What is the evidence for the existence of gravity

Question

I have recently been wondering that what good evidence proves the existence of gravity (besides the Cavendish experiment).

I would like straightforward answers that are on point. I will be willing to listen.

Now for something a little different but still kinda related:　If I change the materials in the Cavendish experiment from lead to a different material (E.g. plastic or another metal), but keep the mass the same & set the experiment up exactly like how Cavendish did, would I get the same results?

I will state that physics is not my strong suit.

Answer 1

The following experiment is a simple one that can be tried at home. First pick up a ball in your hand and then release the ball above the ground. If you observe carefully you may see the ball moving closer towards the Earth. This is evidence of the mysterious gravitational force.

Answer 2

Your second question "If I change the materials in the Cavendish experiment from lead to a different material ...." leaps straight into the Equivalence Principle, the Eötvös Experiment and one of the deep remaining mysteries about gravity: why is it not "choosy"? In more formal language, the gravitational "charge" of a body is, as far as experiment so far can tell, altogether independent of that body's composition and instead depends only on the mass of that body. This is unlike electromagnetism, for example, which only affects matter that bears electric charge. So the Cavendish experiment's result is unaffected by the makeup of the spheres - as long as they have the same mass. As a practical point, it would be hard to do the experiment with plastic spheres as these would need to be so big to have the same mass as metal ones. But it would be easy to do with other heavy metals and the result is unchanged, as long as the mass of the spheres is the same in each case.

This independence of gravity from makeup allows the possibility of the equivalence principle, which is grounded on the experimental observation, through the Eötvös and like experiments, of the equality between inertial and gravitational mass (one couldn't possibly have such a result if gravity depended on makeup).

As for your first question, it is a little hard to know where to begin. The most basic everyday observations show that, relative to the surface of the Earth, something let fall does not undergo uniform motion. Set up a falling object with a high speed camera and scale, and you will easily determine an acceleration. See the Moon undergo its acceleration, relative to the Earth's surface. Now, whether you choose to say that this shows a forcing influence on these bodies, as in Newtonian Gravity, or whether you say that a frame stationary with respect to the Earth's surface is experimentally observed to be non-inertial and follow through to a more Einsteinian, geometric description of the phenomenon, in both cases you conclude that the Earth is acting in some way on falling bodies.

Answer 3

What is the evidence for the existence of gravity

Etymology

If you look in a dictionary, particularly in one that shows etymology (origin of words) you will see that gravity is an old word meaning weight.

It also has the meaning of seriousness (e.g. the notion that serious ideas or subjects weigh heavily on the mind) - but we can ignore that.

Apples

It is everyone's everyday experience that if you let go of an apple, it will tend to move towards the ground.

Science is mostly about measuring this sort of phenomenon and working out how long it takes to move the distance from hand to ground, how fast it travels in each part of the journey and whether the time taken depends on colour, smell, density, day of week, holder's hat-size etc.

So far you shouldn't have any difficulty in believing in

• the notion that things have weight
• the notion that bigger things tend to be heavier
• the notion that heavier things are things which are harder for you to keep from moving from your hand to the ground
• the notion that some materials have more weight than their size would suggest (small iron cannonballs are heavier than similar sized tennis balls)

You should have access to plenty of evidence that all the above are true.

You can just name this tendency of heavy things to move as, their gravity.

Theory of gravity

Many people of a scientific bent studied the apple phenomenon. They hoped to come up with some mathematical way of describing the way apples move. Then you could use the mathematical description to also predict how coconuts and other objects would move. Military types thought this might help them with moving their cannonballs onto the heads of people they didn't like very much.

To scientists a "theory" is the name they give for any sort of mathematical description that actually works to describe something in ways that can be usefully used to predict what happens next.

Newton came up with a theory about how heavy stuff moves (i.e. about stuff with gravity)

Newton's law of universal gravitation states that every mass attracts every other mass in the universe, and the gravitational force between two bodies is proportional to the product of their masses, and inversely proportional to the square of the distance between them.

The notion that the Earth attracts apples shouldn't be too hard to find evidence for. Apples dropped in Australia move in a different direction than apples dropped in Canada, but both move towards the large heavy object we call Earth.

For the part about the inverse square law. You can perform all sorts of tests but one way is to apply the theory to the motion of planets about stars or moons about planets and see how well the theory's predictions of what happens next matches what you actually see happening next.

But note that Newton's theory is a description for two objects only.

Answer 4

To prove gravity all one can do is admitting Newton’s second law. Do the following syllogism:

Preposition 1: Newton's second law (simple version) states that “Accelerations are caused by Forces”.

Preposition 2: Since Galileo (and perhaps much earlier) it has become an everyday life fact that falling objects do so with an acceleration of magnitude $\rm g$.

Conclusion: If forces causes’ acceleration and falling bodies are accelerated then falling must be governed by a force (we happen to call such a force as gravity).

Now the question may go into “proving” Newton’s second law. In this post a question about proving it was made (please read the comments to the question itself); in this answer I tried to explain why forces must be related to accelerations and not other time derivatives.