Action and the Light takes all paths concept.

[Goosebump]

My science minded brother (chemist and engineering grad) sent me a youtube video, lost the link was on steam, about how light and in fact all particles take all possible infinite paths and we only see the one that wasn't canceled out. My questions are these:

1. particles of light and possibly all others, 'consider' all the paths (wtf does a particle considering its path mean?).
2. most get canceled out by each-other like competing waves?
3. We observe the one with the "least action". But then they put the planet Mercury in the video and claimed it was active the same way. So all particles are acting this way?
4. So the particles in my body are "considering" on a path to the fried banana stand but when I don't get a fried banana those paths were canceled out some how? Meaning all the while there was a chance, possibly infinite chances of me getting a fried banana but I didn't because my brother is confusing me?
5. How, if at all, do I make the particles in my body more likely to chose, when considering paths, the one to the friend banana stand?

This might be a tall order. But WTF does any of this mean or have to do with anything? Or should I just go back to my milk and cookies and let the brainiacs lose sleep, and possibly fried bananas, over this one?

[HappySkeptic]

My science minded brother (chemist and engineering grad) sent me a youtube video, lost the link was on steam, about how light and in fact all particles take all possible infinite paths and we only see the one that wasn't canceled out. My questions are these:

1. particles of light and possibly all others, 'consider' all the paths (wtf does a particle considering its path mean?).
2. most get canceled out by each-other like competing waves?
3. We observe the one with the "least action". But then they put the planet Mercury in the video and claimed it was active the same way. So all particles are acting this way?
4. So the particles in my body are "considering" on a path to the fried banana stand but when I don't get a fried banana those paths were canceled out some how? Meaning all the while there was a chance, possibly infinite chances of me getting a fried banana but I didn't because my brother is confusing me?
5. How, if at all, do I make the particles in my body more likely to chose, when considering paths, the one to the friend banana stand?

This might be a tall order. But WTF does any of this mean or have to do with anything? Or should I just go back to my milk and cookies and let the brainiacs lose sleep, and possibly fried bananas, over this one?

I'll give this a try.

1) 

When considering light, it is simplest to consider it a wave until detected as a particle.  

Here is a cool thing about waves - the resulting wave moves as if every point on the wavefront is emitting energy.  The only thing that makes a wavefront seem to move as it does, is the constructive interference from the phase-coherent emission from all the points in the wave packet.  This is the same with quantum wavefunctions or water waves.  Even if we didn't plot the moving wave, we could add up the phase-specific emissions of every point in the wavepacket, assume a least-action (i.e. straight in a constant medium) path, and find the amplitude of the wave at any future point and time.

So, the concept of "all paths" is best solved using wave theory.  That is what the Quantum Wavefunction is.  It is a practical analog to the "all paths" calculation (which I can't imagine solving without using a wavefunction).  People who like to talk about particles taking "all paths" might say that the Quantum Wavefunction doesn't exist (but they would be hard pressed to calculate a result without it).  Others find the Quantum Wavefunction so useful that they might imagine it to be something "real".

2) Yes, if you tried to phase-add all the paths for a particular final position and time, one would find that only the path of least action remains, because the others all cancel out. But as I said, doing wave solutions is equivalent to doing this calculation.

3) The principle of least-action does not just show up in Quantum Physics. It is also a classical thing called a Lagrangian. It is used to calculate paths in Newtonian Physics, and there is something similar in General Relativity.

4) and 5) How do the particles all choose to go to the fried banana stand? Now that's a profound question.

The standard answer is that wavefunction collapse (or possibly wavefunction decoherence) reifies our macroscopic reality, and macroscopic reality mostly obeys classical rules.

The real answer is that no one knows how or why that is the case. Wavefunction collapse has no theory associated with it. It is a required hand-waving add-on to quantum theory. It appears to happen but we don't know why. You "should" be both going the fried banana stand and not, all at once.

[Goosebump]

My science minded brother (chemist and engineering grad) sent me a youtube video, lost the link was on steam, about how light and in fact all particles take all possible infinite paths and we only see the one that wasn't canceled out. My questions are these:

1. particles of light and possibly all others, 'consider' all the paths (wtf does a particle considering its path mean?).
2. most get canceled out by each-other like competing waves?
3. We observe the one with the "least action". But then they put the planet Mercury in the video and claimed it was active the same way. So all particles are acting this way?
4. So the particles in my body are "considering" on a path to the fried banana stand but when I don't get a fried banana those paths were canceled out some how? Meaning all the while there was a chance, possibly infinite chances of me getting a fried banana but I didn't because my brother is confusing me?
5. How, if at all, do I make the particles in my body more likely to chose, when considering paths, the one to the friend banana stand?

This might be a tall order. But WTF does any of this mean or have to do with anything? Or should I just go back to my milk and cookies and let the brainiacs lose sleep, and possibly fried bananas, over this one?

I'll give this a try.

1) 

When considering light, it is simplest to consider it a wave until detected as a particle.  

Here is a cool thing about waves - the resulting wave moves as if every point on the wavefront is emitting energy.  The only thing that makes a wavefront seem to move as it does, is the constructive interference from the phase-coherent emission from all the points in the wave packet.  This is the same with quantum wavefunctions or water waves.  Even if we didn't plot the moving wave, we could add up the phase-specific emissions of every point in the wavepacket, assume a least-action (i.e. straight in a constant medium) path, and find the amplitude of the wave at any future point and time.

So, the concept of "all paths" is best solved using wave theory.  That is what the Quantum Wavefunction is.  It is a practical analog to the "all paths" calculation (which I can't imagine solving without using a wavefunction).  People who like to talk about particles taking "all paths" might say that the Quantum Wavefunction doesn't exist (but they would be hard pressed to calculate a result without it).  Others find the Quantum Wavefunction so useful that they might imagine it to be something "real".

2) Yes, if you tried to phase-add all the paths for a particular final position and time, one would find that only the path of least action remains, because the others all cancel out.  But as I said, doing wave solutions is equivalent to doing this calculation.

3) The principle of least-action does not just show up in Quantum Physics.  It is also a classical thing called a Lagrangian.  It is used to calculate paths in Newtonian Physics, and there is something similar in General Relativity.

4) and 5) How do the particles all choose to go to the fried banana stand?  Now that's a profound question.  

The standard answer is that wavefunction collapse (or possibly wavefunction decoherence) reifies our macroscopic reality, and macroscopic reality mostly obeys classical rules.

The real answer is that no one knows how or why that is the case.  Wavefunction collapse has no theory associated with it.  It is a required hand-waving add-on to quantum theory.  It appears to happen but we don't know why.  You "should" be both going the fried banana stand and not, all at once.

I'm not sure I understand it any better. I still don't get what "action" and "considering" is. But I do understand what adding a, as you so aptly put, "hand-waving add-on" to make sense of things.

Also Thanks for your joke with 5.

What a crazy reality we live in.