# Why two mass attracts each other?

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Who's going to fix that stupid comment farsight? To really understand what Einstein meant is to learn the theory and it's important derivations. There's spacetime around your anus. You speaking about the metric, as if you understand how to use them, is a joke.

Not strictly. Strictly speaking, spacetime is an abstract mathematical space in which motion does not occur because it models space at all times. You can draw world-lines in it, and you can draw them curved, but that worldline represents the motion of a body through space over time. The body doesn't actually move through spacetime. People tend to talk of "the spacetime around the Earth" and suggest that light moves through it, but actually, that's wrong.

Let me rephrase. The rubber sheet (sans bowling ball...just a flat rubber sheet) is a two dimensional(x and y-axis) representation for the entirety of space-time. When you add the bowling ball (mass) to the previously two-dimensional and homogeneous rubber sheet (space-time), the rubber sheet (space-time) bends (becomes inhomogeneous) in a third direction (z-axis).

You need two dimensions to bend a line....you need three dimensions to bend a plane...etc. Wouldn't you need a 5th dimension to bend 4 dimensional space-time? Every analogy or picture I've ever seen to describe how mass alters space-time involves the addition of a new dimension.

Maybe I'm taking the analogy so far it's breaking down. I'm not a physicist so I rely pretty heavily on the analogies and thought experiments to gain understanding.

Who's going to fix that stupid comment farsight? To really understand what Einstein meant is to learn the theory and it's important derivations. There's spacetime around your anus. You speaking about the metric, as if you understand how to use them, is a joke.
Get lost bruce. I answer the questions with robust references, you snipe from the sidelines like some naysaying troll bristling with envy. Now watch and learn...

MeNotYou said:
Let me rephrase. The rubber sheet (sans bowling ball...just a flat rubber sheet) is a two dimensional(x and y-axis) representation for the entirety of space-time. When you add the bowling ball (mass) to the previously two-dimensional and homogeneous rubber sheet (space-time), the rubber sheet (space-time) bends (becomes inhomogeneous) in a third direction (z-axis).
It does, but this isn't a great analogy because it uses gravity acting on the bowling ball to depict gravity. In this respect it's circular. A better analogy would be to put your fingers together, place them on the rubber sheet, then spread 'em. You then create a "tension gradient" in the rubber sheet such that a ripple moving across it would veer towards your hand. An even better analogy would feature a three-dimensional elastic "bulk" with an outward stress or pressure. Like you're inside a big block of gin-clear ghostly silicone rubber pushing outwards in a spherically-symmetric fashion. This gets fairly close to what Einstein was modelling. If you take a look at the Einstein Field Equations and his stress-energy tensor, you can see mention of "shear stress". This tells you that Einstein was thinking in terms of an elastic solid. He thought of space as something that could be curved and stretched and put under pressure, et cetera.

MeNotYou said:
You need two dimensions to bend a line....you need three dimensions to bend a plane...etc. Wouldn't you need a 5th dimension to bend 4 dimensional space-time? Every analogy or picture I've ever seen to describe how mass alters space-time involves the addition of a new dimension.
It isn't a 5th dimension like in some science fiction movie. It's much more mundane than that. Imagine you're a light beam, and that for your morning exercise you swim up and down a swimming pool. But in the dead of night I tip a sack of jello on the West side of the pool. The next day you find your swimming veers West. Then imagine you're underwater in a gin-clear ocean and your swimming always veers down. That's pretty much how gravity is. Swim round in a square to emulate matter falling down.

MeNotYou said:
Maybe I'm taking the analogy so far it's breaking down. I'm not a physicist so I rely pretty heavily on the analogies and thought experiments to gain understanding.
You don't have to be a physicist to understand gravity. It's actually pretty simple.

Let me rephrase. The rubber sheet (sans bowling ball...just a flat rubber sheet) is a two dimensional(x and y-axis) representation for the entirety of space-time. When you add the bowling ball (mass) to the previously two-dimensional and homogeneous rubber sheet (space-time), the rubber sheet (space-time) bends (becomes inhomogeneous) in a third direction (z-axis).

You need two dimensions to bend a line....you need three dimensions to bend a plane...etc. Wouldn't you need a 5th dimension to bend 4 dimensional space-time? Every analogy or picture I've ever seen to describe how mass alters space-time involves the addition of a new dimension.

Maybe I'm taking the analogy so far it's breaking down. I'm not a physicist so I rely pretty heavily on the analogies and thought experiments to gain understanding.

Chapter 2 Curving introduces curved spacetime and the first metric solution. My favorite GR text. Very easy to read.
Click on Chapter 2 Curving.

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Get lost bruce. I answer the questions with robust references, you snipe from the sidelines like some naysaying troll bristling with envy. Now watch and learn...

It does, but this isn't a great analogy because it uses gravity acting on the bowling ball to depict gravity. In this respect it's circular. A better analogy would be to put your fingers together, place them on the rubber sheet, then spread 'em. You then create a "tension gradient" in the rubber sheet such that a ripple moving across it would veer towards your hand. An even better analogy would feature a three-dimensional elastic "bulk" with an outward stress or pressure. Like you're inside a big block of gin-clear ghostly silicone rubber pushing outwards in a spherically-symmetric fashion. This gets fairly close to what Einstein was modelling. If you take a look at the Einstein Field Equations and his stress-energy tensor, you can see mention of "shear stress". This tells you that Einstein was thinking in terms of an elastic solid. He thought of space as something that could be curved and stretched and put under pressure, et cetera.

It isn't a 5th dimension like in some science fiction movie. It's much more mundane than that. Imagine you're a light beam, and that for your morning exercise you swim up and down a swimming pool. But in the dead of night I tip a sack of jello on the West side of the pool. The next day you find your swimming veers West. Then imagine you're underwater in a gin-clear ocean and your swimming always veers down. That's pretty much how gravity is. Swim round in a square to emulate matter falling down.

You don't have to be a physicist to understand gravity. It's actually pretty simple.
So what's your problem? You don't have to be a Physicist to learn GR. It's a lot of work regardless what you think. Work you haven't even started. Troll. Pariah. Intellectual dishonest crank. Quit spreading nonsense.

Chapter 2 Curving introduces curved spacetime and the first metric solution. My favorite GR text. Very easy to read.
All: ignore this guy. He doesn't know any physics. He can't answer any questions or explain anything. He's just a loser who is jealous of those who can.

Go on, beat it bruce. You aren't contributing to this discussion, you're just getting in the way. You want to prove me wrong and talk physics? Don't think so. Keep this up and I will come after you, and I will embarrass you. Capiche?

MeNotYou: sorry about this. Guys like bruce will do anything to stifle a discussion. Don't let him.

All: ignore this guy. He doesn't know any physics. He can't answer any questions or explain anything. He's just a loser who is jealous of those who can.

Go on, beat it bruce. You aren't contributing to this discussion, you're just getting in the way. You want to prove me wrong and talk physics? Don't think so. Keep this up and I will come after you, and I will embarrass you. Capiche?

MeNotYou: sorry about this. Guys like bruce will do anything to stifle a discussion. Don't let him.

Troll. You really are the goofball in the video I reviewed. You should be sorry for your intellectual dishonesty. Have all the discussion you want just don't include the nonsense comments on GR.

You don't want to talk physics, and you don't want anybody else to talk physics, and I will embarrass you. Capiche, bruce?

MeNotYou: guys like this are the bane of the internet. It's important not to let them get in the way of your learning. That's why they're here. No other reason. It's not as if they say actually this guy is wrong, it goes like this... Instead they will start a whole pile of unpleasantness in an attempt to derail a thread and scare you away. Be alert for it.

All: ignore this guy. He doesn't know any physics. He can't answer any questions or explain anything. He's just a loser who is jealous of those who can.

Go on, beat it bruce. You aren't contributing to this discussion, you're just getting in the way. You want to prove me wrong and talk physics? Don't think so. Keep this up and I will come after you, and I will embarrass you. Capiche?

MeNotYou: sorry about this. Guys like bruce will do anything to stifle a discussion. Don't let him.

The following is derived from the Schwarzschild Metric. It's a really interesting derivation. What natural phenomena does it describe?

The derivation

Put the derivative of the effective potential term (from the equation of motion) into quadratic form (to find critical values)

r*^2 - L*^2r + 3L*^2 = 0

Where

r* = r/M, and L* = L/mM

Then divide through by L*^2 and manipulate to get

r*^2/L*^2 = r* - 3 [saving this for a later
substitution]

Setting dr = 0 in the Schwarzschild metric and
substituting dphi = (L*/r*^2)dTau the metric becomes

dTau^2 = (1 - 2/r*)dt^2 - (L*^2/r*^2)dTau^2

To find the ratio dTau^2/dt^2 divide through by the
bookkeeper time dt^2 and simplify to

(dTau/dt)^2 = (1 - 2/r*) / (1 + L*^2/r*^2)

Now substitute 1/(r*-3) for L*^2/r*^2 and simplify to

dTau/dt = (1 - 3M/r)^1/2

What natural phenomena does this describe? It has to do with relativity theory so it should be easy for you.

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You don't want to talk physics, and you don't want anybody else to talk physics, and I will embarrass you. Capiche, bruce?

MeNotYou: guys like this are the bane of the internet. It's important not to let them get in the way of your learning. That's why they're here. No other reason. It's not as if they say actually this guy is wrong, it goes like this... Instead they will start a whole pile of unpleasantness in an attempt to derail a thread and scare you away. Be alert for it.

Embarrass me? Troll.

Troll. You really are the goofball in the video I reviewed.

This one. It is hilarious. The resident crank, John Duffield, at his best.

Troll. You really are the goofball in the video I reviewed. You should be sorry for your intellectual dishonesty. Have all the discussion you want just don't include the nonsense comments on GR.

Bruce you are not talking to the question under discussion, or the physics. Your posts more closely meet the criteria of trolling than Farsight's.

http://curezone.com/forums/troll.asp
An "Internet troll" or "Forum Troll" or "Message Board Troll" is a person who posts outrageous message to bait people to answer. Forum Troll delights in sowing discord on the forums. A troll is someone who inspires flaming rhetoric, someone who is purposely provoking and pulling people into flaming discussion. Flaming discussions usually end with name calling and a flame war.

I am not defending Farsight's comments in this. A person could offer wrong or bad information without trolling. It is far more difficult to avoid the label, when what you are doing is focusing on the poster in a derogatory manner, rather than on the subject under discussion. If you do not agree with how he is explaining the question or subject, offer what you believe is a more accurate answer, or explanation. Explain why his response is wrong or in error. Just flaming anyone's comments or a poster personally, is not constructive. It is by the deffinition above "Trolling".

This one. It is hilarious. The resident crank, John Duffield, at his best.

I guess I should watch the entire interview. I can't get past the comment about his disillusionment concerning the focus of his daughters physic class. That's right after he says he's been interested in physics his entire life. ?. A prelude for his writing his crank book.

No, in context he's talking about electromagnetism. And everybody can read it, so you cannot hope to get away with your assertion.
Apparently anybody can read it but you. Maxwell is clearly talking about the natural inclination of what one thinks in mechanics, but then he doesn't use that inclination in anything that follows.

How about this. You are all half-right? I called cobb and talked him reluctantly into a sneak preview of his treatise. His position was quite simple: If , under the principal of equivalence, one man's kinematically viewed acceleration (inertial observer watching a baseball in flight) is another man's gravitation (the flea on the ball), then both the kinematic (geometrical) and the kinetic (gravitation, the flea) observations are equally valid. Therefore, general relativity, which necessarily has a massless "test" body, cannot in fact account for Einstein's own "relativity of inertia" (increased mass when masses are "piled up") because it cuts out the force dependent / kinetic component. The equation cobb deduced is mindblowing. I am convinced it is right, and that we were always neglecting a term in the conclusion that "g = a" in freefall. This guy a crackpot or genius? (ref: "a third new science"). I just entered his contest, so we shall see.

Bruce you are not talking to the question under discussion, or the physics. Your posts more closely meet the criteria of trolling than Farsight's.

I am not defending Farsight's comments in this. A person could offer wrong or bad information without trolling. It is far more difficult to avoid the label, when what you are doing is focusing on the poster in a derogatory manner, rather than on the subject under discussion. If you do not agree with how he is explaining the question or subject, offer what you believe is a more accurate answer, or explanation. Explain why his response is wrong or in error. Just flaming anyone's comments or a poster personally, is not constructive. It is by the deffinition above "Trolling".
They just naturally take to trolling each other.

All: ignore this guy. He doesn't know any physics. He can't answer any questions or explain anything. He's just a loser who is jealous of those who can.

Go on, beat it bruce. You aren't contributing to this discussion, you're just getting in the way. You want to prove me wrong and talk physics? Don't think so. Keep this up and I will come after you, and I will embarrass you. Capiche?

MeNotYou: sorry about this. Guys like bruce will do anything to stifle a discussion. Don't let him.

Stop trying to polarise the situation. I know bruce is quick to snap but he has read up on GR extensively, he knows some physics.

MeNotYou: guys like this are the bane of the internet. It's important not to let them get in the way of your learning. That's why they're here. No other reason. It's not as if they say actually this guy is wrong, it goes like this... Instead they will start a whole pile of unpleasantness in an attempt to derail a thread and scare you away. Be alert for it.

There does seem to be a lot of nerd machismo on this forum.

An even better analogy would feature a three-dimensional elastic "bulk" with an outward stress or pressure. Like you're inside a big block of gin-clear ghostly silicone rubber pushing outwards in a spherically-symmetric fashion. This gets fairly close to what Einstein was modelling. If you take a look at the Einstein Field Equations and his stress-energy tensor, you can see mention of "shear stress". This tells you that Einstein was thinking in terms of an elastic solid. He thought of space as something that could be curved and stretched and put under pressure, et cetera.

...this sounds a lot like space-time displacement. Like adding a ball to a sealed tank full of air. The ball displaces the air, but since the air can't escape it becomes compressed...the PSI rises and it adds stress to the whole system. Kind of like that? Wait...would that make the Higgs-Boson the ball/tensor/thing that displaces(stresses) space-time??

Likewise in GR. Substitute "observers" by "masses", and "north" by "the future". As time passes, the two masses propagate through space-time, slowly approaching one another due to the presence of curvature, Eventually they collide. There are no forces involved, this is a purely geometric phenomenon, and it is quantified in the so-called Rhaychaudhuri Equation.
This is my understanding as well, but if there is no force particle involved....what is a graviton and why are physicist looking for it?

The following is derived from the Schwarzschild Metric. It's a really interesting derivation. What natural phenomena does it describe?

The derivation

Put the derivative of the effective potential term (from the equation of motion) into quadratic form (to find critical values)

r*^2 - L*^2r + 3L*^2 = 0

Where

r* = r/M, and L* = L/mM

Then divide through by L*^2 and manipulate to get

r*^2/L*^2 = r* - 3 [saving this for a later
substitution]

Setting dr = 0 in the Schwarzschild metric and
substituting dphi = (L*/r*^2)dTau the metric becomes

dTau^2 = (1 - 2/r*)dt^2 - (L*^2/r*^2)dTau^2

To find the ratio dTau^2/dt^2 divide through by the
bookkeeper time dt^2 and simplify to

(dTau/dt)^2 = (1 - 2/r*) / (1 + L*^2/r*^2)

Now substitute 1/(r*-3) for L*^2/r*^2 and simplify to

dTau/dt = (1 - 3M/r)^1/2

What natural phenomena does this describe? It has to do with relativity theory so it should be easy for you.

I'm almost positive this describes pudding. Very delicious.

I'm almost positive this describes pudding. Very delicious.

Nah, the universe is made of cheese.

That's why our galaxy is called the milky way.

This is something I've always struggled with.

Yaa, it is quite difficult to explain attractive force between two mass in terms of spacetime curvature.

If all the large dimensions are accounted for....and gravity is literally all of spacetime(all 4 large dimensions) bending....don't you need another dimension for it to bend into?

Spacetime uses 4 dimensions. Three space dimension and one time dimension. Spacetime bending is a local phenomena where mass is present. Spacetime bending is useful in a relativistic situation. Spacetime bends into itself as is confirmed by gravitational lensing where light bends around a mass. For more details about spacetime see here.

If spacetime(all 4 dimensions) is a line, and that line bends....by definition that line must exist on a 2 dimensional plane.

Mathematically right but quite difficult to visualise or imagine. In advanced version more than 4 dimensions are used to describe spacetime.

Am I correct to say, it's not the stuff IN spacetime that is bending...it's spacetime itself?

If spacetime is bending, all the stuff in spacetime also bends. Thats why light also bends.

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