My problems with quantum theory.

[Gen1GT]

I am still learning as quickly as I can (thank Bob for audiobooks and long drives), but I have some problems with quantum theory. I might as well put them all in one thread, as I'm sure I'm not the only rookie who needs clarification on these concerns.

 

 

1) Quantum entanglement - how to two particles get entangled to begin with?

 

2) Quantum entaglement experiments - I know about the machine (by Clauser?) that fires two particles in opposite directions. One detector is supposed to measure velocity, and one detector is supposed to measure position. Why was this experiment a failure?

 

3) Uncertainty - it doesn't make sense my head that we'll never be able to measure both position and velocity accurately. How about this thought experiment: In a vacuum chamber, you fire a photon at a wall of strings. Isn't the resolution of the string wall high enough that we know exactly where the photon hit? We already know it was going the speed of light, so doesn't this give us an exact position and momentum?

 

4) Uncertainty - If we can't exactly calculate position and momentum of a particle, how does the LHC exist? Don't we need to move a particle at an exact velocity to a known position where it could be measured? If not, how can we can any result at all from the LHC experiments?

 

Please avoid jargon in your explanations ... thanks in advance!

Edited February 10, 2013 by Gen1GT

[swansont]

1) Quantum entanglement - how to two particles get entangled to begin with?

 

You are leveraging conserved quantities and the quantum nature of the system. One way for photons is downconversion — an interaction that gives off two photons. Because energy and both linear and angular momentum must be conserved, the polarizations are coupled.

 

2) Quantum entaglement experiments - I know about the machine (by Clauser?) that fires two particles in opposite directions. One detector is supposed to measure velocity, and one detector is supposed to measure position. Why was this experiment a failure?

 

link?

 

3) Uncertainty - it doesn't make sense my head that we'll never be able to measure both position and velocity accurately. How about this thought experiment: In a vacuum chamber, you fire a photon at a wall of strings. Isn't the resolution of the string wall high enough that we know exactly where the photon hit? We already know it was going the speed of light, so doesn't this give us an exact position and momentum?

 

Momentum is a vector that does not depend on c, (p=E/c for a photon) so you need to know direction as well as energy to know momentum.

 

4) Uncertainty - If we can't exactly calculate position and momentum of a particle, how does the LHC exist? Don't we need to move a particle at an exact velocity to a known position where it could be measured? If not, how can we can any result at all from the LHC experiments?

 

The detectors are large and capture a reasonable fraction of the area surrounding the collision site.

 

Please avoid jargon in your explanations ... thanks in advance!

 

Some jargon is unavoidable.

[derek w]

4) Uncertainty - If we can't exactly calculate position and momentum of a particle, how does the LHC exist? Don't we need to move a particle at an exact velocity to a known position where it could be measured? If not, how can we can any result at all from the LHC experiments?

The LHC works by taking a very large number of particles,confining them into a very thin beam,the odds are you will get a few collisions(not exact).

[Gen1GT]

Response to swansont:

 

1) I don't understand your answer. You're going to have to be analogy heavy. I would submit six more questions to understand your response, and more than likely, each of those six responses would require yet another six questions to understand. You can see how this could get out of control. I'm sure there is an analogeous explanation to how particles become entangled.

 

2) http://en.wikipedia.org/wiki/Clauser_and_Horne%27s_1974_Bell_test

 

Why weren't they able to measure both position and momentum? What does spin have to do with either position or momentum?

 

3) Okay, let's forget about momentum. If we fire a photon (which we know is travelling at c) at a wall of strings, does this not give us exact position and velocity?

 

4) One down, three to go! LOL

[swansont]

Response to swansont:

 

1) I don't understand your answer. You're going to have to be analogy heavy. I would submit six more questions to understand your response, and more than likely, each of those six responses would require yet another six questions to understand. You can see how this could get out of control. I'm sure there is an analogeous explanation to how particles become entangled.

 

OK, let's entangle other particles. A parent particle has spin 0. It decays into two daughters, each with spin 1/2. To conserve angular momentum those particle must be oriented opposite each other (spin up and spin down). They would be entangled.

 

 

2) http://en.wikipedia.org/wiki/Clauser_and_Horne's_1974_Bell_test

 

Why weren't they able to measure both position and momentum? What does spin have to do with either position or momentum?

 

There is no mention of spin or position or momentum in the link.

 

Position and momentum are linked via the Heisenberg uncertainty principle.

 

3) Okay, let's forget about momentum. If we fire a photon (which we know is travelling at c) at a wall of strings, does this not give us exact position and velocity?

 

 

Velocity is not momentum, and strings are not infinitely narrow. See also the explanation in my previous response.

[Gen1GT]

1) Okay, so if a device can use an electron to pop out two photons, they will each have 1/2 opposing spin? That type of thing?

 

2) I can't explain this further...in the books I've read, Clauser tried to disprove Bell's Theorem by using a machine to fire particles in opposite directions with one detector set to measure momentum and the other to measure position. Because the particles were fired at the same time, we could deduce position and momentum at the same time by measuring each particle separately.

 

3) I think you're dancing around my idea...LOL

[imatfaal]

1. electron has spin 1/2 to start with - so two particles with spin up and spin down 1/2 would not work as that is not conserving spin number. you need to start with a spin zero particle - Pions would fit the bill

 

2. [guess] all these decays are probabilistic - how would you take account of the possible decays that produce more than your desired number of particles and thus play havoc with your results. I would think you would have an uncertainty in your measurements - and we are back to square one [/guess]

 

3. how do you exactly localise the string etc for your calculations? ]

 

 

edit Neutral pions [latex] \pi^{0} \rightarrow 2 \gamma [/latex] seem perfect apart from the fact that photons are spin 1 not half

But there is another much less common decay

 

[latex] \pi^{0} \rightarrow 1 \gamma +e^+ + e^-[/latex]

 

But then you have to take the photon into account - ie see point two's guesswork

[swansont]

1) Okay, so if a device can use an electron to pop out two photons, they will each have 1/2 opposing spin? That type of thing?

 

That particular reaction isn't possible, but that's the idea

 

2) I can't explain this further...in the books I've read, Clauser tried to disprove Bell's Theorem by using a machine to fire particles in opposite directions with one detector set to measure momentum and the other to measure position. Because the particles were fired at the same time, we could deduce position and momentum at the same time by measuring each particle separately.

 

Clauser's experiment used polarization entanglement.

 

http://mist.npl.washington.edu/npl/int_rep/tiqm/TI_45.html

3) I think you're dancing around my idea...LOL

 

No. Your idea is is not telling you what you think it's telling you. If a photon is detected, the detector has a physical size, which gives the x uncertainty. The other term is momentum, not speed, and momentum is a vector. So there is a transverse component that has uncertainty, also based on the size of the detector.