Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jeffreyH on 11/12/2013 19:31:51
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This classic experiment brought particle spin to the attention of the physics world, but would the direction of particle deflection be random under zero gravity?
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Compared to the electromagnetic forces action on the particles in the experiment, gravity is tiny, it might as well not be there.
If you did the experiment in zero gravity the outcome would be practically identical.
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Compared to the electromagnetic forces action on the particles in the experiment, gravity is tiny, it might as well not be there.
If you did the experiment in zero gravity the outcome would be practically identical.
So does that mean that the spin interacts with the electromagnetic field rather than the gravitational field? If so then is the up or down orientation of angular momentum affected by the field itself? In other words is the spin oriented generally perpendicular to the electromagnetic field?
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Compared to the electromagnetic forces action on the particles in the experiment, gravity is tiny, it might as well not be there.
If you did the experiment in zero gravity the outcome would be practically identical.
So does that mean that the spin interacts with the electromagnetic field rather than the gravitational field? If so then is the up or down orientation of angular momentum affected by the field itself? In other words is the spin oriented generally perpendicular to the electromagnetic field?
Yes--the potential energy of a particle with magnetic dipole moment in an external magnetic field is proportional to the dot product of the magnetic dipole moment with the field. In a constant magnetic field, the particle will rotate its spin so that its dipole moment aligns with the external field. However, in the Stern-Gerlach experiment, the field is not constant, so the particles are deflected in their path as they move in the field in such a way as to minimize their potential energy. Since there are two allowed spins rather than a continuously variable spin, the amount of deflection is quantized.
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In this context "Up" and "Down" are just handy labels for how the electron spins are aligned with the external field.
I guess they chose the names because the "up" state will "fall" into the "down" state and lose energy.
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In this context "Up" and "Down" are just handy labels for how the electron spins are aligned with the external field.
I guess they chose the names because the "up" state will "fall" into the "down" state and lose energy.
That is not what the wikipedia description says.
http://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment
QUOTE:
"The experiment is normally conducted using electrically neutral particles or atoms. This avoids the large deflection to the orbit of a charged particle moving through a magnetic field and allows spin-dependent effects to dominate. If the particle is treated as a classical spinning dipole, it will precess in a magnetic field because of the torque that the magnetic field exerts on the dipole (see torque-induced precession). If it moves through a homogeneous magnetic field, the forces exerted on opposite ends of the dipole cancel each other out and the trajectory of the particle is unaffected. However, if the magnetic field is inhomogeneous then the force on one end of the dipole will be slightly greater than the opposing force on the other end, so that there is a net force which deflects the particle's trajectory. If the particles were classical spinning objects, one would expect the distribution of their spin angular momentum vectors to be random and continuous. Each particle would be deflected by a different amount, producing some density distribution on the detector screen. Instead, the particles passing through the Stern–Gerlach apparatus are deflected either up or down by a specific amount. This was a measurement of the quantum observable now known as spin, which demonstrated possible outcomes of a measurement where the observable has point spectrum. Although some discrete quantum phenomena, such as atomic spectra, were observed much earlier, the Stern–Gerlach experiment allowed scientists to conduct measurements of deliberately superposed quantum states for the first time in the history of science.
By now it is known theoretically that quantum angular momentum of any kind has a discrete spectrum, which is sometimes imprecisely expressed as "angular momentum is quantized"."
The illustration next to this section also shows an up or down directionality. Is this incorrect?
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If you turned the apparatus on its side, the deflections would be to left and right, rather than up and down.
The deflection is into two streams, one towards, and the other away from the pointed magnet (rather than the flat one)
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If you turned the apparatus on its side, the deflections would be to left and right, rather than up and down.
The deflection is into two streams, one towards, and the other away from the pointed magnet (rather than the flat one)
Thanks for that clarification. It makes sense now. That throws into question my thoughts on particle spin and gravitation. The only thing I can still investigate is how the angle of momentum may flatten in a gravitational field. The question is would the deflection be greater in a weaker gravitational field?
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If this experiment, rather than being performed flat were inclined upward at a 45 degree angle, would the deviation be expected to differ by any amount. How could this be tested. Would the equipment to do this be beyond our current technologies?
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Do you realise just how weak gravity is?
It is about 1,000,000,000,000,000,000,000,000,000,000,000,000,000
(that's 39 zeros, it depends on exactly what particles you are using) times weaker than the electromagnetic forces involved in the S G experiment.
So, the effect of pointing this thing straight up or down, or sideways or running it on the moon would be almost exactly identical. You would not be able to tell the difference.
The experiment itself is quite tricky- the effect is rather small so it's a waste of time looking for a change that's the equivalent of a millionth of a millionth of a millionth of the size of an atom, compared to the distance to the sun (or something like that- I may have lost count of the zeros).
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Do you realise just how weak gravity is?
It is about 1,000,000,000,000,000,000,000,000,000,000,000,000,000
(that's 39 zeros, it depends on exactly what particles you are using) times weaker than the electromagnetic forces involved in the S G experiment.
So, the effect of pointing this thing straight up or down, or sideways or running it on the moon would be almost exactly identical. You would not be able to tell the difference.
The experiment itself is quite tricky- the effect is rather small so it's a waste of time looking for a change that's the equivalent of a millionth of a millionth of a millionth of the size of an atom, compared to the distance to the sun (or something like that- I may have lost count of the zeros).
Yes , gravity is weak, but on a macroscopic scale we see the effects easily. The parabolic trajectory of objects for example. If we cannot detect an influence at a microscopic scale, which is much easier to detect on a macroscopic scale, then what does that tell us about the difference between gravitation acting upon discrete particles and acting upon a solid mass made of molecules. Shouldn't this scale from the quantum level up to the macroscopic level unless something special is happening at the quantum scale?
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This classic experiment brought particle spin to the attention of the physics world, but would the direction of particle deflection be random under zero gravity?
Yes. The experiment has nothing to do with gravity. The only force acting on the particles was a magnetic force. The results are in no way dependant on the presence of a gravitational field. Any interaction with the gravitational field will not show up in the experiments due to the weakness of the gravitatoom field. The spin orientation is totally indepentant of the direction the gravitational field.
If this experiment, rather than being performed flat were inclined upward at a 45 degree angle, would the deviation be expected to differ by any amount. How could this be tested. Would the equipment to do this be beyond our current technologies?
No. The experiment is in no way affected by the presence of a gravitational field.If we cannot detect an influence at a microscopic scale, which is much easier to detect on a macroscopic scale
The macroscopic refers to the size of the source of gravity not the size of what's being studied.
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This classic experiment brought particle spin to the attention of the physics world, but would the direction of particle deflection be random under zero gravity?
Yes. The experiment has nothing to do with gravity. The only force acting on the particles was a magnetic force. The results are in no way dependant on the presence of a gravitational field. Any interaction with the gravitational field will not show up in the experiments due to the weakness of the gravitatoom field. The spin orientation is totally indepentant of the direction the gravitational field.
If this experiment, rather than being performed flat were inclined upward at a 45 degree angle, would the deviation be expected to differ by any amount. How could this be tested. Would the equipment to do this be beyond our current technologies?
No. The experiment is in no way affected by the presence of a gravitational field.
If we cannot detect an influence at a microscopic scale, which is much easier to detect on a macroscopic scale
The macroscopic refers to the size of the source of gravity not the size of what's being studied.
Yes but the field (the earths) is the same for the experiment in question and a beach ball or any other object following a trajectory due to gravitation. You seem to have misunderstood my point. If we can easily see the effect of gravitation on large macroscopic objects then why is it harder to see it with particles? Does gravitation operate differently at these scales? If so then why?
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There's three reasons we ignore gravity in this experiment. The first is that particles are light and they're accelerated to very high speeds. Even if you turned the magnet off, the particles would not fall very far by the time they hit the detector because they're traveling so fast that they simply don't have time to do so. It's almost certain that the experimental apparatus isn't sensitive enough to detect this.
But lets say, for the sake of argument, that it is sensitive enough to detect the falling of the particles. Once we turn on the magnetic field, the magnetic force will dwarf the gravitational force. So the deflection due to the magnetic field will be huge compared to the tiny deflection due to gravity. There's a practical problem of making a detector that could measure both the large shift due to the magnetic field and the small shift due to gravity.
Then there's the final point that you need a good reason to invest the considerable time and money into doing this experiment. Thinking that maybe there's a gravitational version of Stern-Gerlach isn't good enough. You'd need a well-founded theory that made a novel prediction.
People have done experiments where gravity is important for particles. The Milliken oil drop experiment is an important example: http://en.wikipedia.org/wiki/Oil_drop_experiment
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" If we can easily see the effect of gravitation on large macroscopic objects then why is it harder to see it with particles? Does gravitation operate differently at these scales? If so then why?"
A beachball has (at least very nearly) equal numbers of electrons and protons.
Each of those has an electric charge but, from a distance, the effect of each + charge tends to cancel out the effect of each - charge so the overall effect of the electrostatic forces is small (though they are what holds the ball together).
A small charged object near the ball will be attracted by one set of charges, but repelled by the others and so the effects nearly cancel out.
On the other hand, all mass is positive.
So all the particles attract all the other particles.
The effect of each individual one is tiny, but they all add together.
It's not a scale thing, it's a cancellation of two almost equal forces thing.
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Thanks for all the useful information. It is much appreciated.
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On the other hand, all mass is positive.
So all the particles attract all the other particles.
The effect of each individual one is tiny, but they all add together.
It's not a scale thing, it's a cancellation of two almost equal forces thing.
By all mass is positive do you mean it is biased to have an overall positive charge unless in an ionic state?
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I mean nothing has a negative mass, so all mass is positive.
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I mean nothing has a negative mass, so all mass is positive.
I assumed that but had to make sure. The other interpretation didn't make sense.