of spacetime. What you'll get is the same prediction as the that predicted by in Newtonian mechanics. Things get hairier if that linearization isn't valid such as when the spacecraft is in the close proximity to an extremely massive object, but tidal forces can still be calculated. Tidal forces become much stronger than those predicted by Newtonian mechanics in case of extreme curvature of space-time.How can tidal forces not be taken care of in GR? They're only there because of what, presumably, GR predicts.
We are getting very far from OP's question, but the analogy is better than you're giving it credit for being; indeed, I've seen it used in serious introductions to GR. It works because in GR there's no such thing as standing still - in four-dimensional spacetime you're always moving in the time direction.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.
I can see that GR tries to explain the origin of a force like the one that the proximity of two masses causes but, if the effect is the same as that which occurs between two charges, then why is it not allowed to be called a force? What distinguishes the one 'force' from the other force apart from the difficulty in detecting it?
Then we should be allowed to treat centrifugal force as a real force too. In general relativity, you can't have one defined as a real force and not the other.Weight is a readily perceived and measured force - given the appropriate equipment - so why are we not 'allowed' to treat gravity as a force? It is such a tangible thing that it seems to me that the purist tail is wagging the dog of experience.
Strictly speaking GR doesn't identify "real" or "fictitious"; it depends on the used coordinate system and one's interpretation what one calls fictitious and what real. That doesn't mean that gravity and fictitious forces must be one and the same - rather the contrary (see my post #20)!Then we should be allowed to treat centrifugal force as a real force too. In general relativity, you can't have one defined as a real force and not the other.
Edit: that's the beauty of general relativity - you can use whichever coordinate system you want. But this means we must accept that gravity and fictitious forces are one and the same.
Edit again: You could say that the coordinate system comoving with the expansion of the universe is the only true coordinate system, and use this coordinate system to judge what is gravity and what is fictitious force. But this seems like an unnecessary extension to me, without much insight gained.
It probably depends on what one means with "fictitious" in this context. In my world, "fictitious" forces are imaginary (or even occult) forces, without an identified corresponding physical cause
The term "fictitious force" is strictly defined in physics. If you misunderstand it, that is your problem and yours alone.It probably depends on what one means with "fictitious" in this context. In my world, "fictitious" forces are imaginary (or even occult) forces, without an identified corresponding physical cause (see also the footnote in my post #20).