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Jupiter60
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- TL;DR Summary
- Force or not?
According to general relativity, gravity is not a force, however it is referred as one of the four fundamental forces. This seems like a contradiction.
YesJupiter60 said:Summary:: Force or not?
mpresic3 said:If and when electromagnetism and weak and eventually strong forces are understood as well (and physicists believe they eventually will be), these forces will be considered interactions as well.
This is rather confusingly worded, but seems to me to say that because gravity causes acceleration it must be a force. Of course, in the general relativistic picture, gravity does not cause acceleration in any meaningful sense. That is, objects in free-fall are moving inertially. It can be difficult to state precisely what you mean by changing speed in relativity, but you can certainly set up circumstances where two objects meet twice with different relative velocities when they meet. In relativity this is due to the curvature of spacetime, not due to a force. It's roughly analogous to the fact that if you start in front of me facing to my left, and we both walk straight ahead half way around the world and meet again, you will be facing to my right, although neither of us turned once.Gazi said:relative to another object in space which moves at same speed at the beginning, accelerating through Earth with 9.81m/s^2 is a force because both will not have same speed at the end. Space ships use gravity for sling shot so they increase speed with it thus it should be force
The laws of general relativity break in the singularity like a black hole. That’s why physicists try to find a quantum relationship between gravity and general relativity.Jupiter60 said:Summary:: Force or not?
According to general relativity, gravity is not a force, however it is referred as one of the four fundamental forces. This seems like a contradiction.
Jupiter60 said:Summary:: Force or not?
According to general relativity, gravity is not a force, however it is referred as one of the four fundamental forces. This seems like a contradiction.
This was what Einstein and Schrödinger were hoping at the end of their lives too. Unfortunately it didn't work out.mpresic3 said:I anticipate, in the future all interactions will eventually be understood, possibly using differential geometry, perhaps using concepts that are to be determined (TBD).
Then gravity is not a "fictitious force", because a true gravitational field cannot be made to disappear by choosing an appropriate reference frame since if there's a true gravitational field (due to some distribution of energy, momentum, and stress like a star) the curvature tensor doesn't vanish, and there's no way to get the tensor to vanish by choosing any reference frame, because the nonvanishing of the curvature tensor is a frame-independent property.Frodo said:A virtual, pseudo or fictitious force is a force which can be made to disappear by choosing an appropriate reference frame.
Interesting quibble when one sees the generality of what the OP was asking in a sub-forum entitled Classical Physics and at Basic (High school) level.vanhees71 said:In terms of physics: Any true gravitational field leads to some tidal forces which are not vanishing in a free-falling reference frame.
It does seem to me that a number of posters (I have one, not vanheese71, in mind!) are more keen to demonstrate their depth of knowledge than to put themselves in the shoes of the questioner and answer the question without bringing up minutiae which serve only to confuse and digress.Dale said:Just to hopefully nip a pointless semantic argument in the bud:
Frodo said:One gives different answers to primary school children, high school students, sixth formers, undergraduates, graduates and post-doctoral researchers.
Or even: Yes. No. Maybe.Jupiter60 said:I guess the answer to "is gravity a force?" can be "yes and no".
I think it's important to understand the context and limitations of your knowldege. Someone at high school must accept that they have a limited knowledge and it that there is a deeper understanding. What's important is that they think flexibly when confronted with additional knowledge, refinements or subtleties.vanhees71 said:I think, it's important also at the high school level to be precise. Of course you cannot argue with tensor calculus at the high school level, but you can make as precise as possible qualitative statement about the gravitational interaction, and it should be made clear that inertial reference frames only exist locally at any point in spacetime. I don't say, that's a simple task for a high school teacher having not the sharp mathematical language of tensor calculus.
May I suggest you start by reading Fictitious force as I think it will greatly assist you in understanding fictitious forces. I gave a few examples in my post above - they may also help your understanding.vanhees71 said:I never understood, why inertial forces are called fictitious. I never use the notion of "fictitious forces" for "inertial forces".
A fictitious force (also called a pseudo force, d'Alembert force, or inertial force) is a force that appears to act on a mass whose motion is described using a non-inertial frame of reference, such as an accelerating or rotating reference frame. An example is seen in a passenger vehicle that is accelerating in the forward direction – passengers perceive that they are acted upon by a force in the rearward direction pushing them back into their seats. An example in a rotating reference frame is the force that appears to push objects outwards towards the rim of a centrifuge. These apparent forces are examples of fictitious forces.
Did you see my suggestion of considering a semi-infinite slab of mass? You can have your region as big as you want without tidal forces.vanhees71 said:Again, sufficiently small means a region small enough such that tidal forces can be neglected.
The term "inertial force" sounds less like "explaining away".mpresic3 said:... it is unhelpful to "explain away" gravity as a pseudoforce. ...
Well, it needs to be actually infinite since the finite case is only locally a more or less uniform field. It defies the 'as big as you want' part.Frodo said:Did you see my suggestion of considering a semi-infinite slab of mass? You can have your region as big as you want without tidal forces.
Been pondering this question myself, but it seems the correct answer needs to be some solution to Einstein's field equations. Feel free to point out why I'm spouting nonsense, because I'm bound to do that somewhere in this post.vanhees71 said:I don't think that a semi-infinite slab would not be subject to tidal forces. Do you have a concrete model in mind?
That's right. The guys putting a ship on the moon need to get it there with simple computations. They're not looking for the correct answer to this question, they're looking for an easy one that gets their ship there. The Newtonian answer works far better for them.mpresic3 said:If you are sending a rocket to the moon and addressing physicists and engineering colleagues, it is unhelpful to "explain away" gravity as a pseudoforce.
The OP (who is undoubtedly no longer paying attention to this thread or even this site) did not ask if using gravity-as-a-force mathematics will suffice to get a ship to the moon. He asked which is the more correct answer, which is definitely a relativity question even if he's not actually in a graduate level general relativity course. That correct answer should not detract the student from using the simplified models when appropriate, which for practical purposes, is almost always. An exception is the precise timing needed for function of the GPS system, which must use GR mathematics for both satellite and cell phone, or it just doesn't work.mpresic3 said:Clearly, I think most readers should describe a force as a push or pull to a middle to high school student. With this understanding gravity is a force. A few students will go on to take general relativity and learn the refinements.
Once more: Gravity is a fundamental interaction and not a purely inertial force. The latter is an approximation for sufficiently small spacetime regions. The weak equivalence principle tells you that you can in any spacetimepoint use a locally inertial reference frame but not that there exists a global inertial reference frame. If this were true, then all of physics would just be special relativity with a fixed Lorentzian affine space, but GR (in its geometrical interpretation) tells you that at presence of any kind of energy, momentum, and stress distribution (the energy-momentum tensor of all matter and radiation) spacetime is described by a pseudo-Riemannian manifold with non-zero curvature and thus the always existing local inertial frames are only local, which is why they are called local!A.T. said:The term "inertial force" sounds less like "explaining away".