Quantum mechanics/ general relativity

[tuxedofilm6]

Why is it that the theories of quantum mechanics and general relativity can not co-exist?

[El D]

They can because they do. The problem is reconciling where the differences lie and what causes them.

[Davidgrey]

 

Try reading Stephen Hawkings book "A brief history of time" Excellent book written in plain English.

[Loosehead]

They do co exist it is just that, as EI D says they are not unified. Physicists have been searching for what they call the "Grand Unified Theory" for the best part of a century.

Trouble is quantam theory throws up such bizzarre situations that standard Physics is at a loss to explain it. Another excellent book is "The Elegant Universe" by Brian Greene. but I rcommend "A Brief History of time" first. 

[jake-the-peg]

I'd take issue with the statement that quantum theory throws up such bizarre situations that "standard" physics is at a loss to explain it.

Quantum theory *is* standard physics - it is more sucessful even than relativity in explaining the world around us - if anything is bizarre it is the universe we see and not the physics we use to describe it.

I've been trying to find a good site describing the EPR (Einstein-Podolsky-Rosen) paradox but have come up empty handed.

The EPR paradox directly pitted relativity and quantum physics.

Basically if two particles are emitted from a nucleus in exactly opposite directions you can measure the momentum of one and the position of another with sufficient accuracy to violate Heisenberg's uncertainty principle - If you can't then they must communicate the act of measurement and if their seperation is great enough then that communication must exceed the speed of light. (yes I know it's not strictly true - quantum entanglement blah-de-blah)

In 1982 a variation of this experiment was performed at the University of Paris by Alain Aspect - Relativity lost.

Attempts to reconsile this are known as quantum non-locality which will really fry the brain.

There are other conflicts but this is probably the most famous

[Gef]

"Basically if two particles are emitted from a nucleus in exactly opposite directions you can measure the momentum of one and the position of another with sufficient accuracy to violate Heisenberg's uncertainty principle"

Sorry if I am wrong but I thought that Heisenberg's uncertainty principle related to the momentum and position of the same particle.

[jake-the-peg]

That's the whole point

You need a decay which by definition produces two identical particles.

In fact the HUP can be stated in a number of ways - and applies to different measurements not just momentum and position . Aspect's team as I recall measured the polarisation of photons.

This is easier to do, a charged particle changes its' quantum state and emits two photons in opposite directions - they must have the same polarisation however when they are both measured the act of measuring the polarisation of one is found to change the polarisation of the other.

The upshot is the notion of quantum entanglement where entities can be considered to be in related quantum states even though they are physically separated.