NationStates Jolt Archive

Physics question - I had a thought that will probably not interest non-physicists at all (maybe one or two, like me).

If I have source that I can decay into two electrons emitted simultaneously, won't examining the spin of one of them tell me the spin of the other WITHOUT observation?

Further - if I examine the spin of one, AND the position of the other - cannot I immediately ascertain the position and momentum of both?

Heisenberg, why hast thou forsaken me?

(Or - am I completely missing something?)

You are missing something.

It's physics, you always are.

You are missing something.

It's physics, you always are.

Bingo!

The answer to three of your four questions are 'Yes'. *nod*

Dragoneverything-is-alienstide has posted in my Physics thread? My work here is done...

If I have source that I can decay into two electrons emitted simultaneously, won't examining the spin of one of them tell me the spin of the other WITHOUT observation?

Yes: see EPR paradox and entanglement. Though the entanglement has a finite length of coherence - basically, the other particle could interact with something else and assume another spin.


Further - if I examine the spin of one, AND the position of the other - cannot I immediately ascertain the position and momentum of both?
This cannot be.
This is because IF the system of the two particles is in an entangled state of spin , it is a SINGLE system and the Heisenberg uncertainity principle tells you that you cannot measure simultaneously both conjugated quantities (momentum and position or spin and ...cannot remember) with infinite precision.
If the particles aren't entangled, well, you're fucked.

Yes: see EPR paradox and entanglement. Though the entanglement has a finite length of coherence - basically, the other particle could interact with something else and assume another spin.


Looking into EPR paradox and entanglement, now.


This cannot be.
This is because IF the system of the two particles is in an entangled state of spin , it is a SINGLE system and the Heisenberg uncertainity principle tells you that you cannot measure simultaneously both conjugated quantities (momentum and position or spin and ...cannot remember) with infinite precision.
If the particles aren't entangled, well, you're fucked.

If the particles weren't entangled, then you wouldn't gain the information about the one from observing the other. Far as I can tell.

The second concept - the 'single system' thing, I need to look into. I was under the impression Heisenberg held for particles, but I wasn't aware it could account for systems.

The second concept - the 'single system' thing, I need to look into. I was under the impression Heisenberg held for particles, but I wasn't aware it could account for systems.

I'd need to revise that (studied some years ago), but I really think I can remember that while particles are entangled the system can be seen as a "single" waveform, so there you are.
The Heisenberg principle places rules about measures (integrals) you can extract from probability densities - if that probability density describes two particles or one, that's immaterial.

Physics question - I had a thought that will probably not interest non-physicists at all (maybe one or two, like me).

If I have source that I can decay into two electrons emitted simultaneously, won't examining the spin of one of them tell me the spin of the other WITHOUT observation?

Further - if I examine the spin of one, AND the position of the other - cannot I immediately ascertain the position and momentum of both?

Heisenberg, why hast thou forsaken me?

(Or - am I completely missing something?)

You're missing the fact that the process of measuring the spin of an electron alters it's position, and vice versa. You might be able to determine what both of one electron's previous states had been, but not what they are now.

You're missing the fact that the process of measuring the spin of an electron alters it's position, and vice versa. You might be able to determine what both of one electron's previous states had been, but not what they are now.

No, I'm not missing that fact at all.

If you study on particle's momentum, you change it - according to Heisenberg. Okay - fair enough. But the first point I was discussing hinged on the fact that you would then ALSO know the momentum of the OTHER particle, without observing it - so you WOULDN'T alter it's position.

That was the first thought that suddenly crashed and exploded in my head.

The second one dropped straight out of that - if you were simultaneously analysing BOTH particles for different properties, you'd have a snapshot where you would know the position of one (and thus both) and the momentum of the other (and thus both). And Heisenberg suddenly exploded in a puff of logic.



Edit: The reason my head explodes in that first example isn't JUST because you effectively measure without observing, which seems like a problem to the logic - it was also because it effectively (in my head, at least) creates a kind of "Schrödinger's Heisenberg". The measurement of the first particle alters it. The non-observation of the second particle leaves it unaltered. The particles are entangled - and thus both altered AND un-altered.


"pop" brain asplode

No, I'm not missing that fact at all.

If you study on particle's momentum, you change it - according to Heisenberg. Okay - fair enough. But the first point I was discussing hinged on the fact that you would then ALSO know the momentum of the OTHER particle, without observing it - so you WOULDN'T alter it's position.

That was the first thought that suddenly crashed and exploded in my head.

The second one dropped straight out of that - if you were simultaneously analysing BOTH particles for different properties, you'd have a snapshot where you would know the position of one (and thus both) and the momentum of the other (and thus both). And Heisenberg suddenly exploded in a puff of logic.



Edit: The reason my head explodes in that first example isn't JUST because you effectively measure without observing, which seems like a problem to the logic - it was also because it effectively (in my head, at least) creates a kind of "Schrödinger's Heisenberg". The measurement of the first particle alters it. The non-observation of the second particle leaves it unaltered. The particles are entangled - and thus both altered AND un-altered.


"pop" brain asplode

Eww... messy. :D

No, I'm not missing that fact at all.

If you study on particle's momentum, you change it - according to Heisenberg. Okay - fair enough. But the first point I was discussing hinged on the fact that you would then ALSO know the momentum of the OTHER particle, without observing it - so you WOULDN'T alter it's position.

That was the first thought that suddenly crashed and exploded in my head.

The second one dropped straight out of that - if you were simultaneously analysing BOTH particles for different properties, you'd have a snapshot where you would know the position of one (and thus both) and the momentum of the other (and thus both). And Heisenberg suddenly exploded in a puff of logic.



Edit: The reason my head explodes in that first example isn't JUST because you effectively measure without observing, which seems like a problem to the logic - it was also because it effectively (in my head, at least) creates a kind of "Schrödinger's Heisenberg". The measurement of the first particle alters it. The non-observation of the second particle leaves it unaltered. The particles are entangled - and thus both altered AND un-altered.


"pop" brain asplode

Forgive me my idiocy--physics is not my strong point. At all. Is there a reason to assume that the two particles have the same position and momentum? Wouldn't observing (and thus altering) the momentum of one give you, at best, a guess about the other?

And as far as decay... can you actually know when two particles remain? I thought decay was a probability (hence Schrodinger's alive/dead cat).

The source decaying into two electrons simultaneously alone is enough to ask about the parameters.

Add to the fact that either your electrons are entangled, in which case the photons you measure as the observation of the electron's spin or momentum will affect both electrons and thus make the calculations less accurate in the pursuit of accuracy, or they are not entagled, in which case the measurements of the one do not allow you to gain any information on the other.

Compare it to baseballs. You have a source that pitches two baseballs. However, the particle that you bounce off the baseball to measure its momentum and determine its position is another baseball sized object. If the two baseballs are linked to each other in some way, then the measurement of the one affects both. If they are not linked, then the measurements on the one are useless to observing the other. Since photons are the particle that carry electromagnetic potential, bouncing them off the particles that emit and collect them is like bouncing moons in and out of planetary orbits.

surely if you change the result by measuring, then you only get the altered measure - you don't observe the unaltered electron at all, because by the simple fact of observing one of them, you have changed it, and it is now no longer twinned with the other, eh?

Physics question - I had a thought that will probably not interest non-physicists at all (maybe one or two, like me).

If I have source that I can decay into two electrons emitted simultaneously, won't examining the spin of one of them tell me the spin of the other WITHOUT observation?

Further - if I examine the spin of one, AND the position of the other - cannot I immediately ascertain the position and momentum of both?

Heisenberg, why hast thou forsaken me?

(Or - am I completely missing something?)
You're in esteemed group for thinking. Einstein argued the exact same thing.
May I direct to:
http://en.wikipedia.org/wiki/Uncertainty_principle#Critical_reactions
which explains it far better than I ever could.

this is also interesting, as it contains a recording of Heisenberg himself explaining his uncertainty principle:
http://www.thebigview.com/spacetime/uncertainty.html

"Schrödinger's Heisenberg".

So, Heisenberg goes in a box and we set up some poison to *gets shot*

creates a kind of "Schrödinger's Heisenberg".
This is doubly funny, as they hated each other.
Heisenberg basically said Schrödinger's were shit.

I know the part about the "flip one's spin and the other's spin", but why would that mean that they're both going at the same speed in the same direction? After all, we've already established that they're in two different locations...

I know the part about the "flip one's spin and the other's spin", but why would that mean that they're both going at the same speed in the same direction? After all, we've already established that they're in two different locations...

But they're generated in such a fashion that they are entangled - theoretically (as far as I can tell) they're playing a game of equal-but-opposite. So - since they're acting in that way, the fact that particle A is spinning 'clockwise' (yes, I realise that's like the worlds most enormous oversimplification), means particle B must be spinning 'anti-clockwise'. If particle A is 'going up' then particle B is 'going down'. etc.

Physics question - I had a thought that will probably not interest non-physicists at all (maybe one or two, like me).

If I have source that I can decay into two electrons emitted simultaneously, won't examining the spin of one of them tell me the spin of the other WITHOUT observation?

Correct.
Further - if I examine the spin of one, AND the position of the other - cannot I immediately ascertain the position and momentum of both?

Measuring the spin does not tell you EITHER position or momentum; it is a third quantity independent of either.
The units of the Heisenberg-uncertainty limit "h-bar" (Planck's number over 2 pi) are mass length-squared over time. Momentum is mass length over time; momentum uncertainty times position uncertainty <= h-bar. Spin, or "angular momentum", IS in mass length-squared over time; your uncertainty about it is <= h-bar. Electrons have spin "+ 1/2" (meaning it is somewhere from zero to h-bar) or "- 1/2" (somewhere from zero to negative h-bar).

Hmmm...I believe the answer for my locale is, and I quote, "GOOOOOOOOOOOOOOOOOOOOOD" So, I cant be of much help here, lol...

Correct.

Measuring the spin does not tell you EITHER position or momentum; it is a third quantity independent of either.
The units of the Heisenberg-uncertainty limit "h-bar" (Planck's number over 2 pi) are mass length-squared over time. Momentum is mass length over time; momentum uncertainty times position uncertainty <= h-bar. Spin, or "angular momentum", IS in mass length-squared over time; your uncertainty about it is <= h-bar. Electrons have spin "+ 1/2" (meaning it is somewhere from zero to h-bar) or "- 1/2" (somewhere from zero to negative h-bar).

Ah. It's that whole momentum versus angular momentum thing that's messing me up, maybe.