Gravity?

[Antares]

I'm a Freshmen in High School and I have a great interest in Physics. One thing I don't understand is... What IS gravity, really? How does it work? Why does it attract rather than repel? Why does gravity influence massless particles such as photon? It's funny how you can be quite advanced into a subject matter and no know the basics. I've read somewhere that to put gravity in a 2D universe, it would be like a heavy object dent on a trampoline and objects will fall towards the dent. Anyone can explain? :thumbsup:

[blueback]

I heard a theory that if everything was expanding at the same rate it would look like gravity to us since we're expanding too.

Picture a basketball a foot away from a baseball. If both the basketball and the baseball began to get bigger, but you didn't have the floor as a reference point, they would appear to be coming together. When they touch, they would start to push on each other. So, if you pushed the baseball away from the basketball it would appear to "fall" back as they continued to expand

I don't think that theory explains everything, but it is a fun way to break your mind out of preconceptions.

[rwyatt365]

*inserting tongue firmly into cheek*

The primary elementary particle of the universe is the Flying Purple Unicorn (FPU), all matter in the universe is composed of FPUs. Every FPU emits energy waves called "cuteons" which other FPUs find to be irresistible and thus are attracted to. It is because of this cuteon radiation, which permeates the entire universe, that causes FPUs to move towards each other. The strength of this attractive force is directly proportional to the sum of the FPUs (roughly proportional to the mass) in any given object – larger, more massive objects (with large concentrations of FPUs) have greater cuteon radiation and therefore more attractive force, which we sense as "gravity".

Cuteon radiation has never been measured as the only sensors capable of detecting them have been found in the eyes of little girls, and men (which explains the former's affinity for ponies and Cabbage Patch Kids, and the latter's affinity for girls). Medical ethics prevent further study.

*removing tongue*

[Drayakir]

Okay, basically, gravity is one of the four fundamental forces of nature, the other ones being the strong force, weak force, and electromagnetism.

Now what it is- it's a result of the deformation of the space-time continuum. Or rather, gravity is "generated" by the space time continuum, and deforms the space-time continuum at the same time. Fundamental force, baby!

Anyway, why it is able to only attract, I don't know, but if the anthropocentric view of the universe is right, it couldn't have been any other way- otherwise we wouldn't be there to observe it. And as for it affecting photons- it doesn't affect them directly, but it affects the immediate area around them, causing them to move in different ways.

[Hdier]

I've been trying to think of a way to make gravity repel, and have been coming up short. Maybe if I understand exactly why gravity attracts, that would help. Maybe graduating High School would help. Yeah, finishing High School would definitely help.

[Drayakir]

Make gravity repel? Easy. Just make the "charge" on it go from positive (attraction) to negative (repulsion). Although how one would go about, I have no idea.

[Hdier]

Charge? How are charges involved in gravity?

[thegnat]

Charge? How are charges involved in gravity?

I'm quite sure charges *aren't* involved in gravity.

However, I'm not a physics major, and I hadn't thought of gravity this way.

But I do believe that gravitational attraction has to do more with the mass of the object rather than the charge. (think of our solar system as to the less massive things orbiting the more massive things)

[rwyatt365]

I've heard gravity as being a "warp" in the space-time continuum, sort of like what the OP said about gravity in a 2D space.

If one imagines 3D space represented by a membrane then any object with mass will "warp" (or bend) that membrane proportional to it's mass. Gravity is simply the resultant effect of warped space by bodies with mass. Stretch a piece of rubber in a frame, then put a basketball on the rubber. The rubber will bend (or warp) under the weight (mass) of the basketball. Now, roll a marble across the rubber - it will move towards the basketball as it passes (it might even run into the basketball if you didn't roll the marble fast enough) because of the impression (or warp) that the basketball put in the rubber. And if you looked closely you would see that the basketball moved slightly towards the marble as it passed - but only very slightly.

This is a fair analogy for how objects with mass warp space-time and thus induce what seems to be an attractive "force" on one another, when it's all just a function of the "fabric" of space that we exist in.

By this analogy, in order to have anti-gravity we need to find something that is "anti-massive" (anti-matter, perhaps) that will "dimple" space-time instead of indent it (i.e. put in a "bump" in the fabric instead of a depression).

But aren't FPUs more fun?! ;D

[Drayakir]

That why I quoted charge. Not a charge, but like, the setting of gravity.

[helices]

Like many of the real fundamentals of Physics, gravity (like energy and mass) is difficult to define properly (as in, defined in terms of fundamental causes rather than just observed effects).

The analogy of the two dimensional sheet of spacetime is a useful one to visualise the effect of gravity, though it doesn't really explain what's going on.

One theory, is that there exists a sub-atomic exchange particle, called the graviton. This particle has never been observed and is currently only hypothetical (and not universally accepted). Quite how the exchange of these particles leads to attraction between bodies I do not know.


helices

[Nolen]

Like many of the real fundamentals of Physics, gravity (like energy and mass) is difficult to define properly (as in, defined in terms of fundamental causes rather than just observed effects).

Amen!

To the OP,

From the classical point of view gravity is like this. Particles have a property called mass. They exert an attractive force on each other, i.e. the force on each particle is in the direction of the other particle, that is proportional to the product of their masses and the inverse square of the distance between them. This is an observed law and accurately predicts when it is tested. I know that is probably unsatisfying, but it's probably the best place to start from. You might check this out:

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The analogy with charge is as such. Particles have a property called charge. If they aren't moving, they exert a force on each other that is proportional to the product of their charges and the inverse square of the distance between them. When you write this down and compare it to the gravitational force equation, it's basically the same thing but with different letters and a different sign. In this case though the charge can be positive or negative (as compared to mass being nonnegative) hence the force can be attractive or repulsive, and because of the different sign, like charges repel and opposites attract (as compared to like masses attracting). See the link below. However this is only when they aren't moving; in that case things get more complicated.

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As for the description of a sheet bending, this comes from the general relativistic point of view. This is a different (and more accurate according to measurements) way of describing gravity. Think of it like this. In the classical picture, when a particle is moving and passes another particle the gravitational force that the stationary particle exerts on the moving one causes it's path to curve in the direction of the stationary particle. The "force" picture explains this well. However, another way of looking at this is purely in terms of geometry. In this picture, the stationary particle's mass is actually bending space (changing the geometry of space) and the moving particle is still moving "straight," straight here meaning it is taking the shortest distance from A to B (following a geodesic). That is about all I can say about GR as it requires some fancy math and I don't know all that much about it anyway.

[Itsme]

I'm a Freshmen in High School and I have a great interest in Physics. One thing I don't understand is... What IS gravity, really? How does it work? Why does it attract rather than repel? Why does gravity influence massless particles such as photon? It's funny how you can be quite advanced into a subject matter and no know the basics. I've read somewhere that to put gravity in a 2D universe, it would be like a heavy object dent on a trampoline and objects will fall towards the dent. Anyone can explain? :thumbsup:

If you are interested in exploring this question I would suggest that you do check into regularly accepted theories based on Newtonian Physics as well as Einstein's General Theory of Relativity and his Special Theory of Relativity first. These form the basis for for currently accepted theories concerning astrophysics, particle physics and so on and are taught as the dominant principles or theories of physics today.

There is another theory of astrophysics and the forces of the universe which is gaining traction. It is often referred to as the Electric Universe. A query to your favorite search engine should bring up plenty of links both for and against the theory. Another query along these lines would be catastrophism.

[BadMojo]

I've been trying to think of a way to make gravity repel, and have been coming up short. Maybe if I understand exactly why gravity attracts, that would help. Maybe graduating High School would help. Yeah, finishing High School would definitely help.
I found a solution to that damn "gravity problem"... unfortunately it's called jogging :suspicious::p

[HackerX]

Gravity, a rough overview, based on some sort of quantum theory that I'm trying to remember (I did quantum physics, honest!):

Basically, everything is connected to everything else, via a very weak force. Gravity is the sum of that force when comparing one body of mass to another, taking into account proximity.

E.g. Earth has a largeish mass (relatively speaking), thus it has a relatively large gravitational pull compared to a person standing on it. However, the person also has their own gravitational pull, it's just that the earth's one is bigger. Both objects are in relatively close proximity.

Bigger object, bigger pull, everything has an effect on everything else (even if undetectable) etc etc. I.e. You have a gravitational effect on some sun in some distant galaxy, and it has some effect on you, it's just that the lack of proximity means that the effect is negligable.


*inserting tongue firmly into cheek*

The primary elementary particle of the universe is the Flying Purple Unicorn (FPU), all matter in the universe is composed of FPUs. Every FPU emits energy waves called "cuteons" which other FPUs find to be irresistible and thus are attracted to. It is because of this cuteon radiation, which permeates the entire universe, that causes FPUs to move towards each other. The strength of this attractive force is directly proportional to the sum of the FPUs (roughly proportional to the mass) in any given object – larger, more massive objects (with large concentrations of FPUs) have greater cuteon radiation and therefore more attractive force, which we sense as "gravity".

Cuteon radiation has never been measured as the only sensors capable of detecting them have been found in the eyes of little girls, and men (which explains the former's affinity for ponies and Cabbage Patch Kids, and the latter's affinity for girls). Medical ethics prevent further study.

*removing tongue*


*bows* My purple unicorns strike again. Cuteon! lol

It also explains why the cute girls at parties always have all these guys hanging around.


I've been trying to think of a way to make gravity repel, and have been coming up short. Maybe if I understand exactly why gravity attracts, that would help. Maybe graduating High School would help. Yeah, finishing High School would definitely help.


You can't really. As far as we know, we can't reverse the forces involved. You're most welcome of course to apply more force in the opposite direction to overcome it :)

[Hdier]

Think back three centuries ago (or you can just imagine it if you aren't three hundred plus years old):

As far as we know, we can't make a horseless carriage.

The point is that I'm trying to figure out how to do something that we cannot yet do, not do something that we can already do!

[Antares]

I've been trying to think of a way to make gravity repel, and have been coming up short. Maybe if I understand exactly why gravity attracts, that would help. Maybe graduating High School would help. Yeah, finishing High School would definitely help.

That was what I was thinking :cheesy: Finishing High School is a good suggestion. I'm trying to understand why gravity attracts. In my understanding, gravity is the weakest force of the four forces of nature that has infinite range, meaning that it will keep getting weaker, but it will always be there, so I dont think it repels.

[Hdier]

I enjoy trying to figure out ways to make forces work against themselves to my advantage. Figuring out why, exactly, it attracts would most likely assist me in my aspirations to reverse the attraction of matter to one anther.

[HackerX]

It's more like... everything is attached to everything else by a stretchy string (whose elasticity happens to be inverse to normal elastic). It's entirely possibly that scientists will work out how to cut the string, so that the attraction doesn't have an effect anymore, but reversing the effect is impossible. The negative effect of cutting that connection though is once you do, the object stops being affected by it... Which is far more of a negative that you'd think

[karen]

(please forgive me here, I just got over a six month sentence teaching in the "resource room")

Really simply put, gravity pulls or rather clumps things together and dark energy spreads things out. Both are forces in the universe. Without gravity things (such as solar systems, planets, matter) would be more evenly spaced. Without dark energy things would all be squished together.

[Hdier]

It's more like... everything is attached to everything else by a stretchy string (whose elasticity happens to be inverse to normal elastic). It's entirely possibly that scientists will work out how to cut the string, so that the attraction doesn't have an effect anymore, but reversing the effect is impossible. The negative effect of cutting that connection though is once you do, the object stops being affected by it... Which is far more of a negative that you'd think

I have a motto: 'Nothing is impossible. We simply haven't learned how to do it yet.'

[thod]

You drop a stone and it falls. Aha someone says, nothing happens without a cause, I name this cause gravity.

Some people say that gravity is identicle to acceleration and thus alters space time so that the a photon, in its frame, is still traveling is a straight even if an observer says its path is bent.

Now if they realy knew, then they would know that falling is an inherant property of stones like hardness. )