Expert: Jayendra Upadhye - 5/17/2006

Question
I recently finished reading Stephen Hawking's "A brief History of Time" and I am still quite befuddled about what exactly gravity is. I realize it is a force between two (or more) bodies, but what is this force? What is actually acting on us? If there were no matter would gravity still exist as an ambient, background force? It sounds as if gravity is a function of space (or vice-versa) but is it also a function of time? If it is theorized that gravity travels at light speed. Does this mean that for each star whose light eventually reaches earth has an amount of gravitational attraction (repulsion?)acting on us as well?  

Answer
Hi Robbie,
Questions ...questions ..& still more questions..! :)
Makes ones mind whirl..right?

It helps greately if one considers all scientefic theories in two ways.
1 - The practical part that helps us evaluate fairly accurately things such as the distance we hope to travel, the speed at which we do it, the measurable (finite) forces we will encounter on the way etc.
2 - The "pure theory" part sucha as the fact that all inverse square fields never really become zero, they only become "too weak to be significant", that every particle in the universe is "gravitationally bound" (the weak force) to every other point in it, no matter by how "weak" a force.

Imagine all force fields to behave as per a very complex formula, of which we are aware of only those factors that help us calculate and predict things over "finite" small ranges of distance and magnitudes.
Imagine the other factors to be so small (no matter how quaintly they appear in the formula), as to enable us to altogether ignore their existence.

That basically takes care of all things in the newtonian (and comfortable) universe.

now ..
1 - gravity cannot be divorced from mass. It will NOT exist in a massless universe. Period.

2 - gravity as a function of time (varying over time for a universe of fixed mass), falls in the second category and it is fruitless wasting ones energy on factors not even known currently.

3 - each star gravitationally affecting us...i answered at the outset. Every thing in the universe is gravitationally bound to the other.

Regarding gravity (or its warping of space) travelling at the speed of light...i often wonder.
What i say next is strictly my loud thinking and NOT scientefic gospel..
you see, light takes 8 minutes to travel to earth from the sun, across 1 astronomical unit.
Now the sun is hurtling across the galaxy orbiting once in 225 million years roughly.
Also the earth is moving.
As the suns zips around the galaxy, the warpinng of space also has to follow, remaining centered around the sun. (which is the bottom of the potential well).
If it takes 8 minutes for any field relocation to reach earth, the earth should be moving along its "old orbital line" line in the intervening time, as it will continue to see the sun "where it was 8 minutes ago".

This "gravity lag" should actually distort the orbits of distant planets and even the earth. Why does this not happen?

Does the gravity well propagate its movement thru space not in 8 minutes but "instantaneously"? Thus keeping the earth's orbit almost circular, and consistent over the ages?

In other words, for the earth's orbit to remain centered on the sun, it has to somehow know where the sun has moved! For the information on the said movement is ALWAYS 8 minutes late!

Think about that!
sometimes isnt ignorance truly blissful? :)

Jayen