Relativity

The key idea of general relativity, called the equivalence principle, is that gravity pulling in one direction is completely equivalent to an acceleration in the opposite direction. A car accelerating forwards feels just like sideways gravity pushing you back against your seat. An elevator accelerating upwards feels just like gravity pushing you into the floor.

If gravity is equivalent to acceleration, and if motion affects measurements of time and space (as shown in special relativity), then it follows that gravity does so as well. In particular, the gravity of any mass, such as our sun, has the effect of warping the space and time around it. For example, the angles of a triangle no longer add up to 180 degrees, and clocks tick more slowly the closer they are to a gravitational mass like the sun.

Many of the predictions of general relativity, such as the bending of starlight by gravity and a tiny shift in the orbit of the planet Mercury, have been quantitatively confirmed by experiment. Two of the strangest predictions, impossible ever to completely confirm, are the existence of black holes and the effect of gravity on the universe as a whole (cosmology).

No one except Einstein was thinking of gravity as equivalent to acceleration, as a geometrical phenomenon, as a bending of time and space.

This is something that really needs to be sorted out.  FE'ers seem to have many preconceived notions about the implications of general relativity and the equivalence principle that are just untrue.

Einstein didn't deny the existence of gravity, he just said that an acceleration due to gravity feels the same as (thus, is equivalent to) any other form of acceleration in the opposite direction.  This is what the equivalence principle means.  It does NOT by ANY MEANS imply that gravity does not exist.  Thus, gravity creates a field in which objects are accelerated in the way that anything else accelerates. 

Quote from: The Philosopher on March 16, 2007, 08:37:08 PM

This is something that really needs to be sorted out.  FE'ers seem to have many preconceived notions about the implications of general relativity and the equivalence principle that are just untrue.

Einstein didn't deny the existence of gravity, he just said that an acceleration due to gravity feels the same as (thus, is equivalent to) any other form of acceleration in the opposite direction.  This is what the equivalence principle means.  It does NOT by ANY MEANS imply that gravity does not exist.  Thus, gravity creates a field in which objects are accelerated in the way that anything else accelerates. 

The equivalence principle states that acceleration and gravity are locally indistinguishable.  Being in a gravitational field is equivalent to being a non inertial FOR.  In other words, there is no experiment, example or local physical process which is explained by gravitation, that can't be explained by acceleration.  This acceleration, therefore, must also warp space so as to produce the same 'gravitational' effects such as gravitational lensing, and gravitational potential.

General Relativity EXPLICITLY states that gravity as a force does not exist.  It is a pseudo force arising from being in a non inertial FOR.  In this, it is much like centrifugal force and the Coriolis force.

The equivalence principle states that acceleration and gravity are locally indistinguishable.  Being in a gravitational field is equivalent to being a non inertial FOR.  In other words, there is no experiment, example or local physical process which is explained by gravitation, that can't be explained by acceleration.  This acceleration, therefore, must also warp space so as to produce the same 'gravitational' effects such as gravitational lensing, and gravitational potential.

So, to the experiment. If Einstein was right then the sun is bending space and we should be able to observe this by looking at the stars behind the sun. In other words, if the sun were between us and certain stars whose exact positions in the sky we know to six decimal places then we should see the light of those stars bending through space as that light passes the position of the sun on its way to earth. As a result the stars should appear to us to be out of place by a certain calculable number of degrees. Yes, not only did Einstein say the the sun would bend the light of the stars, he also knew how much bending would be done.

Well, you can't see stars that are behind the sun because the sun is too bright! Even Einstein knew that! So, they all had to wait for an eclipse. And when it came there were lots of people praying for a clear sky! And as the sky got darker and darker more and more stars came into view until it got just dark enough the see one particular star that everyone had agreed to measure the exact position of that day. And sure enough! The star was exactly where Einstein said it would appear to be and that place was not where God, nor Newton, put it! It appeared off position by the exact amount that Einstein's theory said it would be. Of course, Einstein, ever the man of humility, pointed out that an infinite number of experiments could prove him right; but it would take only one to prove him wrong!

You are mistaking gravity and gravitation.  This is what no one appears to be able to understand.  Gravity is a pseudo force that was invented to deal with not being in an inertial FOR.  Gravitation is the acceleration caused by the deformation of spacetime.

General Relativity EXPLICITLY states that gravity as a force does not exist.

Einstein combined the equivalence principle with special relativity to predict that clocks run at different rates in a gravitational potential, and light rays bend in a gravitational field, even before he developed the concept of curved spacetime. It is important to note that any accelerated frame of reference has a gravitational potential associated with it. Therefore clocks displaced in the direction of acceleration with respect to an accelerating rocket will be found to be going faster or slower by the observer in the accelerating rocket in accord with gravitational time dilation. The same applies to other gravitaitional effects such as gravitational red shifting and the bending of light.

One of the defining features of general relativity is the idea that gravitational 'force' is replaced by geometry. In general relativity, phenomena that in classical mechanics are ascribed to the action of the force of gravity (such as free-fall, orbital motion, and spacecraft trajectories) are taken in general relativity to represent inertial motion in a curved spacetime. So what people standing on the surface of the Earth perceive as the 'force of gravity' is a result of their undergoing a continuous physical acceleration caused by the mechanical resistance of the surface on which they are standing.

...  Gravitation is the acceleration caused by the deformation of spacetime.

I was hoping you would do a little work yourself, but that seems to be too much to ask.  So, here you go:

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Einstein combined the equivalence principle with special relativity to predict that clocks run at different rates in a gravitational potential, and light rays bend in a gravitational field, even before he developed the concept of curved spacetime. It is important to note that any accelerated frame of reference has a gravitational potential associated with it. Therefore clocks displaced in the direction of acceleration with respect to an accelerating rocket will be found to be going faster or slower by the observer in the accelerating rocket in accord with gravitational time dilation. The same applies to other gravitaitional effects such as gravitational red shifting and the bending of light.

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One of the defining features of general relativity is the idea that gravitational 'force' is replaced by geometry. In general relativity, phenomena that in classical mechanics are ascribed to the action of the force of gravity (such as free-fall, orbital motion, and spacecraft trajectories) are taken in general relativity to represent inertial motion in a curved spacetime. So what people standing on the surface of the Earth perceive as the 'force of gravity' is a result of their undergoing a continuous physical acceleration caused by the mechanical resistance of the surface on which they are standing.

Both from Wiki, and can be found in thousands of other creditable places.

Quote from: TheEngineer on March 16, 2007, 08:53:14 PM

The equivalence principle states that acceleration and gravity are locally indistinguishable.  Being in a gravitational field is equivalent to being a non inertial FOR.  In other words, there is no experiment, example or local physical process which is explained by gravitation, that can't be explained by acceleration.  This acceleration, therefore, must also warp space so as to produce the same 'gravitational' effects such as gravitational lensing, and gravitational potential.

1) By saying that all forms of acceleration warp space to produce gravitational effects (which implies that this gravitation goes in all directions, otherwise it can't explain gravitational lensing), then you're taking Einstein's ideas a step further, are you not?  If this is true, then you better damn well have some form of rationale, evidence, or reference that led you to believe this. If it's not true, then tell me where exactly Einstein said that the equivalence principle worked in the way you're describing.  From what I can tell, you're coming to the conclusion that the equivalence principle (with reference to Gravity) must also work inversely, so as to support the Flat Earth model.  This is purely non sequitur.
2) The experiment:

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So, to the experiment. If Einstein was right then the sun is bending space and we should be able to observe this by looking at the stars behind the sun. In other words, if the sun were between us and certain stars whose exact positions in the sky we know to six decimal places then we should see the light of those stars bending through space as that light passes the position of the sun on its way to earth. As a result the stars should appear to us to be out of place by a certain calculable number of degrees. Yes, not only did Einstein say the the sun would bend the light of the stars, he also knew how much bending would be done.

Well, you can't see stars that are behind the sun because the sun is too bright! Even Einstein knew that! So, they all had to wait for an eclipse. And when it came there were lots of people praying for a clear sky! And as the sky got darker and darker more and more stars came into view until it got just dark enough the see one particular star that everyone had agreed to measure the exact position of that day. And sure enough! The star was exactly where Einstein said it would appear to be and that place was not where God, nor Newton, put it! It appeared off position by the exact amount that Einstein's theory said it would be. Of course, Einstein, ever the man of humility, pointed out that an infinite number of experiments could prove him right; but it would take only one to prove him wrong!

Care to explain how light can be accelerated in this circumstance?  EXPERIENCING gravity is the same as experiencing acceleration (and they are the same when you're dealing with calculations and predictions), but acceleration does not produce all the effects of gravity.

Quote from: TheEngineer on March 16, 2007, 09:08:07 PM

You are mistaking gravity and gravitation.  This is what no one appears to be able to understand.  Gravity is a pseudo force that was invented to deal with not being in an inertial FOR.  Gravitation is the acceleration caused by the deformation of spacetime.

And it can easily be observed that this deformation of spacetime is caused by mass.  Unless, of course, the sun is made of pixie dust.

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General Relativity EXPLICITLY states that gravity as a force does not exist.

1) I provided a credible source (and a quotation from it) that suggest the opposite.  Why are you unable to do so?  You've said this many times, but have yet to reference a credible source... I wonder why.

In a rocket in free space (somewhere far away from stars and planets) a laser beam is emitted from one side of the rocket to a light detector on the opposite side. The astronaut sees the laser beam travel in a straight line from one side to the other. Now consider the same rocket in free fall near a planet. According to the equivalence principle the astronaut will again see the laser beam travel in a straight line across the cabin. So far, nothing strange! But now consider the same experiment viewed from the vantage point of someone who is at rest relative to the planet. The stationary observer also sees the event: laser beam hitting the light detector. (Events can't be changed just by changing your point of view!) But, by the time the light beam has crossed the cabin, the rocket and the light detector will have fallen a small distance. So the observer who is stationary with respect to the planet will see the laser beam follow a curved path.

What this shows is that gravity can be made to vanish merely by going to a frame of reference that is in free fall. If gravity can be so easily banished, Einstein reasoned that what we call the force of gravity may be an illusion; perhaps, gravity is not a force at all, but is somehow related to free motion in spacetime. Einstein went further: since gravity has been transformed away within the elevator all experiments conducted therein should give the same results as experiments carried out in a region far away from gravitational influences.

The fundamental principle of general relativity asserts that accelerated reference frames and reference frames in gravitation fields are equivalent. General relativity states that clocks run slower in strong gravitational fields (or highly accelerated frames), predicting a gravitational redshift. It also predicts the existence of gravitational lensing, gravitational waves, gravitomagnetism, the Lense-Thirring effect, and relativistic precession of orbiting bodies.