Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thebrain13 on 19/12/2007 19:33:44

Title: quantum entanglement
Post by: thebrain13 on 19/12/2007 19:33:44

I dont know much about this topic. What objects can be entangled? How do you entangle them? And how does an entangled object affect another?

And please explain this in a simple manner, dont be a showoff. thanks.

Title: quantum entanglement
Post by: Dick1038 on 19/12/2007 20:30:21

There's a explanation on Wikipedia: http://en.wikipedia.org/wiki/Quantum_entanglement. Check it out. Or, just google the subject.

Title: quantum entanglement
Post by: lyner on 19/12/2007 21:27:04

It's all to do with the basis of Quantum Physics.
Any system - take an atom, as the simplest, can be described by an equation. this equation relates the energy and the arrangement of particles around the atom.  This equation must have a solution which agrees with reality - it would be nonsense otherwise. It turns out that the only reasonable solution which describes what we see is that the electrons are behaving like standing waves around the atom. The equation is called a Wave Equation, for that reason.
Just like a guitar string, which will only vibrate naturally at its fundamental frequency and  at overtones, the electrons, in orbit (as they say) can only be in certain positions or 'energy levels'.  This results in a set of 'Quantum Numbers'  with which you can  describe a system.
For all the common particles, the Pauli Exclusion Principle applies - this says that no two 'Fermions' can have the same identical set of quantum numbers.
Until you actually measure the states of a system, you don't know its quantum numbers. Actually doing the measurement resolves this. Before the measurement they are, not only unknown, but they can exist with any value.
If you have two systems - say two electrons on the outer shell of an atom. They will have so - called spin numbers (s) of +1/2and -1/2. You don't know which is which but you know they are different. We say they are entangled. If you measure one electron - or force it - to have a quantum number of +1/2 the the other's quantum number is instantly -1/2.
This may not sound significant or it may sound 'bleedin obvious' but it is important and it was a revolutionary idea, when it was first recognised. It means that one 'knows' what the other is doing.
Take two particles which are entangled in this way and put them a million km apart. When the state of one of them is determined, the state of the other is instantly known - you could say that this is the transfer of information at instantaneous (faster than light) speed.
It is the basis of quantum computers and, because of the immense speed of communication involved, the quantum computer is, potentially, very fast. Unfortunately, it isn't just as easy as that; you don't program it in the same way as your ordinary PC. It is very much in its infancy but it shows great promise.
Now get reading elsewhere!

Title: quantum entanglement
Post by: that mad man on 19/12/2007 22:50:38

Thanks sophiecentaur, that "clicked" with me [:)]

Title: quantum entanglement
Post by: thebrain13 on 20/12/2007 00:21:25

I want to know, what objects can be entangled, ive heard of photons and electrons before, is that it? How do they get entangled, why are only some particles entangled? And lastly can they affect more than just a particles spin?

Title: quantum entanglement
Post by: Soul Surfer on 20/12/2007 10:52:22

In almost all subatomic particle interactions two particles come together (collide) and interact producing two (and occasionally more)other particles.  These particles are said to be "entangled" and are incompletely defined in their properties until they have another interaction (or measurement) that effectively defines their properties.  When one of the particles has an interaction and its precise properties are defined, this means that the other particle coming out of the interaction must now also be fully defined but this particle is not fully defined until the very instant that the first particle is defined however far apart they are.  This is the prime example of the quantum weirdness of nature because inside a normal causal universe it implies that the "information" that the other particle has been defined has been transferred instantly and not at the speed of light.

The immediate riposte to this is of course that they were really defined absolutely at the instant of collision and we just didn't know until the other particle interacted so we were only seeing something that had already been settled but this is NOT true.  Very careful measurements have been made to prove that until the first interaction actually happens the twoparticles are genuinely undefined and could be in any of the allowable states.

Title: quantum entanglement
Post by: opus on 20/12/2007 18:32:27

String theory could surely become entangled......?