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Wikipedia points out that Gravity is:

most accurately described by the general theory of relativity (proposed by Albert Einstein in 1915) which describes gravity not as a force, but as a consequence of the curvature of spacetime caused by the uneven distribution of mass.

As I am not all that clued up on physics, I don't quite understand this and to add to the confusion, in the top voted answer to the closed question, What is Gravity?, the answer starts of by saying that

Gravity is a fundamental force in addition to the other 3 known forces of nature that are: electromagnetism, the weak force and the strong force.

then later on states that

Einstein's theory treats gravity as something other than force, namely that it isn't a force.

so I have been trying to understand it all with a bit more reading.

In the question, What is Gravity and what causes objects to act against it?, there is a passage with an image which if I understand it correctly, explains the quote from Wikipedia.

So I understand the concept of gravity, in that it's not actually a force, but more of a displacement in the spacetime grid. An object with a big enough mass will bend the spacetime, causing smaller objects to "attract" to it Earth in its spacetime

So the fact that the Earth, the moon and other planets are spinning is not what creates Gravity somehow? Is the bent spacetime why they do not act like a centrifuge and throw everything off the surface?

To add to this, a few years after Einstein’s theory of relativity described gravity as the distortion of space and time, we gained awareness of the confusing world of quantum physics. This led to the discovery of force-carrier particles, or bosons, behind three of the fundamental forces. This is all totally outside my comprehension, and in an attempt to marry gravity with quantum theory, physicists came up with a hypothetical particle — the graviton, and the name is attributed to Dmitrii Blokhintsev and F. M. Gal'perin in 1934.

What I am trying to understand is how is Gravity created?

With all these theories, am I to assume we actually don't know how gravity is created, even though it was discovered in the 17th century and we have had over 400 years to find out?

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    $\begingroup$ No one has ever come close to answering this question. Curved space is just kicking the can down the road. So now the new question becomes “How does a mass curve space time?” It’s still a question but only more complicated. $\endgroup$ – Bill Alsept Apr 5 at 17:45
  • $\begingroup$ Still no answer, we actually don't know how gravity is created. I think they need to re visit push gravity. At least it follows the inverse square law and and has an actual description. $\endgroup$ – Bill Alsept Aug 4 at 16:29
  • $\begingroup$ Gravity will be better understood when quantum gravity theory will be constructed, accepted and validated. Neither it is done, neither graviton is confirmed. So, as about true nature of gravity, we can only conclude that "The truth is out there" as The X-Files main character has said. $\endgroup$ – Agnius Vasiliauskas Aug 4 at 17:27
  • $\begingroup$ @BillAlsept Here's the thing you seem to continue missing in each one of the comments you give. Science in general is in the game of epistemological pragmatism so if it can construct a scientific theory/mathematical model which can be thrown at reality more times before breaking then such a theory is a success. We've gotten to a point, however, that many of our theories deal with describing phenomenon that you cannot even really see nor sensibly interact with closely. $\endgroup$ – The victorious truther Aug 4 at 17:49
  • $\begingroup$ @BillAlsept Given the predictive success of Newtonian gravitation in its own time many physicists came forward with some rather popular "interpretations" of the theory. Some describing it as an "action at a distance force" some such as Lesage giving an extremely mechanical description (without scientific evidence) of its mechanism through "push forces" and others including Newton gave aether like fluid modeling of the phenomenon. All of these were interesting pet models in DESCRIBING the phenomenon but they never much gave actual evidence to lean one way or the other. $\endgroup$ – The victorious truther Aug 4 at 17:53
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To avoid confusion you should not mix up the Newton's theory of gravitation and Einstein's general relativity.

According to Newton gravity is a force which is measured as attractive between massive objects. As per Newton gravity is generated by the mass.

However Einstein via the equivalence principle understood that the trajectory of a free falling object in spacetime is independent of its mass, so gravity could be described as a geometry shaped by the distribution of mass/energy in spacetime. A free falling object follows a geodesic and experiences no acceleration; instead an object stationary on the earth surface is subject to acceleration which deviates it from a geodesic. This acceleration in Newton theory is interpreted as the force of gravity. The spinning earth does act as a centrifuge, but the centrifugal force, luckily for us, is negligible compared to the force of gravity.

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    $\begingroup$ This doesn't seem to answer the question. The OP knows that gravity overcomes the centrifugal force, but he is wondering how the gravity is created. "As per Newton gravity is generated by the mass." - How? $\endgroup$ – Chalmondley Mar 20 '19 at 8:35
  • $\begingroup$ Physics describes how nature works, not why nature works in that way. Your question is not applicable in science. $\endgroup$ – Michele Grosso Mar 20 '19 at 16:23
  • $\begingroup$ You said, "Physics describes how nature works, not why..." I have asked how gravity works. As @Chalmondley says, you haven't said how. You just said it is generated by the mass. How? $\endgroup$ – Chris Rogers Mar 21 '19 at 8:26
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    $\begingroup$ We have to be careful with the phrasing. Science measures relations between physical variables (observables) and then infer physical laws to be validated in successive experiments. Experimentally two bodies attract each other with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers (Newton's law of universal gravitation). That is how things work in nature. Science can not say more, just describe it. A refined description is the general theory of relativity. $\endgroup$ – Michele Grosso Mar 21 '19 at 10:36
  • $\begingroup$ @MicheleGrosso OK....so HOW is gravity created? That is the question after all. $\endgroup$ – Bill Alsept Apr 5 at 17:37
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I think your question is a fair one. The problem is that physics is very mathematical, and in some cases not very intuitive, so there is an inherent difficulty in trying to describe rigorous mathematical relationships with words.

Newton concluded that objects will move at constant speeds in straight lines unless forces act on them to change the value or direction of their speed. Newton classified gravity as a force that caused objects to accelerate in a certain direction, so the mass of the Earth, for example, causes apples to fall from trees. Einstein changed that perspective to one in which masses curve the spacetime in which objects move. That's hard to imagine, and may seem like sophistry, but it means that the orbit of the moon is not a circle in a flat spacetime but a straight line in a curved one. The moon goes around the earth not because gravity is attracting the two, but because gravity is bending spacetime around them and the moon is moving a straight-line in that curved spacetime.

People continue to refer to gravity as a force in part because Newton's model of gravity is easier to understand and apply than Einstein's, and gives a sufficiently precise answer for the sort of day to day problems encountered in textbook physics (eg weights sliding down ramps and so on).

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The force you live with every day, and call gravity, really isn't the force of gravity. The closest thing to actual gravitational forces are the tidal forces (accelerations) which change the relative proximity of freely falling objects.

What you actually feel as the force of "gravity" is the acceleration of Earth pushing you out of your natural free-fall geodesic world line. Einstein's equivalence principle says that when you are in free-fall (also called free-float) you remain in a local Lorentz (inertial) frame without accelerating. So when you stand on Earth's surface, you are continually accelerating upward, thus changing inertial frames. You can think of the rate of acceleration as the rate at which you change inertial frames.

When you are rotated around in a centrifuge, for example, you are also continually changing inertial frames. The net effect of the force acting on you moves you in a circle, but at each instant the force is linearly directed toward the center of rotation.


I should add that the apparent centrifugal acceleration field differs from the apparent acceleration field naively attributed to gravity near Earth's surface.

Let's imagine we are pressed against the inside of a cylinder rotating about its axis in deep space, with the cylinder large and heavy enough that our mass will not significantly affect its rotation. If you were to run in one direction around the cylinder, you would "weigh more" than if you were standing still. If you ran in the opposite direction, you would "weigh less". If you threw a ball "upward", (that is opposite the pull of our pseudo gravity) you would find that the ball would not fall directly back into your hand. These features are called the Coriolis effect.

It's also worth mentioning that (as I understand things) consideration of a rotating frame of reference was an important step is Einstein's development of his General Theory of Relativity.

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  • $\begingroup$ This is an interesting hypothesis. Have you any references to share on where I can read more about this? $\endgroup$ – Chris Rogers Nov 4 '19 at 8:18
  • $\begingroup$ Relativity: The Special and General Theory By Albert Einstein books.google.com/… $\endgroup$ – Steven Thomas Hatton Nov 4 '19 at 13:50

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