You can use physics concepts in your answer because I read a lot of popular science books.I've never seen the answer to my question stated explicitly just that gravity is weak and hard to understand on the micro scale.Why does this make it such a conundrum though?In basic terms why is a theory of quantum gravity so hard to come by?
So, let me try to put it as simply as possible. In quantum mechanics, we know you can have superpositions. If you have a superposition of state A and state B, then there are two universes, one in which state A is the case, and one where B is the case. Now assume state A has a cat located in space at a given location, while state B has the cat 2 meters to the left of that location. The cat, like all matter, will have a gravitational field and a curvature of space-time associated with it. Since there are two states, there are two curvatures of space time for this system. But Einstien will tell you that space-time curvature is relative, and for this to be possible, there must be only ONE curvature of space-time, though the exactly how you observe it is relative to your frame.
Since quantum mechanics requires there be multiple spcae-times for superposition to exist, while relativity requires just one space-time, there is a huge problem where the two theories just can't agree. And this is the root problem.
Roger Penrose has the most understandable literature on this topic that I have found. Try reading the first part of his paper on gravitational decoherence.
There was a two part tv programme in the uk that finished last Monday called Steven Hawking, Master of the Universe on channel 4 and it explained why gravity was difficult to understand and how it was theoretically solved.
The gist was that gravity was weaker than the other three forces as it was "diluted" through 11 dimensions according to the String Theory. This would explain why it was weaker but cannot prove it. But they are hoping that the new accelerator in Switzerland may be able to prove in some way their theory.In basic terms why is a theory of quantum gravity so hard to come by?
Einstein's theories were smooth continuum's in which gravity fit the mold nicely - not so with packets of quanta.
Gravity appears to actually be a smooth continuum and there in lies the conundrum (I love that word).
Perhaps new discoveries will show that gravity is in fact made up of packets or quanta and can not exist in smaller units - presently, however, it remains that word - conundrum.In basic terms why is a theory of quantum gravity so hard to come by?
Although theories such a string and super symmetries have addressed the thorny issue of quantum gravity, there does not at present exist a complete working theory. Many theorists have published contributing elements towards a future theory of quantum gravity but at the present time these 'elements' have not been synthesised into a working theory. It seems that the physics community desperately needs another Einstein like theorist to come along and make the 'quantum jump' necessary.
Steven Hawking approached quantum gravity by suggesting that just inside the event horizon of a Black Hole, quantum fluctuations could result in a pair of virtual particle and anti-particles springing into existence. If one of these virtual particles quantum tunnels through the event horizon then it becomes a real particle and can escape into the universe. The process of Hawking radiation allows a gravitating object such as a Black Hole to 'shine' and evaporate. After many eons the slowly diminishing Black Hole would quantum explode back into the universe, according to Hawking's theory. This theory is perhaps the most well known and accessible version of a partial quantum gravity theory!
I don't think quantum gravity is a conundrum, it is just that all the theoretical 'bits' are not in place yet and the 'right' theorist is just not in place yet!
this is something I've thought about intensely and although I've come to a simple conclusion it still takes quite a bit of explaining
start at the beginning, the forces recognised by physics, are the two atomic forces, magnetism and gravity, gravity being the only one that doesn't fit into Einsteins equation
so gravity doesn't fit!
if you think of the other forces as being the forces of the very small and gravity being the force of the very large, things start to make sense
gravity is a force of mass, in a black hole where gravity increases due to an increase of mass it starts to become a stronger force
now picture a huge black hole (on a universal scale) where the mass has become a singularity, all the forces are in equilibrium momentarily until a large explosion, that the increased gravity can't contain (happens) which causes the formation of a universe and therefore the redistribution of gravity away from a single mass unto the collective mass of the universe
in my opinion there was never a starting point because everything is eternal, if you can get you head round that one then you're a better man than me
