Undergrad Momentum of a stationary particle/wave?

[Nugatory]

If the particle has no momentum until it's measured then the question I've always had is what does that mean ?

It means that it has no momentum the same way that I have no lap when I'm standing up and no fist when my hand is open with fingers outstretched.

and must it involve a sentient being doing the measurement?

No. An inanimate device whose output no one ever looks at is quite sufficient.

 

[bob012345]

It means that it has no momentum the same way that I have no lap when I'm standing up and no fist when my hand is open with fingers outstretched.No. An inanimate device whose output no one ever looks at is quite sufficient.

If an inanimate device is sufficient, then that device could conceivably be any other particle since any interaction might be considered a "measurement". Then, everything would be in a definite state between "measurements" which in the real world continuously happen.

 

[DarMM]

It means that it has no momentum the same way that I have no lap when I'm standing up and no fist when my hand is open with fingers outstretched.

What exactly do you mean by this? Do you mean that fundamentally quantum objects don't have an energy/momentum/position unless forced to adopt one by interaction with a classical object?

 

[Nugatory]

?...since any interaction might be considered a "measurement"...

Not "any" interaction, but "any thermodynamically irreversible" interaction. Yes, and that's why Schrödinger's cat isn't in a superposition of dead and alive before we open the box - the cat is continuously interacting irreversibly with its environment, and this is generally the case for all the macroscopic objects around us. The phenomenon is called "decoherence", and it explains why the macroscopic world generally behaves classically. The particle in a box is a different situation altogether; it is interacting only with the square-well potential, reversibly.

You can google for "quantum decoherence", although the math might be a bit daunting. David Lindley's book "Where does the weirdness go" is a layman-friendly overview worth reading if you don't want to take on the math.

 

[Nugatory]

What exactly do you mean by this? Do you mean that fundamentally quantum objects don't have an energy/momentum/position unless forced to adopt one by interaction with a classical object?

Not quite... but the context for the quote goes back to #22 of the thread.

 

[DarMM]

Not quite... but the context for the quote goes back to #22 of the thread.

I'm still not quite sure what you mean, is it simply that the particle has no position or momentum it just imprints one on the device?

 

[bob012345]

Not "any" interaction, but "any thermodynamically irreversible" interaction. Yes, and that's why Schrödinger's cat isn't in a superposition of dead and alive before we open the box - the cat is continuously interacting irreversibly with its environment, and this is generally the case for all the macroscopic objects around us. The phenomenon is called "decoherence", and it explains why the macroscopic world generally behaves classically. The particle in a box is a different situation altogether; it is interacting only with the square-well potential, reversibly.

You can google for "quantum decoherence", although the math might be a bit daunting. David Lindley's book "Where does the weirdness go" is a layman-friendly overview worth reading if you don't want to take on the math.

Thanks I've heard of it. It's what keeps my brain from already knowing everything to knowing almost nothing...:)