Saturday, 8 September 2012

What is Gravity and why does it work?

Since Issac Newton, scientists have tried to grasp one of the 4 fundamental forces of nature. Gravity. The weaker of the 4 forces but yet little is known about the way it operates. We understand how it has a relationship with mass/matter and that gravity due to relativity can alter or bend space and time if strong enough. Examples of this would be black holes. Infinitely massive particles but reduced to an infinitely small space. Such compression typically makes a black hole. Though as mentioned in my previous posts black holes are not portals to another world or plane of existence. But what is inside them, is a tiny dot with so much mass, and light is unable to escape the clutches of the void.

So what is gravity, in its purest sense? How does it work? Well there are two answers to this question:

1) Nobody knows. It just does.

2) There's a very peculiar property of gravity that people have noticed for a long time. In particular, "gravitational charge" is exactly the same quantity as "inertial mass". That is, the "m" in the formula "F = G m1 m2 / r2" for gravitational attraction is exactly the same stuff as in the formula "F = ma". That is interesting, because there's no particular reason why that should be the case. But, for everything we've been able to measure (to exquisite accuracy) the "m" in the first formula is exactly the same as the "m" in the second formula, so that everything accelerates the same in the same gravitational field.

Well, forces with that property are often called "pseudo forces" and when they are observed they're usually just symptoms that you chose the wrong coordinate system to consider your problem. "Centrifugal force" is an example of a force like that -- if you're inside a sealed centrifuge, you can immediately tell that you are, even if you can't see out, because you stick to the walls. But someone in a near-inertial frame sitting around the outside of the centrifuge can see that the walls of the centrifuge are really just pulling you inward with an acceleration a, and f_centrifugal = m_you * a_centrifuge. (f=ma).

 Fig.1 Matter interacting with Gravity, and is influencing the space time fabric

The cool thing about Einstein's general theory of relativity is that it does away with the coincidence between the two kinds of mass. He instead explained gravity as a curvature of space time itself so that the straightest possible path through space and time curves slightly in the vicinity of mass. To do that, he had to produce a unified theory of space and time, which has us living in a four-dimensional place called "space time". The time axis is special in some ways but just like the spatial axes in other ways, for many things, you can treat time as "just" a different direction than the three we're used to.

The curvature is usually very, very slight, one of the important things to realize about Einsteinian relativity is that everything in the Universe is zipping along the "time" axis at the speed of light -- so if it takes you one second to jump up 4 feet from the ground and fall back down onto it, the amount of curvature of your trajectory is spread out over a complete light-second of motion (30,000 kilometres along your time axis). But because we don't perceive motion on the time axis directly, that curvature is very, very foreshortened and your trajectory looks like a tight parabola, just like you can spot a very slight bend in a pool cue by sighting down the length of the cue to foreshorten the cue itself.

Fig.2 Matter getting sucked in by a sheer gravitational vortices

Now, why does mass bend space time? Nobody knows. There are some very good ideas floating around, but the only solid answer right now is that nobody has been able to figure out why it actually happens. Though the unification of Quantum gravity is the makeup of General Relativity and Quantum Mechanics, which recently has taken a hit in the world of theoretical physics from analysed Gamma Ray Bursts. But in principle, no one theory has been proposed and has been proven, leading to an ever changing battle for the Grand Unified Theory.


  • ray says:
    7 October 2012 at 22:21

    We're living in XXI century and you could think that such "basic" things in physics like gravity are already explained. What a surprise:)

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