General Relativity VS Quantum Theory

[fatty_ashy]

hi..I've been doing some reading on Quantum Cosmology, and read somewhere that General Relativity is inconsistent with Quantum Theory. I have always assumed (at least since i started reading about these higher level physics) that Quantum Theory and General Relativity were one of the few most accurate theories. or at least it is used in many areas of our life because classical (Eulcidean) physics was flawed. Does anyone know why and how are the two theories incosistent with each other?
I read it from here Please tell me what you think about this.

[Tormod]

It's not correct to say that Euclidean geometry is "wrong". It has worked for two millennia and still holds true in most cases. Euclids book on geometry is one of the most read, cherished, and reproduced books in history!

The "problem" with it is that it is a geometry for points, lines and fields in a 3 dimensional world. Modern physics and mathematics need models that will work in multidimensional space. A simple example is the theorem which states that all angles of a triangle gives a total of 180 degrees. This is of course true on a flat sheet of paper, but if you place the triangle on a sphere, you get more degrees. So you need better ways to explain that.

Relativity theory is not a replacement for Euclidean geometry, but rather builds on other theories which are again based on Euclid and others. See it as an extension, a way to explain our world with more accurate terms, without having to throw Euclid out the window. Euclid got a lot of things right, but the dogma that his theorems were the "only truth" is basically dead.

Now, for the problems with general relativity and quantum physics. General relativity is about everything that is larger than a particle, and how energy and mass behaves in relation to each other, while quantum physics explains the (weird) behavior and construction of the particles, right down to the point where we can no longer see anyting - the fabric of space-time itself.

Maybe we can say that general relativity tries to explain what the world looks like in a 4-d space-time, while quantum physics tries to explain what space-time _is_.

From your post you seem to think that relativity and quantum theory are one and the same. But that is absolutely not true. These are perhaps the two main theoretical achievements of the 20th century! They can both be right, and they can both be wrong. But they are not the same theory.

One of the big issues in trying to unify relativity with quantum physics is how to get gravity into the equations. For a very interesting account of this, read Lee Smolin's "Three roads to Quantum Gravity".

I can't get your link to work right now, maybe the site is down. But my answer to your question is basically, "there is no such thing as an accurate theory" and "Quantum Theory and General Relativity are two theories, not one".

Did that help at all?

Tormod

[anonymous]

Tormod,

It is my understanding that relativity and quantum mechanics contradict each other - in other words, they cannot both be right. What is this contradiction or why can't they both be right? In Brian Greene's Elegant Universe, he tries to explain this in his string theory. But I have not gotton to that part of the book yet.

[Tormod]

Dear anonymous -

let me know when you get to that part of the book - then you tell me.



Tormod

[deamonstar]

it is my understanding that they are both right. that general relativity describes properties of the macroscopic (matter... or large groups of quantum effects as a whole), and that quantum theory describes the effects of individual or very small groups of quantum particles.
if I remember correctly (probably not...) Greene describes strings as resonances of quantum effects that combine to form net results that lead to the macroscopic.

[Robust]

"....resonances of quantum effects that combine to form net results that lead to the macrosopic."

Thanks for that, Deamonstar. Greene appears to be da man!....will check him out.


"All things number and harmony." - Pythagoras

[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

[alexander]

Originally Posted by Tormod

It's not correct to say that Euclidean geometry is "wrong". It has worked for two millennia and still holds true in most cases. Euclids book on geometry is one of the most read, cherished, and reproduced books in history!

Not to be rude or disrespectful or anything, and not to get off topic, we have a section to discuss this, so i'll be glad to discuss it there (if you'll start though pm me or something so i notice it, but i'll attend), just thought i'd say this: and so is the Bible, but that doesnt mean that the concept of religion is not entirely wrong... (and please dont mix it with beleif like most people)

[pgrmdave]

It was my understanding that Euclidian geometry is still true within a universe with fewer dimensions and what Einstein did was simply expand upon his theories by adding time as a "physical" dimension. It's similar to the jump from a two dimensional geometry to a three dimensional, the concepts are the same, but the reality is much more complicated.

[TINNY]

a very warm welcome to pgrmdave.
excuse me, what's the difference between QT and QM?

It was my understanding that Euclidian geometry is still true within a universe with fewer dimensions

so how can euclidian geometry be simultaneously right and wrong? It has to be either one of the two. Refering to Bo, it is a matter of application and suitability. In most situations, it is satisfactory and you don't have to be concerned too much about quantum effects and spacetime-gravity.

There are several reasons why QT and GR are extremely hard to bring together

so what's the solution to bring it together? GUTs?