Gravity simply refuses to join in with the other forces in an all-encompassing theory of the Universe.
Gravity seems simple enough to understand – pretty much everyone is taught in school that gravity causes massive objects to pull every other massive object in the universe towards themselves. That, and gravity obeys the inverse square law (the intensity of a thing is diminished by the square of how far you are from the source, so if you double your distance from a massive object, you’ll feel one quarter of the gravity you did when you were half as close).
The equation for determining how much gravity an object possesses, that everyone was taught, is simple enough as well:
G = 6.67 x 10-11 N*m2/kg2 is the Universal Gravitational Constant. M is the mass; F is the force.
As derived by Sir Isaac Newton himself.
First, it was Newton, who derived the equation above. He explained gravity as a force that, unlike all the other forces one deals with in daily life, could act ‘at a distance’ – ie, a massive object could effectively ‘reach out’ and draw another massive object towards it without ever needing to touch it. Newton was rightly disturbed by this thought…
Einstein’s explanation, and his solution to Newton’s conundrum, was that gravity isn’t actually a real force at all (hence the reason why it seemed so troubling), but actually an after-effect of a massive object interacting with the fabric of the Universe itself.
Einstein’s explanation, and his solution to Newton’s conundrum, was that gravity isn’t actually a real force at all
Helpful analogy: Imagine sitting on a trampoline. The elastic material that is the base of the trampoline droops downward where you sit, because of your weight. A ping-pong ball then placed at the edge of the trampoline would roll down into the trampoline and stop at your legs, following the path of least resistance. If the ping-pong ball was heavy enough, it would also dent the fabric as it rolled its merry way down.
Essentially, that’s gravity. You are a planet, and the material is spacetime itself. The ping-pong ball is anything that has been ‘attracted’ to the planet. Every object of mass has its own bubble of deformed spacetime – you, me, the Earth, the Milky Way. Everything.
So long has it has mass.
Now: to compare to the other forces of nature, Gravity stands out like a sore thumb. It is a much weaker force than the others, it’s not mediated by particles of any sort (or so Einstein would agree), and it simply refuses to join in with the other forces in an all-encompassing theory of the Universe.
The last part is especially frustrating to those physicists trying to do just that. As a result, models like String Theory (in all its incarnations), among many others, have been formulated to try to remedy the situation. Unfortunately, not a single one of them can be verified through experiment with modern technology.
So why not just leave gravity alone, if it so refuses to take a place amongst Theories of Everything (ToE’s)?
Try telling that to Stephen Hawking.
If Hawking is right, the way for us to explain how the Universe came to be is lying within black holes. The two heavy-weights in the physics of the Universe are Quantum Mechanics (QM) and Relativity, and neither can handle black holes – QM can’t handle that they’re so massive, while Relativity can’t handle that they’re so tiny (the thing that makes a black hole a black hole is a ball with a radius of about zero).
A ToE that can handle black holes is an awesome theory indeed, and could help explain the Big Bang – if Hawking is right. In his mind, the Big Bang was simply the reverse of what happens in black holes. Instead of eating matter, it spat matter out.
Fitting gravity in with everything else in the Universe should ultimately give scientists insight into how everything came to be. If that doesn’t work, then maybe we should leave gravity alone after all.
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