Science Forums

# Gravity

## Recommended Posts

how does gravity work?

• Replies 104
• Created

#### Popular Days

Who are you? Have you tried googling for "how does gravity work"?

Tormod

##### Share on other sites

who are you?wait I know a retard tormod

##### Share on other sites

uh oh... watch the name calling, please.

as to how gravity works....

gravity is a psuedo-force much in the same way that centrifugal force is a psuedo-force. it is caused by the temporal inertia of mass (its tendancy to remain at rest in a given moment in time) being accelerated through space-time.

the acceleration is provided by the accelerated expansion of the universe. as a body of mass exists in space, the 'outward' expansion of space tries to push the body of mass through time as well. as space expands, it encounters resistance from the temporal inertia of the body of mass resulting in a curvature of space-time around it.

other bodies of mass are not actually 'pulled' towards another body of mass, so much as space-time is expanding around them. 'realatively' speaking, the distance between the bodies of mass will become less. (of course, other factors such as total mass and velocity and trajectory influence the behaviours of the various bodies of mass which interact with each other.

##### Share on other sites

Our present understanding of gravity suggests that it is only a manifestation of spacetime curvature. Mass curves the space around it. The mechanism is unclear, but it happens.

According to General Relativity, a body subject only to gravitation, simply describes a path in the space that corresponds to the shortest distance between two points of spacetime. When there is no body in the neighborhoods, it describes a straight line at constant velocity. When a body happens to be next, the curvature of spacetime makes the shortest path between to points to be distorted and the moving body will simply follow this distortion.

Pretty simple, isn't it? This is equivalent to say that all bodies subject only to gravity travel in geodesics in the spacetime. Matter curves spacetime and distort these geodesics from a straight line to other curves, depending on the distribution of matter.

##### Share on other sites

• 4 months later...

but can you prove it and how?

Jump

##### Share on other sites

Anthony: but can you prove it and how?

What the above people are describing is gravity according to Einstein's general theory of relativity. General relativity has been confirmed by several observations.

Because the Sun is massive, it should warp the spacetime surrounding it. Thus, light that passes by the Sun should be deflected from a straight line course; it should follow the straightest possible path in the curved spacetime. In short, a star that appears to be next to the Sun should have its light bent during its travel past the Sun. To test this, some such stars were observed during a eclipse and their positions were found to be slightly off of where they should have been - their light had been bent by the Sun's mass. Since then other such observations have confirmed the bending of light by mass: one has even come to be known as Einstein's Cross - it is a single star (or is it a galaxy?) that appears to be multiple positions at once, forming a cross-like shape.

Another has to do with orbital precession. Mercury's orbit had been known to have a very slight deviation from that predicted by Newton's theory of universal gravitation. That is, the ellipse of Mercury's orbit slowly moved around the Sun. Einstein's general relativity described Mercury's orbital precession quite nicely whereas Newton's didn't.

Einstein also stated that strong gravitational fields - large curvatures of spacetime due to mass - would cause time dilation, and this too has been experimentally confirmed.

Another prediction of Einstein's general relativity is that gravitational waves exist and that they travel at the speed of light. I know that experiments were planned to detect gravitational waves, but I stopped keeping track of that kind of thing a couple of years ago. If the experiments have been carried out, then they would add to the pile of evidence that confirms Einstein's view of gravity as being warped spacetime.

##### Share on other sites

Someone above mentioned that gravity is a pseudo-force in general relativity...how can that be?

Einstein introduced the equivalence principle which states that gravity and acceleration are indistinguishable - they are equivalent. Mass has two interpretations. Gravitational mass determines how strongly an object attracts others gravitationally and inertial mass determines how much an object resists attempts to change its state of motion: i.e., to accelerate it. Yet these two seemingly unrelated masses turn out to be not only proportional to each other, but to be exactly equal (that's why heavy and light objects fall at the same rate: the gravitational attraction for the more massive object is greater, but it also takes a greater force to accelerate the more massive object - since the two opposing factors are exactly equal, they cancel out). Many people had pondered on why this was but Einstein explained it.

Here's the idea in a nutshell. Picture yourself in a regular elevator. Naturally, you are standing on the floor because gravity pulls you down and holds you to it. Now, imagine that the elevator is not here on Earth but is instead out in deep intergalactic space, where there is no gravity. You are floating around inside the elevator. But if the elevator were to be accelerated at 9.80 m/s^2 in a direction towards the top of the elevator, you would find yourself standing on the floor and being held there by the accelerating elevator. You'd feel a force on your feet equal to the force of gravity if you were on Earth and the elevator motionless. The two situations are indistinguishable. Even if you perform an experiment, you can't tell the difference. For example, if you had a ball and dropped it, in either situation it woud fall towards to the floor with an acceleration of 9.80 m/s^2. Einstein concluded that since there is no experiment you could perform that would tell which is the case - on Earth and motionless or out in space and accelerating - that gravity and acceleration can be considered equivalent and gravity can be transformed away by replacing it with a suitable acceleration (or vice versa).

PS: Technically, tidal forces can be used to determine which is the case if the reference frames are large. For example, if you had 2 balls and dropped them on opposite sides of a very wide and tall elevator, you would see the two balls approach each other horizontally as they fell because of their non parallel paths directed towards Earth's center of mass. However, in space, this would not occur: the balls would fall perfectly parallel to each other. So the equivalence principle holds truly for only very small reference frames.

##### Share on other sites

Here's the idea in a nutshell. Picture yourself in a regular elevator. Naturally, you are standing on the floor because gravity pulls you down and holds you to it. Now, imagine that the elevator is not here on Earth but is instead out in deep intergalactic space, where there is no gravity. You are floating around inside the elevator. But if the elevator were to be accelerated at 9.80 m/s^2 in a direction towards the top of the elevator, you would find yourself standing on the floor and being held there by the accelerating elevator. You'd feel a force on your feet equal to the force of gravity if you were on Earth and the elevator motionless. The two situations are indistinguishable. Even if you perform an experiment, you can't tell the difference. For example, if you had a ball and dropped it, in either situation it woud fall towards to the floor with an acceleration of 9.80 m/s^2. Einstein concluded that since there is no experiment you could perform that would tell which is the case - on Earth and motionless or out in space and accelerating - that gravity and acceleration can be considered equivalent and gravity can be transformed away by replacing it with a suitable acceleration (or vice versa).

This and a good number of other thought experiments are covered in his own book on the subject "Relativity (The Special and General Theory)" Albert Einstein.

Not only do you benefit from sharing his understanding and specific apporach, but just to actually read HIS words as he felt they best explained his ideas. Great book.

##### Share on other sites

There are 2 characteristics of a gravitational field:

!) It attracts matter

2) It dilates time, or to look at it another way, It reduces the speed of light.

These are balancing effects. The forces involved must follow the conversation of mass/energy in just the same way as electromagnetic forces do.

So how does it work out? An object falling into a gravitational field gains energy as velocity. This kinetic energy is balanced by a loss of energy from its mass. That is calculated as E=MC2. The C is the speed of light. Inside a gravitational field C is lowered.

The best way to think of gravity is that mass/energy reduces the speed of light. The formula for this reduction is clear enough, in non-relativistic situations, but it would be brave to extrapolate it further without evidence.

It is more than possible that getting to infinite time dilation, that is C=0, is as possible as accelerating a mass to the speed of light. Personally I consider it almost a certainty. Black holes are paradoxes, and paradoxes are always caused by false underlying assumptions.

The evidence for black holes is not really there. I must remind all that nobody has seen a black hole. The are certainly stellar objects that are small and massive enough that, if black holes are valid, must be black holes. However if black holes are not valid, then we automatically create a cosmology where such objects are possible without being black holes.

##### Share on other sites

Originally posted by: BlameTheEx

It is more than possible that getting to infinite time dilation, that is C=0, is as possible as accelerating a mass to the speed of light. Personally I consider it almost a certainty. Black holes are paradoxes, and paradoxes are always caused by false underlying assumptions.

Quantum Mechanics/ Uncertainty Principle is based on a paradox. That we can know either about the particle or we can know about it's motion. But not both.

So according to you, QM is based on "false underlying assumptions"/.

##### Share on other sites

Originally posted by: BlameTheEx

It is more than possible that getting to infinite time dilation, that is C=0, is as possible as accelerating a mass to the speed of light. Personally I consider it almost a certainty. Black holes are paradoxes, and paradoxes are always caused by false underlying assumptions.

##### Share on other sites

Freethinker.

Nope. According to me there, is no paradox in Quantum Mechanics. The maths works well, and continues to work under all circumstances bar singularities. It is the underlying assumptions behind singularities (Black Holes) alone that I question. They are paradoxes for the simple reason that the maths that justifies them fails within them. That is in no way true for Quantum Mechanics.

GAHD

Best as I can figure it, ANY Black Hole is a paradox.

##### Share on other sites

Originally posted by: BlameTheEx

Freethinker.

Nope. According to me there, is no paradox in Quantum Mechanics.

Ya but ALSO "according to (you)"

Originally posted by: BlameTheEx paradoxes are always caused by false underlying assumptions

And there is no doubt that QM is based on the paradox.

The EPR paradox arises in a thought experiment which shows that quantum mechanics leads to very counter-intuitive and paradoxical consequences."

</a>

</a>

"The Quantum Zeno Effect

The phenomenon is called the quantum Zeno effect, because it resembles the famous paradox raised by the Greek philosopher Zeno, who denied the possibility of motion to an arrow in flight because it appears "frozen" at each instant of its flight. "

</a>

So which is it?

##### Share on other sites

Freethinker.

I stand by my statement.

Quantum Mechanics works. It's just difficult to understand. "counter intuitive" is a valid criticism but "paradoxical" is not. It is very easy to create paradoxes by misusing Quantum Mechanics. Once you fully accept that particles, photons, elephants ect exist only as an infinite field of varying degrees of probability until an interaction defines their position, then there are no more paradoxes. Admittedly in the case of the elephant the probability is VERY high that it's just about where you expect it, but there is no question but that it might next appear on mars. A paradox is a breakdown of logic. There IS no breakdown here.

I am not surprised that a trawl through the net will find those who don't fully accept a theory that includes the possibility of a mars rover being stomped by dumbo, but Quantum Mechanics obeys all the rules of a good theory. It's simple, explains the evidence, and (outside singularities) creates no situations that disobey its own rules, or any more fundamental rules, such as conversation of mass/energy or momentum.

##### Share on other sites

2) [A gravitational field] ... reduces the speed of light.

... That is calculated as E=MC2. The C is the speed of light. Inside a gravitational field C is lowered.

The best way to think of gravity is that mass/energy reduces the speed of light.

Isn't that relativistically repugnant?

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.