Does Einstein's Theory of Relativity Apply to Everyday Gravity on Earth?

[porkncheese]

Hi my background is in mechanical engineering. I use a little bit of motion physics but not often.
Pls excuse any errors or wrong terminology I may use.

Einstein's theory of relativity addresses the force of gravity in space time but does it address or apply to gravity on Earth? What creates or causes the gravity on earth? Gravity which is responsible for holding everything to the surface of the earth. From objects at rest or in motion to liquids and solids.

I believe Newton states that mass creates gravity. The greater the mass the greater the gravity. Isaacs equations are still widely used today in engineering. I have often referred to them myself.

So if I put a bowling ball in a vacuum of space will a small ball bearing be drawn to it due to a gravitational force?

 

[Meir Achuz]

So if I put a bowling ball in a vacuum of space will a small ball bearing be drawn to it due to a gravitational force?

Yes....

 

[porkncheese]

What a genius Isaac Newton really was. He blows my mind

So has this practical example been performed and confirmed?

 

[ModusPwnd]

Look up the cavendish experiment, that may be interesting to you.

 

[porkncheese]

Excellent. I believe the experiment was done under Earth's gravity.
Have other experiments been performed in vacuum enclosure?
I would like to see a huge suspended mass with a tiny object pinned to the bottom by gravity. To simulate myself standing upside down in Australia

 

[phinds]

Newton's equations are what fall out of GR in a limiting case, so there is no difference between the two, really. Anything that Newton's Law of Gravity predicts is also predicted by GR, just more correctly and also in a way that works for the more general case. But it IS way easier to use Newton, so folks who build bridges and such don't use GR because they don't have to (they are working inside the limited case).

 

[ModusPwnd]

Excellent. I believe the experiment was done under Earth's gravity.

It was done near the earth, but it was not testing the gravity from the earth. It was testing the gravitational attraction from one ball to the other perpendicular to the direction the Earth would attract.

Even in the vacuum of space there is gravity from earth.

 

[A.T.]

Einstein's theory of relativity addresses the force of gravity in space time but does it address or apply to gravity on Earth?

Yes. This animation has almost the same title as your post:

https://www.youtube.com/watch?v=DdC0QN6f3G4

 

[porkncheese]

Thanks to everyone for the feed back.

@ModsPwnd. I understand their testing gravitational forces of bodies. Not the Earth's gravity.
I just imagine putting a large mass which represents Earth in a vacuum chamber. Test its gravitational pull on a small object representing a car. No strings attached. Proportionate in scale. That would be cool to see. A scaled down example of the gravitation attraction of earth

 

[porkncheese]

@A.T. Very interesting animation. So GR does in fact apply in Earth's atmosphere also. Thank you for answering one of my questions. Lol. What a freak of nature Albert Einstein. So much perseverance the man had also.

I shouldn't have titled this Einstein vs Newton. I wanted to compare the two. That sounds like boxing match which was never my intention

 

[porkncheese]

@Phinds What do u mean by "a limited case"
I agree Newtons equations of gravity and motion are much easier and more applicable to everyday science and engineering on earth. His equations are still being used everywhere in everyday applications.

GR is much harder to grasp and is really used in astrophysics, true? The precision of Einstein's field equations over great distances into space is just phenomenal.

Thanks again guys for helping me understand a few things.

I'd say these two men are pure geniuses. Ahead of their time. The biggest contributors to an amazing world called Physics

 

[phinds]

Thanks to everyone for the feed back.

@ModsPwnd. I understand their testing gravitational forces of bodies. Not the Earth's gravity.
I just imagine putting a large mass which represents Earth in a vacuum chamber. Test its gravitational pull on a small object representing a car. No strings attached. Proportionate in scale. That would be cool to see. A scaled down example of the gravitation attraction of earth

There are satellite in orbit that are smaller than a car, so this is being demonstrated every day. There is no difference between that and what I think you mean which is to put them both very far removed from other gravitational forces and have the start off stationary relative to each other an watch them come together. That wouldn't prove anything not already shown by current satellites in orbit.

Newton's Law is just GR for small masses traveling slowly.

 

[porkncheese]

Ah yes I was going to state that there wouldn't be anything to learn really. It would just b a cool scaled down experiment to observe I feel.
GR is definitely for larger bodies at greater speeds over larger distances

 

[phinds]

GR is definitely for larger bodies at greater speeds over larger distances

Well, not really LARGE bodies but more massive bodies (which, I realize, does usually come with "larger" but let's get the terminology right) and not over larger distances. Newton works just fine over large distances. Stick with "more massive" and "moving faster" (LOTS faster )

EDIT: actually "more massive" isn't right either. More DENSE is what it is. Newton works just fine without mass limitation, it just doesn't work in very high gravity fields such as near an extremely dense object like a neutron star or a black hole.

 

[porkncheese]

I did mention in my opening paragraph to excuse any incorrect terminology.
I know what ur saying.
Thanks again to all

 

[Chestermiller]

Newton: A falling body has a downward gravitational force acting on it, and it is accelerating.
Einstein: A falling body has no force acting on it, and it is not accelerating.

Newton: A body at rest on the Earth has no net force acting on it, and it is not accelerating.
Einstein: A body at rest on the Earth has an upward force acting on it, and it is accelerating.

Einstein's description is a consequence of the curvature of spacetime and the movement of bodies along their (curved) world lines. The presence of a massive body like the Earth causes the region of spacetime in its proximity to become curved.

Chet

 

[MikeGomez]

Einstein: A falling body has no force acting on it, and it is not accelerating.

Acceleration is the time derivative of velocity.

It sounds incorrect to me to say that a body in freefall is not accelerating. It makes me think I can step off the top of a tall building and everything will be alright because I won’t accelerate.

 

[Chestermiller]

Acceleration is the time derivative of velocity.

It sounds incorrect to me to say that a body in freefall is not accelerating. It makes me think I can step off the top of a tall building and everything will be alright because I won’t accelerate.

In the context of general relativity, what you are referring to here is called "coordinate acceleration", but it is not true acceleration. You know that it is not true acceleration because, if you had an accelerometer attached to the body, the accelerometer would read zero. This has been verified experimentally. When you are in free fall, all you are doing is traveling along the geodesics of curved spacetime.

Chet

 

[phinds]

Acceleration is the time derivative of velocity.

It sounds incorrect to me to say that a body in freefall is not accelerating. It makes me think I can step off the top of a tall building and everything will be alright because I won’t accelerate.

Yes it does sound weird, but you want to watch out for that. Under Newton, You'll die because of acceleration and under GR you'll die because of the curvature of space-time. That's probably a nicety you won't care much about on the way down.

 

[ModusPwnd]

Its not the acceleration due to gravity that kills you... Its the acceleration due to the coulomb force at the pavement. You can accelerate at any rate and be just fine. Thats because all your atoms get accelerated at the same rate (as long as the tidal force is low).

 

[MikeGomez]

In the context of general relativity, what you are referring to here is called "coordinate acceleration",

I'm with you so far...

but it is not true acceleration. You know that it is not true acceleration because, if you had an accelerometer attached to the body, the accelerometer would read zero. This has been verified experimentally. When you are in free fall, all you are doing is traveling along the geodesics of curved spacetime.

Chet

That I don’t get. It sounds like either an improperly designed accelerometer, or an improper use of one.

The basic design for an accelerometer is a box with a mass inside which is connected (to the insides of the box) by springs. When a force is applied to the outside of the box, the springs inside either stretch or compress due to the inertia of the mass inside, and that stretching or compressing is what is used for measuring acceleration.

Here is a link, not to be insulting, but just to show the image as a reference to what we are talking about.
http://www.daviddarling.info/encyclopedia/A/accelerometer.html

So, what if I told you that I could accelerate the accelerometer without making it read anything? I could take off the lid of the box, and apply not a single force to the outside of the box, but several forces along the length of the springs and to the mass as well. Maybe it won’t read exactly zero acceleration, but you get the idea, it would certainly read very much less than its true acceleration due to its change in velocity.

This is not how an accelerometer is intended to be used, but that is exactly what we are doing when we use it to try to measure acceleration of a freely falling body due to gravity. Gravity reaches into the box, and applies a force uniformly to every unit of mass (or perhaps energy). It defeats the design of the accelerometer.

Accelerometers are devices which measure acceleration that is ultimately due to very short range contact forces. Gravity (whether a force or not) is long range in comparison, and acceleration due to gravity cannot be measured in this manner.

 

[MikeGomez]

Yes it does sound weird, but you want to watch out for that. Under Newton, You'll die because of acceleration and under GR you'll die because of the curvature of space-time. That's probably a nicety you won't care much about on the way down.

Understood, but I would prefer to die having had a much better understanding of Einstein’s reasoning (or today’s more modern reasoning) than I do now.

 

[MikeGomez]

Its not the acceleration due to gravity that kills you... Its the acceleration due to the coulomb force at the pavement. You can accelerate at any rate and be just fine. Thats because all your atoms get accelerated at the same rate (as long as the tidal force is low).

Very true indeed, and I think a lot of people don’t appreciate that.

In fact, the same could be said for jerk and joust and all other higher derivatives of position. As long as you are able to apply the changes "at the same time", you will not feel it.

 

[A.T.]

It defeats the design of the accelerometer.

Only in the context Newtonian physics gravity defeats the design of the accelerometer. Acceleration by gravity, which is a real force in Newtonian physics, is not picked up by an accelerometer, unlike all other real forces. So gravity is a rather inconsistent special case in Newtonian physics.

In General Relativty acceleration due to gravity is a coordinate effect, which are never picked up by an accelerometer. So in General Relativty the relation between accelerometers and real interaction forces is more consistent, without special cases.

Accelerometers are devices which measure acceleration that is ultimately due to very short range contact forces.

Nope. Accelerometers can measure acceleration due to long range electromagnetic forces. Accelerometers measure the real physical, frame invariant proper acceleration :
http://en.wikipedia.org/wiki/Proper_acceleration

Gravity (whether a force or not) is long range in comparison, and acceleration due to gravity cannot be measured in this manner.

It has nothing to do with range, but the fact that the force of gravity is always proportional to mass, which is a key feature of inertial forces (coordinate effects), as opposed to real physical effects. That's why General Relativity models acceleration due to gravity as a coordinate effect, consistently with what accelerometers measure.

 

[George Jones]

I am going out with my family right now, so I don't have time to get involved in the discussion, but here a couple of things that I wrote years ago that echo what Chestermiller and A.T. have written:

At first this, it's very difficult warp one's mind around this stuff! Like anyting, though, intuition is built up through experience.

The geodesic ("straight line") along which an object moves when it falls freely is a straight line in spacetime, not a straight line in space. The usual version of Newton's first law refers to a straight line in space.

Whether or not an object is freely falling, it "moves" along a line spacetime. For example, suppose I hold a ball around which a watch is strapped, so that it is "at rest in space". The ball is still "moving" along its worldline in spacetime, i.e., at every reading on the watch, the ball is at a different event on its worldline.

If I hold the ball for a while and then release it so that it falls freely, part of the ball's worldline corresponds to the time when its "at rest" in space, and part of the ball's worldline corresponds to the time when its freely falling. So, the question is, "Which part of the worldline, if any, is straight in spacetime?"

A mathematical model, general relativity, that is backed up by loads of empirical evidence answers "When the ball is freely falling." In this model, objects "fall at the same rate", because they move along (almost) the same grooves in spacetime. Ignoring the spacetime curvature caused by a test object, these "straight line" gooves are intrinsic properties of spacetime, so "falling" at the same rate makes sense.

Now, in special realtivity, consider an accelerometer in a spaceship located deep in in interstellar space.

The accelerometer consists of two main parts - a hollow sphere like a basketball (go Suns!) inside of which is a slightly smaller sphere. Initially, the centres of the spheres coincide, so that there is a small, uniform gap between the spheres.

If the ship is accelerating, the gap will be closed, and contact between the spheres will be made. An alarm that indicates "curved" motion will sound. If the ship is not accelerating, no alarm will sound, and "straight line" motion is indicated.

Now move the accelerometer to a place near the surface of the Earth, and assume that the accelerometer is small enough that tidal forces can be neglected. When the accelerometer is held at rest (in space), the alarm sounds, but if the accelerometer falls freely, no alarm sounds.

The advanced book Relativity on Curved Manifolds by de Felice and Clarke elaborates on DaleSpam's comments in a very readable way.

Take a look at the nice prechaper Geometry and Physics: An Overview; in particular, read sections 3 and 4 from this prechapter.

 

[D H]

Here is a link, not to be insulting, but just to show the image as a reference to what we are talking about.
http://www.daviddarling.info/encyclopedia/A/accelerometer.html

That is an incredibly bad article. The first sentence kills it. Accelerometers do not and cannot measure gravity.

From a Newtonian perspective, accelerometers measure all the real forces acting on the accelerometer except for gravity. Why this special case? There's inevitably a test mass of some sort inside every accelerometer. You can shield that test mass from contact forces and from electromagnetism, but no matter how hard you try, you can't shield it from gravitation. There is no such thing as a gravity shield.

It's so much simpler from a general relativistic perspective. Accelerometers measure all the real forces acting on the accelerometer, period. Accelerometers can't measure the centrifugal or Coriolis forces because they're fictitious forces. The Newtonian gravitational force is a fictitious force in general relativity.

 

[A.T.]

You can shield that test mass from contact forces and from electromagnetism, but no matter how hard you try, you can't shield it from gravitation.

The crucial thing is not the shielding, but how the forces affect objects. Unlike electromagnetism, gravity will always accelerate everything by the same amount, including light. So you can always (locally) "transform away" the acceleration due to gravity by changing the reference frame, just like you can "transform away" inertial (fictitious) forces.

 

[MikeGomez]

The way I see it, the issue is not about shielding, it is about even or uneven acceleration.

Gravity pulls evenly on all parts of a mass. That is why objects of different weights fall at the same rate at the surface of the earth. That is called coordinate acceleration.

On the other hand when you apply a force to one end of a spring, the spring will compress. That is because a force is not applied evenly to all parts, but (more or less) to only a single contact point. That is called proper acceleration.

So when we talk about proper acceleration, we are really talking about accelerating something unevenly, such that a certain amount of compression within the body occurs. An accelerometer is in essence an instrument capable of measuring this compression. It has nothing to say of evenly distributed acceleration (coordinate acceleration).

 

[A.T.]

The way I see it, the issue is not about shielding, it is about even or uneven acceleration.

The issue neither shielding nor even vs. uneven. It is proportionality to mass, which allows you to transform it away.

Gravity pulls evenly on all parts of a mass. That is why objects of different weights fall at the same rate at the surface of the earth. That is called coordinate acceleration.

On the other hand when you apply a force to one end of a spring, the spring will compress. That is because a force is not applied evenly to all parts, but (more or less) to only a single contact point. That is called proper acceleration.

That is not really the distinction. It's more like this:

- Coordinate acceleration is time derivative of velocity, in some frame of reference.
- Proper acceleration is what an acceleromenter measures, which is the same in all frames of reference.

Or:

- Proper acceleration is the coordinate acceleration in a local free-falling frame.

The talk about deformation of some bodies is too specific and misses the general point. You can measure proper acceleration without deformation of bodies, with light rays, which curve in frames undergoing proper acceleration.

 

[Dale]

The way I see it, the issue is not about shielding, it is about even or uneven acceleration.

Gravity pulls evenly on all parts of a mass. That is why objects of different weights fall at the same rate at the surface of the earth.

Yes. This property of accelerating all masses the same (I assume that is what you mean by even) is what allows gravity to be represented geometrically. It is one of the defining propertys of gravity.

 

[ModusPwnd]

I don't see a distinction worth protesting here... Yes, you cannot shield the gravitational force that is why all masses accelerate equally. With an electric force shielding or screening happens regularly. No shielding, even acceleration, proportional to mass - it all describes the same thing.

 

[porkncheese]

This is why I question the practicality of GR on earth.
Perhaps its because I haven't studied GR in depth but I cannot grasp the 2 previous Einstein statements.
They directly contradict Newton which is what I was taught in motion physics.
A body at rest has a velocity that remains at zero. How does something accelerate without even moving?

 

[ModusPwnd]

How does something accelerate without even moving?

By defining "moving" via its position in GR spacetime space rather than Newtonian euclidean space. Thats what I gather from my limited knowledge... That seems to be the distinction being made between "coordinate" and "proper" frames.

 

[porkncheese]

I was taught Newtonian physics. I use it regularly and successfully in engineering on earth.
It is extremely difficult to grasp the theories of Einstein.
Perhaps astrophysics is where it is better used.

 

[WannabeNewton]

I was taught Newtonian physics.

GR isn't Newtonian physics. It's that simple. GR isn't hard to understand once you let go of your Newtonian intuitions.

P.S. we can formulate Newtonian mechanics in exactly the same way as GR modulo relativistic effects i.e. in terms of space-time curvature with free fall = non-accelerating geodesic motion. See Newton-Cartan theory.