Relativity and Quantum theory...?

Question

Physicist are trying to combine quantum theory with relativity.
Both contradict one another in certain areas. One theory will eventually have to give way so that a successful newer theory will emerge ( quantum gravity ).

Do you feel that this new theory will incorporate quantum theory within the general framework of relativity OR

vice versa, that relativity will exist as subset of quantum theory ?

Please explain your answer and please, serious answers only ( preferably by professionals ).

thank you.

Answer 1

Hi fullbony

First, a quick correction: quantum mechanics and relativity (the special theory) work very well together. Quantum electrodynamics (QED) is a relativistic quantum theory which is the most successful scientific theory ever (where "most successful" means greatest correlation between prediction and experiment). But special relativity is a theory of flat space-times without gravitation.

The theories which do not work well together are *General* Relativity (which is a relativistic theory of gravity) and quantum mechanics. It's useful to know why they don't work so well together. The other fundamental forces have been quantised in an approach generally termed quantum field theory. The quantum field theory (QFT) for electromagnetism is QED (mentioned above). When you try and build such a QFT, you encounter a number of difficulties in the formulation - the theories generally diverge, producing infinities in basic areas which blow away your model. Example: in QFT the electron is treated as a point-sized particle. the electron charge at classical distance is shielded by the polarisaiton of the vacuum, but at very close distances it becomes problematic. The field potential, emf, energy, etc, are all proportional to inverse powers of the distance, meaning the closer you get to the "point" the higher these values get. QFTs resolve this mathematical problem through a technique called "renormalisation", including balancing or counterterms to offset the infinities and leave you with a workable answer.

However when this same approach is applied to gravity it fails. Quantised GR generates higher order infinities - effectively infinities of infinities - which defeats renormalisation. This means we can't use the same format for quantum gravity as we do for quantum electrodynamics, weak or strong interactions.

So... what's left? Importantly, neither QFT nor GR will be "tossed out". Each type of theory is very successful in its own domain, so our new quantum gravity theory *must* reduce to GR in the classical (ie non-quantum) limit, and *must* look like QM or the appropriate QFT in the flat space limit. The current leading candidates are any of several string theories, M-theories, or loop gravity. String theories are most in fashion at present but have a single great flaw: they're yet to produce a testable distinguishing prediction (ie there's not yet a test we can perform which would identify a string theory as more accurate than any other quantum gravity candidate). M-theories or brane theories are a re-packaging of n-supergravity (a theory which poses symmetries using branes). Quantum loop gravity is simply a quantised gravity theory, meaning its focus is narrower. Personally I'd like to think that all the time and effort put into string theories won't be wasted - that it will eventually bear some fruit. However it concerns me that there hasn't been much solid empiricism yet.


Hope this helps!
The Chicken

Answer 2

Hello, good question, I love this subject! Modern string theory/M-theory has combined the two. The M stands for Mystery, Magic, or Matrix according to taste coined by the famous physicist Ed Witten. Heralding itself as the Grand Unified Theory introduced mid 80's it is still in its infancy as far as theories go. They have overcome serious problems with the mathematics of the theory. The ongoing problem is the inability to see the strings in a lab because of the size being so small. Though the Fermi Lab particle accelerator has made monumental strides in quantum science they are limited to the power capabilities that it can produce. The new particle accelerator in Cerne, Italy when fully functional in a few more years will have the best chance of finding the holy grail of M-theory. The Graviton. If found that the graviton seeps out of the proposed membrane of the 4-d (three space, 1 time) we live on then the theory will have solved what Einstein could not.
Einstein’s theory works very well on large scale but goes to pieces on quantum levels.
String theory merges the two mathematically and conceptually by stating that gravity truly is as strong as electromagnetism but the shape of the gravitons string is closed therefore not tied down to our dimension so it seeps into a higher dimension. The other particles are attached in half moon strings that are tied down and the force cannot escape. All forces other than gravity are analogous to jelly on toast. It stays. But gravity is more like cinnamon sugar on toast it is able to seep off.
A good source for these theories is Brian Greene's "The Elegant Universe" which I have had to read several times before truly understanding its core concept.
No cut and paste here. I hope I get an "E" for effort. hehe! Thanks for your time and hope this helped. Good day!

Answer 3

Well, I'd have to think that relativity would have to be a subset of quantum theory.

I though tsome of this is resolved in quantum electrodynamics, if you not that the magnetic field is actually a relativistic version of the E-field.

I guess that I don't consider the theory of general relativity to be very broad - not enough to be a framework to handle quantum effects.

E.g. if history repeats, as we discovered Newton's laws as a subset of the relativistic mechanics, then I'd go with my original answer.

Answer 4

The answer seems to be in string theory. Everything is made up of vibrating strings. Of course this requires that the universe contains 11 dimensions, most of which we are unable to perceive.
From NOVA, The Elegant Universe on PBS.
"The fundamental particles of the universe that physicists have identified—electrons, neutrinos, quarks, and so on—are the "letters" of all matter. They appear to have no further internal substructure. String theory proclaims otherwise. According to string theory, if we could examine these particles with even greater precision we would find that each is not pointlike but instead consists of a tiny, one-dimensional loop. Each particle contains a vibrating, oscillating, dancing filament that physicists have named a string.
Although it is by no means obvious, this simple replacement of point-particle material constituents with strings resolves the incompatibility between quantum mechanics and general relativity (which, as currently formulated, cannot both be right). String theory thereby unravels the central Gordian knot of contemporary theoretical physics.
String theory proclaims, for instance, that the observed particle properties associated with the four forces of nature (the strong and weak nuclear forces, electromagnetism, and gravity)—are a reflection of the various ways in which a string can vibrate. Each of the preferred mass and force charges are determined by the string's oscillatory pattern. The electron is a string vibrating one way, the up-quark is a string vibrating another way, and so on.
Far from being a collection of chaotic experimental facts, particle properties in string theory are the manifestation of one and the same physical feature: the resonant patterns of vibration of fundamental loops of string. The same idea applies to the forces of nature as well. Force particles are also associated with particular patterns of string vibration and hence everything, all matter and all forces, is unified under the same rubric of microscopic string oscillations.