Is gravity a force?

  • #1
Summary:
Conflicting opinions on videos I’ve watched
I’ve watched a few videos recently that explained that gravity is not a force rather it is caused by time dilation because clocks tick slower closer to mass. Objects will follow a geodesic through spacetime and require a force to move them away from a geodesic - so the surface of the earth is accelerating everything on the surface. If you fall into a hole you don’t experience a gravitational force pulling you down rather you feel the removal of the force that was pushing you away from your geodesic. I then watched a video on quantum gravity that said gravity was a force (although almost infinitely weaker than the other known forces) and the theoretical particle is the graviton.
So which is true?

On the subject of gravity not being a force - if we are being constantly accelerated by standing on the surface of the earth doesn’t that require some energy? If yes where does that energy come from?
 

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  • #2
phinds
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Summary:: Conflicting opinions on videos I’ve watched

I’ve watched a few videos recently that explained that gravity is not a force rather it is caused by time dilation because clocks tick slower closer to mass.
This, unfortunately, is utter nonsense, since it is backwards. Gravitational time dilation is CAUSED by gravity, it is not a cause OF gravity. Also, clocks do NOT tick slower in a gravity well, they tick at the same one second per second as all clocks. This all gets mildly complicated. I suggest you read some actual physics rather than pop-sci presentations which are misleading you badly.

Gravity is considered a force in Newtonian Physics, but our universe doesn't work on Newtonian physics, it works on General Relativity and gravity is not a force.

Newtonian Physics is a limited range of General Relativity so is used for day to day practical applications like building bridges.
 
  • #3
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I’ve watched a few videos
Videos are generally not good sources if you want to actually learn physics. Textbooks or peer-reviewed papers are better.

gravity is not a force
In General Relativity, this is true; gravity is not a force. Instead it is due to spacetime curvature.

rather it is caused by time dilation because clocks tick slower closer to mass.
This is one aspect of "gravity", and ultimately the time dilation effect being referred to is due to spacetime curvature.

Objects will follow a geodesic through spacetime and require a force to move them away from a geodesic
Yes, this is one way of restating the fact that gravity is not a force--objects that, in pre-relativity physics, we would have said were moving "solely due to gravity", are in fact moving on geodesics; they are in free fall, and feel no force. In fact, the GR view that gravity is not a force is actually simpler than the Newtonian (pre-relativity) view that gravity is a force, because on the Newtonian view one has to explain why objects moving solely under the "force" of gravity, such as falling rocks or planets orbiting the Sun, do not feel any force. In GR, this is obvious: they feel no force because they are not being acted on by any force.

the surface of the earth is accelerating everything on the surface.
Yes.

If you fall into a hole you don’t experience a gravitational force pulling you down rather you feel the removal of the force that was pushing you away from your geodesic.
Yes.

I then watched a video on quantum gravity that said gravity was a force (although almost infinitely weaker than the other known forces)
This is the relativity forum, not the quantum physics forum; and in any case we don't have a good theory of quantum gravity (though trying to find one is an active area of research). The basic quantum gravity idea being referred to here is better described by the term "interaction", not "force"; basically, the idea is that "gravity", aka spacetime curvature, should ultimately be due to the same kind of thing that underlies the three known interactions in the Standard Model of particle physics (strong, weak, electromagnetic).

the theoretical particle is the graviton.
At some level of theory, yes, most physicists expect that quantum aspects of gravity will manifest themselves as a massless, spin-2 gauge field, the particle aspect of which is called the graviton. However, it is extremely unlikely that we will be able to test such quantum aspects of gravity experimentally in the foreseeable future.
 
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  • #4
Dale
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Summary:: Conflicting opinions on videos I’ve watched

I then watched a video on quantum gravity that said gravity was a force (although almost infinitely weaker than the other known forces) and the theoretical particle is the graviton.
So which is true?
Currently there is no accepted and validated quantum theory of gravity. So any claims about gravitons are purely speculative at this time.

In contrast, the other concepts of gravity that you described are on firm theoretical and experimental ground.
 
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  • #5
. I suggest you read some actual physics rather than pop-sci presentations which are misleading you badly.
Seems a little harsh. The videos I’ve been watching are by scientists working in the field eg Don Lincoln from Fermilab not just random bods posting fake science from their shed.

Learning from textbooks would be a challenge as I haven’t done any advanced maths in 35 years so would need to revise that intensively before I could tackle any physic textbooks (unless they were without the maths but then they wouldn’t really be more useful than videos).
 
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  • #6
Orodruin
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Seems a little harsh. The videos I’ve been watching are by scientists working in the field eg Don Lincoln from Fermilab not just random bods posting fake science from their shed.
It really isn’t. Popular scientific presentations are not designed to teach you physics. They are designed to tell you about physics in an entertaining and light fashion without being too far out of bounds. This is true regardless of whether the presenter is a respectable physicist in their own right.
 
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  • #7
PeroK
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On the subject of gravity not being a force - if we are being constantly accelerated by standing on the surface of the earth doesn’t that require some energy? If yes where does that energy come from?
Acceleration does not, in general, require energy. Let's return to the Newtonian model of gravity for an example. The Moon is in an approximately circular orbit of the Earth, hence has continuous centripetal acceleration towards the Earth. No additional energy is needed to maintain this acceleration.

In GR, the situation for someone standing on the surface of the Earth is somewhat analagous: there is continuous upwards acceleration, but it requires no energy input to maintain this.
 
  • #8
pervect
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Summary:: Conflicting opinions on videos I’ve watched

I’ve watched a few videos recently that explained that gravity is not a force rather it is caused by time dilation because clocks tick slower closer to mass. Objects will follow a geodesic through spacetime and require a force to move them away from a geodesic - so the surface of the earth is accelerating everything on the surface. If you fall into a hole you don’t experience a gravitational force pulling you down rather you feel the removal of the force that was pushing you away from your geodesic. I then watched a video on quantum gravity that said gravity was a force (although almost infinitely weaker than the other known forces) and the theoretical particle is the graviton.
So which is true?

On the subject of gravity not being a force - if we are being constantly accelerated by standing on the surface of the earth doesn’t that require some energy? If yes where does that energy come from?

In Newtonian theory, gravity is a force. When Einstein created the theory of special relativity, he observed that the concept of Newtonian gravity was not consistent with special relativity. Einstein was concerned by this - there is now plenty of evidence that his special theory of relativity was correct. Then and now, though, the issue of how the special theory could possibly incorporate gravity remains.

There were several possibilities, but eventually Einstein came up with what is now called General Relativity as a way to incorporate gravity into the framework of special relativity.

You've already given a good lay summary of how GR works, congratulations on doing your research. I'll address a few things you haven't mentioned, though.

In order to incorporate gravity into special relativity, Einstein considered what the reference frame of an accelerating rocket or elevator would act like according to the theory. This is commonly called "Einstein's elevator". The motivation for considering the two cases, the force one feels in an accelerating elevator, and the force one feels due to gravity, as being equivalent is usually called "The Principle of Equivalence". There are several flavors of how the principle of equivalence works, this is just a short and not very detailed summary.

One of the predictions that comes out of a detailed, formal, analysis of Einstein's elevator is that clocks on the floor of Einstein's elevator do not tick at the same rate as clocks on top of the elevator. This effect is not compatible with the idea of considering gravity as "just a force". Forces do not, by themselves, cause clocks to tick at different rates. Now, it is perhaps possible to view gravity as a force plus other effects that cannot be fit into the mold of a "force", effects that would cause clocks to tick at different rates. However, I'm not aware of any such approach that has been published, and in any event it is not how we currently view and use the theory of General Relativity in practice. Einstein thought for quite a long time on the matter, and he eventually came up with a full theory of how he thought gravity could work in a manner that was compatible with special relativity, a theory that we now call "General Relativity".

Experimental results, such as the Pound-Rebka experiment, illustrate that the predictions that General Relativity makes are consistent with experiment, while Newton's theories of gravity fall short. The differences are small, but measurable with sufficiently precise and careful experiments.

Unfortunately, while this is all well and good as far as it goes, it is difficult to proceed further with just popularizations. I don't think it is possible to get a really good understanding of all aspects of General Relativity from the popularizations you have mentioned, though they are good start. In particular, I'm not aware of any way to popularize the treatment of energy in General relativity. There are some advanced graduate level treatments, but reducing them to a popular level has not, as far as I know, been done. Of course, I don't know everything, but I haven't heard of such a treatment.

What I can say though is that it doesn't take any energy to keep an object stationary on the surface of the Earth. It turns out that General Relativity has many different notions of energy - I would reagard ADM, Bondi, and Komar energy as being the "big three", though there are others. All of the formulations of energy in GR that I've mentioned are consistent with the idea that it doesn't take any work to hold an object at rest on the Earth's surface.
 
  • #9
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Acceleration does not, in general, require energy. Let's return to the Newtonian model of gravity for an example. The Moon is in an approximately circular orbit of the Earth, hence has continuous centripetal acceleration towards the Earth. No additional energy is needed to maintain this acceleration.

In GR, the situation for someone standing on the surface of the Earth is somewhat analagous: there is continuous upwards acceleration, but it requires no energy input to maintain this.
My brain always gets messed up by this case. For example, if I fall toward the earth from a substantial height, very bad things will happen to me. Which seems to imply some type of work being done on my fragile body.
 
  • #10
PeroK
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My brain always gets messed up by this case. For example, if I fall toward the earth from a substantial height, very bad things will happen to me. Which seems to imply some type of work being done on my fragile body.
In Newtonian physics, the gravitational force does work on you in that scenario. You have increasing KE in the Earth's frame of reference.

In GR, if we choose your local free falling frame, then the Earth's surface has a force on it and the part that hits you has increasing KE in your frame.

GR locally reduces to SR and to Newtonian mechanics. It's the global picture that you need curved spacetime to explain. E.g. bodies falling on opposite sides of the Earth.
 
  • #11
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In Newtonian physics, the gravitational force does work on you in that scenario. You have increasing KE in the Earth's frame of reference.

In GR, if we choose your local free falling frame, then the Earth's surface has a force on it and the part that hits you has increasing KE in your frame.

GR locally reduces to SR and to Newtonian mechanics. It's the global picture that you need curved spacetime to explain. E.g. bodies falling on opposite sides of the Earth.
That mostly makes sense, especially the earth having increasing KE in my frame (honestly, that seems workable even in Galilean relativity), but what do you mean by "the earth has a force on it?"
 
  • #12
PeroK
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what do you mean by "the earth has a force on it?"
I didn't say that!
 
  • #13
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I didn't say that!
Sorry, I didn't mean to misquote you. You said the Earth's surface has a force on it, specifically if we chose the free falling frame.
 
  • #14
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Sorry, I didn't mean to misquote you. You said the Earth's surface has a force on it, specifically if we chose the free falling frame.
If you land on a bit of ground, that bit of ground has a real upward force from below. That's why (looking at things locally) that bit of ground is accelerating upwards. And, even after you collide with it, it's still being forced upwards.

What doesn't work is if you try to follow those local mechanics down to the centre of the Earth. From the global perspective, things don't add up in purely Newtonian terms without gravity itself as a force.
 
  • #15
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Sorry, I didn't mean to misquote you. You said the Earth's surface has a force on it, specifically if we chose the free falling frame.
In any case, maybe I could pose a better set of questions to describe my confusion. How is gravitational heating explained, or nuclear fusion in stars? What if I where to set down on a planet with something like 30x G(earth)? Would I just turn into a pool of organic sludge? Would it be fair to say that the small patch of earth that I am resting on would experience some (very trivial amount) of gravitational heating?

I can grasp the fact that I am essentially I'm the same frame as the surface of the earth, but why do I have weight, suddenly when I come to rest with respect to the surface?

This is why I said earlier that this specific case (at rest on the surface of a massive body) breaks my brain.
 
  • #16
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If you land on a bit of ground, that bit of ground has a real upward force from below. That's why (looking at things locally) that bit of ground is accelerating upwards. And, even after you collide with it, it's still being forced upwards.

What doesn't work is if you try to follow those local mechanics down to the centre of the Earth. From the global perspective, things don't add up in purely Newtonian terms without gravity itself as a force.
Ok, so I think you mean just the coulomb forces that make the earth a solid mass. Is that correct?
 
  • #17
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And let me please reframe it one other way. If I'm at rest on th
In any case, maybe I could pose a better set of questions to describe my confusion. How is gravitational heating explained, or nuclear fusion in stars? What if I where to set down on a planet with something like 30x G(earth)? Would I just turn into a pool of organic sludge? Would it be fair to say that the small patch of earth that I am resting on would experience some (very trivial amount) of gravitational heating?

I can grasp the fact that I am essentially I'm the same frame as the surface of the earth, but why do I have weight, suddenly when I come to rest with respect to the surface?

This is why I said earlier that this specific case (at rest on the surface of a massive body) breaks my brain.
And one more example that probably mashes things up a little, but hopefully makes some sense. If I am at rest on the surface of the earth, there is no work. And if I am at rest hovering above the surface of the earth, there is no work. In the latter case I would have to expend energy to do so. Is that correct? At least classically?
 
  • #18
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In GR, the situation for someone standing on the surface of the Earth is somewhat analagous: there is continuous upwards acceleration, but it requires no energy input to maintain this.
However, even this statement requires some clarification. Energy is frame variant. So for the scenario of someone standing on the surface of the earth, in the Earth’s frame no energy is required, but in the local free-fall frame energy is required. Where that energy comes from cannot be answered locally, and requires transitioning from a local flat free-fall frame to a global curved frame. At that point there is no longer a nice easy to describe flat analogy, but you are stuck using the full tensor math of GR.
 
  • #19
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Ok, so I think you mean just the coulomb forces that make the earth a solid mass. Is that correct?
I don't care where the forces come from. Kinematically, the Earth's surface is accelerating.

You have two options:

1) Take gravity as a force and do Newtonian mechanics.

2) Use GR.

There are ways (as I have tried to show you) to explain local gravity without curved spacetime or gravity as a force. It's clear that you can't explain everything that way - otherwise, there would be no need for GR at all.

If you don't like that idea, then stick with 1) or 2).
 
  • #20
Dale
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if I am at rest hovering above the surface of the earth, there is no work. In the latter case I would have to expend energy to do so. Is that correct? At least classically?
No, that is not correct classically. For example, a piece of superconductor material can hover over a magnet without expending energy. The fact that a helicopter requires energy just means that it is inefficient, not that energy is always required to hover.
 
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  • #21
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I don't care where the forces come from. Kinematically, the Earth's surface is accelerating.

You have two options:

1) Take gravity as a force and do Newtonian mechanics.

2) Use GR.

There are ways (as I have tried to show you) to explain local gravity without curved spacetime or gravity as a force. It's clear that you can't explain everything that way - otherwise, there would be no need for GR at all.

If you don't like that idea, then stick with 1) or 2).
Just to rewind a bit.... the part that boggles my mind is that from the GR point of view, an object at rest on a massive body experiences constant acceleration, with no transfer of energy. This is very weird to me. it's much easier for me to grasp an orbiting body (technically with Newtonian acceleration) not requiring energy to maintain orbit, as in GR it is in free fall.
 
  • #22
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No, that is not correct classically. For example, a piece of superconductor material can hover over a magnet without expending energy. The fact that a helicopter requires energy just means that it is inefficient, not that energy is always required to hover.
Thanks, great counter example.
 
  • #23
PeroK
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Just to rewind a bit.... the part that boggles my mind is that from the GR point of view, an object at rest on a massive body experiences constant acceleration, with no transfer of energy. This is very weird to me. it's much easier for me to grasp an orbiting body (technically with Newtonian acceleration) not requiring energy to maintain orbit, as in GR it is in free fall.
You can use valenumr's laws of motion if you like:

Newton: ##\vec F = m \vec a##

valenumr: ##E = m|\vec a|##

I know which one I'm sticking with.
 
  • #24
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PS classically, for a body of velocity ##\vec v## under force ##\vec F##: $$\frac{dE}{dt} = \vec F \cdot \vec v$$There is, therefore, no change in kinetic energy due to a force if:

1) ##\vec F## is perpendicular to ##\vec v##

or

2) ##|\vec v| = 0##

In GR, in the reference frame of the Earth, the curvature of spacetime counteracts the real force and results in ##\vec v = 0## for an object at rest in the Earth's frame of reference. Hence ##\frac{dE}{dt} = 0##.

You can't explain this using classical mechanics, but given that curved spacetime does counteract the real force, it's not inconsistent with classical ideas about energy.
 
  • #25
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You can use valenumr's laws of motion if you like:

Newton: ##\vec F = m \vec a##

valenumr: ##E = m|\vec a|##

I know which one I'm sticking with.
Well, that wasn't what I was trying to say at all. I certainly understand that energy is frame dependent. I have no problem with the fact that an object at rest in an inertial frame has zero (kinetic) energy in that frame.
 

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