Science Forums 2

[Bo]

yes the books by brian greene are very good for understanding this.

As for the question: General relativity (GR) and quantum theory (QT) both operate on completely different scales. if you look at very small objects (atoms etc.) gravity is not important, and you can use QT, without bothering on GR. In this way extremely precise measurements have been made. However on larger scales (say the world we see around us). we can forget the quantum effects nd use newtonian mechanics. Which does include gravity and it works incredibly well. If we look at still bigger scales (the complete universe) it turns out that only gravity is important, however on this scale newtonian gravity has it's shortcomings and we need GR to describe everything correctly.
So the main thing here is: most theories are only more or less correct in a certain regime

Now the point is that there are situations that we need both gravity and quantum effects. (Big bang, black holes) so we need to combine QT and GR.

There are several reasons why QT and GR are extremely hard to bring together. I will give here the one that doesn't need any mathematics:

In QT, on the very small scale everything is extremely "wobly" the uncertainty that is crucial in QT says that we can never know everything of a certain point in space, or that points can never be exactly located. The result is that on this extremely small scale, everything -including space itself- is chaotic, spikey, etc. GR on the other hand requires that everything is completely smooth (it may be curved, but the curve should be smooth) (there are subtleties here; see my post in the "quantum singularity" thread). The logical conclusion is that we cannot describe the smooth space-time of GR in the wobly spikey space-time of QT.

Bo