Why Gravity is NOT a Force

Thread Starter

nsaspook

Joined Aug 27, 2009
8,000
Well, It may not be a force, but it sure acts like one.

But then relativity also says that a magnetic field is simply the relativistic result of a moving electric field.
There is only one EM force that's fundamental to the universe as we understand it. The electric and/or magnetic 'fields' we interact (accelerations) with are a consequence of spacetime projections of that force into our 3D world. Gravity is (in a sense) that acceleration.
 

crutschow

Joined Mar 14, 2008
26,417
Well I don't know what "spacetime projections of that force into our 3D world" means but that's okay.

Also I don't believe you can have a magnetic field without an electric field but vice versa is possible.
 

Hymie

Joined Mar 30, 2018
947
I came across this youtube video about a week ago.

To my mind it is analysing gravity under a false premise to claim that it is not a force – it considers an object under ‘free-fall’ conditions and what that object is experiencing.

But consider the situation on earth (or any other planet), as Sir Issac Newton realised, an apple falling from a tree is falling under the force of gravity accelerating at 9.81m/s/s (in a vacuum). Many experiments can be conducted on earth to quantify this force – to claim it does not exist is nonsense.
 

BobTPH

Joined Jun 5, 2013
2,888
But consider the situation on earth (or any other planet), as Sir Issac Newton realised, an apple falling from a tree is falling under the force of gravity accelerating at 9.81m/s/s (in a vacuum). Many experiments can be conducted on earth to quantify this force – to claim it does not exist is nonsense.
Imagine you were inside a closed box accelerating through empty space at 9.8 m/s/s . Your apple would fall exactly the same way, and would would infer The same incorrect conclusion.

Bob
 

Hymie

Joined Mar 30, 2018
947
But if I am accelerating at 9.8 m/s/s through empty space there must be some force causing this acceleration (gravity?).
 

BobTPH

Joined Jun 5, 2013
2,888
In the thought experiment I gave, the apple is not accelerating, yet you would claim it us, that is the point.

Curved space-time requires an acceleration in order to stay in the same place, or to move in a straight line at a constant speed. The earth orbiting the sun us actually moving in a straight line in the space-time curved by the gravity of the sun.

Bob
 

ZCochran98

Joined Jul 24, 2018
125
This debate is the entire reason physicists have been searching for a theory of quantum gravity (AKA the "theory of everything" or "grand unified theory," as they're more well-known) for well over half a century. Relativistically, gravity is a distortion in 4D spacetime, the projection of which into 3D appears to be a force. Quantum mechanically, however, gravity is probably an interaction/exchange of gauge bosons called "gravitons" (which have not yet been detected, as the energy levels required are much higher than what we can produce in LHC or Cern - it took us long enough to be able to detect Higgs bosons, after all!). Which is correct? Physicists have no idea yet, because both appear to be true, on different scales. Thus, the need for a theory (provable by experimentation - that excludes string theory!) that is able to unite the general relativity view of gravity with the quantum mechanics view of gravity.
 

bogosort

Joined Sep 24, 2011
600
But if I am accelerating at 9.8 m/s/s through empty space there must be some force causing this acceleration (gravity?).
Your spaceship's engine causes the acceleration. And since there is no physical experiment that can distinguish between the acceleration caused by the engine thrust or the acceleration caused by standing at sea-level on Earth's surface, the two are physically equivalent.

Let go of an apple in the accelerating spaceship and it will appear to fall at a rate of 9.8 m/s^2. The physics of the situation is straightforward: the engine produces thrust, which moves the ship up to the "floating" apple. It's precisely the same physics on Earth: let go of an apple on Earth and it appears to fall at 9.8 m/s^2. The only difference is in our minds: if we take the Earth to be still, we have to invoke a pseudo-force (gravity) to explain the apple's motion. This the same phenomena that produces the more familiar pseudo-forces, like the Coriolis effect and the centrifugal force.
 

Hymie

Joined Mar 30, 2018
947
In my spaceship, the apple will fall to the floor due to the accelerating force applied by the ship’s rocket engines – on earth the apple will fall in exactly the same way due to the force of gravity.
 
Towards the part where the acceleration equation is explained around 10 mins into the video:
"Our spatial coordinates may not be changing yet we may be accelerating...!! And later he explains the equation (Flat Space Time Acceleration + Curved Space time Equation) = Net Acceleration"

So does it mean that when I am sitting steady (Zero Flat Space Time Acceleration), I am actually experiencing acceleration in time dimension of Curved Space time...?? Is that what he meant...??

Regards,
Rahul
 

bogosort

Joined Sep 24, 2011
600
In my spaceship, the apple will fall to the floor due to the accelerating force applied by the ship’s rocket engines – on earth the apple will fall in exactly the same way due to the force of gravity.
But you wouldn't say that the apple falls to the ship's floor because the engine is producing gravity, would you? The simpler explanation is that the once the apple is let go, there are no net forces acting on it and so it doesn't move. The ship's floor, however, has a force acting on it (the engine), causing it to rush up to meet the apple. Upon coming to rest on the floor, the apple now experiences the normal force of the ship's floor. If we put a scale under the apple, it would weight exactly as much as it does on Earth.

If you didn't realize you were on a ship -- there were no windows and the capsule is sound- and vibration-proof -- you would have no way of determining whether you and the apple were feeling gravity or the normal force of an accelerating vessel. This is a huge clue that they are the same thing. It's what inspired Einstein to work out the details in GR.
 

bogosort

Joined Sep 24, 2011
600
So does it mean that when I am sitting steady (Zero Flat Space Time Acceleration), I am actually experiencing acceleration in time dimension of Curved Space time...?? Is that what he meant...??
Yes, exactly. An object in free-fall experiences no net forces ("weightless") and hence has no proper acceleration. We can always choose a coordinate frame in which the object appears to accelerate, but then we must introduce a ficticious force (gravity) to explain the acceleration.

When you drop an apple on Earth, the apple is in free-fall and has no acceleration; once it hits the ground, the ground stops the apple from continuing in free-fall and so it now has a net force (the ground's normal force) acting on it. Hence, the apple -- sitting there on the floor -- has proper acceleration.
 
Yes, exactly. An object in free-fall experiences no net forces ("weightless") and hence has no proper acceleration. We can always choose a coordinate frame in which the object appears to accelerate, but then we must introduce a ficticious force (gravity) to explain the acceleration.

When you drop an apple on Earth, the apple is in free-fall and has no acceleration; once it hits the ground, the ground stops the apple from continuing in free-fall and so it now has a net force (the ground's normal force) acting on it. Hence, the apple -- sitting there on the floor -- has proper acceleration.
Thank you for picking up my post to reply...!!

But why does the apple has to 'rush' to the surface of the earth....!!

Suppose I just leave the apple without any initial velocity...!!

In case of Newtonian Physics, the apple wud be pulled by the earth...!! Hence it accelerates towards it...!!

But what 'forces' the apple to rush to earth in Curved Space Time (ST)...??

Does curved ST fabric 'forces' EVERYTHING to ALWAYS keep moving on straight paths in curved space time..???
 
Top