Talhaarshad:
“Is gravity a force?
Is gravity a Energy?
tell me anyone pleasez what is it?”
The short answer is, in my opinion, that gravity is a negative energy field that manifests as curved spacetime geometry.
Henrik:
“I have another question regarding curvature of spacetime. Why does the curvature of space seem to depend on the relative velocity of the masses that pass each other.
For example when light passes a large mass it is not deflected from the Cartesian straight line as much as an object passing with half the velocity. And an object passing with very small velocity is deflected even more. How does that fit the description of objects generally traveling in a straight line which just curves because of the curvature of spacetime - why does spacetime seem to curve less for a fast object than for a slow object?”
It’s easy to confuse the term ‘spacetime’ with just ‘space,’ because most of us aren’t accustomed to thinking of time as a dimension equivalent to a space dimension. And this gets even more confusing when we’re trying to visualize special relativistic effects and general relativistic effects simultaneously, which your example demands.
For clarity, let’s start by looking at these two spacetime effects individually.
A motionless body (with respect to the observer) that is far from gravitational influences, according to special relativity, moves through time at the greatest rate. In fact, such a body is moving through time at the equivalent of the speed of light (since 3e10cm/sec is equivalent to 1sec/sec by the constant C). So really you can see that nothing is ever motionless in both space and time – a body can move through space at a velocity approaching C, or move through time at a velocity approaching C, or it can move with some mixture of both factors which yields a net spacetime velocity of C. That’s special relativity in a nutshell.
Now we consider our small test mass in the presence of a body with an appreciable gravitational field. For starters, we know that time is dilated (moves more slowly) within a gravitational field, compared to the scale of time at a point far removed from such a field. And as you might imagine, the space dimension within a gravitational field is dilated by precisely the same factor (although inversely – space is contracted by the same degree that time is expanded within the region of the gravitational field). Together, these alterations in space and time define a vector field of acceleration pointed toward the center of mass of the body and this is the effect that we call gravity.
Now we can answer your question: “why does spacetime seem to curve less for a fast object than for a slow object?”
The path of a fast object passing by the Earth (let’s say that its velocity is 1/2C) will only be slightly curved *in space* but its path will be appreciably curved *in time.* For example, compared to a distant observer stationary wrt (with respect to) the Earth, time aboard our fast-moving test mass is approx .866 seconds elapsed for every second that elapses on the observer’s distant clock. The fast-moving clock passing the Earth is slowed by both the velocity of the body (special relativity) and by the gravitational field of the Earth (general relativity), yielding a significant temporal curvature.
The path of the same body as it passes the Earth slowly, on the other hand, will be curved significantly *in space,* but will be only negligibly curved *in time.*
As you might intuit at this point, the curvatures of both examples are in fact equivalent, because the spacetime curvature around a body of stationary matter is a static quantity.
Things get even more interesting however when we consider the ‘twisting’ of spacetime around a massive rotating body, which is known as ‘frame-dragging,’ the ‘Lense-Thirring effect,’ or ‘gravitomagnetism.’ But that’s a whole other ball o wax.
If you’re interested in learning more about this subject, I suggest reading some good papers on the Global Positioning System (GPS), because the GPS satellite and signaling network is essentially ‘applied special and general relativity,’ and its success is a testament to the validity of Einstein’s model of gravity and spacetime.
http://relativity.livingreviews.org/Articles/lrr-2003-1/
http://arxiv.org/abs/gr-qc/0507121
And as Loren Booda pointed out, Wheeler’s book ‘A Journey into Gravity and Spacetime’ is a wonderful primer for anyone who wants to grasp these concepts, without all of the daunting mathematics that usually accompanies this subject.