physics question #3998

Monty, a 42 year old male from saskatchewan asks on November 12, 2007,


What exactly is gravity? Is it a wave or a ray? What exactly is the attraction that makes gravity work?

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the answer

William George Unruh answered on November 15, 2007, A:

Gravity is not a ray or a force, although in some situations it can act like a force. The source of gravity is the energy, momentum and stresses within matter.

Gravity is chameleon like, and tends to depend on how you look at it. It has many aspects, and can be described in many ways. Like the parable of the elephant where different observers use different analogies to try to describe the elephant, gravity also depends on who describes it, or how you describe it. In one approximation, it can be described, as Newton did, as a force. In another it can be described as the lack of force: particles follow straight lines in the four dimensional spacetime, but the structure of spacetime is such that those straight lines can cross and recross each other.

The best description we have of gravity is Einstein's theory of General Relativity from 1915. Gravity is explained by general relativity as changes to the structure of space-time, changes to the distances defined in this spacetime. While measuring rods are used to measure the distance between two spatial points (at the same time), in space-time distances can also be measured by clocks. Clocks act like odometers measuring off distances. These distances have some strange properties. Lines in the spacetime which extend mainly into time rather than in space, are straight not if they are the shortest distance between two points, but if they are the longest. This is the origin of the so called "Twin's paradox". Two travellers, one of whome stays still and the other of whom moves off in space and eventually returns, travel different distances. The traveller who stays put in this case has a longer path than the one who travels in this concept of distance associated with special relativity.

In General Relativity this notion of distance is generalised. Spatial distances can change from time to time, not because the two bodies move, but the because the distance changes "on its own". The most interesting cases of this are called gravity waves which are travelling disturbances in spatial distances. Gravity waves have been detected by their effect on the source from which they originate. We can detect the loss of energy by the source of the waves, typically a very massive object such as a black hole at the centre of a galaxy. This agrees to remarkable precision (much better than 1%) with what the theory predicts. Though we have built gravity wave detectors on Earth, none have ever been detected because the effect is very small (changes in distances of less than one part in 10^21- one thousand billion billion.) Such small changes are hard to measure.

However, the most surprising attribute of this theory is how it explains the phenomena we usually call gravity, for example the tendency bodies have when thrown into the air away from the earth to come back down and hit the earth. Or the pressure we feel in our feet when we stand on the earth. General Relativity describes these phenomena not by postulating some force that the earth exerts on us, pulling us to the center of the earth. Instead it explains these phenomena by the inequable flow of time from place to place.

To explain what the inequable flow of time means try this thought experiment. Say I have in front of me a clock and I watch it tick. At the same time you have in front of you a clock and I watch it tick. We will assume that both clocks are manufactured in the same plant to the same exacting standards and both are known to tick at exactly the same rate if held side by side. Now, I know you stay a constant distance from me, because if I shine a pulse of light at you, it takes the same amount of time to get back to me after reflecting from you no matter when I shine it at you. However as I look at your clock I see that it is ticking more slowly or more rapidly than mine is. That is what the "inequable flow of time" means. This effect has been observed to incredible precision, and is critical to the proper working of GPS (Global Positioning System) for navigation. This has nothing to do with synchronization, but the rates at which your clock ticks and the rate at which my clock ticks at a different location, the comparison in this case being done by light.

Thus near the earth time flows inequably. Newton told us that objects in the absence of any forces, move along "straight lines". Euclid taught us that a "straight line" is the shortest distance between two points. Then Einstein came along and taught us that straight lines in spacetime are lines along which a clock would measure the longest time between two points, not the shortest. Thus objects in spacetime try to travel so as to maximize the time they spend on the trip. Only if one exerts forces on the body does the time get shorter (like the travelling twin in the twin's paradox). When one throws a ball from one place to the other, the ball, in the absence of forces (and General Relativity says that gravity is not a force), tries to maximize the time. Since time ticks faster further from the earth, the object likes to get further away from the earth in order to maximize the time. Thus, the path near the earth of a ball flying from one point to the other looks like a parabola, rather than what we would usually consider a "straight line" in space.

One can show that trying to maximize the time also explains why the moon circles the earth, the earth the sun, etc. It also explains why we feel the pressure under our feet as we stand on the surface of the earth. This is force that the earth exerts from us preventing us from following the "maximum time" path through spacetime.

An analogy is the force we feel on our seats in a car as it goes around a curve. we feel a force of the car seat on our body pushing us toward the inside of the curve.  People sometimes say that this is because there is some sort of centripital force acting on us pushing us out away from the inside of the curve. But as physicists, we know that there is no such extra force. The force of the car seat on our bottoms is needed to accelerate us around the curve. There is no centripital force. Similarly the force we feel on our feet while standing is the force needed to prevent us from following a "straight line", a Maximal Time line, which would go into the earth toward its centre.
For more, try the Wikipedia entry on gravity.

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